By Don Hewes (EAA 32101) minimize problems in the future. This article, therefore, is in- 12 Meadow Drive tended to be an indepth assessment of the phenomenon to help Newport News, VA 23606 the designer and builder gain a better overall understanding. In general, the several instances of extreme flight behavior referred to in these magazine articles represent the extreme or Overview most severe cases of a phenomenon which I've chosen to call This three part article concerns the effects of rain and bugs "Flight Behavior Change" or "FBC" for convenience in this article and for lack of a proper term. To say that these cases represent on the flight behavior of canard and tandem-wing airplanes which have been of concern to a number of homebuilders for quite some the norm and apply equally to all current designs would be grossly time. In the past few months a number of rather harrowing incorrect and would be a great injustice to the designers who have inflight experiences have been related in some of the aviation made very sincere efforts to minimize or eliminate this type of magazines and the apparent causes for them have been discussed behavior. It is important, however, to relate to these cases because to some extent. This article covers the subject in considerably they tend to "show us the way", but bear in mind that there are more technical detail and addresses some aspects that have not many tail-first airplanes flying which show only minimal changes been treated elsewhere. Some recent NASA data pertinent to the in behavior or none at all. Although FBC does occur with conventional airplanes, it is subject are presented along with some information supplied by generally of little concern for them. Sailplane enthusiasts have designers of these airplanes. An analysis and interpretation of long recognized and dealt with rain and bugs but the concern the available information is included along with a number of there apparently has been primarily with the effects on perfor- suggestions and recommendations for interested designers and mance at cruise conditions. Now, however, the phenomenon has homebuilders. become something of a problem with the new canard and tandem There are some areas which are not yet documented and some wing designs not only from the standpoint of performance but, which are still not clearly understood. However, based on availa- more importantly, from that of control and maneuvering. ble information, it is judged that this behavior is not necessarily Because both canard and tandem-wing configurations are dangerous BUT proper attention must be given to it in the design, generically similar, I'll generally use the term "tail-first" hereaf- building and operation of a tail-first airplane. Because of the ter for convenience to refer to either a canard or a tandem-wing complex interaction of the aerodynamic characteristics involved, design. By "generically similar", I mean that they both have two there are different forms of behavior that can be encountered with lifting surfaces each carrying a significant portion of the airplane's airplanes of different design. Moreover, it appears that the be- weight and both have the elevator located on the forward surface. havior of airplanes of the same design may differ depending on It is understood, of course, that the area and load ratios of the the specific design features, construction techniques and work- surfaces are different and this may be important from the manship details. Selection of the airfoils and accuracy in duplicat- standpoint of severity of the FBC, but this is not important for ing the shape, smoothness and alignment of the lifting surfaces purposes of classification. The difference between pusher and are considered to be of prime importance. tractor configuration is believed to be of relatively little signifi- Because each homebuilt aiplane is more or less unique in the cance for this phenomenon. aerodynamic sense due to the variables introduced during con- FBC can be triggered by any one or more of several agents struction, the builder must exercise his responsible role in assur- including rain, bugs, frost, snow, ice or any other material that ing the airworthiness of his own airplane. The builders and disrupts the boundary layer airflow of the lifting surfaces. There- designers can be of great mutual benefit by conducting specific fore, for convenience, I am simply using the term "contamination" flight tests and exchanging detailed information in those areas to refer to the condition brought on by any of these agents. not yet well documented and understood. Contamination may produce either a pitchdown or pitchup trim change and may be apparent to some degree over the complete Introduction flight envelope. In some cases, it may become very serious as Recent issues of SPORT AVIATION, Homebuilt Aircraft and airspeed is reduced and landing conditions are established or as Aviation Consumer have highlighted a number of instances in a takeoff is attempted. Furthermore, the nature and seriousness which there have been extreme changes in the longitudinal flight of the problem can vary from one copy to another of the same behavior of different homebuilt tail-first airplanes with particular design. emphasis on the tandem-wing configuration. Accumulation of We will cover this subject in three installments with the first rain, bugs or some other material that disrupts the laminar presenting an analysis of some of the aerodynamic factors in- airflow over the lifting surfaces has been attributed as the primary volved. The second installment will discuss some pertinent wind cause for these changes that, in some cases, have resulted in forced tunnel and flight test data, and the last will discuss a number of landings and injuries. Although it is important to understand related subjects including suggestions and recommendations. Al- that this is the primary cause, it is equally important to under- though considerable technical details and data are involved, we stand many of the other factors that are involved and how these have tried to include definitions and explanations of those items can be controlled by the designer and builder so as to avoid or which may not be readily understood by non-technical readers. 36 MAY 1983 Unfortunately, this led to a fairly lengthy text that many may of airplanes and attempting to find the cause for and solutions to find tedious to read, but I encourage those who are interested in the problems encountered. The end product of such work was a tail-first airplanes to read it all the way to the end. report of this effort written in an objective and unbiased manner, insofar as possible, and distributed to appropriate interested per- Background sons and organizations. With this background, it was only natural for me to apply the same approach to this particular subject as I first took note of the phenomenon several years ago at one an extension of my interests in the homebuilt airplane movement of the forums at Oshkosh where a description was given of a very and my desire to help advance the principles of safe flight. mild change in trimmed airspeed along with a gradual change in I'll conclude this discussion by noting that in the past few altitude when rain was encountered. Over the intervening years, months indirect references have been made in two separate news- I had been hearing of an increasing number of similar experiences letters to statements that I have made concerning the behavior and noted that some of the designers cautioned about flying in of tail-first airplanes as well as to the material presented in this rain or with dirty wings. Finally, when I started to build my article and in both cases I have been incorrectly quoted. I hope Dragonfly airplane over a year ago, I decided to took further into that those who may have noted those references will reassess the the problem myself. contents of those newsletters in the light of the material presented I've been a pilot since college days with essentially all my time in this article. Please bear in mind, of course, that there are in lightplanes but I have briefly flown several larger airplanes several areas in which personal opinion or judgment has to be including helicopers and heavy twins. I have briefly flown four used because there is insufficient information on which to base different tail-first airplanes, the prototype VariEze, Dick Rutan's firm conclusions. In these cases, you must allow room for honest Long-EZ, the RAF Long-EZ and the prototype Drangonfly This differences of opinion. experience certainly does not qualify me as a test pilot by any stretch of the imagination but I felt that my past 33 years experi- Definition of Problem Areas ence as an aero research engineer specializing in dynamic stability Our first concern is to identify just what flight behavior prob- and control at NASA's Langley Research Center provided me with the background to approach the problem on a fairly sound techni- lem areas we are going to be concerned with. In general, there cal basis. (I am now retired and have no direct connection with appear to be seven areas in which contamination effects can play NASA other than as a retiree.) a significant role; that is, 1) takeoff distance and speed, 2) rate Fortunately, some of my former associates at NASA had just of climb, 3) landing speed and distance, 4) stick forces and pitch completed a wind tunnel study using a full-scale model of the trim travel, 51 maneuvers at takeoff and approach speeds, 6) cruise VariEze tested in the Langley 30x60 foot wind tunnel and some efficiency, and 7) maximum speed. Of these, the last two are not of the data were published recently (see Reference 1). The model considered pertinent to the concerns of this article although they had been made in the Langley model shops and was a very may be of general interest. accurate replica of the VariEze design as depicted by the plans. Contamination effects should increase the distance and Since the tests were also run at speeds very close to the approach airspeed required to takeoff and, in an extreme case, could actually and landing speeds for the VariEze, the wind tunnel data could prevent takeoff altogether. Although takeoff may not be pre- be considered as being directly applicable to the actual airplane vented, the rate of climb after takeoff could be reduced to the without worry about scale effects. The canard had been mounted point that avoidance of ground obstructions could be difficult. The to the fuselage with a force measuring balance so that the direct same contamination effects that come into play at takeoff also lift, drag and pitch contributions of the canard were recorded are involved in landing and, as a result, landing speed and distance along with total aerodynamic contributions of the complete air- can be increased markedly. Perhaps of equal concern is the fact frame as measured by the primary balance system. that the ability to maneuver effectively at the takeoff and landing Some special tests were made in which water was sprayed on speeds may be somewhat limited. Whether or not these areas the airplane so as to represent the conditions which appeared to become real problems depends, of course, on the magnitudes of cause the pitch-change problems. Others were made with the the various contamination effects. leading edges coated with coarse grit to act as a boundary layer Small changes of a few ounces to a couple pounds in stick force tripping device. can be uncomfortable and disturbing for sustained periods of The researchers also had conducted some flight tests using a cruising flight, especially if the stick trim system is inadequate, VariEze built by EAA member Bob Woodall from Baltimore and but these should not be much of a bother during landing or takeoff. flown by him at the request of the NASA researchers. Other However, if the force changes are in the order of several pounds related flight tests were made at Mojave, CA, through the cooper- or more, the impact on the pilot may be significant. ation of Bun Rutan using a Long-EZ. These flight tests provided While the effects themselves in some cases may not be too some very important verification of the wind tunnel test results severe, just the distraction of having to cope with unknown or (see Reference 2). unfamiliar behavior at a critical moment could be serious, espec- The VariEze was selected for testing because it incorporated cially in the case of an inexperienced pilot. Thus, seemingly small a number of unique and interesting advanced design features and changes in the flight behavior due to contamination should not NOT because it had any significant problem. I have used these be regarded lightly. data because they are my only convenient source of information directly applicable to the subject of this article. By examining Discussion of Aerodynamic Factors these data it was possible to observe some trends which help Which Influcence Flight Behavior explain certain aspects of the FBC phenomenon. Two basic conditions under which an airplane is flown are 1) At this point, I want to comment about the reasons for writing with pilot holding the controls, and 2) with the controls free. this article. First, there is very little technical data available on Although the controls can be easily moved by the pilot when he this subject and very little of this has been presented in the current is holding them, we will refer to this as the STICK-FIXED condi- literature available to most homebuilders. Some of the canard tion because he does hold the stick in a more or less fixed position designers have spent much effort to evaluate the behavior of their in normal steady flight. We will refer to the latter condition as respective designs and have devoted considerable space in their STICK-FREE. Two aspects of the stick-fixed condition will be newsletter to this subject. However, there appear to be areas discussed as follows: 1) the aerodynamics of the airplane without which some have not fully evaluated. consideration of the forces that the pilot must apply to hold the Next, I'm sure that there is a very large group of potential controls fixed, and 2) specific reference to the control forces and designers, builders and pilots who do not have access to the the trim system used to balance the aerodynamic hinge moments. information made available by the designers through their news- In the following discussions, we will be considering the possible letters and direct contacts with their builders. This group needs effects of contamination on various aerodynamic characteristics information from some source to help in making decisions and and the resulting changes in the flight path of the airplane that taking actions which could affect their safety and their satisfaction would be induced by the aerodynamic changes. It is recognized with the airplane. that we will be discussing these characteristics relative to more Consequently, it appeared appropriate that the subject be or less steady level flight conditions, however, the results have a reviewed in an open manner so as to create an avenue for greater direct bearing on the dynamic or non-steady cases of landing and interchange of information. My background is that of a research takeoff which are of more concern. We will not be concerned at engineer working with the primary objective of conducting analyt- this time with the magnitude of the effects but with the possible ical, wind tunnel and flight studies to identify unusual behavior trends or changes involved. SPORT AVIATION 37 Stick-Fixed Aerodynamic Characteristics FIG. I - LEVEL FLIGHT BALANCE We need to concern ourselves with three of the basic aerodynamic characteristics of a tail-first airplane which regulate OF FORCES. the longitudinal motion of the airplane, that is, lift L, drag D and pitching moment M. We use the stick-fixed case so that we don't have to worry at this point about the effects of a free floating control surface on the pitching moments of the airplane. Actually, we will be concerned not only with the TOTAL lift, drag and pitching moment but, also, with the contributions to each of these made mainly by the TAIL and the WING. For this part of the discussion, DEFLECTION and TRIM of the elevator and POWER or THRUST T remain CONSTANT unless otherwise noted. In general, the effects of contamination of the wing and canard normally would be expected to INCREASE DRAG and to DE- CREASE LIFT at a given airspeed. Although the pitching mo- ments of the individual components might not be altered signific- antly, these drag and lift changes of the wing and tail result in significant changes in the contributions of the components to the total PITCHING MOMENT of the airplane. Furthermore, the resultant pitch change, if there is one, MAY BE EITHER IN THE NOSE-UP OR NOSE-DOWN direction depending on the location of each component relative to the center of gravity. To understand how these effects can influence the behavior of the airplane, we'll first introduce the familiar simple force diag- ram in Figure 1 which shows the TOTAL forces acting on an APPROX. EQUATIONS airplane TRIMMED for STRAIGHT and LEVEL FLIGHT at some arbitrary airspeed. The flight path is aligned with the airspeed vector VI and is at a zero angle relative to the horizon. We'll L - W = O assume that the airplane, in this case, is clear of contamination (no rain, bugs or what have you). The main point of this diagram is to remind you that, in level flight, the weight W of the airplane is balanced by the total lift L and the thrust T from the propeller is balanced by the total drag D for level flight, (for simplification, we are ignoring the fact that the thrust may not be aligned exactly opposite to the drag. Also, the simplified equations for the assumed steady flight conditions are included in this and the following two FLIGHT PATH ANGLE (%)- 0 figures for those of you who are more technically oriented.) Be- cause the airplane is assumed to be in trimmed flight, the pitching moment is necessarily zero. Next, in Figure 2, we show the diagram and equations for the FIG. 2 - EFFECTS OF LIFT AND case where the airplane has been subjected to the assumed DRAG and LIFT changes produced by the contamination. Taking the DRAG CHANGES (*L AND effects one at a time, we first note that the direct effect of the total DRAG INCREASE is to cause the airplane to PITCHDOWN L. somewhat and establish a RATE OF DESCENT denoted by the inclined flight path. Note that although the pitch ATTITUDE has changed, the ANGLE OF ATTACK HAS NOT CHANGED. (A pilot cannot determine angle of attack unless he has a suitable sensor and indicator available.) The additional thrust required to balance the higher drag was provided by a component of the weight which now acts along the descending flight path. If there had been no change in lift, there would have been NO ' J *\ "^^^*^^ __ — • — — SIGNIFICANT CHANGE IN AIRSPEED. However, since the ^r=^ total LIFT has to remain essentially equal to the weight of the l airplane for our assumed condition of STEADY FLIGHT, the effect of the LIFT DECREASE caused by contamination was to \ INCREASE THE AIRSPEED so as to restore the full amount of \ lift required for balance. (Note that, at a given angle of attack, lift is a function of both airspeed and lift coefficient. When we \^W cosX2 say here that lift is decreased, we actually mean that lift coefficient is decreased, thus airspeed must increase to regain the original W ^ total lift.) This, in turn, resulted in additional DRAG and, since both drag and airspeed were further increased, the FLIGHT PATH and RATE OF DESCENT INCREASED further also. Thus, we can see that a lift change tends to have a more complex influence on the behavior of the airplane than a drag change. If ONLY the lift had changed, the airspeed would have L 2-W cos changed just as much and the rate of descent would have changed '?... *-"'/7TIZ also because of the added induced drag. Up to this point, we have not changed the pitching moment or permitted the pitch control surface to move. So, we have the airplane at the SAME ANGLE OF ATTACK to the airstream as in the original situation. However, the PITCH ATTITUDE in Figure 2 has changed to a slight nose down condition because of the descending flight path angle. We now have the elements of a pitchdown behavior, that is, INCREASED AIRSPEED and RATE OF DESCENT with a NOSEDOWN ATTITUDE change even though the pitching moment of the airplane has not been changed. RATE OF DESCENT = If we had started originally with a climbing condition rather than 38 MAY 1983 level flight, the trends would have been the same, that is, the FIG. 3 - EFFECTS OF PITCH airspeed would have increased and the rate of climb and noseup attitude would have been reduced. The fundamental effect of a PITCHING MOMENT CHANGE TRIM CHANGE for steady flight conditions is to cause the airplane to CHANGE TO A NEW ANGLE OF ATTACK where the total moment is returned to zero. Of course, when this happens, BOTH the lift and drag characteristics are altered EVEN FURTHER due to the combined effects of angle of attack and airspeed. The nature of the changes will depend on both the magnitude and direction of the moment change, as well as the original angle of attack or airspeed. If it is a noseup moment change, then the airplane will tend to slow down and climb when operating close to cruise conditions or slow down and descend when near landing condi- tions. (The crossover point is the speed for best rate of climb, that is, maximum lift/drag ratio.) The opposite tendencies will happen if the pitching moment changes in the nosedown direction. It is quite evident now that there is a rather large range of possible combinations of effects for the STICK-FIXED condition we asumed. In some cases, the pitching moment change may tend to cancel the direct pitchdown effects of the contamination on lift and drag, but in others, the moment change will further increase those effects The MOST CRITICAL combination from the standpoint of the pitchdown behavior appears to be a NOSEDOWN MOMENT CHANGE ENCOUNTERED AT LOW AIRSPEEDS This case is depicted in Figure 3 As we noted previously, the pitching moment changes are dependent on the location of the individual components of the airplane, so we'll now refer you to Figure 4 where we have a L3- W cos &3 = O diagram for a TAIL-FIRST arrangement. Here we are considering only the lift contributions of the wing and tail to the pitching W moment of the airplane as these are major factors involved. One of the features of current tail-first designs is that the tail carries a significant share of the airplane's weight. For instance, the VariEze tail carries about 30 percent of the weight and the Quick ie, Q2 and Dragonfly about 60 percent. The amount depends on the location of the center of gravity which is dictated by stability =0 AT considerations. Obviously, any changes in the lift of either the wing or tail are going to have a significant effect on the pitching moment of the airplane. It is evident that a pitchdown moment change will result if there is a decrease in the tail lift and that an opposite moment change will result from a loss in wing lift. There will be no net CURVE moment change if both lifts change by the same percentage. Thus, the actual net moment change will depend to a large extent on SLOPE the degree to which the tail and wing are each affected by the contamination. This is an important point which will be referred to later. RATE OF DESCENT V3si*33 Stability and control considerations dictate that the FOR- WARD LIFTING SURFACE MUST STALL BEFORE THE AFT SURFACE, that is, that tail must stall before the wing. All current tail-first designs have the primary elevator surface mounted on the forward tail surface; consequently, at low speeds FIG. 4- SIMPLIFIED DIAGRAM OF the elevator TENDS TO OPERATE IN STALLED FLOW CON- DITIONS AND LOOSE EFFECTIVENESS. The term "elevator PITCHING MOMENTS FOR effectivenss" refers to the ability of the elevator to generate a pitching moment change with a change in elevator deflection. TAIL-FIRST DESIGN. This tendency to loose effectveness has been used in some, but not necessarily all, tail-first designs to provide stall-proof charac- teristics for the airplane as a whole. "Stall-proof means that the wing of the airplane cannot be stalled and that adverse behavior L. associated with a partically stalled wing is avoided by LIMITING ,, '-w THE MAXIMUM ANGLE OF ATTACK that can be reached. Very careful attention to design details is required to provide this feature and, if the design is compromised by poor workmanship or other factors, the benefits of this feature may be lost. Whether the airplane is or is not stall-proofed, the tail group is subject to partial or full-stalled conditions near the minimum airspeed of the airplane. Any factors which can influence pitch trim of the airplane AND stall of the lifting surfaces very likely will influence the MINIMUM AIRSPEED at which the airplane can be flown and possibly lead to some serious stability and control problems. If contamination by itself or in combination with factors W associated with poor workmanship can cause the tail to lose lift and stall at an angle of attack lower than intended by the designer, it is possible that the minimum speed will not be low enough to permit a safe takeoff or landing. On the other hand, if these factors Lc+ LW-W = O have a strong influence on the wing, it is possible that the minimum speed will be reduced to the point where other undesir- (LcxXc) - CLW* Xw)= able effects, such as wing rock or divergence, might be encoun- tered. SPORT AVIATION 3« From the several points covered in the last few paragraphs, also had been changed by contamination, as discussed in the it appears that the aerodynamic characteristics of the tail can be previous section, the stick force or trim control change would very important in determining the nature and magnitude of the reflect these other changes as well as the hinge moment changes. FBC. More will be said about this later. Up to this point, we have assumed that the throttle, elevator You probably are aware by this time that we have been talking and pitch trim controls have not been moved by the pilot and about two different kinds of "trim" relative to pitch behavior of have looked at what the resulting behavior of the airplane might the airplane. One has to do with pitching moments about the CG be. Rather than let the airplane respond in these ways, the pilot of the airplane, and I prefer to call this PITCH TRIM. The other most likely will move the controls to try to restore the airplane has to do with the pitch stick forces felt by the pilot and I prefer to its original trimmed flight condition. The movement of the to distinguish this from the other by calling it STICK TRIM. In controls from their original trimmed positions to the final posi- affect, pitch trim has to do with the POSITION of the pitch control, tions will be direct indications of the effects of contamination on that is, the position required to adjust the pitching moment to the basic aerodynamic characteristics of the airplane. The subject zero at the desired airspeed. Also, stick trim has to do with the of measurements of these movements in a series of flight tests FORCE exerted by the pilot to maintain pitch trim. If a pitch will be addressed later in this article. trim system is used, then the stick force can be reduced to zero in which case you can refer to stick trim in terms of trim-system Control Forces For the Stick-Fixed Conditions POSITION. Now we need to consider the forces that the pilot must exert When we say the plane is FULLY TRIMMED, then we should to hold the stick fixed or to move it because they provide the mean that both PITCH and STICK trim have been achieved under pilot's "feel" of the airplane and influence the manner in which steady flight conditions. In this case, the controls can be released he controls its motions. Obviously, the forces must not exceed the without disturbing the airplane. force capabilities of the pilot's arm, wrist and hand. But of equal I have found that a great deal of confusion and misunderstand- importance, they must stay within rather narrow limits to be ing often arises when a clear distinction is not made between considered acceptable and they should be adjustable by the pilot these different trim conditions. THIS DISTINCTION IS IMPOR- for reasons of comfort. The pitch control forces come from four TANT in our discussions of FBC. different forces — 1) the aerodynamic hinge moment of the elevator, 2) the pitch trim system, 3) the control system friction, Stick-Free Condition and 4) the mass or inertial moments of the system. We will not Basically, if an elevator is completely free to float, it will float worry about the last two because they are not pertinent to this at the specific deflection where the net hinge moment is zero for discussion. any given flight condition. As long as the net hinge moment is Please note that the hinge moment of a FIXED elevator has zero at the desired speed, then the airplane will be fully trimmed no connection with the pitching moment of the airplane and will and there will be no tendency for the airspeed to change. However, not influence the motion of the airplane in anyway whatsoever. if the aerodynamic hinge moment or the trim springs are altered, Of course, if the elevator is deflected as a result of the hinge then the elevator will tend to change to another angle thereby moment, then a pitching moment will be produced and the causing the airspeed to change usless the stick is restrained from airplane's motion will be affected. But in this situation, it is the moving. DEFLECTION not the HINGE MOMENT that causes the motion. Let's assume for the moment that contamination of the That is not the case we are discussing at this time. airplane affects ONLY the hinge moments of the elevator, just The hinge moment of the fixed elevator generally increases as we did initially in the last section. Also, let's assume that the with both angle of attack of the tail and deflection of the elevator airplane has been fully trimmed for level flight and the stick but the actual variation is dependent on the specific shape of the released before encountering contamination. Obviously, the elevator and location of the hinge line. We won't worry about the elevator will move and the airplane will respond to the elevator details other than to note that at larger angles of attack (lower motion in some manner as soon as the hinge moment changes airspeed) the elevator generally will have a moment in the trail- with contamination. For instance, if the elevator tends to float ing-edge-up direction. Consequently, the pilot would have to pro- further downward when contamination is encountered, the duce a continuous rearward-pull force to hold the elevator fixed airplane will pitch up and slow down to some lower airspeed. It at lower speed unless some form of a pitch trim system is provided. will also be climbing if the final speed is above that for best rate Although some trim systems may balance part of the hinge mo- of climb, or it will be descending if the speed is lower. Naturally, ment using some aerodynamic means, such as a tab, there usually if the elevator tends to float further upward, the airplane will are some mechanical springs included to facilitate adjustment of pitch down and the airspeed will increase. If the floating angle the stick forces. is changed too much, it will be necessary for the pilot to intercede Once the control force is balanced to zero for a given airspeed, either by readjusting the trim control or holding the stick if the any attempt by the pilot to move the stick from the trimmed trim control is not sufficient. position will be resisted by the aerodynamic hinge moments and If the phugoid motion of the airplane (the long-period oscilla- the springs of the trim system. These forces provide the "feel" for tory pitching motion) is very lightly damped, it is possible that a the stick. sudden encounter with the contamination will tend to initiate an The aerodynamic hinge moment is the direct result of essen- oscillatory response. However, this should impose no serious prob- tially the same pressure forces that act on the elevator to produce lem unless the speed change is significant, in which case, the pilot it's contributions to lift of the complete tail surface. However, the most likely will want to grab the stick while he retrims the pressure forces acting at the trailing edge are more effective in airplane. producing a moment about the hinge line than those close to the If the other aerodynamic characteristics are also changed with hinge line, consequently, the hinge moment is very sensitive to contamination, the resultant behavior of the airplane with con- small surface or airflow changes at the elevator that may have trols free would not necessarily be any more severe but the be- very little effect on the lift generated by the tail. havior could be completely different depending on which charac- Let's assume then that contamination causes ONLY the teristic was affected the most and which was the most influential. aerodynamic hinge moment of the elevator to change without Because of the many variables involved for the stick-free case, changing any other aerodynamic characteristic of the airplane. not much useful information can be learned about the sources of Now, if the pilot had the stick force originally trimmed to zero the changes produced by contamination. while in steady flight, a force will be exerted at the stick trying to move it to a new position. If the pilot wants to maintain the Use of the Controls As Indicators of Contamination Effects original airspeed, he must resist that force with his hand, at least until he can readjust the trim system. As long as the surface does In the previous section, we have been discussing primarily the not move, the change in only the aerodynamic hinge moment will possible effects of contamination on the aerodynamics of the not produce an effect on the motion of the airplane BUT the pilot airplane and the resulting types of flight path responses without will be feeling an effect through his stick. If the change in the regard to the magnitude of these possible effects and how to hinge moment may be exceeded and the pilot will have to apply document their impact on behavior of the airplane. The answer the extra force needed to completely balance the stick force. is to utilize wind tunnel data, if available, for some of the informa- Take note that either the change in the stick force or the trim tion and to extract some of it from actual flight tests. Fortunately, control position will be an indication or measure of the hinge we have some pertinent wind tunnel data available and will moment change. If some of the other aerodynamic characteristics discuss that in the next installment. There also is a limited 40 MAY 1983 amount of flight data and that will be discussed later as well. At less affected by contamination is normally used. Also, the elevator this point, however, we need to discuss just what type of flight in its tail-aft location tends to maintain sufficient power at low data is needed and how best to use it. speeds so that the pilot can override any serious moment changes To obtain all of the desired information from flight tests, we induced by the contamination would need a complete set of instrumentation that will record accurately all of the flight path variables involved along with the Discussion Summary and Comments positions of all the flight controls. This would permit us to calcu- The two physical features that make most current canard and late all the changes in lift, drag and pitching moment and relate tandem wing airplanes generically similar also make their pitch them to the actual motions that resulted from the effects of con- behavior susceptible to the effects of contamination. These fea- tamination. Of course, this would be a very costly and time con- tures are 11 carrying of a significant portion of the weight by each suming process and is not suitable for our purposes. Therefore, of two lifting surfaces, and 2) location of the elevator or pitch we must rely on some simpler flight test data that relate directly control on the forward lifting surface. Because of these features, to the behavior of the airplane but can be obtained with very the aerodynamics of the forward surface or the tail can have a simple instrumentation. major influence on the behavior of the airplane when contamina- The use of the control movements required to reestablish the — tion is encountered. However, there is a large number aero- original flight conditions, as mentioned briefly in the prior section, dynamic characteristics and their combinations for the airplane provides us with a suitable method which can be easily im- as a whole that can be altered by contamination and produce plemented. All that is required for much of the information are numerous types of pitch behavior. calibrated scales mounted on the appropriate controls so that the Specific areas to be concerned with are 11 takeoff distance and pilot can observe the before and after control settings and a speed, 2) rate of climb following takeoff. 3i landing speed and notebook and pencil to record the measurements. Even if measure- distance, 4( stick force and trim control travel, and 51 maneuvers ments are not taken, some useful qualitative information can be during takeoffand approach. Knowledge of the specific responses obtained from the pilot's impressions of the control movements of the airplane to encounters with contamination under carefully as long as he is properly prepared to make the observations. controlled conditions will help identify the specific aerodynamic To use these observations or measurements it is important to factors involved and should aid in finding methods for correcting understand the significance of the various control movements serious problems in these areas of concern. The following is a involved. For instance, if ONLY the trim position or stick force summary of the responses to changes in specific aerodynamic changed, then it is known that the hinge moment was the only forces: aerodynamic characteristic that was influenced by contamination. 1) An increase in ONLY drag caused by contamination will It is apparent then that only a modification of the elevator will cause an increase in the rate of descent (decrease rate of climb) be effective if the FBC is serious enough to require a fix. On the and cause the airplane's pitch attitude to change to some extent other hand, if movements of the stick or throttle were also in- in the nose down direction. Angle of attack and airspeed less than volved, then other aerodynamic characteristics were affected by maximum will not be affected to any noticeable degree. the contamination and other types of modifications might be 2) A decrease on ONLY lift will cause a direct increase in dictated. airspeed which, in turn, causes the drag to increase further. Con- As long as the airspeed and flight path are returned to their sequently, the rate of descent and pitch attitude will change also, original conditions, the change in throttle position reflects the BUT there will be no angle of attack change. TOTAL drag change caused by the contamination assuming, of 3) A change in ONLY pitching moment will cause the angle course, that operation of the engine and propeller have not also been influenced. I've emphasized the word "total" because we of attack to change which in turn will cause changes in the pitch cannot obtain separate indications of the drag contributions of attitude, airspeed and rate of descent. The direction of the change will be determined primarily by the loss of lift of the wing and the individual components of the total airplane with this very tail. A pitch-down change will be produced by a lift loss at the simple method. tail and a pitch-up change by a loss at the wing. The changes in The change in the stick position reflects a NET change in lift, rate of descent will depend on the trimmed airspeed relative to pitching moment and elevator effectiveness. To separate the com- the speed for best rate of climb (maximum lift drag ratio) of the bined change, it is necessary to add an angle of attack sensor and airplane. The most critical condition for a pitch-down behavior indicator so as to determine if the angle of attack had changed appears to be the loss of tail lift at low airspeeds. or not. If there had been no angle of attack change, then there 4)A change in ONLY the elevator hinge moment will cause would have been no change in the total lift. In this case, the stick the airspeed to change ONLY if the stick is free. If the stick is position reflects only the net changes in pitching moment and held fixed, the change in only the hinge moment will cause the elevator effectiveness. force required to hold the stick fixed to change, or the pitch trim The change in angle of attack indicates a change in total lift control to be changed by the pilot so as to reduce the force back of the airplane but does not differentiate between the wing and to zero. tail lift contributions. This differentiation, however, can be infer- Movements of the controls, that is, the changes in elevator red from the indicated change in pitching moment changes derived stick position (pitch trim and trim control position (stick trim) as from the stick position measurements. For instance, a pitchup well as the throttle, required to return the airplane to the original change would suggest that the wing lost more lift than the tail, trimmed flight condition, can be used to document the FBC and and vice versa. to aid in finding the possible causes of the behavior. As you have no doubt realized, this kind of analysis is not In this first installment, we have been concentrating on basic exactly easy and a fair amount of judgment is involved because the measurements involved are only INDICATIONS of the con- aerodynamic and flight dynamic principles without regard to specific details of a particular tail-first configuration. Next month tamination effects and not direct accurate measurements. Even we will cover pertinent NASA wind tunnel and flight tests of the though they are relatively crude, they should help to document VariEze airplane and discuss those construction and operational the type of behavior involved and to isolate the possible source factors which appear to contribute to FBC. We will also discuss of the major aerodynamic changes that cause that behavior. Such the relation of these findings to the several other current tail-first information is helpful in comparing various airplanes and in designs. trying to develop fixes for major problems.

Tail-Aft Versus Tail-First Pitch Behavior References The question as to why FBC is not as apparent with conven- 1. Yip, Long P. and Coy, Paul F.: Wind-Tunnel Investigation tional tail-aft designs as with tail-first designs can be answered of a Full-Scale Canard-Configured General Aviation Aircraft. to a large extent by noting that, in the case of the TAIL-AFT 13th ICAS Congress/AIAA Aircraft Systems and Technology Con- design, the wing is intended to carry essentially all of the weight ference, Seattle, Washington, Aug. 22-27, 1982. ICAS Paper of the airplane. The CG is therefore located very close to the lift Number 82-6.8.2. of the wing (near the wing quarter-chord location). Although a 2. Holmes, Dr. B. J. Obara, C. J.: Observations and Implica- noticeable lift change may result from contamination of the wing, tions of Natural Flow on Practical Airplane Surfaces. 13th ICAS its effect on the pitching moment will be small because of the Congress/AIAA Aircraft Systems and Technology Conference, negligible moment arm. Furthermore, the design requirements Seattle, Washington. August 22-27, 1982. ICAS Paper Number for the aft-tail are different than previously and an airfoil shape 82-5.1.1. SPORT AVIATION 41 the simpler plain flap used in other tail-first designs. The general By Don Hewes (EAA 32101) trends in the data as illustrated here should be fairly similar, 12 Meadow Drive however. Newport News, VA 23606 In Figure 6, three sets of data are shown, one for the tail with a dry smooth surface (FREE), one with the leading edges artifi- cially roughened with a small amount of coarse grit glued to both left and right tail panels (FIXED), and the last with both panels IN THE FIRST installment of this 3 part article, we discussed smooth but with one panel wetted with water sprayed from nozzles primarily aerodynamic characteristics that could influence the to simulate rain (FREE, WATER ON). The elevator deflection is flight behavior of tail-first airplanes when they encounter some zero for all cases. (The terms "free" and "fixed" refer to the types form of contamination; that is, rain, snow, frost, bugs, dirt or of air flow transition associated with the smooth and roughened what have you. The term "Flight Behavior Change" or "FBC" leading edges.) was coined to identify this influence. This month we will look There is a LOSS OF ABOUT 27% of the TAIL LIFT through- specifically at some pertinent wind tunnel and flight test data out the normal flight range of angles of attack for the second obtained by NASA using the VariEze airplane and relate these set of data compared with the first set showing that the ap- findings to our previous discussions of FBC and to the behavior plication of the coarse grit, representing a "bad case of the of several other current tail-first designs. We will also discuss bugs", had a very pronounced effect on the aerodynamic charac- various design, construction and operational factors that can have teristics of the canard. The loss of lift produced by the WATER some significant effect on FBC. SPRAY IS ROUGHLY EQUIVALENT TO THAT OF THE GRIT Before proceeding, however, I would like to restate what I said when you consider that only half of the canard was sprayed with previously: that the VariEze was selected by NASA because it water. There are also corresponding increases in the drag of the incorporated a number of unique and interesting advanced design canard associated with these losses of life. features and NOT because it has some particular problem. The These are very large changes in the aerodynamic characteris- data were used for this article because they were the only data tics and certainly indicate a significant effect of contamination available pertinent to the subject of this article. The specific data on the characteristics of the tail surface. (Remember, however, applies only to the VariEze design and should not be applied to we are looking only at the tail contribution and not the total aero the other designs without proper consideration being given to the characteristics of the airplane.) Of course, the influence of surface physical differences. However, some of the trends and effects noted contamination on the drag of sailplanes has been recognized for can be applied in a more or less straight forward manner as will many years and is associated with the premature TRANSITION be discussed in this installment. from laminar to turbulent flow which causes the drag to increase significantly but does not alter the lift to any extent at cruise Wind Tunnel Data For VariEze Tail Surface conditions. The somewhat unusual aspect of these data is the In the first installment we focused our attention on the critical relatively large loss of lift at the low angles of attack due to the nature of the tail surface, so let's examine this further by looking roughness but this type of loss can be found with some other at the NASA wind tunnel data obtained from Reference 1 for the airfoils that are unusually thick. The loss is attributed to prema- VariEze's tail surface. Figure 5 shows how the lift of the tail and ture turbulent flow SEPARATION near the trailing edge occur- the complete airplane varies in the wind tunnel as the angle of ring at the lower angles of attack. attack of the airplane is varied with the elevator set at several The trends noted in the data do correspond to those assumed deflections for the clean or uncontaminated condition. (These data in our earlier discussions, however, we are missing elevator hinge moment data which unfortunately were not available. for the tail are given in terms of the area of the main wing and, consequently, have smaller values than you would normally ex- Discussion of Tail Sensitivity to Contamination pect to see. The difference between the two curves represents the lift contribution of the wing and the fuselage combination.) What is it that makes the tail aerodynamics so sensitive to Note that the increments of tail lift produced by deflection of the effect of rain and bugs? The quick answer to this question is the elevator (the spacing between the tail lift curves) decrease as that it is the particular type of airfoil that is being used for the the elevator is deflected further and further downward. This is tail surface and is not the location of the surface on the airplane. especially evident for angles of attack above about 10°. Also, note But this needs explanation. that the canard begins to stall at about 14° with zero deflection As air flows past an airfoil, the molecules of air next to the BUT it begins at about 8° with maximum down deflection. Thus, surface are slowed by the frictional forces acting between the DOWNWARD deflection of the elevator REDUCES THE LIFT molecules and the airfoil surface as well as the molecules them- EFFECTIVENESS OF THE ELEVATOR AND CAUSES EAR- selves. This action forms what is referred to as a boundary layer LIER STALL of the complete canard surface. Bear in mind that on both the top and lower surfaces. We will concern ourselves this surface uses a slotted flap as the elevator which generally is with the layer over the upper surface. This boundary layer and more effective in generating lift increments with deflection than what happens to it are the key factors to the FBC phenomenon. 48 JUNE 1983 FIG. 5 - LIFT CHARACTERISTICS OF •8f VARIEZE (REF I.) .6 Q-«.

m

-8-404 8 12 16 20 24 28 32 36 40 44 WING STALL

CANARD STALL

12 16 20 24 28 32 36 40 44 a, cleg

SPORT AVIATION 49 CANARD TRANSITION

.4 O FREE D FIXED .3 O FREE, WATER ON .2

.1

0 ,01 .02 .03 .04 .05 .06 .07

-404 8 12 16 20 24 28 32 36 40 44 a,deg FIG. 6 - EFFECTS OF CONTAMINATION ON CANARD.

The boundary layer grows in thickness from just a few point where this occurs is referred to as the "transition point". At molecules at the nose to several tenths of an inch as the flow this point, the layer thickens, as illustrated in Sketch C, and the progresses toward the trailing edge. If we were to measure the flow from here on to the trailing edge and beyond is very random velocity distribution across the boundary layer with a very small in nature and referred to as "turbulent". The velocity profile for velocity probe and plot the variation of the velocity as a function this condition, also shown in Sketch B, indicates that not only of the distance from the airfoil surface, we would be looking at has thickness increased but the velocity of the molecules at a what is called a "velocity profile" as shown in Sketch A. As you given distance has decreased greatly. This condition creates much can see, the actual outer edge of the layer where the velocity more drag on the airfoil and the flow finds it much more difficult becomes constant is very difficult to locate, so for convenience we to follow the contour of the airfoil exactly. Although the lift being actually refer to the thickness as that point where the velocity generated by the airfoil is not altered to any significant degree has reached some arbitrary percentage of the free stream velocity by the transition to turbulent flow, the POTENTIAL FOR SEP- above the surface. We are not really interested in the actual ARATION of the flow from the airfoil altogether is increased. If thickness for this discussion but rather in the manner in which separation does occur, the result is then a marked loss of lift over it changes and the character of the flow within this layer. those portions of the airfoil aft of the "separation point". In effect, The flow in the boundary layer near the nose of the airfoil is the flow at the separation point starts to reverse direction and very orderly as the molecules slide smoothly, one over the other, the air from the lower surface starts to spill over the trailing edge. and we have what is called "laminar" flow as long as the surface Two of the key factors affecting the SEPARATION POINT are itself is relatively smooth. This results in a low drag or shearing the THICKNESS of the boundary layer and the LOCATION OF force on the airfoil. The velocity profile for laminar flow shows a THE TRANSITION POINT. If the transition point is located very rapid increase in velocity in a distance of only a few FORWARD on the airfoil where the boundary layer is quite thin, molecules. The shape of the profile is depicted in Sketch B. Al- then the boundary layer becomes thicker immediately behind the though the boundary layer gets progressively thicker as the flow transition point, as indicated in Sketch C, but the overall velocity moves aft from the nose, the shape of the profile does not change. or energy level within the turbulent boundary layer is relatively The significance of this is that the molecules within the boundary high. The potential for separation in this case is RELATIVELY layer maintain most of their energy and are able to continue LOW. If, on the other hand, the transition point is located much moving in a uniform manner following the contour of the airfoil further AFT where the laminar boundary layer has thickened for some distance. The greater the distance the laminar flow considerably and become "aged", that is, the molecules close to continues, the lower the drag will be. the surface have lost most of their energy, then the turbulent At some point along the airfoil, however, the molecules are layer that now forms is very much thicker, as indicated in Sketch going to start loosing their uniform motion and begin to tumble D, and its energy level is greatly reduced. The potential for over each other. This action is referred to as "transition" and the separation is now RELATIVELY HIGH. 50 JUNE 1983 The expanding shape of the forward portions of all airfoils the airplane flying in steady flight. The other points represent creates a positive pressure gradient which helps the air molecules non-steady flight, but the distinction is usually not made in this move out of the way of the wing as it passes through the air. This type of figure by the engineer because of normal practice. favorable gradient also helps to damp or stabilize the small As pointed out earlier, the "clean" canard stalled in the range amounts of airflow turbulence that tend to build up in the boun- of about 8 to 14 degrees angle of attack but it can be clearly seen dary layer. This causes the TRANSITION POINT to be further in Figure 5 that the complete airplane is not fully stalled until aft on the airfoil than it would be if there were no favorable about 24° is reached. Furthermore, the figure shows that the gradient. ELEVATOR CANNOT TRIM THE AIRPLANE TO ANGLES OF The tapering shape of the rear portion of the airfoil creates a ATTACK HIGHER THAN ABOUT 17°. There appears to be a negative or adverse pressure gradient which tends to cause the wide safety margin for stall of the wing built into this design, airflow to become turbulent sooner than it would if there were however, the margin actually is not as wide as it appears because no adverse gradient. Thus the TRANSITION POINT tends to there are aerodynamic problems associated with a partially stalled move forward in the presence of such a gradient. Not only this wing which must be considered. Departure of the lift curves from but also the negative gradient, if strong enough, will cause the a straight line at about 17° identifies the initiation of the partially SEPARATION TO OCCUR PREMATURELY. stalled condition. Thus, the margin actually is very small. Increasing angle of attack to produce greater lift increases the The loss in elevator effectiveness associated with the stall- pressure gradients over both portions of the airfoil. This increase proofing feature discussed earlier is evidenced by the bunching in the negative gradients over the aft portion is what causes the of the curves for the higher elevator deflections which occurs in airfoil to stall ultimately. If one airfoil at zero lift has a greater the range of 15 to 17° angle of attack. (Refer to the portions of negative gradient than another airfoil due to greater trailing-edge the pitching moment curves close to the CM = 0 axis in the upper taper, then that airfoil will stall at a lower angle of attack and figure.) The significance of this loss can be seen with the aid of develop less lift than the other. the solid curve of Figure 8 which shows the ability of the elevator The first of the two airfoils shown in Figure 7 is known as the to trim the airplane. This curve is a cross-plot of the data given GU 25-5(11)8 and is the one used on the VariEze tail. The GU in Figure 5 for the "clean" condition. It shows that the elevator 25 and variations of it are used on most all of the other canard cannot trim the airplane to a trimmed lift coefficient greater than designs currently flying as well. It is a laminar-flow airfoil de- about 1.5 which corresponds to the 17° angle of attack limit just veloped to give high lift at quite low Reynolds numbers (low mentioned. (An airspeed scale in knots has been added to the airspeed) and also to have some unique stall characteristics which original figure for reference purposes. These airspeeds refer to make it very useful for the highly loaded condition unique to the non-maneuvering level flight for the aft CG position at gross tail-first designs. weight at sea level and may not agree directly with the airspeeds The GU airfoil obtains its special characteristics by having a indicated in flight.) This indicates that a speed lower than about rather large thickness (20%) and having this thickness located 61 knots could not be reached even though the elevator was quite far aft. These features cause the boundary layer to remain deflected to relatively large downward (positive) angles. Changes laminar over about 55% of the upper surface and much further in weight and center of gravity position will alter the curve aft on the lower surface under normal conditions thereby produc- somewhat but the general trend will still be evident. ing very low drag. Comparison with the modified NASA GAW-1 This figure also shows a dashed curve for the "dirty" condition airfoil used for the less heavily loaded main wing of the VariEze, in which the flow was tripped by grit at the leading edges of the also given in Figure 7, shows that the maximum thickness of the wing and tail. Comparison of the two curves shows that the effect GU airfoil is greater than that of the GAW airfoil. of contamination was to cause a noticeable downward shift in the For the GU airfoil, as long as the flow is laminar before elevator deflection required to trim at a given lift coefficient or reaching the negative-gradient region and the angle of attack is airspeed and that an increase of about 8 to 10 knots for the not too large, the flow will continue to stay attached to the surface minimum airspeed due to contamination is indicated. This result so that the full chord length of the airfoil can produce lift until can be traced directly to the loss of tail lift illustrated in Figure the normal stall angle is reached. If, however, the flow has become turbulent due to roughness on the leading edge surface before reaching the region of adverse pressure gradient, then premature separation or stalling may occur even though the angle of attack is far below that for the normal stall, perhaps at zero angle of attack or less. Consequently, the GU airfoil will experience PRE- CANARD MATURE SEPARATION when there is sufficient accumulation of any type of material near the leading edge that will induce turbulence. A small amount of roughness or waviness existing in the surface may not, by itself, cause the separation because of the stabilizing effect of the positive pressure gradients along the forward portions of the airfoil. But this should tend to make the tail susceptible to much smaller amounts of contamination than otherwise. It should be noted that other types of airfoils, such as that used for the wing, are not as critical from the standpoint of premature separation because 1) the shape of the forward position GU 25-5(11)8 tends to cause transition to occur further forward, 2) the adverse pressure gradients due to the aft shape are not as severe as that for the GU airfoil, or 3) there is a combination of both. However, these other airfoils are not necessarily immune to the problem. It is possible that there are a number of other airfoils, including modifications of the GU section less critical as far as contamina- WING tion is concerned, that would be better suited for use on tail-first designs.

Complete-Airplane Wind Tunnel Data The pitching-moment data for several elevator settings as measured for the "clean" condition of the complete airplane are also presented in Figure 5 along with the previously mentioned lift data. The pitch data are given in coefficient form based on the wing area and mean chord of the main wing with the center MODIFIED NASA GAW-I of gravity located at the design aft limit. For those of you who are not familiar with wind tunnel data, I'll point out that only the data points for lift and drag for which the pitching moment FIG. 7-AIRFOIL COMPARISON. coefficient is zero, that is, trimmed lift and drag, correspond to SPORT AVIATION 51 15 CANARD TRANSITION FREE ——— FIXED max 10

0

-5 150 106 87 75 67 61 58 53 AIRSPEED, KNOTS -10 -0.5 0 0.5 1.0 1.5 2.0 Ltrim FIG. 8 - ELEVATOR TRIM CHARACTERISTICS FOR AFT C.G.CREF. 0

6 and its associated effect on the pitching moment produced by shape, incidence or twist distribution of the lifting surfaces and the tail relative to the CG of the airplane. should be corrected before considering the airplane to be fully The results of the wind tunnel tests are consistent with the airworthy. We will say more on these items a little bit later, trends that were discussed in the previous installment. but the reason for mentioning them here is to make the point Some of you who recall that an airfoil normally stalls at about that the amount of ELEVATOR DEFLECTION required for a 14 to 16 degrees may wonder about the rather high 24° angle of given airspeed can be used as a PARAMETER in evaluating how attack at which the VariEze main wing stalls as shown in Figure closely a given copy of a particular design matches the original 5. Bear in mind that the VariEze main wing is quite highly swept or other copies. If the deflections match within 2 or 3 degrees for and this has the effect of reducing the slope of the lift curve and the same airspeed, weight and CG location, then there is some extending the stall to the higher angles of attack. You will find assurance that the airplane is properly constructed and rigged. that the straight or nearly straight wings used in the other tail-first designs will stall at the lower angles. Comparison With Flight Data Discussion On Use Of Excessive Elevator Deflections Now, let's look at some VariEze flight test data to see if they The marked upswing of the solid curve shown in Figure 8 is agree with the wind tunnel data. attributed partly to the significant stable break (nose down) in Figure 9 shows a plot of the variations of elevator deflection the pitching moment curve as angle of attack is increased beyond with airspeed for the conditions of level flight with both "clean" about 14° with zero elevator deflection. This break is associated and "dirty" leading edges. These data were obtained from a series with the initial stalling of the canard. However, the upswing of of unpublished NASA tests of the VariEze that were related to the curve is also attributed to the loss of effectiveness of the those reported in Reference 2 for the Long-EZ. The figure reveals elevator as the elevator is deflected further and further from its that the airspeed was reduced, the minimum airspeed was in- zero position, noted earlier. It is especially important that suffi- creased and more down elevator deflection was required to main- cient elevator effectiveness be retained at speeds just above tain a given speed within the speed band when the leading edges minimum so that the airplane can be safely maneuvered for were contaminated. Note that very little additional deflection was take-off or landing. This being the case, then it is important to required at the high speed end whereas added deflections of 3 to not "use up" the elevator for purposes of trimming the airplane 5 degrees were required at the low speed end. Aside from an offset in cruise and high speed flight. In other words, it is important to of about 3° at the high speed end of the curves, there is reasonably adjust the relative incidence of the tail and wing so that the good agreement between the curves given in Figure 8 for the wind elevator will be trimmed close to zero deflection or, perhaps, tunnel tests and those of Figure 9 for the flight tests. Thus, it slightly up for cruise conditions. appears that there is good qualitative agreement between the If flight tests reveal that excessive amounts of down elevator wind tunnel results and those obtained from flight test. (There are being used to maintain steady conditions in the normal flight are a number of technical reasons for the data from these two envelope, there are problems with the construction and/or rigging different types of testing to differ to some extent, consequently of the airplane. These problems can be traced to improper airfoil engineers never expect to get exact agreement.) 52 JUNE 1983 14 Transition

free 12 o V MIN. fixed

10 ELEVATOR DEFLECTION 8 DE6. 6

V MAX.

I_

60 80 100 120 140 160

INDICATEDAIRSPEED, KNOTS

FIG. 9 -FLIGHT TEST DATA FOR VARIEZE.

A reasonable question to raise at this point is, "Well, did the Just as I was completing this article Burt invited me to come flight tests reveal any significant FBC as far as the pilot was to Mojave and fly the Long-EZ to check on its flight behavior. As concerned?" The answer to this is a qualified "Perhaps". It was it turned out, I arrived there in the midst of the series of heavy observed that the landing and take-off speeds with the "dirty" rain storms that hit California at the end of January and had an edges were noticeably higher than normal. This might present a excellent opportunity to fly in both rain and dry air. Dick Rutan problem to a pilot unfamiliar with the airplane, especially if he provided me with a very effective demonstration of aerobatics tried to operate from a short field. However, it should be noted with his plane in the rain and then I explored low speed behavior that the tests really were inconclusive relative to FBC behavior in both rain and dry air and found no significant deterioration in because they were not tailored to study this phenomenon and the the flight characteristics. I essentially repeated the same thing test pilot was not specifically asked to address this problem. with Mike Melvill in the factory Long-EZ because Dick's bird is The tests were made primarily to obtain the performance data not a stock Long-EZ, but there was very little difference between for straight and level flight with power required to sustain con- the two as far as rain effects were concerned. Inasmuch as I was stant altitude. No tests were made to determine effects on rate of riding in the back seat, I did not make the take-offs and landings climb and on the stick forces or trim. Also, banked and accelerated but these did appear to be reasonable even though it was raining maneuvers at low airspeeds, such as might be performed in abnor- at the time. Although these flights do not represent exhaustive mal landings and take-offs, and forward CG loading conditions testing under all conditions they did serve to demonstrate to me were not performed because they were not considered pertinent that the Long-EZ design and these particular copies of it have no to the primary objective of these particular tests. Inasmuch as significant FBC piloting problems even though the flight tests do these maneuvers and conditions require additional down-elevator show some influence on the elevator characteristics. deflections from those required for normal level flight, they are pertinent to the FBC problem and need to be performed. More on this subject will be said later. Comparison of FBC Characteristics An additional comparison of flight data can be made by exa- For Copies of the Same Design mining the Long-EZ test data given in Reference 2. We should There are no specific data available to me for comparing the note that the Long-EZ appears to be similar to the VariEze but FBC characteristics of various copies of the VariEze design. How- does differ in several respects. The wing is considerably larger ever, in recent discussions, Burt Rutan has stated that there have and carries more of the total weight of the airplane. It also has been noticeable differences in these characteristics observed for less sweepback and uses a different airfoil section. The canard, the large group of VariEze airplanes currently flying. In the case however, appears to be quite similar. The data, shown in Figure of a few of these airplanes, very definite pitchdown tendencies 10, were obtained for the same conditions as those for the VariEze have been encountered while, for the majority, only relatively and indicate somewhat similar results. Some later data provided mild tendencies have occurred having no significant effects on by Burt Rutan for the Long-EZ show essentially the same trends flight behavior. Surprisingly, in a few instances, a very mild as in Figure 10 but indicate that the minimum airspeed was pitchup has been encountered. It would appear that the results affected only slightly by loss of the laminar flow. obtained from the NASA tests were consistent with those obtained SPORT AVIATION 53 for the majority of these airplanes currently in use, but these tests Discussion of Possible Factors do not account for the extremes noted in a limited number of cases. We will discuss the following: 1) airfoil shape and surface skin As I'm sure you're already aware, no two of these homebuilt conditions, 2) elevator hinge gap and trailing edge shape, 3) copies can be built EXACTLY alike and none can be exactly like incidence and twist, 4) center of gravity location, and 5) weight. the original version because of the building tolerances involved First, we should consider how close the wing and tail surfaces and the small changes introduced by each builder to suit his can be duplicated by the many different homebuilders, a larger individual requirements. It, therefore, should be no surprise to number of them building their very first airplane. Good workman- anyone in learning that the flight behavior of these do differ to ship is vital in building to a high degree of accuracy and an some extent. The real problem is the large range of this variability. excellent finish is always a very good index of the quality of While some of these extreme cases may be attributed directly to workmanship. But a smooth and glossy finish and good workman- rather extensive modifications to the original design or very poor ship do not necessarily guarantee that the airfoil itself is made workmanship, there apparently are some where there are no to the required accuracy. Much can depend on the construction readily apparent differences to explain the behavior. techniques involved in addition to the skill and experience of the In our earlier discussions, we spent much time showing that builder. different forms of FBC could be produced by each of the basic Airfoil designers will tell you that departures of only a few aerodynamic forces and moments. Also we showed that the exact thousandths to hundredths of an inch in critical areas of the airfoil form would depend on the extent that the aerodynamic contribu- can alter basic characteristics very markedly. The leading edge tions of the components of the airplane were influenced by con- and upper surface are particularly critical. If the construction tamination. Discussions of the tail and its airfoil, which have been techniques do not permit the builder to work to required ac- identified as being a primary source for the FBC phenomenon, curacies in these areas, then inconsistent results may be obtained revealed that the tail's contributions were very sensitive to small even by very careful and experienced builders. irregularities on the surface of the tail. It is only reasonable, Prior to working with the fiberglass-foam type of construction therefore, to conclude that some cases of extreme behavior may myself, I thought that it should be a very accurate way of repro- have resulted from SMALL DIFFERENCES in various features ducing a given airfoil surface. Now that I have been involved to of the airplane or other factors that are not necessarily readily some extent in building several of them, I have changed my apparent. If so, then, we need to evaluate the effects of small opinion, at least concerning the current homebuilding techniques variations of a number of design, construction and operational I have used or observed. factors. There are several steps and working conditions in the process Unfortunately, there are no technical data to show us the that can influence the final shape. Some of these are 1) copying relative effects of the numerous factors that might be involved. template shape, 2) excessive melting due to cutting the cores too Consequently, we can only list those that we think might be slowly or with the wire too hot, 3) under cutting or scalloping due involved and discuss their possible impact. Obviously, this to wire lag (wire too slack, wire too cold or cutting speed too fast), involves a high degree of personal opinion and I freely admit that 4) wire bowing due to variable density at core joints, 5) foam core I am merely making educated guesses at this stage. The reader bowing (after wire cutting) due to internal and surface stresses, is invited to make his own assessment after reviewing this 6) assembling final structure from many core sections without article, and the author will be glad to hear from anyone who has accurate jigs, 7) surface irregularities caused by core defects and some constructive information to offer. cloth texture, overlap and joints.

20 18 Vmin trim TRANS ITION 16 OFR 14 D FIXED 12 ELEVATOR DEFLECTION 10 6e ' de9 8 6 V 4 max 2 0 60 80 100 120 140 160 180

INDICATED AIRSPEED, V( , knots FIG. 10 -FLIGHT TEST DATA FOR LONG-EZ. CFtEF. 2)

54 JUNE 1983 Some of these items are beyond the control of even a very VariEze or modified versions of it. The first three designs employ careful builder. Furthermore, there are no procedures using Eppler airfoils for the main wing which I believe is similar to templates for checking the final airfoil shape after completion. that used with the Long-EZ. The Retro uses a NACA 74-series The builder has no way of knowing what the exact shape is unless section for the wing. The first three designs also use a plain flap he goes to a great deal of extra work. But if he does, he has no design for the elevator rather than the slotted flap design way of knowing that the final shape is acceptable. employed by the others. Based on my own experience and observations, I believe that There are numerous copies of both the Quickie and Q2 deviations of 1/16 to 1/8 inch or more from the design shapes for airplanes and there are several reported encounters with signifi- both the tail and wing are possible and that deviations of this cant pitchdown. As a matter of fact, most all of the reported cases size are sufficient to account for some of the extremes in FBC that covered in the previously mentioned magazines have involved have been noted. There is little doubt in my mind that the CON- one or the other of these two designs. In reviewing the literature STRUCTION TECHNIQUE is responsible for these deviations to that Gene Sheehan, current president of Quickie Aircraft Co.,, a large extent. very kindly sent me, I found that he and the late Tom Jewett Aside from producing significant differences in the shape of have devoted a significant amount of effort studying the pitchdown the airfoil, the fiberglass-foam construction methods also result behavior experienced with their designs. Gene indicated that his in roughness and waviness of the surface skin which requires company had provided the builders with considerable information extensive filling and sanding to achieve an acceptable condition. on the subject warning against flying in rain or taking off with Of the two, WAVINESS probably is the more critical factor from wet or dirty wing and tail surfaces. Also he mentioned that the the aerodynamic standpoint and probably is the more difficult to design modification permitting the ailerons to be reflexed to detect and eliminate. I know that at least one of the designers provide additional pitch control power has proved to be very has gone to great lengths to get the builders to pay attention to effective in minimizing or eliminating the pitchdown behavior. the waviness problem, but I expect that there is a wide variation He also mentioned the design modification developed by Carry in these two factors for the final surfaces from one plane to the LeGare, a former associate, was also effective and was available next. to builders. This modification consists of a small horizontal trim- Another surface factor to consider is that of the actual SUR- ming surface mounted on the vertical tail. FACE FINISH. If the surface is covered with a very glossy and In my recent visit to Mojave, I also visited Gene and saw the slick finish, the rain will tend to adhere in the form of raised Q2 prototype which has been fitted with the NASA LS(1>- drops which will cause significant turbulence. On the other hand, 0417MOD airfoil for the front lifting surface. He reported that if the finish is dull or satiny, the rain may tend to spread out in rain has been found to have very little effect on the behavior of a thin smooth layer with little influence on the airflow. Also, the the modified Q2 and that the company plans to make the new quantity of bugs, ice or other materials that adhere to the surface surface available for retrofitting on current Q2 airplanes. He also may depend to a large extent on the type of surface treatment. said that they would be evaluating this airfoil for the Quickie Closely related to the airfoil "shape" problem are those of the also but he was not sure that it would be as effective because of ELEVATOR GAP and TRAILING EDGE. The shape and width the lower Reynolds number invovled. of the gap between the elevator and the fixed portion of the tail Bob Walters, designer of the Dragonfly, states that there is has a very strong influence on the flow separation behavior over only a mild pitchdown with his airplane and claims that it is not the elevator. Consequently, small differences here can cause two a significant problem. He does caution builders about flying in otherwise identical tails to have significant lift and elevator rain and warns not to take-off with wet or dirty surfaces. It should control power differences. The manner in which the elevator is be pointed out that, up until very recently, his airplane has been mounted with the external hinges does lend itself to possible the only Dragonfly flying, consequently he does not know yet how variations in the gap, but I really have no feel for how much of typical the behavior is for the numerous copies that are now a problem this may be. approaching flight status. The shape of the trailing edge has a very powerful effect on One of the first builders to fly his Dragonfly, Terry Nichols, the hinge moments of the elevator although the lift is not influ- reported in the recent Dragonfly newsletter that he performed enced to any appreciable degree. Here again I do not have any stalls in and out of rain and noted a 10 mph increase in stall information relative to the variability of these factors in the field, speed and apparently a more pronounced stall break due to the but I have a feeling that little attention is paid to the trailing rain. This result is consistent with Bob's recent comment that he edge shape. As a matter of fact, I believe that some of the early found the stall speed of his prototype was increased about 8 to 10 VariEze airplanes had a rather blunt trailing edge and that this mph. While on my recent trip, I was able to discuss this further was later changed to a sharp edge. This certainly could account with Terry as well as with Rex Taylor who has taken over the for noticeable differences in the stick trim characteristics between Dragonfly company. Rex has had similar experiences in rain but some of the airplanes. indicated he has not tested the airplane extensively in the rain. As noted earlier, variations in INCIDENCE and spanwise None of these people feel there is any significant problem with TWIST in both the tail and the main wing can be a source of trim the Dragonfly. Although I was able to fly the prototype Dragonfly differences from one airplane to another. Furthermore, inasmuch through some extreme maneuvers at low speed with no problem, as LOCATION of the CG has a direct effect on trim of the airplane, I was not able to do this with contaminated surfaces and, therefore, we can see that differences in CG location as well as lifting surface cannot verify their opinions. incidence will lead directly to differences in elevator deflection Examination of the Dragonfly templates given in the plans required to maintain a given airspeed. Also, differences in revealed that the tail airfoil is different from the standard GU WEIGHT of the airplane will cause the elevator to be trimmed 25 airfoil in that it is slightly thinner and the maximum thickness differently for a given airspeed. Discussions with two of the desig- appears to be moved slightly forward. When I asked Bob about ners have indicated that there have been cases where the elevator this, he stated that he was concerned about the abrupt contour deflections required for cruising flight have been down from the change of the GU 25 section and made the modification to help normal settings by several degrees, indicating some significant eliminate the separation tendencies encountered with the VariEze and unusual variations in the rigging and loading of the various tail. (He is one of the early builders of a VariEze.) He did not airplanes. I strongly suspect that these particular airplanes have explain whether this was merely to reduce drag for better cruise significantly different FBC than those that were rigged and loaded performance or to minimize the effects of rain, but regardless of uniformly. which was his intention, it appears that his modification to the airfoil may have been effective to some degree in alleviating FBC Discussion of Other Current Tail-First Designs characteristics which otherwise might have been more evident. Now, let's try to relate our information on some of the other It is pertinent to note here that an airfoil developed indepen- current tail-first designs to our discussions of the VariEze and dently for another application by airfoil designer John Roncz, Long-EZ. We are going to talk about the Quickie, Q2, Dragonfly whom I have consulted, matches closely the contours of the and a new single-place design called the "Retro" by its designer Dragonfly airfoil. John's study shows that his airfoil is not as and builder, Gion Bezzola of Estavazer Le Lac, Switzerland (see sensitive to flow separation problems as the original GU 25 sec- February 1982 SPORT AVIATION). Unfortunately, we have only tion. Thus, since the two airfoils appear to be quite similar, it is limited and very sketchy information on these designs. probable that the Dragonfly airfoil truly is effective in minimizing While these designs differ considerably from each other and the pitchdown problem as Bob's results have indicated. But until from the VariEze configuration, they all have heavily loaded front more Dragonflys have been test flown, this supposition will not lifting surfaces which employ the GU 25 airfoil used by the have been proven. SPORT AVIATION 55 DISTANCE

REFERENCE THICKNESS LAMINAR

VELOCITY f SKETCH A SKETCH B

TRANSITION

SKETCH C

SKETCH D SEPARATION

56 JUNE 1983 On a recent personal trip to Switzerland, I was fortunate to The process of evaluating the acceptable behavior of a have the opportunity to talk with Gion Bezzola and observe him homebuilt airplane is strictly a qualitative process utilizing the flying his prototype version of the Retro. He stated that it is judgment of one or more knowledgeable and experienced pilots. subject to pitchdown much the same as the VariEze but, unfortu- Strictly speaking there are no firm quantitative parameters that nately, we did not have time to discuss the subject in depth. He must be met, however, the pilot should make his judgment of FBC is a pilot for the Swiss Air Force and has many hours in numerous based on the extent that the "numbers" are changed for the airplanes of all types, including the VariEze. His views on the following items: 1) take-off distance and speed, 2) rate of climb factors involved in the pitchdown behavior were consistent with following take-off, 3) landing speed and distance, 4) stick force those I have expressed in this article. and trim travel, and 5) maneuvers at take-off and approach speeds.. Let's summarize our discussion at this point by observing that we have covered a total of six different designs and have found that each does demonstrate some form of FBC but the type and Closing Remarks severity are not necessarily the same for all designs. Certainly, The discussion covered in this article leads to the following there are basic design differences between the various designs statements: that are important and we should not expect that the behavior 1) FBC is a premature-stall phenomenon associated with tail- of all would be necessarily the same. first airplanes in which a significant portion of the weight is I would like to point out that most of the FBC encounters that carried by the two lifting surfaces and the elevator is located on I have heard about for any of these designs have been isolated the forward surface. Because thick laminar-flow airfoil sections incidents with the airplane flying into rain while in NORMAL such as the GU 25 section, which are quite sensitive to surface CRUISING flight and were NOT SPECIFIC TEST flights designed roughness or contamination have been used for the forward tail to isolate and identify the behavior. In some cases, these encoun- surface, the tail can have a major influence on the behavior. ters have been fairly mild and the persons reporting these events However, there are a number of aerodynamic and physical factors seem to be left with the impression that the phenomenon is of which influence the nature of this phenomenon and these factors little or no concern. Inasmuch as FBC may be much more critical can vary significantly from one copy to the next of a given design. for low speed flight conditions, there is the POSSIBILITY that Use of other airfoils less sensitive to the effects of contamination some airplanes that have been judged as having no problem will, should minimize or eliminate this behavior. in fact, demonstrate undesirable or unacceptable behavior. 2) Builders should be particularly aware of the critical nature The "bottom line" for this discussion is that, regardless of the of the various factors involved and are cautioned to follow the specific design, the builder or pilot must be aware of the POSSI- designers instructions and design details as closely as possible. BILITY that his particular airplane may have some unusual They should avoid making any changes in the airfoil shape or characteristics. Therefore, the only way to know for sure is to control surface configuration without thorough knowledge of the conduct proper flight test with his own airplane. influence of these changes on the flight behavior of their airplane. Such changes may cause the airplane to fly completely different from the original design with severe degradation in performance, flight behavior and safety. Discussion On Potential Dangerous Behavior 3) FBC appears to be more critical for low speed flight condi- We have been discussing at great lengths "FBC", "potential tions than for cruise and high speed because of loss of pitch control problems", and "undesirable or unacceptable behavior". But what power and lifting capability along with increased drag. Maneuv- do we mean when we use these words when applied to airplanes? ering at low speed may aggravate the behavior. Pilot control Do we mean that their behavior is dangerous, and what is it that inputs in response to the FBC encounter will depend on the type actually makes it dangerous? of behavior. Pilots who are unfamiliar with this phenomenon may To answer these questions, let's first define "dangerous be- not be able to recognize aggravated behavior and may apply wrong havior" as being "motions of the airplane, either controlled or recovery inputs. uncontrolled, that expose the occupants to the threat of immediate 4) Although this phenomenon is not necessarily a serious injury or loss of life". Next, "unacceptable behavior" will be defined problem for a particular design, FBC of each copy should be as "motions that require extraordinary or exceptional pilot skill evaluated completely with flight tests in much the same manner for safe controlled flight". Finally, we will define "undesirable as stalls and other flight behavior. These tests should be made behavior" as "motions that require normal pilot skill but impose during the initial airworthiness fly-off period required for certifi- an extra or disconcerting workload". cation of the airplane in the experimental category. I prefer not to use the term "dangerous" to describe the be- 5) Care should be exercised by the pilot when flying a tail-first havior of an airplane because it is imprecise and can be misun- airplane for the first time and he should check with others who derstood by many people. For instance, the act of getting out of may have flown it previously to learn about its specific behavior. bed is "dangerous" if it results in falling and breaking your leg. He should be aware of the potential for encountering some unusual Of course, many people also consider that flying in any airplane behavior and the proper technique for recovering. is "dangerous" but many others do not. Actually, it is more approp- The statements made here and throughout this article have riate to use "dangerous" to describe the total situation; that is, been based on an analysis of a number of basic aerodynamic facts the airplane PLUS the specific flight conditions or environment. and well established principles of flight dynamics and pilot be- By these definitions, then, we can say that, if the behavior of havior, as well as a very limited amount of experimental wind an airplane is judged to be "unacceptable", it may or may not be tunnel and flight test data. However, there are areas of personal dangerous because the skill level of the pilot in command must judgment and speculation, and consequently, these statements be taken into account. At this point, we must be a little bit careful should not be treated as firm conclusions until a much broader about assuming what the term "pilot skill" actually means. Many base of experimental data can be gathered and a more thorough pilots consider themselves highly skilled because they have sev- analysis made. eral thousand hours in many different type airplanes, and they Next month we will conclude with a number of suggestions certainly are justified to do so. However, in the case of flying an and recommendations pertaining to flight testing and operation airplane that is not familiar to them and that has an unusual of tail-first airplanes. behavior under some flight condition, some of these pilots may not be able to cope because they may not understand what is happening or the situation does not permit them the time to "feel out" the problem. If they were provided the information about References the nature of the problem beforehand, then it is highly likely that 1. Yip, Long P. and Coy, Paul F.: Wind-Tunnel Investigation all of them would be able to cope. Thus, we can see the importance of a Full-Scale Canard-Configured General Aviation Aircraft. of being ADEQUATELY INFORMED, as well as SKILLED, in 13th ICAS Congress; AIAA Aircraft Systems and Technology Con- cases of dealing with airplanes having UNUSUAL CHARAC- ference, Seattle, Washington, Aug. 22-27, 1982. ICAS Paper TERISTICS. Number 82-6.8.2. There should be little confusion in dealing with the term 2. Holmes, Dr. B. J.., Obara, C. J.: Observations and Implica- "undesirable". You can consider it as referring to nuisance traits. tions of Natural Flow on Practical Airplane Surfaces. 13th ICAS There are a number of airplanes that have one or more characteris- Congress—AIAA Aircraft Systems and Technology Conference, tics that can be termed "undesirable" yet the airplanes are consi- Seattle, Washington. August 22-27, 1982. ICAS Paper Number dered acceptable on the basis of their overall behavior. 82-5.1.1. SPORT AVIATION 57 By Don Hewes (EAA 32101) builder of a tail-first airplane can be certain of the 12 Meadow Drive airplane's flight behavior with contaminated surfaces is Newport News, VA 23606 to conduct flight tests designed specifically to evaluate this condition. I believe that such testing is in keeping with the builder's responsibility for the airworthiness of his airplane. Furthermore, it is a matter of the builder being _L HIS IS THE final installment of a three-part article fair not only with himself but his family and anyone who on the subject dealing with the responses of tail-first rides in or flies his airplane that this be done. airplanes to rain, snow, bugs or whatever. The term "Flight This recommendation applies to any canard or tandem Behavior Change", or "FBC" for short, has been coined to wing airplane whether it be the first copy of a new design refer to this type behavior. This behavior results primarily or the umpteenth copy of a proven design. In making the from changes in the aerodynamic characteristics of the decision to conduct the flight tests, it is recommended that lifting surfaces. This part covers several recommendations the builder contact the designer to discuss the tests and and suggestions for people who are designing, building or obtain his advice. The builder should be familiar with flying this type airplane. conducting such experimental test flights and have some recent flight time. As is true for initial flight testing of any homebuilt airplane, if the builder does not fit the role of test pilot, he should find some qualified person to do the flying for him. Recommendations and Suggestions It is suggested that the builder prepare a flight report of the tests and submit them to the designer. The report should cover the effects of contamination on at least the The keyword for conducting FBC tests, as in the case following items: 1) take-off distance, 2) rate of climb follow- of any flight testing, is BE PREPARED. Initial flight ing take-off, 3) landing speed and distance, 4) stick force testing of any homebuilt airplane, even if it is the 500th and trim control travel, and 5) maneuvers at approach copy of a well proven design, should always be treated as speed. Other factors such as elevator deflections and angle a truly experimental flight test operation with all approp- of attack changes should also be included. The designer riate precautions taken. After all, what is on the sign that can then correlate this information with his own tests and you had to place on the door? If you don't remember, it is those of the other builders as a way of isolating those spelled E-X-P-E-R-I-M-E-N-T-A-L. factors which are most important in affecting the FBC. It is suggested that the critical portions of the tests be Included in the builder's report on an airplane with a delayed until all others are completed. However, you can significant FBC problem should be detailed measurements perform the preliminary portions of the tests while doing of the various critical parts of the airplane. the other normal tests but be sure that the lifting surfaces Concerning the accuracy required to provide repeatable are dry and clean and avoid flying in the rain. Most of the aerodynamic characteristics for these airplanes, it is following tests should be done using a mid-location for the suggested that designers 1) evaluate the need for greater CG to begin with and then repeated with the CG moved accuracy, and 2) provide information on how to achieve progressively forward and rearward. The test pilot may the greater accuracy, if it is required, taking into account elect to eliminate some of the intermediate steps depending the wide spectrum of builder skill involved in the current on the observed behavior from the previous tests. homebuilding movement. Incorporation of final contour Although there are many tail-first airplanes currently templates and alignment jigs should be considered. flying which show no significant problems when they en- In the following sections, a series of flight tests to counter rain, bugs or whatever, there are enough cases of evaluate FBC is suggested for use in lieu of any other FBC problems to indicate that caution should be exercised specific information supplied by the designer. The actual when flying an airplane of this type with unknown charac- procedures will need to be developed by the builder and teristics. Because of the many variables associated with pilot following these suggestions. Also, suggestions are current techniques used in constructing homebuilt made regarding pilot preparedness and possible cures for fiberglass-foam airplanes, I believe that the only way a severe FBC problems. SPORT AVIATION 61 Flight Testing loading condition. Normally the elevator should be very close to zero deflection or whatever was specified by the designer. If there is greater than a couple degrees from Before commencing these tests, the airplane should be the desired setting, carefully inspect the airplane for im- equipped with accurate airspeed and rate of climb indi- proper rigging or some of the other factors covered in the cators and inflight calibrations of the airspeed installation previous sections. should be made to insure reliable airspeed measurements Following this test, conduct a series of baseline data within a couple MPH or better. An outside air temperature tests to measure take-off speed and distance as well as the indicator should be available for airspeed corrections. Re- rates of climb at take-off power in the range from take-off member that these measurements need to be accurate so speed to somewhat greater than that for maximum rate that reasonable comparisons can be made of other mea- of climb using increments of about 5 mph or so. The mea- sured quantities obtained from various different airplanes surements should be taken as you climb through the same using airspeed as the basis for comparison. If airspeed is altitude for each different speed so a series of saw-tooth in error, then the comparisons will not be reliable. Note climbs and descents will have to be made. If you wish, at that airspeed probes located in different positions of the the same time you can conduct idle-power tests which airplane may produce significantly different readings be- should also be made to measure the rates of descent for cause of local pressure differences (position error). the same speed range. (You may have to be concerned with An angle of attack indicator will be quite useful but is excessive cooling and heating cycles involved.) Be sure not an absolute necessity. A relatively simple vane that you have a well stabilized climb or descent established mounted on a boom extending about a foot or two forward before reaching the specific altitude for taking the mea- from the front surface near the tip can be used but it must surement. You will probably need several hundred feet to be calibrated to account for flow upwash. To do this, fly do so. It takes practice . . . and a passenger to take the the airplane at constant altitude for a series of speeds over readings is very handy. A small hand held tape recorder the speed range and compare the vane reading with an is also useful in place of the passenger. Note touchdown inclinometer mounted in the cockpit. You will be dealing speed and landing distance. with only a fairly small angular range of about 15° and The next step is to make a complete elevator vs. you should obtain each reading with an accuracy of about airspeed calibration (similar to that shown in Figures 9 1A° or better. You can use a small electrical potentiometer and 10) for each of the loading conditions at power required attached to the vane and a meter hooked up in simple for straight and level flight as part of the baseline data Wheatstone- bridge circuit to obtain the vane readings. for the subsequent special tests. All that is required is a Because the existence of a laminar boundary layer is scale on the elevator control calibrated in terms of elevator the key to the FBC phenomenon, it is important to deter- position and on the trim control calibrated in terms of any mine the amount of laminar flow that exists on your convenient scale appropriate for the type of control handle particular airplane for the various flight conditions. There- used. If the trim control is a crank type, then you will need fore, it is recommended that the first test should be one to keep track of the number of turns. The elevator scale to obtain a visual indication of the boundary layer flow should be accurate to within about one-quarter of a degree conditions on the wing and tail surfaces. This can be done and should be read to at least one-half a degree. The trim with either of two fairly simple techniques, the first is the system scale should be accurate to about 1 or 2 percent of one described in Reference 3 using sublimating chemicals. the full travel. It is recommended that all these tests be This has been used very successfully by NASA and is flown at safe altitude of at least three thousand feet above highly recommended. the local terrain. The second technique is similar but uses plain motor The next step is to repeat the last step taking data for oil in place of the chemicals. It has been used extensively coordinated banked turns of 30 and 60 degrees. in wind tunnels but I have not had any personal experience Then the following step is to perform a series of banked with it. Also, I don't have any convenient reference for the coordinated turns at the normal approach and touchdown steps involved, so a bit of experimenting will be involved. speeds using slow and then rapid control inputs. Do this You should be able to find a couple quarts of old used motor with power to maintain essentially constant altitude and oil that are heavily loaded with soot so that the oil is very then with normal reduced power for landing. Start with black. Wipe this uniformly over the lifting surfaces to shallow turns and observe any increased tendency to pitch provide a thin layer of oil which will tend to migrate to down or up. Carefully note any additional aileron/rudder the region just aft of the TRANSITION POINT. Be sure deflections required to correct for possible roll/yaw tenden- that the oil does not tend to form drops that linger other- cies caused by partial flow separation on only one side of wise you are creating the same effect as rain or bugs. You the tail or wing. If a pitch break occurs, hold controls may have to adjust the viscosity of the oil by thinning with steady if possible to observe the motions of the airplane. kerosene or thickening with heavier weight oil to get the Note rates of descent. proper effect. You will probably need someone to help Attempt to recover by pulling further aft on the control. observe the flow patterns which will undoubtedly change This may result in an aggravated FBC with the airspeed with the different flight conditions. increasing significantly. In this case, you may need to You should expect to see flow transition point some- PUSH THE CONTROL FORWARD as you would in the where near the mid-chord position on the tail and probably case of the stall of a conventional airplane. .'Remember further forward on the wing. If this test shows that the that the tail is in a stalled condition with the elevator transition point is near the leading edge, then there is deflected downward. Raising the elevator will allow the relatively little laminar flow and the subsequent tests will tail to become unstalled.) Note altitude lost in recovering. probably show only a mild FBC if any. On the other hand, Increase the bank angle in small increments and ob- if the boundary layer is laminar near or past the mid-chord serve any further tendencies for FBC, roll or yaw. Note position, it is possible the behavior will be much more the elevator deflections and airspeed at which they occur. evident. It should be necessary to conduct this test for only Bank angles in excess of the limits normally observed need one loading condition but at least three speeds from land- not be reached. ing to cruise conditions should be covered. Remove the CAUTION: The objective of the following steps is to flow visualization material at end of test. fly the airplane with an artificially induced turbulent During this test, observe the position of the elevator boundary layer on portions of the tail so as to simulate required to maintain straight and level flight for a given the effects of contamination. These steps pose some addi- 62 JULY 1983 the last step checking to see that the elevator deflections and airplane flight behavior are still within acceptable limits. If the incremental changes experienced with the two different lengths of tape are insignificant, then you can proceed repeating the tests with the tape applied full span. ELEVATOR Otherwise, proceed with smaller increments until the re- DEFLECTION DE6. sults indicate that you should proceed no further. Repeat these tests with the CG moved to the mid-point

V UAX. and then the forward locations. If the full-aft limit for the CG was not tested previously, it should be done also unless prior tests indicate otherwise. Be aware that these tests will probably be more critical so proceed with caution.

• 0 100 110 140

INDICATEDAinSPfCO, KNOTS

20 FIG. 9 -FLIGHT TEST DATA FOR VARIE2E. 18 TRANSITION 16 min trim OFREE 14 D FIXED 12 ELEVATOR tional hazard and should not be performed by anyone who DEFLECTION 10 is not fully qualified and prepared to handle the airplane 6 deq under conditions requiring emergency actions. When you are ready to start the final series of tests, carefully clean off the first 3 inches of the WING and TAIL leading edges using a cleaning agent to remove any traces of dust or oil so that a strip of masking tape will stay firmly attached in flight. Apply a double thickness strip 60 80 100 120 140 160 180 (about .008 to .010 in.) of'/»to 3A inch masking tape about INDICATED AIRSPEED, V( . knots 2 inches from the leading edge, both top and bottom, FIG. 10 -FLIGHT TEST DATA FOP LONG-EZ Cfi£F. 2) starting inboard and extending out to 1A the span of each tail panel. Be sure that the tape used has very good adhe- sive qualities and press it firmly onto the surface. It is necessary that the tape be applied in short sections of about 12 inches each so as to eliminate the possibility of one Having performed all of these steps, you should have loose end causing the whole length to peel off. Tape is fairly well defined the operating characteristics of your applied inboard only for the first flight to minimize the airplane FOR THE MOST SEVERE CASE OF RAIN OR possibility of large rolling moments caused by unsymmet- BUGS. If you were unable to complete the full series, then rical separation, and to approach the most severe condition you know that some operational limitations should be (full span trip) in a careful manner. placed on the airplane for conditions where the airplane With the tape in place, conduct a series of high speed might become contaminated. taxi runs checking to see that there is sufficient elevator Be sure to remove the masking tape from the surfaces power to lift off within the first quarter of the runway. after a few days of testing to avoid possible damage to your Use the mid CG loading condition for these tests. If it is finish due to the "curing" of the adhesive over a longer determined that a satisfactory liftoff can be made without period of time. reaching excessive airspeeds, apply a small amount of the boundary layer flow visualization material used previously so as to check to see that the tape is tripping the flow. Place it in a location where you can easily see it, and then proceed with the take-off. Carefully note and record take- off distance and airspeed. If the flow appears not to be Pilot Preparedness tripped, land and add another layer of tape. Otherwise, proceed with repeating the previous rate of climb, rate of descent and elevator vs. airspeed tests for comparison with If you are flying a tail-first airplane that has not been the initial data. Make a careful note of the minimum trim thoroughly tested for its FBC, you should be aware that speed of the airplane for the landing condition. Then repeat you may encounter a FBC problem unexpectedly and you the coordinated-turn maneuvers. Note any unusual should be prepared to take the proper corrective action characteristics that may be associated with the tripped immediately. Do not attempt a flight if rain or snow are flow condition. If any unsatisfactory or unsafe effects are threatening or if the field is heavily infested with flying noted, terminate the testing immediately. bugs. If possible, load the airplane so that the CG is in the When landing, make only gradual turns and maintain mid to aft portion of design range. an approach speed about 1.3 times the noted minimum Inspect the lifting surfaces and remove any surface trim speed. Note and record touchdown speed and landing contamination that could cause flow turbulence. Inspect distance. Compare the data and check to see that the the elevator travel for proper down travel limits. The elevator deflections required with the tripped flow condi- travel stop should be positive with no "spongy" tendencies. tion are not excessive. There should be only very small Do not take-off from a field with long grass or weeds changes from the original data of 1 or 2 degrees at the which can strike the leading edge. On take-off, check to most. see that excessive speed is not required to reach liftoff. If the results are judged to be acceptable, add more Abort the take-off if in doubt. If there is sudden power tape span wise and extend it to the Vz-span location. Repeat loss, avoid abrupt elevator inputs and banked turns. SPORT AVIATION 63 If rain, drizzle, mist or bugs are encountered during techniques now offer some hope for obtaining a suitable flight, avoid slow flight and steeply banked turns. Make section. These steps represent major redesign effort and a SHALLOW STRAIGHT-IN approach with airspeed 10 should not be taken unless all else fails, and then should to 15 knots higher than normal. Be prepared to ADD be taken only after consultation with the designer. POWER if the plane suddenly starts to pitchdown and pick Of course, any modifications to the airplane should be up speed. Avoid making abrupt aft stick inputs and allow checked with the FAA and thoroughly flight tested for all speed to increase significantly before attempting to apply flight conditions. any aft stick. Allow for a longer than normal landing runout. In the event of a go around, remember that the airplane has higher drag and lower lift than normal and will not climb as rapidly as normal. Expect a shallow Closing Comments climbout and avoid any abrupt turns. We have specifically aimed this article at the problem of a more or less symmetrical stalling phenomenon in which there are no significant rolling or yawing moments Curing Severe FBC Problems present. However, it is quite possible that an unsymmetri- cal condition of the airplane exists so that the stall itself will be unsymmetrical. The pilot, therefore, should also Finally, we will address the question of what to do to think in terms of the potential for a ROLLOFF or SLEW- the airplane if it demonstrates UNACCEPTABLE be- ING behavior associated with the FBC. havior. This article has been presented to the reader for the The first thing to do is review all information supplied purpose of exchanging information. Because of the highly by the designer (owner's manual, newsletters, etc.) and experimental nature of both the information of this article then contact him if you do not find an obvious source of and the flight activity associated with homebuilt airplanes, the problem. If this is an original design, review the earlier the author cannot assume responsibility for actions taken discussions in this article. as the result of using this information, the suggestions or Check for misalignment. Check the tail for insufficient the recommendations presented herein. incidence and the wing for excessive incidence. (Both will cause excessive down elevator settings.) Shim the surfaces or make new attachment fittings. Carefully check the airfoil shapes using external templates to see that they conform to the design airfoils. Acknowledgements Unless the designer can provide specific data for the de- sired contours of the airfoil, you will have to develop them from the normal construction templates. In this case, you I would like to thank all of the following persons for will have to make allowances for the added thicknesses their help in obtaining some of the information and data of fiberglass, resin and surface filler. presented herein, and some of them for the advise and An alternate method to making templates from the comments pertaining to this presentation: drawings is to make a series of exact half-templates (upper Dr. Bruce Holmes (Flight Research Engineer), Joseph and lower) of the actual surface contour. These are then L. Johnson (Head, Dynamic Stability Branch — full scale compared with a drawing of the desired airfoil. A simple wind tunnel), Long Yip (Wind Tunnel Research Engineer) procedure is to mask the chordwise section of the upper and Dan Somers (Airfoil Research Engineer), all of NASA's surface to be checked with a narrow strip of Saran Wrap Langley Research Center. Also John Roncz (Airfoil De- or wax paper and then lay down a thin strip of "Bondo" signer), 1510 E. Colfax Ave., South Bend, IN; Burt Rutan plastic body filler about Vz inch wide. Before the filler (President, Rutan Aircraft Factory); Gene Sheehan (Pres- hardens, press in a piece of plywood or hardboard previ- ident, Quickie Aircraft Co.); Bob Walters (Dragonfly De- ously rough cut to the approximate contour of the upper signer) and Rex Taylor (Viking Aircraft Ltd.). surface. The piece should extend below the upper surface This acknowledgement should not necessarily be con- at the leading and trailing edges. After the plastic hardens, strued as representing an endorsement on the part of any carefully mark on the template the position of the trailing one of the individuals or organizations mentioned. edge and a reference mark placed on the leading edge of the surface. Repeat the process for the bottom surface and then match the two parts of the template using the leading and trailing edge marks on the two parts. This process is References quite easy and fairly quick but requires rework of the template whenever the surface is reworked. Some contour errors can be corrected by filling with 1. Yip, Long P. and Coy, Paul F.: Wind-Tunnel Investi- microballoon slurry and refinishing. Others may require gation of a Full-Scale Canard-Configured General building a complete new surface. Aviation Aircraft. 13th ICAS Congress/AIAA Air- If possible, the CG forward limit could be set further craft Systems and Technology Conference, Seattle, aft so as to avoid the higher loading of the tail. Washington, Aug. 22-27, 1982. ICAS Paper Number A possible redesign solution is an aileron reflex 82-6.8.2. mechanism such as the system developed for the Quickie 2. Holmes, Dr. B. J., Obara, C. J.: Observations and Im- airplanes. Also, a small moveable horizontal surface lo- plications of Natural Flow on Practical Airplane Sur- cated at the aft end of the fuselage to provide an inflight faces. 13th ICAS Congress/AIAA Aircraft Systems adjustable pitch trim moment could be installed in some and Technology Conference, Seattle, Washington. cases. Perhaps the most drastic but most satisfactory solu- Aug. 22-27, 1982. ICAS Paper Number 82-5.1.1. tion would be to build a new tail surface with an airfoil 3. Holmes, Dr. B. J., Croom, C. C., Obara, C. J.: Sub- less susceptible to the flow separation problem. Unfortu- limating Chemical Method for Detecting Laminar nately, there is relatively little data available on which Boundary-Layer Transition. Handout available from to base a decision for selecting an alternate airfoil section. Dr. Holmes, Mail Stop 286, Langley Research Center, However, some of the latest airfoil computer design Hampton, VA 23665. 64 JULY 1983