BY JONY BINGELIS, 8509 GREENFLINT LANE, AUSTIN, TEXAS 78759

tirst flight, that is a logical enough conclusion. After all, A LOOK AT STALL WARNING homebuilts do feature a wide range of airfoil types and DEVICES wing configurations. And, since plans rarely, if ever, detail the installation of stall warning devices, the builder is on his own . . . with a number of questions to resolve. Just where should the stall warning device be located? i.NY AIRPLANE THAT does not give the pilot unmis- How far from the wing root? How high or how low on the takable warning (buffeting, shaking, etc.) that a complete leading edge should it be? Is it safe to cut a hole in the stall is developing is a dangerous airplane to fly ... no leading edge and to gouge out a rectangular opening large matter how delightful its other flight characteristics might enough for the stall warning horn relay? With such seem- be. An airplane like that is dangerous to any pilot in the ingly unanswerable questions and the unattractive price traffic pattern where a sudden distraction or a miscalcula- of a new stall warning unit, it is not surprising that the tion due to a momentary lapse of attention can lead to a installation of an aural stall warning unit is not a high stall, especially while "turning final" (an ominous term, priority item with homebuilders. isn't it?). One moment the pilot is in complete control and the Stall Warning Devices? Who Needs Them? next he is staring at the ground in shocked disbelief. He Hold on now, let's not dismiss the subject too casually. realizes, too late, that he allowed the airplane to stall! In Nobody can tell whether or not a particular airplane will spite of his frantic effort to raise the nose by yanking back need some sort of stall warning device until after the on the control stick so hard the elevator hits the stops, the airplane's stall characteristics have been experienced in nose down pitching remains uncontrollable, as uncontroll- flight tests. able as the sudden wing drop. The accompanying rapid Here's some guidance. If your airplane exhibits a loss of altitude becomes excessive. If the airplane's altitude marked buffeting or a shaking sensation felt through the is too low and its recovery is too slow, too bad! (That's no airframe or the control system, you don't really need a way to recover from a stall, is it?) stall warning device. If your airplane requires a very Sitting comfortably in your easy chair you know that pronounced and deliberate rearward pull force on the con- trying to raise the nose of a stalled aircraft is a suicidal trol stick to make it stall, you don't need a stall warning act. The correct action is to pop the nose down, isn't it? It device. On the other hand, if your airplane, without warn- seems we all know that except for the guys who have spun ing, characteristically tucks its nose and/or exhibits a very in. pronounced rolling tendency anytime you let your airspeed Aircraft manufacturers are very much concerned over get a bit low, you have an airplane that must be considered inadvertent stalls and go to great lengths to build into to have dangerous stall characteristics. Such an airplane their aircraft well announced and gentle stall charactris- should be re-rigged and/or equipped with stall strips or at tics. Consequently, many of their aircraft models have two least a stall warning horn. different types of stall warning devices. One is the stall Actually, the installation of stall strips or a stall warn- strip and the other an aural warning horn that is often ing horn need not be limited to aircraft that exhibit unde- augmented by a light to also alert the pilot's visual sense. sirable stall characteristics. Airplanes can be stalled under Piper's 1983 Saratoga model even has two separate stall a variety of conditions of speed and attitude, therefore, warning horn systems installed, one for a flaps up condition even an airplane with normally docile stall characteristics and the other for flaps down. could, under some conditions of flight, develop a beastly However important such devices might be, a newly behavior. certificated homebuilt is not likely to be similarly equip- ped. It's not that the homebuilders have anything against Stall Strips Vs. Stall Warning Horns stall warning devices; it is just that they assume the things cost too much and that they cannot be accurately installed A stall warning horn, correctly installed and adjusted, ... for the initial flight test anyway. Well, although the will start its unearthly raucous blare anytime the aircraft aural stall warning horn devices are moderately expensive, wing approaches within 5 to 10 mph of a stalled condition. the stall strips are not. Certainly, one such device is better This type of warning device does not change the airplane's than none — if needed. As for the argument that the stall stall characteristics at all. It does not, cannot, make the warning device may not be accurately installed for the stall more gentle. All it can do is announce with its strident 20 SEPTEMBER 1983 sound that if you don't do something pronto, the airplane leading edge than you might expect. Since the unit has is going to stall. slotted screw holes in the attachment plate a degree of Stall strips, on the other hand, when properly fabri- vertical adjustment is possible after the initial installation. cated and installed can serve to warn you of an impending This feature may not be as good as you would believe as stall and, to a degree, may even alter the stall characteris- any adjustment of the lift detector up or down from the tics of the airplane. So, although stall strips are less expen- original placement will probably require the enlargement sive and easier to install, believe it or not, they are, in my of the hole in the wing. Another thing . . . since that lift estimation, more functional than are the aural warning detector unit's plate must be screwed onto the leading horn systems. This may explain why manufacturers often edge, its installation will seem crude to the homebuilder equip their aircraft with both systems. who has a nice slick airplane with no drag producing bumps on it. That could, indeed, be the real reason we The Lift Detector Stall Warning Horn don't see more of these units on homebuilts. This is the device that has been around a long time. It consists of a horn mounted in the cockpit (usually behind The Reed Horn Stall Warning System the instrument panel) and an electrical relay unit operated Here is an improvement over the lift detector type mechanically by changes in the slipstream. This unit re- warning horn. This system will be found on Cessnas man- quires an electrical system (12 volts or 24 volts DC). ufactured after the late 1970's. It, too, is operated by a These units may be purchased from any of the larger change in airflow over the wing's leading edge. This device homebuilt supply outlets. If you prefer, you can save a lot does look like the older lift detector warning horn instal- of money by buying only the lift detector unit and get a lation except for the impression it gives that some vandal door buzzer from Radio Shack as a substitute. Of course, has pulled the flapper (lift detector) out leaving only a it is not FAA sanctioned but is about $70 cheaper. It won't vacant rectangular opening. This appearance is mislead- be loud enough, you say? You could always use two buzzers ing. Actually, that vacant port is a tip-off that a simplified for a total cost of less than $8.00 (1983 prices). For that and more economical stall warning reed horn warning matter, you could go all the way and wire in a warning system is installed. light if you want one . . . again Radio Shack. The simple construction of the device and the equally The lift detector unit is mounted in the leading edge simple installation is very much evident. The external part of the wing, usually in an area of undisturbed air flow. of this lightweight system fastens to the leading edge of This, of course, means outside the propeller disc area. The the wing by means of an adjustable slotted plate that can lift detector will not react to the normal airflow over the be adjusted up or down about 3/16". Behind the adjustable wing, regardless of the airspeed. However, anytime the plate is a scoop-like piece to which a bug screen is affixed. airflow is altered, as when the angle of attack increases This, in turn, is attached to an adapter to which a plastic in an approach to a stall, the disrupted airflow will force tube is fitted. The plastic tube, being very flexible, is easily the lift detector upwards and activate the relay to which routed up to the wing root where a plastic reed horn is it is attached. This sends an electrical current to the warn- stuck into the other end of the plastic tube. That is all ing buzzer and/or light. there is to the system. No electric wiring or power is Well, how useful is this unit? It will warn you of an needed. As for the stall warning horn, it is nothing more approaching stall provided it is properly installed. This than a simple reed type horn . . . something like a party warning will come regardless of the aircraft's speed or type noise maker. of maneuver in progress. If you deliberately continue to The horn actuates just before the wing stalls, very invite a stall, the warning horn will continue its irritating much like the other type of stall warning horn — 5-10 blare as long as the wing is in an abnormally high angle mph above the stalling speed. The horn sounds off when of attack and the flow of air over the device is disrupted. a negative air pressure is induced at the wing's leading Obviously, the unit cannot warn you of an impending edge. This causes a reverse airflow through the horn. stall if it is not working. You can minimize this risk if you The adjustable plate mentioned earlier controls the include the check of the stall warning horn in your pre- speed at which the horn blowing takes place. Moving the flight inspection — you do make 'em, of course. Simply plate upward will cause the horn to actuate at a higher turn on your Master Switch and walk over to the lift airspeed. Moving the plate downward conversely causes it detector and raise that little tab and listen for the horn to to blow at a slower speed. The reed horn stall warning go off. If you hear it you know that the unit is working. systems adjustable plate is installed in approximately the You also learn that the battery is still alive. same position on the leading edge as the other model would Incidentally, this same check can be used for another be. purpose. Just before you close and lock the hangar door, Unless you luck out on the first try, final adjustment go over to your trusty stall warning lift detector and gently can only be obtained after one or more flight tests. raise the little tab. If nothing happens (no horn noise) you You can check out the horn operation on the ground know positively that your Master Switch if OFF. Isn't that by covering the opening in the adjustable plate with a much easier than climbing back on the airplane for a look cloth, putting your mouth to it, and inhaling to create a at the switch? slight suction in the system. This intimate action causes How do you determine where to locate that warning the horn to make its characteristically audible raspberry. horn lift detector? One thing is certain, the stalling charac- A reed horn warning system is better suited for a high teristics of an airplane cannot be precisely predicted by wing airplane where it is possible to locate the horn at ear mathematical methods. As you know, the stalling charac- level in the cockpit. It probably wouldn't be as audible in teristics of most any airplane are markedly different under a low wing airplane where the horn would most likely be different flight conditions of power application and flap situated somewhere behind the instrument panel. At any setting. The best guidance I can offer is to suggest that rate, the reed horn warning system is lightweight, easy the photos accompanying this article are quite representa- to install and to adjust. tive of such installations. A trip to the airport for a first hand viewing of lift detector installations would also be Stall Strips helpful. An aircraft's wing (airfoil) will influence the lift detec- Stall strips are located where they are on the wing tor location more than any one other factor. Ordinarily, because the builder (manufacturer) has determined that that is where they work best. . . for a particular airplane. you will find the device positioned somewhat lower on the

SPORT AVIATION 21 The sharp edge of a stall strip is intended to disturb the smooth flow of air over the wing surface in the area where it is installed . . . and do it just before the rest of the wing PERMANENT INSTALLATION begins to stall. This disturbed flow or burbling will cause IS WITH SCREWS OR the airplane to shake or shudder slightly, a fair enough POF RWETS x warning for any pilot that a stalled attitude is being ap- proached. If set properly, the stall strips will do their thing about 5-10 mph above the normal stall speed of the aircraft. Stall strips are usually located near the inboard leading edge area of the wing. Their purpose being (supposedly) to cause the inboard area of the wing to begin to stall first. This, theoretically, provides a more gradual stall and en- ables the ailerons to be effective longer. However, don't be STALL STRIPS - surprised to find stall strips located almost anywhere on (MAKE Of WOOD OR ALUMINUM ANGLE) the leading edge . . . even out toward the wing tips on some aircraft. Since the purpose of stall strips is to an- nounce the impending stall, their function is best served by creating a disturbance in airflow where it will be most noticeable to the pilot. Apparently, sometimes this is most effective at the ailerons. Because of this non-standardized use of stall strips, you can only conclude that you too will have to experiment with your own stall strip requirement.

Stall strips are triangular lengths of aluminum angle, CABIN BUG SCREEN plastic or wood. Wood strips are more practical to use in running preliminary tests. A typical stall strip is either %" x %" or %" x %" in PLASTIC HORN ASSEMBLY width and has an average length of about 12". The lengths observed varied from 6%" to 20".

When installing stall strips, a cardboard template dup- V SAFETY WIRE licating the first 6-8" of the leading edge portion of your £>- wing can be most useful. Cut a triangular notch for the stall strip where you want it positioned. The template will help assure a symmetrical installation on both wings. Attach your stall strips initially with masking tape or gray duct tape but do not fair the strips into the wing. Your objective is to maintain a sharp triangular cross section. Next, make a flight check of the newly installed strips. The result you want is a slight buffeting or vibration in the airplane or the controls. This should ideally occur about 5-10 mph before the stall occurs. If you aren't satisfied with your first efforts try different locations along the wing and place the stall strips higher or lower in an attempt to obtain the best results. Taping the strips on will permit such experimentation without hurting the finish on your airplane. After you obtain the pre-stall response you want, secure the strips to both wings with screws or pop rivets. The installation of stall strips makes an interesting experiment, particularly if your aircraft is bashful about displaying any pre-stall shake or shimmy. SLOTTED ADJUSTABLE PLATE FIGURE 2. AIR INLET I would assume that all stall tests would be done at a safe altitude and not in the traffic pattern. That's what I REED HORN STALL WARNING SYSTEM would assume, wouldn't you?

A 9" stall strip located in the most conventional of locations. On the extreme right you will notice the installation of a regular Note how high this stall strip is located on the wing. It too is in lift detector stall warning device (Bellanca). the conventional inboard section of the wing. 22 SEPTEMBER 1983 Two stall strips grace this airplane. A short 9" one inboard and a long 20" strip at the juncture of the flaps and aileron area. Note that one is located low on the wing and the other consid- erably higher. An aluminum angle serves as a stall strip on this Mooney. The angle is much thinner than appears here because the ends are tapered.

Look closely and you will see a long stall strip well camouflaged Who said high wing aircraft don't need stall strips. Apparently this one does. (Cessna center line twin) by the paint stripes. These stall strips are at the wing tips . . . how come? (Piper)

Here's one good reason homebuilders aren't too eager to install Here's the other reason the lift detector warning system is not conventional stall warning devices . . . ugly isn't it. Must also more popular. Get a load of the gaping hole that has to be hacked disrupt the airflow over the wing behind it. into the leading edge of the wing.

A reed horn stall warning installation in a Cessna. It differs from the regular stall warning device in that it has no lift detector tab This triangular stall strip is a stamped aluminum piece mounted showing. (A lot of bugs around here.) in the wing root juncture (Comanche). SPORT AVIATION 23