2-CYCLE VERSUS 4-CYCLE Comparing the 582 and 912 in the Pulsar

Editor's Note: Mark Brown is By MARK BROWN practically negligible. Therefore, the the designer of the highly efficient Aero Design, Inc. extra power of the 912 remains the Pulsar and has done extensive predominant effect in this segment test work with both 2-cycle and 4- 11910 Radium St. of flight which gives the 912 pow- cycle powered versions of the San Antonio, TX 78216 ered Pulsar a 10 mph higher cruise airplane. speed than a Pulsar with a 582. The primary advantages for the Every homebuilt airplane is a Since takeoff and climb perfor- 582 are its low cost and its simplic- research vehicle to some degree mance are directly influenced by ity which are, of course, directly and the Pulsar is no exception. weight and the 582 engine is 40 Ibs. related. The 582 costs less than half Hundreds of hours of flight time on lighter, it has the advantage in this as much as the 912, basically many different Pulsars have revealed area. Surprisingly, even though the because it has about half as many some valuable information about 912 has 14 more horsepower than parts. Correspondingly, the mainte- engines and flying qualities. the 582, the Pulsar actually takes off nance, installation and overhaul Since the Pulsar has been flown shorter using the 582 and climbs requirements are simpler and less extensively with both 2-cycle and 4- about the same on either engine. expensive for the 582. All these fac- cycle engines, a side-by-side Cruise performance isn't so sen- tors are particularly important for the comparison is helpful. The Rotax sitive to weight because the only homebuilderto consider. 582 2-cycle is still the most popular force opposing thrust is drag and Of course not all of the advan- engine for the Pulsar because of its the only component of drag affected tages favor the 582. The 912 is a low cost and simplicity. However, by weight is the drag due to lift very sophisticated 4-cycle engine the 4-cycle is increas- (called induced drag). At low angles with many impressive characteris- ingly used in Europe and Australia of attack induced drag is very small tics of its own. First, it uses less fuel where it is certified to JAR 22 motor- because the wing lift vector is than the 582 while producing more glider standards. almost vertical. Since the angle of horsepower. At 75% power the 912 Comparing the performance of attack in cruise is only about 2 burns 3.5 GPH at 140 mph while the the 582 to the 912 depends on the degrees, the induced drag due to 582 burns 3.8 at 130 mph. This fuel segment of flight being considered. the extra weight of the 912 engine is flow advantage for the 912 also

58 FEBRUARY 1992 increases the range of the Pulsar, so the benefit is two fold. Another advantage for the 912 is how smoothly it idles. At 1100 rpm the instrument panel doesn't even shake. With its standard 2.27 to 1 gear ratio the prop is only turning 485 rpm which makes it an excellent airbrake for glide path control. On the other hand, as with most 2-cycle engines, the 582 doesn't idle very smoothly at low rpm. To compen- sate, Rotax has recently introduced a vibration damper for the 582 which now allows a reasonably smooth idle at 2200 rpm. Its 2.0 to 1 gear ratio gives 1100 rpm on the prop, but that's still too high a prop speed to help much as an airbrake. Fortunately, the Pulsar has large flaps that provide adequate glide path control even with the high idle speed of the 582. Nose to nose, the powered Pulsar on the left, and the Rotax 912 powered Comparing the vibration levels at Pulsar XP on the right. cruise rpm is actually a lot more meaningful than comparing at idle due to its higher rpm. caused by system failures related to speed because cruise is where an The most important issue in any installation problems or operator spends most of its engine comparison is in the area of error, not the basic engine. Clearly, life. In this comparison the results reliability. Unfortunately, this is a failure in any one of the systems are a little surprising. such as fuel, ignition, cool- The moving parts of the ing, lubrication, controls, 582 are so light that it is or even instrumentation just as smooth as the 4 "The most important issue in can lead to an engine fail- cylinder 912. With re- any engine comparison is in ure. spect to noise level, the Assuming for a minute 582 has another surprise. the area of reliability." that the installation is per- Most 2-cycle engines fect, a basic comparison use a tuned exhaust pipe of the reliability of the 2- to maximize power which cycle and 4-cycle engine also maximizes noise. The 582 always a difficult subject because is still a valid (although sometimes exhaust system is only mildly tuned so many factors like maintenance controversial) question. Unfortu- and incorporates an after muffler and operator error are involved. In nately, many opinions on the subject that actually makes it as quiet as the fact, all the failure studies done by are more a matter of prejudice 912. The only difference is the Aero Designs indicate that the vast based on isolated cases than of higher frequency sound of the 582 majority of engine failures are actual facts. Everyone naturally has more confidence in the type of engine they are most familiar with. Because of this, the two-cycle has a distinct disadvantage since its appli- cation to aircraft (in any numbers) is relatively recent. Also, because of the high power to weight ratio, the 2- cycle engine is often used in racing and other demanding applications where the engines are pushed to extreme limits. Even applications in ultralight aircraft are demanding in their requirement for high continu- ous power levels to overcome their inherent high drag. Operating under such conditions, the 2-cycle engine has clearly suffered some abuse that has affected its reputation. Conversely, when 2-cycle en- gines are operated at 75% power as they are in the Pulsar, and given the same care as a 4-cycle engine, Aero Designs has found that the 2-cycle The Pulsar's prop was developed to match the aircraft's drag profile and engine var- engine is just as reliable as the 4- iables. Note the narrow chord tips cycle. Nothing in the 2-cycle has

SPORT AVIATION 59 asa

The combination of the Pulsar's big plain flaps, NASA airfoil and high aspect ratio wing . . . and overall light airframe weight . . . give the Pulsar great short field performance and excellent cruise performance.

craft engine. For example, a typical 4-cycle, 80 hp aircraft engine weighs 175 Ibs. compared to 124 Ibs. for the Rotax 912 with comparable equipment. This 50 Ib. additional engine weight increases the loads in every part of the airplane. In fact, the structure suffers six times the weight increase because of the G-load factor. This requires more structure which fur- been found to be inherently unreli- mary consideration on this issue is ther increases the weight. In able. This evaluation is, of course, rpm because high tip general, airplanes suffer from the limited to engines specifically devel- speeds result in very noisy and inef- "snowball" effect more than any oped for aircraft, like the Rotax 582. ficient operation. Of course, both other machine because every part is Other engines that aren't adapted to the 582 and the 912 use gearboxes inter-related. A 50 Ib. heavier engine tolerate the special cooling, loading to keep the prop speed down while typically requires another 50 Ibs. of and altitude conditions of aircraft most aircraft engines are just structure throughout the airplane. operations are a different subject designed to run, direct drive, at low This 100 Ib. increase in empty altogether. rpm. However, the problem with weight requires more wing area to One characteristic of the 2-cycle low rpm engines is weight. Since maintain a safe stall speed which engine that concerns some people is horsepower is directly related to in turn takes more power to main- its high operating rpm. Again, the how many times the cylinder fires tain the same cruise speed which anxiety is mainly due to comparison per minute, a slower engine is takes a heavier engine and more with the more familiar 4-cycle engine. always heavier for the same power. fuel . . . and so on. That's why Considering the small number of That's why the 2-cycle engine has engine weight is so important. moving parts in the 2-cycle and the almost twice the power for a given However, the snowball effect works absence of valves and cams, the weight as a 4-cycle engine and why just as well in reverse. A lighter rpm is really conservative. In fact, a high rpm 4-cycle (even with a engine requires less structure, which some 4-cycle engines rev even gearbox) has more power per requires less wing area, which higher than the Rotax 582. The pri- pound than the typical low rpm air- cruises faster, on less power, etc. If 60 FEBRUARY 1992 there's any secret to the Pulsar's outstanding performance on such low horsepower, the lightweight Rotax engines are it. Getting back to the basic 2- cycle/4-cycle comparison. All 2-cycle engines have a reputation for being rather sensitive to operat- ing conditions and loading. To a certain degree, this characteristic has proven to be true for the Rotax 582. The prop selection and design of the engine cooling system were

«>"All• two-cycle engines have a reputation for being rather sensitive to operating conditions and loading."

The Rotax 582 installation in the Pulsar. certainly more challenging than for the Rotax 912. The reason for the challenge lies in the basic function of the 2-cycle engine. Since it pro- duces power on every , it also produces heat on every stroke. However, if the engine overheats, the induction air passing through the heats up. This causes the induction air to expand, leaving less oxygen to mix with a set quantity of fuel. The result is an excessively rich mixture which causes the power output to drop. This heat related power loss is no worse than operating a 4-cycle air- craft engine with the heat on. However, if the heat affects are combined with a heavy propeller loading and a narrow power band, then a real problem can develop. First let's consider the propeller load. A fixed pitch prop is a won- derfully simple and inexpensive device, but it's like having a car with only one gear. The engine is over- loaded as it tries to accelerate and get underloaded at high speed. Of The Rotax 912 installation in the Pulsar XP. course, the effect is less severe for a propeller than for an automobile, but fuel/air charge is very critical since Now consider what would hap- the same forces are at work. A low both events must occur on the same pen if the engine were loaded right pitch prop is needed for takeoff, stroke. In fact, the pressure pulses to its limit on takeoff by a propeller while a high pitch improves cruise. in the exhaust system are an impor- selected to maximize cruise speed. Obviously a compromise is in order tant part of the cycle. Unfortunately, Then add to that the induction air but the point to remember is that the these pressure pulses only occur at heating effect. As the engine heats engine is still always loaded to its one discrete frequency which up and loses a little power, the prop limit on takeoff. means a 2-cycle engine only puts loading is too high, so the rpm drops Next, the narrow power band of out its best power at a specific rpm. off. As the rpm drops off, the power the 2-cycle must be brought into the If the engine is not allowed to oper- output continues to decrease. Of picture. The timing required to expel ate at its best rpm, its power output course, the load from the prop also the exhaust gas and induct a new is severely reduced. decreases as its speed is reduced, SPORT AVIATION 61 but what would happen if the power F. in less than a minute. Such ther- Aero Designs, the engines are output is falling faster than the prop mal shock can and does cause equally reliable. Since both engines load? Well, that's just what can cylinders to crack and many other are very light weight for their respec- happen on a 2-cycle engine if it's problems. Liquid cooled engines in tive power, either engine will given not set up correctly, and needless to general run much cooler and main- the Pulsar the high efficiency and say the takeoff doesn't get very far. tain a more constant temperature performance it was designed for. The equilibrium load where the drag which greatly improves the longevity Therefore, either engine will continue on the prop equals the engine out- and efficiency of the engine. This is to be available in the Pulsar kit put is typically around half power. why the Voyager's flight around the depending on individual preference. The Pulsar's solution to this diffi- world used a liquid cooled engine On a new subject, but still cult situation is two-fold. First, a for its primary power. strongly related to light weight lengthy propeller development pro- Back to the 2-cycle sensitivity engines, is the STOL capability of gram was carried out which involved question. The specially designed the Pulsar. The light weight along designing, building and flight testing propeller and cooling system in the with high lift and controllable drag many different types of props. Pulsar effectively eliminate any of are the most important factors for Various combinations of blade plan- the complications normally associ- short takeoffs and landings and form, twist distributions, blade ated with operating a 2-cycle engine. these are all strong ponts in the sweep, diameter and pitch were In fact, one positive result of the Pulsar design. In fact, with respect tested until, after more than a year 582's load sensitivity is the way it to light weight, the Pulsar is one of of work, a very specialized prop was reacts to altitude. As the air thins the lightest two place airplanes in developed. This prop uniquely out at altitude, the prop unloads, the world. In its performance class, matchs the Pulsar's drag profile and which allows the engine to turn a it may be the lightest. engine variables, to provide a more higher rpm for a given throttle set- High lift is even more important in consistent load on the engine. One ting. This causes the engine to pull STOL performance but this factor noticeable characteristic of this spe- in more air for a set amount of fuel, must be considered carefully to limit cial prop is its very narrow tip chord. which makes the engine lean out the compromises in other areas. For This reduces the drag of the blades automatically at higher altitudes. example, increasing the wing area on takeoff while maintaining the Therefore, the Rotax 582 doesn't reduces cruise speed and causes a large disk area needed for good have and doesn't need a mixture rough ride in bumpy air. More sig- static thrust and efficient nificantly, a very large wing cruise. The final result is a on a very light airplane can be prop that doesn't overload the "...all air-cooled aircraft a dangerous combination in engine at any temperature. strong gusty winds. In the To eliminate even the small engines suffer severe Pulsar design, a new high lift power loss due to induction air airfoil from NASA Langley and heating, a special cooling system thermal stresses due to the effective flap system provide was designed that incorpo- large temperature good STOL performance rates dual radiators located without the penalties of an directly inside the air inlets of variations." oversize wing. the cowling. The location, size Of course, a high lift airfoil and design of these radiators and effective flap system are are all key elements of the sys- no excuse to undersize a wing tem. The location is important control when operated below about either. A careful balance is neces- because the full force of the incom- 12,000 ft. For operations above this sary to maintain a safe stall speed in ing air is directed squarely into the altitude, Rotax sells an optional alti- the rare case of a forced landing off face of the radiators. The size of the tude compensating carburetor which field. The occupants can usually radiators and this high energy air has been tested on the Pulsar to walk away from an off field landing at directly behind the prop provides so 16,000 ft. with excellent results. 45 mph but at 65 the risk is much much cooling capacity that a max The only real limitation caused by higher because the kinetic energy to performance climb on a 100 degree load sensitivity is in the use of a dissipate is more than double. day will not produce more than a controllable pitch propeller. If a pilot For the landing segment of STOL 165 degree F. coolant temperature. ever tried to execute a go-around operation, drag is even more impor- In cruise the temp stays around 140 with a controllable prop set at cruise tant than high lift or light weight. degrees F. Such effective cooling pitch, a 2-cycle engine will suffer the Sailplanes are a good example. All virtually eliminates the problem with overload syndrome described ear- sailplanes have high lift wings and induction air heating. lier. Of course, this is such a dangerous may are light weight, but without To prevent over-cooling at cruise situation that Aero Designs strongly spoilers for drag, a sailplane might speed, the airflow through the radia- opposes the use of a controllable use more runway than a 747. Drag tor is restricted by the design of the pitch prop with a 2-cycle engine dissipates energy and that's almost cooling core. As the speed in- unless some automatic system is as important to a landing as getting creases, the back pressure in the employed to reduce the prop pitch on the ground. Of course, in most radiator increases, which keeps the when full throttle is applied. A 4- other phases of flight drag is totally airflow fairly constant. This constant cycle engine would also suffer from undesirable. Consequently, the con- airflow results in a very consistent this type of pilot error, but not to trollable drag produced by an coolant temperature. such a dangerous degree. effective flap system is the most On a side issue, all air-cooled air- To summarize, the Rotax 582 has desirable because flaps also craft engines suffer severe thermal advantages in simplicity and econ- increase lift exactly when you need stresses due to the large temperature omy while the 912 offers higher it. The Pulsar uses the simplest form variations. The engine temperature performance, smoother idle and bet- of flap design called plain flaps. can change as much as 250 degrees ter fuel economy. In the view of These flaps are hinged on the bot-

62 FEBRUARY 1992 torn so even small deflections increase drag immediately by open- ing a large gap on the top of the wing. With this system, and a slow PULSAR SPECIFICATIONS approach speed, the Pulsar can maintain a steep, controllable glide PULSAR PULSAR XP path for a very predictable spot land- ing and short ground roll. EXTERNAL DIMENSIONS Even at gross weight with no wind Wing Span ...... 25.0 Feet ...... 25.0 Feet the Pulsar uses no more than 800 feet for either takeoff or landing. Length Overall ...... 19.5 Feet ...... 19.5 Feet Also, obstruction clearance is very Height Overall ...... 6.3 Feet ...... 6.3 Feet good because the climb angle and approach path are relatively steep. Wing Area ...... 80 Sq. Ft...... 80 Sq. Ft. To open up even more fields to the Pulsar, Aero Designs now has a tail- Wing Aspect Ratio ...... 7.8 ...... 7.8 dragger option for grass runway operation and also a large tire option WEIGHTS AND LOADINGS for the tri-gear version to allow oper- Empty Weight ...... 460 Pounds ...... 510 Pounds ation on smooth grass runways. The large tires do slow the Pulsar Useful Load ...... 440 Pounds ...... 450 Pounds down about 4 mph in cruise but in Gross Weight ...... 900 Pounds ...... 960 Pounds combination with the flexible fiber- glass main gear, the shock loads are Power Loading ...... 13.6 Pounds/HP ...... 12 Pounds/HP reduced considerably. A recording accelerometer in the cockpit mea- Wing Loading ...... 11.2 Pounds/Sq. Ft. ....12 Pounds/Sq. Ft. sure 2.1 G's with the standard five Fuel Capacity ...... 16 Gallons ...... 18 Gallons inch wheels but only 1.6 G's with the optional six inch wheels. Both G Limits (tested) ...... +6.0 and -4.0 ...... +6.0 and -4.0 tests were done at the same speed Seats ...... 2 ...... 2 on the same grass runway for com- parision. POWER PLANT On the general subject of han- dling qualities, we've learned a good Engine ...... Rotax 582 ...... Rotax 912 bit from the literally hundreds of pilots who have flown the Pulsar Max. Power ...... 66 HP ...... 80 HP demonstrator. Almost without fail Propeller ...... 56 in. Wood Laminate ...60 in. Wood Laminate they make some comment on how easy it is to fly, and how responsive PERFORMANCE and maneuverable it is. Most pilots are understandably surprised that Cruise Speed ...... 130 MPH ...... 140 MPH an airplane can have both qualities. Stall Speed ...... 45 MPH ...... 46 MPH Stability and maneuverability are usually opposing characteristics. Rate-of-Climb ...... 1000 Feet/Minute ...... 1000 Feet/Minute The reason the Pulsar is so stable is strictly a function of its tail volume Service Ceiling ...... 15,000 Feet ...... 15,000 Feet and center of gravity position. Tail Takeoff Roll ...... 800 Feet ...... 800 Feet volume is the area of the empen- nage multiplied by its distance to Landing Roll ...... 800 Feet ...... 800 Feet the CG of the airplane. The effect it Range ...... 400 Miles ...... 500 Miles has on stability is exactly like feath- ers on an arrow. A large tail volume does compromise cruise speed slightly but since it is the key ele- ment in stability, the Pulsar doesn't called inertia and the less an object these lengthy trips aren't really tir- cut corners here. As a result, the weighs the less inertia it has. Being ing because the seats are reclined Pulsar will fly hands off in smooth so light in weight, the Pulsar has about 30 degrees which helps to air for as far as you want to go. very little inertia so it offers little prevent pressure points and Such high levels of stability are resistance to changes in its motion. fatigue. The pilot and passenger great for flying in straight lines but Consequently, the controls feel can just sit back, relax and enjoy most airplanes with this much sta- very light and crisp making maneu- the excellent visibility of the coun- bility don't maneuver very well. The vers a joy and corrections in bumpy tryside through the large single controls are heavy and sluggish air almost effortless. piece canopy. As the ol' saying and very tiring for the pilot in bumpy Aside from its excellent handling goes - Time flies when you're hav- air just to keep the wings level. The qualities, the Pulsar has also proven ing fun. reason the Pulsar remains so over time to be a true traveling For a complete information pack- maneuverable is strictly due to its machine. Many single day trips of age on the Pulsar, send $5 to Aero light weight. Newton's laws define over 1200 miles have been accom- Designs, Inc., 11910 Radium, San how objects in motion tend to main- plished in the demonstrator Pulsar Antonio, TX 78216, phone 512- tain that motion. This affect is enroute to the airshows. Even 308/9332.

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