The title is for effect. It means flying a giant scale model airplane on the back side of the power required curve. Few model flyers go into this flight regime on purpose. Occasionally we see a novice or even an experienced flyer rotate his model at take-off with too low airspeed. This is followed by either an ungraceful settling back on the runway or a snap-roll ending in a crash. The approach and landing may be similarly exciting or disastrous with too low airspeed. However, we can design our model aerodynamics to provide less risk in the low airspeed - high angle of attack flight regime. In fact, we can have a lot of fun flying in the extremes of this unique area. Flight on the back side of the power required curve is quite different from our ordinary flying, accordingly, knowledge about the regime is important.
A relatively easy reading treatment of power
available - power required may be found in "Aerodynamics of the Airplane"
by Millikan published during WWII. The power required to fly in
un-accelerated flight results from two basic sources of drag. These are
induced drag and parasite drag. Induced drag is associated with
high angle of attack flight. Induced drag increases rapidly with
increasing angle of attack. Parasite drag is low at low speeds but
increases rapidly as airspeed increases These two sources of
drag are inseparable and combine to produce the effective drag
that power or thrust must equal for un-accelerated flight.
The figure is conceptual, illustrating in general how drag varies with airplane speed. Since drag is a force tending to slow the airplane, an equal thrust force generated by the engine produces a condition of un-accelerated flight. Accordingly, the drag curves represent the thrust required at any particular speed for un-accelerated flight. The dashed lines grossly represent the nature of induced and parasite drag. Two drag curves are shown to roughly illustrate the effect of wing aspect ratio upon model airplane fly-ability in the low speed back side of the thrust required curve. The definition of back side of the thrust required curve is simply the part of the curve where more thrust is required to fly slower. The higher aspect ratio wing is more efficient and has less drag at any given airspeed except near the abrupt stall point (about 15 degrees angle of attack). The low aspect ratio wing is less efficient and has greater drag at most airspeeds. However, the maximum lift may obtained at angles of attack up to 30 degrees with no abrupt stall characteristics. It is seen that low aspect ratio wing models offer the potential for better controllability at lower speeds since no abrupt stall occurs in this regime. The efficiency of our giant models is usually unimportant since most of us are getting 100-300 miles per gallon. Therefore, if we use a lower efficiency wing we simply use the next size larger engine. This moves the thrust available line higher on the curve and expands the overall speed range of the model.
The POGO and OLE TIGER models were flown in the back side of the thrust required region for many hours over the years. Even full-up elevator landings are common place when the wind isn't too gusty. When flying in this region, throttle-elevator coordination is required as in a full scale airplane. The back side approach and landing is flown using the elevator for airspeed control and the throttle for vertical velocity control as in aircraft carrier landings. Although the ailerons are yet somewhat effective at very low airspeeds, the rudder is used mainly for heading control since it is quite effective due to the relatively high throttle for flare-out if landing with near full up elevator.
A thrust to weight ratio of about 1.1 with a low aspect ratio wing model will enable flight down to zero speed by carefully piloting the plane to a vertical nose up attitude. This may be done by slowly moving the elevator nose up, while holding altitude nearly constant with increasing throttle. If you are skillful and have a little luck, you can do a torque roll at zero speed. In flying this maneuver the airplane probably never exceeds an angle of attack of 25 degrees since a lot of lift is generated with a low aspect ratio tapered wing because a lot of wing area is in the propeller slipstream. Additionally, less lift is required of the wing as the pitch attitude exceeds 45 degrees because the thrust vector is beginning to replace the lift vector in supporting the airplane.
It is easy to park the airplane with a ten to fifteen mile per hour wind. This requires throttle - elevator coordination to hold the altitude constant and to not move forward or backward. Under this moderate wind condition one may make essentially a vertical landing with little or no ground roll. This requires more skill and the wind must not be gusty.
Another interesting maneuver is to fly the power required curve. This has to be done in segments in order to keep the model within sight. Although ideal un-accelerated flight cannot be achieved, a slowly de-accelerated flight while maintaining constant altitude will provide some idea of the power required at the various airspeeds.
Start with altitude and airspeed stabilized at about one-half throttle. The object is to observe the power required to maintain level flight as the elevator is very slowly and continuously moved in the nose up direction. With some practice it should become apparent that initially less throttle is necessary but as the airplane begins to slow down more throttle becomes necessary to hold the altitude. This test will end with the airplane either stalling or hanging on the propeller if the wing doesn't stall and if enough power is available. This maneuver has provided an approximation of the drag of your model as a function of airspeed. All maneuvers may also be flown inverted (except for landing!). Try it (at altitude) you might like it!
A pattern plane still has enough speed after the pilot reduces the power to have rudder effectiveness. Speed and a barn door rudder will get you a stall turn every time. On the other hand, if you fly a plane with a lot of drag, like a scale plane or a "stick" type plane that stops dead every time you cut the power, you have to cheat a little. Feed the rudder in simultaneously with the reduction of power. On really slow planes, you may have to put the rudder in first, or only reduce to half power. Experiment to see what works best for your plane. Some planes, even pattern planes, like a burst of power to kick the tail around. For this, you climb, cut power, put in rudder, then give the engine a quick burst of full power and go right back to idle, remember to keep the rudder in. The prop blast will hit the rudder and start the tail around. Gravity will do the rest.
To recover from a stall turn, release the rudder and
let the plane descend. Add power smoothly and blend in elevator for a smooth
pull out. Try to make the exit altitude the same as the entry. Good
flying.