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Trimming for thrust On your first flight trim your plane to fly flat and level during hands off flight at about 1/2 throttle. Speed effects trim so trim at the speed you fly at the most which we will call cruising speed from this point on. Once you are satisfied with the trim take the plane up high and get into a nice long run for hands off flight then cut the throttle and watch you plane for changes in direction. It should remain straight and go into a gentle dive. If it dives too steeply you should recheck your CG. Go back to level flight at cruising speed again on a nice long straight run and give it full throttle. The plane should stay straight and go into a gradual climb. Land your plane and check the position of all your control surfaces. If everything is as it should be all the controls should be pretty well centered. If not you may want to take time to study your plane and see why the controls have to be off center to fly your plane straight and level. It is important to understand thrust only effects your aircraft under power. For instance if your plane pulls to the left, right, tail up or tail down under throttle try cutting the power and let the plane glide. If the plane then tends to fly straight and level this would indicate you need to work on the thrust. Invert your plane and take note of how much down elevator in takes to keep the plane flying level. It should take a small amount of down. Too much means it is tail heavy. Too little or none can indicate a nose heavy plane. It should take a small amount of down. |
Too much means it is tail heavy. To little or none can
indicate a nose heavy plane. Now let's get serious about checking thrust From flat level flight heading into the wind at full throttle pull a "straight" vertical up line, the key being "straight" vertical. Careful not to make any rudder or aileron inputs to cause any lateral movement. The plane should track straight for 4-5 seconds then fall off slightly left. This being caused by the slight mechanical effect of rotation mentioned earlier. If it pulls off too early, or pulls abruptly, your thrust is off. Make adjustments the opposite direction it pulls. I.E., plane pulls left, add right thrust. Of course side thrust works in conjunction with both lateral balance and rudder trim. Either of these being off can cause the same effects, so be sure those are correct before checking side thrust. For down thrust we also start in a straight level flight path at full throttle. Chop the throttle and watch for pitch changes. What you're really looking for is a nose up/nose down attitude. Off throttle you should get a slow level decent. If the nose pitches up the engine was holding the nose down. Reduce down thrust. A sharp nose down means not enough down thrust. Another check for down thrust is to simply fly the plane straight and level under power and notice if the plane is flying level and not in a nose down |
or nose up position. Why take the time to balance and perfectly trim the control surfaces, spend high dollars for the smallest lightest everything on that 3D plane only to neglect its handling potential by adding aileron, rudder &/or elevator to counter improper thrust. Or how about we spend hours making a sleek thin airframe, drop the big dough on that high revving hot rod mill, then start adding drag to get it to fly straight with mis-aligned surfaces rather than getting the thrust right. As mentioned before the planes that most benefit from thrust trimming are the high power, low weight hot rods but even the docile flying run planes will benefit from proper trimming. The plane only uses the engine for propulsion, allowing the airframe to do the flying. So power it up, trim it out and make that bird fly the best it can. Till next month Lew
APRIL 08
OCRC Cloud Busters : Spring Fun Fly |
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