Denny Dart and AMA Cub

“I have been testing the DD II and I believe it flies better than the AMA Cub.” – Ding Zarate
I am not surprised, Ding.  The DD II has higher aspect ratio wings than the AMA Cub.  That gives it an aerodynamic advantage.  The triangular wingtips are poor on both.  The DD would benefit from trapezoidal wingtips, tapering the leading edge, trailing edge or both.  The same applies to the tailplane and fin.  The sole advantage of the triangular wing is structural reliability; they don’t twist when built by kids.  Use of the rectangular center center section voids that warranty.
Two other considerations come into play.  The angular relationships between the surfaces and the thrust line are important.  The zero lift line of the tailplane should be approximately parallel with the thrustline.  The wing should be set at the angle to the thrustline that produces minimum sink.  For a flat wing, that is somewhere in the vicinity of 5 to 7 degrees.  Then the wing position should be set so the CG balances the plane to fly at that wing attack angle.  Glide testing for minimum sink with the prop replaced by ballast is a good way to get that.  None of this has to be done to exacting standards.  Practical eyeball judgement will get a good flying plane.  You know a slow, steady descending glide when you see it.
A neglected part of balance is the vertical alignment of forces.  The thrustline ideally passes through the CG.  You need to know the vertical position of the CG.  That can be found by balancing the leading edge of the fin on a razor blade.  If thrustline passes below the CG, there will be an up moment during the power burst.  Correcting that with CG or angle changes will result in a faster descent during the entire flight, especially when the power comes off; the plane will come down fast.  Similar inefficiencies result when the thrustline goes over the CG.  In both cases you are using energy to correct the imbalance.  Also, the relationship of the drag to CG will affect pitch trim.  This is included in the glide tests, but only at glide speed.  Under power, the lift and drag increase, but the weight remains the same, leading to imbalances.  It is hard to tell where the drag acts.  If you are pretty sure the thrustline is close to CG, the plane glides well, and the plane shows up pitch under power, you know the drag is above the CG and the thrustline can also go above the CG. Vice versa if the plane shows down pitch under power.  It would be nice if there was a way to isolate a “center of drag”, but there isn’t.  Even if you could, it is impossible to maintain perfect balance throughout the entire flight because the aerodynamic forces change while the weight does not.  The best is to get everything well balanced during the cruise.  The imbalances may be used to control the climb and glide.  Indoors you want a slower climb to stay off the ceiling, so a bit of down pitch from thrustline over CG may help.  Outdoors, you may want a faster climb, but a slower descent in glide.  

A final consideration is the size of the motor.  The motor cross section controls the torque.  You want a motor that flies the plane level a little above the middle of the torque curve.  Length controls the number of turns and the motor run.  For longest flights, you want the longest motor.  There are limitations on how long the motor can be.  Twice the hook distance is my rule of thumb, and that is pushing things a bit.  Longer than that, the motor will not unwind properly and you don’t really gain from the greater length.

Gary  Hinze

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