enter image description hereI've put together a model plane in the form of an angel, and the planform is quite tail heavy (please see pic). I'm trying to balance the plane with a lifting tail prop that makes the "apparent" CG in the place where the CG should be. I've launched the plane twice. The first time it seemed like there wasn't enough forward thrust. The second time the tail rotor didn't have enough thrust and it pitched upwards. (Please see video links). Will this concept work if tweaked properly?

  • $\begingroup$ Why add a complicated prop at the rear when you could just add the same weight as...well, just weight at front? $\endgroup$ – J... May 1 '16 at 11:21
  • $\begingroup$ I don't like weight. $\endgroup$ – djacob7 May 2 '16 at 6:33
  • $\begingroup$ Yes, but I'm sure you don't like your aircraft to be un-flyable either. The point is that it is vastly easier to design the aircraft to be stable in the first place than it is to add in artificial stability control later. Unless your goal is to have an unstable aircraft and you did this on purpose? $\endgroup$ – J... May 2 '16 at 11:10
  • $\begingroup$ Consider it an invention. $\endgroup$ – djacob7 May 2 '16 at 19:00

Tryin' to cheat the laws of stability, aren't we?

In both videos it is evident that the plane is unstable in pitch. Adding a lifting prop will not change this, because changes in the angle of attack will not affect this prop's lift force much. Stability is achieved by shifting the center of gravity forward, so that changes in the angle of attack will produce additional forces which create a counteracting pitch moment.

In fact, adding the motor will shift the center of gravity even further back. In some cases this helped because the increased pitch inertia made it easier for the pilot to keep the aircraft under control.

If you need to keep the center of gravity so far backward, add artificial stability with a pitch rate sensor and a microcontroller which deflects the elevator appropriately. There are many pages with Arduino-controlled artificial stability solutions - take your pick!

  • $\begingroup$ Thanks for the links.I might use a Hobbyking 3-axis stabilizer using only the pitch axis. $\endgroup$ – djacob7 May 2 '16 at 19:02

Yes, if tweaked properly. You need it to be able to articulate, and employ the same type of software that makes Ospreys not crash so much anymore fly, and either have two of them or an additional tail rotor.

enter image description here

  • $\begingroup$ Yes, I'll be adding an electronic pitch stabilizer if it proves to be unstable. $\endgroup$ – djacob7 May 1 '16 at 17:25

Maybe. Pitch stability could be the biggest problem. The "tail rotor" will have to vary its thrust according to airspeed in order to match what the wing is doing. Let's say the aircraft is in straight and level flight, with the tail rotor delivering an appropriate amount of thrust. Now if the airspeed increases only slightly, the wings will generate slightly more lift, but because of their position, will create a significant nose-up moment, so the aircraft will pitch up rather abruptly. Of course, only a slight increase in tail rotor thrust will offset it and keep the aircraft in balance. You might want to ditch the traditional tail surface altogether and control the tail rotor via a feedback control system cued by angle of attack. Your flight control input would then adjust the target angle.

  • $\begingroup$ That's a very good explanation, Anthony. Thanks. I was thinking of eliminating the tail surface and using the tail motor as an elevator. But first I'll try to verify the scenario you outlined. Assuming I kept the elevator, would a pitch axis stabilizer help in this instance? $\endgroup$ – djacob7 May 1 '16 at 6:38
  • $\begingroup$ Anthony, when the wings get more lift and pitches up, doesn't the drag also increase somewhat, slowing down the plane? $\endgroup$ – djacob7 May 1 '16 at 6:47
  • $\begingroup$ You are right. Lift and drag are the two "virtual" components of the same aerodynamic force which is normal to the wing, so increasing the angle of attack to increase lift inevitably increases drag. That cannot be changed (though the ratio L/D -- i.e. the verticality of the force -- can be increased by the wing design). $\endgroup$ – mins May 1 '16 at 8:50
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    $\begingroup$ Using the prop to add artificial stability looks attractive initially, until you look at the inertia of the prop and the inevitable delay in its response. The consequence is a phase shift in the reaction which could well excite the pitch oscillations you want to damp. $\endgroup$ – Peter Kämpf May 1 '16 at 11:03
  • $\begingroup$ I don't expect the tail rotor to stabilize the plane. It's only function is to provide lift. Pitch stability will be achieved as usual: make it a bit nose heavy and provide reflex on the elevator. I might also add an electronic pitch stabilizer. $\endgroup$ – djacob7 May 1 '16 at 17:23

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