Comment by Ryan Beall on February 8, 2010 at 12:06pm

Or you could just buy this:

http://www.tech-mp.com/Twinn_Rexx_support.htm

Comment by Jack Crossfire on February 8, 2010 at 12:31pm

This is called vectored thrust. People normally use 3 rotors & vector 1 rotor for yaw control. The biggest win in multi rotor design would be a way to connect 3 rotors to a single motor & use collective pitch for attitude control. That would be mechanically simpler than a single rotor & would make gas turbines economical. 1 guy managed to do it using belts only to use an electric motor.

Comment by bGatti on February 8, 2010 at 12:47pm

I'm pretty sure this is not an actively stable design, so it must be passively stable (weight lower than thrust line). Given a passively stable design, there are far fewer servos required. You could probably get away with a single servo to control rotation. My prediction - uncontrollable oscillation (weight based stability), unusable in wind, and generally uninteresting. What is missing is meaningful vectoring - what I see is only relative vectoring - pointing the thrust in one direction while creating gyroscopic torsion in the other is not a recipe for stability. What you think of as vectoring the motor, is really only weight shifting.

Comment by Jeff on February 8, 2010 at 3:12pm

bGatti- i recognize your concern, and I share the same concern. for such a small craft, I am going to need very sensitive sensors, a very responsive (and advanced) flight control program, and precise servos. However look at Gress Aero http://www.gressaero.com/, they have successfully used twin rotors to control pitch of the craft (granted, at a far larger scale).

jack- My previous post uses twin rotors on a single motor, basically reaping the benefits of quad/tricopter development, adding inherent stability, and adding a more sophisticated flight control program to keep it stable. However I do not know if it is possible to run two rotors off the same motor, AND make it flybarless.

ryan- I will not buy that (too big :-/) , but I will be looking at that in depth. Thank you for the link! I am trying to make this as small as possible, so it will be a "safe" indoor UAV (I'll add rotor guards/ducts later).

I really appreciate the thoughts. I have not discounted anything that you guys have said. I'll think this all through and post again when I make some forward progress.

Comment by Alex on February 8, 2010 at 10:48pm

I like how you are building you own airframe instead of just buying one :). But may I ask why you want to fly in the direction of the arrow? I am guessing that its way better to fly with with the rotors on in front of another because during foward flight you will be much more efficient as less frontal area, but im sure you have a reason for this :P

I think that as you mentioned the gyroscopic effects of tilting the rotors will be a concern, for sure it will be a major factor if you tilt both of the rotors forwards/back as you might find the body will rotate instead of the rotors as they are relatively large compared to body, and the body does not have any mass beyond the rotation axis (not sure if i explain this very well) I would recommend to just go for a tricopter configuration, or a coaxial config (one rotor above the other).. just my 2cents :-)

Comment by Jeff on February 9, 2010 at 7:16am

Eventually I would like to add an airfoil and convert this into a VTOL tilt-wing or tailsitter, which is why right now it looks like a sideways Chinook.

My inspiration came from the movie Avatar (the physics are vaguely rooted in reality), but my design inspiration was copied from V-22 Osprey and other successful RC VTOL's.

I will be doing some math, but i believe that if I shrink my blade length (and mass), the design will look (and be) more feasible. Also, in this configuration, the rotors will rarely be pitched in the same direction. They will be rolled inwards and outwards (in relation to the center plane) to control pitch of the craft. They will be differentially pitched forward and backwards to control yaw. Rotor speed will control roll. Only to control forward/backward flight will the rotors need to be pitched in the same direction, and this movement will be very small. However I agree with you Alex, I may need to incorporate some other stabilization to encourage the body of the airframe to resist the collective pitching of the motors. This would probably be in the form of either

a) a third rotor - as you and others have suggested
b) a horizontal tail - do you think that a horizontal elevator at the rear of the craft would be adequate?? I think it might be....

Comment by bGatti on February 9, 2010 at 10:11am

I guess the Horiz tail is sufficient. Still looks like a weight-shifting flight control schema. More stable with airflow over the H-Stab.

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