I am designing a model aircraft that will have 4 propellers in a square configuration (think like a quadcopter) so that it can take off and move around like a quadcopter, then transition into plane flight like a VTOL aircraft.

For operating in quadcopter mode, it would be ideal for the center of mass to be in the center of the airframe (behind the main wing) for motor efficiency and agility reasons. If the CG is over the wing the front two motors will have to work harder than the back two, and while the PID controls will stabilize for this, it will decrease throttle headroom.

  • $\begingroup$ It depends on how much, and if your tail plane has enough down force to counter-act the natural tendency to pitch up. This can become a bigger problem in VTOL mode if the CoM is behind the rearmost propellers because the only thing left to make it pitch down is to run the front props in reverse... This is why aircraft like the Osprey have a very narrow CG margin. $\endgroup$ – Ron Beyer Feb 11 at 19:19


One relatively trivial example is a tandem wing (or a canard with a very large forward surface, if you will); the center of mass will be between the lifting surfaces, positioned so that the forward surface flies at a higher coefficient of lift than the rear (to give static longitudinal stability).

If you build your model with the motors on short arms, so that they're a bit above the wings when in quadcopter mode, you may be able to manage the CoM shift so that you automatically get the balance right for forward flight in wing lift.

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