I am currently designing an electrically powered RC plane with one propeller. The wingspan is c. 2.5 m and the length c. 1.6 m.

Right now the plan is to put the propeller at the top of the VS where the HS is also situated. However, as I've advanced further with my design, it has become clear that it is hard in all possible ways, starting from the structural traits (foam covered with fibreglass) to fitting the 40 mm engine along with the elevator servos.

So, should I just move the propeller + engine down to the aft of the fuselage or is there something about the performance that this move might adversely affect?

E: As mentioned in the comments, moving the motor would affect the pitching moment characteristics of the A/C. "Luckily" I have the option of changing the battery placement within the fuselage to counter this effect so the pitching moment consideration is made void by this.

  • $\begingroup$ The direction of the torque moment will be reversed, from nose up pitching to nose down pitching, affecting the stability, and requiring adjustments to balance this change. $\endgroup$ – mins Jul 19 '16 at 9:30
  • $\begingroup$ Good question -- the Trislander's center engine got shoved exactly where you're talking about (i.e. top of the VS), so it's not completely unheard-of to do... $\endgroup$ – UnrecognizedFallingObject Jul 19 '16 at 11:47

To place a heavy mass, part of which is even rotating, on a long, flexible beam is generally not a good idea. The engine mass will reduce the eigenfrequency of the vertical tail, inviting early flutter, and any imbalance in the propeller will provide the initial excitation which flutter needs to develop. Also, placing heavy masses away from the center of gravity increases the moments of inertia and makes the handling of the aircraft needlessly sluggish. The designers must feel some pain to pick this unattractive engine position.

Reasons for this unusual engine placement could be:

  • Better aerodynamic control for ground handling. With the control surfaces squat in the propeller's slipstream, ground handling of seaplanes on water is much improved.
  • Better visibility for the pilot: Instead of a center engine placement in the nose, a tail location gives much improved visibility.
  • Less FOD (foreign object damage): The raised prop position makes it less susceptible to being damaged by debris kicked up from the ground.

The first reason explains why most designs with tail-mounted engines are seaplanes. The engine location on the Trislander, the three-engined version of the Britten-Norman Islander, was picked for the last two reasons and because the designers felt that flutter was not much of an issue with that compact, slow utility aircraft.

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I can find two examples of aircraft (Yes, using that term loosely) with a propeller engine mounted at the top of the vertical stabilizer:

In both cases, this design is chosen due to the conditions where the craft will be operating. The Trislander is designed for short take-off and landing on unprepared surfaces (island hopping is referenced in the name). The Orlyonok is designed to operate in ground effect, which is even more of an "unprepared surface." Engines and propellers far from the ground is a desired design trait in either case.

You don't mention if the propeller is in front of or behind the stabilizer. I would think (IANAA*) that the propeller in front of the stabilizer would make the control surface less effective due to the disturbance of the propeller. The control surfaces on both examples above seem oversized, which I will take as definitive proof of my hypothesis.

*I am not an aerodynamicist.

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    $\begingroup$ IANAA but I would have thought that the wind created by the propeller would make the control surfaces more efficient (greater airspeed) $\endgroup$ – Manu H Jul 20 '16 at 19:54
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    $\begingroup$ There is even a third one: The Seawind seaplane $\endgroup$ – Peter Kämpf Jul 20 '16 at 21:03
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    $\begingroup$ Another pair: the Stuttgart e-genius and Icaré 2. $\endgroup$ – yankeekilo Jul 21 '16 at 5:27
  • $\begingroup$ @ManuH I would argue this airflow would be more turbulent due to the propeller. I'm sure this effect is variable based on air speed. I honestly don't know. $\endgroup$ – blaughw Jul 21 '16 at 16:17

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