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-   -   Placing the motors in Pull or Push configuration? (https://www.rcuniverse.com/forum/aerodynamics-76/11685817-placing-motors-pull-push-configuration.html)

makehobbyrc 12-18-2020 09:13 AM

Placing the motors in Pull or Push configuration?
I need some advice... about my RC Foam Home Made Seaplane (I hope will fly one day...).
About the motors... I will have 2 at least maybe 3. I have bought 2 ducted fans but also 2 simple brushless motors (with 15cm = 6 inch props...). I might use 2 ducted fans or / 2 or 3 propeller motors...

My questions are: 1) Any preference in putting the motor in Push or Pull configuration? Propeller in front or behind then motors...
2) And any need to put the motors at a slight angle from the longitudinal axis of the aircraft... It is a Seaplane, and the motors will be placed slightly above the CG, to be above the wing and fare from the water... Any need to have a slight angle pitching downward (for the rotational axis of the motors) to compensate for the "nose down pitching effect" whilst accelerating....?
Thanks a lot!
PS: pictures will come as soon as I am allowed to post ;)

BMatthews 12-22-2020 01:26 AM

Objects in front of the prop tends to reduce their effectiveness. Props work best when they see clear air free from turbulence that occurs from parts of the model. So I vote for normal tractor use.

And yes, you will want to angle the motors so they face up by a good angle. On the flying boat I built last spring I went with +2.5 incidence in the wing and the motor pointed up another +2 more than the wing. It still had a slight tendency to dig in the nose when on the water during the slow part of the takeoff run. But in the air it was just about right. No strong tendency to climb with throttle.

I thought the prop was up high enough too. But the nose digging in even the amount it did caused water to come up around the nose and into the prop. So plans are to add on a pair of 1/4" (6mm) wide by 1/2" (12mm) tall extra chines on the nose both to aid with floatation in the nose as well as act like to direct the water more to the sides. I learned my lesson. LOTS Of flotation up front ! ! !
Bubbles original design flying boat.
Bubbles flying boat.

mithrandir 02-05-2021 07:50 PM

there are books written... multi-volume... addressing these very questions....

Tractor: clean air in, accelerated air causing thrust loss as it sweeps over the fuselage and tails....

Pusher: No fuselage to funk up the accelerated air... but a lot of crudded up air going into the prop from fuselage and tails... actually can be an irritating source of noise and potential prop fatigue...
there is a theory that pushers have added pitch stability if the prop is at the back of the plane....

Trying to run the thrust line through the CG is a myth..... you want to align the thrust line with the center of DRAG. (As much as is practicable)
this can be attenuated if you have a simple gyro on pitch, or even a throttle-elevator mix....

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