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Question

Gleb

New member
#1
I saw some planes with their engines above the wing, like the porco Rosso plane or the ft otter. What are the physics behind them? Where does the center of mass need to be to make it balanced. I am needing this not for scratch building so it would be best if the answers were a bit more precise. Thank you!
 

FDS

Well-known member
#2
The centre of mass will move depending on how forces are acting on the aircraft. If you mean the centre of gravity (CG) then that is determined by the wing chord.
Handy NASA stuff and the maths. You design the wing, then you can use a calculator like this one to work out the CG.
There is a limit to how far back you can mount a pusher motor, which will be determined by how heavy the motor is. The bigger the motor, the nearer the wing chord it will have to be, since you won’t be able to compensate for its mass if it’s further back unless the nose is correspondingly very long and has a bigger battery in it.
You will also have to look at the thrust line vs wing position as this can affect handling. I am not clever enough to understand that part. I get CG, it’s less complicated! Think of the plane as a beam balance, with loads being added to each end and the pivot point determined by the wing chord etc, as shown in the CG calculation.
Pushers also require higher take off speeds than tractors since the prop wash doesn’t go over the majority of the controls, so you might need a good throw, elastic launcher or longer runway to get airborne. Prop efficiency is typically lower in pushers.
I hope this is the start of an answer, if you give more detail of what you are going to apply the info to and what you want to build, people here will give better answers.
 

Merv

Well-known member
#3
I am needing this not for scratch building so it would be best if the answers were a bit more precise.
I agree with @FDS, If you let us know what you are going for, it would help.

There are three things that interplay, the location and thrust angle of the motor, the angle of the wing and the angle of the tail. There are multiple combinations of these three that will result in a good flying plane. But change any one of them and you will need to change another to offset.
 

BS projects inc.

Well-known member
#4
Remember that the farther away the motor is from the CG the more "leverage" it will have. For example if your motor was a foot above the cg it would want to rotate down.
 

evranch

Well-known member
#5
It's possible to have a pusher prop at the very rear of a large plane - like this:


Note the huge, long nose that's required to balance it out. The wing can be moved back and forth depending on payload, but mine is still mounted much like this, right at the back, even with cameras and 8000mAh sitting close to the nose.

When the motor is above the wing (like the planes you described, or the Maja), the thrustline is off. This induces pitching moment that is linked to throttle. You can compensate by mixing elevator with throttle, so that you end up with a net neutral pitch. In the case of the Maja, which has a flight controller, I programmed a small fixed mix into the FC, to allow flying it manually. When flying in auto, it just uses the gyros to pull enough elevator to maintain the proper attitude. Without a FC, you can set this mix in your transmitter.

FDS is correct about the takeoff speed, this plane needed a long runway with gear, and now takes a full two-handed pitch to get it airborne as a belly lander. It still wallows badly until it gets up some airspeed. Without the flight controller to run the throttle and level the wings, it used to be a 2-man launch.
 

jaredstrees

Well-known member
#6
So, I think the OP was asking why motors are sometimes mounted on the top of planes over the fuse. Generally this is done to obtain more clearance from the prop to the surface. Such as in a sea plane (the Otter). Or it can be done to increase the field of view such as in FPV. some trainer planes have this feature as it reduces the chances of the prop striking the ground while learning.