Front and Rear Propeller

Front mounted propellers are tractor propellers. They are mounted in front of the engine and pull the aircraft forward. Rear mounted propellers are pusher propellers. They are mounted behind the engine and push the aircraft forward.

Both propellers are trying to move the shaft forward this force is then transfered to the fuselage via the bearings (or lack thereof) the forces would be the same.

Placing the driving force before or after the balancing point may however have an effect on the stability or maneuverability.

I cannot remember any back-push kits from my model aircraft flying days (50 years ago) and does not have any first hand experience.

do they differ in shape , size and material etc?

Little if any. The control arms are different that are used to slow the aircraft for landing. The rear mounted engines require stronger control arms to flatten the props or reverse the air flow.

If both props are on the same power shaft then they both turn the sams direction and the full offset trim for all of the torque is needed. If there are two engines/motors tem they turn opposite each other and small torque representing small dynamic differences are present but may require no trimming at all. The propellers are the same unless they are located one imediatly behind the other in which case the frant propeller discharge angle is part of the equation for the rear propeller entrance angle

I would imagine that using a single pusher or tractor propeller = nothing to worry about.

But if you want to use both on the same model, the first speeds the air up considerably, so to "add" power, you need to speed it up a lot more with the rear prop, therefore a much coarser pitch would be needed on the second motor, I would imagine.

It may turn out best if the front prop has a slightly "finer" pitch and the second "coarser" as already mentioned.

The next question is the direction of rotation of the second engine to the first. That is, when both motors are mounted, the props revolve in the same direction when viewed from in front (or behind!) the aircraft.

I would imagine that it may be better to have the second engines rotation to be reversed, so it will not be wasting power in changing the rotation of the flow from the first engine....just a thought. I do not know if in practise there will be much actual difference though....

Andy, if you reverse the rotation of the rear engine so that the two props are rotating in the same direction in respect to the axis of travel, you will be doubling the reactionary torque imparted to the aircraft. Either way you are also doubling the gyroscopic mass ( probably not that big a deal for a full sized aircraft but it might be for a model.). adding the two issues together will result in a major stability problem. All aircraft I am aware of that have coaxial push/pull props have their engines counter-rotating so that the reactionary torque is cancelled. The Russian 'Bear" bomber had counterrotating props. The Convair B-36 were pusher only.

Good point, I had forgotten torque completely........

Suggest you check out http://www.rcgroups.com/forums/index.php

and perform search for tractor pusher

Enjoy

We used to use a pusher propellor on front engined control line planes. This allowed us to run the engines "backward" so that the engine torque actually helped keep the lines tight instead of trying to make the plane turn in to us. This allowed us to use less weight on the the outboard wing tip and less right rudder to keep the lines tight. Never saw this written up in a magazine or anywhere else, it was just something which occurred to one of our group which we all thought was a great invention.

The props were the same material as the normal tractor props same design etc, just mirror images!

Hi all,

I did not think any "puller/pusher" dicussion would be complete without reference to the O2 or 337 Skymaster in the General aviation world.

Try this link for further info : http://www.337skymaster.com/

Warmest regards