Motor angles for pusher planes

Summary of some experiences in setting-up the right motor angles on scratchbuilt pusher planes. 

The video shows the maiden "flights" of a scratchbuilt wing with no motor angle. As you can see it did not fly so well! This is the second plane with which I have to experiment with different motor angles before finding a good solution. 

The results of these experiments have often been counter intuitive but with a great effect on the plane's ability to fly. Does it need some up-thrust? down-thrust? why? Here are some tentative answers.

Why pusher planes?

I like pusher planes for several reasons:

• The propeller is well protected. It will not break on a unfortunate nose first landing and it will not hurt the things it crashes on too much.
• It is great of aerial video or FPV since there is no propeller to block the view in the front.
• In some cases it allows to put the motor very close to the CG which can give very good flight characteristics.

Of course all of these can be discussed and there are also some drawbacks with pusher planes but anyway, lets say that you have decided on a pusher design.

Why is there sometimes a need for some up/down-thrust?

Not all pusher designs will need up-thrust or down-thrust to fly properly. In fact the problem is not related to pusher/tractor configuration or the position of the motor in the front or back of the plane. The problem is actually related to the position of the motor on a vertical axis: if the thrust line is higher or lower than the wings.

With pusher planes, since the motor is not on the front of the plane there are usually some structures of the plane (wing, fuselage, tail, etc) which prevent the motor of being mounted at the level of the wings. The result is that the motor/propeller typically ends-up somewhere above the wing or fuselage. It can also end up bellow the wings but that is not so common so I am going to focus on motors mounted over the wings: planes with a high thrust line.

The result of a high thrust line is always the same, whenever power is applied the plane has a tendency to pitch down. Depending on where the motor is the effect can be enough to completely prevent the plane from flying. This is what happened with the last flying wing I built.

How to avoid the problem?

The only way is to design the plane is to have the thrust line horizontal and "on the same level as" the wings (or very close to it). This can be done on flying wings and deltas by butting the motor at the rear, it can also be done by putting the motor/prop in a slot or having a twin tail, having the wing high above the tail, etc. If some of such option fits the design than it will definitely make the plane fly better. 

In some cases, none of these motor positions are desirable. For example on a sea planes or on a belly landing plane we do not want the propeller to be exposed bellow the plane. In those case (and others) we need to have a high thrust line.

What is the problem with a high thrust line?

If the thrust line is horizontal and above the level of the wing the plane will pitch down when power is applied. The reason is quite simple and illustrated bellow:

So the problem is the rotation force which makes the plane pitch down. The solution is quite simple, we need to somehow produce a force that counter acts this "pitch down" force, something that makes the plane pitch up. 

If the motor is not too high (and the motor not too powerful) than the force can be counter acted by using the elevator. By applying some up elevator when power is applied you can compensate the pitching down moment. That can work but creates drag and requires constant adjustments of the elevator for every power change(could use some mixers or something like that to help).

On more severe cases, if the motor is quite high or has a lot of power the elevator does not have enough authority to counter act the pitching forces.

Mounting the motor at an angle allow using a fraction of the thrust to counter act the pitching down force. The question is the should the motor be tilted up? down? by how much?

The thrust line through the CG?

One way of doing it is to directly use part of the thrust to counter act the rotation. By making the thrust line go through the CG, the is no rotation around the CG anymore.

Basically the horizontal part of the thrust is still used to power the plane forward and some down force is used to push the tail down. That works quite well because when the power of the motor varies, all the forces vary in the same way and always compensate each other. The problem is that the blue part of the force is wasted to propel the plane (and tends to push the plane down). This is a good solution as long as the blue force remains small.

That worked for my flying wing:

There are two things which contribute to making this a good solution: keeping the motor as low as possible and as far as possible from the CG. If the motor is too close to the CG or too high then the situation looks like this...

... and the plane will not fly.

This was the case for this plane: 

This is however not the end of the story: in some cases some up-thrust can actually also solve the problem especially if the motor is close to the CG.

Going the other way? up-thrust?

Remember that all we need is some force to push the tail down. Let's just blow on it!

And there it is! the air flow of the propeller wash creates a down force on the tail off the plane which is proportional to the thrust of the motor and can counter act the rotational force around the CG. This works very well both on model planes and real planes when the motor is close to the CG and if there is some tail surfaces in the prop wash.

It sounds quite obvious but it took me quite some time to figure out why one of my pusher plane was flying better with some up-thrust.

Conclusion

Even if we limit ourselves to pusher planes with the motor above the wing and behind the CG, there is no single answer to the question "what motor angle should be set for the plane to fly?". Depending on the design and the exact position of the motor the best solution can be some up-thrust or some down-thrust:

• Up-thrust if the plane has a tail in the prop wash and the motor is not too far back.
• Down-thrust if the motor is not too close to the CG and/or the plane has no tail.

If there is no tail and the motor is mounted high above the CG it will probably never fly very well.

I hope these experiences can be useful for someone, leave some comment if you have experienced different things or have other explanations!

Cheers!

Just for fun, here are the thrust vectoring system I built to easily test different motor angles.

Here are some additional pictures of the wing's thrust vectoring system. It is inspired from the yaw servo system on rcexplorer's tricopter but with a push rod instead of a direct-drive servo.