How I Set Prop Thrust Angle.By Eric McAuley Most folks like to have their plane fly in slow, sweet circles. The reason for this, I think, is to see it better and not have to chase it across the county. It is also very handy for indoor flying to keep it off the walls and ceiling.
There are three main ways to achieve a circle. One way is to deliberately warp a wing. This is called "washout" or "washin" depending on which way you warp it. The Wright bros. plane was controlled this way. Using a system of cables and such, they had the ability to cause "wing warp" and that made one wing produce more lift than the other. This caused the plane to "bank", { one wing rises higher than the other} thus turning the plane.
Another way is to deflect the rudder. This works because air passing the rudder pushes more on one side than the other. Much like a weather vane, or the flute of an arrow. Another good example is when you hold your flattened hand out the window of the car while moving down the road.
Both of those ways are valid and have merit. You may need to use one or both, in some cases. Try them, if you wish. They may work fine for you, as they work well for many people. The biggest trick is to find what works for you.
The way I prefer to make my planes turn is with prop thrust angle. There are other ways, perhaps better ways, but this works for me because it is simple and consistent. You must set the prop at a slight angle to the center line of the airplane anyway. If you are careful, you can use that angle to turn the plane.
When your plane is balanced, and gliding, you will most likely notice it has a tendency to fly to one side or the other. That's fine, don't worry about it. Go with it. There are several possible reasons that it does that.
Could be that:
One wing is shorter than the other. {I have done that.}
One wing is heavier than the other. {Done that too}
Accidentally built in washout. {Yep}
Air current you are not aware of. {lots of things I am not aware of}
Rudder might not be dead on straight. {many times}
For whatever reason, if it's not too severe, its fine. You can correct it with prop thrust angle. Try as I might, I have yet to build a perfect airplane. These flaws will cause your plane to have a natural tendency to turn one way or the other. It's normal. It's nearly impossible to eliminate completely. Instead of pulling your hair out over it, use it to turn your plane.
Here's how it works. As a prop turns, it tries to rotate the plane in the opposite direction. This is called rotational torque. The only thing that stops your plane from flipping over, is the lift provided by the wings. On a plane with a large wing and a small underpowered prop, the problem is not so drastic. On a plane with a small wing and a large, powerful prop, the problem is much more severe. Don't worry about whether your prop is big or small. The principle is the same and it still works.
If you want to see this in real life, hang your plane from a single string by the tail so that the nose is pointed straight down. Wind it up and see what happens as it hangs there. The prop will go one way and the plane the other. With no air going over the wings, it has nothing to counteract the prop torque. Try it once, it's fun.
Some folks call this the P-Factor. When the plane is moving, the factor is less and the wings lift counteracts it, but not totally. You can use the remaining P-Factor to turn your plane.
If you build your plane and mount the prop straight and true on the nose, it will try to turn up and to the left under power. It will keep doing that until it is upside down or hits the ground. Or both. If you mount your prop with 3 degrees down thrust and 2 degrees right thrust, [measured as if you were sitting in the cockpit, facing forward.] it stands a better chance of level flight. The reason for this is that the prop thrust is now pulling and twisting the plane in the opposite direction of the spinning prop. {In a nutshell} We use down thrust because both wings are trying to pull up do to lift. We use right thrust because the plane is trying to bank left, against the prop torque. For the purpose of turning, we will concern ourselves with the right thrust. We will talk about down thrust later. More right thrust equals less left bank.
Lets put this into practice. Lets assume that your plane is balanced and the glide is pretty much stable. Set your prop at 3 degrees down and 2 degrees right. Put just a few turns on that rubber motor and try it.{50 or so} Did the flight change?
If not, try more winds on the motor. Prop torque gets stronger with more turns. The tighter the rubber motor, the faster the prop wants to turn, and thus, more prop torque. And hopefully, a longer stable flight.
At some point, the prop starts actually pulling the plane and not just riding along. {freewheeling} You will kn
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