The following article is taken from the AMA National Newsletter.
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RIGHT RUDDER

Have you ever wondered about the effects of power setting and thrust on a single engine propeller airplane?

We try hard to balance everything on our models with symmetrical construction, exact center of gravity (CG) placement, and perfect lateral balance. If we were building a glider or jet, the perfect balance would hold true, but a single engine and rotating propeller really messes things up!

On a calm day you probably have to hold right rudder on takeoff for a straight climbout. I know this is caused by the single engine and propeller because I took some acrobatic lessons in a British Tiger Moth (with the opposite engine rotation) and I definitely had to hold left rudder during climbout!

I used to think this phenomenon was caused by engine torque. From the pilot's seat the engine turns clockwise, so engine torque would want to make the plane roll counterclockwise. This would be corrected with a bit of right aileron, and doesn't explain the need for right rudder.

Well, here's the definitive explanation of all propeller and engine effects from Steve Smith of NASA.

There are five significant effects of the propeller and slipstream on aircraft trim and stability.

First, we better clarify the distinction between stability and trim.

Trim has to do with balancing all the moments and forces so you get steady flight. A propeller-induced yaw moment can be trimmed with the rudder. That's trim.

Stability has to do with the way the moments and forces change as the aircraft attitude changes. For example, if a gust disturbs the aircraft in a bank, do the forces and moments change to recover or diverge from the bank?

An example of longitudinal stability and trim would be like this: You position the CG of your model so that you get a steady glide with the tail incidence set at a certain angle-probably carrying a small upward lift on the tail. This is trimmed.

If the CG is too far back, and the upward lift is small, it may also be stable, but perhaps not. The key is how the pitching moment changes with angle of attack. When an upward gust causes an increase in angle of attack, both the wing and the tail see the angle increase and correspondingly increase their lift. The added lift is applied at a point 25% of the average chord length behind the leading edge for each surface.

For your models, it is likely that the CG is behind the wing 25% chord point, but the airplane is still stable. The moment arm of the tail is longer, so even though the tail is smaller, the added lift on the tail may overpower the nose-up moment caused by the added lift on the wing, so the net moment is nose-down, restoring the glide from the gust upset. If the CG were a bit too far aft, with more tail lift to trim, a point would be reached where the incremental lift on the tail was not enough to balance the nose-up moment from the incremental wing lift, and the airplane would diverge nose-up, stall, then probably pitch down and dive violently.

Now I'll see if I can cover propeller effects, not in any specific order. The direction of forces and moments are described for a tractor (not a pusher) propeller, turning counterclockwise from the front (normal right hand rotation).

First, there is the propeller itself, which causes gyroscopic coupling in maneuvers. If the airplane pitches up, the gyroscopic coupling on the rotating mass of the propeller tends to yaw the plane nose right. So you can make climbing right turns easier than climbing left turns. Fortunately, props are pretty light. This effect was really a dominant one on old WW I fighters with rotary radial engines, where the crankshaft was mounted to the fuselage and the entire cylinder head assembly rotated with the propeller. This was a large rotating mass, and strongly affected the maneuverability.

Second, there are the propeller thrust forces, which are affected by the flow-angle entering the propeller. Suppose you mount your engine so the thrust line is parallel to the body axis, and the airplane is trimmed to fly with a small angle of attack of the body axis. Now the downward-moving blade sees the added pitch angle from the angle of attack, and the upward-moving blade sees a reduction in pitch angle. So the right blade (moving down) produces more thrust, and this produces a left yaw moment. You can trim this left yaw moment by inclining the thrust axis of the prop downward slightly, so it is near zero angle of attack to the flow. I think you would get a right turn at high speed, and a left turn at low speed.

Note that this effect is also a stability effect, since the yaw moment varies with angle of attack. If you have wing dihedral, this yaw will couple into rolling moment, so as the airplane flies slower, angle of attack increases, and the left turning tendency will increase.

Third, there is the propeller slipstream swirl effect. This swirl tends to change the flow direction over the rest of the plane, especially the tail surfaces. The effect is strongest when the airplane is moving slowly with high power, since this causes the greatest swirl angles. As the airplane speeds up, the swirl is diluted by the faster flow, and the angles are reduced. Thus slipstream effect is reduced as speed increases. Direction of the slipstream swirl moment is nose left.

Fourth is simply the engine torque, which will tend to roll the airplane to the left. The roll will couple through any wing dihedral to cause a left turn. Note that this is not a stability item, since the torque doesn't change very much with airplane attitude- it's just an unbalancing moment that must be trimmed.

Fifth is a real stability effect. The propeller has a certain amount of lifting area, from the projected area of the blades. When you pitch a propeller, it produces some lift (beyond just inclining the thrust axis upward) so it acts like a small canard surface. When you yaw the prop, you get a side force, just as you would from a small fin. For a propeller installed ahead of the CG, this effect reduces stability, so that the fin and tail area are slightly larger than would be required if the prop were not there. This is usually a very small effect and rarely influences airplane design, but there are a few good examples where it did.

Through the development of the Spitfire, they put bigger and bigger engines in it, and it went faster and faster, with more and more propeller pitch angle. Eventually the plane got snakey-it would hunt from side to side. The propeller was having enough of a destabilizing effect on the directional stability that they had to increase the fin area.

The Red Barron unlimited racer of the early 1970s was a P-51 fitted with a Griffon engine. The big engine allowed higher prop pitch, and directional stability was poor until they increased the fin area.

The early Northrop flying wings were pusher prop designs, and the propellers provided enough directional stability that fins were not needed. When they built a jet-powered flying wing, they had to put fins on it. The fins were about the size of the projected area of the propellers it used to have. I suppose one could argue that those props also added longitudinal stability which resulted in less elevon download and lower drag than would be calculated, ignoring the props.

from The Glitch
Cape Ann RC Model Club