There are four forces that conspire against you to add right rudder control input to your model airplane. Learn what they are.
No, it is not when you really have to use the bathroom and it becomes a factor in your ability to control the airplane. The P stands for propeller. You know, that thing up front that bites when you get too close.
A propeller blade is like a spinning wing. Just like a regular wing, the amount of lift or thrust produced depends on its angle of attack. When you have full throttle and the nose of the airplane is tilted up, like during a takeoff, the angle of attack of a propeller blade depends on which side of the airplane it is on at the moment.
Propellers normally spin clockwise when viewed from the pilot’s seat. If the nose of the airplane is pitched up relative to the direction of motion, then the rising blade will have a lower angle of attack than the dropping blade on the right side. Again, this is when viewed from the pilot’s seat. The propeller pulls the airplane to the left, so apply right rudder to correct.
Bottom line? P-factor will be strongest right after lifting off from the ground and whenever you are in a steep climb. Of the four forces, this is the best known one because it is the most noticeable.
2. Spiral Slipstream
Since a propeller generates thrust by rotating, the accelerated air coming out the back of the propeller follows a spiral around the airplane fuselage. But the vertical stabilizer and rudder normally only extend vertically up, not down. This means that this spiraling air column will push on the vertical tail only from the left side. Once again, we have to use right rudder to compensate.
This effect will be most pronounced at high power settings.
This one is easy to understand. When the motor (or engine) puts a rotational force on the propeller, the rest of the airplane will be rotated in the opposite direction. With a normal tractor propeller, the airplane will tend to roll to the left. While on the ground, the left wheel might end up with more rotational drag and pull the airplane to the left, too. To correct it (you guessed it), apply right rudder.
You are more likely to notice this force when the throttle is advanced rapidly. Since an airplane normally weighs a lot more than the propeller, this force is not normally noticed. The roll stability (dihedral) of the airplane will do a decent job on its own to compensate.
4. Gyroscopic Precession
A spinning propeller acts like a gyroscope. The faster it spins and the heavier the propeller, the stronger the gyroscopic forces at play. If the airplane pitches down rapidly, the nose of the airplane will tend to be pushed to the left. This is a left yaw and requires right rudder to correct. Conversely, a rapid pitch up by the airplane will yield right yaw.
I ordered the forces roughly in descending order of importance. Learn what they are and how they might affect your airplane. These four forces also interact with each other in complex ways. If your model airplane all of a sudden starts acting funny, it could be one or more of the four dark horsemen.
You say that your model airplane doesn’t have a rudder? Or worse, that it has one but you never use it? In a pinch, using ailerons to correct these forces is an option but definitely suboptimal. If you have a rudder, hopefully you now have four more reasons to learn how to use it.