Last week, we looked at some basic principles of full-scale tail rotor design. This week, let’s investigate more into the four weights that are used in a full-scale helicopter rotor to reduce control power. But first, let’s review some engineering principals we already know. If you were to tie a small rock to the end of a string, and then twirl the rock, it would produce a centrifugal force, pulling the rock to the outside of the circle. This force can be increased in one of three ways: (1) increase the weight of the rock, (2) increase the speed of the rock, or (3) increase the length of string, while keeping the same rotational speed. It’s this last case, increasing the length of the string, which will become important later in this discussion.
The first photo is of my Fury tail rotor where the rotor blades are in the zero pitch, or streamlined position. This is a very stable position of least aerodynamic drag, and is where the blades will go if no outside force (from the tail rotor servo) were present. Now let’s call the distance from the center of the tail rotor hub to the tail rotor blade mounting bolt “R” for radius.
Referring to the second photo of my tail rotor, you will now notice the tail rotor blade is 90 degrees to that in the first photo. This is the position of maximum aerodynamic drag, and is certainly not a good position for the blade. And, as you can imagine, it would take quite a bit of force from the tail rotor servo to keep the blade in this position. We can therefore see that when the tail rotor blades are streamlined as in the first photo, very little, if any, force is needed from the tail rotor servo to keep them in place. And, as the tail rotor blades are displaced to a greater angle of attack, the force required by the tail rotor servo is also greatly increased.
Again in the second photo of the tail rotor, the blade mounting bolt has been greatly extended to a distance “D” for demonstration purposes, and is now in the plane of the tail rotor. A right triangle has been formed with the two sides R and D. The hypotenuse of this right triangle is from the extended tip of the bolt to the center of the tail rotor hub, which I have chosen to call “X”. Also, since the hypotenuse of a right triangle is the longest side, it follows that X is greater than R.
Now let’s refer back to the rock on the string. We said a longer string will produce a greater centrifugal force, and that is exactly what we have when we compare R to X. Disregarding the tail rotor blades themselves for a moment, the centrifugal force produced by the weight on the blade mounting bolt in the first photo is proportional to the distance R. In the second photo the distance R has now been increased to X, which produces a greater centrifugal force. This greater centrifugal force tries to rotate the tail rotor blades from the streamlined position in the first photo to the 90 degree position of the second photo.
Stand by for more in my next blog to see where all this is going.