The Chinese Weight Mod Explained

A discussion arose (again) about the chinese weight mod in the Heliguy forums, and the same old question was asked again: "what is the chinese weight mod, and why do I need it".

So, having read this thread and others in days gone by, and having been amazed by all the weird theories and term-tossing, I decided to make some figures to explain this thing once and for all. I hope I have been successful!

The whole mod is very simple, even in terms of physics. I know it can sound weird when people talk of the tennis-racket force etc. but give the below figures some thought and I'm sure you'll get it. Its basically very simple. You only have to understand centrifugal force (carousel turning, you feel like someone is pushing you away from the center axis), and simple aerodynamic force (when you stand in the wind, you feel like its pushing you).

So, the Chinese weight mod is done to counter problematic forces opposing the servo arising from:

• centrifugal forces in the tail grip system when the blade is at an angle.


• aerodynamic forces from the turning blades.

When the blade is at zero angle, no such forces are involved:




Fc = centrifugal force, in this picture it causes no problems as the blade grip control arms are already at their farthest distance from the center.

But when the blades are at and angle (as they usually are), the blade grips want to move away from the shaft (centrifugal force) and thus cause a rotating tendency (torque, marked with a tau, around the red marker) that tries to bring the blade grip control arms level with the rest of the grips. This works against the servo.



Fc = centrifugal force on the blade grip control arm
τ = torque, caused by the centrifugal force
Fa = aerodynamic force caused by the pressure of air in front of the blade
τa = torque caused by the aerodynamic force
Fp = problematic force caused by the two kinds of torques, pulling on the servo

Now, to counter the above harmful torque, we can attach weights to the blade grips as shown in the below figure.


tau = torque
Fc = centrifugal force, this is causing the problems
Fc2 = another centrifugal force, from the chinese weight mod
tau2 = another torque, caused by the chinese weight mod

This, of course, illustrates an optimal situation where the torques cancel out. In practice we usually just get close to that, thus alleviating the pressure on the servo.

Then, again, when the angle is zero, the weight mod causes no extra forces because the weight is uniformly distributed around the axis of rotation (tail shaft):




Also, you may not agree with Fc, tau, etc. being the same on both sides. I agree, but it was just so much simpler to illustrate like this, and they are practically the same.

I hope this helps!