Jim Nordin’s excerpt from the Lancair 320 build manual says it pretty clearly. What it leaves out is that the linkages need to be disconnected from the aileron. Leave the pushrod mounting bracket attached to the aileron but don’t connect a pushrod to it. Don’t even have a bolt installed in the mounting brackets, as you want to be flutter-free even if that bolt were to fall out somehow.
The essential purpose of balancing the aileron is, as was mentioned previously, to have the inertial reaction of the aileron to wing flexing be in a direction to not amplify the wing deflection. Okay, let me say that differently. Wing flexes up (maybe due to a gust) and the aileron lags behind due to inertia. If the aileron is heavier BEHIND the hinge line, then when the wing flexes up, the aileron will be deflected trailing edge down due to inertia. This increases the lift on the wing, causing more wing deflection. The wing eventually springs back, and when flexing downwards the trailing edge deflects up, reducing the lift loads and amplifying the elastic response of the wing structure.
On the other hand, if the aileron is heavier AHEAD of the hinge, then when the wing flexes upwards due to a gust, the inertial reaction of the aileron is to deflect trailing edge up, reducing the lift and therefore reducing the structural flexing.
When the center of mass is AT the hinge line, the inertial reaction neither amplifies nor reduces the additional lift which caused the flexing. This is called “100% mass balanced”.
If your ailerons are not balanced, the self-amplified flexing can grow catastrophically. Fast. That’s flutter. But there is some structural damping in the system which keeps this from happening. But damping decreases with airspeed, so you will eventually get to an airspeed where flutter will happen. But that’s above redline speed, sometimes a lot, but sometimes a little. (Depends on the airplane.)
When you have more engineering resources at your disposal, you can measure the structural damping and allow a little less lead to be used. You basically allow a little amplification – not too much – and let the existing structural damping keep flutter from happening below redline speed. But you are essentially guaranteed to be free of flutter if the ailerons are 100% mass balanced, so that’s what homebuilders do.
Using the wing to hold the aileron while measuring the balance point is convenient, but there cannot be any friction. You can do the same for the elevator. A little tough to do with the rudder, as the airplane would have to be on it’s side. But a better way is to take the surface off the airplane, insert the hinge pins, and use some kind if “knife edge” to support the hinge pins. This gives a virtually friction-free condition. But if the hinges are low-friction enough, you don’t need to do that, and if the aileron hinges had that much friction, you’d probably want to fix it anyway.
As to whether the aileron should stay in position due to friction when you place it somewhere other than in-trail, well, when the system is fully assembled it doesn’t matter for flutter. It only matters for handling qualities.
- Rob Wolf