Neutral Stability - W & B

[Bob Kuykendall]

Originally Posted by billdianne

How do you measure friction on the ground? If I lift the elevators by hand should they fall back under there own weight? Can you use a fish scale to measure the pull at the stick?...

You make a good point that control system friction can be very hard to quantify. I think that whether or not the elevator reliable falls under its own weight, and the ease with which it does so, is probably a reasonable measurement criteria.

I suppose that you could, as you suggest, use a fish scale to measure the "breakout" force required to overcome the friction. But without a more specialized measuring tool I bet that it would be very hard to establish repeatable measurements that way. If you could make such a machine, it would be interesting to bring it to RV get-togethers and take measurements from a bunch of different aircraft and see what the range is.

...When I first installed the elevators they were very stiff. I literally spent hours getting rid of all the friction I could, aligning the bearings, torquing the bolts to the minimum setting and making custom spacers for the center bearing. It made a huge difference but I still don't know how to quantify what is considered to be normal.

I know that when I was helping Steve Smith build his RV-8 (see Steve's post elsewhere in this thread) we also went to great lengths to reduce the elevator friction to the lowest possible level. I know that we played around with the spacing at the center bearing and at the drive arms; we may have even made custom spacers for them on the lathe. We eventually got almost all of the friction out of the elevator system. It wasn't completely frictionless, but the elevator would easily fall to the nose-down stop when released.

I know that on the glider I'm developing now, I've gone to very great lengths to keep elevator friction down to the absolute minimum. With modern sailplanes, the elevators tend to be very small and the hinge moments very low, so any friction or slop will mask what little natural elevator feel there is. Of the nine pivots in my elevator control circuit, there are only two that don't use ball bearings. All five push-pull tube guides use linear ball bearings.

Thanks, Bob K.

[49clipper]

My -6 with an O-320-D1A and metal sens prop, is 1067# @ 70.8" EWCG with a tail wheel weight of 71#.

[Bad Sheila]

Wow, so mine is not as bad as yours for aft CG because of the wooden prop. I feel better already. Thanks for the input.