Swash Link Geometry Question

[Ray K.]

Hey everyone!

I own a pretty heavily modified and fully separated Axe CP, and I'm really enjoying it. One of the only mods that I left to do is to convert the swash links so that they don't have z-bends on the servo hons anymore. I've found several ways that I can do this, but I need to know how important it is that the links are at perfect perpendicular angles to the swash. It seems to me like you can be off a few degrees before it becomes a problem, but I'd like to know for sure. The method that I'm favoring to do this is going to make it such that the link is going to tilt inboard a little bit as it goes from the servo to the swash, and it's likely that the link won't be perfectly parallel with the mainshaft either, but it will be pretty close. Also, with a CCPM setup, it's pretty critical that the link balls on the horns are all exactly the same distance out on the horn, isn't it? I suppose that minor differences could be compensated for by adjusting servo endpoints, by I'd rather not do that if I don't have to.

Just thought I'd try to get some information from people who know more about this than I do...

Thanks!

-Ray

[vera]

It's kinda important for the control rods to meet the servo arm at 90 deg when the swashplate is in the center of its range of motion. But if it isn't 90 deg then at least all the rods should meet their servo arm at the same angle, and that's really more important because otherwise changes of collective will tilt the swash by some amount. This is because the linear motion of the control rod depends on the cosine function of the angle of the servo arm. This is fairly linear in the neighborhood of 90 degrees but it craps out as the shaft turns. You want all the functions to crap out by the same amount to keep the swash level. If one servo is farther from 90 deg than another to start with, then that one will run out of steam, so to speak, sooner and the swash will tilt by some amount as you run the collective up and down.

Anyway, whatever deviation from perfection there is, if it's the same with all 3 servos it won't matter much.

[Ray K.]

vera-

Thank you! That's pretty much what I thought, but I really didn't pay that much attention in math class way back in high school. It occurred to me that I can shim the servos and the horns a little bit, and I should be able to make them all at least the same, if not right on. At what point does the rate of linear motion drop off so badly that it's no longer helpful? My guess is that somewhere between 30° and 35° of servo horn rotation is going to be where one would start to notice it rolling off. So am I right in setting up the horns so that I have 0° of collective pitch with the servo horns at 90°? Thanks for your help!

-Ray

[vera]

So am I right in setting up the horns so that I have 0° of collective pitch with the servo horns at 90°?


That would be my choice. I like the swash to be in the center of its vertical range of motion, servos centered, with 90 deg between rods and arms, 0 deg pitch on the blades, and level washout and mixers.


I should be able to make them all at least the same, if not right on.


That should do the trick. The thing that introduces bogus tilt to the swash is different angles between arms and rods for the 3 servos. The swash can be made to be level at any position, but when you move it up or down from there you want it to stay level. This requires sameness among the servos. But gee whizz... a little difference won't be a catastrophe. If you make a lot of extreme collective inputs and it isn't exactly set up right you may need a little cyclic to cancel whatever error occurs. Even if you get the mechanics perfect, a servo itself might be a little different than another and that can cause bogus tilting of the swash with changes of collective input. That's life with the CCPM contraptions.