RCU Forums - rate/linkage geometry

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rate/linkage geometry

I have an Sbach that has rates that go from 45deg. On high rates to 20 deg. for low rates. I started with 140% for he high on the radio but I hav to go down to 30% to get the 20 deg. The servo arm and the horn are the same distance from center (hing or servo spline) to connection point, approx. 1.5". The arms are 90deg to the pushrods, not necessarily to the servo body. Conection on horn is in line with the hinge pivot. Is this correct to have such a low number not close to 100%? Had to start at 140% or it would be almost 0% for low rate. I have not had a plane with 3D rates before. Can take some pics if that would help.

RE: rate/linkage geometry

Pictures will help for sure. Rule of thumb is to use the shortest servo arm that will give you the deflection you need. As you reduce the % on dual rate you reduce your resolution. Most of us would not notice this until you get below 50%. If your main focus is 3D then it's not really a big beal as the lost resolution is not a major deal just as long as you are getting plenty of power to your control surface. What you have now is a 1:1 ratio. Never go longer on the servo arm then that.

RE: rate/linkage geometry

Fair warning on the advice above. As you shorten the surface arm, you increase the stress on the hinges.
Most 3D airframes, high rate surface deflection will be sufficient with a 1:1 ratio (servo arm=surface horn length).
Servos have usually 90-100 degrees of travel. That's usually enough for surface deflection for 3D.

Here's what I mean illustrated.
The bottom example will put more stress on the hinges than the top even though the 1:1 ratio and amount of deflection is maintained.

http://smilexpress.net/photos/RC/advantages.jpg

RE: rate/linkage geometry

I just had an Aha moment. I really dont do 3-D. The instructions have highs for 3-D rates. I think I will put my highs at half that. I will get smaller servo arms and adjust from there. I really would like to have more resolution than a need for 45 deg. elevator. The most I ever do is lomcavacs and tumbles, the rest is IMAC style (not that I'm that good). So thanks guys. but here is what I have it set up as anyway

RE: rate/linkage geometry

If you have more throw than you need, do one of two things. Move the linkage on the servo horn closer to the center. And/or move the linkage on the control surface horn further from center. Both of things will reduce throw while giving you a better mechanical advantage. IE: your servo will be more effective.

david

RE: rate/linkage geometry

Quote:

ORIGINAL: xcz5785

I have an Sbach that has rates that go from 45deg. On high rates to 20 deg. for low rates. I started with 140% for he high on the radio but I hav to go down to 30% to get the 20 deg. The servo arm and the horn are the same distance from center (hing or servo spline) to connection point, approx. 1.5". The arms are 90deg to the pushrods, not necessarily to the servo body. Conection on horn is in line with the hinge pivot. Is this correct to have such a low number not close to 100%? Had to start at 140% or it would be almost 0% for low rate. I have not had a plane with 3D rates before. Can take some pics if that would help.

thats the nature of the 3d beast you cant have low rate resolution and 45deg on the flippers. Its not going to fly like a pattern ship with a 3d cg anyway so dont worry.if you want to piddle with geometry to get it correct I would highly recomend spending 9 bucks on this http://www.envisiondesignusa.com/evd..._Overview.html. your drawing has perfect geometry. shortining the servo arm will require the horn to move forward of the hinge line and still keep the arms and linkage parallel. with that said it doesnt have to be perfect unless your anal like me