Servo arm to horn ratio?

[Flyinrazrback]

Quick question, I pulled the servo tray out of my dalton (pull/pull) and am now putting 2 servos in the tail. The horn in the tail is an SWB offset rudder horn, and is 2" from the center of hinge to the ball link attachment, my question, should I go with 1.25 or 1.5" arms to get max throw? Servos will be Futaba 9152s, thanks!

[bodywerks]

Are you saying the control horn one the rudder is an SWB arm, I can't help you because I never knew SWB made control arms for the surfaces. If yor are saying you have an offset servo arm that is 2" from the servo axis to the outer hole, you need 2" or greater distance from the rudder's hinge line to where the pushrod connects. I don't know the thickness of the rudder so I don't know how long the control horn needs to be.
Also, since your servo arm is offset, but does not need to be, you can form an imaginary line between the servo arm's outer hole and the screw hole with a ruler, ad set the arm up such that the imaginary line is purpendicular to the servo, lengthwise.

[Flyinrazrback]

Attached is a picture of a swb horn. From the center of the bevel out to the ball link attachment is 2". The rudder servos are mounted in the tail, just trying to figure out with the horn distance being 2" from center to ball link, if I will get enough throw with a 1.25" servo arm or if a 1.5" would be necessary.

[bodywerks]

OK, I re-read it and, if the distance from the hingeline to the ball link attachment on the rudder is 2", then even a 1.5" servo arm will not be enough, especially since I assume the servos will be mounted in the side of the fuse, which will put the servo arm's and rudder arm's axis are off by 90 degrees, in relation to each other (this results in a loss of effective throw). I would say you need at least a 1-3/4" arm and 150% ATV to achieve 45 degrees of travel.

[bodywerks]

And if the arm on the servo is, indeed, offset, then you need to measure the amount of offset and match it with an opposite amount of offset on the servo arm. In other words, if the rudder arm sweeps forward of the hinge line by 1/4", then the servo arm needs to sweep aft of the servo's axis by 1/4". This will get you as close as possible to achieving equal throws in both directions.

[Flyinrazrback]

The arm on the servo will be a standard airwild arm, that is just the rudder horn that was already in the plane being used with a swb tray on pull/pull. It would be a lot of work at this stage to pull the horn out in favor of a 10/32 type horn.

[Michael Glavin]

At 60 degrees servo angle;
1.5/2 nets 40 degrees throw
1.75/2 nets 50 degrees throw
2/2 nets a 1:1 ratio, 60 degrees throw is plausible

The SWB arm pictured centers the pivots on the hinge line if properly installed. However this does not ensure equal travel arcs, you'll need to maintain a 1" offset/space between the control arm pivot horizontally and the servo output shaft or axis to realize same, its all good.

[bodywerks]

That doesn't appear to be an offset arm, i.e., all three attach points appear to form a straight line. That doesn't take anything away from Mglavin's recommendation, though, assuming that the servo axis and rudder axis are 90 degrees in relation to eatch other...

[Flyinrazrback]

Ok cool, I do not want to run a 1.75" arm, dont feel comfortable running anything over a 1.5" arm, so need to figure something out. May just have to cut the rudder off and rehinge it, and set it up with a 10/32 horn, but was trying to avoid that.

[Michael Glavin]

I'd run the 1.75" arms, the linkage ratio is not a problem. The actual ratio is very similar to a .75" servo arm and 1" CA .

1/.75=1.3
2/1.75=1.14

1.14 x 300 = 342.857

60*/ 50* x 300 = 360 or Input/output x Torque = Force

A 300oz servo will introduce 343 oz-in's Force. The rated Torque of the servo will always remain a constant.

[SleepyC]

DUDE.. I'd ask Tony.... I'm sure he would have an answer or solution for ya!

[Flyinrazrback]

I did ask Tony, he said a bolt might be better. I think I am just going to leave the pull/pull in so I dont have to rip the rudder off.