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Control Horn help

#1
Hey all, I recently got into a debate on control horn setup. I was under the impression that having the push rod in the top/highest slot in the control horn would give the most throw, and moving it down the control horn would decrease the throw.

However, when I sat down and looked at it, if the travel is the same at both the top and bottom, the bottom would have further throw.

Does the orientation of the push rod factor in? Whether it is above the arm or below? Also the same would probably go for servo arms. I think that is why I thought the top slot was higher throw than the bottom because on a servo the outermost hole is the biggest throw and the inner is the least. Wouldn't that theory translate due to the face that they are both just range of motion above a joint?

After the question was brought up and I had too much time to think about it, I am extremely confused.

Any insight would be great!
 

tamuct01

Active member
#2
The farther out from the control surface that you put the linkage, the more travel will be required for the same surface deflection. Due to the larger travel of the servo, you will also have the highest resolution and more control of the surface deflection. Above, below, or to the sides doesn't really matter (for a hinge placed in the center of the control surface).

Back in the day before digital radios and fancy electronics, you could adjust the proportion between the servo throw and surface deflection by moving the pushrod connections in or out of the various holes. Close to the servo and away from the surface got you the least deflection; away from the servo and close to the surface got you the most. Typically, I try to setup my planes using the outermost holes on both the servo and control surface for a more 1:1 movement with the highest resolution.
 

kdobson83

Well-known member
#3
I always use the outer most holes as well, you can always adjust your throws in the remote, if u have a programmable remote that is. And yes, Tamuct01 is right, farther out on servo = more throw, farther down on the control horns(closer to control surface) = more throw as well.
 

Hai-Lee

Old and Bold RC PILOT
#4
Wow! This topic is so appropriate and rarely visited.

The things to consider when setting up control horns, pushrods and servo output arms are at best a trade off of performance Vs damage risk.

With a control horn if you use the innermost hole, (hole closest to the control surface or hinge line), you will definitely get the most possible movement of that surface, BUT where there is any slop or movement in the pushrod fitting into the control horn hole then at the innermost position you will find that the control surface has greater free play and being precise in position will be difficult. It may even fly erratically.

The other thing to consider is the forces applied by the control surface through the control horn and pushrod to the output gearing of the servo itself. It is just a simple matter of leverage. When the pushrod is fitted in the innermost hole the force applied to the servo gears during a crash can be greatly amplified and even destroy the servo gears themselves.

Whilst we setup control surfaces for a certain amount of deflection the available power, (torque), available must be considered. On the servo output arm the outermost hole has the least torque and applies the greatest amount of forces to the gearing in the event of a crash.

Regardless of crashes and play in the pushrod mounting holes the positioning of the pushrods makes little difference because the distance traveled by the control surface will be set up to be the same. The power used by the servo would also be the same.

Servo transit times should also be the same regardless of pushrod setup as it is an angular measure so distance from the output gear axis is irrelevant.

Personally I start with the pushrod in the outermost hole in the control horn and select the hole in the servo output arm to give the appropriate deflection. If I still require more throw then I bring the control horn end of the pushrod a single hole closer to the hinge line and so on.

If I find that I am at the outermost hole in the servo output arm and a little too close to the control surface hinge line I revise the servos I am using or seriously consider the use of metal geared servos for the installation.

As a rule, (where possible), I try to always use the outermost hole on the control horns as it has more Pros than Cons and could help my servos survive that inevitable crash!

Just what works for me!

Have fun!