Perfect control horn position

[birdseed]

Hi all,
Im trying to find out where the ideal control horn position is.
I always thought the clevis should go on the horn, ideally over the centre of the hinge line.

But does this change if you are using a live hinge, where the top surface of the wing blends into the top of the aileron.

any help would be appreciated.

(Its a savex L39 jet im putting together)

[harveyweewax]

This is my understanding: Ideally you want the control horn hole to be directly in line with the hinge line however; in practice it is okay to have the bellcrank hole slightly behind the hinge line. If the control horn hole is in front of the hinge line then there is a possibility that the controls could bind up. This applies regardless whether the hinge is mounted on the centerline or the top line.

But, if you are using a pull-pull system and an offset hinge line then the control horn geometry must be centered around the hinge line and not the center of the control surface. This is not really a consideration with control surfaces driven by push rods.

If you are interested, here is a thread with a lot of info on pull-pull controls. Some of the info can also be applied to push rod setups too.

http://www.rcgroups.com/forums/showthread.php?t=615748

Hope this helps,

Sven

[vintage1]

This is my understanding: Ideally you want the control horn hole to be directly in line with the hinge line however; in practice it is okay to have the bellcrank hole slightly behind the hinge line. If the control horn hole is in front of the hinge line then there is a possibility that the controls could bind up. This applies regardless whether the hinge is mounted on the centerline or the top line.



I don't think its an issue with pushrods. You put the clevis hole where you will get the differential action you want. With rudders, that is bang on the hinge line since you don't want differential action.

With ailerons and elevators,its a choice - a wide choice.


But, if you are using a pull-pull system and an offset hinge line then the control horn geometry must be centered around the hinge line and not the center of the control surface. This is not really a consideration with control surfaces driven by push rods.

If you are interested, here is a thread with a lot of info on pull-pull controls. Some of the info can also be applied to push rod setups too.

http://www.rcgroups.com/forums/showthread.php?t=615748

Hope this helps,

Sven

Pull pull is a bit of a nightmare to set up with no slop.

[Ollie]

Think differently.

From Dr. Mark Drela:
"All four wing servos have RDS drive on them . I used large-diameter hypo tubing for the shafts rather than the commercial solid wire shafts. The linkage is amazingly tight -- tighter than is possible with horns and pushrods I think. It also eliminates the need for electrical connectors across the outer wing joint. The drawback is that it's considerably more work than a horn setup, but not too bad I think."

From:
http://www.charlesriverrc.org/articles/supra/supra.htm

Also:
http://www.irfmachineworks.com/rds/
http://www.genie.rchomepage.com/
File 06, Rotary Driver System

[Rodney]

On pull-pull it is sometimes helpful to not have the hinge center and the point at which the lines attach directly aligned (this offset is sometimes refered to as Ackerman) and is set up so that, as the surfaces are moved off natural, the line not being pulled goes slight loose. This is usually accomplished by putting the line attachment points slightly aft of the hinge centerline. However, NEVER set up reverse Ackerman, where the line not being pulled gets tighter as you move the surface away from neutral as that will load down the servos and increase current consumption excessively.

[harveyweewax]

Think differently.

From Dr. Mark Drela:
"All four wing servos have RDS drive on them . I used large-diameter hypo tubing for the shafts rather than the commercial solid wire shafts. The linkage is amazingly tight -- tighter than is possible with horns and pushrods I think. It also eliminates the need for electrical connectors across the outer wing joint. The drawback is that it's considerably more work than a horn setup, but not too bad I think."

From:
http://www.charlesriverrc.org/articles/supra/supra.htm

Also:
http://www.irfmachineworks.com/rds/
http://www.genie.rchomepage.com/
File 06, Rotary Driver System

Good Idea! Thanks Ollie and Dr. Drela :)

[Bill Mixon]

Im trying to find out where the ideal control horn position is.
I always thought the clevis should go on the horn, ideally over the centre of the hinge line.
This isn't always the case. What you should look at is the relationship between the output arm of the servo, and the hinge line.
Provided that your servo is directly ahead of the control surface, then yes a 90 degree horn geometry would be correct.
If it's above or below it, then you need to use a horn location that places the connection point perpendicular to your control linkage relative the hinge line.
See example in sketch.
But does this change if you are using a live hinge, where the top surface of the wing blends into the top of the aileron.
One important issue here is to not go by the airfoiled top surface to base the 90 degree (or whatever is needed) horn geometry off of the surface, but rather the centerline of your wing.

Another issue with skin hinged setups is differential. This is due to the effective surface area difference at equal amounts of travel up and down. This is barely noticable unless the surfaces are very thick.
This may be worth taking into consideration on some setups

[Bill Mixon]

Here is the same with the horn and servo arms incorrectly placed.
Both of these setups would result in poor control.

[BMatthews]

Excellent illustrats Bill. And you make a good point about how it's not just the geometry at the one end.

But there is one case you show where it would be OK even though you say it isn't. The first one in the second panel showing the horn and output arm but with an angled pushrod. It's actually OK and would give a linear response since both the output arm and control horn are parallel to each other.

The key to equal throws is actually that the output arm and the hingeline to control horn holes have to be parallel to each other.

For example in that first illustration in the second panel. The angled pushrod is the part that makes it seem bad. But if you were to replace it with a Z shaped rod with two 90 angles and the legs of the rod coming off each end straight on you'll see what I mean.

Also note that both the first examples in each panel meet the parallel axis requirement but by using different conditions. It may be an odd way to look at it but if you think of it as both are set up with a differential geometry but such that they are opposite and complimentary then that may make more sense.

[Bill Mixon]

Excellent illustrats Bill. And you make a good point about how it's not just the geometry at the one end.
But, there is one case you show where it would be OK even though you say it isn't. The first one in the second panel showing the horn and output arm but with an angled pushrod. It's actually OK and would give a linear response since both the output arm and control horn are parallel to each other.

The key to equal throws is actually that the output arm and the hingeline to control horn holes have to be parallel to each other.
I probably should have went to greater lengths to explain what I was getting at. I may need to do some other sketches that will speak for themselves, as I have difficulty with putting this info into words.

The problem with the example you referenced is that the linkage system gets into a mechanical disadvantage when deflected in one direction.
Having the output arm and control horn parallel with each other is just one part of the setup. If I were to exagerate this example with the servo directly under the control surface, with the same horn and arm geometry, the result would be no control at all.

In the deflected sketch here, the surface does move the same amount as the servo. However, in the down deflection direction you can see that the geometry is about to go over-center. this results in a setup with lots "slop" and less precision in the down direction.
Another thing here is that I'm showing a horn and arm of the same length (1:1 ratio). If these were different then the problem could be much worse.

When I setup a control surface, I like to construct a rectangle, with one long edge going from the servo arm screw to the hinge line. Then I use the short sides of my rectangle to setup horn and arm geometry.

[BMatthews]

Ah, I see now. Yeah, when it runs onto the extremes of the arc then it gets messy again. But even so your last sketch shows that it's not that far out for deflection angle of the surface even with the close to binding issue occuring. As I suggested the two differential actions are complements of each other and the results are close to normal and equal. But when you reach a point where the pushrod is very close to being in line with the horn axis small differences have big effects.

FWIW we SHOULD try to set things up like you did in the first illustration of the first panel. That's the ideal. And I like your rectangle setup suggestion. It nicely describes what we need to do for proper and equal operation. It's even more important when the horn and arm are of different lengths where any differential throw won't compensate for each end as much as possible.