Konrad
Very Strong User
I've been seeing a lot of concern with the proper set up of the classic control horn and clevis. Most of these are over factory installed control horns. This just goes to reinforce my concern with factory installed control horns (see this thread). To be fair to the OEM and designers unless they are supplying all the hardware it really is difficult for them to control the installation of the control system. All the more reason not to install the control horn.
Now some might think that the factory might have some insight into installing the control horn. I'd have thought that the factory might yield a better installation with the use of fixtures and such. But generally this isn't proving to pan out. In my the last purchase of a glider with factory installed control horns I was shocked to see that the horn were placed such that they impinged on the hinge line. I don't mean just some glue, but rather the horn itself.
You can see this best when you flex the control surface to close the hinge gap. On the hinged side you will see the horn or glue distorting the surface. This causes a huge amount of stress for the servo. It also is the stress riser that often starts the failure with the live hinge (Kevlar).
Attached are some photos of this. In one you can see that the horn has crossed the hinge line. I have to admit I didn't see this at first as there wasn't much glue usually associated with this king of hinge binding. It wasn't until I started to raise the aileron that I saw the control horn causing a bulge in the lower wing skin. Looking at the top side of the wing and control surface it became clear what was going on.
Now I don't have access to the assembly drawings of process sheets. So some of this is conjecture. But looking at the slot cut into the aileron, I can see that the horn wasn't pushed aft as the slot would indicate it should have been. Also the surface has a dramatic bulge near the control horn indication that it was jammed as far back as the installer could get the horn. This got me thinking why does this designer's models have such a propensity for this problem. Then the solution dawned on my. The designer uses a set of airfoils often referred to as cusped. His are often double cusped (both top and bottom). This can result in the aileron cavity being rather small for the tail of the control horn. I suspect the OEM is using vendor supplied control horns. As such they aren't cut to have the aft facet of the horn shaped with a concaved profile. This results in the horn jutting past the hinge. I believe that the root cause for this horn installation failure can be traced to the improper profile of the control horn for cusped airfoils.
This jutting control horn will also cause issues with the basic geometry of the control system installation. It is generally considered best practices to place the control horn clevis pivot over the hinge line. This is done to minimize unwanted surface differential as the servo tries to actuate the surface. If this control horn was for bottom actuated ailerons the mechanical differential would be revered for what one would normally want in the way of aileron differential.
I've found that for flaps to be set up per the American preference (90° downward motion) that one actually need to bias the flap control horn clevis pivot rather dramatically forward for top driven flaps. This is done to allow the pivot to be above the hinge line avoiding the 3 point line control bind. Far too often the Factory installed flap control horn doesn't have this much needed forward offset.
Another pet peeve of mine is that if the horn is installed correctly, geometrically, there often isn't thought given to the installation of the clevis. If the factory is going to install the control horn it should provide the needed clearance to allow the clevis to reach the control horn. And then there is the issue with the factory provided clevis. Far too often these are of the staked pin style. To get these to go into the horn often requires that the pin cut its own added clearance as the pin can't inter the horn square. This results in a lot of control slop. For this reason I prefer the clevis with the discrete pin.
I don't know why I have to say this but here is the disclaimer: These observations are mine and mine alone and may not reflect the management of this forum or any other individule posting on this forum.
Now some might think that the factory might have some insight into installing the control horn. I'd have thought that the factory might yield a better installation with the use of fixtures and such. But generally this isn't proving to pan out. In my the last purchase of a glider with factory installed control horns I was shocked to see that the horn were placed such that they impinged on the hinge line. I don't mean just some glue, but rather the horn itself.
You can see this best when you flex the control surface to close the hinge gap. On the hinged side you will see the horn or glue distorting the surface. This causes a huge amount of stress for the servo. It also is the stress riser that often starts the failure with the live hinge (Kevlar).
Attached are some photos of this. In one you can see that the horn has crossed the hinge line. I have to admit I didn't see this at first as there wasn't much glue usually associated with this king of hinge binding. It wasn't until I started to raise the aileron that I saw the control horn causing a bulge in the lower wing skin. Looking at the top side of the wing and control surface it became clear what was going on.
Now I don't have access to the assembly drawings of process sheets. So some of this is conjecture. But looking at the slot cut into the aileron, I can see that the horn wasn't pushed aft as the slot would indicate it should have been. Also the surface has a dramatic bulge near the control horn indication that it was jammed as far back as the installer could get the horn. This got me thinking why does this designer's models have such a propensity for this problem. Then the solution dawned on my. The designer uses a set of airfoils often referred to as cusped. His are often double cusped (both top and bottom). This can result in the aileron cavity being rather small for the tail of the control horn. I suspect the OEM is using vendor supplied control horns. As such they aren't cut to have the aft facet of the horn shaped with a concaved profile. This results in the horn jutting past the hinge. I believe that the root cause for this horn installation failure can be traced to the improper profile of the control horn for cusped airfoils.
This jutting control horn will also cause issues with the basic geometry of the control system installation. It is generally considered best practices to place the control horn clevis pivot over the hinge line. This is done to minimize unwanted surface differential as the servo tries to actuate the surface. If this control horn was for bottom actuated ailerons the mechanical differential would be revered for what one would normally want in the way of aileron differential.
I've found that for flaps to be set up per the American preference (90° downward motion) that one actually need to bias the flap control horn clevis pivot rather dramatically forward for top driven flaps. This is done to allow the pivot to be above the hinge line avoiding the 3 point line control bind. Far too often the Factory installed flap control horn doesn't have this much needed forward offset.
Another pet peeve of mine is that if the horn is installed correctly, geometrically, there often isn't thought given to the installation of the clevis. If the factory is going to install the control horn it should provide the needed clearance to allow the clevis to reach the control horn. And then there is the issue with the factory provided clevis. Far too often these are of the staked pin style. To get these to go into the horn often requires that the pin cut its own added clearance as the pin can't inter the horn square. This results in a lot of control slop. For this reason I prefer the clevis with the discrete pin.
I don't know why I have to say this but here is the disclaimer: These observations are mine and mine alone and may not reflect the management of this forum or any other individule posting on this forum.
Last edited:
