establishing proper linkage ratios and servo requirements

[Oilsands]

If you are able to post a photo of what your talking about, may make things easier for all. What servos do you have know?

[Rishelman]

I'll try to get a photo up after work today. I have HD DS120MHV in ail, and a 5625 in each ele half. I've heard other reports of the DS120s getting sloppy pretty quickly, and maybe they don't have the torque as advertised. So, that's kinda where I'm coming from. Not sure if the servos are junk, or if I wore them out quickly with crappy geometry. And the ELE are almost as sloppy and they are Hitecs.

Rishelman

[Rishelman]

Trying to upload photo

[Rishelman]

There we go. You can see the distance from hinge line center to horn is in the 1/2" area. I can see the hardpoint through the ultracote, and I could move the post back another 1/2" safely. This is the aileron, and the ELE is set up the same way.

So, whadya think?

Rishelman

[Oilsands]

So with the pivot point on the control horn being over the control surface hinge line, what that does is sets it up to have equal deflection both ways.
When setting up the horn length (which is measured from the hinge line to pivot point) do not make it less than the servo arm length that you use. Adjust the length of the control horn from the hinge line so that you get the deflection that you would like and the horn does not contact the surface while deflected. The with the sevo at max deflection, find out how short of a servo arm you can go and still give you the deflection you are looking for. Never make the servo arm longer than the control horn from hinge line distance. You want to have at least the same length servo arm as the distance between your pivot point on your control horn to hinge line. That will give you a 1:1 ration. If you end up with a shorter servo arm than control horn distance then you are even better than the 1:1 ratio.
Most manufactures will give you a spec on control horn distance, as well as a servo arm length. The are a good starting point. Hope this helps and you can understand what I am trying to say.

[Rishelman]

So, you are talking about the length of the threaded rod? The "height" of the control horn from the control surface? I was asking more about the distance of the control horn away from the hinge line. Which of course in this case would require different control horns (like the Dubros) to keep the pivot over the hinge line. The "height" of my control horn could be adjusted easily by just threading it up or down the rod. I may need to install longer rod in my surface though, as I cut them off about 3/8" above the top of the horn. Am I following your train of thought?

Rishelman

[dhawks]

With all this info about setting up precise geometry, why do I see most servos on ailerons and elevators mounted with the servo axis perpendicular to hinge line of the surface? In other words, the servo is mounted vertically instead of on it's side to allow the servo arm to move in the same plane as the horn on the surface.

Dennis

[66stang]

You want the pivot point on the hinge line like you have it. The height is what you want to adjust.

[Bill Clark]

	Quote: Originally Posted by dhawks
	With all this info about setting up precise geometry, why do I see most servos on ailerons and elevators mounted with the servo axis perpendicular to hinge line of the surface? In other words, the servo is mounted vertically instead of on it's side to allow the servo arm to move in the same plane as the horn on the surface. Dennis

my edge has that set up (arms travel in same plane). I would say for the sake of reduced manufacturing cost and ease of servo installation to be the main reasons for the "servo on top", still being able to create good geometry although not as straight forward of a process.

[Mithrandir]

though not as elegant as the Servo axis and the hinge axis being parallel... I prefer the above mentioned configuration as it is easier to get the full travel out of the servo without hitting the skin... for example.. the comp-arfs.... the arm hits the skin before full travel...

[Rishelman]

Well, I installed my new 7954SHs this weekend and raised the horn as high as I could off the control surface. It only turned out to be an inch (1") total before I ran out of thread. I went flying and those servos really rock. I don't really think my geometry is going to matter much with the availiable torque of those babies, as long as I have full deflection and no flutter or blowback I'm happy. However, I learned something new, and the next plane will be done right.

Thanks to all who replied.

Rishelman

[Oilsands]

	Quote: Originally Posted by Rishelman
	So, you are talking about the length of the threaded rod? The "height" of the control horn from the control surface? I was asking more about the distance of the control horn away from the hinge line. Which of course in this case would require different control horns (like the Dubros) to keep the pivot over the hinge line. The "height" of my control horn could be adjusted easily by just threading it up or down the rod. I may need to install longer rod in my surface though, as I cut them off about 3/8" above the top of the horn. Am I following your train of thought? Rishelman

You want your servo arm when at netrual to follow the same line as your hinge line. Then using the control rod linkage to contect from the control horn (which is at a distance from the hinge to the pivot on the control arm distance that you are gong to try) to the servo arms. You want to end up using the most travel supplied by the servo to move your control surface to what you want. If you end up using a small amount of your servo travel, then you are not getting the full benefit of the servo that you paid good money for. Not to mention you will end up with a smoother control.

[Mithrandir]

	Quote: Originally Posted by Oilsands
	You want your servo arm when at netrual to follow the same line as your hinge line. Then using the control rod linkage to contect from the control horn (which is at a distance from the hinge to the pivot on the control arm distance that you are gong to try) to the servo arms. You want to end up using the most travel supplied by the servo to move your control surface to what you want. If you end up using a small amount of your servo travel, then you are not getting the full benefit of the servo that you paid good money for. Not to mention you will end up with a smoother control.

I have sen others do this arrangement with the servo arm at an angle to the pushrod at Neutral... so that at about 50% throw, the pushrod was at about perpendicular to the hinge line...

I think this is to avoid extreme angles at full deflection... sorta... splittin' the error down the middle

[Oilsands]

Are you saying something like this? I have also seen this setup as well. One of my planes were set up the first way and another this way, it was more due to the hard point location. I like the 2nd one a little better, since I end up with a short servo arm it lines up better than the angle being the otherway.

[Rishelman]

That was actually going to be my next question. I didn't want to ask about to setup theories at once to avoid confusion.

I recently saw on FG somebody doing a Carden (I think), and they set up their linkage so the pushrod was at 90 degress to the hinge line at full deflection. Which, actually made some sense to me. Seemed like that would offer the most power at full deflection for that particular parameter (aside from all the other aspects of linkage setup). Instead of the pushrod being off to an angle while under the most stress it could see, it would just be a straight shot to the servo arm. I realize given the preinstalled hardpoints in an ARF this may not be an option; but, if it were, or if building from a kit, is this the way to go?

I feel like you can kindof get away with a little more in the 50cc and smaller planes, but I intend to move up to 100cc as soon as humanly possible, so I'm studying up.

Thanks again guys

Rishelman