Servo arm length??

[digger44]

Is there a common method or equation for determining what length servo arm to use for a particular surface? I know this has a lot to do with the throws required for the type flying that you will do but whats the next step? I need to buy some arms and not sure where to start as far as the correct length for the best mechanical advantage at the particular throws involved. I also know that the power of the servo is in question if the length of the arm is long enough to provide excess leverage on the servo shaft. Does this question make any sence? I am really looking for a more general application answer rather than spacific lengths. Something that could be used on all applications.

Thanks,
Bob

[TheTank]

You want to achieve a 1:1 mechanical ratio. This means the the distance from the hinge line (center of rotation or pivot) to the connection point on the horn (ie where the clevis or ball link attaches on the horn) is the same as the length of the servo arm. For a 1.25" servo arm, the distance from the hinge line to the connection on the horn should be 1.25"

[Rolling Thunder]

1 to 1 ratio is true if you want 60 degrees of throw. If you want 45 degree of throw and use a 1 " arm you will need to have a control horn 1.22" from the hinge line. If you want 30 degrees the control horn should be 1.73" from the hinge line.

[digger44]

What he said? Lol, jk. Thanks for the responce.

This is what I caught (or didn't) from both responces. The 1 to 1 ratio makes perfect sence but does that mean that if I use a 1" servo arm that I will also attach the horn at 1" away from the hinge line and get 60 deg throw? Also if I use 1 1/4" servo arm and attach the horn 1 1/4" away form the hinge line I will get 60 deg throw. So in this case the servo power would be the limiting factor in servo arm length? The longer the servo arm length, the more power is required. No matter what the arm length at 1 to 1 ratio you will get 60 degrees throw or whatever the servo is capable of ?

I can see in my minds eye the relationship between the length of the servo arm and the distance from the hinge line to the horn connection. The example of a 1" servo arm and the horn connection at 1.73" away from the hinge line resulting in less throw vs. a 1" horn connection, further away from the pivit point. So what are the other variables in the equation? I know control rod length(adjustable) and probably others. So what is the determining factor in servo arm length. Does every one just get the longest arms that the servo can adequately handle and set up the geometry and throws from there, hoping that nothing flutters?

[digger44]

I just found this spreadsheet on the web written by a guy in Italy named Keith Black. Looks like he did a great Job with this and I would personally like to thank him if he is around. Tried this out with some of the above numbers and it seems to be accurate. Hopefully this will be useful to someone else as well.

Thanks,
Bob

[JoeAirPort]

See my comments in blue.

	Quote: Originally Posted by digger44
	What he said? Lol, jk. Thanks for the responce. This is what I caught (or didn't) from both responces. The 1 to 1 ratio makes perfect sence but does that mean that if I use a 1" servo arm that I will also attach the horn at 1" away from the hinge line and get 60 deg throw? Yes Also if I use 1 1/4" servo arm and attach the horn 1 1/4" away form the hinge line I will get 60 deg throw. So in this case the servo power would be the limiting factor in servo arm length? No, if you keep the 1 to 1 ratio the servo torque required is the same regardless of servo arm length. Really the servo arm length is determined by whichever one clears the stab at full deflections. The control rod can get very close to the wing or stab at full deflections so the longer arm keeps the rod away from the wing/stab. The longer the arm the longer your control horn needs to be, that ratio has to be maintained. And really I never use a 1 to 1 because none of my planes have 60 degree surface deflections. The most I have is just over 50 degrees so my ratio ius more like 60/50 = 1.2 to 1. That requires a little less servo power at that ratio. The longer the servo arm length, the more power is required. Only if your servo arm is length is more than your control horn height. No matter what the arm length at 1 to 1 ratio you will get 60 degrees throw or whatever the servo is capable of ? Yes, because you kept the control horn the same height as servo arm length. I can see in my minds eye the relationship between the length of the servo arm and the distance from the hinge line to the horn connection. The example of a 1" servo arm and the horn connection at 1.73" away from the hinge line resulting in less throw vs. a 1" horn connection, further away from the pivit point. So what are the other variables in the equation? I know control rod length(adjustable) and probably others. So what is the determining factor in servo arm length. Does every one just get the longest arms that the servo can adequately handle and set up the geometry and throws from there, hoping that nothing flutters? Find the shortest servo arm that clears the wing/stab at full deflections. On bigger planes always 1.25 or 1.5 inch. Then set your control horn height to match the ratio you want. Using a 1.25 inch servo arm, a 60 degree servo throw, and 45 surface deflection gives you a 60/45 = 1.33 ratio. 1.33 times the 1.25 inch arm = 1.66 inch control horn (from hing line). That's your formula and those are the only variables. Assume 60 degrees for all servo throws, that's what my 9C gives me with any servo. Plug in your surface deflection into the ratio. Calculate the horn height and you're all set.

[digger44]

Thanks Joe, very good info. I think it's starting to sink in now. One other question if you'all don't mind, with an offset control horn, what is the best way to compensate for the built in differential, and how does this effect the geometry? Feels like I'm back in school,
Thanks again,
Bob

[JoeAirPort]

The best way to compensate for (ie get rid of) the built in differential on an offset control horn is to shorten the control rod to get the up and down deflections the same. Then you need to change the servo's center and endpoints since you shortened control rod. This requires either a matchbox/synchronizer or a Hitech programmable servo (my choice). If it's just one servo per surface you can use sub trim and endpoint adjustment (if your radio permits it, my 9C does). That worked on my last 30%'er with Nelson RCL 70/71 control horns. The only draw back to this method is that you require more torque from the servo on the up or down swing to hold the surface from blow back.

	Quote: Originally Posted by digger44
	Thanks Joe, very good info. I think it's starting to sink in now. One other question if you'all don't mind, with an offset control horn, what is the best way to compensate for the built in differential, and how does this effect the geometry? Feels like I'm back in school, Thanks again, Bob

[digger44]

Thanks again Joe,
I am using the 9C as well as one servo per surface. The 5945's I have are way overkill for the plane so I probably won't need to worry to much about differential, just wanted to try and understand these relationships better as I put this plane togeather.

[rcfury]

If i can add a quick little question also that involves the same idea. But what about ATV's or endpoints as 9c has it listed? Should you max out your end point and adjust the arms mechanically to achieve your recommended throws?
Im asking because im trying to figure out the setup for my showtime 4d. The instructions call for some 50 deg down elevator throws, and i can barely get 40 deg with 1 inch arms with the control horn at their stock height.