Ok simple question.
Regardless of servo torque, linkage friction, surface loads ........
Can the speed of movement of control surfaces be changed by moving control linkages in or out on the horn and servo arm? This problem has confused me for awhile. On the one hand, the outer edge of a servo horn is moving faster, but the entire arm ends up at its endpoint at the same time....so it wouldn't change? But the outer end is moving faster, right? So it does change?
What setup will produce greatest speed? My guess: out on the servo horn and in on the linkage -worst mechanical advantage though[]
Help!

Yes, you can make the edge of the control surface move further than the servo arm but at the expense of torque. In other words, if the distance from the center of rotation of the servo arm to the connection to the push rod is twice the distance from the pivot point of the control surface to the hole on the control horn or whatever you use on the control surface, the control surface will move twice the ANGLE of the servo horn. If the servo arm moved 20 degrees, the control surface will move 40 degrees, but the torque will be 1/2 of the torque of the servo. You can trade off torgue for angular motion or vice versa.

Hello,

Bill is right, speed and torque, choose what you want! The fastest setup would be the one in which on the servo arm you choose the outermost hole and on the control horn you put the clevis on the innermost.What matters is the ratio between servo arm and control horn: you could have long control horn and long servo arm and this situation is equal to a short control horn and a short servo arm.

Since Bill is speaking about servo and surface deflection, perhaps this formula that I've found (posted also on IMAC thread) could be interesting: it relates servo arm length and deflection angle with control horn ones. I find it useful when choosing control horn lengths on my new planes, considering the amount of surface deflection I want.

Here they are. They are pretty simple, which is so since work with the assumption that control horn clevis hole is on hinge line.

If we want SURFACE DEFLECTION ANGLE:

a_2 = arcsin { l_1 * sin (a_1) / l_2 }

where:

a_1 := servo deflection angle
l_1 := servo arm length

a_2 := surface deflection angle
l_2 := surface control horn length (measured from hinge line!)

If we want to know the CONTROL HORN LENGTH needed to have a given surface deflection angle:

l_2 = l_1 * sin (a_1) / sin(l_2)


Hope you find them useful!

Ooops... errata corrige! Last formula is:

l_2 = l_1 * sin (a_1) / sin(a_2)

Example:

On my new pattern plane I have servos on the stab. I want to use futaba little round servo arms, which give a servo arm length of about 8 mm. I want to have 25 deg of maximum surface deflection.

The required control horn length, from hinge line, considering max servo angle = 60 deg:

l_2 = 8 mm * sin ( 60 deg ) / sin ( 25 deg ) = 16,4 mm

Simple answer ... yes. The rotation (angular velocity) is the same for inner and outer holes. However, during the same time period, the outer hole on the arm moves through a larger arc than the inner hole. Think of being in the center of a merry-go-round as opposed to being on the outer edge.

John VB

Wow!
You guys sure can make things complicated and wonderful.
I, on the other hand, am simple-minded.
Anything you do with the linkages, that gives you the same throw in degrees at the control surface, but has fewer percent of ATV or AFR will increase speed at the expense of torque.
best regards,
Dean Pappas

Hmmm............. Dean, which is more important, speed, torque, or accuracy?
Gordie

Hi Gordie.
In general, torque and accuracy are both improved together, and most important.
Speed is critical (in my opinion) on the elevator. A fast elevator enhances the pitch break in the snaps.
Of course, if the elevator doesn't have enough torque behind the quick movement, then you lose the speed benefit, anyway.
The only option, in that case, is a servo that is both fast and powerful.
Maybe you could use two fast servos, one on each elevator half.
Later,
Dean

I guess one has to determine how much torque is enough -- hmmm. If there is more than enough then you can go for more speed until you hit the torque limit that is acceptable for our application.

Dean -- I don't think you could ever be accused of being simple minded! I have read your FM pattern articles with great interest since the mid-80's when finishing was the big discussion (I actually ordered some back issues to fill in the gaps for the lost, stolen or strayed issues). Right now I am hanging on every word that has to do with trimming. Some of the principles are beginning to make some sense (the second or third time around for me!) and I have seen some benefit in my own planes. Keep the good stuff coming! I'm a pattern-orphan in Minnesota...

5 cells make them go faster/stronger also.

Good flying (if you dress warmly enough!)
Tom

Hello Tom and Dean,

the informations you are speaking about regarding trimming are available? I'm too interested in trimming issues!

Dean, unluckyly I din't read your articles! It would be good to know if they are still available somewhere...

Thank you!