>Simultaneously Increasing the length of the surface horn by a
>similar amount will not increase the load on the servo. Servo linkage
>should always be designed to use the maximum amount of servo travel to give
>the desired control surface throw.
We should distinguish between different types of loads:
servomotor/gear loads
bearing/casing/mount/hinge/pushrod loads
Servomotor/gear loads depend only on the ratio: surface_horn/servo_horn
Doubling or halving both horn lengths has no effect on these loads.
Bearing/casing/mount/hinge/pushrod loads depend only on the surface horn length,
in inverse proportion. Doubling the surface horn length will reduce these
loads by half.
The following linkage design procedure will minimize all the types of loads:
1) Determine the maximum servo travel range. Set the horn length ratio
so as to get the desired resulting maximum surface travel range.
Assuming the travel of the servo and surface are symmetric,
the equation for the horn length ratio is:
surface_horn/servo_horn = sin(surface_throw/2) / sin(servo_throw/2)
Example:
100 deg servo throw (+/-50)
70 deg rudder throw (+/-35)
rudder_horn/servo_horn = sin(100/2) / sin(70/2) = 1.33
2) While keeping this horn ratio fixed, make the horns as long as possible.
Let's say the longest horn I can fit on my rudder servo is 12mm.
The corresponding rudder horn should then be 12mm x 1.33 = 16mm
Nonsymmetric throws require a more elaborate analysis.
Blaine Rawdon's spreadsheet does the job well:
http://members.cox.net/evdesign/
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