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Increase Mechanical Advantage

1/22/2003 by Len Alessi

Copyright:© 2003 Horizon Hobby, Inc.

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By Len Alessi
Team JR Member

Mechanical Advantage is a very important concept when setting-up linkages - especially with larger aircraft. It refers to the leverage that the servo can exert on the control surface. Since the control surfaces are rather large it is important for the servo to have enough mechanical advantage or leverage to control them, regardless of the servo's rated torque. A large amount of torque is of little value if there is not enough leverage to use it. Insufficient leverage can lead to control surface flutter (usually a catastrophic event) and blow-back, where the air flow pushes the control surface backwards resulting in mushy or no control at higher speeds.

There are two ways to increase the mechanical advantage of the servo. One is to make sure that the control horn device is long enough, whether it be a horn as shown in the illustration above or a bolt with a Rocket City-type fastener. The horn is the lever that the servo uses to control the surface. The longer the horn, the more leverage the servo has. It's like a lug wrench - when you can't get a lug nut loose you put a piece of pipe over the end of the lug wrench to extend the handle and that gives you more leverage to break the lug nut free. It's the same thing - the lug nut is the control surface and you are the servo trying to move it. As a general rule-of-thumb, try to attach the linkage at the control surface so that it is at least 1" away from the surface - longer is better.

The second way to increase the mechanical advantage for the servo is to attach the linkage at the servo arm as far inward (towards the servo arm retaining screw) as possible while still providing enough throw. It's the "lever thing" again, but in reverse, as we are trying to take leverage away from the control surface by providing it with a shorter lever to work against the servo.

Ensure that the attach-point is the same distance from the hinge line for like surfaces (two ailerons, two elevators and two rudder horns). If the attach points are not the same distance from the hinge line there will be unequal throw and it will be more difficult to synchronize the surfaces for equal deflection. This is especially critical for the rudder where two servos are attached to the same surface - unequal throw will

JR MatchBox (JRPA900)
cause the servos to fight one another causing excessive battery drain, and in severe cases may cause servo damage.

JR produces a device called a Match Box that is designed to make multiple servo set-up on a single control surface very easy.

Always try to use the maximum amount of travel (100%-150%) that the radio provides. If it is too much travel, don't reduce the percentage of travel in the radio. Instead, move the linkage further away from the hinge line at the control surface and/or move the linkage inward on the servo arm or use a shorter arm. If you use high percentages of travel you maintain resolution (fine movements of the stick result in fine positive movements of the control surface). When we decrease travel percentages we lose resolution.

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