Linkages and Mechanical Advantages

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Linkages and Mechanical Advantages

New postby retiredVTT on Thu Oct 01, 2009 9:36 pm

..here is some excellent info regarding servo hookup..read the last paragraph TWICE...are you doing this?
I was NOT...but I will be in future..!
Bill


Linkages and Mechanical Advantage
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
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.
Bill
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Re: Linkages and Mechanical Advantages

New postby yorcram on Fri Oct 02, 2009 12:09 pm

Sounds like Bill has a post stutter :lol: :lol:

Marc


(oops, sorry Marc, accidently deleted part of your post, while I was trying to answer you. I got the stutter fixed!)
Bill
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Re: Linkages and Mechanical Advantages

New postby Enforcer on Fri Oct 02, 2009 12:22 pm

The article mentions this is most important with larger planes. What does "larger" refer to exactly? Also, is this only a concern if you are using "high torgue" servos?
Murray Wheten

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Re: Linkages and Mechanical Advantages

New postby retiredVTT on Fri Oct 02, 2009 7:53 pm

Just the opposite Murray...
..if your using high torque servos, the control surface will likely move and "hold" against the pressure of the air over it. But with regular or low torque servos there would be a tendency for the wind to "blow back" against the servo and deflect the control surface in the opposite direction to what you want.
This could show up in a knife edge attempt...you change the servo to a higher torque one and the plane knife edges much better.
Or you change the linkage setup as described and get additional mechanical advantage (more force) on the control surface from the same servo.

Some of us, as we move up to larger planes, tend to reuse the smaller servos we had in the smaller planes and that's when we can run into trouble with either stripped servo gears or a case of "blow back".

Bill
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Re: Linkages and Mechanical Advantages

New postby steve on Sun Oct 04, 2009 3:28 pm

On my "over powered" hyperflea, I have the issue of not enough servo torque at WOT. As a result I have augmented my throtlle end point to 90% so as to allow for servos (hs-55) to deflect the elevons at that maximum setting. If I leave my throttle to 100% and go WOT, the servos do not deflect the elevons and i crash...........everytime.

Funny I have never stripped a servo on that aircraft..................even on one of its many dirt naps....

Steve
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