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Old Dec 02, 2012, 03:01 PM
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United States, OH, Amesville
Joined Nov 2012
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More thoughts on MEGA SUE

Well, I was looking through the catalogs for cheap servos that don't weigh much and have lots of torque... and look, it's the holy grail right next to it !!!!

I was looking at the specs for the 9 gram servos I've been using (1.7 kg/cm) and then I looked at 14, 18, etc gram servos. It sorta surprised me that the 18 grams had LESS than 2x the torque of the 9 gram servos. Then it struck me ... why not use TWO 9 gram servos...

Of course objections flooded into my mind - two inexpensive servos wouldn't necessarily have the same response, so that one might want to turn 64 degrees and the other might want to turn 61 degrees and they'd fight each other, until somebody gave up. SO, we need a compliant linkage. I thought of a number of different ones, and then I thought "Ackerman" and so I did some back of the Excel calcs... (shown below, but eminently ignorable).

The idea behind this spread sheet is to put two servos side by side 10 cm apart, hook them with a Y connector, and then put a T-bar between them. The control surface would be hooked to the T-bar. So the question was... if the linkage had 0.5 mm slop, how much angular difference could one have between two servos at about 60 degrees deflection. The chart shows the difference between the the ideal distance (10 cm) and the real distance with the mismatched angles. The allowable difference is 0.05 cm (.5 mm). The graph shows that the answer is 3 to 3.5 degrees difference between the two servos (given the same input). I checked for other angles of deflection and came up with similar answers. They don't have to be precise if there is a bit of compliance in the linkage.

THEN I had an even better idea. How about a TOTALLY compliant linkage ??

Instead of making a MEGA SUE with a 12 inch wide elevator on each side... make TWO independent 6 inch wide elevators on each side. Each one has it's own 9 gram servo. If one uses a Y connector then one has a big 12" wide elevator (or elevons if one has a suitable computer transmitter). That will double the available torque for the total elevator, at the cost of double the weight, but that's better than what I was finding in the catalogs... and it's SIMPLE. BUT, if one has a computer radio with a 8 channel receiver (like me, thanks to FE in Dayton ) one can play other games. Make one set the main controls, and use the other like mixed flaperons or whatever... all sorts of options.

And the nice thing... they are cheap servos, and being analog they don't draw a lot of power when not being activated. And they won't fight each other.

And how about SUE with CROW ???

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Last edited by Dan_D; Dec 02, 2012 at 03:06 PM.
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