Servo Size for 120 sized plane? - RC Groups
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Feb 19, 2007, 09:33 AM
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Servo Size for 120 sized plane?

I have an Ultra Stick 120 Lite with O.S. 120 2 stroke on it. What size servos should I use? The current JR 537 analog ( 5 of them) have nylon gears and do not there position under load. These servos are the ones shown in the illustrations in the assembly manual. I also think I get flutter in the elevator control surface which has lead to elevator failure .They are rated at 40 ozs. I think this may be to low.

I have never seen another 120 sized plane with such small and low powered analog servos.

Ideas , thoughts , suggestions - PLEASE.
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Feb 19, 2007, 11:09 AM
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JR ds821 digital's $27 each and they're rated pretty high in torque and are fast. I have 20 of them in planes up to 27% and they all work perfectly.
Feb 19, 2007, 11:55 AM
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BlazerB52's Avatar
I use this calc to decide on what size I need for a plane.

Put in control surface size and max. speed you think you will be going.
Feb 19, 2007, 12:01 PM
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Thank you, for the reply. I have some of those. The look pretty strong . Are you sure they have enough power? It seems all of the 120 sized planes that are either fun fly or aerobatic have more powerful ( over 100 ozs). I think the 821 is around 70 ozs, I am not sure.

I am really curious why Horizon Hobby would show the low end JR servos in there assembly manual , if they were not adequate?

I think for long term use servos with nylon gears are not a good bet.
Feb 19, 2007, 12:10 PM
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Thank you for the calculator. Its looks great. When it refers to the chord of the control surface I assuming that means both the fixed and movable part of the elevator. And the length is the measurement perpendicular to the fuse ( the long way down the elevator).

Yes ? to both ?
Feb 19, 2007, 01:11 PM
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BlazerB52's Avatar
The cord on the control surface would be its width .
And then length is longest dimension.
If you have a control surface that is tapered. Lets say 1 1/2" at one end and 1" at the other. I put in the largest measurment which would
be the 1 1/2" figure to be safe. You only are calculating the surface area dimensions that moves.
Feb 19, 2007, 02:25 PM
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Thank you very much . The calculator has given me an excellent idea of what I need. I know it might have some error in it but this looks like a very good stating point.
Feb 22, 2007, 07:17 PM
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There are a ton of us flying the 821's on planes uo to 13 pounds with excellent results.
Feb 22, 2007, 08:11 PM
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Thank you for the response. I have installed a bunch of 821 ( both JR and Spectrum - which look, curiously, very much alike) in my plane. My bad experience is with the 537 which is woefully too weak and improperly geared ( nylon) to do the job. I learned this the hard way .

Now I ask you , what servo is used in the illustrations in the assembly manual for the Ultra Stick 120 ? The analog , 40 oz torque , nylon geared 537.
Horizon distributes both Hanger 9 Aircraft and JR radios. Surprise , surprise.

The assembly manual show the 120 sized engine as the bottom end of the recommended engines. There are even illustrations of these larger engines in the final pages of the manual.

By the way, to there credit , Horizon replaced the Ultra Stick with no questions asked once I mailed the crash airframe back to them . Didn't say boo.

The new Ultra Stick flies beautifully . The new digital servos are big improvement over analog servos. Like the handling of radial tires over bias ply.

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