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Control Surface Springs


BigT
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Mike is correct, nearest gauge = 22 24, 22 is US equivalent!

I bought some 1.5mm o/d aluminium tube to make short 'sleeves' for the wire to stop the thin wire breaking through the foam.

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To cut thin aluminium tube, roll it under a knife blade then deburr the ends with a pointed end blade.

General note for torsion springs of a given material.  The stiffness is proportional to the diameter to the power of 4 divided by the length.   IE If you make the spring shorter it will be stiffer.  If you increase the wire diametre it will be stiffer.

There's spring info here:-

https://www.barcs.co.uk/forums/topic/8785-help-needed-for-dlg-build-thread/?tab=comments#comment-164414

The thread shows a spring that has the ends at 90 degrees to each other looking along the twisting bit of the spring.    IE this came with the Twister II - 0.5mm wire.  Relaxed position shown.  This spring felt too strong to me.

Spring.jpg.1a5f2cb1bc9002292829accd2a684df7.jpg

I prefer a folded spring so it doesn't apply a twisting load to the components - the ends of the spring can be put at the horn.  Here's my set-up, it's not as stiff as most but it seems to work for me.  Pictures are the relaxed position and the installed position :-

 

Picture2.jpg.550a18fd1b789d106c29319cc009b12f.jpgPicture1.jpg.5cb3fb0d131767ebd01045eed534f0c5.jpg

 

The alloy tube near the bend at the outer end of the spring stops it popping out of the hinge line.  This set-up gives about 50 grams tension on a 12mm horn with the surface neutral. YMMV.

 

 

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I tried that folded hinge multiple times but was never happy with the way it sat in the hinge line, the tube retainer was probably what I was missing and my wire too thick.   I like how the spring ends can be at the horn for added skin strength.

 

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I just remembered the reason I prefer the 180 degree spring to the 90 degree spring*.  There is less variation in torque for a given range of control movement.  For +/- 20 deg movement :-

90 degree spring goes 70-110   +/- 22%

180 degree spring goes 160-200 +/- 11%

*The angle is the amount of twist to get to neutral.


The folded vs. straight choice is a separate consideration.

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My  preference for spring tube is to use small bore heatshrink that I put on to the spring legs and then shrink down. Slide it off the legs and it's an exact size tube. I glue in by piercing the skin of flying surface with a pin and let a drop of runny cyno dribble down the pin.

You can just see the ends of the red heat shrink tube in the photo and a small piece of diamond tape to make sure spring doesn't pop out.

Springs are made from guitar string.

You can just see the pin hole where cyno was dribbled in on the tube just below the control horn.

20190906_082533.thumb.jpg.176adc195465609fd700647a9ac95baa.jpg

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  • 3 months later...

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