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Old Aug 31, 2012, 06:39 AM
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ottootts's Avatar
Germany
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Servo throw vs control horn throw

I need some advice about the best setup for clean and precise control throws.

I've read several times now, that you should choose the longest arm throw at the control surface and the shortest throw at the servo, to achieve the required control surface deflection.

As the moments are equal, due to the rigid push-rod, what is the reason to have the above recommended setup?

eg 2 cm (throw) x 10 Ncm (torque) == 1 cm (throw) x 20 Ncm (torque)

regardless of which end is the servo or horn.

And as the precision is defined only by the angular displacement of the servo, the length of the throw is immaterial.

Or am I missing something more subtle or just plain obvious?
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Old Aug 31, 2012, 03:07 PM
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I'm interested in how an engineer would answer the question but to me it's a common-sense type of situation; for max precision without the need for large throws, think of a a bicycle in low gear. Move the pedals a lot = move the wheels very little. This is achieved with a small gear on the pedals (servo arm) and a large gear on the rear wheel (control horn).

Bill

Quote:
Originally Posted by ottootts View Post
I need some advice about the best setup for clean and precise control throws.

I've read several times now, that you should choose the longest arm throw at the control surface and the shortest throw at the servo, to achieve the required control surface deflection.

As the moments are equal, due to the rigid push-rod, what is the reason to have the above recommended setup?

eg 2 cm (throw) x 10 Ncm (torque) == 1 cm (throw) x 20 Ncm (torque)

regardless of which end is the servo or horn.

And as the precision is defined only by the angular displacement of the servo, the length of the throw is immaterial.

Or am I missing something more subtle or just plain obvious?
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Old Sep 01, 2012, 02:06 AM
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ottootts's Avatar
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Quote:
Originally Posted by HouseOf4Doors View Post
think of a a bicycle in low gear. Move the pedals a lot = move the wheels very little. This is achieved with a small gear on the pedals (servo arm) and a large gear on the rear wheel (control horn).

Bill
That makes sense to me - use as much servo movement as possible for a given angular displacement of the control horn.

Thanks, Bill
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Old Sep 01, 2012, 03:04 AM
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Toowoomba, QLD, AUSTRALIA
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But to throw a spanner into the works, Naruke, a well known Japanese F3A pilot and designer, has a spec sheet for at leat one of his designs that give mechanical linkage arrangements that require the ATV/AFR on "normal" rates on the ailerons set to 45%

This arrangement sacrifices torque and resolution for gains in speed, surface deflection linearity and matching surface deflections at the small angles around neutral.

So like all things in F3A, compromises are made and not always the linkage arrangement that provides the most mechanical advantage the best arrangement to use.
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Old Sep 01, 2012, 07:09 PM
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You can think of the servo as a device which produces a given torque and Torque = Force x Lever Arm. In this case the Lever Arm = Radius of Servo horn. No matter what radius you use the same torque will be produced. So as you move out on the servo horn the force decreases. If you want maximum force applied to the pushrod you would use a short servo horn.
Now looking at the control surface, you would want maximum torque. Since Torque = Force x Lever Arm you would want a large Force and a large Lever Arm. We’ve already maximized the Force by picking a short servo horn. By using a long control servo horn we maximize the Lever Arm as well.
Of course in practice we are limited since the servo horn has to be long enough to give the required throws.
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Old Sep 02, 2012, 02:03 AM
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That makes sense to me, thanks. I asked at the field yesterday, and I was told that a long throw at the control horn, minimizes the effects of any slack in the control horn's pushrod linkage. In any event, this arrangement seems to have universal approval among the experts. Thank you all ;-)
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Old Sep 02, 2012, 03:05 AM
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Originally Posted by wingster View Post
Now looking at the control surface, you would want maximum torque.
No not always, just as F3A ships are not propped for maximum static thrust.

In a dual servo elevator arrangement, using full servo travel to maximise your control surface torque WILL amplify any small linkage geometry differences for small angles around neutral.

Small horn on servo, large horn on the surface is a good rule of thumb, but however you chose to do it, just double check that your control throws are identical at 10% travel as well as 100% travel....
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Old Sep 02, 2012, 09:17 PM
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As with everything in F3A it seems, everyone else is wrong except yourself
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Old Sep 03, 2012, 09:24 AM
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Nah, I just get a bit tired of hearing the same old "MUST use full servo resolution" and "control linkages MUST be at 90deg angles" wives tales.

I thought you started working on some 1/2A thingy back when I was building the Albatross, how's that working out?

Oh and just for kicks I bought a Scorpion 4035-400 for the Curare, I've spun up an 11x10 on 10S with it and it's a bit of an overkill but you can always throttle back. Although I'm looking hard at pulling the Hanno from the SlingShot and and running this motor with a 12x12 on 8S?
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Old Sep 03, 2012, 09:17 PM
KE your cub.
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Ugh dude, you can't talk to me about that, I feel queasy. Replacing a hanno with an electric is like putting a 4 cylinder turbo into a mustang. It may work better but it's all wrong!

1/2a thing? Which one? I seem to have filled the house with teeny buzz bombs. They're hilarious, and the smaller they are the funnier they get. It's amuses me greatly do to 3 loops about 3m in diameter about 2m away from myself with an .010 screaming it's little box off

Back to controls, I used to be super anal about geometry, but a few dodgy bARF setups have meant I've had to stop being so anal.
Nowadays my mantra is "do WHATEVER is needed to achieve your outcome"
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Old Sep 04, 2012, 12:57 AM
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The reason Naruke has 45 percent on ailerons in Normal mode is that they need 100 percent for snaps.
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Old Sep 04, 2012, 09:26 PM
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Quote:
Originally Posted by jnred123 View Post
The reason Naruke has 45 percent on ailerons in Normal mode is that they need 100 percent for snaps.
Yeah I know, that's why I had judges "asking" me about my "snap rolls" when I first started flying my new ship. However for the vast majority of the flight the aileron linkage arrangement is only taking advantage of less than half of the available servo resolution and force, which has been deemed by someone greater than myself as just fine for precision F3A work.

Full resolution and linkages at 90deg are great "rules of thumb" in an ideal world but as Curare pointed out "do WHATEVER is needed to achieve your outcome" is the best advise and double check the control surface is doing what you want it to do throughout the full range of travel.
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Last edited by bjr_93tz; Sep 04, 2012 at 09:31 PM.
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