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Fullflying Control Surfaces

Old 02-02-2010, 01:10 AM
  #1  
Gohmer
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Does anyone know of a formula for the pivot position of fullflying control surfaces?

Thanks
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Old 02-02-2010, 07:18 AM
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Larry3215
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You should be ok if the pivot is somewhere in the 25%-33% range back from the leading edge.

I normally do mine about 33% back.

Its more important than normal to mass balance full-flying surfaces.

In other words - the surface should balance on the pivot point. That normally means adding weight to the leading edge of the surface.

If its a really slow flyer the mass balancing is not so critical but its still a good idea.

Be extra carefull with your linkages to be sure they are strong enough and have zero slop. Dont cheep out on the servos either

Is this for an elevator or rudder or?
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Old 02-02-2010, 07:36 AM
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Elevator and rudder on a slow flyer. Thanks for your help.
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Old 02-03-2010, 12:57 AM
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Pitching moments for airfoils are measured at the 25% chord line.
With a flying tail, placing the pivot any further aft can lead to a divergent oscillation.
The surface can also deflect to full one way or the other, overpowering the servo.
Don't go any further back than 25%, to be safe.
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Old 02-03-2010, 01:44 AM
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Well, that depends on the airfoil and where max thickness is IIRC.

There was a NASA or NACA paper on that subject some years ago but now I cant find it.

No question that 25% is safer. It takes stronger servos though.

Ive done a number of planes with all flying stabs, rudders and/or wings. All have been pivoted in the 30%-33% range and mass balanced.
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Old 02-16-2010, 03:04 AM
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cyclops2
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Default Sounds Like a boat rudder .

Only caution is if you move WAY too far back, in a unexpected high speed dive you may lose control in the direction the servo is PUSHING the control rod. Add the slop of the push rod to the too far back pivot point & you are SPLAT.
Good reasons to always have the servo PULLING on the rod when you need UP.

Rich
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Old 02-16-2010, 07:27 PM
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Originally Posted by Cyclops View Post
Only caution is if you move WAY too far back, in a unexpected high speed dive you may lose control in the direction the servo is PUSHING the control rod. Add the slop of the push rod to the too far back pivot point & you are SPLAT.
Good reasons to always have the servo PULLING on the rod when you need UP.

Rich
Where do you place the pivot on the ones you fly, please?
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Old 02-16-2010, 07:46 PM
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Default May sound like a stupid way.

I Have a GWS F 15 with full flying elevons. The elevators control all flying. Ran my own test of the pivot point by putting a 1/8 rod into the 1/2 elevator piece and moving into the breeze from a running floor fan. 3 holes and I found a stabile spot. That does elimanate any error. Pushing it to a 45 degree angle gives you the exact amount of servo force needed.
I think I have a picture of the tail setup. Will post it, if I can find it.

Rich
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Old 02-16-2010, 10:06 PM
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This beauty (and it was) showed up at the 2003 Society of Automotive Engineers Collegiate Lifter event at the Antelope Valley Tailwinds Field in June of 2003.
Everything about the plane superb; the design showed a lot of influence from Dr. Drela's work, the motor had a nifty gear box, and it featured an all-flying horizontal.
On it's one flight, the thing was a galloper that was almost uncontrollable, due to the position of the pivot for the horizontal which went into a non-stop limit cycle.
The plane managed a couple of 180 degree turns until it became unflyable, and crashed.
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Old 02-16-2010, 10:15 PM
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Originally Posted by Sparky Paul View Post
This beauty (and it was) showed up at the 2003 Society of Automotive Engineers Collegiate Lifter event at the Antelope Valley Tailwinds Field in June of 2003.
Everything about the plane superb; the design showed a lot of influence from Dr. Drela's work, the motor had a nifty gear box, and it featured an all-flying horizontal.
On it's one flight, the thing was a galloper that was almost uncontrollable, due to the position of the pivot for the horizontal which went into a non-stop limit cycle.
The plane managed a couple of 180 degree turns until it became unflyable, and crashed.
nifty
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Old 02-16-2010, 10:33 PM
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A symmetrical airfoil has no pitching moment around the 25 percent chord line.
Any deflection at that location requires little effort, the airfoil will tend to remain at whatever position it is moved to.
To prevent limit cycling, the practice is to place the hinge point ahead of the 25% chord line.
This requires some effort from the mover.. i.e. a servo in this case. The built-in centering of the servo will prevent any limit cycling, as the aerodynamic force on the surface will also tend to prevent any further movement of the surface.
With the hinge axis aft of the 25% chord line, any force on the surface will try to move the surface -further-.
In the referenced instance, this destabilizing force fought the centering force of the servo, with the surface moving rapidly from one extreme to the other, due to flexure in the control horn and the servo arm.
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Old 02-16-2010, 10:44 PM
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Originally Posted by Sparky Paul View Post
A symmetrical airfoil has no pitching moment around the 25 percent chord line.
Any deflection at that location requires little effort, the airfoil will tend to remain at whatever position it is moved to.
To prevent limit cycling, the practice is to place the hinge point ahead of the 25% chord line.
This requires some effort from the mover.. i.e. a servo in this case. The built-in centering of the servo will prevent any limit cycling, as the aerodynamic force on the surface will also tend to prevent any further movement of the surface.
With the hinge axis aft of the 25% chord line, any force on the surface will try to move the surface -further-.
In the referenced instance, this destabilizing force fought the centering force of the servo, with the surface moving rapidly from one extreme to the other, due to flexure in the control horn and the servo arm.
thanks
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Old 05-17-2010, 06:49 PM
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Figuring the 25% mean aerodynamic chord for flying tails...
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Old 05-17-2010, 07:59 PM
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Nicely done!!
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