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Old May 11, 2011, 08:55 AM
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Slovenia, Ljubljana, Ljubljana
Joined May 2011
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Discussion
Rudder pivoted centrally

Ok, I'm an absolute beginner, so ...

I'm doing Multiplex Fox mod with AR6400
variant from E-Flite mSR heli. Since its
linear servos (looks like AS2000) are very
weak I was thinking how to make a rudder
control design for them.

My idea is to pivot rudder not at its edge
but more to its center. This way, servo would
have less work to fight with rudder since little
less than half of the front part of rudder will be
pushed to opposite direction by air stream.

Once again, this is for cca 40g Fox and for
light wind conditions.

Is this idea very bad ?
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Old May 11, 2011, 09:50 AM
Slopeaholic
Hutch's Avatar
Roseville, California, United States
Joined Mar 2001
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Not sure if this would be a good idea. If I understand what you are trying to do correctly, you will not be making your servo's life any easier. Any movement would cause the rudder to want to increase it's deflection as soon as the leading edge was caught by the slipstream, and would probably make your small servo work harder. The second reason why this would be a not so good idea is that you will be creating a lot of drag, as what you propose is actually a good way to create aerodynamic braking. The Salto for example uses these style airbrakes. That Tiny fox shouldn't require a lot of servo power, so maybe give it a go using a standard rudder hinge?

-hutch
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Old May 11, 2011, 10:03 AM
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Bradford West Yorkshire, UK
Joined Apr 2010
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Quote:
Is this idea very bad ?
YEP.

The nearer the pivot is to the centre of any control surface means you have to connect the servo to the pivot point.

Worst case, the control surface flips and the control is reversed.

How do I know? Vee tailed Thermal soarer with near centre pivoted tail feathers. NO Ailerons.

After the flip, up becomes down and left becomes right. Managed to get down safely the FIRST time I did it, Terra firma became Firmly Terrible second time.



Regards Ian.
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Old May 11, 2011, 01:19 PM
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Florida
Joined Aug 2004
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This can work quite well, just keep the area ahead of the hinge line to 10% of the total movable area. If you put the hinge point to far back, the surface will hunt and be hard on your servo. This is called dynamic balance and is often used in full scale aircraft. It will also help to have a good static balance; i.e. the weight of the area ahead of the hinge point equal to the weight aft of the hinge point. Another way to take the load off a servo is to use boost tabs but they are more difficult to install and also need to be limited to not more than 10% of the movable area.
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Old May 11, 2011, 01:24 PM
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South Wales U.K.
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Balanced control surfaces have been around for years, it's determining how much balance that can be important.

I have flown a few models with 'all flying tails', (horizontal stab/tailplane), they all worked quite well. But certainly not 50/50, more like 40/60 or less.
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Old May 11, 2011, 09:37 PM
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in the gutter, again....
Joined Jun 2005
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IN all truthfulness, on a relatively slow flying aircraft like the fox, you may turn a nice flying plane into an out-of-control bomb.

why? angle of attack.

It's been known for some time in the sailplane world that all flying tails (used regularly on horizontal stabs) are VERY effective at small angles of deflection, which basically changes the angle of attack of the wing. they become very unpredictable at high angles of attack, and will stall.

If you apply this to a rudder. at full throw you may very well stall the rudder and end up with no control or directional stability at all.

Better to have a fin with a hinged rudder, as this changes the angle of attack less, but provides more camber, resulting in a better lifting surface.

If your servo is having trouble pushing and pulling the rudder of a fox that size, I don't think it's the servo's fault. check you hinging and your linkages to make sure they're slop free and not sticky or tight before you point the finger at the servo.
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Old May 12, 2011, 12:29 AM
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United States, MA, Waltham
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Hypothetically, if you had the pivot point at 25 percent MAC, you'd need very little force. But 20 percent is probably safer. If you had a rectangular fin, you'd pivot it at 1/5 of the way back from the leading edge.
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Old May 12, 2011, 07:12 AM
An itch?. Scratch build.
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South Wales U.K.
Joined Mar 2003
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Here's a discussion thread from the DLG (Discus Launch Glider) forum -

All flying rudder

Many of the early aircraft, also WWI fighters used all flying horizontal and vertical rear stabilizer surfaces. The Fokker Dr.1 is a good example of the all flying fin/rudder.
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Old May 12, 2011, 03:16 PM
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The 'Wack, BC, Canada
Joined Oct 2002
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The idea of pivoting the surface part way back at between the 20 to 25% MAC point only works if there isn't a fixed forward fin. If there is the airflow through the gap that is created could cause some oddball control responses and make it harder on the servo than just putting the hinge at the joint line like normal.

The one exception to this is where there's a big aerodynamic balancing horn such as seen on the full size Piper Cub's rudder. But as already mentioned if you copy that setup then you do not want more than 20 to 25% of the total area ahead of the hinge line. Otherwise the rudder will try to move to full throw all the time. 20 to 25% of the total area in the forward portion should balance the loading so the servo needs only very little power.
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Old May 12, 2011, 04:08 PM
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Slovenia, Ljubljana, Ljubljana
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Thanks for interesting insights from all. This is my
first encounter with building anything like this myself
and with this forum. And I really like the shivering feeling
despite the fact that I didn't even start yet changing
anything physically ...

Quote:
Originally Posted by BMatthews View Post
The one exception to this is where there's a big aerodynamic balancing horn such as seen on the full size Piper Cub's rudder. But as already mentioned if you copy that setup then you do not want more than 20 to 25% of the total area ahead of the hinge line. Otherwise the rudder will try to move to full throw all the time. 20 to 25% of the total area in the forward portion should balance the loading so the servo needs only very little power.
This solution, with balancing horn, from Piper Cub looks
very promising. I'll report back with results as soon as have
it done . Thanks
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Old May 13, 2011, 09:43 AM
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Florida
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I did quite a bit of experimenting with having various amounts of the area ahead of the hinge line. I found that anything over 10% usually caused problems with over control or what I called hunting (the surface would not stay at neutral but would try to oscillate about the neutral position when you wanted no movement off center. To much area in front of the hinge line could overpower the servo and cause over control.
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Old May 15, 2011, 01:54 AM
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United States, AZ, Gilbert
Joined Nov 2009
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Quote:
Originally Posted by glajder View Post
Ok, I'm an absolute beginner, so ...

Since its
linear servos (looks like AS2000) are very
weak I was thinking how to make a rudder
control design for them.
In the case of your rudder, I'd suggest simply using a regular 5g or less rotational servo, I hold the belief that they are more effective at creating torque required than the linear servos.

I am not sure if this would be a viable option, but with a rotational servo, even a smaller one (as long as it does not make the plane too tail heavy) you could mount the rudder directly on the servo arm, no linkages, with a small hole through the rudder so that you could screw it onto the servo.
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