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All-moving tail surfaces on pivoting, tubular spars

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cluttonfred

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I have always liked the way Bud Evans did the rudder on the VP-1 and VP-2, where a single aluminum tube serves as both spar and pivot for the all-moving rudder. Here is the VP-2 rudder:
51.jpg 15.jpg 53.jpg
It seems like it would be pretty straightforward to make three such surfaces to create a horizontal stabilizer as well. In fact, it might be possible to epoxy the ribs to the tube and eliminate the blocks and little bolts and make all three surfaces literally identical and interchangeable, such as on the Boulton Paul Phoenix.
boulton_phoenix_1.jpg
Any thoughts on the pros and cons of such a system applied to horizontal tails or V-tails as well as vertical ones?
 
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Victor Bravo

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If you have not already though of it, offset the tubes for the H-stab and the V-stab, such that you can run the support tubes (bushing tubes) all the way thru the fuselage both vertically and horizontally. So the horizontal tube is anchored to both left and right fuselage sides or longerons, and the vertical tube is anchored to the top and bottom fuselage skin. This will allow the lightest and most efficient structure, while still allowing identical and interchangeable tails.

There will have to be some consideration of how you drive/actuate the surfaces in order for those parts and hookups to be interchangeable, without making it complicated.
 

jedi

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Any thoughts on the pros and cons of such a system applied to horizontal tails or V-tails as well as vertical ones?
The rudder is generally unloaded, IE. centered, while the stabilator generally has a down load and this may cause additional friction and wear concerns that need to be addressed. With the rudder locked in position the aircraft is still flyable. With the elevator locked in position it is not so flyable.

Good idea but make sure to do it right.
 

cluttonfred

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Thanks, guys.

VP, you can see in drawing 2 above that Evans did it exactly that way using PTFE bushings top and bottom. In fact, the tube passes all the way through the bottom skin to support a section of threaded bar for the steering chains to the tailwheel. There are lots of way to actually drive the surfaces and I wouldn't mind "interchangeable but drill a couple of holes here for the rudder horn or there for the elevator horn". Here's how Evans did it.

22.jpg

For a horizontal stabilator, I could see the left and right spar tubes sleeved over over a central tube that could go out as far as necessary to reinforce each side.

jedi, I see your point, though hopefully the generous bearing areas (two bearings, 2" diameter x 3/4" width each) would minimize the problem. Some dry Teflon lubricant probably wouldn't hurt either. Still, it would certainly be an inspection item but also an easy thing to check for free movement and undesirable play as part of every preflight.
 

jedi

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Thanks, guys.
........

jedi, I see your point, though hopefully the generous bearing areas (two bearings, 2" diameter x 3/4" width each) would minimize the problem. Some dry Teflon lubricant probably wouldn't hurt either. Still, it would certainly be an inspection item but also an easy thing to check for free movement and undesirable play as part of every preflight.
Thanks to you too. I did not look thru the drawings to verify the "using PTFE bushings top and bottom".
 

mullacharjak

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Round tube spar in a bearing(plastic) was a feature of the Canadian Chinook Ultralight of the early 80s both rudder and elevator.There were no hinges as such.
 

cluttonfred

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Off the top of my head I can't think of many planes that actually had interchangeable vertical and horizontal tail surfaces: Boulton Paul Phoenix: Emigh Trojan, Ted Smith/Piper Aerostar. Any more come to mind? For what it's worth, the VP-2 has 19 sq ft of horizontal tail area and 9.6 sq ft of vertical tail area, so the proportions are pretty much perfect.
 

lr27

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Seems like a good idea if you can afford the weight.

Would TFE dry lube help much on TFE bushings? If you were going to do that, why not UHMW polyethylene bushings?

I wonder how much the friction affects the control feel.

Anyone know why the rudder anti-servo tab is mounted at an angle? Is it to counter torque and propwash?
 

Dan Thomas

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Seems like a good idea if you can afford the weight.

Would TFE dry lube help much on TFE bushings? If you were going to do that, why not UHMW polyethylene bushings?

I wonder how much the friction affects the control feel.

Anyone know why the rudder anti-servo tab is mounted at an angle? Is it to counter torque and propwash?
Stabilators on certified airplanes like the Cherokees and Cessna Cardinal ride on bolts, not big tubes. Bolts in bushings will offer much less friction under load than a large-diameter tube in bushings, and the previously-made point about the constant downforce on the stab making friction a hassle is perfectly valid. One might find a good bit of stickiness in flight, making trimming for level flight a real pain. You'd have to mount that tube in ball or needle bearings, or in a well-greased and sealed bushing. More complexity. Unsealed bushings have the very real problem of seizure if grit gets into them, and making things with more bearing clearance raises the spectre of flutter.

The tab angle is probably for reducing rudder pedal load in the climb. It's arranged to deflect the rudder to the left, appropriate for the VW's rotation.

The Jodel's rudder is the entire vertical surface and rides on two bolts held by brackets on the fuselage sternpost. No stickiness. It didn't need any trim tab, at least with my 65 HP. It was enormously powerful and needed very little pedal effort.
 

lr27

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It works well enough on the Volksplane, so I'm guessing it can't be horrible.
 

dino

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Peterson J-4 Javelin sailplane had identical horizontal and vertical tail surfaces
 

pwood66889

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"Stabilators on certified airplanes like the Cherokees and Cessna Cardinal ride on bolts..."
Thanks, Dan. Been noodling a "Full Flying" tail for some time myself.
 

cluttonfred

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Thanks, Dan, that’s very clear. I guess Bud Evans knew a thing or two when he only used the aluminum tube trick for the rudder. I’ll need to go with a different solution if I want interchangeable surfaces. The Phoenix horizontal knife-edge and small vertical fin might be worth exploring. Cheers, Matthew
 

lr27

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I wonder why Evans used the tube trick in the first place? With conventional, separate control surfaces he could have used three identical tail pieces. The spars could still extend through the fuselage, they just wouldn't have to move. They'd probably be lighter.
 

cluttonfred

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I am not sure why Evans went with all-moving tail surfaces, but by all accounts they are effective and pleasant, and whatever additional complication they come with is offset by eliminating the fixed tail surfaces. The ailerons...not so much.
 

Tiger Tim

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What benefits are there to having three identical tail surfaces on a plane that will be made one at a time by hand? Genuinely curious here.
 

Victor Bravo

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I believe the Pilatus Porter had interchangeable tails, and the original Bede/American Aviation AA-1 had all the stabilizers and tail controls identical. The flaps and ailerons were also all the same too IIRC.
 

cheapracer

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The Morgan is a simple setup, 100+ flying can't be too bad.

The main spar is of steel sitting in 2 PTFE bushes, held to the frame in 2 'U' clamps.

The fiberglass ribs are 'glassed to aluminium tubes that are snug fits over the steel tube, and of course pinned to the steel tube, as is the lever inbetween the support bushes. Morse Cable (push pull cable) operated.

Morgan stab.jpg

P1010201.jpg

S5001687.JPG
 

David Lewis

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I believe it is standard practice for the stabilator to be mass balanced, and have an anti-servo tab so the axis of rotation can be moved farther back.
 

dkwflight

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Years ago I flew a prototype ultra lite wit an all flying elevator.
The designer missed the design a bit.
The control stick forces were so light as to be non existent.
The pivot point was too close to the center and no travel stops
Absolutely could not fly hands off.
SCARY
Dennis
 
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