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Old Sep 08, 2012, 08:46 AM
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Estonia, Harju, Tallinn
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Inverted V-tail, no ailerons, no dihedral

I was looking into inverted V-tails and stumbled upon this page --> http://www.niquette.com/puzzles/v-tailss.htm
The author claims that the Predator drone has no ailerons and from the drawings I can't see any dihedral either.
As far as I understand rudder steered models have to have quite a bit of dihedral.
So, is it possible to have a plane with inverted v-tail, no ailerons and no dihedral that's also steerable?
I think I haven't seen any planes with only an inverted v-tail for control. Is the v-tail suitable for a rudder steered model (with dihedral)?
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Old Sep 08, 2012, 11:23 AM
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A flying buddy built an inverted V-tail airplane. Its wings have no dihedral. I've seen it fly, and it can do turns with no problems. The tail is a bit bigger than normal, because my pal was designing by guess and by golly, erring on the large side. It's no big trick to take a pair of scissors to those parts of a foamy you don't want.

If, indeed, the Predator has no ailerons, it's probably because the flight profile doesn't depend on doing aileron rolls or very tight turns. If it has no dihedral, it may be that the wings seem to have no dihedral when the drone is at rest on the runway, but when it's aloft, the wings may be flexible enough to bow upward, thanks to their bearing a 1 G load, thus gaining some dihedral in flight.

Bottom line: inverted V-tail with no wing dihedral is good. Go for it.

Jim R.
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Old Sep 08, 2012, 11:27 AM
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BTW, I see you are a newcomer (I hate the word "nooby".) Welcome to RC Groups.

Jim R.
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Old Sep 08, 2012, 11:42 AM
A witty saying proves nothing.
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I believe the Predator UAV has both ailerons and dihedral. There are a lot of versions of it, and the newest one does not have an inverted V-tail either.

It looks to have a small dihedral angle, maybe 2 degrees per wing? If an airplane has zero dihedral it always looks like the wings are drooping.

Many photos show the aileron surfaces and control connection below the wing. It appears to have flaps as well.

Predator B:

"Redundant flight surfaces
4 ailerons
4 flaps
4 ruddervators
1 rudder"

http://www.dtic.mil/cgi-bin/GetTRDoc...f&AD=ADA427459

Kevin
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Old Sep 08, 2012, 03:56 PM
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This pic shows ailerons on the MQ-1 Predator. The Predator B has a Y-tail configuration. (Also a turboprop engine.) You pays yer money...

Jim R.
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Old Sep 09, 2012, 04:51 AM
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Thank you, Jim. I've been reading RC Groups for years. This is the first time I had a question I didn't find an answer to.

The question was more about inverted v-tails than the Predador drone. I was pretty sure that the Predator had ailerons. Just wanted to check.

I'm thinking of building a twin boom model that has only ruddervators for control and no ailerons. So I was just wondering if maybe someone had experience with that particular configuration - are there any specific issues to consider when designing the plane and so on.

Martin
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Old Sep 09, 2012, 09:50 AM
A witty saying proves nothing.
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The wording of your question will influence the answers you get.

It depends.

An inverted V-tail will develop a rolling moment in the correct direction with "rudder" deflections, unlike an upright V-tail that has roll and yaw fighting each other. Whether this rolling moment is enough for control of the airplane will depend on the rest of the aircraft design. A low span model with a big tail might work quite well. Some EDF jets just use a flat stabilizer for roll and pitch successfully. A high aspect ratio, large span like a Predator UAV would be quite sluggish with a lot of skid because the rolling moment will be relatively weak compared to ailerons. Dihedral would add a rolling moment from the yaw, which would help.

An AVL analysis could sort it out, and then you could test fly it in CRRCSim.

Kevin
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Old Sep 09, 2012, 11:55 AM
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Sorry, I didn't mean to imply that I didn't like the answers.

There's probably no mystery here. I'll do what you suggested and analyze the model in AVL and CRRCSim.

Martin
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Old Sep 10, 2012, 12:31 PM
An itch?. Scratch build.
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I just tried a thread title search on, inverted V-tail, it came back with 19 threads.
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Old Sep 10, 2012, 01:32 PM
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One thing is fer shere
The predator is easily the worst looking aircraft in existance
-it is a classic example of "purpose built"
(form follows function)
Using present stability systems , it probably could be flown sans ailerons.
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Old Sep 10, 2012, 04:22 PM
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Yes, but could it be flown with a Spektrum DX8?

Jim R.
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Old Sep 10, 2012, 04:45 PM
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It could -provided you knew how to do the req,d mixes
That tx is quite good at allowing for multiple mixing.
I did a Phantom jet - years back- using only controls on th vertical and the two drooping stabs -
I mixed the elevators to aileron and treated the vertical seperately - If it was interconnected - teh model just yawed with no roll
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Old Sep 11, 2012, 02:37 AM
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Quote:
Originally Posted by martig View Post
....I'm thinking of building a twin boom model that has only ruddervators for control and no ailerons. So I was just wondering if maybe someone had experience with that particular configuration - are there any specific issues to consider when designing the plane and so on.

Martin
I've never tried it but I can tell you that there's more than a few folks that have tried to use elevons on the tail of a more or less regular planform and it didn't work.

For starters there is absolutely nothing at all magical about an inverted V tail. It won't make up for a lack of dihedral or anything else that a regular separate stab and fin or upright V will not do. The ONLY thing it has going for it is a minor amount of proverse roll when you deflect the surfaces for some yaw. But that minor amount of rolling torque won't do anything unless the planform of the design has an unusual tail span or uses an extremely small aspect ratio.

Here's why. If the roll control surfaces are not on the wing itself then the wing will tend to try to damp out the roll. This happens because the the downward traveling wing sees what amounts to an increase in the angle of attack and thus generates more lift which fights the rolling action. The upward traveling sees and does the opposite.

This action of the wings fights the roll being induced by the elevons on a regular flat stabilizer or a V tail of any sort. And unless the stabilizer has a span that is well over more than half the wing then the wing's action will win and any roll response you get is going to be extremely lazy and slow.

But put in a bunch of dihedral and the yawing forces from the tail will produce a good amount of rolling couple due to the dihedral and how it responds to some yaw angle.
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Old Sep 11, 2012, 02:48 AM
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Bruce,

Great explanation. There's a reason ailerons tend to be placed toward the outboard part of the wing, and a reason for using polyhedral in a rudder-elevator design... the extra displacement of the rolling surface from the roll axis offers more moment.

Even deflected ailerons will contribute to roll damping... not much you can do about that.
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Old Sep 11, 2012, 06:13 AM
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The little Phantom had dihedral in outer panels -I knew it needed some stability
The Predator -like lots of new stuff - won't fly without a computer .
We have some of the new Bind n fly models - a Mig and a Gee Bee -which have the stability system (AS3X) which works on 3 axis.
This setup REALLY works.
Either model will fly knife edge with very little corrective control and more sedate flying is smooth.

Using this type system - a model like the Predator could be done -very scale like -with docile flying character.
The Phantom was done in the early 1980's powered by a Tee Dee- - weighed 14 ounces - Looks pretty ugly but flew quite well
still have it hanging on a nail
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