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Old Sep 11, 2012, 07:22 AM
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A flight control system (combined with "fast enough" control actuators) can stabilize an airplane by moving the flight controls to keep the airplane from diverging in pitch or yaw. If you have an airplane that is lacking in control power on a particular axis, all the computer and actuator bandwidth in the world won't solve that problem.
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Old Sep 11, 2012, 07:26 AM
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Thats true for any setup
These setups DO work-
have you tried one?
even duffers can fly em!
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Old Sep 11, 2012, 08:15 AM
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Wasn't suggesting they don't work. Just suggesting that the reason they work is there is a control surface that provides sufficient power in each axis.

In the case of a Predator-like design with no dihedral and no ailerons, you would have little if any ability to command roll. If full deflection of the V-tail surfaces won't cause the airplane to roll, it won't help to put the control surfaces under computer control.
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Old Sep 11, 2012, 09:04 AM
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My little model would not roll but there was enough power to bank -which is all that was needed - I suspect the Predator would respond the same way - - I build a lot of stuff - try n fly - -I think the predator would fly- just not a n aerobat - I suspect the full scale is much the same - just enough maneuverability to do it's job.
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Old Sep 11, 2012, 10:26 AM
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The difference is that your model has geometric dihedral at the tips. This contributes significantly to the overall dihedral effect which causes any sideslip to couple into roll. With this coupling you can use rudder deflection (sideslip) to control roll/bank. A Predator without dihedral would have very little coupling between yaw and roll.
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Old Sep 11, 2012, 11:04 AM
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The Predator did have a little dihedral in some versions -according to published 3 views
it doesn't take much -
IF the craft flies at some AOA (a little ) the wing taper ( le sweep) adds a wee bit more stability.
it certianly isn't a desireable setup fora no aileron setup but I think it could be flown
If you fudged and added tiny spoilers - it would help----.
Having done a bunch of low dihedral designs for aerobatics , I found fin placement etc., could resolve couple -sometimes
This stuff is cut n try - look at the permutations of the predator
The designers calculated - designed -built - flew---- THEN reconfigured based on actual results.
sorta like the old WW11 fighters - it always took a few tries to get a really good setup.
Some guys went on endlessly about the perfect Spitfire wings - they ended up clipped to a blunt end.
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Old Sep 11, 2012, 04:22 PM
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Years ago a buddy of mine built an electric model that had a Clark Y'ish wing with no dihedral. Simply long upward bevels under the lower side that created a "sliced" off bevel in the wing. And no ailerons. The tip bevels, which are similar to what Dick did with his F4, was enough to provide a smooth sort of rolling action. But it meant that it needed to be handled carefully for landing approaches or I'd find that it would not roll back up to land with wings level.

Because it relied on the dihedral effect of the beveled tips it lost ALL rolling response when the wing was loaded with negative G. I'd roll it inverted and it would yaw with no roll at all. I'd actually have to then pull up elevator to produce some positive G on the model so it would roll out. This meant that all the rolls had to be positive G barrel style rolls. No axial rolls allowed it the rolling simply stopped as soon as it was upside down and I'd apply a nudge of down to hold it level. Needless to say low inverted passes were RIGHT out....

All in all an interesting design to fly. But one which was so full of quirks that I have no idea how it made it to be published in the model magazine at the time. It was so full of quirks that his own amount of stick time was quite a bit less than mine on it. The key was to work WITh the quirks..... that and a lot of thinking ahead to avoid being caught in a situation where I could not pull positive G while upside down without hitting the deck.

All of which ties into this thread to show that a model design either needs ailerons or it needs PROPER dihedral and yaw ability.

A low wing such as a Predator would require quite a high angle to have any sort of reasonable yaw to roll response. Even if we're only after UAV like gentle maneuvering I'd suggest that with the long wings and odd setup we'd still be looking at something over 5 degrees per side to get decent yaw to roll response and a reasonable roll rate sufficient to recover from a 30 degree bank in the landing approach to level the wings in a matter of around 2 seconds or so.
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Old Sep 11, 2012, 05:14 PM
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Another concern with the Predator is that, unlike most rudder-elevator planes, it has to be able to land on a runway in a crosswind. You can't do a steady-heading sideslip to align your landing gear with your flight path in a crosswind if you don't have ailerons and rudder.

The Predator's crosswind limits are pretty restrictive because it can't bank too far before the wingtip is on the runway.
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Old Sep 11, 2012, 05:47 PM
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I doubt that the 3-view pictures of the predator take into account wing flex either. As a drone it is really a one speed design, with maximum efficiency and minimum manoeuvrability as design specifications. I don't really expect it ever to handle negative G loads for more than a fraction of a second.
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Old Sep 11, 2012, 05:47 PM
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Quote:
Originally Posted by ShoeDLG View Post
Another concern with the Predator is that, unlike most rudder-elevator planes, it has to be able to land on a runway in a crosswind. You can't do a steady-heading sideslip to align your landing gear with your flight path in a crosswind if you don't have ailerons and rudder.

The Predator's crosswind limits are pretty restrictive because it can't bank too far before the wingtip is on the runway.
The bigger Bombardier regional jets have a VERY restrictive bank angle for landings -Friend flys em says it can be exciting.
back to models -
The quirky model Matthews noted is not too much different from some 3 ch trainer types
Once you get em inverted you are sunk-unless you have enough altitude to pull thru a loop.
My tiny (30gram bipe with 100 sq) will not roll -except for torque roll -which it does very nicely
However the generous sweep and low aspect ratio makes for a very stable model even with minimal dihedral'
The model is typically flown slowly (high AOA) so that the sweep provides more stability.
More than one way to skin a cat.
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Old Sep 11, 2012, 07:20 PM
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I found out that not enough dihedral on a R/E V-tail can have vicious effects. If you've glanced at my glide test thread, you'll see it was unflyable until I added enough dihedral. Now, if someone would help get the unwanted climb corrected, I can say that design is nearly (are they ever?)complete.
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Old Sep 12, 2012, 06:41 AM
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An inverted V effectively adds anhedral to the plane as a whole. The sideslip considerations hold for the tail as well as the wing. The small span of the tail compared to the wing might make the effect go unnoticed, unless the wing itself has no dihedral or sweep to start with.
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Old Sep 12, 2012, 01:03 PM
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The assertions that they would use this tail configuration so that they can get more fuel is ridiculous. They could have just built it with dihederal and made it rudder/elevator with a standard tail. There are reasons for the inverted V, but I doubt that's it. I'm sure it has ailerons.
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Old Sep 12, 2012, 02:55 PM
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There's always anhedral and anhedral -

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Old Sep 12, 2012, 03:10 PM
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Well, the harrier has a strongly swept and high set wing. the anhedral in that case is to obtain neutral stability in a roll. The high set wing is to minimize the "suck down" effect that can happen in close proximity of the ground, when the accelerated air exiting the nozzles causes a low pressure area between the wing and the ground, and to allow easier handling of wing mounted mission loads.
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