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Old Jan 28, 2014, 04:50 PM
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"Plank" gliders, dihedral, roll trim-- what is known?

Hi, I'm looking for some info on what others have experienced with "Plank" wings (unswept flying wings). Links to papers or articles where these issues are discussed would also be welcome.

Thinking specifically about a thermal soarer sailplane. Roll control achieved via ailerons or elevons. No rudders or tip-draggers

Is it generally beneficial to incorporate some dihedral into the design?

In a thermal turn, will the pilot typically need to maintain a rolling-out aileron input, or not necessarily? (Let's say for simplicity that the updraft speed is uniform across the whole circle-- i.e. we could do the tests same in still air if we wish, and get the same results.)

Can we say with confidence that the more dihedral we incorporate into the design, the less rolling-out aileron input, or the more rolling-in aileron input, will be required to hold the glider in a constant-banked turn at the min. sink angle-of-attack? Or do we sometimes observe the opposite effect? For example is it ever the case that increasing the dihedral angle would make the wing require more rolling-out aileron input, or less rolling-in aileron input?

Can we say with confidence that the wing is experiencing at least a slight sideslip during a constant-bank thermal turn, at the min. sink angle-of-attack? With the ailerons (elevons) deflected as needed to hold the bank angle constant? Or is there reason to believe that this is not always true?

Is it common practice to try to tailor the dihedral angle such that no roll control input either toward the inside or the outside is needed when thermalling at the min. sink angle-of-attack, working under the assumption that increasing the dihedral angle will cause the pilot to need to give less rolling-out aileron input, or more rolling-in aileron input? Or is it not really practical to try to achieve a "neutral" roll trim for thermalling, in this manner? If this is not practical, why not-- because such balance simply could not be achieved, or rather because it is better to optimize the dihedral angle for other considerations such as roll rate, mandating the use of less dihedral? Is there any reason to believe that such a balance could not be achieved at least in theory, if the designer were willing to settle for a poor roll rate?

PS-- noted-- "A couple of climb and glides and hooked a good thermal and rode it way up. A little up elevator and some opposite aileron to keep flattening it out.....Wow..." ( http://www.rcgroups.com/forums/showt...2059254&page=2 ) -- this on a plank wing with very modest dihedral-- presumably, more dihedral would have allowed the ailerons to be centered or even required inside aileron, to hold the same bank angle? Can we be certain of these relationships? In other words the situation is really no different than in a more "conventional" design, with a longer fuselage giving more moment-arm for the vertical fin?

Steve
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Old Jan 30, 2014, 03:43 PM
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Plank gliders normally have a vertical fin. Are you planning to have such fins, but not controllable, so no active rudder?

Because of the short lever arm on the fin, Planks have limited yaw stability compared to conventional aircraft. That is why I like zero dihedral. Dihedral will create adverse yaw that must be countered with rudder input. With limited yaw stability, why create problems? My Plank 101 has a fin but no rudder and it has no bad habits. It seems to turn fine with the bank and yank technique. I've never been accused of performing smooth thermal turns, but that may be more about the short wing span and my slow reactions.

Herk has build a few thermal Planks with dihedral and rudder to effect I'm told.

Kent
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Old Jan 30, 2014, 10:41 PM
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Thanks for the notes above...

Makes sense, that dihedral and rudder would tend to go hand-in-hand...

Pilots with experience flying Planks with rudders can help shed light on my questions, so long as they have some experience on what happens in a constant-bank thermal turn if the rudder is kept centered (no mixing with ailerons).

Basically what I want to know, based on experience not just theory, is this-- with the rudder centered, can we be confident that increasing the dihedral angle will cause the Plank glider to require less outside aileron or more inside aileron, in a constant-bank thermal turn? Even when circling near min sink to optimize the climb rate? Or do we sometimes see the opposite?

I'm not talking about circling so slow that one tip is stalling, just a good efficient slow circle...

Thanks
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Old Jan 31, 2014, 02:00 PM
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Steve, Not really an answer to your question --- But

This old model I built years ago, had quite a lot of dihedral. It needed it because it was a RES design. It took positive rudder to keep it in a thermal turn. Presumably if it had elevons and a fixed fin, it would still take some turn positive aileron to keep it in a thermal turn. There would be some adverse yaw, but top aileron would not be needed until the turn got rather steep.

The later model has very little dihedral - just enough to keep the wing from looking droopy. It was a drag rudder experiment, but later converted to ordinary ailerons. It has a removable fin-rudder on the centerline - removed in this picture. It does take a slight bit of top aileron in a thermal type of turn -- which might produce a bit of pro-turn yaw. As you said - this is talking about rather shallow banked steady state turns at modest speed.

Dihedral isn't the only factor influencing this behavior. Any old free flighter knows that the balance between vertical tail size and dihedral is pretty critical. So the fin placement and area (Vv) will have an effect on the bank angle at which a spiral dive tendency will develop - requiring top aileron to restrain it.

Even without an active rudder, the fin plays this role in the answer to your question.

A plank is just a conventional layout with short coupled tail surfaces - so the dynamics are going to be pretty much the same. If the Vv is small and there is significant dihedral you can expect dutch roll and a tendency to roll out of a shallow bank. If the Vv is large and the dihedral is small then top aileron will be required even at rather shallow bank angles.

With the center fin removed my newer plank had both very little dihedral and a very small Vv so it kind of really didn't know what to do.
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Old Jan 31, 2014, 02:42 PM
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So Herk, do you still own the "old RES plank"?......still fly it? I, for one, would very much like to see this floater in action, as in carving a nice thermal turn. To my modest understanding of plank dynamics, having dihedral for an RES configuration seems problematic at best, yet you say this one flew well. I believe that Ken Bates also had a similar design.....I'll do poking around on youtube.

Kent
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Old Jan 31, 2014, 03:10 PM
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Quote:
Originally Posted by Knoll53 View Post
So Herk, do you still own the "old RES plank"?......still fly it? I, for one, would very much like to see this floater in action, as in carving a nice thermal turn. To my modest understanding of plank dynamics, having dihedral for an RES configuration seems problematic at best, yet you say this one flew well. I believe that Ken Bates also had a similar design.....I'll do poking around on youtube.

Kent
No Kent, You and I have discussed this model before. After flying from the winch for a while I electified it and flew it that way quite a bit more - was back in the 80s.

It died when I folded it trying to get down from a boomer thermal. I never found the whole thing. I did show you a piece of the broken wing when you asked about the odd construction method. That was back in one of the drag rudder threads. Or maybe it was email - I can't remember.
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Old Jan 31, 2014, 05:52 PM
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Windfreak

Couple of images from the Windfreak (Roger Sanders) building thread at the Dutch forum.
http://www.modelbouwforum.nl/forums/...-voor-vgg.html







Some other threads about Windfreak:
http://www.modelflying.co.uk/forums/...s.asp?th=50941
http://www.rcgroups.com/forums/showthread.php?t=982885
http://www.rcgroups.com/forums/showt...ght=zon+zoeker
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Old Feb 01, 2014, 11:05 AM
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Thanks for posting Papa. Love the rudder design on the WindFreak. That's what I call a full flying rudder!

I had never seen the WindFreak before and this entire category of thermal planks is new to me. It is a fascinating configuration and, to me, an unlikely combination, that is dihedral with a short coupled rudder. I would not think that there would be enough rudder "authority" to control the roll and adverse yaw created by the dihedral when a cross wing gust hits the plane. But many have proved that it is a viable configuration. I found a video of a short slope flight of a similar glider.

Steve may or may not get the direct answer he is looking for to his question, but these examples of this configuration are great!


Kent
the guy who missed this glider back in the 1980's, when they were "new".

Windlord-Br (1 min 1 sec)
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Old Feb 03, 2014, 12:26 PM
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Some of the questions I'm trying to better understand...

* We know that dihedral only creates a rolling-out torque if the wing is experiencing some amount of sideslip.

* Re the comment above that dynamics in Planks are similar to dynamics in more "conventional" gliders--

* In a more conventional glider the vertical fin, being so far aft, would force some amount of sideslip at the wing, even if the vertical fin simply aligned itself fully with the local flow. (Note that the relative wind is curved in turning flight, by virtue of the fact that the flight path is curved.)

* Even if the wing tended to contribute a pro-skid yaw torque for some reason (imagine that inboard tip created more drag than the outboard tip, perhaps because the pilot is needing to hold outside aileron), the pro-slip influence of the vertical fin might still cause the wing to experience a slight sideslip.

* On the other hand, if the wing contributed a strong pro-slip influence, then a large vertical tail "volume" would tend to limit the slip-- for the same tail "volume", a large tail area on a shorter arm should allow less slip than a small tail area mounted far aft (because the further aft we mount the fin, the more the flow has curved in a way that limits the fin's power to fight the slip.)

* What I'm interested in understanding, is does the wing itself (including any required control surface deflections) contribute a pro-slip or a pro-skid influence, in a constant-banked thermal turn?

* The apparent similarities in dynamics between more "conventional" gliders and Planks, seems to suggest that the wing itself contributes a pro-slip influence in a constant-bank thermal turn, regardless of the tail geometry. And even in the case where the ailerons must be deflected somewhat toward the outside of the turn.

* But could it happen that when thermalling at the min sink speed / a-o-a, or at any speed slower than the best L/D speed, the inboard (slower) wing makes more drag than the outboard (faster) wing, making the aircraft fly in a skid rather than a slip? Presumably the pilot would need to be holding outside aileron to keep the glider from rolling into a tighter turn. If the glider is in fact skidding around the (constant-bank) turn, then the more dihedral built into the wing, the more the (skidding) glider is going to want to roll into a tighter turn, and the more outside aileron will be needed to hold the bank angle constant.

* Do we ever see this in Planks? A need for a great deal of outside aileron when thermalling slowly? Is it possible that dihedral actually makes the situation worse, because the aircraft is in fact skidding not slipping? Is it possible that anhedral could actually improve the low-speed thermalling characteristics, requiring the pilot to hold less outside aileron?

* If not, why not?

* Regardless of the underlying theory, does practical experience with aileron-controlled Planks suggest that the above situation never arises, and that the more the dihedral, the less outside aileron (or the more inside aileron) will be required to hold the bank angle constant, even when flying very slowly, well below the best L/D speed (adjusted as appropriate for the bank angle)?

* The dynamics of a spiral dive with increasing bank angle are surely different than the dynamics of a constant-bank turn. I'm interested in focussing especially on the constant-bank turn, with aileron and elevator positioned as needed to hold the bank angle constant and the a-o-a near min. sink.

* Thanks for the continued input and thoughts...

Steve
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Old Feb 04, 2014, 05:43 PM
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thinking it over...

Quote:
Originally Posted by HerkS View Post
A plank is just a conventional layout with short coupled tail surfaces - so the dynamics are going to be pretty much the same. If the Vv is small and there is significant dihedral you can expect dutch roll and a tendency to roll out of a shallow bank. If the Vv is large and the dihedral is small then top aileron will be required even at rather shallow bank angles.
I guess another extension of my question is, in a "conventional" layout, with rudder centered and pilot holding ailerons as needed to command a constant-bank turn, does the combined yaw effect of the wing and ailerons always tend to yaw the nose outboard of where it would be if the vertical fin (or more precisely the center of side area?) were full streamlined to the local flow? In which case reducing the size of the vertical fin would always increase the slip angle, which if the aircraft has dihedral, will require the pilot to hold less outside aileron or more inside aileron. The aircraft is becoming more positively stable in roll.

Or could it sometimes happen that the combined effect of the wing and ailerons tends to yaw the nose inboard of where it would be if the vertical fin (or center of side area) were fully streamlined to the local flow? In this case, reducing the size of the fin would decrease the slip angle, which if the aircraft has dihedral, will require the pilot to hold MORE outside aileron or less inside aileron-- the aircraft is becoming more unstable in roll, at least in the context of a constant-banked turn, where the pilot is making roll inputs as needed. Spiral stability in the case where the ailerons are centered and some gust has disturbed the aircraft, is a completely different matter.

The most extreme case of the situation described immediately above, would be that the combined effect of the wing and ailerons yaws the nose so far inboard that the aircraft is experiencing a skidding flow, not a slipping flow, over the wing. In this case, any increase in dihedral would require the pilot to hold MORE outside aileron or less inside aileron, and any decrease in dihedral would require the pilot to hold less outside aileron or more inside aileron.

This would seem most likely to happen when the tail moment-arm is short-- because in this case, even when the vertical fin is fully streamlined to the local flow, only a small slip angle results at the wing (say the quarter-chord point). It wouldn't take much pro-skid contribution from the wing to overcome this and cause the wing to experience some degree of skidding flow. If this phenomenon happens in any aircraft, Planks would seem the logical place to look for it.

Of course we can always make the situation even more extreme by imagining that our ailerons have wrong-way differential or only move down and not up-- in that case the combined yaw effect of the wing and ailerons would be almost certain to be a pro-skid influence, no? In this case it seems almost certain that dihedral would generate not only an unfavorable roll torque due to adverse yaw whenever the aircraft is maneuvered in roll (giving a terrible roll rate), but also an unfavorable rolling-in torque as the pilot maintained an outside aileron input in a constant-banked turn, possibly making it impossible to hold a constant-banked turn. Unless-- the question arises again-- if the wing naturally tends to slip rather than skid in a constant-banked turn, maybe when we give the wing extreme dihedral, yes we have a terrible roll rate, but we need to hold a rolling-in aileron input to keep the turning going, in which case the drag from our deflected ailerons is going to cause more slip, which will require still more rolling-in input, till we end up with ailerons fully deflected and only a very modest bank angle.

It's all rather confusing. I guess the way to explore it hands-on would be to make a Plank with adjustable dihedral. Use spoilerons as the primary roll control, but also have ailerons that can be used for roll trim. Explore whether or not its the case that when circling very slowly, with lots of dihedral, the aircraft is untrimmable in roll, because when the ailerons are deflected for a rolling-out torque, this increases the drag of the inside wingtip, which interacts with dihedral to create still more rolling-in torque, requiring still more rolling-out aileron deflection, etc.

Again, if this doesn't happen, I don't understand why not. When flying slowly, below best L/D speed, doesn't the slower-flying wingtip have to create more drag than the faster-flying wingtip, if the ailerons are set so that both wings are creating the same amount of lift?

Steve
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Old Feb 07, 2014, 12:14 AM
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An old flying buddy of mine built and flew a Windfreak for a few years. It flew quite well with the rudder to roll coupling being on par with a polyhedral model of the same sort of wing span. Same with the rudder to hold a turn. Namely a moderate amount to slow roll into a turn then a small "trim" to hold the model in the turn and then opposite to come back to level. Or if left alone it would slowly level itself.

There was something about the angled down tips that was written up in the article. It related to how the angled tips SHOULD produce a proverse roll but in actual practice trying to fly the model without the tips produced some odd reactions. It's all rather hazy since we're talking 30 something years ago that I read and re-read the article.
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Old Feb 07, 2014, 11:35 AM
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OK Bruce, your recollection is hazy......I get that.

What does your gut tell you about the tips? Do you think that they add something to handling?

Personally, I don't see it. You've got dihedral in the wing that creates roll and adverse yaw from rudder input (or a cross wind gust), then anhedral at the tips to do just the opposite.

Kent
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Old Feb 07, 2014, 12:18 PM
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http://www.rcsoaring.com/rcsd/RCSD-2006-01.pdf
Always a good read.Shame the articles aren't all together.
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Old Feb 09, 2014, 03:17 PM
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Quote:
Originally Posted by BMatthews View Post
An old flying buddy of mine built and flew a Windfreak for a few years. It flew quite well with the rudder to roll coupling being on par with a polyhedral model of the same sort of wing span. Same with the rudder to hold a turn. Namely a moderate amount to slow roll into a turn then a small "trim" to hold the model in the turn and then opposite to come back to level. Or if left alone it would slowly level itself...
Yes that is the kind of information I'm looking for, thanks.
It seems to me that the following must be true:

1) If the rudder were centered, and the point where the aircraft centerline was tangent to the (circular) flight path occurred at the rudder, then the wing (say the quarter-chord point) would experience an extremely slight slideslip.

2) If the rudder were deflected to the inside of the turn, and some point on the centerline of the rudder itself were tangent to the (circular) flight path, then the wing (say the quarter-chord point) would experience a skid, which would contribute a rolling-in torque. That's not consistent with a constant-banked turn-- because the outboard wingtip flies faster, and tend to create more lift, than the inboard wingtip. The roll torques can only balance if the wing is experiencing a slipping flow, which interacts with dihedral to contribute a rolling-out torque.

3) In a constant-banked turn, if the wing is experiencing a slip not a skid-- as we know must be the case-- and yet the rudder is deflected inward, not outward--then it's certain that the airflow is striking the inside face of the rudder. The outboard wingtip must be creating more drag than the inboard wingtip-- how else could the yaw torques balance?

4) In other words, the tail moment-arm is so short that there is negligible curvature in the flight path and relative wind, between the quarter-chord point of the wing and the tail. The direction of the flight path and relative wind as measured at the tail is essentially the same as the direction of the flight path and relative wind as measured at the wing's quarter-chord point, or at the aircraft C.G.. If the wing is experiencing any significant degree of slip, then rudder must also experience a slip-- the airflow MUST be striking the low side or inside of the rudder-- UNLESS the rudder is deflected significantly toward the outboard / high/ "downwind" wingtip rather than toward the inboard / low / "upwind" wingtip.

(We can't make the same statement for a more conventional sailplane with a much longer tail moment-arm-- in this case if the wing experiences a slipping airflow, the airflow might still be striking the outside or high side of the fin and rudder, contributing a nose-yawing-out-torque, even if the rudder is centered.)

4) When the whole aircraft is experiencing a slip, this means that the point where the (extended) centerline of the aircraft is tangent to the curving line of the flight path, lies somewhere behind the rear of the aircraft.

5) Re 3), why does the outboard wingtip create more drag than the inboard wingtip? The outboard wingtip is moving faster. But the inboard wingtip is flying at a higher angle-of-attack and lift coefficient (due to the geometrical interaction between slip and dihedral), which would seem likely to increase drag. At speeds near or below min. sink, might not the inboard wing tend to create more drag than the outside wing, in a constant-banked turn? Requiring outside rudder, not inside rudder, to keep the wing flying at the required slip angle, and hold the bank angle constant? If not why not?

After all, when flying at speeds below best L/D speed, for a given wing loading, if we decrease airspeed and increase angle-of-attack, while keeping lift constant, we'll end up increasing drag, right? The slower inboard wing, flying at a higher angle-of-attack, ought to make more drag than the faster outboard wingtip, flying at a lower angle-of-attack. Right? If not, why not?

6) Similarly I re-read Blaine Beron-Rawdon's articles on "Dihedral"-- in "Model Aviation"-- he notes that reducing the size of the vertical fin will cause the aircraft to fly at a larger slip angle when circling-- because the outboard wingtip is making more drag than the inboard wingtip, so the model always flies yawed more outboard than the attitude that would completely streamline the flow at the vertical fin-- and more so if the fin is small than if the fin is large -- again one might ask why this is so, especially when circling very slowly. (These articles were aimed specifically at rudder-controlled sailplanes with dihedral and no ailerons.)

Steve
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Old Feb 09, 2014, 03:21 PM
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Quote:
Originally Posted by BMatthews View Post
An old flying buddy of mine built and flew a Windfreak for a few years. It flew quite well with the rudder to roll coupling being on par with a polyhedral model of the same sort of wing span. Same with the rudder to hold a turn. Namely a moderate amount to slow roll into a turn then a small "trim" to hold the model in the turn and then opposite to come back to level. Or if left alone it would slowly level itself.
Did the model ever require the pilot to hold a steady outside rudder deflection rather than inside rudder deflection, to circle at a constant bank angle? Particularly when circling quite slowly, i.e. at the min. sink airspeed, and below the max L/D airspeed (as adjusted for the bank angle)? That question really gets to the heart of the observational data I'm trying to collect. (See previous post for what I'm thinking is going on terms of the theory here-- and for what I'm having a hard time understanding...)

Thanks for the note...

Steve
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