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Old Dec 08, 2010, 09:13 PM
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Skydream anyone?

Here is an article from Quiet & Electric Flight by Dereck Woodward with an unusual rubber model that he feels could be built as an EP. Although it is not a canard the long nose is just begging for one. I wouldn't use the winglets on the end of the wing either.
This was part of the article that also included info about my X-Wing.
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Old Dec 08, 2010, 10:09 PM
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Those are a form of winglets on the tips. A number of folks have experimented with them, including me, and out at Stanford Dr. Kroo and his team have done quite a bit of work with them. He refers to it as a "C wing". While still providing the benefits of winglets, they also act as both vertical and horizontal stabilizers. The idea works very well, but stiffness is critical if you want to avoid aeroelasticity problems.

Replacing them with a canard would most likely NOT be an improvement.
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Old Dec 09, 2010, 05:51 PM
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Delta Duck Mark 2

Here are a few shots of Mark 2. The build is fitting together well.
Charles
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Old Dec 09, 2010, 11:22 PM
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Quote:
Originally Posted by Don Stackhouse View Post
Those are a form of winglets on the tips. A number of folks have experimented with them, including me, and out at Stanford Dr. Kroo and his team have done quite a bit of work with them. He refers to it as a "C wing". While still providing the benefits of winglets, they also act as both vertical and horizontal stabilizers. The idea works very well, but stiffness is critical if you want to avoid aeroelasticity problems.

Replacing them with a canard would most likely NOT be an improvement.
DON, This is a Canard Forum so any new plane HAS to be a Canard congfiguration. I was just looking at the overall plan-form of the Skydream. With the really long fuselage there is plenty of room for a minimal size canard. The winglets on the Skydream would have to be rigid or they will flop all over the place especially at speed. A standard Slow Stick wing is considerably more substantial then those little winglets and it will easily flutter at speed (but it doesn't come apart).
I just found the Skydream to be a stepping off place for a new canard design. The Skydream as shown is a low speed rubber design.
If I was any good at 3D CAD I would attach a drawing. After working in AutoCAD for years before retiring I am also too lazy (better things to do) to do 3D.

Stan
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Old Dec 09, 2010, 11:34 PM
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Contra - Twin Canard

Traplet Plans UK is offering this design.
http://shop.traplet.com/product.aspx?c=2886
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Old Dec 10, 2010, 12:29 AM
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Thanks for that link Red! I have the magazine and plans, am looking at it as being 4 birds away from Crimson! (present project)
Johnny
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Old Dec 10, 2010, 02:17 AM
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Delta Duck Mark II Nice work as always Charles. I'm looking forward to seeing it in flight. Do you think the flat spins will work with this one?

Don
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And don't even think of getting me started on the subject of pushers! With extemely few exceptions (almost all of which are driven by issues other than performance, and suffer performance losses in order to deal with that other issue) those should be banned.
Yes, there are other issues than performance, loads of them. They make life more interesting.

cheers

Nick
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Old Dec 10, 2010, 06:06 AM
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On the Stanford website there is some information about a C-wing plane that includes a canard wing surface. It is explained that "This design comprises a three-surface configuration providing a large allowable c.g. range".



More info here:
http://aero.stanford.edu/Reports/Non...iguration.html
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Old Dec 10, 2010, 06:25 AM
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DON, This is a Canard Forum so any new plane HAS to be a Canard congfiguration....
I'm only too fully aware of that. However, just because it's supposedly a requirement here does not mean that all of us commenting on it have to "toe the partly line" and support something that will not be an improvement. Someone has to serve the purpose of providing a "reality check" when one is needed.

You cannot help the acceptance of a particular design concept by forcing it into applications where it will be a disadvantage.

As far as aeroelasticiy, yes, my own experiments with C wings pointed that out. However, that wing design lends itself to this, with a short span and plenty of chord.

Both the horizontal and vertical sections of the winglets recover induced drag energy from the wing's tip vortices, and in fact the horizontal segments, being further aft, will probably be more effective in this than the verticals. If you replace them with a canard, you remove this winglet recovery, hurting the overall induced drag, you replace some of the wing's lift with lift from the less effficient canard, again hurting the overall efficiency, and you disturb the lift distribution on the wing with the downwash from the canard, causing further reductions in the aircraft's efficiency. In addition, you add quite a bit of surface way out in front of the C/G, some of which will have a negative influence on yaw stability, which through a variety of processes is also likely to hurt overall efficiency, as well as handling qualities. It will probably help dynamic stability in pitch, but probably not in yaw, in fact probably a detriment there.

If you want to do it because you think it will "look cool", for models that is a perfectly acceptable reason to try it. However, don't fool yourself into thinking it will fly better. People care how cool it looks. Air molecules do not.
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Old Dec 10, 2010, 06:26 AM
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That's what I'm talkin' 'bout!

Looks very like a Starship / LongEzy wing in that picture. When does a wing with seats in it become a lifting body? I can imagine problems with cabin pressurizing as soon as we depart from the traditional cigar shape. Have they found a way around that? The last picture on your link suggests a solution with 9 cigar tubes fixed side by side inside the wing.

hmmmm

Nick
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Old Dec 10, 2010, 06:45 AM
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[QUOTE=John235;16784523]On the Stanford website there is some information about a C-wing plane that includes a canard wing surface. It is explained that "This design comprises a three-surface configuration providing a large allowable c.g. range"...QUOTE]

Yes, in that case they are distributing a lot of the plane's mass, in particular the highly variable payload mass, along the length of the plane. That calls for a longer moment arm for the "tail" surfaces. However, other than some sort of electronic active yaw stabilization, the same approach is not viable for the yaw stability problem, so the plane would need larger vertical fins to compensate.

The C-wing is by nature short-coupled, although not as short-coupled as a pure flying wing would tend to be. Short-coupled airplanes tend to do better by minimizing the lengthwise distribution of mass, spreading the mass along the span instead. They have done some of that in this case by putting some of the mass in those pods on either side of the fuselage. However, the concept as sketched still has an awful lot of lengthwise mass distribution, which is where the three-surface configuration can help.
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Old Dec 10, 2010, 10:18 AM
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Originally Posted by Don Stackhouse View Post
Those are a form of winglets on the tips. A number of folks have experimented with them, including me, and out at Stanford Dr. Kroo and his team have done quite a bit of work with them. He refers to it as a "C wing". While still providing the benefits of winglets, they also act as both vertical and horizontal stabilizers. The idea works very well, but stiffness is critical if you want to avoid aeroelasticity problems.

Replacing them with a canard would most likely NOT be an improvement.
Have you been able to show benefits from winglets at model size/speed? I always hear how they have to be tailored so carefully to give more benefit than they cost.
RE
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Old Dec 10, 2010, 02:14 PM
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Red Runner, Thank you for showing that really breathtaking Contra twin! Something like it might be a future project without the dihedral.
John, the C wing discussion is beyond me but at least they are showing a canard wing for it. I appreciate the show and discuss efforts here. You never know what may develop in our minds.
Charles
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Old Dec 10, 2010, 02:20 PM
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Have you been able to show benefits from winglets at model size/speed? I always hear how they have to be tailored so carefully to give more benefit than they cost.
RE
Another beautiful example of a simple question with an answer that is anything but.

The short answer is "yes and no."

Now the "longer" answer:

Winglets recover energy from the tip vortices of the wing, in effect reducing the induced drag of the wing. However, as lifting surfaces in their own right, they have induced and parasite drag of their own. The trick is in making sure the latter does not cancel out or completely overshadow the former. If the winglets make more drag of their own than they recover, then you would be better off without them.

For that net benefit to happen, first of all you have to have enough induced drag from the wing that there is something to be recovered. On a well-designed sailplane there generally isn't much induced drag to begin with, so that doesn't help the case for winglets.

The cynical way to look at it would be that in order for winglets to do a good job, it helps if the wing design is lousy to begin with.

The other problem that crops up is that induced drag is inversely related to airspeed. As you go faster, the amount of air being "processed" by the wing to make lift increases, and therefore the induced drag decreases, just as induced drag decreases with the square of the wing span. The wing makes lift by grabbing chunks of air and shoving them downwards. If the chunks of air get bigger due to more span, more airspeed, or if their density increases due to lower air temperature or altitude, the induced drag goes down.

This is why winglets are also often an easy sell on jet airliners and business jets. Those spend most of the mission either climbing, which is a low-speed, high lift and induced drag condition, or cruising at high altitude, where the air is so thin that it ends up being a high lift/high induced drag condition where winglets can help significantly.

OTOH, our models, particularly sailplanes, usually have a wide range of operating points where efficiency matters, everything from low speed/high lift (thermalling, launching, landing) to high speed/low lift (the end phases of launch, penetrating upwind). Winglets always have a "crossover velocity", the velocity above which the winglet's own parasite and induced drags add up to more than the portion of the wing's induced drag they are recovering. Our models typically spend a significant part of their mission profile above the crossover velocity, and so the overall benefit of wingets tends to be questionable.

There have been a number of tests by various modelers (Chuck Anderson's tests in particular) that seemed to demonstrate an advantage at thermalling speeds. The way Chuck likes to fly, he spends most of his time at low speeds, which makes the winglets an easier sell for his case. His tests involved making removable winglets on a "floater" type sailplane, flying the plane with both and trimming the plane for level flight, then removing the winglet on one side and noting which way the plane wanted to turn. He did show that the plane consistently tried to turn away from the winglet in a mostly yawing motion, suggesting a drag benefit. However, winglets do create rolling moments, so there is still room for debate on whether his results in part or in whole were due to that factor. What is not in question is that at high speeds, the winglets would not have helped.

In the case of the Skydream, we have a very short span, wide chord wing that is likely to have fairly high induced drag, making the ground for a crop of winglets somewhat more fertile.

However, there is another way to deal with the winglet's own parasite and induced drag: make it do double duty. If you can make the winglet perform in place of something else that you would need to have anyway (typically a vertical fin to provide yaw stability), then the winglet's drag is already "paid for", and any recovery from the wing's induced drag is then a bonus.

In addition, if we can situate the winglet further aft of the wing tip, where the wing's tip vortex is more fully developed, it's easier to recover more of the energy from the wing's induced drag. The C-wing configuration does exactly this, using the vertical portion of the winglet to provide a further-aft mounting point for the horizontal portion, essentially "for free" from a structural cost standpoint.

In the case of the C-wing such as the Skydream, removing the horizontal portion of the winglet would remove the part of the winglet most likely to be providing a benefit, and replacing it with a canard that does not have any recovery-of-induced-drag benefits. At best you can hope for a break-even, although most likely there will be a net loss.
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Old Dec 10, 2010, 11:21 PM
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Nickchud
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Delta Duck Mark II Nice work as always Charles. I'm looking forward to seeing it in flight. Do you think the flat spins will work with this one?
Maybe so, Nick, The rudder is larger and the tips are smaller. With an aft CG, it will hopefully do a slower spin. I feel it will be easier to see with lighter colors and more fun to fly as more of a floater.
Charles
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