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Old Jul 15, 2014, 12:13 AM
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Dihedral vs. polyhedral in F3X

Just a quick question, hoping someone knows the answer. One thing I've noticed in looking at the various F3X style designs is that F3B/F3F generally have more models with straight V-shaped dihedral compared to F3J. I'm just curious why?

Is it that the two-man tow launch is just that much harder on the wing for F3J compared to the other classes so a center joiner is impractical?

Even still, why not just have a center V-dihedral and have straight plug-in tips?

Anyone?
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Old Jul 15, 2014, 01:56 AM
F3B
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Hi

Some current f3j models have a 2 piece wing - e.g pike perfection and egida. F3b models are stronger than f3j models (generally) so it isn't due to strength. In f3b/ f3f the models have a lot less dihedral over all, because they are speed based, rather than duration/handling like f3j.

In terms of why poly is not popular in f3b any more, I honestly don't know from an aerodynamic perspective. Old school f3b models are often 3 piece wings. I dare say it's easier to mould a flat wing.

Anyway it's not because 2 piece wings are not able to take a 2 man tow. A 2 piece f3j wing is rather large nowadays also.

T
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Old Jul 15, 2014, 05:20 AM
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Maybe you didn't understand my question. I'm not asking about 2 vs. 3 piece wings. I'm asking about V-dihedral (each wing is straight) vs. polyhedral (at least 1 extra bend in each wing). The Pike Perfection and Egida both have polyhedral.

Here are some V-dihedral F3B ships (and there are a TON more):



[IMG]www.f3x.eu/Dingo/Dingo.jpg[/IMG]






And here's a polyhedral F3J ship for reference:




I wonder if the reason is that the F3B/F3F ships need to go faster, so extra dihedral breaks in the wing would add too much drag? But in an F3J glider, speed isn't as important as stability in thermals, so the extra breaks do more good than harm.
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Old Jul 15, 2014, 02:58 PM
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Quote:
Originally Posted by satinet View Post
Hi

...In f3b/ f3f the models have a lot less dihedral over all, because they are speed based, rather than duration/handling like f3j.

In terms of why poly is not popular in f3b any more, I honestly don't know from an aerodynamic perspective. Old school f3b models are often 3 piece wings. I dare say it's easier to mould a flat wing.

T
Satinet answered it pretty well, I thought.
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Old Jul 15, 2014, 04:38 PM
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Satinet answered it pretty well, I thought.

It's not the AMOUNT of dihedral that I'm asking about. It's the SHAPE of the dihedral. That is, either 1 bend (in the center) or 2-3 bends (in the wingtips or wingtips + center).
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Old Jul 15, 2014, 04:46 PM
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Poly - As far as I know you get more effect with up turned tips because of the same reason ailerons are more effective than flaps at rolling the model (further outboard)

I think the idea is you also also effect the centre of the wing less. I.e to get the same effective angle with dihedral you need a lot of dihedral.

How designers decide on the angles I don't know. Probably lots of clever xfoil stuff (or tlar!)
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Old Jul 15, 2014, 05:01 PM
R2R
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Originally Posted by DrewV View Post
It's not the AMOUNT of dihedral that I'm asking about. It's the SHAPE of the dihedral. That is, either 1 bend (in the center) or 2-3 bends (in the wingtips or wingtips + center).
That's what the 2nd and 3rd bends do is add more effective dihedral without increasing dihedral in the center panel. Increasing dihedral in the center panel would decrease effective lift, I would think. Poly also probably aids in approaching the optimal elliptical lift distribution without excessively narrowing the tips.
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Old Jul 15, 2014, 05:12 PM
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I understand the nature of dihedral and the theory behind EDA calculations, but why don't F3J models use straight one-bend dihedral like the F3B models? That's my question.

I don't think I've seen even ONE single F3J design with a one-bend (straight V-shaped) dihedral. Yet there are dozens of top F3B designs with it. They all have a similar AMOUNT of EDA, though. There must be a fundamental reason for why F3J planes have polyhedral while many F3B ships only have dihedral.
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Old Jul 15, 2014, 05:38 PM
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I hear you. I'm curious as well. If you look at a Supra, the center bend is 2*, and the tip joiner with the greatest bend is 5*. Comes with 3 different tip joiners (0*, 2.5*, and 5*).
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Old Jul 15, 2014, 05:43 PM
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With the tip joiners at 0 degrees, then obviously there would be a 2-degree V-shaped dihedral. That seems a bit low, but not horrible.

I'd love to find out why F3J ships don't use 2-3 degrees V-shaped dihedral as a baseline, though. Every V-dihedral plane I've ever flown has thermalled beautifully.
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Old Jul 15, 2014, 05:47 PM
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I think that from a optimal F3J glider perspective the several point dihedral is perhaps an advantage both in performance and handling, as indicated by the previous posters.

But if you need to make an extremely quick roll turn at high speed with very small margins, then roll stability might become a negative factor.

Production factors are involved, because there is nothing that escapes compromises, so if the overall intentional performance lets you chose to make the wing more complicated or less complicated, you would choose less complicated if the complication is not particularly beneficial or perhaps even negative according to the priority task.

tk
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Old Jul 15, 2014, 06:40 PM
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Interesting question.

On the full size side Schempp-Hirth revised their wing designs to polyhedral where Schleicher have stuck with flat wing panels.

At the bottom of this page there are five other threads suggested with the same question, the answer might be in there!!

GB
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Old Jul 15, 2014, 07:11 PM
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I have both a 2 piece poly sailplane (Tanga F3J) and a 3 piece (Pike Perfect), they are very similar in flight, basically you can take either one and stand it on a tip if you have to for a tight thermal turn, my Tanga F3B/F3F not so much. It likes larger faster circles but in a footrace obviously the F3B wins. The poly is much more stable and conducive to thermal type flying. The F3B turns and burns, it will thermal but not as easily as the poly. I don't know much about winch launches as I fly only slope but I can load the Tanga F3J (2piece) up with 4 lbs of tungsten and fly it in 50 without folding the wings so I don't think strength is an issue.
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Old Jul 15, 2014, 07:14 PM
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I think the reason is that tasks involving speed require fewer breaks in the wing, so a simple straight V-dihedral is preferred. But for lower-speed thermal duration tasks, it is not as critical to avoid additional dihedral breaks. In the case of F3J thermal duration, a slight increase in EDA is preferred without sacrificing much in terms of added drag from the additional dihedral breaks.

But I'd love to hear from the actual designers and experts on this topic.
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Old Jul 15, 2014, 07:22 PM
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Originally Posted by bobbyjack View Post
I have both a 2 piece poly sailplane (Tanga F3J) and a 3 piece (Pike Perfect), they are very similar in flight, basically you can take either one and stand it on a tip if you have to for a tight thermal turn, my Tanga F3B/F3F not so much. It likes larger faster circles but in a footrace obviously the F3B wins. The poly is much more stable and conducive to thermal type flying. The F3B turns and burns, it will thermal but not as easily as the poly. I don't know much about winch launches as I fly only slope but I can load the Tanga F3J (2piece) up with 4 lbs of tungsten and fly it in 50 without folding the wings so I don't think strength is an issue.

The Tanga is an interesting example. There is both an F3B and an F3J version. The F3B version has a straight V-shaped dihedral, but the F3J version has polyhedral. So obviously there is a specific design goal for each class of plane. I think my guess of fewer breaks = faster flight is probably correct. It's not about strength, it's about the tradeoff of speed vs. stability in comparing F3B vs. F3J. Very small changes, but in today's age of fine-tuned composite molded planes, very small differences actually can make a difference between 1st and 15th place in a contest.
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