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How does one come up with the best dihedral angle on a R/E glider?

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How does one come up with the best dihedral angle on a R/E glider?

Old 04-23-2011, 06:52 AM
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aeajr
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Default How does one come up with the best dihedral angle on a R/E glider?

How does one come up with the best dihedral angle on a R/E glider?

* Does it change for a polyhedral wing with two angles?

* How about a poly wing with 3 angles? Does that change things?


I am building a Bird of Time kit. In round numbers, the center panel calls for about 4 degrees with the outer tips at 8 degrees. I assume we would call that 12 degrees. Someone told me that Thornberg later said it would have been better if he had put more dihedral into the wings for better turning.

So, this raised the question in my mind of how one comes up with the right dihedral angle for a R/E glider. Or how do you find a starting point for testing?

* Is there a forumula that gives you a starting point?

* Is it all about TLAR, that looks about right?

* Do you build 12 sets of dihedral braces and test till your retirement?

* For polyhedral angle wings, is there a ratio of inner to outer dihedral?


Is there a difference between a flat center and 12 degree tips vs. a 4 degree center and 8 degree tips? Or would it be better to go 4 + 4 + 4 degrees in three sections?

It appears that most poly wings tend to have more dihedral per section as you go out from the root. Is this required, preferred or does it just look better that way? Is there a golden ratio?


So I am asking all you designers out there, how do you do this? Inquiring minds want to know, not because we want to be sailplane designers, but because we marvel at the miracles you create and want a peak behind the curtain.
Old 04-23-2011, 08:18 AM
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Default RE: How does one come up with the best dihedral angle on a R/E glider?

There's no formula that I know of. It comes down to a case of "the last model was OK but it was a little slow to respond to rudder so let's put in a little more dihedral". And to some extent the amount used is linked to how heavy the model is and how fast it flies. A light but fast flyer will respond to yaw angles with a faster roll rate than a slower and heavy model.

And there's no doubt that putting the greater angle at the tips results in a stronger roll rate than using simple V or gull wing dihedral. In fact gull wings typically roll very poorly in response to yawing with rudder. So while a poly, simple V and gull wing may all have the same total value of root to tip dihedral the nod certainly and enthusiastically goes to the polyhedral option for getting the most rolling couple from a given yaw angle.

This would suggest that going to a multi break 5 or 7 panel wing would be even better. But I'd suggest that once you are at a basic 4 panel "Gentle Lady" wing layout that adding more dihedral breaks is only going to give you a very little extra for yaw to roll effectiveness. The 5 panel wings are popular for a lot of RES models. I'd suggest that the main gain over a 4 panel wing is the use of a flat center for an uninterrupted center panel wing spar and a natural place to mount a single spoiler surface and control linkage for simplicity.

So why would we bother with more than 4 or 5 panel arrangements? Dave Thornberg suggested that perhaps the elephant ear Wolf wingtips are somehow more efficient at contraining and using the flow of air around the dihedral break. Do they? We'd need to do some serious wind tunnel or thread tuft experiments to find out if his claim is true or not. But given that we know there's a significant amount of span wise flow on any real wing I'd say that using more dihedral breaks of smaller angles between panels SHOULD result in less drag. Ideally an RES model should likely use a smooth elliptical curved wing like that found on the old Hobie Hawk. And is it better to break up the tripped air that flows over these joints over more joints of smaller angle or is it better to "just get it over with" and have one mid span dihedral break? Not having access to a wind tunnel I'll have to invoke the Heisenburg Uncertainty Principle on this topic and say "I just don't know.... "

Getting back to the BoT.... For some years I flew a somewhat heavy BoT and a T tailed RO-8 2 meter. Both shared the use of rather short tail moment lengths and both have the tendency to "level and leave" a thermal if I slow down a little too much. When I got into electric flying I built a new fuselage for the RO-8 wings and stabilators. I made the new fuselage about 2.5 inches longer and reduced the size of the fin and rudder by about 15 to 20% to make up for the added length. The longer fuselage even with the smaller fin and rudder totally changed for the better how the model flew. Granted some of this was the slightly higher flying speed needed by the weight. But on two occasions I had ballasted the old original glider for windy days and the new electric fuselage let the wings fly SOOOO much better. The model was more responsive to the rudder inputs. Some of the wing wobbles where the tips wobble around for a moment or so that occured on both the BoT and short glider RO-8 when flying with lots of rudder in turbulent conditions was gone. The "level and leave" if the model was slowed down in lift was gone. All in all the model was far more delightful to fly in all conditions.

Based on all this if I were to build another BoT the one mod I would do is to stretch the fuselage in a similar way as what I did with the RO-8. My thinking is to make the tail about 2.5 to 3 inches longer and slightly reduce the size of the fin and rudder by maybe 10% in compensation.

As for the dihedral adding another 1/2 to 3/4 inch to the tips would likely not be a bad thing either. While I suspect that much of the improvement in the yaw-roll response would likely come from the tail length increase a small amount of added dihedral would be like adding suspenders to a better fitting belt. And a paltry 1/2 or 3/4 inch of added dihedral would not have any significant bad effects in either appearance or added air drag around the dihedral joint.

The BoT is known for needing a fairly sizable lump of lead in the nose. So along with the tail length increase adding an inch or little more to the nose would not be a bad thing. That and working hard to keep the weight of the tail under control by building light and then "carbon it up" with a layer of carbon cloth and resin over the whole fuselage and especially over the tail. And building up the rudder and fin instead of the solid sheet versions in the plan and kit should further work to reduce the need for lead in the nose. And for contest work with the typical landing "arrivals" it would not even be a bad thing to include stiffener stringers of carbon flat rectangular rod inset into grooves in the fuselage wood before the cloth and resin is applied. The fuselage wood could be nicely carved and sanded and then the stringers inlaid in grooves cut into the surface. A slick little grooving tool can be easily made by cutting and glueing pieces of coarse tooth hacksaw blades together to make a short "wood rasp" for a job of this sort. Then the carbon cloth can be pulled to produce a long axis diagonal weave pattern wich both lends some added longitudinal strength but also gives a lot of torsional stiffness by acting like diagonals joining the main load carrying stringers together.
Old 04-24-2011, 04:16 AM
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Default RE: How does one come up with the best dihedral angle on a R/E glider?

There is a formula. It's called Equivalent Dihedral Angle. The idea is you can have a flat center panel with bent up tips or you could have multiple dihedral angles across the span or just one dihedral angle and they should all perform similarly. There is a little math involved and it's not very complex but I wrote a small program in MS Excel called "Sailplane Calc" along with an article for Radio Controlled Soaring Digest (RCSD) on how to use it. The formulas for tail sizing come from Dr. Mark Drela. He wrote an article in the August 2004 Issue of RCSD called "Basic sizing checks for homebrew RC thermal gliders".

In short he suggests an EDA of 6 for an aileron Thermal Duration model and an EDA of 12 for a polyhedral glider. It's not an exact science but these suggestions are a good place to start.

Here is the link to Dr. Drela Tail Sizing article:
http://www.rcsoaringdigest.com/pdfs/RCSD-2004/

and the link to my spreadsheet:
www.TailwindGliders.com

Curtis
Old 04-24-2011, 04:49 AM
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Default RE: How does one come up with the best dihedral angle on a R/E glider?

CloudyIFR, that is outstanding. Thanks so much!
Old 04-24-2011, 05:03 AM
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Default RE: How does one come up with the best dihedral angle on a R/E glider?

Thanks for the info and links CloudyIFR and also for the question aeajr. I was thinking about that the other day.

I'm going to build a heavily bashed Bird of Time as well. I'm changing the wings to dihedral and installing flaparions to help with turning as well as going to a pylon mounted wing. aeajr be sure to incorporate spoilers in that thing if you can, that plane does not want to land!
Old 04-24-2011, 05:07 AM
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Default RE: How does one come up with the best dihedral angle on a R/E glider?

Here is a link for an easy spoiler set up.

http://www.rcgroups.com/forums/showt...ht=ez+spoilers
Old 04-24-2011, 05:15 AM
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Default RE: How does one come up with the best dihedral angle on a R/E glider?

Hey B I like your idea about stretching the BOT out 2.5" I'll do that on mine. One other mod I'm planning is going to a V-tail on my BOT. If I stretch the fuse 2.5" and go to a V-tail do I need to decrease the surface area of the rear stabilizers as well like you did on the RO-8?
Old 04-24-2011, 12:45 PM
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Default RE: How does one come up with the best dihedral angle on a R/E glider?

Well, you're doing a lot of changes so you won't be able to just use the stuff as it comes on the original design. For example because you're lowering the outer dihedral and going with flaps and ailerons you'll need to reduce the size of the vertical fin or lower the angle of the V to reduce your vertical tail volume coefficient or the model will want to tighten the turns and try to wind into a spiral dive all the time. Also because you'll be using ailerons for the rolling function you don't so much need the longer tail for the yaw to roll coupling. However a longer tail does aid in more easily damping any disturbances compared to a short tail.

The bottom line is that with so many things changing you're best off to treat your new model as just that, a whole new design that just happens to use BoT wing outlines.
Old 04-27-2011, 04:39 PM
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Default RE: How does one come up with the best dihedral angle on a R/E glider?

You know I was wondering how to incorporate flaps and ailerons into the Bird of Time's wing. Then I look up and see Esprit Models banner ad for their totally new Albatross 3000 glider. It's like nothing you've ever seen before!

Basic membership has it's rewards.

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