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Old Apr 12, 2015, 05:53 AM
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Question on soaring with a wing...

I want to build a 2 meter swept wing for thermals.

My question is how much dihedral to use? Anyone been there yet?
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Old Apr 12, 2015, 08:23 AM
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No dihedral is necessary in a swept wing (or "chevron") design. Spiral stability is provided by the sweep angle. This is due to magic, because plank wings with no sweep also have spiral stability. "Magic" is defined as anything I don't intuitively understand.

Swept wings for thermal soaring do in fact work very well, but typically are best suited for cruising around a lot of sky to find the lift, rather than relying on slow sink speeds while waiting for a thermal to drift under your plane.

Wing design is subtle, but in general, thermal wings have more twist (root to tip washout) than slopers, and often have a high-lift airfoil root airfoil and low-camber or zero-camber airfoil at the tip.

If you must fly from a flat field, a bungee launch is simpler than a high start, which requires a double tow hook arrangement, as the wing is unstable when a tow hook location not far in front of the CG is used. A bungee with a short line attached well in front of the CG will work, but the launch height is gained by the velocity you can achieve off the line. It will be disappointing compared to a conventional planform's launch height off a high start, but better than a DLG.

An electric power pod can be used for launching, but the pusher configuration that is best adapted to swept wing planforms has some potential for cut fingers! A combination of bungee and electric power is good... if you don't have a slope in front of you!

Your best bet is to read everything you can on this forum. There are some real experts posting here. Especially refer to the excellent thread on the SW-11 Swept Wing Prototype by Knoll53, which includes good videos of bungee launches of a wing designed for thermal soaring.

Best of luck! If you stick with it, you will soon leave fuselages and rudders and stabs in your past!
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Old Apr 12, 2015, 09:59 AM
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:-)
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Old Apr 12, 2015, 10:00 AM
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Whoa! How do I delete this double-post?
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Old Apr 13, 2015, 12:12 PM
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Quote:
Originally Posted by EdSoars View Post
Whoa! How do I delete this double-post?
You can't.
but you can edit it to be 3 characters long, which explains the popular "..." post
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Old Apr 14, 2015, 08:12 AM
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It works!
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Old Apr 14, 2015, 10:23 AM
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Quote:
Originally Posted by RCWorks View Post
I want to build a 2 meter swept wing for thermals.

My question is how much dihedral to use? Anyone been there yet?
If this were a German forum, the question would be how much anhedral to use. I understand that .5 degree is popular. I build all of mine flat.
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Old Apr 14, 2015, 10:27 AM
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Originally Posted by Knoll53 View Post
If this were a German forum, the question would be how much anhedral to use. I understand that .5 degree is popular. I build all of mine flat.
On my kits it's about -.5...

We have them built on a flat surface which works out to a -.5
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Old Apr 14, 2015, 11:07 AM
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I have experienced spiraling down with dihedral on swept wing design. Flattening the dihedral got rid of that tendency.
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Old Apr 14, 2015, 11:54 AM
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Swept wings have inherent spiral stability from the sweep. Anhedral on higher aspect ratios keeps roll response more linear and quicker. I don't use aspect ratios above 12, so flat works best for me. It also has the advantage of having the same stability right side or upside-down!
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Old Apr 15, 2015, 04:56 AM
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Some years ago the "combat wings" were very popular. The Nurflugels major saving grace, "design economy", contributed to this popularity.

I used to fly them, although I was never much of a fan of combat flying.

I liked flying them with the tip fins removed. A length of strapping tape, applied with enough tension to the lower surface, would bend whatever anhedral angle I wanted into the foam wings.

I liked flying them with just enough anhedral to make them roll unstable. This is also the way I "tune" my hang gliders, so that while circling I have to constantly "high side" to keep the glider from exceeding my desired bank angle.

With the RC gliders, this meant circling flight with the inside wing having the least reflex, and the outside wing having the most reflex.

This configuration limited, or eliminated entirely any "adverse" yaw. I feel that I got better climb rates while thermaling with this set up.

I put "adverse" in quotation marks because I feel that an all-wing glider with no fuselage doesn't experience a slip or skid angle that is "adverse" It's just a slip/skid angle. It means that one wing has more sweep than the other wing. With no fuselage experiencing a lateral AoA that increases drag, what's "adverse" about it?

It is kinda fun (if you don't have to hike too awful far down the hill) to see the limits of using sweep as your only source of directional stability.

Simply perform a too-slow wingover, so that the glider experiences a slip angle somewhere around 90 degrees. Watch it descend rapidly in a spanwise dive, tumbling about the pitch axis, all the way to the ground.

My favorite high-performance hang glider is my Sensor 610F 135. It has a wide nose angle (low sweep angle), and a wedge-shaped vertical fin. That fin has saved my butt a couple times, because I get carried away by the joy of flight sometimes get a bit too "spirited" in my maneuverings. The tail fin pulls the nose back into alignment with the airflow and allows me to relax once again.

There's still a place in my heart for the Nurflugel, the pure flying wing.

But for practicality's sake, I've found that a fin improves visibility of an RC model so much that I'm now willing to sacrifice some of my idealism.

I highly recommend that those wishing to explore the aeronautic intricacies of this design build in a way to adjust the anhedral angle on-site. Joiner spars with different geometries can be taken to the flying site, and you can fine tune the glider for the various weather conditions. I prefer more anhedral for thermal flying, and a bit less for smooth ridge lift.

The Nurflugels possible aerodynamic advantages will likely never be realised fully without computer driven artificial stability. However, they can be configured to give great performance within a desired range of performance.

This range can be increased if you have a variable CG.

Have a nice time, but I'm willing to bet you one thing:

The primary purpose of the pure flying wing is to teach the value of a tail.

Birds have 'em. And they have "artificial" stability to boot.

The Nurflugels sure are beautiful and graceful in the air though. Unless the slip angle gets too far out of whack.
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Old Apr 15, 2015, 08:05 AM
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Dayhead: I'm glad the fin saved your butt, because otherwise you wouldn't have been able to make that great post! Educational!

Great suggestion about multiple joiner rods to test anhedral angles; will try it soon. Beats hammering on one at the flying site.

To "design economy" I would add "survivor economy". The damage resistance of swept wings is sometimes pretty amazing. However, I'm learning that a parts-simple wing contains some complex subtleties, what with spanwise airflow, etc.

Question: how does anhedral produce less reflex on the inside wing of a circling glider? I can't get my head to wrap around the geometry.

keeping my fins on,
ed
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Old Apr 16, 2015, 10:48 AM
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Ed, the anhedral doesn't create the reflex, it creates a stability profile that results in the pilot having to "high side" the stick while in circling flight.

I was referring to "up elevon" as reflex, and "down elevon" as camber.

The wing's sweep back angle creates a dihedral effect. The Nurflugel actually needs a noticeable amount of actual geometric anhedral before this sweep generated dihedral effect is neutralized.

What makes this all the more fun is that the dihedral effect varies with AoA.

I have contemplated a glider with in-flight variable anhedral. This would likely require momentarily going to a zero or slightly negative lift condition, moving an indexer, and then returning to normal flight with the new anhedral setting.

It would be fun to try controlling an established bank angle by simply varying the anhedral.
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Old Apr 16, 2015, 03:40 PM
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In-flight changes of CG and an-di-hedral scare me. The Swiss F3B team used variable CG years ago in the international championships, and the resultant changes in handling made their planes hard to control accurately. These were not flying wings though.

That dihedral effect varying with AoA is part of why chevron wings need yokes for histart or winch launches. Which in turn is why wings haven't been developed nearly as highly as conventional planforms, which in turn is why wings haven't been as popular (in general) as conventionals. Competitions dominate the high-performance end of our sport.

And maybe electric power and FPV flying is changing all that.
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Old Apr 16, 2015, 07:20 PM
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There is a balance between fin effects and dihedral. Too much dihedral results in dutch roll. Too much fin results in spiral instability. Since flying wings as a group tend to have trouble getting enough yaw stability, they can tend to be unusually sensitive to even small changes in dihedral and/or sweep.

Because the dihedral effect of sweep varies with angle of attack and lift coefficient, it's very possible to have a plane that has spiral stability problems at higher speeds AND dutch roll problems at lower speeds.

However, if the plane has a really bad dutch roll problem, enough to make it divergent in less than one cycle, it can look like a spiral instability problem.

Sweep acts like dihedral at high lift coefficients, but the effect decreases as lift coefficient decreases. All of this is true for essentially any airplane, not just flying wings. In the case of sweep, enough sweep to get good pitch stability in a tailless design can be enough to act like too much dihedral, causing dutch roll problems. Adding anhedral (such as on the VariEZE and LongEze, among others) cancels out some of the excess dihedral effect from the sweep.

I have Hi-started pure flying wings (no tip fins or vertical surfaces at all) successfully with a single tow hook in the usual position, just ahead of the C/G. One of them was my Klingberg 6 ft. flying wing, and Hi-starting was one of the few things it did reasonably normally. In just about every other building and flying characteristic it was the pure definition of "awful".
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