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Old May 29, 2016, 03:22 PM
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Prandtl BSLD design

Although our swept wing designs with elliptical lift distributions and tiplets fly very well, It is always encouraging to discover room for improvement. In this case, it is the Bell-Shaped Lift Distribution discovered by Ludwig Prandtl back in the early nineteen-teens, expanded by the Horten brothers on into the 1950's and now being further fleshed out and explored by the NASA Prandtl Project under Al Bowers. The possibility of eliminating two significant sources of drag and simplifying the structure even more is enticing.

Right now, there is one major obstacle to progress in the modeling community: how to calculate the non-linear twist and elevon dimensions needed to create yaw stability and eliminate adverse yaw.

I hope this thread will attract contributions from some of our more advanced aerodynamicists (I can't even pronounce it) and get more of us TLAR designers building these fascinating wings.

It would be a dream come true to have a spreadsheet like Nur.exe to enter tip and root chord, and sweep values; airfoil zero lift incidences, span, sweep, and weight, and

Here's a kick-off question; to what extent can I use nur.exe to lay out a BSLD wing? Let's say it's to be a 2 meter span wing aimed at thermal soaring, so probably using a design lift coefficient of about 0.5. I chose thermal soaring because these pure wings are very hard to see at distance if they aren't overhead. And maybe we should stick to readily available airfoils that can be produced in either balsa or foam.

How about it? Iron Eagle has been very helpful in Private Messages, but it's time to unleash The Power of NURFLUGEL!

May the Twist be with you!

ed
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Old May 29, 2016, 08:47 PM
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Quote:
Originally Posted by EdSoars View Post
Right now, there is one major obstacle to progress in the modeling community: how to calculate the non-linear twist and elevon dimensions needed to create yaw stability and eliminate adverse yaw.

I hope this thread will attract contributions from some of our more advanced aerodynamicists (I can't even pronounce it) and get more of us TLAR designers building these fascinating wings.

It would be a dream come true to have a spreadsheet like Nur.exe to enter tip and root chord, and sweep values; airfoil zero lift incidences, span, sweep, and weight, and
There's a spreadsheet at the bottom of this page for that. The author of that spreadsheet, Marko Stamenovic, had an article in the March 2016 issue of Radio Controlled Soaring Digest. There's another similar spreadsheet in the links at the bottom of this page. Also XFLR5 has recently added a sin^n curve to the <Show Target Curve> option in the <Wing and Plane Design> window.

--------.~.
--------/V\
------//----\\
-----/(------)\
----(^^)---(^^)--Norm
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Old May 29, 2016, 09:23 PM
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This looks perfect. Thanks Norm! Onward to building techniques...

Looking over the Horten wings on the Show Your Nurflugel thread, almost all are built-up, I guess due to the non-linear twist. Looking at the twist distribution on the Nest of Dragons spreadsheet recommend by Norm above, it looks like a decent approximation can be done with a hotwire on a single panel, using a wedge to force the blank into the departure from linear.

Others may not have my structural constraints of extreme durability and simplicity of build.
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Old May 30, 2016, 01:43 PM
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Ed,
Cant wait to see what you come up with.
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Old May 30, 2016, 10:18 PM
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I can't wait either! But there seem to be a lot of chores stacking up...

These Horten/Prandtl design parameters are WAY different than what I'm used to. For instance, the static margin on my "conventional" swept wing designs is 5%. The Horten IV uses 13%! At least that is what is shown for the 5 meter monster in the Nest of Dragons spreadsheet by Marco Stamenovic. The taper ratio is 0.2, that is, the tip chord is 20% of the root. I am used to no taper or at most the tip chord is 75% of the root.

Horten wings use high-lift, high-camber, high pitching moment airfoils at the root, transitioning to a low pitching-moment (or symmetrical) airfoil at the tip with lots of twist (8-10 degrees); I use low-pitching moment airfoils from root to tip with little twist (2.5 -3 degrees).

Even the elevons will be different: I've been using constant-chord elevons; the high-twist wings can use an elevon that tapers from almost zero inboard, up to the full tip chord outboard.

This will be interesting. Perhaps the best approach is a small test bed to get used to the more complex ture-nurflugel designs.

Off to the spreadsheets...
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Old May 31, 2016, 03:08 AM
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Ed,

I built 2 Horten wings several years ago the 100" has 8 degs. of twist. It flies great. The 110" version I used 10 degs. of twist. It to flies very well. the big one has no yaw problems. Attached is the basis for these wings. they both are hot wired foam with 1/16" balsa sheeting.
Very easy to make. Each panel is cut in 3 sections, with progressive twist. So short panels to hot wire.


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Old May 31, 2016, 06:47 AM
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Untitled (1 min 29 sec)


Here's a video of a bungee launch of the 110" Horten. Crappy windy day. But I flew it anyway. I landed exactly on the bungee ring. Videos of the small and large Horten. Plus my CO 8.

Vern
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Old May 31, 2016, 06:18 PM
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Quote:
Originally Posted by EdSoars View Post
...These Horten/Prandtl design parameters are WAY different than what I'm used to. For instance, the static margin on my "conventional" swept wing designs is 5%. The Horten IV uses 13%! At least that is what is shown for the 5 meter monster in the Nest of Dragons spreadsheet by Marco Stamenovic. ...
Be careful. Is that the static margin for the MSU tests, or the original as-designed static margin? The MSU aircraft had a fairing added over the nose skid, which added so much lateral area to the nose that it forced the plane into a badly noseheavy condition to maintain yaw stability. This in turn messed up the lift distribution so badly that it substantially impacted performance.
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Old Jun 01, 2016, 09:22 AM
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Thanks Don and Vern! I'm encouraged.

Here is the first-pass layout. I started designing with Nur.exe to get the basic parameters of geometry, size and wing loading in the ballpark, then put the geometry into NOD (Nest Of Dragons) spreadsheet.

Enough graphs to satisfy a real Horten geek. Let me know how they look. There is good agreement between Nur and NOD in the numbers, but the shape of the curves is different.

This will be a 1.6 meter wing with no flaps to start out.

Vern, I hope to be able to clamp the foam blank onto a trailing edge wedge that provides the non-linear twist. We'll see how it goes. The other possibility is to cut the basic linear twist first and add the extra twist that departs from linear when I sheet the wing.

Haven't decided whether to use balsa sheet or just FG + CF tow. It has to be simple. The extreme taper should limit flutter.

The reverse-tapered elevons have worked for me before, although they tapered to 50% of the tip chord, not 80%. They could be easily modified.
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Old Jun 01, 2016, 12:53 PM
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Maybe gamma is a sin^2.5 lift distribution?

-Dave
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Old Jun 01, 2016, 05:40 PM
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The Greek letter Gama stands for circulation. Basically Gamma is the local lift. Find Gamma for several points along the span and fit a curve through them to see the lift distribution.
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Old Jun 01, 2016, 10:10 PM
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Yet the lift distribution curve shows a reduction in Cl at the root, with the max Cl at about 25% span, while the Gamma curve shows the max Cl at the root. There's something fundamentally different between the two curves.

I don't know what the sin curves are. ?? What do the different powers mean?

Vernon: as I find time, I'll enter the Kolibri into both Nur and NOD to see how they compare. The plan shows 4 sections, not three, unless I misunderstand the plan. ??? What are the airfoils? Alpha zero and Cm/4 for the airfoils would be nice, but not totally needed.
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Old Jun 02, 2016, 12:56 AM
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Yet the lift distribution curve shows a reduction in Cl at the root, with the max Cl at about 25% span, while the Gamma curve shows the max Cl at the root. There's something fundamentally different between the two curves.

I don't know what the sin curves are. ?? What do the different powers mean?

Vernon: as I find time, I'll enter the Kolibri into both Nur and NOD to see how they compare. The plan shows 4 sections, not three, unless I misunderstand the plan. ??? What are the airfoils? Alpha zero and Cm/4 for the airfoils would be nice, but not totally needed.
Both builds are on here.Vern has a great tow spar system.
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Old Jun 02, 2016, 06:55 AM
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The Greek letter Gama stands for circulation. Basically Gamma is the local lift. Find Gamma for several points along the span and fit a curve through them to see the lift distribution.
Norm, thanks for the reminder. I tend not to think of circulation as it was not really a part of my (long past) education.



-Dave
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Old Jun 02, 2016, 06:56 AM
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Yet the lift distribution curve shows a reduction in Cl at the root, with the max Cl at about 25% span, while the Gamma curve shows the max Cl at the root. There's something fundamentally different between the two curves.
"Middle effect" ...?

Quote:
I don't know what the sin curves are. ?? What do the different powers mean?
Ways to define the lift distribution. Sin distribution is for an elliptical distribution, sin^3 is for a BLSD.
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