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Nov 30, 2021, 10:55 PM
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EdSoars's Avatar
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A thermal chevron wing?


Having recently rediscovered thermal soaring, i.e. F3RES, I've found that looking for thermals on calm, cold winter mornings beats cold, windy afternoons on the slope.

In designing fast, rugged slope planes I had moved away from chevrons due to the difficulty of making them sufficiently torsion-resistant to fly on the slopes. Then I saw the RES-Dart snag thermals at fingertip heights and it got me thinking. It's too windy here in the middle of the Rockies for flying ultra-light RES models in anything but near calm, so why not build a chevron that can fly thermals in breezy conditions? It wouldn't need to be covered in $50/yd carbon, and could be electric or hi-start launched.

The RES-Dart uses something similar to an AG-35 airfoil, with a hefty negative pitching moment, and uses 5 degrees of twist to overcome the negative pitch, as well as handle tight thermal turns. If I wanted to build a similar chevron that would get back upwind in 20mph breezes, could I use a near-zero pitching moment airfoil and 2-3 degrees of twist? I would use flaps for camber control, since I fly for sport only.
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Dec 01, 2021, 04:32 PM
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Dec 01, 2021, 05:04 PM
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EdSoars's Avatar
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Miniphase, whaddya mean,?? What about my burning question about twist and pitching moment? I've used Nur.exe for past designs using very slightly negative to slightly positive pitching moment (reflexed) airfoils and less than 3 degrees twist. Nur doesn't give me useable info about strongly twisted wings' performance. The wing in my brain at the moment will be much like your own zero-taper chevrons but adapted to thermal flying.
Dec 02, 2021, 08:03 AM
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Ed,

You know what my setup is: HS522 airfoil entire span, 30 deg sweep, no dihedral, 3 deg washout.

I've messed with XFLR5 and spreadsheet calculators but my feeble brain doesn't trust the results. I have a lot more faith in empirical data.

I just ordered a video camera to mount on a baseball cap so I can document my flights. I'll use it to record the initial flights of my new "thermal flying wing" when it's finished in about a week and post the results on my thread.

~Bryan
Dec 04, 2021, 06:25 AM
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Quote:
Originally Posted by EdSoars
Miniphase, whaddya mean,?? What about my burning question about twist and pitching moment? .


Quote:
Originally Posted by EdSoars
If I wanted to build a similar chevron that would get back upwind in 20mph breezes, could I use a near-zero pitching moment airfoil and 2-3 degrees of twist?
That sounds eminently sensible Ed... though I'd err more towards 3 degrees (all depend your target AUW and flying speed)
Dec 06, 2021, 12:30 AM
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The first article I ever saw about chevron wings was in the February 2006 issue of Quiet Flyer, by Neil Pollock. He advised low pitching moment, 2-3 degrees of twist. Sounds very familiar, eh, Bryan?

I like Uwe Heuer's 2m Marumba design: almost zero lift at zero flap and elevon to maximize top end speed, with flaps mixed to elevator, so up trim gives down flap. It's a nice way to extend the speed range for thermaling and landing. I call my smaller trial version the Marimba, since it's based on, but not identical to, the Marumba. Anyway, the Marimba is back on the workbench after a year's layoff.

Stay tuned.
Dec 06, 2021, 12:14 PM
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Bryan C's Avatar
Reminds me of the design philosophy of the Switchblade DLG.

The designer went from a high lift, negative pitch airfoil and significant washout to a neutral pitch airfoil and almost no washout. DLG launch speeds are much greater than thermalling speeds. His goal was to maximize the launch height at the sacrifice of thermal performance.

He also recommends snap flaps (flaps slaved to elevator).


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