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Under Chamber drag


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Posted · Report post

Hey Avideers:

Haven't visited in a long while and would like to direct Avid Flyer owners to a video I have on You Tube.

There has been much discussion as to how much drag is caused by the under-chambered wing on the Avids. So I did some tests with yarn trails under the wing especially at the front strut connection to the wing.

As you will see on the video, the yarn tells fly smoothly to the rear on take off at a high angle of attack, but as soo as I start to level out the yarn tells actually turn and face FORWARD. 

I kludged up a strut to wing fairing and as you will see on the video this minimized the under wing turbulence. I am not that skilled at fabrication so I didn't install them permanently on my Model B or on my current MK4, but am  have yet to put them on my MK4, but may do so soon.

Also, I need to put the yarn tells all along the underside of the wing to see if this turbulence is just at the strut or all along the wing.

These tests were done( on my model B which has since been totaled on a back country strip. I now fly a MK4.

It is quite obvious that proper wing/strut faring would substantially reduce some drag and give a few more nano knots of speed.

(See You Tube Skypics234 for video)

Lil' John

skypics234 at aol.com

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Posted · Report post

Just ran some more yarn tests and to my delight I found the only turbulence to be behind the junction of the forward wing strut and the wing.

 

The is very little turbulence behind the rear strut which surprised me.

the rest of the under part of the forward wing area shoed the yarn running streamlined so now I will get back to designing a wing strut faring.

 

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Posted (edited) · Report post

Harry Riblett, of wing rib design fame, ran tests on the Eppler modified undercamber ribs many years ago.  I don't know if that was a computer analysis or actual wind tunnel tests.  He said that about 90 mph a bubble formed in the undercamber and caused enough drag to keep the wing from flying faster than that with standard power.  He redesigned the rib for the Kitfox 4 and the later models have flown at around 120 mph, although one might wonder if that was with more HP than the 582 engine.  His letter and rib measurements have been posted on this site.   It will be interesting to see what you can find out on the drag.   EDMO 

Edited by EDMO

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Posted · Report post

I can say one thing about the Kitfox 4 wing.  It is definatly a faster wing than the undercambered ones.  The Kitfox 4 I flew in out of Pennsecola with a 503 clipped right along.  At least as fast if not faster than the Avid with way less HP.  JImChuk

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