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Old May 21, 2012, 06:26 PM
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Originally Posted by kcaldwel View Post
Despite the 6 degrees of aero washout from the inverted tip airfoil, I still had to add 1 degree more washout to get it to trim with a stable CG position. This doesn't agree with the original chart very well, but I'm not sure how they derived that.
That appears to be a page from an article in the "On The 'Wing" column of Radio Controlled Soaring Digest. The data is derived from the Panknin twist formula. As I understand it that formula is empirically derived. I've always suspected that it doesn't fully account for sweep effect which actually decreases the lift slope on the center section and increases the slope near the tip. Taper also has that effect so combining sweep and taper adds a bit of -Cm regardless of the airfoils chosen. An Eiffel tower planform with wash-in on the center section and wash-out on the outboard panel can fix that.

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The lift distribution with the inverted airfoil ends up a bit bell shaped, and the the tip separates on the top surface and stalls before the glider really slows down.
Yep. The mean line at the leading edge slopes up and the upper surface is too flat. You could fix that by sloping the mean line down on the front 15 or 20% of the airfoil but then you'd have a reflexed section.

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Unless the tips were lifting down with a substantial negative Cl at all flight speeds, using an inverted cambered airfoil at the tips will be a bad idea. If the tips have that much down load on them, then the overall performance will be poor in any case. I can't see how it would ever be a good idea.
That's probably why the full size designers stopped experimenting with inverted tip sections around 1930

--Norm
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Old May 26, 2012, 02:09 AM
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i know on a normal wing/tail aircraft, the washout helps keep the tips from stalling as they have a shallower aoa and a lower stall speed as a result. so if they had inverted airfoils on the tips, the tips would not create the lift as the center section stalls, instead they keep pushing down, deepening the stall and *crunch* thats the end of that experiment lol
does that sound right guys?
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Old May 26, 2012, 11:36 AM
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Airfoils produce lift both ways, no matter what the direction the camber is. An inverted airfoil just isn't very efficient at producing upwards lift - it's drag is higher than an upright cambered section when producing upwards lift, and the stall AoA from the chord line is lower. An airplane with a cambered airfoil can still fly inverted.

I have a little plank flying wing with an upside down Clark Y airfoil that flies pretty well. It doesn't need a reflexed airfoil because the negative Cm of the Clark Y becomes a positive Cm when it is inverted.

The stall behaviour of the airplane would likely not be noticeably different. You would still want to set the relative angles, including the induced angle of attack especially on a swept wing, so that the tips aren't stalled when the centre section stalls to prevent tip drop and incipient spins.

Kevin
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Old Jun 06, 2012, 09:10 AM
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Quote:
Originally Posted by kcaldwel View Post

I have a little plank flying wing with an upside down Clark Y airfoil that flies pretty well. It doesn't need a reflexed airfoil because the negative Cm of the Clark Y becomes a positive Cm when it is inverted.

Kevin
So referring this back to my original question; would your plank have been 'more draggy' if you had used the Clark Y right side up and just reflexed the trailing edge for stability....I suspect it would have
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Old Jun 06, 2012, 09:44 AM
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Good quetion!

The Clark Y has a lot of camber, so it needs a lot of reflex to try and get the Cm positive. The inverted Clark Y has quite a large positive Cm which requires even more reflex to try and match, which seems to be the only fair comparison.

The XFOIL results for Cm is very approximate at the best of times, and with with that much reflex I'm not sure I trust it at all, but it looks like the inverted Clark Y has lower drag at low lift coefficients - less than 0.2 - but the upright airfoil with reflex is better at any higher Cl, and also has a much higher max Cl.

I believe a properly designed reflexed airfoil will be better than any inverted cambered airfoil.

Kevin
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Old Jun 06, 2012, 09:30 PM
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I think in the end it becomes a compromise. Reflexed may be more efficient, but you have progressive twist and need to find a straight line for hinging purposes, The reflex may make this slightly harder to achieve.
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Old Jun 06, 2012, 10:57 PM
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If you have a swept wing, probably neither a reflexed section or an inverted one is best. A symmetrical tip, or a cambered section all along the span with section changes for the local Cl and Re would be best. Maybe that makes the hinge line easier.

Kevin
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Old Jun 07, 2012, 02:58 AM
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Interesting concept guys.

I'm into slope and DS, always after max efficiency (speed) over a broad wind range an air speed. I like the idea of replacing wing twist with a symmetrical tip.

This is a 41" Starfire EPP wing with 1 degree wing twist on a PW51 airfoil. I've had it flying in winds from 5-40 knots (wide range).

Do you think I could squeeze a little more speed (less drag) out of this type of wing over a wide speed range, by using symmetrical tip as opposed to washout ?
.
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Old Jun 08, 2012, 02:44 AM
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if you had a semi-symmetrical tip, and could fix that to a rod to twist the wing, ala wing warping, you could adjust the amount of washout for the flying speed.
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Old Jun 08, 2012, 05:12 AM
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sounds feasible, but I think I'll be to busy just flying acro to adjust for speed.

I'm doing a similar build (all PW51 airfoil) with the outer panels having zero LE sweep (like a plank with a pointy nose section). I should be able to get away with zero wing twist on that shouldn't I ?
Slope/DS planks usually have zero washout right ?
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Old Jan 12, 2013, 11:37 PM
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A couple days ago I ran across NACA-TM-564 "Recent tests of tailless airplanes" by Alexander Lippisch, published 1930. This paper describes the Storch series of aircraft that Lippisch developed in the '20s. These tailless aircraft used inverted airfoil sections at the wing tips.

Holy zombie thread, Batman

--Norm
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Old Jan 14, 2013, 10:09 AM
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Good find Norm. I thought I was doing something new when I built my Boomerang in the '90's. I guess I invented it 60 years after Lippisch did!

I don't think there's a reason to do this unless your tip section is behind the cg.

My first real flying wing had a bunch of reflex and while it flew ok, it would not loop because moving the trailing edge that far up caused it to stall. Also, reflexed airfoil wings tend to change trim a lot with speed. When I designed the Boomerang, I figured there must be a better way to trim the plane. The tip had to push down somehow and the usual solution was with either reflex and/or twist. I figured what could be better at pushing down than an inverted airfoil? This seemed like it would have less drag flying in this condition and I was building a pylon racer, so that's what I did.

I've learned a lot since designing the Boomerang, so I'm not sure I would use a regular airfoil and just stick it upside down again, but I can't ignore how well it worked either. Airplanes all basically work the same. They have to be nose heavy for stability and then something either has to hold that nose up, like a canard, or push down at the back. Swept flying wings can do either or both of these things to find trim. BTW, the Mubu uses a center elevator, so it's working like a canard.

Steve C
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