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May 20, 2012, 10:30 AM
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miniphase's Avatar

washout vs inverted tip section

I'm hoping the more learned contributors can shed some light on the merits of using an inverted wing section at the tip of a swept 'wing (rather than using washout).

I'm thinking that a wing without washout will be more prone to tip stalling, but will have a wider speed range and better energy retention (ie less drag).

The tip stalling could be dealt with by using a thicker section (or blunter leading edge) at the tips and maybe wing fences.

I recall an 'On the Wing' article with some graphs showing the E205 section with a linear transition to the tip (ie symetrical at half semi span) and how at certain angles of sweep this would actually require wash in.

So any ideas on how to calculate what outboard portion of the wing needs to be inverted? (a product of pitching moment cg and sweep?) I'm thinking about dealing with the junction using a fence rather than a gradual transition along the semi-span.
Last edited by miniphase; May 20, 2012 at 10:36 AM.
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May 20, 2012, 10:43 AM
Registered User
miniphase's Avatar
found the graph....
May 20, 2012, 11:12 AM
HerkS's Avatar
Whatever arrangement of airfoils, sweep and twist (or no twist) you choose, it will be ideal for only one flight speed. (assuming weight and CG position don't change). For other speeds the elevator surfaces must produce reflex or camber to compensate.

So the whole question begins with the purpose of the design. You mention a wide speed range so I would be inclined to a more symmetrical tip section and fairly low twist. Let the elevator surface take care of adjustments to the airfoil shape at outer portion of the wing - when needed. Northrop's designs used symmetrical airfoils all the way and had only 4 degrees of total twist (zero degrees of aerodynamic twist and 4 degrees of geometric). High AR designs like yours would need less I think if the mission is for the plane to be speedy and agile - more if it was to be a floater.

I have seen models built with the inverted airfoil at the tip, but (personally) never thought it a particularly good idea. If I desired an airfoil with a positive Cm out there, I'd probably choose one that was designed specifically for that feature.
May 20, 2012, 11:14 AM
Cognitive dissonance
kcaldwel's Avatar
Keep in mind the zero lift line (the angle at which an airfoil makes zero lift) is about -3 degrees for the E205. If you just invert the airfoil and align the chord lines, you automatically have about 6 degrees of washout. It will look like you need wash-in using the chord lines as reference if the sweep angle is steep enough to need less than 6 degrees of washout. The Cm effect is secondary.

The E205 is very poor below Re 100k.

A better approach might be to rough out a planform, and select from Dr. Drela's AG airfoils for the best fit of Re and design Cl. If you need negative Cl near the tips, then an inverted airfoil will have lower drag to produce the negative lift.

Although this can be analyzed by hand, using one of the free computer analysis programs will get you an estimation of the results easier. As Herk said, you can really only optimize for one speed, but using good airfoils will give you the widest speed range.



If you are changing airfoils along the span, then it is always good to look at their zero lift lines as a reference. You can end up with a lot of effective twist in a wing otherwise, without really realizing it. The chord line is not aerodynamically significant, it is just convenient for airfoil construction. It is the zero lift lines that matter. There will be about 1 degree difference between the chord line and the zero lift line for every 1% of camber.

There will also always be added angle of attack as you move outboard on a swept wing, and even a straight one to a lesser extent.

Using the right airfoil for the design Cl and Re will add a lot of performance. Dr. Drela's designs typically use 3 or 4 airfoils along the span.
Last edited by kcaldwel; May 20, 2012 at 11:33 AM.
May 20, 2012, 12:54 PM
internet gadfly
nmasters's Avatar
Lippisch did this on his Storchs. It works but it can't be very efficient for the reasons already described (or can it?). What I don't like about it is that most airfoils don't have very good inverted characteristics. If you do this you should get the polars of your selected airfoil inverted to make sure that its stall isn't horrible. The L/D will most likely be in the toilet unless you start with something really low camber but ,hey, lots of form drag at the tips of swept 'wings contributes to directional stability so, if you can avoid a tip stalling monster, it should handle nicely.

BTW I was watching some videos of a friend's home movies and spotted an airplane that looked kind of like a Storch but with the help of the Nurflügel mailing list we identified it as the "Krischan" by Waldemar Beyer. It's the last plane in this video.

TSA Wiberg Part 11 of 11 (4 min 48 sec)
Last edited by nmasters; May 21, 2012 at 11:35 PM. Reason: Added a link to a drawing of the Storch IV
May 20, 2012, 01:08 PM
Registered User
miniphase's Avatar
The second of that series has colour footage of the HoIV (from about the 1min 19sec mark and again at 2min 10)
May 20, 2012, 05:25 PM
Cognitive dissonance
kcaldwel's Avatar
I think I got it backwards? Going from an upright to an inverted E205 with the chord lines aligned should give 6 degrees of aero wash-in?

Maybe the Cm effect is much bigger than I guestimated? I'll have to look at this in more detail.

Anyway, the bit about using the zero lift lines is right, I think!

May 20, 2012, 06:18 PM
Cognitive dissonance
kcaldwel's Avatar
I'm more confused than usual today. I think I had it right the first time?

Last edited by kcaldwel; May 20, 2012 at 06:38 PM.
May 20, 2012, 09:06 PM
Registered User
Frankly Mate, if yer looking for insight.. in here,.. then you are clearly on your own.
Read, learn, experiment.. a lot ! the Only realistic approach to genuine knowledge is to deal with it firsthand imo..
May 21, 2012, 12:30 AM
Crazy Cessna Owner
Robbie d's Avatar
Tail Saw, in the design of his 'Micro Elli' and possibly others, has used both washout and a 'negative lift' airfoil. the design flies well (although mine hasn't been flown yet).
May 21, 2012, 04:01 AM
the answer 42 is
Hey Paul,

long time no see from my brief experience I would say that the best on the tip would be to use a modern "horizontal stabilizer", airfoil HT series for instance. At the final section of the wing is working as Horiz Stab anyway. This prooved to work very well on my experiments using standard airfoils on swept wings, it might work as well in combination with FW airfoils

May 21, 2012, 09:38 AM
less is more
Knoll53's Avatar
Originally Posted by nmasters
BTW I was watching some videos of a friend's home movies and spotted an airplane ...
Now THOSE are home movies ! Cool.
Ask her to post more.
Finally, a good use for YouTube.

May 21, 2012, 12:48 PM
internet gadfly
nmasters's Avatar
Originally Posted by Knoll53
Now THOSE are home movies ! Cool.
Ask her to post more.
I wish I could. She died a few years ago. She seemed to know everybody who ever flew or built sailplanes in California. When I visited them we'd spend all day at Bob's hangar talking and then on the way back to their home she'd play tour guide pointing out where all the soaring pioneers lived. I miss her and Bob. I meet too many interesting people near the ends of their lives
Finally, a good use for YouTube.
There's a lot of crap on Youtube but there's also a lot of good information too. The TSA documentary is great. It's a shame that the film was so old by the time she copied it to video tape and then the tape was so old that the VCR had tracking problems in spots when it was copied to DVD. Here's a great old home movie that June would have loved:
Flying Wing Northrop N1M & N1M-2 (4 min 55 sec)
May 21, 2012, 01:14 PM
You know nothing....
Stuart A's Avatar
Thanks for that Norm;good music too for a change.
May 21, 2012, 04:17 PM
Cognitive dissonance
kcaldwel's Avatar
I ran a quick XFLR analysis just to make sure my original thinking was right. It appears to be. The airfoil Cm effect is small, the inverted E205 performs very poorly at the tip. 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.

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.

I ran a version with a symmetrical tip from the mid-span out to compare. I adjusted the twist for a similar level of stability and trim speed. The CG can be further back, because the tips work better. The lift distribution is much more elliptical, even with linear twist (from zero lift lines), the flow doesn't separate at the tips so it can fly much slower, and the performance is much better as well.

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.


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