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Old Sep 24, 2008, 03:58 PM
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Hamburg, Germany
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Aerodynamics of Flying Wings and Planform/Airfoil Issues

Hey Guys!
If if figured it out correctly the following is what i think to know:
Sweep is primarily applied to achieve Yaw stability and in combination with
washout it provides a positive zero lift pitching moment.
But you can also achieve a positive pitching moment by using reflexed airfoils.
Without any sweep. But to have better stabilty on pitch and yaw you use sweep anyways.
This leads me to the follwing question:
Reflexed Airfoils and washout serve the same purpose?!
If so, do you combine them or use just one of them???
Greetings,
Marvin
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Old Sep 24, 2008, 04:53 PM
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Depends on the sections you are using. The more lift the root section produces, the more washout you will need at the tip as a general rule. The more 'lifty' foils tend to produce a stronger, negative pitching moment.

If you cut a wing without enough washout, you can overcome the negative pitching by reflexing the elevons, this does tend to produce more drag and affect handling though. Most commercial epp wings are cut without any washout, instead they rely on the tapered elevon being reflexed up a few degrees. The taper therefore providing more reflex at the tip- if you like, an artificial way of producing linear washout
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Old Sep 24, 2008, 04:55 PM
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if you know the pitching moment, zero lift angle and camber of the sections (along with the dimesions of your wing) you can use the spread sheet from the Bsquared website to calculate the washout for you. I think there is also a German language programme on dasnurflugelteam website.
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Old Sep 24, 2008, 05:07 PM
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Hamburg, Germany
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Quote:
Originally Posted by miniphase
if you know the pitching moment, zero lift angle and camber of the sections (along with the dimesions of your wing) you can use the spread sheet from the Bsquared website to calculate the washout for you. I think there is also a German language programme on dasnurflugelteam website.
I used a program for matlab called tornado.
Now i am using Nurflügel. It is better for the purpose of designig wings.
Right now i am tweaking wings it that program and try to understand what difference diffrent airfoils make.
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Old Sep 24, 2008, 07:48 PM
Herk
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Virginia USA
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Marvin -- Reflexed airfoils have a couple of disadvantages for models.

First, they are usually thin at the rear and difficult to build accurately and strong.

Also, the reflex produces a slowdown of the flow on the upper surface which has a negative influence on both lift and drag with low Reynolds Number flow.
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Old Sep 24, 2008, 10:34 PM
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Actually, rearward sweep generally produces more roll stability than yaw but it is not really used in our models for either purpose. Sweep is typically used in a flying wing as a way of increasing the distance between the A.C. and the upward angled trailing edge (at the washed out wingtips). Plus you have a great place to mount fins, which maybe kinda work as winglets as well.

One of the core requirements for pitch stability is Cm0>0, which means that at a zero-lift angle of attack the aircraft must tend to produce a pitch-up moment measured from the aerodynamic center. For wings the A.C. is always at the 1/4 chord, for other planes it is in other places.

There are then basically 3 approaches to flying wing stability:
1. Use a conventional high-lift, Cm<0 (pitch down) airfoil, sweep it, then twist it a hell of a lot to create the pitch up force. A good example of this is the Boeing Scaneagle but this approach is seldom seen in models.
2. Use a fairly neutral airfoil with a little bit of sweep and twist (2deg) like most "Zagi" type wings. Great stability and ruggedness at the expense of drag and tip-stalls from the sweep.
3. Use a Cm>0 (reflexed) airfoil with no sweep. Very little pitch stability is possible with this approach and these designs are extremely sensitive to elevator trim/slop/damage but have the best performance.
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Old Sep 25, 2008, 12:53 AM
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Quote:
Originally Posted by somt1033
Hey Guys!
If if figured it out correctly the following is what i think to know:
Sweep is primarily applied to achieve Yaw stability and in combination with
washout it provides a positive zero lift pitching moment.
Sweep allows the tips to perform the same function as a conventional empennage. The sweep has the same influence as the tail moment arm of a conventional airplane i.e. The dynamic stability is proportional the square of the distance from the AC of the lifting surface to the AC of the stabilizing surface(represented by the blue lines in this drawing).

Quote:
But you can also achieve a positive pitching moment by using reflexed airfoils.
Without any sweep. But to have better stabilty on pitch and yaw you use sweep anyways.
Yes, in pitch but not yaw. A reflexed airfoil can provide static stability but not dynamic. Sweep + twist provides both.


Quote:
This leads me to the follwing question:
Reflexed Airfoils and washout serve the same purpose?!
To some extent, yes.


Quote:
If so, do you combine them or use just one of them???
That's a design decision. The designer's choices can get pretty complicated. That's what makes flying wings more interesting than “normal” airplanes

--Norm
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Last edited by nmasters; Sep 26, 2008 at 12:02 AM. Reason: added "in pitch but not yaw"
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Old Sep 26, 2008, 03:46 PM
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Swept wings

There is another approach to "wings" which is slightly more efficient.
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Old Sep 26, 2008, 03:52 PM
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"Straight" wings.

Sraight, reflexed wings work ok for full size also!
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Old Sep 26, 2008, 11:51 PM
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Neat! I hadn't seen that picture of the Facet Opel before. Most I have seen are unpainted

--Norm
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