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Old Jan 15, 2016, 02:24 AM
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PRANDTL-D No. 3 Takes Flight (1 min 45 sec)


NASA Armstrong PRANDTL-D Interns, Summer 2015 (5 min 48 sec)
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Old Jan 15, 2016, 08:09 AM
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Thanks nmaster...very interesting.....any idea of airfoil used...???
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Old Jan 15, 2016, 12:24 PM
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No idea what the airfoils are but that's not the key to this design. The overall wing geometry and lift distribution are. All airfoils have approximately the same slope to the cl over alpha curve so once you know the zero lift AoA you can calculate the washout and planform needed to get a desired lift distribution. If the airfoils have negative pitching moments the wing will need more washout so the airfoil Cm should be close to zero or a bit positive. There are already lots of low Cm airfoils designed for model scale Reynolds numbers so they didn't have to design new ones but since this is a student project it wouldn't be surprising if they did. Hopefully the Prandtl-D technical report will be available soon and we can read all about it.

--------.~.
--------/V\
------//----\\
-----/(------)\
----(^^)---(^^) --Norm
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Old Jan 15, 2016, 04:11 PM
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thanks .....i'm waiting for....seem to be a reflexed
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Old Jan 16, 2016, 09:12 AM
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At low Reynolds numbers it's common to have the aft portions of the upper surface flat or even very slightly concave to minimize the adverse pressure gradients back there and reduce the tendency for airflow separation. This does not make those airfoils "reflexed", the camber lines are still flat or concave-downwards back there.
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Old Jan 16, 2016, 01:49 PM
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Also notice the shape of the elevons on the big plane in the first video above. The short and tapered elevons are not a Horten feature, they're based on Edward Udens experiments. Here's a table of his results:
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Old Jan 16, 2016, 03:41 PM
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Interesting certainly.
But what purpose in the single wing design.. beyond a training exercise for volunteers.?
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Old Jan 16, 2016, 10:10 PM
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Quote:
Originally Posted by nmasters View Post
No idea what the airfoils are but that's not the key to this design. The overall wing geometry and lift distribution are.
Quote:
Originally Posted by Bare View Post
Interesting certainly.
But what purpose in the single wing design.. beyond a training exercise for volunteers.?
Watch this video (and listen to Norm!) if you want to understand why they are running these tests, it's pretty interesting. The real-world applicability is hotly contested of course, but we science these things because they are awesome.

NASA's Albion H. Bowers - " Why Birds Don't Have Vertical Tails" - AMA EXPO 2014 (38 min 48 sec)
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Old Jan 17, 2016, 03:44 AM
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Quote:
Originally Posted by Don Stackhouse View Post
At low Reynolds numbers it's common to have the aft portions of the upper surface flat or even very slightly concave to minimize the adverse pressure gradients back there and reduce the tendency for airflow separation. This does not make those airfoils "reflexed", the camber lines are still flat or concave-downwards back there.
thanks guys for interesting infos

Yes Don Stackhouse.....you seem right....
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Old Jan 17, 2016, 10:34 AM
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Quote:
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Also notice the shape of the elevons on the big plane in the first video above. The short and tapered elevons are not a Horten feature, they're based on Edward Udens experiments. Here's a table of his results:
All of this is a fall-out from Prandtl's work on BSLD's, and the elevon span is a result of that span load. The Hortens used multiple elevons covering most of the span for some combined effects needed to preserve the lift distribution shape at off-design-point operating conditions during elevator inputs, but the same thing that limits elevon span for roll inputs applied to their designs as well.

Note, not all of their earlier designs in particular used a BSLD, but the later ones did.

As I may have mentioned here before, one of the main effects the Hortens were exploring was the use of a BSLD to minimize or eliminate adverse yaw, which would then eliminate the need for a vertical fin and rudder. Prandtl's original work with BSLD's was just to reduce induced drag below what could be achieved with an elliptical span load. He was looking at it just as an application on a straight, unswept wing on a tailed aircraft. The Hortens were the ones who found that by implementing Prandtl's BSLD as a swept, tapered wing they could eliminate the need for a tail, and all the drag and weight that went with it. Bare, that's also the answer to your question, by using the same approach as the Hortens, performance can be improved for a given weight, which makes it a better approach for its intended mission, which in this case is as an aerial surveyor on a Mars probe.

Getting back to the elevon span constraints, due to the BSLD there is a portion of the tip section of the wing that actually produces induced THRUST instead of induced drag. If the elevon span is short enough to stay within that zone of the tip, aileron inputs will cause proverse yaw (into the turn) instead of the adverse yaw (away from the turn, which requires a rudder input to correct) typical with ailerons on an elliptical lift distribution.

However, we generally want zero yaw with an aileron input. To achieve that, we need the proverse yaw from that negative-induced drag (i.e.: induced thrust) portion of the tip, plus just enough of the adverse yaw from the positive induced drag zone just inboard of there, so that the two exactly cancel each other out.

This is all related to the shape of the lift distribution, and gives you a very specific amount of elevon span required for the yaw from aileron inputs to come out exactly zero. This tends to result in fairly short elevon spans, and if you try to increase the elevon span beyond that amount, the elevons begin causing adverse yaw, which then interacts with the sweep to cancel out the rolling effects of the elevons. Yes, that means if you increase the elevon span beyond the optimum amount needed for zero-adverse-yaw, you can actually make the roll response WORSE.

However, in my experience at least, keeping the elevon span at the zero-adverse-yaw span generally results in excellent roll rates and aileron response, with no need for any more aileron authority than that.
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Old Jan 20, 2016, 03:02 PM
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does The Prandtl-D use tip washout.???
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Old Jan 20, 2016, 05:07 PM
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Quote:
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does The Prandtl-D use tip washout.???
Yes. Very.
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Old Jan 20, 2016, 06:19 PM
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BSLD's like that used for the Prandtl-D generally use very complex and non-linear washout, which is present in varying amounts over the entire wing, but tends to be highest at the tips. A BSLD tends to have lift values near zero for the region near the tips, and if you don't want to have ridiculously low chords (near zero) at the tip region, that means lots of washout in that area. I have made models with a BSLD that's implemented strictly through chord, with near-zero washout, but the tips end up looking like flat toothpicks. Extremely low Reynolds numbers, and not very practical from a structural nor an aerodynamic standpoint.
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Last edited by Don Stackhouse; Jan 20, 2016 at 06:25 PM.
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Old Jan 21, 2016, 12:41 AM
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very interesting.....

any links about BSLD..??
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