NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This?

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Norman

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

For those of you who aren't already aware, Koen Van de Kerckhove over at Nest of Dragons was provided the specs for the Prandtl-D by Albion Bowers and subsequently built both several scale models and the molds for a full-sized sailplane. You can find the spreadsheet of his build- containing that data -here:

http://www.nestofdragons.net/media/44663/2016-03-10-dragonwing-shared-information.xls
Why, yes, there has been some interest in that sort of thing. Welcome to HBA, Mavigogun. It's a great forum.
 

Kingfisher

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

Sounds like a ridiculous number to me even for airliners with relatively huge tails.

I don't buy it. Fundamental issue is that the math/theory that's pertinent to this is called lifting line method for a reason. It is founded on a planar wing. A BSLD is a tiny bit superior under some conditions compared to an ideal elliptical wing. But a wingletted wing is far superior, span restrictions or not and will be both lighter and have less profile drag than a down-lifting tip. Note that every single airplane in existence with an AR over 35 (and no span limitations) uses winglets.

We see this in practise as well. No design I'm aware of uses down-lifting tips, military, airliners or flying experimental.

It certainly makes sense if you're constrained to a planar wing, due to stealth for example, but that's the only condition upon which it makes sense to me. If you can have kinks in the wing, a winglet, if tailless a C-tip makes way more sense. That's why those futuristic Boeing/Airbus renders look the way they do.

Birds most definitely have stall/spin problems. I've seen dozens of them stall/spin in thermals. Many even use it to dive instantly.
Without trying to understand the math and theory, my gut feeling tells me the same. Why not have a vertical fin and winglets, especially if one needs a fuselage to carry the pay load anyway, and additional wingspan just takes up more space on the ground. I think it is important to note that for a wing with the same span, the built in twist resulting in a BSLD will always result in less lift, so common sense says add a tail and make the wing create all the lift it can.

On the other hand, looking at nature is always interesting. Why is it that no recent aerial animals (birds and bats) have evolved vertical fins (other than for display, like cockatoos)? Many aquatic animals have, for example dolphins and sharks (and almost all fish for that matter). Even manta rays maintain a small dorsal fin near the tail. Interestingly faster fish have less fins sticking out than slower ones (or at least they can fully retract all non-essential fins for fast swimming, like tuna and marlin), so that they look more like dolphins. And then some pterosaurs did have vertical fins, but they were at the back of their heads!?
One important difference between aquatic and aerial propulsion is that in water a creature does not generate much lift, but in the air it does. So does one conclude that generating lots of lift is most efficiently done by a single planar surface that does everything including stability? It is interesting that the rudder on an aircraft is really not needed to fly an aircraft, it just “enhances” the turns and helps steering on the ground. So it seems an aircraft is indeed improperly designed, and if the wing twist can take the vertical fins function, maybe it should. If airliners were just huge flying wings, they could potentially hold a lot of freight and people, too, but would definitely not fit into existing airport structure.
 

autoreply

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

Birds have one advantage, they can be unstable and be made stable by active control. Just like blended wings, that don't work without a V-tail, unless you have a computer flying it (think the B2 bomber).

That's a huge potential gain in aircraft efficiency we haven't explored so far, except for fighters (F16 paved the way). The other difference with birds is that on their scale Dutch roll (pun intended) isn't significant.
 

Norman

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

On the other hand, looking at nature is always interesting. Why is it that no recent aerial animals (birds and bats) have evolved vertical fins (other than for display, like cockatoos)? Many aquatic animals have, for example dolphins and sharks (and almost all fish for that matter).
:ponder: Submarines also have a dorsal fin. Without a big conning tower a sub will roll excessively with rudder input. Fish probably wouldn't mind banking into the turns but submariners find it very inconvenient. Large "wings" at the CG, like a fish's pectoral fins, could also counter this rolling moment but they would get in the way of docking and they need a place to stow the periscope so a conning tower serves multiple needs. It's also a handy place for the captain to stand when it's on the surface.

Even manta rays maintain a small dorsal fin near the tail.
Actually the main dorsal fin has completely disappeared from the rays. Most fish have a smaller second dorsal fin (and anal fin) at the base of the tail. The second dorsal fin probably does function exactly as an airplane vertical fin with the tail itself functioning as a propeller.

One important difference between aquatic and aerial propulsion is that in water a creature does not generate much lift, but in the air it does. So does one conclude that generating lots of lift is most efficiently done by a single planar surface that does everything including stability? It is interesting that the rudder on an aircraft is really not needed to fly an aircraft, it just “enhances” the turns and helps steering on the ground. So it seems an aircraft is indeed improperly designed, and if the wing twist can take the vertical fins function, maybe it should. If airliners were just huge flying wings, they could potentially hold a lot of freight and people, too, but would definitely not fit into existing airport structure.
On most airplanes the fuselage creates more than 1/2 of the zero lift drag. When you get to really big airliner sizes it's possible to build a wing with enough room for people to walk around inside it. At that point it becomes economical to switch to a flying wing. The BWBs currently under development all have vertical fins and are still too big for the terminals. If there's ever a flying wing airliner it will probably have folding wing tips. That's not as far fetched as it sounds. The 777X has folding wing tips as an option but as far as I know nobody has ordered it with that option, probably because the fold mechanism weighs as much as several passengers.
 

Mavigogun

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

Welcome to HBA, Mavigogun. It's a great forum.
Thanks! That it is. I've been lurking for some time while conducting my own informal education- a never ending process; during that time, I've appreciated your contributions here and on a number of other forum for flying things.

I think it is important to note that for a wing with the same span, the built in twist resulting in a BSLD will always result in less lift, so common sense says add a tail and make the wing create all the lift it can.
Without confronting or testing that "always", there is no "common sense"- only design goals, right? Sink and climb rates, best glide and best glide speed- I reckon aside from handling, one might have good reason for favoring best glide over maximized lift, powered or soaring.

---

Actually, it seems that "always" is a bit of a mental thorn I'm not past yet. My understanding is that "twist" is tailored to account for changes in airflow induced by the wing itself- so, the airfoil can be presented to the flow/"relative wind" to capture a particular quality or qualities- in which case, there would be an ideal shape for maximizing lift *at that point on the wing*- and that shape wouldn't likely be uniform along the length of the wing, but adjusted to suit the local flow. Thoughts?
 
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timberwolf8199

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

...additional wingspan just takes up more space on the ground. ... If airliners were just huge flying wings, they...would definitely not fit into existing airport structure.
Similar thoughts had occurred to me. Below is the sequence that my mind took:

1) The outer extents of the span are not contributing to lift but rather to control. The size of the lifting portion of your wing is approximately the same (assumed), therefore your design trade is increased span for reduced drag.

2) Added span could make it more difficult to fit these designs into storage and/or move them around on the ground

3) Birds get around this difficulty by having folding wings

4) One of the points he makes is that the structural requirements of a wing are reduced

5) Perhaps the weight reduction made possible by the efficiency gain (assumed benefit) could then be consumed by a folding mechanism that results in an equal weight, equal storage, high efficiency design.
 

Norman

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

My understanding is that "twist" is tailored to account for changes in airflow induced by the wing itself- so, the airfoil can be presented to the flow/"relative wind" to capture a particular quality or qualities- in which case, there would be an ideal shape for maximizing lift *at that point on the wing*- and that shape wouldn't likely be uniform along the length of the wing, but adjusted to suit the local flow. Thoughts?
Yes... The pressure field induces upwash ahead of the wing. When you sweep the wing this upwash turns toward the tip so each point along the span sees a little extra upwash from its inboard neighbor. The result is that if the wing is not twisted the tips will see a higher AoA than the root which has bad consequences at stall. This induced upwash, and thus spanwise flow, is dependent on CL. Low CL induces smaller upwash angle than high CL. Since wings are fairly rigid they can only be optimized for one speed but this is true of any wing with washout, not just BSLD wings.
 

Kingfisher

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

Yes... The pressure field induces upwash ahead of the wing. When you sweep the wing this upwash turns toward the tip so each point along the span sees a little extra upwash from its inboard neighbor. The result is that if the wing is not twisted the tips will see a higher AoA than the root which has bad consequences at stall. This induced upwash, and thus spanwise flow, is dependent on CL. Low CL induces smaller upwash angle than high CL. Since wings are fairly rigid they can only be optimized for one speed but this is true of any wing with washout, not just BSLD wings.
Yes good point. I was trying to find pictures of gliders flying at high speed with their wingtips bending down while doing a high speed pass because the outer wings have negative angle of attack in that situation. I suppose my statement regarding the BSLD wing of the same span and area always having lower lift could be wrong. If one added enough positive AOA to the inner section, than the total lift could be the same as a wing with elliptical lift distribution at a certain speed, but would the drag be higher? Most likely this would become undesirable at a higher speed, when the overall angle of attack is reduced and the outer wing panels start to create negative lift.

A friend of mine build various models of Fw190 in different sizes. On the larger ones he added more washout to give them more docile stall behaviour. I got to fly them all and found that the ones with the washout seem to loose a lot of momentum in a tight turn, and thought the added wing twist was unnecessary, taking away from the performance. But maybe the reasons for that where elsewhere.
 
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Kingfisher

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

:ponder: Submarines also have a dorsal fin. Without a big conning tower a sub will roll excessively with rudder input. Fish probably wouldn't mind banking into the turns but submariners find it very inconvenient. Large "wings" at the CG, like a fish's pectoral fins, could also counter this rolling moment but they would get in the way of docking and they need a place to stow the periscope so a conning tower serves multiple needs. It's also a handy place for the captain to stand when it's on the surface.
Interestingly I don't agree with these observations. I find that fish do not bank at all. A shark or carp glides through the water like on rails, and when it turns it stays perfectly vertical, just deflects its tail fin (rudder) briefly and continues in the new direction. I had an aquarium for years, and found only when fish get sick they start to roll and wobble, because their tail fin starts to hang down and they retract most of their other fins when they don't feel well. Less well adapted creatures, like dolphins and penguins, do bank, but mostly to the outside of the turn. I think it has to do with that they are positively buoyant and/or have a horizontal tail fin. Although dolphins are no doubt amazingly agile swimmers, the shark has a much more precise line in the water.

Regarding submarines, their rudder is often part of a cruciform tail. If a submarine did not have the tower, it would be perfectly symmetric and I don't see a rolling moment induced by the rudder. The tower, on the other hand, adds a lot of side area above the body. If one deflects the rudder, one could expect that the submarine would bank into the turn because of this added area, not despite it! In fact, some Russian submarines have a rudder that sticks out the top much more than the bottom (see the movie with Sean Connery as commander of "Red October"), just like a shark. This may be to counteract the bank induced by the tower, but I'm guessing now.
 
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Norman

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

Yes good point. I was trying to find pictures of gliders flying at high speed with their wingtips bending down while doing a high speed pass because the outer wings have negative angle of attack in that situation.
The tips of a swept flying wing are the horizontal stabilizers. If they are lifting downward they will cause the plane to pitch up. That's how the stab works, right?

I suppose my statement regarding the BSLD wing of the same span and area always having lower lift could be wrong. If one added enough positive AOA to the inner section, than the total lift could be the same as a wing with elliptical lift distribution at a certain speed, but would the drag be higher?
If span is the constraint an elliptical lift distribution produces lower induced drag but if weight is constrained and span if unconstrained a bell shaped lift distribution with 20% more span has lower induced drag. Then you look at how to steer these wings. The answer to the control problem is that you fiddle with the lift distribution: there goes that beautiful elliptical lift distribution

Most definitely this would become undesirable at a higher speed.
So design it for the speed you want to fly at. All machines have a range where efficiency is at a maximum. If you have to operator outside of that range you have the wrong machine.
 

homebuilderfan

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homebuilderfan

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

The Prandtl formula looks like is creating interest in the latest 10 years. Here also. Prof. Frediani has been working at this idea quite since a long time. Here some links
http://http://www.maxpinucci.com/frediani/prandtlplane.htm
DIA - Dipartimento di Ingegneria Aerospaziale, Università di Pisa - Multidisciplinare
IDINTOS
DIA - Dipartimento di Ingegneria Aerospaziale, Università di Pisa - Faculty
Università di Pisa - uniMap
 

Inverted Vantage

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

So correct me if I'm wrong but my understanding is that if they lay the winglet flat and then move the control surface out to the tip of the winglet, then yaw somehow becomes propulsive?

Can anyone explain to me how that's just not making the wing a little longer with a different airfoil at the last two stations?
 

RPM314

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

So correct me if I'm wrong but my understanding is that if they lay the winglet flat and then move the control surface out to the tip of the winglet, then yaw somehow becomes propulsive?

Can anyone explain to me how that's just not making the wing a little longer with a different airfoil at the last two stations?
Yes, and it is the same thing! You don't necessarily need different foils though.
 

SpainCub

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

One question is, where can I find the geometry for this? I'm time limited to build full scale airplanes at the moment, but this could be a fun project to do when the wife and kids are a sleep!
regards,
 

Norman

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

One question is, where can I find the geometry for this? I'm time limited to build full scale airplanes at the moment, but this could be a fun project to do when the wife and kids are a sleep!
regards,
Did you look at the spreadsheets on the Nest of Dragons site that were linked to in post #61 of this thread?
 
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Mavigogun

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

I wonder what the definitive element of realizing proverse yaw is. Twist, tapper, sweep, airfoil- which might be impinged upon and still capture the drag and yaw characteristics boasted by the Prandtl-D? Might a lower-aspect wing with a high-lift airfoil work?
 

Aerowerx

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Re: NASA Prandtl-D Project - Yaw is propulsive, not drag --- Has Anyone Followed This

I wonder what the definitive element of realizing proverse yaw is. Twist, tapper, sweep, airfoil- which might be impinged upon and still capture the drag and yaw characteristics boasted by the Prandtl-D? Might a lower-aspect wing with a high-lift airfoil work?
I am no expert on this, but will hazard a guess that a larger aspect ratio would be a lot easier. Look at the AR on the Horten wings.

IIRC, they used a low-moment airfoil in the center, morphing to a symmetrical airfoil at the tips. And, yes the secret is in the proper selection of taper, twist, and sweep.
 
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