PRANDTL lift distribution as it relates to flying wings.

[Doc J]

Hi

There is a kit of a BSLD plank designed by Marko Stamenovic already.

FVT3 (v3) Prandtl Wing

1200mm span Unique finless flying wing using Bell Shaped Lift DistributionComprehensive Deluxe Kit Contains High quality Laser cut balsa parts Selected 1.5mm Balsa wing sheet Preformed weighted nose block (approx 50g weight) Carbon Fibre Rod trailing edge 32awg Servo extension wire Servo covers...

www.angelwingdesigns.co.uk

He also has a YouTube channel.
I think here he shows the operation of that spreadsheet:

For quick design iterations you can also use the program "Nurflügel" by Frank Ranis. It's Freeware available here:

Nurfluegel

The aero code might be a bit old and not that precise, but it runs very quickly.

There also is a guide on RC-Network on how to use this program:

Hortenauslegung mit „Nurflügel“ von Frank Ranis

Hortenauslegung mit „Nurflügel“ von Frank Ranis Uwe Heuer (Reloadet aus Magazin-Archiv. Erstveröffentlichung: 26.08.2010) Hortenflugzeuge polarisieren. Sie beschränken sich auf das Minimale und bestehen bis auf die für den Flugbetrieb unbedingt notwendigen Anbauteilen nur aus einem Flügel...

www.rc-network.de

In German, just like the program, but Google Translate does a good job.

Peter Wick is working on a book on the design of flying wings. It has a chapter on BSLD, too, which he sent me. But he asked me not to share it, since it's not finished, yet.

Thanks Marc,
I did a little bit of work on a plank with Peter Wick a few years ago - nothing came of it at that time though as othe things took over.

Peter is a great guy - I'll send him an email and see how he is doing.

I might also contact Marko Stamenovic.

Really interesting stuff and a bit different from my more conventional designs!

Cheers,

Doc.

 

[Doc J]

This is another quite informative paper with good explanations:

Cheers,

Doc.

 

[jmf11]

Hello, I join the discussion after some travel between Al Bower youtube AMA talks, browsing the NASA website, the nestofdragons webpages, the RCgroup thread and some academic papers. I'm more of an IT guy, but with a strong curiosity for what flies or sail, efficiency in structures and ressources, less is more... Prandtl theory and Prantl-D project tick a lot of boxes here

Through my searches I tried to understand what were the benefits and limitations. At some horizon a possible design and construction project.

I would be happy to have some help to check if my understanding is OK for those benefits and limitations when applied to RC gliders. Errors expected there ;-)

My understanding:

• BSLD results from an attempt to minimize drag for a fixed bending moment <=> structural constraint <=> weight,
• BSLD should allow to have better performance for fixed weight contraint (as less structure),
• For a fixed span, Eliptical lift distrubution will have lower drag, so BSLD is not best for class rules with strong limit on span,
• If we build a BSLD wing without taking that structural gain (ex with a spar suitable for ESLD), then we miss a big part of the point,
• The BSLD comes with a proverse (inverse of "adverse yaw") that allows to flying without a vertical surface (as birds),
• Absence of vertical surface + flying wing configuration should allow to save for additional weight (boom, elevator, rudder),
• I have seen very different figures about the "efficiency" gain of a BSLD flying wing compqred to classic configuration. From 60% in early Al bower talks (possibly very optimistic" to 5-12%. I don't know if jury is still out there, or if a consensus has been reached,
• The BSLD in Prandtl-D wing is addressed by wing tapering and twist, and the twist is the trick,
• The twist is set for a specific CL <=> a specific fly mode / operation point (ex Thermal or Speed, but not both),
• This is good for a "single mission aircraft" / fixed speed like UAV looking of optimize distance at given speed,
• Not as good for "multi task" design objective, like F3K for ex where flaps and balast would allow more versatility / flight modes <= is this understqnding correct?
• For Rudder Elevator Spoiler (RES) classes, then it makes more sense (no flaps). If there was an equivqlent of F3RES/F3L with a constraint on mass instead of span, BSLD flying wing could be a strong candidate,
• Birds may have some wing morphing ability to trick the twist in their different flight mode, which looks quite remote in RC models,
• Mark Drela AVL tool should be valid to design a BSLD flying wing.

Comments / complements warmly welcomed

I have seen https://www.amaflightschool.org/PRANDTL. I have not seen models in the 2-3 meters range. Is there an available Nasa Prandtl design in that size range (and in that case are the 3D models available)?

Best regards,

JMF

 

[Doc J]

Hello, I join the discussion after some travel between Al Bower youtube AMA talks, browsing the NASA website, the nestofdragons webpages, the RCgroup thread and some academic papers. I'm more of an IT guy, but with a strong curiosity for what flies or sail, efficiency in structures and ressources, less is more... Prandtl theory and Prantl-D project tick a lot of boxes here

Through my searches I tried to understand what were the benefits and limitations. At some horizon a possible design and construction project.

I would be happy to have some help to check if my understanding is OK for those benefits and limitations when applied to RC gliders. Errors expected there ;-)

My understanding:

• BSLD results from an attempt to minimize drag for a fixed bending moment <=> structural constraint <=> weight,
• BSLD should allow to have better performance for fixed weight contraint (as less structure),
• For a fixed span, Eliptical lift distrubution will have lower drag, so BSLD is not best for class rules with strong limit on span,
• If we build a BSLD wing without taking that structural gain (ex with a spar suitable for ESLD), then we miss a big part of the point,
• The BSLD comes with a proverse (inverse of "adverse yaw") that allows to flying without a vertical surface (as birds),
• Absence of vertical surface + flying wing configuration should allow to save for additional weight (boom, elevator, rudder),
• I have seen very different figures about the "efficiency" gain of a BSLD flying wing compqred to classic configuration. From 60% in early Al bower talks (possibly very optimistic" to 5-12%. I don't know if jury is still out there, or if a consensus has been reached,
• The BSLD in Prandtl-D wing is addressed by wing tapering and twist, and the twist is the trick,
• The twist is set for a specific CL <=> a specific fly mode / operation point (ex Thermal or Speed, but not both),
• This is good for a "single mission aircraft" / fixed speed like UAV looking of optimize distance at given speed,
• Not as good for "multi task" design objective, like F3K for ex where flaps and balast would allow more versatility / flight modes <= is this understqnding correct?
• For Rudder Elevator Spoiler (RES) classes, then it makes more sense (no flaps). If there was an equivqlent of F3RES/F3L with a constraint on mass instead of span, BSLD flying wing could be a strong candidate,
• Birds may have some wing morphing ability to trick the twist in their different flight mode, which looks quite remote in RC models,
• Mark Drela AVL tool should be valid to design a BSLD flying wing.

Comments / complements warmly welcomed

I have seen https://www.amaflightschool.org/PRANDTL. I have not seen models in the 2-3 meters range. Is there an available Nasa Prandtl design in that size range (and in that case are the 3D models available)?

Best regards,

JMF

I'll get into this with what I know when I get back from Chinese China. - If someone doesn't beat me to it.

Cheers,

Doc.

 

[Red]

Hello, I join the discussion after some travel between Al Bower youtube AMA talks, browsing the NASA website, the nestofdragons webpages, the RCgroup thread and some academic papers. I'm more of an IT guy, but with a strong curiosity for what flies or sail, efficiency in structures and ressources, less is more... Prandtl theory and Prantl-D project tick a lot of boxes here

Through my searches I tried to understand what were the benefits and limitations. At some horizon a possible design and construction project.

I would be happy to have some help to check if my understanding is OK for those benefits and limitations when applied to RC gliders. Errors expected there ;-)

My understanding:

• BSLD results from an attempt to minimize drag for a fixed bending moment <=> structural constraint <=> weight,
• BSLD should allow to have better performance for fixed weight contraint (as less structure),
• For a fixed span, Eliptical lift distrubution will have lower drag, so BSLD is not best for class rules with strong limit on span,
• If we build a BSLD wing without taking that structural gain (ex with a spar suitable for ESLD), then we miss a big part of the point,
• The BSLD comes with a proverse (inverse of "adverse yaw") that allows to flying without a vertical surface (as birds),
• Absence of vertical surface + flying wing configuration should allow to save for additional weight (boom, elevator, rudder),
• I have seen very different figures about the "efficiency" gain of a BSLD flying wing compqred to classic configuration. From 60% in early Al bower talks (possibly very optimistic" to 5-12%. I don't know if jury is still out there, or if a consensus has been reached,

The 60% number is all inclusive of the whole system. For instance taking a tube and wing airliner type aircraft and making it a pure BSLD flying wing. For the PRANDTL aircraft, a number of about 11% reduction in induced drag seems to be about right.

• The BSLD in Prandtl-D wing is addressed by wing tapering and twist, and the twist is the trick,
• The twist is set for a specific CL <=> a specific fly mode / operation point (ex Thermal or Speed, but not both),
• This is good for a "single mission aircraft" / fixed speed like UAV looking of optimize distance at given speed,
• Not as good for "multi task" design objective, like F3K for ex where flaps and balast would allow more versatility / flight modes <= is this understqnding correct?

We have wanted to build an aircraft to demonstrate the ability to tailor CL for the conditions through a live trailing edge. It is strongly suspected that it would be beneficial but no work yet towards that has been completed.

• For Rudder Elevator Spoiler (RES) classes, then it makes more sense (no flaps). If there was an equivqlent of F3RES/F3L with a constraint on mass instead of span, BSLD flying wing could be a strong candidate,
• Birds may have some wing morphing ability to trick the twist in their different flight mode, which looks quite remote in RC models,

Some land soaring birds (hawks, falcons etc. ) may be able to, but there is not much evidence beyond "folding" in a dive. Lots of people confuse active camber changing (morphing) with feathers naturally bending under load. The Great Wandering Albatross which dynamic soars around the earth 1-2 times per year that we reference does not. The solution is convincingly BSLD based on the avian data sets from many independent researchers.

• Mark Drela AVL tool should be valid to design a BSLD flying wing.

Marko Stamenovic (Horton Flying Believers group on FB) has a wing design tool available

Comments / complements warmly welcomed

I have seen https://www.amaflightschool.org/PRANDTL. I have not seen models in the 2-3 meters range. Is there an available Nasa Prandtl design in that size range (and in that case are the 3D models available)?

Best regards,

JMF

Hi John,

Sorry I missed this earlier. I've addressed some of your point's in your quote, most of your assumptions are correct. I have a 25' span OML I could probably share.

Red

 

[jmf11]

Hi Red,

This was a quite fresh message. Thanks for all the answers. So birds would use an efficient but somehow single configuration (no wing morphing, at least for the albatros). Good to know as it would validate the BSLD in "real life conditions"

You state "We have wanted to build an aircraft to demonstrate the ability to tailor CL for the conditions through a live trailing edge.". Do you mean by sort of flaps on the wing, or something different ?

I have seen Marko Stamenovic tool/Excel program on nestofdragon website, but I don't know if it is his last version (I can try to contact him with messenger, except if someone knows).

JM

 

[Red]

Hi Red,

This was a quite fresh message. Thanks for all the answers. So birds would use an efficient but somehow single configuration (no wing morphing, at least for the albatros). Good to know as it would validate the BSLD in "real life conditions"

You state "We have wanted to build an aircraft to demonstrate the ability to tailor CL for the conditions through a live trailing edge.". Do you mean by sort of flaps on the wing, or something different ?

I have seen Marko Stamenovic tool/Excel program on nestofdragon website, but I don't know if it is his last version (I can try to contact him with messenger, except if someone knows).

JM

I guess you could call it flaps, but it would be multi-segmented and effect a large part of the chord.

 

[Doc J]

Hi John, I asked Al Bowers if BSLD would work on a 'conventional' i.e. "tailed" airframe and he told me that it should indeed work, and offered to calculate the curve for me. I have not taken him up on his kind offer yet but its in my mind. I have also spoken a bit with Red on this as he was actively involved in the experiments.

I think that a POC model would be pretty complicated and maybe not a flyer. Possibly a wind tunnel tester might work as you'd probbely only need to optimize the dynamics in the beginning.

Curved trailing edges are a common sight on full-sized aircraft especially large Airliners, but its quite a step to imagine them on a model where normally the first thing a flyer does is to check that the trailing edge is STRAIGHT! I think its a kind of muscle memory.

If its not a model of a bird that is.

More when I escape.

Cheers,

Doc.

 

[jmf11]

I guess you could call it flaps, but it would be multi-segmented and effect a large part of the chord.

Thanks for the confirmation. Complex to do and control well Better to start with something simpler.

JMF

 

[jmf11]

Hi John, I asked Al Bowers if BSLD would work on a 'conventional' i.e. "tailed" airframe and he told me that it should indeed work, and offered to calculate the curve for me. I have not taken him up on his kind offer yet but its in my mind. I have also spoken a bit with Red on this as he was actively involved in the experiments.

Hello Doc J,

What would be the intention behind a tailed air frame ? Do you have in mind a tailed one with a vertical surface (rudder) or just elevator?

This would eat part of the Prandtl wing benefits for at least weight and drag perspectives. But if you consider it, it is to gain on other aspects. Which ones?

JMF

 

[Doc J]

Hello Doc J,

What would be the intention behind a tailed air frame ? Do you have in mind a tailed one with a vertical surface (rudder) or just elevator?

This would eat part of the Prandtl wing benefits for at least weight and drag perspectives. But if you consider it, it is to gain on other aspects. Which ones?

JMF

I'm hoping that the wing will have less drag in a conventional setup.

If so, its a very big deal.

Doc.

 

[Marc Frank]

Plenty RC Planes have been built that do not have a vertical tail.

Here is a very minimal one:

And here is a bird shaped one:

Then all the Horten / Prandtl models...

They fly, quite nicely actually, but yaw stability is not what you might be used to.
Placing the elevator further aft on a boom allows it to be smaller for the same effectiveness. But the boom adds drag. There is a balance for minimal drag in there somewhere. Not 0 and not ∞.
Similarly, the rudder can be smaller the further aft. Same Balance calculation can be used. And if you move from elliptical to BSLD the vertical stabilizer can be made even smaller.
If you are willing to go very low on the stability factor you can even live without one. But certain maneuvers / flying style requires a higher stability factor, so you might still need a fin.
But it would be smaller than if the lift distribution was elliptical.

 

[Doc J]

Plenty RC Planes have been built that do not have a vertical tail.

Here is a very minimal one:

And here is a bird shaped one:

Then all the Horten / Prandtl models...

They fly, quite nicely actually, but yaw stability is not what you might be used to.
Placing the elevator further aft on a boom allows it to be smaller for the same effectiveness. But the boom adds drag. There is a balance for minimal drag in there somewhere. Not 0 and not ∞.
Similarly, the rudder can be smaller the further aft. Same Balance calculation can be used. And if you move from elliptical to BSLD the vertical stabilizer can be made even smaller.
If you are willing to go very low on the stability factor you can even live without one. But certain maneuvers / flying style requires a higher stability factor, so you might still need a fin.
But it would be smaller than if the lift distribution was elliptical.

Hi Marc,
I'm not sure if the BSLD as applied to the wings only would be a good solution for a conventional aircraft, but to me at least there is enough merit from the studies to date to think that it might, at least to some degree.

While in theoretical thinking we all strive to get the minimum drag for the maximim wing - the holy grail - which normally indicates going 'all the way' and disposing of the entire empennage, i.e. the flying wing, there remains good reason to think that this type of lift arrangement might yield a lower drag wing in flight.

Is the Grail just up the road? Probably not.

There are quite a few problems with actual scientific testing, one of which is scale effect as the airframe sees the same or at least similar air viscosity and thickness, be it a model or full-sized. This is why I thought that an actual flying airframe where the trailing edge geometry etc could be altered during or before testing would be pretty hard to do and to quantatively measure - hence the suggestion of wind tunnel model where weight restrictions etc were not concerns. I'm sure a flyer, or a series if flyers would get there, yay or nay, but how many would have to be made? Is it worth it? I dont know.

In case of the BSLD wing and its advantages, or not, I am only looking at the wing set itself - as though I substituted a BSLD wing set, for an exisiting wing set on a proven airframe - which seems to me to be the fastest and most practical method of POC.

The 64,000$ question is: If the BSLD regime was used on a conventional glider, with empennage, how much dvantage - if any - would be gained?

As mentioned, there is enough evidence to think that there could be, and probably would be a good advantage - but without committing to a wind tunnel test model and a flyer, or several, after that; all we have to work with is the NASA data, the gathering of which used a flying wing as the test model. This is why when the idea occurred to me after seeing the NASA report; I was sure it would have occurred to Al Bowers and to Red Jensen who led the project from the theoretical and through testing phases. Both indicated that it was quite possible.

This continues to go through my mind. I any case its a heck of a way to dissolve a couple of hours at an airport!

Thoughts are a bit disjointed and disorgnized as they came to me, but there might be treasure there somewhere.

Asks self: Is it worth making a set of BSLD wings (after a bit of collaboration with Red and Al) to plug into say a Forza and chucking it of a cliff?

Hmmmm.

Doc.

 

[jure]

... Asks self: Is it worth making a set of BSLD wings (after a bit of collaboration with Red and Al) to plug into say a Forza and chucking it of a cliff? ..

Doc.

candidate : DH108 + ?

 

[Doc J]

candidate : DH108 + ?

Jure, M'Main 3-D man - well I suppose DH108 could be a test model, but probably wouldn't prove anything as to the suitability for BSLD on conventional airframes. However, It would be a lot of fun to plug in a set of BSLD wings after thoroughly test flying the ESLD type on the model though. Hmmm.

Cheers,

Doc, windmilling around in Chinese China.

 

[jure]

of course, Doc, not for a comparison in a conventional aircraft family type,
but to potentially improve the DH108 widow-maker reputation, perhaps...

 

[Doc J]

of course, Doc, not for a comparison in a conventional aircraft family type,
but to potentially improve the DH108 widow-maker reputation, perhaps...

I think we should put a model widow in the cockpit...

Doc.

 

[jmf11]

Hi Red,
Marc Frank comment on tail less designs yaw stability being "different" (which make sense) popped a question in my head: do the Nasa prandtl wings felt special for the pilot? Do they fly like classic flying wings with vertical surfaces? Rudder Elevator config? Or full classical config?

Do the Prandtl-D and Prandtl-M fly differently?

And would the 35" 3d model be possible to share? The ones available on AMA academy are big ones. And I imagine they can't be scaled down "as is" considering the difference in reynolds numbers. The 35" version would be a better candidate.

Jmf

 

[jure]

I think we should put a model widow in the cockpit...

Doc.

naaah ...

 

[jmf11]

Hi,

A complementary question on the one from Thursday about how Prandtl-D and M fly,

BSLD is a way to optimize drag for structure weight. I'm looking for a plot or equation for the bending moment along the span in the BSLD, to try to understand how the spar should evolve.

Mark Drela analyze spar "tapering" for Elliptical lift distribution as reported in https://www.rcgroups.com/forums/showpost.php?p=5151778&postcount=3

I wonder what should be considered for BSLD.

Seems that XFLR5 can calculate that, but the a XFLR5 model is needed ;-)

For Doc J: A thesis on the application of BSLD to gider's straight wings : Project Monarch: The application of Ludwig Prandtl's bell-curve span loading to a straight, high performance sailplane wing