PRANDTL lift distribution as it relates to flying wings.

[Red]

The CF as Wayne mentions above achieves yaw stability with washout very effectively This is a bit of brute force method but works quite well. BSLD is a different animal though, it is very non linear unlike washout. This is the minimum drag solution for the same weight structure and root bending moment. It really only is beneficial if you actually optimize the whole structure, otherwise a washout approximation is much easier to achieve for our models.

 

[nhedler]

Wayne: I was already interested - no pitch needed!
Red: My suspicion is that the high degree of washout does, in fact, make use of the "vortex lift" or "upwash span" that gives the BSLD its performance benefits, even if the actual curve is less-than-perfectly bell-shaped.

 

[Red]

Wayne: I was already interested - no pitch needed!
Red: My suspicion is that the high degree of washout does, in fact, make use of the "vortex lift" or "upwash span" that gives the BSLD its performance benefits, even if the actual curve is less-than-perfectly bell-shaped.

Agreed on the upwash/stability with washout. I was just making the distinction that in's not BSLD due to the straight slope. In the end the they achieve similar results for yaw stability, but after that they start to diverge.

 

[nhedler]

Closer to BSLD than anything else - seems more pedantic than useful to argue otherwise.

 

[Doc J]

I'm wondering if you'd get a lot of the useful effect just by washing out the wings a little on a conventional glider.

I had a 2M V-tail glider about 35 years ago that had a tinge of washout and it was faster than a fully-greased racing snake.

Doc.

 

[nhedler]

I'm also curious how speed relates to washout angle - I'd imagine that (to a point) more washout would limit speed (outrunning vortices) but might allow slower flight/higher AoA, while less washout would move the speed envelope faster but maybe with poorer stall characteristics.

 

[Red]

Closer to BSLD than anything else - seems more pedantic than useful to argue otherwise.

Well no actually . Washout is simply that, washout. BSLD is a whole different animal entirely. While you can mimic yaw stability with washout, you also generate far greater induced drag. If you want to hang your hat on the yaw stability only, and calling difference pedantic, so be it. If it was that easy much smarter people wouldn't have wasted some much time and resources on understanding it.

The real benefit (among other things) to true BSLD is that is the minimum induced drag for a given structural weight. That means if you were to construct a wing for a given weight holding the root bending moment constant, then you would end up with a wing with something like a 22% increase in span, and high single digit, low double digit reduction in induced drag. We have wind tunnel data to back this up, it is not a fantasy number. For reference the winglet most airliners tow around is good for 3% reduction in induced drag. This is compared to the universally accepted "best possible" elliptical lift distribution (ESLD). Interestingly, a solution also put forth by Ludwig Prandtl, who later called the ESLD solution "incomplete" and wrote his 1933 paper where he introduced BSLD.

Red

 

[Red]

I'm wondering if you'd get a lot of the useful effect just by washing out the wings a little on a conventional glider.

I had a 2M V-tail glider about 35 years ago that had a tinge of washout and it was faster than a fully-greased racing snake.

Doc.

In short, no. You just end up generating massive amounts of drag. You can cheat it as above and get some yaw stability, but with a large penalty.

Red

 

[Red]

I'm also curious how speed relates to washout angle - I'd imagine that (to a point) more washout would limit speed (outrunning vortices) but might allow slower flight/higher AoA, while less washout would move the speed envelope faster but maybe with poorer stall characteristics.

Hard to follow this, as BSLD is not a washout angle. It's unique per design non linear twist (in our case, but can also possible with planform a la Horten Bros.) that is tailored to suit the desired CL. If you stray far off of that the efficiencies and benefits go away rapidly, so they are currently sort of a single point design with a very narrow CL. One of the things that is being currently proposed is a design with the whole trailing edge being live and multi-segmented to allow for tweaking of the spanload for a wider CL range. Not sure what you mean about outrunning vorticies, that's not a thing.

Red

 

[Doc J]

In short, no. You just end up generating massive amounts of drag. You can cheat it as above and get some yaw stability, but with a large penalty.

Red

Ahhh...as usual laziness never works...

Doc.

 

[nhedler]

Well no actually . Washout is simply that, washout. BSLD is a whole different animal entirely. While you can mimic yaw stability with washout, you also generate far greater induced drag. If you want to hang your hat on the yaw stability only, and calling difference pedantic, so be it. If it was that easy much smarter people wouldn't have wasted some much time and resources on understanding it.

The real benefit (among other things) to true BSLD is that is the minimum induced drag for a given structural weight. That means if you were to construct a wing for a given weight holding the root bending moment constant, then you would end up with a wing with something like a 22% increase in span, and high single digit, low double digit reduction in induced drag. We have wind tunnel data to back this up, it is not a fantasy number. For reference the winglet most airliners tow around is good for 3% reduction in induced drag. This is compared to the universally accepted "best possible" elliptical lift distribution (ESLD). Interestingly, a solution also put forth by Ludwig Prandtl, who later called the ESLD solution "incomplete" and wrote his 1933 paper where he introduced BSLD.

Red

No True Scotsman - imperfect BSLD is still different than ESLD, and you seem insistent on arguing against an opinion I don't have. Just because it's not perfectly optimized doesn't mean the principle doesn't apply. I am not ignorant of the ideal characteristics of the concept, and the fact that you've copy-pasted the sales pitch suggests you believe I'm disagreeing with its validity entirely.

Hard to follow this, as BSLD is not a washout angle. It's unique per design non linear twist (in our case, but can also possible with planform a la Horten Bros.) that is tailored to suit the desired CL. If you stray far off of that the efficiencies and benefits go away rapidly, so they are currently sort of a single point design with a very narrow CL. One of the things that is being currently proposed is a design with the whole trailing edge being live and multi-segmented to allow for tweaking of the spanload for a wider CL range. Not sure what you mean about outrunning vorticies, that's not a thing.

Red

BSLD does *use* washout, regardless of its (non)linearity; twist which reduces incidence angle is the definition of washout, in my book. Just because *ideal* BSLD uses an oh-so-special secret-sauce twist doesn't mean it's not a type of washout.

When I say "outrunning" it's a shorthand for "flying at a speed such that the wingtip AoA becomes negative due to insufficient upwash angle." The amount and direction of the upwash under the wingtip is (at least partly) dependent on the plane's AoA, which (for a given design and flight configuration) varies with airspeed; if you exceed the optimal airspeed of a given (BSLD) wing, the AoA of the wingtip eventually becomes locally negative, at which point it can be described as having outrun the vortex that it relies on to generate lift.

Whether you intend it or not, your comments give the impression that you don't intend to actually educate - just prognosticate about how much better you can argue. If you actually intended to respond constructively, you might want to take another look at my previous comments and reconsider whether you've actually responded to what I *said* instead of what you wanted to argue against.

 

[Doc J]

No True Scotsman - imperfect BSLD is still different than ESLD, and you seem insistent on arguing against an opinion I don't have. Just because it's not perfectly optimized doesn't mean the principle doesn't apply. I am not ignorant of the ideal characteristics of the concept, and the fact that you've copy-pasted the sales pitch suggests you believe I'm disagreeing with its validity entirely.

BSLD does *use* washout, regardless of its (non)linearity; twist which reduces incidence angle is the definition of washout, in my book. Just because *ideal* BSLD uses an oh-so-special secret-sauce twist doesn't mean it's not a type of washout.

When I say "outrunning" it's a shorthand for "flying at a speed such that the wingtip AoA becomes negative due to insufficient upwash angle." The amount and direction of the upwash under the wingtip is (at least partly) dependent on the plane's AoA, which (for a given design and flight configuration) varies with airspeed; if you exceed the optimal airspeed of a given (BSLD) wing, the AoA of the wingtip eventually becomes locally negative, at which point it can be described as having outrun the vortex that it relies on to generate lift.

Whether you intend it or not, your comments give the impression that you don't intend to actually educate - just prognosticate about how much better you can argue. If you actually intended to respond constructively, you might want to take another look at my previous comments and reconsider whether you've actually responded to what I *said* instead of what you wanted to argue against.

OMG Deja Vu...

Doc.

 

[Red]

Ah yes, silly me. I’ll try to refrain from copy and paste in the future.

 

[Doc J]

Hey Red, as many of us know, you were one of the key people involved with the invesigation, development and subsequent verification of the BSLD effect, in fact as I know you were on the sticks for many if not most of the flight trials of the various airfames used.

As clearly stated in the title, this thread pertains to flying wings. In fact probably more specifically to swept flying wings

However, as often mentioned in the evolving literature stack, birds certainly use the BSLD effect to fly and to manoeuvre as they don't have vertical fins. Added to that, very few birds with higher aspect ratio wing planforms actually have naturally swept flying surfaces, although most of them can at will sweep their wings back - interestingly almost always for faster flying speeds. I can think of one bird which naturally has sweepback and that is the Martin family i.e. Swallows, Martins, and Swifts etc, but I'm sure there are many more.

This indicates to me - an ardent student of nature - that since BSLD occurrs naturally then there is a pretty good chance that this might be of significant value for types of aircraft other than the flying wing configuration.

With these facts firmly in mind, what do you think the possibility of it being useful in the case of planks or even conventional tailgroup models would be? We have CNC so any wing configuration or trailing edge shape is easily done - though perhaps not so easily worked out! Of course its possible to get a similar affect by messing about with the chord lengths etc - but if this were to be a way to do it then I'm sure birds would have done it millions of years ago. This logic - if indeed it is; leads me back to the optimised trailing edge.

I have the idea that a wavy TE could be a really interesting experiment.

So, rather than a stack of formulae and numbers etc, as one of the key 'players' - if I may use that word, and as someone who has been around experimental aircraft most of your life, I'd be interested in your gut-feel "SWAGGY" or "Hunchy" thoughts and feelings.

Cheers,

Doc

 

[Red]

Hey Red, as many of us know, you were one of the key people involved with the invesigation, development and subsequent verification of the BSLD effect, in fact as I know you were on the sticks for many if not most of the flight trials of the various airfames used.

As clearly stated in the title, this thread pertains to flying wings. In fact probably more specifically to swept flying wings

However, as often mentioned in the evolving literature stack, birds certainly use the BSLD effect to fly and to manoeuvre as they don't have vertical fins. Added to that, very few birds with higher aspect ratio wing planforms actually have naturally swept flying surfaces, although most of them can at will sweep their wings back - interestingly almost always for faster flying speeds. I can think of one bird which naturally has sweepback and that is the Martin family i.e. Swallows, Martins, and Swifts etc, but I'm sure there are many more.

This indicates to me - an ardent student of nature - that since BSLD occurrs naturally then there is a pretty good chance that this might be of significant value for types of aircraft other than the flying wing configuration.

With these facts firmly in mind, what do you think the possibility of it being useful in the case of planks or even conventional tailgroup models would be? We have CNC so any wing configuration or trailing edge shape is easily done - though perhaps not so easily worked out! Of course its possible to get a similar affect by messing about with the chord lengths etc - but if this were to be a way to do it then I'm sure birds would have done it millions of years ago. This logic - if indeed it is; leads me back to the optimised trailing edge.

I have the idea that a wavy TE could be a really interesting experiment.

So, rather than a stack of formulae and numbers etc, as one of the key 'players' - if I may use that word, and as someone who has been around experimental aircraft most of your life, I'd be interested in your gut-feel "SWAGGY" or "Hunchy" thoughts and feelings.

Cheers,

Doc

Doc,

We (myself, Al Bowers and many other folks involved from the beginning) have talked about this quite a bit. In theory it should work with planks and conventional aircraft according to the math and the design tool that we developed. However, none up as have ever tried it, and you know what they say about theory. My gut says it is worth investigation. There is a somewhat automated spread sheet that will kick out the proper twist in about an hours time now, whereas the first several iterations were all hand calcs. If you want to play with it, I can see if there is a releasable version. Also, I would join this FB group. Marko Stamenovic (SP) is probably the leading authority outside NASA. He does have a design tool that works really well.

Log into Facebook

Log into Facebook to start sharing and connecting with your friends, family, and people you know.

www.facebook.com

Red

 

[Doc J]

Gotcha - thanks Red - I have joined the Horten group and I would like to take a swing at the twist calc if it is releasable.

Cheers,

Doc.

 

[Marc Frank]

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.

 

[Doc J]

This paper is quite useful.

Vehicle Design Study of a Straight Flying-Wing with Bell-Shaped Spanload

https://openscholarship.wustl.edu/cgi/viewcontent.cgi?article=1564&context=eng_etds

Doc.

 

[jure]

Vehicle Design Study of a Straight Flying-Wing with Bell-Shaped Spanload
Doc.

nice Thesis paper, thanks !
I thought that the author was leaning towards full flying "outerons" , but in the end KISS prevailed...

 

[Doc J]

nice Thesis paper, thanks !
I thought that the author was leaning towards full flying "outerons" , but in the end KISS prevailed...

Hey Jure! Dont make it bad...sing a.. wrong email...

Yep - loads of good stuff and I did get some more from Red too.

Maybe we should start a new thread "Strictly for the birds" - they know what they are doing.

Anyway I found another couple of papers on BSLD as applied to 'conventional' wings which more or less backs up my thinking that the pro thrust part of the theory should in fact be applicable. So much so that I am going to try it on the next "conventional" 2M model.

Of course the other half of the effect: elimination of fins wouldn't strictly apply here, but...I'm also a flying wing fan so who knows what might happen. If the flight control and flight visibility problems could be solved and a decent configuration developed, can you imagine how fast it would be for F3f or similar? I any case I would cheat and add a stinger fin, possibly with a rudder at the back.

Who knows? Maybe a well-designed BSLD wing could solve some of the flying wing unpredictability problems, while some kind of high-brightness LED light arrangement for the visibility might work...don't know yet, but my grey matter is beginning to creak into action. Maybe an optimised lift distribution/geometrical wing twist hybrid? Either/or/and???

In the worst case I'll waste some time and money, but in either case we will know more.

I know because a little bird told me...

Cheers,

Doc.