Flying Wing with "Diffuser Tips" AKA "Weltensegler Crank" AKA "Stromburg Wing"

I've always wanted to explore the properties of this unique, bird-like wing configuration.

The wing is usually sweptback with dihedral over most of the span, but the wing is bent (or cranked) downward near the tips with substantial anhedral.. this bent down portion is usually equipped with control surface, and can serve as both rudder and elevon.

This is essentially splitting an inverted-V tail and putting each half of the inverted V at the wingtips.

This arrangement is not commonly seen... but it's at least a century-old idea.. some examples through history:

1. J.W. Dunne's D.6 tailless monoplane in 1911
2. Fritz Wenk's "Weltensegler" tailless sailplane in 1921.
3. A. Lippisch's Delta IVB and DFS40
4. Blohm & Voss P212 & P215
5. Jack Northrup's N1M

This arrangement is sometimes touted as having many advantages in performance, stability and control.




Some examples in recent ultralight soaring craft with this arrangement were inspired by Richard Miller, a diffuser tip proponent, leading to Bob Trampenau's Sunseed and an experimental prototype of the Fledgling hang glider sporting diffuser tips.

The more modern Exulans hang glider also has a similar arrangement..
And the Stromburg Wing model aircraft group in Germany seems active, with many varieties of Weltensegler-cranked wings being flown.

However, I am puzzled as to why this arrangement remains such a rarity.. What am I missing?
The vulnerability to wing tip damage is sometimes mentioned as a criticism .. but is that it?
Are there inherent disadvantages compared to other tailless aircraft arrangements?

 

I suspect the yawing moment (with the short moment arm) may be more than counteracted by the adverse yaw (with a large moment arm) and a sideslip effect, both to the opposite direction.

 

6. XP-56
7. Stromer et al

It's a very good feature for swept wings in the air but very inconvenient on the ground

 

Don't forget the flying wing from RAIDERS OF THE LOST ARK. :gig:

 

That one certainly was a disaster on the ground

 

There are a couple of Styrofoam tailless chuck gliders with this layout that seem to behave quite well --the amount of 'delta three' (skewing of the fold axis in planform view ) is pretty influential and determines the apparent alpha of each tip and the effective airfoil across the kink region is a problem potentially --it seems that a gentle curve rather than a discrete kink might be better (approximating the curved tip feathers of a soaring bird in reverse ) but harder to fit control surfaces to if not all flying . There was a Polish tailess glider with the same type of tips in the early sixties or late fifties (Neopteryx or something like that ) but it wasn't configured for high performance or reported on in the literature . AV Weyhl wrote a long series of papers in The Aeroplane on tailless aircraft including this type of config --might look for them and see if he makes the analogy to the "V tail on the tips" or comments at all.

 

It seems there are a few key parameters to get the wing stabilized correctly..
These RC model guys with the "Dethgull" and "Robbe Vampir" seem to have workable designs.

DethGull: a tailless gull wing - RC Groups

And the Stromburg designs too..

http://www.m-schoenherr.de/StromerE/default.htm

I seems that the drooped down outer section needs to be toed-in a few degrees to lower the angle of attack... effectively inducing "jump twist."
It does change in angle of attack in proportion with the rest of the wing.
This could permit desirable lift distribution and low induced drag throughout the entire speed range.

On the issue of ground clearance, having it as a high wing with a taller undercarriage/fuselage could alleviate the problem.
Looking at the Sunseed HG, ground clearance compared to other flex & rigid wings today seems about the same.

 

This "Gull-wing" configuration is closely related to the concept of "hyper-elliptic camber, " which has been recently determined to have some very interesting aerodynamic efficiencies:

http://dnc.tamu.edu/projects/flowcontrol/Morphing/public_html/papers/virginiatech.pdf
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.117.7340&rep=rep1&type=pdf

Seabirds seem to have this cranked downwards "gull-wing" configuration, while many land birds like hawks, eagles and vultures seem to prefer the cranked upwards wing tip feathers.

Both of these non-planar wing surface arrangements seem to have performance advantages over a strictly planar wing. Dr. Ilan Kroo has also studied this.. and derived the C-wing as a particularly efficient wing shape as well.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960023622_1996039477.pdf

 

This is a big deal of free flight models but why should it be necessary if you have elevons?

From the poind of view of a slug of air moving along a streamline it would look like a flap deflected 1 or 2 degrees, would it not?

I don't recollect a Polish glider matching that description. The name you mentioned is similar to "Nietoperz" which can look a bit gull-winged from some angles but is in fact flat. Alexander Lippisch used drooping tips a few times like on the DFS39 and 40. The Boeing "Bird of Prey" has them. I have the flying wing articles by A. R. Weyl. I don't remember any mention of V-tails

 

It was the Nietperz --only mildly double cranked and reverse swept -- the reference to "vtail at the tips' was from the first poster on this thread I think --however in the thread on unjoined tails or twin hull configs I mentioned and posted a link to Luigi Pellarini's twin boom sesquiplane having two 'half V ' tails -- that is / on one boom and \ on the other side which has the same pro roll with yaw input as for downbent wing tips and approximates the analogy to the "V tail" on the wing tips (meaning I think the split V as just described ) 'Pellarini Air sedan' will probably pull up a picture of it hanging in a museum --the Pellarini designed transavia Airtruk (or Airtruck) PL 12 has two T tails on separate booms --in both cases the boom attaches mid span to the tail surface .

The Blohm and Voss design with an actual -horizontal V tail ( < and > ) at each tip of a "tailess" swept wing --sometime called diffuser tips but don't know why -- is another case of control and stabilizing surfaces attached to but outside of a swept wing --small booms appeared on another version and it was recently revived and one SCALED design used the config (unswept though)

I figured you might be up to speed on the Wehyl articles Norman --there is almost nothing of technical substance published in any of the mainstream aviation mags or Sport Aviation these days and only the archives (or books that collate the scattered remnants on obscure topics like flying wings or flying cars ) to refer to. Shame really.

 

I think Klingon battleships also have the cranked down tips ( only guessing- don't read Klingon ) --I used to share a house in Ohio (in 1974) with a guy from Missouri who went on to work on the bird of prey . The mock up NAZI flying wing mentioned from Raiders of the lost ark is also a real design that I seem to recall got covered by Wehyl --I have some old Luftsport and Adler etc with many flying wing models of the gulled variety and the fact that they all seem to fly reasonably well with big variations in all the angles seems to indicate some tolerance . The skew airfoil section on the leading wing in a sideslip sees a really bad separation shape at the kink like ( as I write I have my styrofoam "hyperglider" chuck model in hand and have just tested it against my usually blase cat and can confirm both amazing stability and effectiveness against feline targets )--making cardboard mock ups and looking at them along a flight path gives a good idea of what the air sees and some idea of what might be going on to a first approximation I find --then balsa flying versions -- trying to sort out orthogonal axis and derivatives etc etc is also not as much fun and doesn't work on cats.

 

More recently, the "Opensky project" by Kazuhiko Hachiya is using quite the same configuration.

Projet OpenSky: les essais en vol continuent « Ne Cede Malis

 

Exulans (tailless foot-launched glider with adjustable wing sweep)
www.infotech.co.za/exulans/exulans.htm
This foot-launched design has a unique feature. Variable sweep! Check it out!

-South Africa design=now I cant find them...

http://www.delta-club-82.com/bible/708-hang-glider-exulans.htm

Photo Hang glider : EXULANS (University Of Pretoria)

 

I'm curious if anyone has pilot reports on how aircraft with these Weltensegler-crank or diffuser tips perform in flight, and more importantly, how they handle.
From what I've read about them:
1. They are immune from Dutch roll problems.
2. Tip controls produce favorable rolling & yawing moments when initiating a turn.
3. Tip controls can be deployed (trailing edge deflected upward & outward) simultaneously to act as an airbrake while twist, pitch stability, and yaw stability are increased, just when they are needed most for landing.
4. Very good yaw stability whether tracking in a straight line or turning.

The Northrup N1M reportedly handled extremely well... early test flight problems were related to a weak powerplant and flow separation due to a thick airfoil causing a small elevator dead zone.. both problems were readily fixed.

 

I haven't seen any comments from people who actually flew the N-1M. The only comment I recall reading about their experience with the turned down tips was from Jack Northrop. He said that they didn't help as much as hoped but did decrease lift significantly. The Northrop tips were not really diffuser tips according to some people because the break lines were parallel instead of converging ahead of the aircraft. This converging break line is why they're called "diffuser tips" because the lines diverge aft of the plane and someone thought this resembled a diffuser section. Even without the AoA change that the skewing of the break lines affords there's still a reduction of lift because the pressure vectors act perpendicular to the surface and all the pictures I've seen of the N-1M with the tips deflected show a pretty large angle. The lose of lift would move the center of lift forward making it necessary to move the CG forward to allow the plane to trim at the same AoA: Northrop may not have anticipated that and probably didn't like it

I wouldn't say they're immune as much as that the onset of dutch roll can be pushed to a lower speed and the speed range that is free of dutch roll may be be wider.

The skid/roll moment is proportional to the lift differential of the left and right wings but the skid/yaw moment is proportional to the drag differential (drag increases faster than lift). No tip configuration is going to eliminate that or even change it very much. What dose change it significantly is a longer static margin.

I agree with that. Cardenas said he won many bets with other pilots who didn't believe that the YB-49 could do spot landings. Those bets provided the practice that allowed him to land on the little runway at Winslow

I have no doubt that the turned down tips provided better directional stability and control than the drag rudders alone but Northrop must have decided that the design was adequate without them since none of the N-9Ms had provision for such

 

I'm not an engener ,but I have an interest in understanding about air craft and the rules that can keep them in the air and whith out causing unwanted disaster know what the limits of the onvelope of design in a word look at the taradon which lived 70 milion years ago that flew up and down what was to become the Mississippi river if you wish but i like your thoughts to action. Hobart

 

Interesting.. I didn't realize the origin of the name "diffuser tips".. a bit of a misnomer since it really doesn't function the same as a diffuser. Whether or not the break lines diverge, most implementations have the drooped down panel at a somewhat lower angle of attack... in other words, toed-in... which is another form of jump-twist... so the break lines intersect in front of the aircraft, as you described.

Nickel & Wohlfahrt explain that the Weltenseger crank eliminates Dutch roll in 2 ways..
1. the inner part of the wing with high dihedral diminishes the large skid-roll moment.
2. the drooped tips increase the yaw damping significantly..
the second effect reinforces the need to make the drooped panels toed-in.

Skid-roll and skid-yaw moments can be produced by lift differential and drag differential, as you say, but for turning and lift distribution control, we can use other means. "Longer static margin" sounds like the Horten approach of unloading the tips and reducing effective span... which I do not believe is necessary or desirable.

This Weltensegler crank arrangement is so bloody elegant: with control surfaces only on the drooped panels, a turn with correct yaw and roll moments can be initiated by deflecting only one control surface upward .. you get both rudder and differential aileron in one.. Using both control surfaces in unison, you also have an elevator.. deflect them even more, and you have airbrakes too!.. aileron, rudder, elevator, and airbrakes all in one!

you may be right, but we don't know for sure what drove Northrup's decisions. Perhaps it's a moot point anyway. Northrup was trying to develop militarily aircraft for a very rapidly evolving post WW2 world.. my objective is to create a fly-like-a-bird machine. :gig:

 

Dear friends,

tho most important thing for good flight onditions with the Stromburg-types is the use of slotted flaps at the wingtips. They reliaby avoid flow separation and improve the controllability. They work well fram small to big scales.

Unfortunately they are often forgotten in the replicas!

On the following link you find a construction plan of the "Stromer", including the assembly of the slotted flaps, so called "Junkers flaps". Below the plan you find more information and see their realisation on a small wing. Sorry, that all is in German, but the pictures are mostly self-explaning.

Stromer-Plan

Sincerely Michael Schönherr

 

That flying wing design is interesting Michael --the airfoils are all non reflexed and the root end has a flap which conforms to the inboard flap to trim type (S.W.I.F.T =Swept Wing Inbord Flap to Trim) principle --the B52 wing and the Armstrong Whitworth AW 52 embody the same sort of layout --the inner wing root is ahead of the CG enough to cancel the pitching moments . Norman is very familiar with the Mitchell wing which also uses the external airfoil flap as elevon and might have some comments on the need for such surfaces -- it probably avoids the danger of a separated flow at the tips of a swept wing interfering with the boundary layer on a trailing edge surface although the case for delta wings still seems to work (with the crank in the wing a discrete 'crease' in the airfoil, when yawed, looks like it could trip the flow as well -- it seems not to though . ) Such tips on a joined wing might have some usefulness as well .

(actually Norman I was wrong about the Nietperz --only a swept crank gulled wing and might have had the Kazka (sic) canard glider also by SZD in mind -(-the Wampyr tailess glider of about the same time was another of the experimentals that tried something out of the usual sailplane layout and only the SB 13 seems to have investigated rear swept tailess config with upturned wingtips as winglets --combining these would result in a sort of streched out Vari eze --canard plus rear swept double tapered wing with tip fins .

Julian Wolkovitch did the prelimianry design of a carrier based aircraft for Vought that had both a low large foreplane (bigger than a canard) with a rear wing of Weltensegler shape --there is an AIAA paper featuring it --in that case the downturned tips act through a larger moment arm as almost tip fins in addition to their roll yaw effect. The miles Libellula (or was it the M39 ? the twin engined tractor ) had something in common to this Wolkovitch design and might have even influenced him --it also, I believe led to the joined wing via a later rear parasol "T" tailed version -- the loss of lift Norman refers to on the down cranked part of the wing would be partly compensated by the upwash from the sweep and being a non planar wing would it not potentially have a lower Cdi factor (inverse of e ) like the "C" wing tested and found to have the lowest Cdi ? should it also work if 'inverted' --curling downwards --like winglets can ?
The deliberate double cranking in some birds with big anhedral outboard might give some sort of benefit perhaps ?