Facts about flexwing flight

Facts about flexwing flight

An article written by Giles Bru that defines the reasons and philosophy behind Air Creation wing designs. Very sound knowledge, experience and wisdom that may allow us all to understand the considerations about wing twist, reflex bridles, strutted wings, the corset and other aerodynamic truths about flexwings.


  • Jozinko

    Very good article!

  • cburg

    Good article...agree with almost all of it.  I like this point:

    "Pitch tests on a specialized vehicle, such as that of the DHV in Germany, does not guarantee faultless stability. You should know that the DHV that certifies hang-gliders was forced to revise downward the minimum value of Cm required for homologation in order to approve the topless wings, under pressure from hang glider manufacturers. It is also well known that customers’ wings do not keep the same sprogs adjustments as the wings that are successful in these tests, otherwise they would lack performance and especially handling ease. It is a fact that hang-gliding favors performance as the essence of its activity and that many pilots are willing to sacrifice safety for performance, knowing that, in addition, the parachute is for this activity a proven way to obtain a serious second chance. In addition, it’s a fact that no vehicle test can actually measure the stability of modern weightshift wings up to their maximum speed. And it is precisely with the increase in speed that sprogs lose their effectiveness, unlike reflex bridles. We do not claim that topless wings are dangerous. (Some, yes, it's obvious.) But this formula simply does not currently correspond to our (Air Creation) own standards of safety. And we do not see the point of sacrificing those standards in order to gain a few km/h of maximum speed. Again, this is a manufacturer’s choice. A choice that is actually confirmed by the excessive proportion of accidents involving topless wings around the world."


    The following comment is wrong (but I understand his point):


    "In fact, a completely rigid wing can not be controlled by shifting the center of gravity!"

    I guess he's never tried it...because it does...but wing warping/billow shifting (as he described) helps a lot to be sure.




  • Tussock

    Yes, the latter point is interesting.  Back when Moses wore shorts, Bill Moyes was trying to get higher aspect ratio Rogallo wings to turn.  He rigged a fancy arrangement which let him activate a keel pocket with his feet.  In tests he found the system worked just as well without any input from him, and the keel pocket as we know it was born. It was the secret to getting higher performance at the time - the tricks for improving performance were known, but lack of controllability from higher aspect ratio wings held them back.  When Wills Wing introduced the keel pocketless HP some time later, the joke became that HP stood for handling problems. That a completely rigid wing can be controlled by weight shift is demonstrated by the weight and balance needs on most aircraft.

    Getting adequate stability from topless wings is certainly a more challenging exercise than using luff lines. Anyone remember the Progressive Aircraft Dawn?  It can be done, but it takes weight - not just in the sprogs, but it's needed to give adequate strength in the leading edges against the torque they generate. 

    Great to read the article, thanks for posting.

  • cburg

    The Canadian ASG 23 in (1979) came well before the Dawn, but was expensive to make.  The ASG-23 had no crossbars, just a very large "super-noseplate" which the leading edges were plugged into, during set-up. The struts were a very important part of the structure, making a very strong airframe. I believe that he still would have had a market, even at the higher prices, but the market would have been small.

    There was also a strutted German hang glider, a Firebird CX. Heiner "Pete" Bissel ("the Baron") flew one, around the same time frame.


    Albatross Sails “ASG 23”



    Pro-Air “Dawn::



  • Tussock

    Cburg, you're a great historian! I hadn't heard of the ASG 23 - do you know if it used anything more than tip struts for dive recovery? 

    I seem to recall (and my recollection of thirty years + may be questionable) that the Dawns suffered from a few tumbling accidents and wouldn't have passed pitch requirements for certification (and it was never certified).

  • cburg

    Here is some re-posted info on the ASG-23.  I have more but need to dig it up.



    Randy Rauck

    This photo was taken at the American Cup in 1979.
    Myself and Larry Croome were flying the only two production topless models ever in existance, with the Canadian Team. Not sure if the photo is me or Larry. The ASG 23 design needed more mods and tests to be competative with the just released and well finished, Comet 1 by UP which was half thw price and flying well. Tom got rid of crossbar drag by eliminating it and UP (Roy Hagard) covered it up to achieve similar results and a lot less cost.


    John LaTorre

    I'm not sure Randy's right about this being an ASG-23. If it was made in 1979, I think it was an ASG-25, since Tom Price had already been hired by Electra Flyer by spring 1978. (That was when I came to work for Electra Flyer, and Tom was already there.) I remember helping Tom make a couple of these sails and fit them to a crossbar-less frame with fiberglass leading edges.

    The ASG-23, as I recollect, was a more conventional glider with crossbar, preformed ribs, and fiberglass leading edge tips to tension the trailing edge at the tips, much like the Stratus gliders. (I don't think the crossbar was enclosed.) It went out of production in the winter of 1977-1978, when Tom closed his own sail loft and went to work for Electra Flyer. I remember seeing some stored ASG-23 patterns in the sail loft there, but no sails were made from them.


    Larry Croome

    Randy has the story correct except the year was 1980 I believe. Tom brought the ASG 23 up to Canada with its preliminary build done -it needed some shape refinements as well as new spars. Randy has a quancet shop and most of the financing in the 3 way partnership to develop the worlds most advanced and unbeatable hang gliding machine.
    Randy and I set in a concrete floor in his shop in late June thanks to the support of Randies incredible folks (Bless and thanks John & Denise) who were happy to pay for the concrete for their building.
    This flying life of Randy's with the grand central of activity was fun and exciting especially for Randy's mom. Cowboy coffee and three meals a day for us -no conditions and don't dare try and pay for it.
    Tom arrived several weeks late and about one week before Grouse for which reason we did not have time to be ready for Grouse and best to keep it under wraps until the nationals. The ASG 23 version 1 was small and very efficient at speed. We built a whole new sale for the rocket on the old spars with the dubious extra 1/2" holes in them that Tom assured us were in the nuetral axis. I was lucky enough to fly the new prototype in the 1980 Canadian Nationals Gaspe, Quebec. The wind blew 25 mph + for 8 days and I had a comfortable lead -though not enough to take the last day off. The wind virtually stopped on the last day and it turned into a sink rate contest. I went from 1st to 7th. Oh well, two years in a row may have been too much for my head anyways.
    Randy was graciously but openly envious that I flew it for the competition and in hind-site Randy would have been the better pilot for this event and likely would have pulled off a win that year. I was great at pylons and tight closed courses, getting that last pylon with the wing a foot of the ground and bulls eye landings. Randy could core out a thermal in most places better than me and certainly anything cross country and high strategy he was better at. I will say I am pretty sure this is me flying due the arch of the body -that is what I used to do -as if it would make me fly better!
    After the nationals we built two new ASG23's with two new sets of spars and nose units starting in the second week of September. Only 4 weeks later we had two new gliders very much redesigned. Tom for design and sailmaking, me for building, sailmaking and flying and Randy for flying and promoting and sales). Would have been a good team if Roy Haggard wasn't so damm smart with his comet.
    Tom stayed home in Lumby while Randy and I and a four others piled into my dad's motor home and drove non stop from Lumby to Montreal plus another 5 hours around the north road and down the Gaspe Penninsula. Amazing trip and non of us had been this far east. My dad really enjoyed the pictures -especially the one in the motorhome with the centerfold pinned on the wall. Everyone will have remembered this trip to the land of Micheal Trembley and crew.
    Our new ASG's had More span and about the aspect ratio and a better(?) wing tip. While we were at the nationals Tom stayed home and worked on the ASG 23 v2 -some specs based on our feedback from the nationals. The comet appeared at Grouse mountain the same time we were ready to go -so wouldn't you know it, but we thought we could out do it -but were always leary of the sale price difference which proved to be true. It just seemed we could not make enough difference and so it went -though it was a year of working most days and evenings and a lot of learning and traveling -thankyou Tom and Randy and your incredible folks that I know Tom won't forget either.



  • cburg

    Here's some info on the Dawn.  It had sprogs (before they were called sprogs) and a raised keel pocket strut to provide trailing edge reflex.  I wonder how they would compare with a modern strutted trike wing on a test vehicle at nuetral/negative angles.  My guess...just about as good.  Note it was HGMA Certified...requiring pitch testing.  And yes, they got tucked and tumbled.  I think if a modern trike wing was made light for hang gliding...it would get tucked in the same conditions...maybe sooner.  At least the Dawn had the braced raised keel pocket = more reflex.  Low CGs of heavy trikes and less "big air" help them from tumbling.

    Maggi Boone

    Just wanted to mention this is a picture of the very "first" Dawn being flown by it's designer and company owner, Richard "Dick" Boone.

    Surprised him to find this on the net, let alone a non-us site.


    Terry Hackbart

    Dick Boone designed this and many other inovative gliders. He should be credited for much of the design work that came out of Bill Bennetts company Delta Wing. Dick was an early inovator in the sport, and many of his ideas are credited to other companies. He does not get the credit he deserves for helping this sport progress in the early years.

    Mike Sandlin

    Dick Boone made some beautiful gliders. I had a Dawn (156?)and liked it very much. You could pack the struts up into the sail without detaching them, so the setup was fast.They had no special flight aspects that I can recall.

    wayne smith

    I own the black comp in your pics, have several dawns including a brand new never flown one, I would be glad to send more pics, I still fly the comp and my 175 big one.

    John Minnick

    I was a friend of Dicks' back in the 70s & 80s. Flew this glider and many more of designs.

    Kestutis Zelnys

    Interesting, never knew about it. I designed and built and flew strutted hangglider in 1983.


    i put struts on my rogallo sl cohen 220 back in the mid 1970's in canberra australia and i still recon its a better way than topless cross bars less sophistication for a small price in drag i saw the french gye at buffalo victoria australia 1986 nationals fly his version of a topless with struts before topless came out it seems every one is on a trip were even if it is one ounce of drag they will go for it and pay ten times the price its not gliding its the upper class image and expence .. there is many ways to reduce drag but it has to be their way ..i desinged triple surface to lower camber o the bottom surface by foam padding for moyes csx named after me but give it ten years he might spend money on it .. skeatesy


  • Tussock

    Just dug through up old hang gliding magazines looking for the Dawn ads... and found a pic I can't identify (HG, Nov 84, page 25) showing strutted wing at Telluride. The Dawn was a pretty glider, looks good in the ads. I didn't realise (or my memory is fading) it was certified.

    Yes, those keel pocket struts (we called them sting posts, but that may be a local term) became another helpful item in the pitch stability armoury that were sacrificed for performance.  I currently fly a kingpostless, small, drum-tight, low washout trike wing and I'm very conscious of its behaviour in turbulence - the tips will stall in bigger bumps, and the low pitch pressures that let it go fast when you stuff the bar are no longer an asset when you're getting biffed about. I'm not convinced by it.

    Unfortunately dive recovery is an impediment, right up to the point when you need it and can't get enough of it. 


  • cburg


    Like we've discussed on this forum regarding aspect ratio and big tips versus small tips on small strutted wings.  Flexwings in general are a compromise in every respect, and particularly small strutted wings.  Tip area, like everything else has both positive and negative effects.  It just boils down to comfort level and priorities.  Low aspect ratio big tip wings have good pitch and yaw stability, but this stability comes at price in speed range, bar pressure and bar travel.  There’s no right answer.


  • cburg


    Can you post the photo?

    Apparently there is a guy who owns several of them and still flies one.  He's even got a brand new (never flown) Dawn.  I bet she's a beauty.

  • Tussock

    I'll have to hunt for a camera, but the photo doesn't show much - it's straight on, from a distance. I'll see if I can get something posted soon.

    Something that Giles touches on in his article, and is very apparent with my wing, is the influence of anhedral.  With a drum tight, all Mylar sail on stiff leading edges the lift distribution extends a tad more outboard than many wings, and to get it to roll at reasonable rates and pressures the maker has given a bit of anhedral to the wing.  It's easy to fly and vice free around trim speeds, just very sensitive to pitch,  but things change at very low angles of attack. It goes from being roll neutral to roll and spiral instability when the speed gets up, particularly at high throttle settings.  If it's allowed to sideslip when going fast, the slip generates yaw, which generates more roll, lowers the nose, and gets into a kind of positive feedback where speed, yaw, and roll increase and the nose keeps dropping. This happens very fast and will roll the trike beyond vertical in a few seconds. It isn't possible to recover from this unless the throttle is pulled.  While this doesn't happen in normal flight regimes, it's quite possible to get into this situation inadvertently, and in my opinion this condition is dangerous.

    i don't understand VATS system - as Giles says, it seems back to front. At high speeds you want to be heading towards dihedral, not anhedral.

    At some point it seems the search for performance takes too much away from what flex wings do best - simple, straightforward flight.

  • Tussock

    Here's that photo...

    See if this works...

    Sorry, it's not a great rendition.

  • cburg

    Thanks for posting...I'll add it to my collection.

    Yes I’ve commented several times on the “tip walking” issue, in conjunction with “Speed Range”.  Some wings lack yaw stability and when flown at the upper end of their speed range the handling qualities severely suffer.  I call these “one-speed wings”…and evaluate them based on their “usable speed-range”…they only fly well at one speed.  Many popular wings are this way and I would never own one.  I dislike this trait.

    When I own/evaluate a wing, I judge this trait very harshly.  I demand good stability throughout the entire speed range.  I’ve found that for triking, small nose angle, low aspect ratio, big tip wings are my preference.  The opposite is true for hang gliding…because of the improved efficiency of a higher aspect ratio.  I willingly and knowingly sacrifice efficiency for directional stability when triking.

    There are some small tip wings that have excellent directional stabilty at all speeds, but usually a big tip wing is better in this regard.  Everything is a compromise…speed…stability…performance.

    I wrote several threads and posted several pictures on the other trike forum on the subject of Dihedral and anhedral.  I think is useful info for folks new to flex-wing flying.

  • cburg

    I've got a scan and hard copy of most of the old HG rags, but it's somewhere in my well organized (not) database.  Perhaps I can sumble into some more info on the Dawn and ASG-23.

  • Tussock


    A review of the Dawn:


    I gather you've tried a three-dimensional hang block?  Was the base yawed by thrust? I can imagine a tall windscreened, wide podded base giving a little trouble... how did it go?

    and btw, is the gyke project still rolling?

  • cburg

    Thanks I pasted the above post on the Dawn from Delta club.  I love that site.  I go there often.

    Not much progress on the gyro...too busy flying the Mosquito turbine.  What a hoot!

    The three-axis bracket aspect just happened to be discussed with the other topics in this thread...so I pasted the links anyway.

    Mostly is was about sweep, twist, washout, billow shift and yaw/roll stability.