Date: Wed, 10 Aug 1994 00:15:15 -1000 From: andrewh@holly.harvard.edu (Andrew Hawken) Message-Id: <32a9bj$79v@beta.qmw.ac.uk> Organization: Queen Mary & Westfield College, London, UK Subject: Re: Flexifoil Physics?? MR SC LAWRIE (spsl@giraffe.ru.ac.za) wrote: : I have a 6ft version, and I'm intrigued at how much lift it generates. I : thought that the centre of lift for a wing was about 1/3 back from the : leading edge. Since the flying lines come off the leading edge, I would : have expected that the whole thing would simply drift downwind, as opposed : to climbing into the sky. Look carefully at the profile, it is what is known as an S-profile. (Thanks Nop). It generates "lift" on the underside towards the back, pulling the kite back towards you, as well as the conventional lift on the top surface. The result is some kind of a leveage effect:down at the back, up in the middle gives up at the front. This removes the need to bridle towards the trailing edge to maintain the correct attitude while still contributing to the pull. I am not an aero-enginneer, and figured the above out myself, before I bought Stunt Kites II. In the book there is a better explanation (probably more accurate too!), but I dont have it to hand, so I am afraid that you get my version. Handle with care :-) Andy -- ____________________________Andrew Hawken______________________________ "However many ways there may be of being alive, it is certain that there are vastly more ways of being dead" R Dawkins. Home : 0895 420110 QMW : 071 975 5542 AIIT : 0494 677045 Email : A.Hawken@QMW.AC.UK = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Tue, 9 Aug 1994 23:29:46 -1000 From: spsl@giraffe.ru.ac.za (MR SC LAWRIE) Message-Id: Organization: Rhodes University Subject: Flexifoil Physics?? I'm interested to hear from anyone who can shed some light on the physics of a flexifoil.... I have a 6ft version, and I'm intrigued at how much lift it generates. I thought that the centre of lift for a wing was about 1/3 back from the leading edge. Since the flying lines come off the leading edge, I would have expected that the whole thing would simply drift downwind, as opposed to climbing into the sky. I also can't see why it should produce the pull it does, even when its at its zenith...again, my perception of the centre-of-lift thing is that the kite should be tethered directly below this, not at the leading edge. Any suggestions?? Steve +-----------------------------------------------------------------------------+ | Steve Lawrie P.O.Box 360 Grahamstown 6140 South Africa Tel +27 461 25631 | +-----------------------------------------------------------------------------+ = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Wed, 10 Aug 1994 07:44:26 -1000 From: travell@comics.enet.dec.com (I want to Go Fly a Kite!) Message-Id: <9408101644.AA27080@vbormc.vbo.dec.com> Organization: Division of Applied Sciences, Harvard University Subject: RE: Flexifoil Physics?? >I'm interested to hear from anyone who can shed some light on the physics of >a flexifoil.... >>Look carefully at the profile, it is what is known as an S-profile. I presume that the expression "S profile" comes from the shape of the median centreline of the foil shape. In aerodynamic terms it is a reflex aerofoil similar in principle to those used on various flying wing aircraft in the past. To re-phrase what Andrew Hawken says, it generates normal lift from the front half of the foil, and `negative' lift from the rear half, just enough to control the attitude of the foil to the apparent wind seen by the kite. In all, a very clever design. John Travell. = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Mon, 22 Aug 1994 10:30:48 -1000 From: pwmeek@garnet.msen.com (Peter W. Meek) Message-Id: <33b1to$i7u$1@heifetz.msen.com> Organization: Msen, Inc. -- Ann Arbor, MI (account info: +1 313 998-4562) Subject: Re: Flexifoil Physics?? MR SC LAWRIE (spsl@giraffe.ru.ac.za) wrote: : I'm interested to hear from anyone who can shed some light on the physics of : a flexifoil.... : I have a 6ft version, and I'm intrigued at how much lift it generates. I : thought that the centre of lift for a wing was about 1/3 back from the : leading edge. Since the flying lines come off the leading edge, I would : have expected that the whole thing would simply drift downwind, as opposed : to climbing into the sky. Well, I can't help with the physics, but I can help with a point to consider: in flight, the Flexi's spar is bowed. In fact, if you try to fly a Flexi with a rigid spar, it will fly (actually, fail to fly) exactly as you describe. -- --Pete Time flies like an arrow; time durations like a stopwatch. = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Tue, 23 Aug 1994 07:50:45 -1000 From: gaffer@ibmpcug.co.uk (Andrew Beattie) Message-Id: Organization: /usr/lib/news/organiszation Subject: Re: Flexifoil Physics?? Peter W. Meek wrote: >MR SC LAWRIE (spsl@giraffe.ru.ac.za) wrote: >: I'm interested to hear from anyone who can shed some light on the physics of >: a flexifoil.... >Well, I can't help with the physics, but I can help with a point to consider: >in flight, the Flexi's spar is bowed. In fact, if you try to fly a Flexi with >a rigid spar, it will fly (actually, fail to fly) exactly as you describe. The bending of the spar is essential for proper steering response. The reason that it can be bridled at the leading edge is that the flexi has a wierd cross-section. It produces a lot of lift close to the leading edge, but the rear of the foil pushes *down*, with less force, but more leverage, to maintain proper angle of attack. Andrew -- This is not the account that I usually Email from. Please continue to use my usual addresses, thankyou: Work -> gaffer@rec.com Play -> andrew@tug.com The Kite FAQs are at ftp.hawaii.edu:/pub/rec/kites/faq/* "Kite Jumping fucks more knees than the IRA" (Lynn) = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Wed, 24 Aug 1994 13:10:23 -1000 From: pwmeek@garnet.msen.com (Peter W. Meek) Message-Id: <33gk0v$272$1@heifetz.msen.com> Organization: Msen, Inc. -- Ann Arbor, MI (account info: +1 313 998-4562) Subject: Re: Flexifoil Physics?? Andrew Beattie (gaffer@ibmpcug.co.uk) wrote: : Peter W. Meek wrote: : >MR SC LAWRIE (spsl@giraffe.ru.ac.za) wrote: : >: I'm interested to hear from anyone who can shed some light on the physics of : >: a flexifoil.... : >Well, I can't help with the physics, but I can help with a point to consider: : >in flight, the Flexi's spar is bowed. In fact, if you try to fly a Flexi with : >a rigid spar, it will fly (actually, fail to fly) exactly as you describe. : The bending of the spar is essential for proper steering response. The reason : that it can be bridled at the leading edge is that the flexi has a wierd : cross-section. It produces a lot of lift close to the leading edge, but the : rear of the foil pushes *down*, with less force, but more leverage, to : maintain proper angle of attack. Do you mean that a Flexi *WILL* fly with a rigid spar (although steering poorly)? My few experiences with too-stiffly sparred Flexi's was that the foil trailed flat out behind the spar and never developed any lift to speak of. -- --Pete "Things fall apart; the center cannot hold. But what the hell, it's home." = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Fri, 26 Aug 1994 20:10:03 -1000 From: andrew@tug.com (Andrew Beattie) Message-Id: Organization: /usr/lib/news/organisation Subject: Re: Flexifoil Physics?? pwmeek@garnet.msen.com (Peter W. Meek) writes: >Do you mean that a Flexi *WILL* fly with a rigid spar (although steering >poorly)? My few experiences with too-stiffly sparred Flexi's was that >the foil trailed flat out behind the spar and never developed any lift >to speak of. That is exactly what I mean. I suppose I'll have to go and prove it now. I'll make up a (un)flexi with 3 carbon sticks and fly it when I get a decent wind. Andrew -- Kite jumping is for morons. If you want to fly, get a hang-glider. Netiquette-of-the-day: Think twice before posting! You can FTP the kite FAQ's from ftp.hawaii.edu (directory /pub/rec/kites/faq) It is generally accepted that the ravings of Frank Kenisky should be ignored. = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Mon, 29 Aug 1994 04:29:36 -1000 From: spsl@warthog.ru.ac.za (MR SC LAWRIE) Message-Id: Organization: Rhodes University Subject: Re: Flexifoil Physics?? In article andrew@tug.com (Andrew Beattie) writes: >From: andrew@tug.com (Andrew Beattie) >Subject: Re: Flexifoil Physics?? >Date: Sat, 27 Aug 1994 06:10:03 GMT >pwmeek@garnet.msen.com (Peter W. Meek) writes: >>Do you mean that a Flexi *WILL* fly with a rigid spar (although steering >>poorly)? My few experiences with too-stiffly sparred Flexi's was that >>the foil trailed flat out behind the spar and never developed any lift >>to speak of. >That is exactly what I mean. I suppose I'll have to go and prove it now. >I'll make up a (un)flexi with 3 carbon sticks and fly it when I get a >decent wind. I'll easily believe the flexi will fly if it isnt flexed. In a light wind, if you let it ride straight up, it flies almost directly above you with practically no pull. As far as I can see, it is flying "unflexed" up there; it is also generating "no lift to speak of". >Andrew Steve +-----------------------------------------------------------------------------+ | Steve Lawrie P.O.Box 360 Grahamstown 6140 South Africa Tel +27 461 25631 | +-----------------------------------------------------------------------------+ = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Mon, 29 Aug 1994 08:24:37 -1000 From: salanne@convex.csc.FI (Simo Salanne) Message-Id: <33t955$mva@pobox.csc.fi> Organization: STACK Finland Subject: Re: Flexifoil Physics?? In andrew@tug.com (Andrew Beattie) writes: >pwmeek@garnet.msen.com (Peter W. Meek) writes: >>Do you mean that a Flexi *WILL* fly with a rigid spar (although steering >>poorly)? My few experiences with too-stiffly sparred Flexi's was that >>the foil trailed flat out behind the spar and never developed any lift >>to speak of. >That is exactly what I mean. I suppose I'll have to go and prove it now. >I'll make up a (un)flexi with 3 carbon sticks and fly it when I get a >decent wind. Flexi can be flown in "self steering" mode. Flying lines are tethered on ground about half of their length apart. And there it flies and corrects itself in the middle if something in the wind disturbes it. It's a long time since I have flown Flexi this way, but I tend to remember the spar keeps pretty straight. Smooth Winds Simo -- Simo.Salanne@csc.fi STACK Finland = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Wed, 31 Aug 1994 02:53:56 -1000 From: ces1at@surrey.ac.uk (Andy Tate) Message-Id: Organization: University of Surrey, Dept Chem & Proc Eng,UK Subject: Re: Flexifoil Physics?? MR SC LAWRIE (spsl@giraffe.ru.ac.za) wrote: : I'm interested to hear from anyone who can shed some light on the physics : a flexifoil.... : I have a 6ft version, and I'm intrigued at how much lift it generates. I : thought that the centre of lift for a wing was about 1/3 back from the : leading edge. Since the flying lines come off the leading edge, I would : have expected that the whole thing would simply drift downwind, as opposed : to climbing into the sky. Firstly the flying lines don't quite come off the leading edge. The spar pocket is in the base of the kite set back slightly from the nose. When you try to build/repair a flexi these minor points become more apparant. >From what I can deduce from study of the flexi and some books on aerodymanics the flexifoil profile is one of a class of 'reflex aerofoils'. Look carefully at its profile and although the front is more or less conventional you will see that the tail section is flat on the top rather than the bottom as with a more conventional aerofoil. A reflex aerofoil has the attribute that as the angle of attack increases so the centre of lift moves backwards causing the aerofoil to reduce its angle of attack and thus to remain at a stable angle of attack. With a conventional profile the centre of lift would move forward causing the angle of attack to increase until the aerofoil stalls. Some model airplane programs on wing design will draw aerofoil profiles for you and the properties of this reflex aerofoil have been known about for ages although it has not been widely adopted or even given much coverage in books. Andy Tate.... _____________________________________________________________________________ Dr Andrew H.J. Tate /\ Computer, Control & Instrumentation Support, \/ Dept of Chemical & Process Eng, / University of Surrey, Q / Guildford, Surrey GU2 5XH Tel: +44 483 259515 (&V32bis) |-+/ Fax: +44 483 259510 / \ Go fly a kite ! It's fun. Email: ces1at@surrey.ac.uk __________-_-_-_-______________________________________________________________________________ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Fri, 2 Sep 1994 21:37:51 -1000 From: windwiz@rain.org (Dan Rubesh) Message-Id: <34994f$rnl@rain.org> Organization: Regional Access Information Network (RAIN) Subject: Re: Flexifoil Physics?? In article <3439r6$cae$1@heifetz.msen.com> pwmeek@garnet.msen.com (Peter W. Meek) writes: >Andrew Beattie (andrew@tug.com) wrote: >: pwmeek@garnet.msen.com (Peter W. Meek) writes: >: >Do you mean that a Flexi *WILL* fly with a rigid spar (although steering > >: That is exactly what I mean. I suppose I'll have to go and prove it now. > >I will await your results with interest, and might even have to retry >the experiment myself. I never had any luck with it before. Maybe the >wind was too light to lift the *heavy* rigid spar I was using. This >was many years ago, when carbon spars were too exotic (read expensive) >to be playing with. >-- > --Pete >If there was something I'd rather be doing, > I'd be doing it. I've wanted to add this bit to the discussion about Flexifoil physics for a while and haven't had the time to type it in.. It's directly lifted (without permission) from the front of the Flexifoil catalog.. The catalog has some drawings that help to explain some of the following. If anyone wants a Flexifoil catalog they can either get one from their local dealer, e-mail me, or call Cobra Kites (908) 506-0461 _after_ September 21 as they are shut down between 9/2 and 9/21.. ========= Begin Flexifoil catalog excerpt ======= The Flexifoil Power Kite. How it flies: Precision made from specially constructed tough Rip-Stop Nylon fabric, glass-fibre and carbon-fibre components, the Flexifoil Power Kite works like an aeroplane wing. The unique airfoil is inflated by the wind through the gauze vent at the front. Underneath this vent is a multi-tapered spar which gives the unique self-adjusting action for maximum efficiency in all flying conditions. Twin control lines are simply secured to the spar ends and run down to the controls. Pull, Sizes and Aspect ratios: The pull of a Flexifoil Power Kite is linked to the 'relative wind' by the wind and the speed of the kite moving through it. As the Flexifoil Power Kite accelerates upwards from launch, the pull increases (even if the wind speed is constant). As this happens, the spar ends bend downwards so that the curved outer sections provide stability and steering capability while then centre section provides the power. The wingspan of a Flexifoil Stacker is 6 ft (180 cm) and the wingspan of a Flexifoil Super-10 is 10 ft (280 cm). Both have a depth or 'chord' of 2 ft (60 cm). The ratio of wingspan to chord is the 'aspect ratio'. It is an aerodynamic fact that the higher aspect ratio the more efficient the wing will be. This is achieved with the higher aspect ratio of the Flexifoil Super-10 compared to the Flexifoil Stacker. The Flexifoil Super-10 utilizes an extra stiff 100 cm carbon-fibre center spar giving it a superior 'power section'. This, combined with the high aspect ratio efficiency, provides a performance of up to four times that of a Flexifoil Stacker for only 1.5 times the wing area. ======= End of Flexifoil catalog excerpt ========= I'm not sure if the above will help to clarify or further muddy the waters.. :-) As I sees it, the super stiff spar that's been discussed _may_ pull harder (from a longer, approaching full wingspan, 'power section'), but at the expense of stability and steering/tracking ability due to loss/downsizing of the curve that the tapered spar creates.. As a side note, there's an excellent section on the development of the Flexifoil kite and a _real_ early buggy in Kiteworks, by Maxwell Eden, pp 79-86.. I'll try to gather up all of the thread(s) of this discussion and forward them to Ray Merry, co-inventor of the Flexifoil for his possible comments when he returns.. Ob. Disclaimer: I am a stunt kite dealer and carry some of the above mentioned product(s). As such, my opinions may tend to be somewhat subjective. -- Tako Buggy Naked Kichi!! Dan Rubesh AKA Member/Merchant Wind Wizard, Purveyor of windwiz@coyote.rain.org GO Stunt Kites & Accessories danr@crash.cts.com FLY A P.O. Box 5747 (805) 659-5654 (voice) STUNT Ventura, CA 93005 (805) 659-5769 (fax) KITE! Member: Kite Trade Assn. = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Sat, 10 Sep 1994 11:19:02 -1000 From: pete_leaback@cix.compulink.co.uk (Peter Leaback) Message-Id: Subject: Re:Flexifoil Physics In Paraglider design, a vertically curved leading edge is essential for yaw stability and to discourage cells collapsing. Looking at a Flexi, the curvature will certainly give yaw stability. But, as there is a control line attached to each tip, I'm not sure how important this stability is. Pete Leaback = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =