PEEL HISTORY One day in 1988 I sat down and listed the ideal features for a traction kite, attempting to anticipate in advance what the traction kites of 2015 will look like. Some of the listed features, in addition to the fundamentals of good Lift to Drag Ratio and satisfactory M.M.R. (maximum pull/minimum full ratio - see later explanations) were: Ability to scale up to large sizes without deterioration of the weight/area ratio, full options for airfoil profile; robustness in crashes (especially when being swilled around in the surf) and at least a potential for automatic reefing (some way of making the kite smaller automatically as apparent wind increases). Ease of packing and "soft is safe" were bonus features after I had already decided to attempt a soft stunter. The direct lineage of the Peel design was an evolution from the old 20sq.m. two line parafoil I had been given by John Waters while at Lincoln City in 1987. The problems faced were to dramatically improve L.D.R. and control without causing a luffing problem. Soft kites also have the inherent problems of necessary bridle complexity and a tendency to collapse when flown into wind pockets (their form holds only when there is positive air pressure available from relative motion). After thirty three prototypes and numerous frustrating re-cuts and re-rigs the first satisfactory Peels were coming through intermittently during 1990 and I started using them at festivals during that year. Chasing those essential characteristics for traction kites L.D.R. and M.M.R., I tried every possible airfoil profile from a wedge to a Flexifoil clone. At first I could find no window between poor tracking (the ability of the kite to hold a horizontal traverse, without "mushing out" especially in light winds - kites that fail by this measure will gradually require to be pointed more and more upwards even while maintaining a horizontal course) and luffing (tendency of the kite to collapse catastrophically when the angle of attack becomes negative). I differentiate two forms of luffing. One occurs when or if air flow ever sets up a "couple" with one force lifting the trailing edge while another force pushes the leading edge down. This form of luffing is inherent to the profiles used and to the bridling setup and can occur even during steady flight in smooth wind. It is absolutely unacceptable. I call this form of luffing Centre of Pressure migration luffing, C.P. luffing for short. The other form of luffing occurs when the kite experiences negative angle of attack. This can easily occur when flying in turbulent winds or even just when high speed/momentum carries the kite out passed that point at the edge of the wind where it is, in the limit, able to hold steady state flight. This is a huge problem for kitesailing because unless you can walk on water there's not much you can do to stop it once it starts happening. When flying statically (i.e. standing on the ground) fliers instinctively, unconsciously take a quick step backwards to hook the kite back into some movement relative to the air - the prerequisite for continuing kite inflation. I call this form of luffing Apparent Wind luffing. A.W. Luffing for short. For a while I despaired of usable solutions and went back to developing delta form rigid frame stunt kites. For reasons explained later this kite style has fundamentally good M.M.R. a feature that I am sure is well understood by Don Tabor from San Diego to whom the kite world owes a hugh debt for the development and popularising of this kite style. Building larger and larger delta form kites (up to 5.5m windspan) I was eventually defeated by a scale effect problem. Lift increases as the square of span but strength requirements increase as the cube of span. Big rigid kites suffer terminal handling problems because they must be too heavy for their area. For all this, large rigid structure kites (carbon) could yet become the dominant traction kites as their shortcomings are circumvented or if the alternatives prove to have worse failings. Likewise for stacks of smaller rigid frame delta form kites which I have, at least temporarily, given up on because of launching problems and their tangle susceptibility when crashed - especially in rolling surf. Also tube breakages can be a nuisance. Returning to soft stunter development I decided to just put up with poor tracking and bridle the kites back to eliminate C.P. luffing while I concentrated on control/steering development for a time. By October 1990 I had tried a large number of systems starting with just separate bridles to each side of the canopy as for the original John Waters parafoil and the Paraflex. This caused a loss of performance through canopy distortion. The next attempt was to use a semiflexible bar spaced out along which were bridles to the individual cells and below which were various criss- cross or linked "Y" bridles connecting to the flying lines. This worked very well but was "impure"-use of a rigid member in a soft kite. Next came the caternary bridle; the flying lines are set up as a continuous loop with individual cell bridles attached to it at various spacings. With this I could not get tight turning without unacceptable canopy distortion and large arm movement. But I have since seen this type of bridle used entirely successfully by Nop Velthuizen in Holland even on 5m2 soft stunters. Next I tried a complicated cascading pulley system which worked satisfactorily but then hit on the current (and copyright protected) cross bridle system which is simple, causes minor if any canopy distortion and allows tight turning without excessive arm movement even for 7.5m wingspan kites. In early 1991, researching airfoil profiles I was told of the profile forms used on flying wing airplanes to prevent basically the same C.P. luffing Peels were suffering from. This quickly led to the (copyright protected) humped trailing edge profile we now use that opened the window to good tracking and the elimination of C.P. luffing. I now believe that it is possible to use a profile without a trailing edge hump and not have serious C.P. luffing but there are currently other reasons for not doing so; the assumed M.M.R. benefit from the hump and also a simple but poorly understood limitation of "skins and ribs" soft kite construction. The average kite airfoil profile is not the rib profile but is always deeper particularly at points where the cell width is much greater than the rib depth. I am always much amused by the mega precision and mega computer optimisation that is claimed for some soft kite rib profile airfoils. The rib profile applies for less than 1% of the kite! To accomplish a perfect wedge shaped trailing edge, an infinite number of ribs would be required. Practicable rib spacings always cause "coal sack" type distortions along the trailing edge, seriously disturbing airflow exit and placing a limitation on attainable L.D.R. of maybe only 15 or so (good sailplanes manage a wing L.D.R. of 100+!) the humped trailing edge may cause less form drag than conventional trailing edge forms - think of the wedge shaped low drag shapes currently fashionable in car design. Using the humped trailing edge the window has opened very wide. I can now bridle Peels far forward of the position where the lift coefficient drops out of the useful range without C.P. luffing becoming a problem. There was always a solution to the C.P. luffing problem. The "Paraflex" design from Wolfgang Schimmelpfennig and Wolkensturmer uses bridles only in the forward 25% of its chord and an airfoil which seems to be evolving towards the "Flexifoil" profile. This is an excellent vice free and exciting kite to fly but does not seem to have a satisfactory combination of L.D.R. and lift coefficient to allow satisfactory upwind performance for kite traction application. As for Flexifoil though, it could be quite suitable for speed record use where sufficient "runup" is available for apparent wind to build. A basic explanation of this limitation may well be to think of only the forward 30% (the bridle supported portion) of the Paraflex as being the effective lift creating area with the remaining 70% just along for the ride, for visual effect (as used in the very effective "Snoopy" and "Big Boss" etc type decorative kites) and to act as a sort of anti luffing "whales tail". To complete the Peel history requires introduction to the final member of the Peel innovation troika: Cross Bridle, Humped trailing edge and, as from the patent filing date of June 1992, the Reefing Bridles. First must come a thorough explanation of how the characteristics of L.D.R. and M.M.R. can be combined to make a good traction kite. Accepting that as buggy/boat speed reaches true wind speed and above (see Kite Traction), all courses tend to be upwind courses from the kites perspective. It is essential that the kite has good L.D.R. and as much pull as possible when it is "at the edge" (see definitions). L.D.R. is a measure of how far around the "edge" is for a given kite and apparent wind. 80 degrees, representing L.D.R. of 5.7 is currently very good L.D.R. for a kite. If a second kite can only manage 70 degrees at which "edge" it has the same pull as the first kite at 80 degrees then, quite accurately, a buggy/boat powered by the first kite will be able to maintain the same velocity to windward as the second kite but on a 10 degree higher course. This is a huge difference and would probably result in the second buggy/boat being lapped after 3 laps of a typical circuit. Of course, if the second kite had more pull but still at only 70 degrees (L.D.R. 2.75) It's buggy/boat won't be able to go as high as its rival but it will go faster and may well prevail. This compromise is the essence of traction kite optimisation. Kite L.D.R. is largely a matter of basic design and is often limited by the imperatives of retaining good control (steering) avoiding all C.P.M. luffing and most A.W. luffing. The biggest factor effecting kite pull at the edge is kite size, in general bigger kites generate more pull. Simple answer then: use a bigger kite! Unfortunately kite size is limited by its maximum pull (see M.M.R. definition) not by its pull at the edge (minimum pull). Too much pull and you capsize (boat) slide sideways (buggy), or fly into the air (boat and buggy), all of which are highly detrimental to progress! To allow the use of a bigger kite for superior performance upwind we need to also have a kite with low M.M.R. Of course, for buggying/boating off the wind (apparent wind hitting your back) high M.M.R. is no great disadvantage, but in most buggying and much kitesailing this rarely occurs. There are many ways of improving M.M.R. The ideal system would retain constant high L.D.R. while holding pull constant at its maximum usable value for all significant wind speeds. There are no available systems on any available kite types which even get close to this ideal. Now back to Peel development history. In its standard 1991 form Peels have good L.D.R. quite satisfactory M.M.R., no C.P. luffing and can, with experienced fliers, avoid A.W. luffing at least as well as other available traction kites. I have tried very hard for some years to improve the Peel M.M.R. still further. It is now possible on 1992 peels to use spring bridling that moves the bridle point automatically forward as pull increases thereby decreasing theoretical maximum pull by at least 25% without causing C.P. luffing. This seems like an ideal solution to better M.M.R. but it has an (expected) side effect. Bridling forward increases the speed of the kite through the air which increases the pull proportionally, offsetting much of the pull reduction accomplished by forward bridling in the first place. Accordingly the best solution seems to be to make the kite smaller as pull increases - a reefing system. My diaries from 1988 onwards are peppered with unsuccessful attempts at Peel reefing. I developed what I thought was a clever, organised search system just for the purpose of finding a usable peel reefing system but after four years of rigorous application I had exhausted all possibilities. The next night a solution just occurred to me in the middle of the night! It does cause a deterioration of L.D.R. as reefing progresses but so does every available system on other kite styles (except, I suspect that wing tip twist off as used on delta style stunt kites probably helps control tip vortex losses at the same time as reducing pull) and it works! Currently achieving about 25% pull reduction over non reefing layouts it has the potential of achieving 50% reduction or more. It also appears to have the bonus of reducing the occurrence of A.W. luffing. I surmise that as the kite's momentum carries it into a lower apparent wind zone, the kite responds by increasing its effective lift area, allowing the kite to climb out through the danger zone. Experience seems to be confirming the result at least if not the hypothesis. As things stand I believe that effective reefing is one of the two great breakthroughs that are required if kitesailing is ever to become a viable recreational activity. After 3 months use I believe this Peel reefing system is at least the beginnings of that breakthrough. (The other required innovation is an on-the-water launching and retrieval system which I believe is also close to being achieved.)