Date: Tue, 21 Sep 1993 22:07:23 -1000 From: engel@irisa.fr (Jean-Christophe Engel) Message-Id: <27p13r$2iv@irisa.irisa.fr> Organization: Irisa, Rennes (FR) Subject: How to determine the pull of a kite This is a rather technical and more or less theoretical question. I wonder whether and how it is possible to determine the pull of a kite with a given area, making a given angle with the wind, blowing with a given velocity ? I dot not mean by empirical trial/error process, but rather with aerodynamics theory. Is there a (simple or simplified) formula that gives the relationship between kite area, angle and wind velocity (and maybe other factors that I'm missing). I must admit that my classes of physics have already faded away, but that I'd be glad to refresh them. Any aerodynamics wizzard out there in the sky ??? kite -------> / / /. <=== / . <=== WIND / . <=== / . / . / . . . . . . . . . . . . . . <---- line Jean-Christophe Engel - engel@irisa.fr = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Wed, 22 Sep 1993 06:34:16 -1000 From: andrew@tug.com (Andrew Beattie) Message-Id: Organization: /usr/lib/news/organisation Subject: Re: How to determine the pull of a kite In article <27p13r$2iv@irisa.irisa.fr> engel@irisa.fr writes: >Is there a (simple or simplified) formula that gives the relationship >between kite area, angle and wind velocity (and maybe other factors that >I'm missing). Some missing factors that come to mind: Sail Drag Bridle Drag Line Drag Keel Drag Wing-tip-vorex drag Aspect ratio Surface texture Billow Aerofoil lift coeficient Reefing effects Flying line elasticity Movement of the anchor point Apparent wind (and thus, line length at launch) Luffing Stalling Venting Wind turbulence Weight Reynold's number Very few people have a good understanding of the *problem*. A rudimentary solution may be available for flying wings, but they still use wind-tunnels. The problem for kites is *much* more complex. On a more useful level, I seem to recall that Patrick Prosser uses 10 pounds per square foot as a rool of thumb. Andrew -- Work: gaffer@plx.com Phone: +44 793 614 110 Fax: +44 793 614 297 Play: andrew@tug.com Phone: +44 256 464 912 Car: +44 831 527 614 Interested in kite traction? mail kites@tug.com = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Wed, 22 Sep 1993 15:06:24 -1000 From: wjon@okcforum.osrhe.edu (Jon Wagner) Message-Id: Organization: Okcforum Unix Users Group Subject: Re: How to determine the pull of a kite Taken from a nifty little book called "Flights of Imagination" and introduction to Aerodynamics by Wayne Hosking. The Standard textbook formula for lift is: L = .5(Lc)(p)(s)(V*V) where L = Lift force Lc= lift coefficient p = Air density s = Area of the kite (square meters) v = Wind Velocity (m/s) The weight of the kite (W) is related to its mass (M) by W = Mg where g is the acceleration due to gravity (g = 9.8 m/sec*sec). Also, typical values for the lift coefficient and air density are 0.6 and 1.3Kg/m. With thes values, the lift formula can be rearranged to predict wind speed for a kite to fly: v = (5 meters/second)the square root of M/S wish I had the right stuff to show formulas correctly. but you should have a starting point from here. have fun jon wagner = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Thu, 23 Sep 1993 05:24:11 -1000 From: engel@irisa.fr (Jean-Christophe Engel) Message-Id: <27sf2r$d5n@irisa.irisa.fr> Organization: Irisa, Rennes (FR) Subject: Re: How to determine the pull of a kite I asked: ------- Is there a (simple or simplified) formula that gives the relationship between kite area, angle and wind velocity (and maybe other factors that I'm missing). Andrew Beattie answered: ------------------------ Some missing factors that come to mind: Sail Drag Bridle Drag Line Drag Keel Drag Wing-tip-vorex drag Aspect ratio Surface texture Billow Aerofoil lift coeficient Reefing effects Flying line elasticity Movement of the anchor point Apparent wind (and thus, line length at launch) Luffing Stalling Venting Wind turbulence Weight Reynold's number -------------------- It looks that you've missed some factors that come to my mind: Coriolis force due to earth rotation Planet attraction from the x billion planets in our galaxy (not to speak of other galaxies) Perturbations generated by lost satellites or kiter inexpectedly sneezing ... and a few other that don't come to mind, so I won't quote them now ... Nevertheless, I found your comments very constructive and encouraging, and I decided to incorporate them at ounce in a new theoretical aerodynamic model that I'm going to send to NASA. Of course, I won't forget to give you due credit for all your helpful remarks ... Jean-Christophe Engel PS: Oh yes, I forgot: ^ = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Thu, 23 Sep 1993 23:05:50 -1000 From: andrew@tug.com (Andrew Beattie) Message-Id: Organization: /usr/lib/news/organisation Subject: Re: How to determine the pull of a kite In article <27sf2r$d5n@irisa.irisa.fr> engel@irisa.fr writes: >Nevertheless, I found your comments very constructive and encouraging, and >I decided to incorporate them at ounce in a new theoretical aerodynamic model >that I'm going to send to NASA. Of course, I won't forget to give you due >credit for all your helpful remarks ... Ok, Ok, I asked for that, however let me look further into your question: You are looking to work out the pull. Why? Let me take some guesses: 1) You want to ensure that your line is strong enough. Important factors include: Line length, apparent wind during launch and method of launch. If you long-line launch the kite, it will build up apparent wind, increasing pull dramatically, perhaps ripping the kite or bridle apart (Ask Patrick), pulling anchors out of the ground or breaking lines. Drag. Watch Martin Lester's Legs or most of Peter Lynn's big toys. Much of the pull on the line is due to drag, not lift. Flying line elasticity This is important. Cheap stretchy line will absorb shocks due to luffing. Expensive non-stretch lines can snap due to increased shock loading. Movement of the anchor point. A tree or ground-stake has no give, leading to maximum shock loading. A sandbag will move under heavy load, relieving the stress. 2) You want to use the kite for traction: sailing downwind. If you want to do this with a single-line kite, the best method seems to be to maximise the drag. Put up a large kite as a mast and add loads of junk to the line, to pull you along, not up. 3) You want to use the kite for traction: sailing upwind. This starts to get difficult: You want maximum lift, minimum drag without the problem of overflying (and without being too powerful away from the edge of the windowm) Andrew -- Work: gaffer@plx.com Phone: +44 793 614 110 Fax: +44 793 614 297 Play: andrew@tug.com Phone: +44 256 464 912 Car: +44 831 527 614 Interested in kite traction? mail kites@tug.com = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Thu, 23 Sep 1993 18:26:40 -1000 From: rock@netcom.com (Anne Rock) Message-Id: Organization: NETCOM On-line Communication Services Subject: Re: How to determine the pull of a kite To add a bit to Jon Wagner's post: >From Mark Cottrell's _Kite Store Book of Kites_ >From the intro: "The final section of the book contains 'some rules for flight' being a collection of the methods I use to determine . . . lift capabilities ...." ********* begin excerpt ************ Lift Capabilities and Line Strength. The lift capability of the kite and the stress this will place on your flying line has to be considered carefully especially if you are to undertake any sort of Aerial Photography .... or any other sort of load carrying activity. For most purposes it is sufficient to calculate the "lift" developed by a kite as a percentage of the wind's static pressure (don't start on about moving fluid not exerting only static pressure or there'll be trouble -- the term is classical and goes back to the beginning of aerodynamics) and assess the kite's pull and required line strength from here. Static pressure on a flat rectangular plate (ie your average kite at launch) is normally given as : P = .002214 * A * V squared Where P is in pounds, A is area in square feet, and V is windspeed in MPH (sorry, no metrication here). Since your kite will take off very nearly perpendicular to the wind this equation will approximate to the initial take off load. As the kite accelerates away from the ground the apparent windspeed experienced by the kite will increase as it climbs and moves upwind ..., thus up to about the 50 degree angle of flight point the static pressure effectively acting on the kite increases rapidly. As you go past this point the plane of the kite becomes sufficiently tilted for the effective area to be reduced and the static pressure goes down. Finally when the kite has very nearly reached its proper flying position the lift is derived from more normal aerodynamics (ie flying as what is normally thought of as a wing) and the load declines to its steady flight state. So what actually breaks/stresses your line is that rapid acceleration and consequent build up of pressure (windspeed *squared* remember) up to the 50 degree point and this is what you have to allow for when estimating a suitable line strength. The following table is given as a guide to estimating line strength for a number of different kites: Kite Type Min Wind Max Wind Factor per sq ft of area Parafoil 10 25 8-10 Delta 8 18 6 Hargrave Box 12 30 5 Cody War Kite 12 25 5 Malay 8 18 5 Tri-D Box 10 20 5 The factors indicate the number of pounds of breaking strain per square foot of lifting area you should allow for flight within the limits given as minimum and maximum wind. As mentioned initially lift capability can also be estimated on a similar basis and on the [enclosed] disc you will find a program called KITESIZE.BAS that estimates the approximate size, of a number of different kites, you would need to lift a given load. The basic premise in the KITESIZE.BAS program is that the steady flight lift of a kite is equal to the static pressure of the wind at a given speed exerted on the kite multiplied by a factor which varies from kite to kite. Since the flyable wind speed range of kites is quite small this works quite well in practice. The basic equation used is: AR = (FW * SF) / (CL * (WS squared) * .002214) Where AR is the area in square feet, FW is the flying weight in lbs, SF is safety factor (use 2.5), CL is the kite's factor and WS is wind speed in mph. **********end excerpt ********* At the back of the book are listings for several programs, and in the listing for KITESIZE.BAS are these "factors" or lift coefficients: Parafoil 1.3 Delta .9 Hargrave Box .7 Cody .8 Malay 1 Tri-D .9 Sanjo Rokkaku .8 Use these with the above equation (CL values). The program requires you to enter the FW (flying weight or load) in pounds, and then calculates the area of the type of kite you specified to lift that load. (An aside: a while back Andrew Beattle wrote that he subscribed only to rec.kites and rec.humor -- I assume that's the play account list -- but I find the rec.kites has been providing plenty of laughs of late :-) Anne - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - * Anne Rock | * * Berkeley, California | Oh Lord, let the words I speak today * * rock@netcom.com | be sweet, for tomorrow I may have to * * | eat them. * * | - sign in a Honduran cafe * - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Fri, 24 Sep 1993 09:11:29 -1000 From: Colin_Douthwaite@equinox.gen.nz (Colin Douthwaite) Message-Id: Organization: Equinox Networks Subject: Re: How to determine the pull of a kite Jean-Christophe Engel (engel@irisa.fr) wrote: > I asked: > ------- > Is there a (simple or simplified) formula that gives the relationship > between kite area, angle and wind velocity (and maybe other factors that > I'm missing). What we need now is for a bright physicist/mathematician to develop a complete formula to include all the things the Standard Formula missed out. All the additional parameters - Drag being most important - might be expressed as a table of constants which you could fit into your Simple Formula for quick mental calculations on the flying field :-) :-) Cheers, = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Sat, 25 Sep 1993 04:04:03 -1000 From: wjon@okcforum.osrhe.edu (Jon Wagner) Message-Id: Organization: Okcforum Unix Users Group Subject: Re: How to determine the pull of a kite Well physicist Have been working on the Grand Unification Theory of the entire universe, I don't see why we can't get a GUT's for kite flying?? or better maybe a pocket or palm top program... I mean absolutly no disrepect, having had students manage to break supposed unbreakable line or at least line that was suppose to be 'more' than needed in the winds with the kite! oh well jon wagner = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Mon, 27 Sep 1993 07:00:57 -1000 From: sasaki@netop3.harvard.edu (Marty Sasaki) Message-Id: <1993Sep27.170057.4387@das.harvard.edu> Organization: Harvard University Subject: Re: How to determine the pull of a kite In article , wjon@okcforum.osrhe.edu (Jon Wagner) writes: >Well physicist Have been working on the Grand Unification Theory of the >entire universe, I don't see why we can't get a GUT's for kite >flying?? or better maybe a pocket or palm top program... > >I mean absolutly no disrepect, having had students manage to break >supposed unbreakable line or at least line that was suppose to be >'more' than needed in the winds with the kite! I imagine that if you look around long enough you will uncover information about the Hargrave box kite and the Eddy kite (both were used by the US Weather Service). You should also be able to get some information about parafoils since that configuration is used in paraplanes and parachutes I suspect that the question will never have a general answer because the general answer is very complicated and you really don't need an answer. Why do you want to know the pull anyway? Usually it's to determine how strong your line has to be, or whether you need help flying the kite, or how large an anchor you need to hold the kite down. After flying kites for a while you can usually get "close enough" (this is an official engineering term ;-). Besides, even if you came up with a number, you would have to calibrate yourself to what the numbers mean. I remember flying a pretty good sized rokkaku once in moderate wind. It was on 135 pound dacron line and I thought it was pulling fairly hard. I was surprised to see that the kite was pulling around 30 pounds (spring scale used for weighing fish). I thought it was pulling much harder than this. The number is just a starting point for choosing flying line strength. Knots and line damage can easily half the total strength of the system. -- Marty Sasaki Harvard University Sasaki Kite Fabrications sasaki@noc.harvard.edu Network Services Division 26 Green Street 617-496-4320 10 Ware Street Jamaica Plain, MA 02130 Cambridge, MA 02138-4002 phone/fax: 617-522-8546 = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Date: Thu, 30 Sep 1993 21:48:20 -1000 From: salanne@convex.csc.FI (Simo Salanne) Message-Id: <1993Oct1.074820.6779@nic.funet.fi> Organization: Finnish Academic and Research Network Project - FUNET Subject: Re: How to determine the pull of a kite In rock@netcom.com (Anne Rock) writes: >To add a bit to Jon Wagner's post: > >From Mark Cottrell's _Kite Store Book of Kites_ [text deleted] >At the back of the book are listings for several programs, and in the >The program requires you to enter the FW (flying weight or load) in >pounds, and then calculates the area of the type of kite you specified >to lift that load. I have Mark's book with the programs on diskette. They don't have any copyright statement so I enclose KITESIZE.BAS. Smooth Winds Simo.Salanne@csc.fi ----- cut here ------ 10 CLS 20 REM Kite size estimating program 30 RESTORE 40 REM following data statements contain sort key, CL, min wind, max wind, flying angle and kite name 50 DATA 1,1.3,10,25,70,Parafoil 60 DATA 2,.9,8,18,75,Delta 70 DATA 3,.7,12,30,55,Hargrave Box 80 DATA 4,.8,12,25,60,Cody War Kite 90 DATA 5,1,8,18,65,Malay 100 DATA 6,.9,10,20,60,Peter Lynn Tri-D Box 110 DATA 7,.8,9,25,55,Sanjo Rokkaku 120 PRINT 130 PRINT 140 PRINT" Kite Size Estimator!" 150 PRINT 160 PRINT" Make your selection by number, then press enter" 170 PRINT 180 PRINT" 1= Parafoil Kite" 190 PRINT" 2= Delta Kite" 200 PRINT" 3= Hargrave Box Kite" 210 PRINT" 4= Cody War Kite" 220 PRINT" 5= Malay Kite" 230 PRINT" 6= Peter Lynn Tri-D Box Kite" 240 PRINT" 7= Sanjo Rokakku Kite" 250 PRINT 260 PRINT 270 INPUT" Your Choice";CS$ 280 CS=VAL(CS$) 290 IF CS<1 OR CS>7 THEN 10 300 READ N,CL,MN,MX,AF,NM$ 310 IF N=CS THEN 320 ELSE GOTO 300 320 CLS 330 PRINT 340 PRINT 350 PRINT" Kite selected is ";NM$ 360 PRINT 370 PRINT" Wind range =";MN;"to";MX;"mph" 380 PRINT 390 INPUT" Operating Wind Speed=";WS 400 IF WS>MX OR WS