Article 27560 of rec.kites: Path: tug!ibmpcug!laura.pcug.co.uk!hole.news.pipex.net!pipex!news.be.innet.net!INbe.net!news.nl.innet.net!INnl.net!hunter.premier.net!news-res.gsl.net!news.gsl.net!nntp.coast.net!howland.reston.ans.net!surfnet.nl!sun4nl!knoware.nl!usenet From: pdjnjvt@knoware.nl (Peter de Jong) Newsgroups: rec.kites Subject: Re: Chevron progress (was Soft kite tuning) Date: Fri, 12 Jul 1996 14:43:43 GMT Organization: Knoware Internet Lines: 93 Message-ID: <31e66492.262816@news.knoware.nl> References: <9607100941.aa03230@tug.com> <4s10os$6h4@news.euro.net> NNTP-Posting-Host: boompie-ppp12.knoware.nl X-Newsreader: Forte Agent .99d/16.182 On Thu, 11 Jul 1996 22:43:44 GMT, andrew@tug.com (Andrew Beattie) wrote: >andrew@tug.com (Andrew Beattie) writes: >>It seems that pulling the nose down makes the kite faster and loads more >>powerful (for the part of the sky where you can get away with it), but the >>stall speed seems to be too high. > >Which leads me to a fundamental question for you aerodynamicists out there... > >What do I need to do to an airfoil to reduce it's stall speed? > >If I wanted a wing that stalled way down at 3mph of apparent wind, what would >it look like? > >How do I make the onset of stall less abrupt (Hmmm... is there any way to >come *out* of the stall more quickly too?) > Andrew, I'm not an aerodynamicist, but lately I've been collecting everything I could lay my hands on to find out what makes an airfoil suitable for a kite... I'm not sure what you mean by stalling. My understanding is ( it maybe a language problem ) that it's the situation where the (apparent) AOA gets to high and you loose the smooth airflow over the top of the wing, causing turbulence and a dramatic loss of lift. I think it's an interesting problem that one can't solve easily, cause there is a big difference in kiting and "normal" flying. A common flying machine has a very limited range in the angle of attack: it starts, flies and lands horizontally. A (normally bridled) kite starts at an AOA of 90, which is reduced to almost 0 overhead. For most airfoils a 0 degree AOA is no problem, but the 90 degrees is. When launching such a kite it is *always* in a stalled position until it's gained enough speed to reduce the apparent AOA. The best solution ( and the most difficult one ) is to find an airfoil that has better performance at low airspeeds and at a high angle of attack... Cause I'm experimenting also I have been searching the web for airfoils and airfoil theory. One of the things that struck me most was that the profiles used mostly in kites, the clark y and the naca series are rather "antique": they date from the 1920's, and since then a lot of airfoils have been added that are designed for specific purposes. Finding the most suitable one is the problem, there are so many.... ( Found some parawing airfoils that look a lot like the Quad Comp profile: very promising ) I'll leave my search criteria for now, but if someone is interested to compare ideas they're very welcome. Normally, you would give the wing some washout to decrease stalling. Knowing you are using a kind of Sput profile (a symmetrical naca) it seems to me that this is no option because when the aoa should get negative during normal flight the section will get a negative lift and the tips will collaps inward. One solution is to use an reflex-type airfoil like a flexifoil does but then you sacrifice some lift. Another thing is bridling. The longer the primary bridle is the harder it is for the kite to adapt to changing conditions. IMO your best bet with an given profile is to try to make the primary as short as possible. Hope this helps, regards, Peter Notes: A lot of airfoil info and theoretics can http://aero.stanford.edu/OnLineAero/ Airfoils, including the mentioned parawing ( HM92 and HM93 ): http://beadec1.ea.bs.dlr.de/Airfoils Found Winfoil: Design and alter airfoils, print them true size under windows ( a blessing when you make elliptical wings ), design wingshapes and you can type in the coordinates for your favorite foil... Shareware at http://www.ozemail.com.au/~malhardy/ Greetings from Holland Peter de Jong A&F Custom Kites Werkhoven NL