Anthony Many thanks for your referrals and detailed reply.
Regards Michael On Mon, 24 Dec 2007 13:33:18 +1030 "Anthony Smith" <[EMAIL PROTECTED]> wrote: > Hi Michael > > Some people have indeed tried to quantify the effect. > > Fred Thomas "Fundamentals of Sailplane Design" only briefly mentions the > subject and is more to do with stability. > > Fred does quote a range of references including many of Frank Irving's > below: > - Irving FG "Centre of Gravity Position and Performance" Sailplane and > Gliding Oct/Nov 1981 > - Kroo I "Trim drag, tail sizing and soaring performance" Technical > Soaring July 1984 > > Frank Irving "The paths of soaring flight" goes into more depth and > discusses optimization in some detail. Frank does a lot of technical > math and I don't recommend this book to many people. Frank does go on > to suggest that the optimal cross country CG pt is slightly aft of the > middle of the CG range as having the CG to the rear gives you a decrease > in cruise performance. (He mentions a cruise of 80 kts, so I assume this > was for a 5 kt day or thereabouts) > > Frank does quote the following references: > - Irving FG "The optimum centre of gravity position for minimum overall > energy loss" OSTIV XVI, 1981 > - Jones RT "Minimising induced drag" Soaring Oct 1979 > - Vernon CO "Trim drag" Technical Soaring Jan 1992 > > Have a search for some of these for a bit of light Christmas reading. > > Some thoughts from myself as well: > > - Gliders can be optimised for either climb, cruise or somewhere in > between. > > - I expect the optimum CG location is going to be a bit like water > ballast and needs to be changed for expected thermal strength. A weak > day, have the CG further aft to maximise the climb. A strong day, move > it forward to maximise the cruise. > > - The effect of CG location is going to be quite dependant on the > individual design of the aircraft. So doing some generic numbers may > not necessarily give you an accurate result for an Astir. The basic > numbers are not that hard to work out - particularly if you already have > an estimate of the drag breakdown of your aircraft. > > - The original Astir series had an Eppler profile that was quite draggy > at higher CLs (ie low speeds) but quite exceptional performance (for its > day) at low CLs (high speeds). This is why they don't seem to climb as > well as other makes, but cruise along really well. Having a forward CG > will increase the down load needed at the tail. At low speed you need > greater elevator deflection (so more profile drag there). As you > mention, you also increase the induced drag of the tailplane. You also > increase the load on the wing! Lift = weight + down load from tail. So > you will need a slightly higher CL for the same thermalling speed. With > the original Eppler profile that is particularly bad news as it > increases your profile drag and wing induced drag. I think this is > where you will most notice the effects in the original Astirs - the wing > is being worked a little harder, but it is producing a lot more drag. > > - Wortmann designed his profiles with a different optimisation than > Eppler. Wortmann's airfoils tended to be excellent climbers, but a bit > more draggy in the cruise. So Glasflugel and other makes that used the > Wortmann profiles will have a slightly different response to CG > location. > > > I hope this helps a little bit. > > Have a merry Christmas and a great soaring new year! > > Anthony > > > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Michael > Derry > Sent: Monday, 24 December 2007 12:26 PM > To: aus-soaring@lists.internode.on.net > Subject: [Aus-soaring] Cockpit load effect on climb performance > > Hi All > > I once did a test flying an Astir just with me in it and then landed > loaded up with parachute, tie down kit, lots of drinking water and > outlanding kit and my climb rates compared to the other gliders (same > gliders and pilots) were significantly reduced. > > ie all variables were the same except an extra 15 kilos or so were in > the cockpit. > > In the loaded up case I needed lots more backstick to maintain the same > speed in a thermal. So with more weight I had a drag penalty from the > elevator. > > I wonder if anyone has any info on the average climb penalty you can > expect from being at the max end of the cockpit load compared to the min > cockpit load ? > > Maybe in % or knots climb ? > Effect on average cross country speed ? > Links to any published articles about this subject ? > > Some people have attacked the problem by putting weight in the tail of > the glider (of course this needs to be checked out by W & B people > first) > or even taking off their shoes and putting them behind them to help. > > So far I have heard a bit of 'Pub Talk' about this subject but nothing > where anybody has a least half seriously tried to qauntify the actual > effect. > > Cheers > > Michael Derry > _______________________________________________ > Aus-soaring mailing list > Aus-soaring@lists.internode.on.net > To check or change subscription details, visit: > http://lists.internode.on.net/mailman/listinfo/aus-soaring > _______________________________________________ > Aus-soaring mailing list > Aus-soaring@lists.internode.on.net > To check or change subscription details, visit: > http://lists.internode.on.net/mailman/listinfo/aus-soaring _______________________________________________ Aus-soaring mailing list Aus-soaring@lists.internode.on.net To check or change subscription details, visit: http://lists.internode.on.net/mailman/listinfo/aus-soaring
