That is quite interesting about the Discus suffering loss of performance above 80 knots. I thought the polar curves were more or less the same with progressive loss of L/D with speed. Do any other gliders have the same problem? Is the issue understood?
On Mon, Jul 14, 2014 at 4:10 PM, Harry <[email protected]> wrote: > Hi Mike, > > Thanks for the erudite explanation of drag, Reynolds numbers etc. I can > only write as a pilot fairly ignorant of what factors influence a gliders > performance but the following may be pertinent. > > Glider manufacturers optimise design, particularly wing design, to be at > greatest efficiency over a quite small speed range. Better to be highly > efficient over a small speed range than less efficient over a large speed > range. Manufacturers used to look at peak efficiency over 50 to 80 knots > dry but I suspect modern aerofoils may compress this range even more and > maybe look at optimisation towards the higher end of the speed range. > > Manufacturers tend to be coy about actual polar curves but the original > Discus published polar curve was more honest than most. It showed a > distinct break and deterioration in performance at about 80 knots dry.. I > assumed this was the point where the reduction in angle of attack reached a > point where the airflow over the nearly flat lower side of the wing > resulted in a break up of the laminar airflow. This reduction in > performance was so severe that it was a waste of time climbing in a strong > thermal once you could final glide at 80 knots dry and proportionally more > if ballasted. The gliders performance degraded so much that it was waste of > time.climbing higher. even if a very strong thermal. once the correct > final glide speed could be flown. > > Drag on the fuselage must be related to the angle of the fuselage to the > airflow. It could well be that some fuselages are less affected than > others. Schleicher fuselages tend to be quite slim past the cockpit. > Perhaps drag varies not only with speed but also with fuselage design with > some fuselages less affected by changes of angles of attack to the incoming > airflow. > > Easy to see why glider designers have such a hard time designing the > optimum performance glider. Get it wrong and couple of millions worth of > Euros would be wasted and maybe the company goes broke. > > Harry Medlicott > > *From:* Mike Borgelt <[email protected]> > *Sent:* Monday, July 14, 2014 12:00 PM > *To:* Discussion of issues relating to Soaring in Australia. > <[email protected]> > *Subject:* Re: [Aus-soaring] 20M gliders > > Rob, > I've done enough 2 seat cross country flying to realise the fun involved, > I'm talking aerodynamics. > > Harry, > > There may be more wetted area and cross section on the 2 seat fuselage but > comparing a Discus2 B to an Arcus (this necessarily approximate) I get > about 32% more cross section on the Arcus fuselage and about 49% more > wetted area. Shape is similar so I'd expect similar drag coefficients. The > mass is 800 Kg vs 525 at gross which is 52% greater so at any given sink > rate the POWER is 52% greater. The wing area is 15.6 M^2 vs 10.16 M^2 so a > ratio of 1.54 (rounded up). > No large differences (slightly worse at 750Kg) and as the Arcus has flaps > I'd expect it to perform the same at mid range speeds and better at high > speeds where the Standard Class glider starts to go out of the low drag > region of the airfoil. > Span loading is different though (mass per unit span) for the Arcus 800/20 > =40, for the D2 525/15 35. Induced drag is dependent on the square of the > span loading - derived here > http://aerocrafty.blogspot.com.au/2013/06/span-loading.html (weird > website behaviour on my office PC but works Ok in the iPad in Chrome) so > yes, the two seat Arcus and ASG32Mi likely will climb worse than the 15M > standard class glider even though the Reynolds numbers on the Arcus wing > are 15% higher (lower profile drag coefficient). Why the high speed > performance is worse is a mystery. > > I don't have any numbers on the height and width of the ASG32 fuselage but > if less than that of the Arcus I'd expect an improvement. > > I wouldn't draw any conclusion about the ASG32 performance from Finland > except that it is clearly not a terrible glider in performance compared to > the Arcus and looks nice. > > Mike > > > > > At 10:33 PM 12/07/2014, you wrote: > > Mike, > > It’s all about driving a large fuselage through the air. The quite small > size difference between say, a Discus A and B fuselage makes an appreciable > difference in performance, particularly at higher speeds. Compare the > massive size difference between an ASG 29 and a two seater fuselage. I > don’t know what the actual drag figures are but they must be a large > difference. Likewise the two seater ASH 25 and Nimbus 3DMs and 4DMs are > left far behind the ballasted 18 metre gliders when the speeds get up a > bit. The actual Arcus fuselage is very similar to the 20 year old Nimbus 3D > fuselages so I guess there was not much scope to improve them much.The > Jonkers JS fuselage is reputed to be an exact copy of an earlier German > glider. Actually expected the new Schleicher 32 fuselage, being a new > design, to have lesser drag but the information from Finland is not > indicative of a substantial improvement. Time will tell. Am sure you could > give us some useful information on drag calculations, > > Harry Medlicott > *From:* Rob Izatt <[email protected]> > *Sent:* Saturday, July 12, 2014 7:09 PM > *To:* Discussion of issues relating to Soaring in Australia. > <[email protected]> > *Subject:* Re: [Aus-soaring] 20M gliders > > You can get two people in a two seater and share the fun which is the > wholepoint of said two seaters. Without handicaps glider comps would be > even less viable. > > On 12 Jul 2014, at 5:59 pm, Mike Borgelt < > mailto:[email protected] <[email protected]>> > wrote: > > From what has been written here over the last few days, it is > disappointing that a new flapped 20M two seater doesn't have as good > performance as a 15M unflapped glider. > > Mike > > > *Borgelt Instruments* - > *design & manufacture of quality soaring instrumentation since 1978* > www.borgeltinstruments.com > tel: 07 4635 5784 overseas: int+61-7-4635 5784 > mob: 042835 5784 : int+61-42835 5784 > P O Box 4607, Toowoomba East, QLD 4350, Australia > _______________________________________________ > Aus-soaring mailing list > [email protected] > To check or change subscription details, visit: > http://lists.internode.on.net/mailman/listinfo/aus-soaring > > > ------------------------------ > _______________________________________________ > Aus-soaring mailing list > [email protected] > To check or change subscription details, visit: > http://lists.internode.on.net/mailman/listinfo/aus-soaring > _______________________________________________ > Aus-soaring mailing list > [email protected] > To check or change subscription details, visit: > http://lists.internode.on.net/mailman/listinfo/aus-soaring > > *Borgelt Instruments* - > *design & manufacture of quality soaring instrumentation since 1978* > www.borgeltinstruments.com > tel: 07 4635 5784 overseas: int+61-7-4635 5784 > mob: 042835 5784 : int+61-42835 5784 > P O Box 4607, Toowoomba East, QLD 4350, Australia > > ------------------------------ > _______________________________________________ > Aus-soaring mailing list > [email protected] > To check or change subscription details, visit: > http://lists.internode.on.net/mailman/listinfo/aus-soaring > > > _______________________________________________ > Aus-soaring mailing list > [email protected] > To check or change subscription details, visit: > http://lists.internode.on.net/mailman/listinfo/aus-soaring >
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