What's erudite? What Reynolds numbers Michael
> On 14 Jul 2014, at 4:03 pm, "Harry " <hw.medlic...@optusnet.com.au> 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 larger 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 someone designs a slightly better glider > and a couple of millions worth of Euros would be wasted by way of moulds etc. > and maybe the company goes broke. > > Harry Medlicott > > From: Mike Borgelt > Sent: Monday, July 14, 2014 12:00 PM > To: Discussion of issues relating to Soaring in Australia. > 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 >> Sent: Saturday, July 12, 2014 7:09 PM >> To: Discussion of issues relating to Soaring in Australia. >> 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:mborg...@borgeltinstruments.com> 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 >>> 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 > 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 > 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
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