For any given volume, a narrower drag bucket must perforce have a lower CofG and a commensurate increase in the moment needed to empty the contents.
From: [email protected] [mailto:[email protected]] On Behalf Of Anthony Smith Sent: Monday, 14 July 2014 9:03 PM To: 'Discussion of issues relating to Soaring in Australia.' Subject: Re: [Aus-soaring] Discus - was 20M gliders Typically an non-flapped airfoil will have curvature on the lower side of the airfoil to help keep the airflow accelerated on the lower surface as the angle of attack is reduced. An airfoil with laminar flow on the lower surface all the way to near CL=0 will have a fair degree of curve to the lower surface. Look at many of the non-flapped Wortmann sections. As you make the lower surface flatter, the laminar flow will drop out at increasing CL’s. Martin Simons lists the airfoil as HX-83-Nase 80. I suspect that it was part of the first generation inverse design using a computer. It does not appear to have a lot of lower surface curvature. From: [email protected] [mailto:[email protected]] On Behalf Of Peter Champness Sent: Monday, 14 July 2014 7:27 PM To: Discussion of issues relating to Soaring in Australia. Subject: Re: [Aus-soaring] Discus - was 20M gliders Very Interesting. Do we know what characteristics of the airfoil lead the narrow drag bucket? On Mon, Jul 14, 2014 at 7:38 PM, Anthony Smith <[email protected]> wrote: The Discus was the first to explore a new concept in glider design. Previously designers had tried to provide a very wide laminar drag bucket from min sink through to VNE (or thereabouts). The Discus designers decided to optimise the airfoil from around min sink speed to a reasonable inter thermal speed (ie 80 kts without ballast). The idea was to have a great polar performance between climb and cruise speeds and then use water ballast to optimise the aircraft for a given day. The result was an exceptionally low drag airfoil within that CL range, that then got very draggy beyond the 80 kts (low CL end when empty) of the curve. It shows up as quite a pronounced kink in the drag polar. The result was impressive for its day with L/D for a std class ship jumping from 38 to 44:1 (claimed) with quite a gain in the cruise L/D too. The downside was that if you got the ballast too light for a day, you really couldn’t go faster than your inter thermal speed from the kink in the polar without a pretty big drag penalty. To get the best out of the aircraft you had to get it correctly ballasted for the conditions . It obviously worked as the majority - if not all - of the std class gliders today follow the same principal in the design of the airfoil. From: [email protected] [mailto:[email protected]] On Behalf Of Peter Champness Sent: Monday, 14 July 2014 6:37 PM To: Discussion of issues relating to Soaring in Australia. Subject: Re: [Aus-soaring] 20M gliders 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 <mailto:[email protected]> Sent: Monday, July 14, 2014 12:00 PM To: Discussion of issues relating to Soaring in Australia. <mailto:[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 <mailto:[email protected]> Sent: Saturday, July 12, 2014 7:09 PM To: Discussion of issues relating to Soaring in Australia. <mailto:[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]> 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 <http://www.borgeltinstruments.com/> tel: 07 4635 5784 overseas: int+61-7-4635 5784 <tel:%2B61-7-4635%205784> mob: 042835 5784 : int+61-42835 5784 <tel:%2B61-42835%205784> 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 <http://www.borgeltinstruments.com/> tel: 07 4635 5784 overseas: int+61-7-4635 5784 <tel:%2B61-7-4635%205784> mob: 042835 5784 : int+61-42835 5784 <tel:%2B61-42835%205784> 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
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