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 <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 5784overseas: int+61-7-4635 5784
mob: 042835 5784: int+61-42835 5784
P O Box 4607, Toowoomba East, QLD 4350, Australia
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[email protected]
<mailto:[email protected]>
To check or change subscription details, visit:
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*Borgelt Instruments***- /design & manufacture of quality soaring
instrumentation since 1978
/www.borgeltinstruments.com
<http://www.borgeltinstruments.com/>tel: 07 4635 5784overseas:
int+61-7-4635 5784
mob: 042835 5784: int+61-42835 5784
P O Box 4607, Toowoomba East, QLD 4350, Australia
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