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
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,
Itâs 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 donât 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
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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
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