Harry,
The problem with gliders is that the wing profile
drag must be low over a very wide range of angles
of attack (same as lift coefficient). It has been
known for over 60 years how to build a very low
drag laminar airfoil at low angles of attack but
the trick has been to get one of these that also
produces very high maximum lift coefficients
while preserving low profile drag coefficients.
Apart from jet fighters, powered aircraft are
normally designed to fly efficiently over a very
narrow range of lift coefficients. While most of
the problem while circling is induced drag which
is a function of span loading and lift
distribution, profile drag is still important and
more so as the speed increases as angle of attack
decreases. NACA 6 series airfoils were used on
gliders for quite a while (K6, Blanik, Foka etc)
until Wortmann worked on the problem and improved
the low speed performance of similar airfoils.
Camber changing flaps help. The reason we don't
see slotted flaps much on gliders is because the
profile drag is greatly increased by slots. There
have been some experiments in that direction though.
You are right about the Discus 1 which is why I
was told in New Zealand that at 110 knots on the
ridge the Std Cirrus is better than the D1. All
the modern unflapped gliders have a "knee" in the
polar to some extent, when the lower wing surface goes turbulent.
Yes the fuselage drag coefficient will likely
change with AoA. That is one of the nice things
about flaps though. If you are in the correct
flap setting the main part of the wing (and hence
the fuselage) meets the air at about the same
angle. Typically for modern gliders at 100 KIAS
the AoA is around 3 degrees at 71 knots it is 6
degrees and at 140 knots about 1.5 degrees so at
the higher speeds it doesn't take much change
anyway over most of the usual cruising range. In
this case it is best to design so the fuselage
lines up with the airflow in the cruising range
as profile drag is less important at the lower
speeds where induced drag dominates.
Yes it is hard designing for optimum performance
as the "optimum" changes with the anticipated
thermal strength. There are experimental
manufacturer's prototypes that don't see the
light of day for long. Wanna bet against a V2
with a different wing either flying or about to?
Or one with an interesting glove over the outer wing?
I ran across the design rationale for the LS6 the
other day. Pretty conservative but thin airfoil
with no turbulators, blowholes etc that worked
very well in the real world with non critical
flap settings. So non critical it became the LS8
with the LS6 flaps set in a small negative
setting and still competitive against the D2 with
a late design advanced airfoil. This must be some
what depressing for glider designers.
Mike
At 04:10 PM 14/07/2014, you 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: <mailto:mborg...@borgeltinstruments.com>Mike Borgelt
Sent: Monday, July 14, 2014 12:00 PM
To:
<mailto:aus-soaring@lists.internode.on.net>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: <mailto:thebunyipboo...@gmail.com>Rob Izatt
Sent: Saturday, July 12, 2014 7:09 PM
To:
<mailto:aus-soaring@lists.internode.on.net>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>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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To check or change subscription details, visit:
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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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Aus-soaring@lists.internode.on.net
To check or change subscription details, visit:
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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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