If you read the Abbott and von Doenhoff book
"Theory of Wing sections" you can look at the
NACA airfoils and the low drag regions. In
general, the narrower the drag bucket the lower the drag when in the bucket.
The airfoils like on the Discus have a problem in
that as the angle of attack reduces, the
stagnation point moves up around the leading
edge. Eventually the part of the leading edge
below the stagnation point causes a suction peak
right at the leading edge with an adverse
pressure gradient aft of that. The adverse
pressure gradient causes the transition to
turbulent flow. So there is a pretty sudden
transition from reasonable laminar run on the
lower surface to no laminar flow with attendant
drag rise, hence the kink in the polar.
Mike
At 07:56 PM 14/07/2014, you wrote:
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
<<mailto:anthony.sm...@adelaide.on.net>anthony.sm...@adelaide.on.net> wrote:
The Discus was the first to explore a new concept in glider design.
Â
Previously designers had tried to provide a very
wide laminar drag bucket from min sink through to VNE (or thereabouts).
Â
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.
Â
The result was impressive for its day with L/D
for a std class ship jumping from 38 to 44:1 Â
(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
couldnât 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 .
Â
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.
Â
Â
Â
From:
<mailto:aus-soaring-boun...@lists.internode.on.net>aus-soaring-boun...@lists.internode.on.net
[mailto:aus-soaring-boun...@lists.internode.on.net]
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
Â
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?
Â
Â
On Mon, Jul 14, 2014 at 4:10 PM, Harry
<<mailto:hw.medlic...@optusnet.com.au>hw.medlic...@optusnet.com.au> wrote:
Hi Mike,
Â
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.
Â
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.
Â
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.
Â
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.
Â
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.
Â
Harry Medlicott
Â
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
Â
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>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,
Â
Itâs all about driving a large fuselage th
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 dr 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,
Â
Harry Medlicott  Â
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
Â
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.Â
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
<http://www.borgeltinstruments.com/>www.borgeltinstruments.com
tel:Â Â 07 4635 5784Â Â Â Â overseas:
int<tel:%2B61-7-4635%205784>+61-7-4635 5784
mob: 042835 5784Â Â Â Â Â Â Â Â Â Â Â Â Â Â
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<http://www.borgeltinstruments.com/>www.borgeltinstruments.com
tel:Â Â 07 4635 5784Â Â Â Â overseas:
int<tel:%2B61-7-4635%205784>+61-7-4635 5784
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