Polar numbers for Discus A

[Serge Serfaty]

Can anybody tell me the numbers to enter in the Cambridge
302 for the Discus A, or where to find these numbers?
Thanks.

[cfinn]

You can review the poplars that are available from WinPilot at
http://winpilot.com/Gliders.htm. Go to the bottom of the page and
select "Polars". This list was created by Carl Herold.

I hope that helps.

Charlie

[cfinn]

[googleb...@yahoo.com]

This isn't everything you need, but there is a WinPilot polar file you
can get for the Discus A at http://www.winpilot.com/polar.asp.

Unfortunately, different manufacturers like to ask for polars in
different ways. My LX5000 wants the polar's (a,b,c) coefficients and
min and max wing loadings. Cambridge wants (1) best L/D, (2) wing area
in sq meter, (3) best L/D speed in kph, (4) dry weight in kg, (5) speed
at 2 m/s sink in kph, and (6) ballast capacity in liters. You can get
some of that from your glider's manual.

What's the difference between the Discus and the Discus A? SeeYou (my
3.1 version anyway) has polars for "Discus" and "Discus 2". I derived
the C-302 polar numbers by plugging the SeeYou polar numbers for H304CZ
into LXe's Polar tool to get a graph of the polar. From that I read off
the numbers for best L/D speed and speed at 2 m/s sink. I can do that
for you, if "Discus" and "Discus A" have the same polar.

-ted/2NO

[googleb...@yahoo.com]

Something I forgot to add: the Cambridge 302 utility has a "From File"
button that looks for files of type "Polar Files (*.plr)". Do not
confuse those with WinPilot .plr files, they have completely different
formats.

[Serge Serfaty]

That's my problem. I can get all these different numbers
but they are not the ones that I need for the Cambridge.
I can get SeeYou polars too.

The Discus or Discus 2 come with 2 different fuselages:
A and B.

[Marc Ramsey]

You may wish to give some consider that any polar coefficient numbers
you obtain are, at best, simply an approximate curve fit to numbers
measured off of one particular clean glider on one particular day, that
flight computers use these approximate numbers to calculate a further
approximation for speed to fly, and that the real world difference in
performance between an A and B model Discus is quite likely smaller than
errors inherent in these approximations. Use some middle of the road B
polar numbers, the result will be close enough to reality of the day to
day performance of your ship...

Marc

[Serge Serfaty]

Marc<
I am totaly aware of that but I don't have these numbers.
Do you know them?

[Udo Rumpf]

The CAI numbers for the Discus A as per R. Johnson.

Wing loading 32kg/m^2
42.5 @ 98km/h
V 2 163km

You will have to make adjustment for your wing loading
I hope this will help.
Udo

"Serge Serfaty" <REMOVE_TO_REPL...@lycos.com> wrote in message
news:3d8jdhF...@individual.net...

[Serge Serfaty]

At 15:00 27 April 2005, Udo Rumpf wrote:
>The CAI numbers for the Discus A as per R. Johnson.
>
>Wing loading 32kg/m^2
>42.5 @ 98km/h
>V 2 163km
>
>You will have to make adjustment for your wing loading
>I hope this will help.
>Udo

Thanks Udo,
The Cambridge requires the wing area. Do you know that
number by any chance?

[Chris]

Discus A wing area :-

10.58 sq Metres
113.88 sq Feet
aspect ratio 21.3

Regards

Chris Runeckles
Discus A TOP VH -GUF

"Serge Serfaty" <REMOVE_TO_REPL...@lycos.com> wrote in message

news:3d7b9uF...@individual.net...

[xcnick]

11 years later and now I have bought Serge's Discus. Udo, if you have your ears on, are the R. Johnson numbers from his 1986 article? The way I read the article he tested at 700 lbs which is 30.2kg/m^2 rather than 32.

thanks
nick

[C-FFKQ (42)]

On Monday, 11 April 2016 15:11:19 UTC-4, xcnick wrote:
> 11 years later and now I have bought Serge's Discus. Udo, if you have your ears on, are the R. Johnson numbers from his 1986 article? The way I read the article he tested at 700 lbs which is 30.2kg/m^2 rather than 32.
>
> thanks
> nick

Nick, I'm sad to inform you that Udo died a few years ago after a lengthy illness.

[xcnick]

On Monday, April 11, 2016 at 12:26:20 PM UTC-7, C-FFKQ (42) wrote:
> On Monday, 11 April 2016 15:11:19 UTC-4, xcnick wrote:

> Nick, I'm sad to inform you that Udo died a few years ago after a lengthy illness.

That is sad.

In the Johnson article he works very hard to get to published L/D. However what struck me is how far off the manual's numbers are at the high end and he doesn't mention it.

Are many manufactures too optimistic at the fast end?

[herbk...@gmail.com]

No Nick, those polars are gospel to the manufacturers, they wouldn't fake'em! In case you are wondering, there's absolutely not gambling going on at Rick's Cafe, either.

[xcnick]

Casablanca is the best. A saying that keeps coming to mind as I do this is: Garbage in, garbage out. Another is measure with a micrometer and cut with an axe.

Who uses what polar?

From what I can tell xcsoar and seeyou are using the manufacturer and winpilot is using Carl Herold, which is closer to Johnson.

The book value at 100kts is 550 ft/min, Johnson shows 650. See my concern? I would like to bend xcsoar, seeyou and the cambridge away from the book values.

[Dan Marotta]

You can make a custom polar in XCSoar and use your own numbers.

--
Dan, 5J

[xcnick]

On Saturday, April 16, 2016 at 4:55:01 PM UTC-7, Dan Marotta wrote:
> You can make a custom polar in XCSoar and use your own numbers.

thanks, I put 51=1.22 76=2.5 97=5 and we will see.

seeyou I did at 50=1.21 65=1.8 80=3.07

but the Cambridge has only two points. Not sure what to fudge. Took me a week to find a 1995 computer with a serial port. Finally got the down audio to shut up.

[Dan Marotta]

I have a Belkin USB to Serial adapter for my laptop.  It worked great with Vista, but doesn't work at all with 8.1.  Fortunately for me, I had an old desktop computer that I dropped off in my hangar.

--
Dan, 5J

[Bruce Hoult]

Cambridge uses only two points, but one of the speeds has two constraints:

1) the L/D at that speed, AND
2) this is *best* L/D speed

Therefore, together with the 2nd speed and sink rate, you have three constraints, same as the other programs. Three constraints uniquely specifies a parabola, which is what virtually all software assumes the polar curve is (at least above min sink):

X*speed^2 + Y*speed + Z = sink

The programs will use the information you input to calculate X, Y, and Z.

For example for your XCSoar numbers 51=1.22 76=2.5 97=5

2601*X + 51*Y + 1*Z = 1.22
5776*X + 76*Y + 1*Z = 2.5
9409*X + 97*Y + 1*Z = 5

Enter these into any linear equation solver, for example at

http://wims.unice.fr/wims/wims.cgi?+module=tool%2Flinear%2Flinsolver.en&+method=coef

x = 0.0014749482401656, y = -0.13611842650104, z = 4.325699378881989

Put those into a spreadsheet and try with 51, 76, 97 and you'll see you get exactly 1.22, 2.5 and 5.

However with 50, 65, 80 you get 1.2071, 1.7097, 2.8759 so the numbers you've put into seeyou are not exactly consistent.

Putting your seeyou numbers into the solver:

2500*X + 50*Y + 1*Z = 1.21
4225*X + 65*Y + 1*Z = 1.8
6400*X + 80*Y + 1*Z = 3.07

x = 0.0015111111111111, y = -0.13444444444444, z = 4.154444444444445

I don't know which of those sets of inputs is best for you. If you use a SVD solver (Singular Value Decomposition) then you can input all six equations (or more!) and get a best fit solution for X, Y, and Z.

Or, we could just say that something between them is close enough. Say:

X=0.001485, Y=-0.135, Z=4.245

These give sink rates different from your inputs by +/- 0.25 fpm at 50&51 knots, +/- 6 fpm at 65&76 knots, and +/- 12 fpm at 80&97 knots.

So, lets work with:

0.001485*speed^2 - 0.135*speed + 4.245 = sink

Cambridge want different kinds of numbers.

Best L/D speed
Best L/D

speed at 2 m/s sink

The L/D is the reciprocal of the glide slope. i.e. if the L/D is 40 then the glide slope is 0.025. If the L/D is 100 then the glide slope is 0.01. etc. We'll work with glide slope for the moment.

Glide slope is sink/speed.

sink = 0.001485*speed^2 - 0.135*speed + 4.245

so

glide slope = 0.001485*speed - 0.135 + 4.245/speed

Best L/D means minimum glide slope, which happens where the 1st derivative of the slide slope is zero.

Remembering high school calculus:

0.001485 - 4.245/speed^2 = 0

=> best L/D speed = sqrt(4.245/0.001485) = 53.466

Glide slope at 53.466 = 0.001485*53.466 - 0.135 + 4.245/53.466 = 0.0238

Best L/D = 1/0.0238 = 42.03

For speed at 2 m/s sink, solve:

0.001485*speed^2 - 0.135*speed + 4.245 = 3.88769 (knots)

or

0.001485*speed^2 - 0.135*speed + 0.35731 = 0

Any quadratic equation solver will give 2.73 knots and 88.18 knots. Ignore the answer that is below stall speed :-)

So for Cambridge:

Best L/D speed = 53.466 knots = 99.02 km/h
Best L/D = 42.03
speed at 2 m/s sink = 88.18 knots = 163.3 km/h.

[xcnick]

Dan, I have the USB to serial working in modern windows, but the Cambridge software only had ports 1 to 4 as an option. The USB to serial wire was assigned a higher number like 8.

Were you able to reassign the wire to a lower number or find later Cambridge software with a larger selection of port numbers?

Bruce, I did give you garbage. When the xcsoar numbers came out as a parabola I knew I screwed up big time. I guess those are the number generated when you choose discus.

The actual numbers from Johnson show the kink some talk about so I don't expect them to fit a parabola.

So I took my school boy ruler to the little graph on page 3 of the Johnson article and come up with the following. I guess I am looking for a quadratic equation near these numbers except 97 knots. Because of the kink 97 knots should be off the most.

max L/d 42.5 @ 53kts min sink 115 ft/min @ 45kts
50kts 121 ft/min or 1.195kts or .615 m/s
60kts 160 ft/min or 1.58kts or .813m/s
65kts 195 ft/min or 1.925kts or .99m/s
76kts 270 ft/min or 2.666kts or 1.372m/s
80kts 308ft/min or 3.05kts or 1.569m/s
97kts 650ft/min or 6.42kts or 3.3m/s

You gave me enough I should be able to learn some calculus. I was not able to get there this morning, but I will try again.

Is the A,B,C of see you x y and z of a quadratic?

[Bruce Hoult]

On Monday, April 18, 2016 at 7:18:31 AM UTC+12, xcnick wrote:
> Bruce, I did give you garbage. When the xcsoar numbers came out as a parabola I knew I screwed up big time. I guess those are the number generated when you choose discus.

Any three constraints will always give you a parabola. That's just maths. Unless they lie on a straight line. Which you could look at as a section of a veeery flat parabola.

You can of course fit a cubic or higher to those three constraints, but there are an infinite number that will fit, and you've got no basis on which to choose between them. So parabola is the best choice.

> The actual numbers from Johnson show the kink some talk about so I don't expect them to fit a parabola.

Yes. The famous Discus "laminar flow drag bucket". Also present in many later designs.

I *think* (without having analysed it) that both the portion below the kink and the portion above the kink correspond very closely to parabolas -- just two different parabolas.

Does any flight optimisation software actually model that? I suspect not. Except in very strong conditions (or final glide), it's the part slower than the kink that you care about.

A bit of a bugger that the CAI software wants the 2 m/s sink speed, as that's past the kink. I reckon the thing to do would be to solve for the parabola from min sink to the kink, and then figure out where 2 m/s sink would occur if the kink didn't happen.

> Is the A,B,C of see you x y and z of a quadratic?

Very probably. I don't have seeyou handy. The names don't matter. A,B,C being z,y,x would matter :-) Do the values look similar?

[Dan Marotta]

Nick,

I haven't been able to make my USB to Serial interface work since I upgraded(???) to Windows 8.1.  Maybe if I could find Windows 7 installation media...  But it's all moot to me now since I'm using a ClearNav XC vario so I can create my profile, including polar, on my laptop and copy it to a USB stick which the ClearNav reads directly.

Dan

--
Dan, 5J

[Bruce Hoult]

On Monday, April 18, 2016 at 7:18:31 AM UTC+12, xcnick wrote:

About the kink...

If this copy of a Discus polar is roughly accurate ..

http://www.fsglider.de/polaren/pol_dcs.gif

.. then you never ever want to fly above the kink (at the lighter weight) unless the MC setting plus current airmass sink is 10 knots or more! In which case you should go straight past the end of the curve to .. I don't know .. whatever point between rough air speed and Vne makes you comfortable?

Meantime, you're only flying slower than the kink if MC plus airmass sink is less than 3 knots.

For every situation between MC=3 and MC=10 (in still air), right on the kink is the only place to be.

(assuming no wind)

[xcnick]

Thanks everybody for the help.

I should probably start a new thread, but how do I get the down audio to quit?

I unchecked the box for down audio in the cambridge software and that worked on the ground, but once it went into circling mode in the air the down audio came back.

330 kg 100 and 160 at 42L/D worked pretty well. With McCready at 2 I ran some 30 40 mile runs at a point and it came out close enough. Water for tomorrow.

[Dan Marotta]

I would guess that the down audio that you can turn off is for cruise mode, not circling.  Is the vario quiet while running through sink?

--
Dan, 5J

[xcnick]

On Tuesday, April 19, 2016 at 8:20:29 AM UTC-7, Dan Marotta wrote:
> I would guess that the down audio that you can turn off is for
> cruise mode, not circling.  Is the vario quiet while running through
> sink?

Down audio works in cruise. I don't mind that too much. But in climb I just have to turn it off.

Got a polar work around. I just loaded the program's polar and then added bugs to make seeyou and xcsoar say the same thing. I could do this in the air.

[JS]

It's a while since setting up a 302 and I don't have one to connect to, but in the CAI utility software for PC or IPAQ the "sink tone on" checkbox is for climb mode. The "speed up" tone is always on.
This was the opposite of the SN10 software when I owned one of those just prior to my first 302. You couldn't turn off the SN10 sink tone in climb mode, but could turn off the "speed up" tone.
The polar just comes out of the glider manual. If it doesn't agree with real life change the numbers. Lowering the 2m/s speed helps make it realistic.
Check that the configuration is not locked on the "Edit Glider ID and Polar" page, otherwise all you can change to fix it will be bugs.
Jim

[xcnick]

On Tuesday, April 19, 2016 at 9:31:53 PM UTC-7, JS wrote:
> It's a while since setting up a 302 and I don't have one to connect to, but in the CAI utility software for PC or IPAQ the "sink tone on" checkbox is for climb mode. The "speed up" tone is always on.

> Check that the configuration is not locked on the "Edit Glider ID and Polar" page, otherwise all you can change to fix it will be bugs.
> Jim

Thanks, I will check that the configuration is not locked on. When I click from 300 it does show down audio unchecked, but it still beeps down in climb. It is good to know I have at least tried to do the right thing.