Polar numbers for Discus A

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

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

On Wednesday, April 27, 2005 at 7:30:17 AM UTC-7, 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

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.

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?

On Thursday, April 14, 2016 at 12:19:25 AM UTC-5, xcnick wrote:
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?

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.

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.

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

On 4/16/2016 11:01 AM, xcnick wrote:
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, 5J

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.

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.

On 4/16/2016 8:32 PM, xcnick wrote:
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, 5J

On Sunday, April 17, 2016 at 2:32:17 PM UTC+12, xcnick wrote:
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.

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?+...+met hod=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.