If you go to our website there's a nice little
polar coefficient calculator. You need the polar
at any weight (and know what that weight
is) Any other desired weight such as the minimum
flying weight and it just calculates the
coefficients for you. It includes the speeds for
min sink and best L/D and the performance at
those speeds for a sanity check on the numbers.
The 3 coefficient approximation isn't necessarily
very good a very low speeds but if you get it
right from best L/D to the maximum useful speed
(usually about 4 knots sink at that speed) you
will be doing well. There's a small text file
that explains how to use the tool.
It also calculates what I call the D coefficient
which is the relative netto offset which is the
CIRCLING sink rate of the glider. Around 1.6
times the min sink rate at that weight is good.
It does vary with bank angle of course. Some
people have been "educating" others with the
wrong definition of relative netto which just shows they didn't understand it.
The numbers the program gives are actually for
our instruments. Left out is that the A coeff is
multiplied by 10^-4, the B coefficient by
10^-2 and B is ALWAYS a negative number.
Also note that this approximation is not based on
physics. It is a curve fitting exercise. In
reality the polar sink is closer to sink = A*V^3 + (B/V)
Also see the Basic Sailplane instruments article on the website.
So once again the lack of proper theoretical
training rears its head. The effect of weight on
the polar should just be a normal part of every glider pilot's training.
Congratulations on your win, Adam!
Mike
At 04:01 PM 10/25/2016, you wrote:
Content-Language: en-AU
Content-Type: multipart/alternative;
boundary="_000_HK2PR04MB08979EBE201C68E51AD6F9A3A2A80HK2PR04MB0897apcp_"
Gâday All,
Just wondering whether anyone knows the formula
for converting a baseline empty polar curve
(speeds/sink rates) to a ballasted polar curve?
Iâve read it somewhere in the past, but cant find it easily now.
Cheers,
WPP
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