Max, I understand your theory why we should not use MC > 0 if below glide
unless we plan is to climb, but as you noted, most (all?) legacy flight
computers, such as 302, do not take head wind into account for STF, since it
does not know when you are in final glide. This is based on the theory that
wind should not affect STF, but this is only true in flatland, not in mountains
nor in final glide. So yes, I agree, flight computers should have the option to
calculate STF taking wind into account, but there is no much we can do about it.
To make up for it, I (and I believe others) are using an empiric MC value, of
5% of wind speed (roughly 0.5 knot up to 10 knots head wind, 1 knot for 20
knots headwind, 1.5 for 30 knots and so forth). Yes, this is not 100% accurate
but seem to be close enough for all practical purpose.
And since 302 gives my audio STF command, there is no other way I can easily
make it to tell me to fly a little faster than tweaking the MC, and since this
in turns update the MC in XCSoar, hence we have this discussion...
Max, we both use the same setup of 302/xcsoar, so I am curious how you address
this scenario of a marginal final glide at the end of the day into a buoyant 20
knots headwind without causing XCSoar to assume that you plan to climb and
recalculate your arrival altitude. The only way to do it will be perhaps to add
bug factor but this is not what it was designed for and takes more effort. MC
is more intuitive.
Ramy
>________________________________
> From: Max Kellermann <[email protected]>
>To: Ramy Yanetz <[email protected]>
>Cc: "[email protected]" <[email protected]>
>Sent: Tuesday, November 22, 2011 11:32 AM
>Subject: Re: [Xcsoar-user] MC and thermalling drift
>
>On 2011/11/22 20:13, Ramy Yanetz <[email protected]> wrote:
>> 2 -MC is not just for expected thermal. If your final glide is against the
>> wind as in these examples, you need to set your MC > 0 in your flight
>> computer (eg 302) to get the best glide speed, even if you don't expect to
>> circle again. This is in fact why I noticed this problem, since I increased
>> my MC to 0.5 in my 302 (which automatically updates XCSoar) to compensate
>> for the 12 knots headwind, and as a result my arrival altitude
>> instantaneously dropped by over 5000 feet. This can never be correct in any
>> logical way. if it was correct I would have landed out.
>
>So your legacy glide computer is incapable of considering the headwind
>properly in a glide calculation. And you found a way to fool it by
>setting some random MacCready value.
>
>Again, this is doing it wrong: you see that the 302 result is not
>correct, and then you edit the MacCready value until the result meets
>your expectations.
>
>Please tell me your formula that turns "12 knots headwind" into
>"MacCready 0.5 m/s". And tell me how this is relevant to XCSoar.
>
>Max
>
>
>
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