"Peter Duniho" wrote in message
...

I'd be suspicious of a formula given without any explanation of its
derivation. This formula in particular seems odd, as the break-even wind
speed as interpreted by you decreases as takeoff speed goes up. This is
opposite what I'd have intuitively thought (that is, an airplane with a
higher takeoff speed is less-affected by wind, requiring higher wind speed
before it matters which way one takes off).

No, I think the inverse relationship is the right one, although
I'm not making any claims that Sparky's formula is correct.
I agree that a higher take-off speed means you're less concerned
about what the wind is doing. In the limit with a truly phenomenal
take off speed, who cares at all about wind or even what grade
you're on? You're cranking out lots of power, ignoring wind
and grade and so the break-even speed would be close to zero.


That suggests that maybe the formula as given in the previous post is
correct (that is, you really do multiply the (s * d) / 5 by V) and that the
units are what are missing. Though, why a formula would be given that
requires a unit conversion rather than just including the conversion factor
in the formula, I can't say.

The other thing I'd point out is that even if the formula is correct, it's
obviously an approximation, as the term taking into account runway slope is
stated to be in degrees, but is used in a linear fashion (rather than using
some trigonometric function).

Since you have a copy of Imeson's book, perhaps you'll be able to find the
same formula and see whether the previous post left out some important
information given in the book. Otherwise, I'm not sure I see how to apply
the formula. You make a reasonable attempt to get the result into some
sensible magnitude, but it changes the formula in a way so as to make it
counter-intuitive as to how it applies to different airplanes of different
capabilities.

I've got the 1998 3rd edition right here, and I still can't find
the formula. In the chapter on "Takeoff", S claims that 1%
downslope is equivalent to 10% more runway, that winds
over 15 knots take off uphill (no grad specified), and that
increased drag on a 1% uphill grade results in 2 to 4 %
increase in takeoff distance and subsequent climb. No idea
what to make of all that. Rather disappointing, I'm afraid.

It makes me discount his supposed formula, as reported,
even if I could find it.