On Mon, 6 Jun 2005 10:58:57 -0700, "Peter Duniho"
wrote in
::

"Larry Dighera" wrote in message
.. .
[...]
The point I'm making is, that of the 360 degrees available for winds
to intersect the intended course, only about 15% are able to result in
a net ground speed increase.

This is from memory, so I'm sure someone will correct me with a more
detailed analysis.

Of course.

It depends on the strength of the wind.

For example, if you are flying 100 knots, a 20 knot wind from 10 degrees aft
of a direct crosswind gives you a 1.5 knot boost in speed, but a 40 knot
wind from the same direction slows you by 1.1 knots.

The stronger the wind, the more directly behind you it can be and still slow
you down.

That said, your statement that only 15% of the available degrees result in a
true tailwind is plainly false. That would be an arc of only 7.5% degrees
to either direction of straight aft of your heading, when in fact modest
wind speeds even only slight aft of your heading result in a net increase in
groundspeed. And it ignores the fact that it's not simply the direction of
the wind, but also the speed.

It's true that more than 50% of all wind directions and speeds result in a
headwind, but it's only *slightly* more than 50%. Certainly not nearly
enough to explain the original poster's experience.

Pete


Around 1998 or so, The High Ground column in Plant & Pilot contained
an article titled Estimating Surface Winds. It provided five
paragraphs each dealing with a different aspect of winds, and four
figures. Figure C is titled Estimating Tailwind Component. It shows
wind from astern (0 degrees), 30 degrees off the tail, 60 degrees off
the tail, and wind from off one wing tip (90 degrees). Here are the
captions of each:

0 Degrees: Estimate tailwind component at full wind velocity.

30 Degrees: Estimate tailwind component at full wind velocity.

60 Degrees: Estimate tailwind component at three-quarter wind
velocity

90 Degrees: Estimate tail wind component at one-half wind
velocity.

So I appears that my recollection was faulty. But it seems counter
intuitive, that a 90-degree crosswind contributes half its velocity to
a tailwind component.


Here is the text of the article:

ESTIMATING SURFACE WINDS

An awareness of the surface wind is all-important to successful
mountain arrivals and departures. A few rules of thumb are
useful.

1 Estimating Headwind Component. If the wind sock is swinging
within 30 degrees of your runway's alignment, consider the
headwind component at three- fourth the wind velocity.
(Mountain winds are seldom steady; a direction and velocity one
moment may change the next.( Allow one-half the wind's velocity
as your component when the sock swings 30 to 60 degrees off the
runway. And, when the sock's angle to the runway exceeds 60
degrees, count the headwind zero.

2 Estimating Crosswind Component. If the wind lies within 30
degrees of runway alignment, estimate your crosswind component at
one-half the wind's velocity. Estimate your component at
three-fourths the wind's velocity if the wind crosses your runway
at 30 to 60 degrees. If the wind angle exceeds 60 degrees,
estimate your crosswind component to equal the velocity.

3 Estimating Tailwind component. If the wind is blowing within
30 degrees of your tail, consider the wind's full strength as your
tailwind component. A wind 30 to 60 degrees of the tail calls for an
estimated component of three-fourths the wind's velocity. Estimate
your component at one-half the velocity if the wind angle exceeds 60
degrees.

4 Estimating wind velocity. Most wind socks used at small airports
are designed to stiffen at 15 knots. Estimate lesser velocities by
the sock's angle of droop. A sock drooping at a 45-degree angle, for
example, shows a velocity of seven or eight knots.

5 Estimating Wind Correction Angle. Knowing at the outset the
approximate wind correction needed on final approach or initial
climbout is helpful. At typical light plane liftoff or approach
speeds of 55 to 65 knots, correct one degree for each knot of
crosswind component. Thus, an approximate 10-degree correction should
keep you on track when lifting off or landing into a 10-knot crosswind
component.