Headwinds, always

Why [don't tailwinds exist]? Ginsberg's Theorem, which paraphrases the
three fundementals of thermodynamics. may be a clue.

First Law - You can't win
Second Law - You can't even break even
Third Law - You can't get out of the game

.... and the three great philosophies of the world are based on the
negation of one of these laws:

Capitalism is based on the idea that you can win.
Communism is based on the idea that you can break even. And
Mysticism is based on the idea that you can get out of the game.

Jose
--
The price of freedom is... well... freedom.
for Email, make the obvious change in the address.

On 6 Jun 2005 07:27:50 -0700, "Paul kgyy" wrote
in .com::

never once a tailwind.

Most winds have a head wind component; in fact, only winds directly
from directly behind have no head wind component. Quartering winds at
90 degrees to the intended course act to reduce on-course ground
speed, and those quartering winds of up to 60 degrees aft of course
can have a significant head wind component.

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.

"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

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.

"Larry Dighera" wrote in message
news
[...]
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.

That's because you need to take into account the application of that
particular resource. Applying that sort of thinking to cruise flight IS
counter-intuitive, because it's not correct in that context.

It's not even literally correct in the context of the article you quoted,
but nevertheless the article you quoted has useful information in it.
First, it's a discussion of landing, not cruising. Second, it's a
collection of rules of thumb, not a precise analysis of reality.

It is easy to show that mathematically, a 90 degree crosswind results in no
tailwind component. Without a correction, it results in no headwind
component as well.

But when dealing with mountain flying, and in particular landing on a short
runway, assuming a tailwind component for a 90 degree crosswind is
conservative approach. That is, a 90 degree crosswind clearly doesn't add
half the wind speed to your groundspeed, but the crosswind does create other
effects that could result in a lengthening of the room required to land,
roughly equivalent to a similar increase in groundspeed.

Note that while a tailwind is estimated at full strength, when coming from
within a 30 degree angle, a headwind is estimated only a 3/4 strength, even
when coming from the same angle (in the other direction, of course).

I believe that is the true nature of the article you've quoted: to provide
rules of thumb that offer safe guidance to pilots landing in constrained
areas, especially when the landing area is defined not by prevailing winds
but by terrain restrictions, preventing the pilot from taking best advantage
of the current winds. Where the winds increase the landing distance, they
are assumed to be greater than actual, and where the winds might shorten the
landing distance, they are assumed to be lesser than actual. In neither
case do the estimates provide any assistance in judging the effects of winds
aloft during cruise flight.

Hope that helps.

Pete

Peter Duniho wrote:

"Larry Dighera" wrote in message
news
[...]
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.


That's because you need to take into account the application of that
particular resource. Applying that sort of thinking to cruise flight IS
counter-intuitive, because it's not correct in that context.

It's not even literally correct in the context of the article you quoted,
but nevertheless the article you quoted has useful information in it.
First, it's a discussion of landing, not cruising. Second, it's a
collection of rules of thumb, not a precise analysis of reality.

It is easy to show that mathematically, a 90 degree crosswind results in no
tailwind component. Without a correction, it results in no headwind
component as well.

I'd like you to show that since it is easy. And a crosswind is relative
to your track, not your heading. OK, now show us the math! :-)

Matt

On Tue, 7 Jun 2005 11:19:29 -0700, "Peter Duniho"
wrote in
::

[...]

I believe that is the true nature of the article you've quoted: to provide
rules of thumb that offer safe guidance to pilots landing in constrained
areas, especially when the landing area is defined not by prevailing winds
but by terrain restrictions, preventing the pilot from taking best advantage
of the current winds. Where the winds increase the landing distance, they
are assumed to be greater than actual, and where the winds might shorten the
landing distance, they are assumed to be lesser than actual. In neither
case do the estimates provide any assistance in judging the effects of winds
aloft during cruise flight.

Yes. I can see now, that you're right about the article's
inappropriateness in this discussion due to it's intentional bias
toward conservatism. It only serves to further confuse the issue.

Instead, let's look at a Crosswind Correction Table (I hope the
formatting works in your browser):
http://www.auf.asn.au/navigation/wind.html

Table 1: Wind components
Headwind component [for ground speed]
Crosswind component [for WCA]

Wind Speed Wind Speed
WA | 5 10 15 20 25 30 | 5 10 15 20 25 30
----+--------------------------+--------------------
0° | -5 -10 -15 -20 -25 -30 | 0 0 0 0 0 0
15° | -5 -10 -15 -20 -25 -30 | 1 2 4 5 6 7
30° | -4 -9 -13 -17 -21 -25 | 2 5 7 10 12 15
45° | -3 -7 -10 -14 -17 -21 | 3 7 10 14 17 21
60° | -2 -5 -7 -10 -13 -15 | 4 9 13 17 21 25
75° | -1 -2 -4 -5 -6 -7 | 5 10 15 20 25 30
90° | 0 0 0 0 0 0 | 5 10 15 20 25 30
105°| +1 +2 +4 +5 +6 +7 | 5 10 15 20 25 30
120°| +2 +5 +7 +10 +13 +15 | 4 9 13 17 21 25
135°| +3 +7 +10 +14 +17 +21 | 3 7 10 14 17 21
150°| +4 +9 +13 +17 +21 +25 | 2 5 7 10 12 15
165°| +5 +10 +15 +20 +25 +30 | 1 2 4 5 6 7
180°| +5 +10 +15 +20 +25 +30 | 0 0 0 0 0 0
----+--------------------------+--------------------
| 5 10 15 20 25 30 | 5 10 15 20 25 30

ground speed* = TAS + value shown. WCA = value shown / TAS × 60


As an example of the limited increase in ground speed provided by a
quartering tailwind, let's take the case of a 30 knot wind from
135-degrees. The table indicates an increase of +21 knots can be
expected, but that +21 knot increase in forward velocity must be used
to overcome a 21 knot crosswind to track the desired course line,
which results in a net 0 knot increase in ground speed. So it appears
to me, that only those winds within 45-degrees of directly aft (or a
90-degree arc) will actually result in a real increase in ground
speed. Or stated differently, the probability of encountering a
tailwind sufficient to increase ground speed is 1 in 4; only 25% of
the time wind will result in a net increase in ground speed.

Do you agree with that?

"Paul kgyy" wrote in message
oups.com...
I just returned from an 8-day tour of the midwest. This was a 6-leg
circular route west to Des Moines from Chicago, then up to the Dakotas,
back through Duluth, Green Bay. We mostly flew at 7000 ft. We had
20-30 knot headwinds on 5 of the 6 legs and never once a tailwind.


Do your eastbound legs in the afternoon.

Matt Barrow wrote:

Do your eastbound legs in the afternoon.

And go high (usually).

George Patterson
Why do men's hearts beat faster, knees get weak, throats become dry,
and they think irrationally when a woman wears leather clothing?
Because she smells like a new truck.

"Paul kgyy" wrote in message
oups.com...
I just returned from an 8-day tour of the midwest. This was a 6-leg
circular route west to Des Moines from Chicago, then up to the Dakotas,
back through Duluth, Green Bay. We mostly flew at 7000 ft. We had
20-30 knot headwinds on 5 of the 6 legs and never once a tailwind.

Flying the wrong way around a pressure center?

Not that it explains an 82% headwind rate, but all else being equal you'll
have headwinds more than tailwinds, because any wind not parallel to your
heading will force you to crab, which always turns direct crosswinds into
headwinds, never tailwinds.

Pete