Herk Stokely
Jun 01, 1996, 01:00 AM
<html>
<head>
<title>A Tongue-In-Cheek Article on Down-Wind Turns</title>
</head>
<pre><strong>Ed Note: This article originally appeared in rec.models.rc.air</strong></pre>
<pre>THE DOWN-WIND TURN - the last word on the subject!
I recently saw a brief item, in a model flying newsgroup,
which instructed novice flyers on the basics of flying in wind.
Among the excellent items of good advice was a technical
explanation of the problem of turning downwind. It is so good to
see again that there are people who understand this difficult
phenomenon properly. Educating new pilots about this insidious
and dangerous condition is a very important activity.
As a former Navy pilot I have spent thousands of hours
flying in the dangerous low altitude environment, at sea, where
the winds are strong and a single mistake can be fatal! Those of
us who have experienced this kind of flying agree that the "down-
wind turn" is a very real danger. If I was to make such a turn
in my Navy aircraft while flying low and slow; say at 100 knots
while flying into a 40 knot headwind, I had to "accelerate from a
ground speed of 60 knots (upwind) to 140 knots (down-wind) in a
matter of seconds. An inattentive pilot could lose it right
there and spin in with his whole crew.
I'm really writing however, to tell you about a related, but
less well known, and even more subtle danger to flight which we
Navy pilots call the "Calm Day Carrier Deck Effect".
The sea is a restless and unstable reference system at best,
but to those of us who fly from carriers, the ship itself (our
home base) is our single most important point of reference.
Flying around the ship on the occasional calm day at sea, we have
observed that even though there is no wind, the ship by its
movement is creating a wind (over the deck) which can be 30 knots
or more. Flying in the landing pattern at 100 knots we're moving
parallel to the ship at a deck speed of 70 knots; then turning
down-ship to land, we rapidly accelerate to 130 knots deck speed,
producing exactly the same effect as if a wind were blowing. The
discovery of this effect has helped to explain many mysterious
accidents. Further, a wind makes this situation even more
dramatic. For instance - 30 knots ship speed into a 30 knot wind
gives a combined wind over the deck of 60 knots. Now we must
accelerate from 40 knots deck speed upwind to 160 knots downwind:
an unimaginably dangerous maneuver.
Originally we thought that on a calm day this effect was
limited to flights conducted within sight of the ship. Careful
radar tracking of aircraft performing a down-ship turn at great
distances has shown exactly the same accelerations as when it was
done right over the ship. Sit back and imagine the complexity of
this situation when there are several ships and each steaming on
a different course. If I'm not incredibly skillful, I could make
a down-ship turn (off one ship) and an up-ship turn (off another)
at exactly the same time - literally ripping the plane in half.
Needless to say it's important to train new pilots in this
critical matter.
Now that we have laid to rest the question of the down-wind-
turn, we're going to devote our energies to even more subtle and
dangerous phenomenon that lurk in waiting for the unwary pilot.
The first we'll tackle is the tidal-gravity-gradient-effect, or
the deadly "down-Moon-turn!
</pre>
<hr>
<p><strong>The following is a letter I received from Herk concerning the above material:<br>
<br>
From HERKSTOK(at)aol.com Thu Jun 13 12:48:29 1996<br>
Subject: Re: Downwind turns<br>
<br>
Hello Jim,<br>
<br>
Certainly, you may feel free to use that piece on your web page. I'm glad<br>
that you found it interesting. Since it is a sarcastic piece, I'm a little<br>
bit uncomfortable with it because it can make a person feel foolish without<br>
really helping them understand what is really going on. So, you might<br>
consider adding this to the end of the piece - since it's the (I hope)<br>
helpful part of the article. -- Herk Stokely<br>
</strong></p>
<hr>
<pre>The error most people make when they visualize a downwind turn is that they
forget that velocity and acceleration are vector quatities. That means they
have BOTH direction, and magnitude. In terms of math and physics the
direction part means that in the simplest case, there has to be a direction
reference system that ends up giving them a + or - value.
I'll give you an example of both the mistaken and the correct way of looking
at this.
If a plane is flying at 15 mph into a 5 mph wind - it has a ground speed of
10 mph. If it then makes a turn and begins to fly downwind its ground speed
changes to 20 mph. Since the plane has inertia it has to accelerate to the
new ground speed, and this can cause it to lose flying speed. Although there
is an element of this analysis that is correct, it is misleading because it
infers that the situation is different because the wind is blowing.
If it was stated more correctly the analysis would show that the plane
accelerated from (plus) 10 mph to (minus) 20 mph ground speed. This is a
total acceleration (in the minus direction) of 30 mph - the magnitude of the
difference between +10 and -20. This is a true acceleration and the laws of
physics require that a force be applied to the model to make this
acceleration happen. The force comes from the sideways component of the lift
force generated by the banked wing of the plane.
What's correct about the misleading analysis is that the plane has to
generate extra lift to be able to make the turn and take up level flight in
another direction. It's also correct that this affects the performance of the
plane while it's turning.
What's incorrect (as I said) is that the impression is given that the
situation is different because of the wind. In fact, the acceleration is
exactly the same when there is no wind. In this case the plane accelerates
from plus 15 mph to minus 15 mph for a total acceleration in the minus
direction of 30 mph. The acceleration is the same, the amount of inertia to
be overcome is the same and the wind has nothing to do with it. In fact you
could observe the turn from a passing aircraft - or from the moon, and the
acceleration wouldn't be affected by the apparent speed of the reference
point.
Important things affect the flight of a plane on a windy day - particularly
at low altitude. The problem with this old down-wind turn business is that
the endless discussion of it, kind of covers up a clearer understanding ot
the things that are really affecting the plane.
I hope this helps make clear why the flames erupt every time this comes up.
Some people get tired of going over it repeatedly while some others insist on
retaining an incomprehension of the physics involved. -- All the best --
Herk Stokely
<a href="http://rcgroups.com/shared/nospam.php?u=herkstok&d=aol.com">herkstok(at)aol.com</a></pre>
</body>
</html>
<head>
<title>A Tongue-In-Cheek Article on Down-Wind Turns</title>
</head>
<pre><strong>Ed Note: This article originally appeared in rec.models.rc.air</strong></pre>
<pre>THE DOWN-WIND TURN - the last word on the subject!
I recently saw a brief item, in a model flying newsgroup,
which instructed novice flyers on the basics of flying in wind.
Among the excellent items of good advice was a technical
explanation of the problem of turning downwind. It is so good to
see again that there are people who understand this difficult
phenomenon properly. Educating new pilots about this insidious
and dangerous condition is a very important activity.
As a former Navy pilot I have spent thousands of hours
flying in the dangerous low altitude environment, at sea, where
the winds are strong and a single mistake can be fatal! Those of
us who have experienced this kind of flying agree that the "down-
wind turn" is a very real danger. If I was to make such a turn
in my Navy aircraft while flying low and slow; say at 100 knots
while flying into a 40 knot headwind, I had to "accelerate from a
ground speed of 60 knots (upwind) to 140 knots (down-wind) in a
matter of seconds. An inattentive pilot could lose it right
there and spin in with his whole crew.
I'm really writing however, to tell you about a related, but
less well known, and even more subtle danger to flight which we
Navy pilots call the "Calm Day Carrier Deck Effect".
The sea is a restless and unstable reference system at best,
but to those of us who fly from carriers, the ship itself (our
home base) is our single most important point of reference.
Flying around the ship on the occasional calm day at sea, we have
observed that even though there is no wind, the ship by its
movement is creating a wind (over the deck) which can be 30 knots
or more. Flying in the landing pattern at 100 knots we're moving
parallel to the ship at a deck speed of 70 knots; then turning
down-ship to land, we rapidly accelerate to 130 knots deck speed,
producing exactly the same effect as if a wind were blowing. The
discovery of this effect has helped to explain many mysterious
accidents. Further, a wind makes this situation even more
dramatic. For instance - 30 knots ship speed into a 30 knot wind
gives a combined wind over the deck of 60 knots. Now we must
accelerate from 40 knots deck speed upwind to 160 knots downwind:
an unimaginably dangerous maneuver.
Originally we thought that on a calm day this effect was
limited to flights conducted within sight of the ship. Careful
radar tracking of aircraft performing a down-ship turn at great
distances has shown exactly the same accelerations as when it was
done right over the ship. Sit back and imagine the complexity of
this situation when there are several ships and each steaming on
a different course. If I'm not incredibly skillful, I could make
a down-ship turn (off one ship) and an up-ship turn (off another)
at exactly the same time - literally ripping the plane in half.
Needless to say it's important to train new pilots in this
critical matter.
Now that we have laid to rest the question of the down-wind-
turn, we're going to devote our energies to even more subtle and
dangerous phenomenon that lurk in waiting for the unwary pilot.
The first we'll tackle is the tidal-gravity-gradient-effect, or
the deadly "down-Moon-turn!
</pre>
<hr>
<p><strong>The following is a letter I received from Herk concerning the above material:<br>
<br>
From HERKSTOK(at)aol.com Thu Jun 13 12:48:29 1996<br>
Subject: Re: Downwind turns<br>
<br>
Hello Jim,<br>
<br>
Certainly, you may feel free to use that piece on your web page. I'm glad<br>
that you found it interesting. Since it is a sarcastic piece, I'm a little<br>
bit uncomfortable with it because it can make a person feel foolish without<br>
really helping them understand what is really going on. So, you might<br>
consider adding this to the end of the piece - since it's the (I hope)<br>
helpful part of the article. -- Herk Stokely<br>
</strong></p>
<hr>
<pre>The error most people make when they visualize a downwind turn is that they
forget that velocity and acceleration are vector quatities. That means they
have BOTH direction, and magnitude. In terms of math and physics the
direction part means that in the simplest case, there has to be a direction
reference system that ends up giving them a + or - value.
I'll give you an example of both the mistaken and the correct way of looking
at this.
If a plane is flying at 15 mph into a 5 mph wind - it has a ground speed of
10 mph. If it then makes a turn and begins to fly downwind its ground speed
changes to 20 mph. Since the plane has inertia it has to accelerate to the
new ground speed, and this can cause it to lose flying speed. Although there
is an element of this analysis that is correct, it is misleading because it
infers that the situation is different because the wind is blowing.
If it was stated more correctly the analysis would show that the plane
accelerated from (plus) 10 mph to (minus) 20 mph ground speed. This is a
total acceleration (in the minus direction) of 30 mph - the magnitude of the
difference between +10 and -20. This is a true acceleration and the laws of
physics require that a force be applied to the model to make this
acceleration happen. The force comes from the sideways component of the lift
force generated by the banked wing of the plane.
What's correct about the misleading analysis is that the plane has to
generate extra lift to be able to make the turn and take up level flight in
another direction. It's also correct that this affects the performance of the
plane while it's turning.
What's incorrect (as I said) is that the impression is given that the
situation is different because of the wind. In fact, the acceleration is
exactly the same when there is no wind. In this case the plane accelerates
from plus 15 mph to minus 15 mph for a total acceleration in the minus
direction of 30 mph. The acceleration is the same, the amount of inertia to
be overcome is the same and the wind has nothing to do with it. In fact you
could observe the turn from a passing aircraft - or from the moon, and the
acceleration wouldn't be affected by the apparent speed of the reference
point.
Important things affect the flight of a plane on a windy day - particularly
at low altitude. The problem with this old down-wind turn business is that
the endless discussion of it, kind of covers up a clearer understanding ot
the things that are really affecting the plane.
I hope this helps make clear why the flames erupt every time this comes up.
Some people get tired of going over it repeatedly while some others insist on
retaining an incomprehension of the physics involved. -- All the best --
Herk Stokely
<a href="http://rcgroups.com/shared/nospam.php?u=herkstok&d=aol.com">herkstok(at)aol.com</a></pre>
</body>
</html>