Rotten advice given in a magazine

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Larry Phillips

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Sep 20, 1990, 7:24:16 AM9/20/90
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> Assuming an aircraft can complete a turn, either upwind or downwind, its >
GROUND speed will essentially be the same(Newton's Laws).

Ye gads! There's one in this hemisphere too! Quick, Henry, the flit!

Sir, would you be so kind as to explain how you think Newton's laws can
possibly be twisted in such a way as to make your statement sound even
remotely true?

-larry

Greg Rose

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Sep 20, 1990, 2:25:52 AM9/20/90
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I was riding home in the train yesterday when I read one of the most
ridiculous things in the Australian AOPA journal. It made me so angry, for
reasons I think will become obvious, that I've spent a large chunk of today
replying to it. Below are the letter and editor's comment which prompted
this response, and my reply to it. (I drew some diagrams but don't feel
that they can be reproduced on a keyboard, you'll have to use
some imagination.)

The letter: (His, not mine)
-----------
The Editor, A.O.P.A. [Australian one, not AOPA Pilot]
Dear sir,

The circumstances of a recent aircraft crash in which a friend was
involved, as a passenger and fortunately without injury, led me to
discuss it with various flying colleagues.

Low altitude and low speed were the main contributors but the final
act, which made the position irretrievable, was a quick turn down
wind. When I mentioned the last point, I was surprised that all except
one of the people I spoke to did not realise its significance until I
explained what old Isaac Newton had expounded about inertia and
momentum about 300 years ago. Except for a glider pilot, who cut his
teeth on the old primary gliders, no one had been told or ever
considered the effect of turning out of wind. Most readily agreed with
what I was saying, accepting it as an explanation of some strange
occurences when under certain conditions the aeroplane seemed to fall
away during a sharp manouvre at slow speed or in strong wind,
especially when stock spotting or similar. Others were skeptical and
one, in a position in CAA [Civil Aviation Authority of Australia]
where he should have known better and could exert a positive influence
on pilots, ridiculed my proposition.

I know flouting this law of physics has killed people in the past,
including too many friends, but assumed they forgot what they had been
taught. Since the crash I referred to above, there have been more
which I am sure may be attributed ultimately to the same effect.

It appears no one is taught today why NOT to make a tight turn out of
the wind especially if at low altitude and low speed, such as just
after takeoff. Probably even more distressing is that no one has been
told that if one finds oneself, for whatever reason, in a position of
low height and critically low speed, a quick turn into the wind may
well be a life saver. [ARRGH - even retyping it I can't let it rest.
DON'T DO THIS! - Greg.] Therefore, when operating at low heights
always be aware of wind direction and speed -- which can change
quickly and catch you out. Don't forget whirlwinds and thermals.

This topic could be discussed at the Safety Awareness Seminars
[Seminars jointly organised by the CAA and AOPA, venues all over
Australia] (I seem to always be too far away to attend). I hope more
light than heat will be generated on the subject as I am sure that a
lot of people would still be here if knowledge of this insidious
danger had been drummed into them.

I drafted the above part of this letter in N.W. Queensland [remote
part of Australia with HUGE cattle ranches where they use aircraft
for roundups] after hearing of another crash a few days before on a
station property in which two men were killed. It seems the same basic
rule had been flouted or was unknown to the pilot. To cap this off,
when I flew back south and enquired about having a 100 hourly done on
my plane, I found that the chief engineer had been killed the day
before, apparently in an incident which sounds all too horribly
familiar -- low speed, low height and turning out of wind.

How much longer will this go on?

Yours Faithfully,
J.H.Treloar
Member No. 2485

[This was then followed by an editor's comment]
(One hopes that the questions raised by this letter are NOT in fact
the true situation, especially with regard to the instructional
aspects. The suggestion that this subject be raised as [sic] Safety
Seminars is good and will be recommended for inclusion in future
agendas ... EDITOR.

[End of quote from AOPA Journal]
--------------

My letter which gives my opinion on the subject is presented below.
I think there is food for thought in it, and I especially want to know
if anyone finds problems with my understanding or explanations.

Troff stuff left embedded: the sequence \fI (or \f2) means italics,
and \(de means a degree sign.

Start my letter:
-------------
Greg Rose
AOPA number 34104
8 Meadow St
Concord NSW 2137
1990 Thursday September 9
The Editor, A.O.P.A.

Dear Sir,

I am writing in response to the letter by J.H. Treloar, member 2485,
published in your September issue.
In this letter, Treloar talks about accidents caused by turning in a
downwind direction while at low altitude and airspeed.
I am sure that the writer meant well
by this letter, but I am seriously concerned that the mistakes therein
might lead to further accidents.

I am disturbed at the lack of understanding shown in this letter,
and appalled by the editor's comment which appears to endorse it.
A student pilot might actually be led to believe the advice therein,

... if one finds oneself, for whatever reason, in a position of low height
and critically low speed, a quick turn into the wind might be a life saver.

I think that the best reason for turning upwind rather than downwind is
that the groundspeed will be lower when you crash.

I note that a representative of the CAA ridiculed this
proposition. A ray of sanity prevails...

It is not, however, my aim to ridicule. I am much
more concerned with getting the reality down on paper
(preferably AOPAs) so that less experienced pilots can
benefit.

When airborne, there is only one meaningful frame of
reference for an aircraft, and that is \f2relative\fP \f2to\fP
\f2the\fP \f2air\fP \f2mass\fP. To put it another way, the aircraft itself
does not "feel" the wind. (Pilots "feel" the wind by
noting the need to crab to maintain heading, or by
differences between TAS [True Air Speed] and GS [Ground Speed].) To the aircraft, then,
there is no difference at all between turning left and
right, upwind or downwind.

More importantly, Treloar seems to have forgotten
that \f2any\fP turn requires energy. Usually this is
supplied by a small drop in airspeed in a gentle turn at
altitude. However we are taught to increase power in tight
turns to supply this extra energy. Alternatively, there is
the glider's solution, to lose more altitude. The energy
must come from somewhere!
Relative to the air mass, all
momentum in the aircraft is directly ahead, and, as Newton
states, you need force to change that, which consumes
energy.

This is why it is important \f2never\fP to make any
significant turn in
situations such as engine-out after takeoff. The laws of
physics are implacable. You will lose energy, either in the
form of airspeed (kinetic energy) or altitude (potential
energy). When low and slow, neither can be safely
sacrificed.

In case any reader may have forgotten it, I will also
remind them that \f2stall\fP \f2speed\fP \f2increases\fP
\f2in\fP \f2a\fP \f2turn\fP, because of the increase in G forces.
The effects are slight in turns
with less than 30\(de of bank, but they are there, and must
not be ignored in a low and slow situation.

To quote the writer again, \fI"... an incident which sounds
all too familiar \- low speed, low height and turning out of
wind"\fP. The fact of the matter is that the words
"out of wind" are completely irrelevant to the
sentence, as it is highly likely that turning in the other
direction would have had the same result.

To perhaps belabour the point about momentum, it is worth
mentioning that there \f2is\fP one way that the wind can
affect the apparent airflow, and that is wind shear. Perhaps this is what the
writer meant, but if so I think it is still backwards. The
wind is likely to be slower nearer the surface, so if a turn
is made from crosswind to downwind, and airspeed is kept
constant, height will be lost, and this could actually
result in a favourable windshear as the aircraft entered a
region of slower air movement with its own momentum
unimpaired. But I certainly wouldn't want to count on this
effect. Note that a turn upwind with a loss of height could
encounter the reverse of this effect, which is exactly why
windshear is a problem on landings.

That is the situation with respect to the air mass and the
aircraft, but I
think it is worth examining the situation from the more
biased viewpoint of the pilot.
Imagine that the aircraft is tracking north under the
influence
of a westerly wind. The aircraft's heading is, say, 350; it
is "crabbing" into the wind. This is illustrated in
Figure 1.
When flying slowly for whatever reason, the required
"crabbing" will be even more pronounced, as will the
effects I discuss below.

To turn onto a heading of 270, which will acheive a square
turn from the point of view of the ground, only involves
making the aircraft rotate by 80\(de. Also, as more and more
of the velocity points into the wind, the groundspeed will
decrease, foreshortening the turn (again from the point of
view of an observer on the ground). So the pilot has acheived
a 90\(de course change into the wind in quite a small area,
and has only used 80\(de worth of turning energy. This is
illustrated in Figure 2, which shows the track as it would
appear on a map.

Conversely, to change course east requires a 100\(de change
in orientation of the aircraft, and a lengthening of the
ground path of the turn, illustrated in Figure 3. (Both of
these figures assume the same constant rate of turn.)

Of course the pilot has a tendency to want to make the turn
smooth, but their point of reference is usually the ground,
and the result of this is more bank to make the turn tighter
and quicker (and potentially more dangerous) when turning downwind,
and to use less bank when turning upwind.

After the turn is complete, there is a different perception
of things. Having turned upwind, the pilot might notice a
lower groundspeed (even though the airspeed is the same). To
counter this, the reaction might be to add more power and/or
lower the nose. Note that both of these reactions decrease
the likelihood of stall. Turning downwind again has the
converse effect, the ground whizzes by, and the pilot might
subconsciously reduce the airspeed by throttling back or
pitching up, with potentially dire consequences.
Being closer to the ground makes the perception of relative
speed much more powerful.

I hasten to stress that these are \f2human\fP \f2factors\fP,
to use BASI's [Australian Bureau of Air Safety Investigation] (least) favourite phrase, and not aerodynamic
factors. They are also the reason we are taught to use the
instruments to keep airspeed and attitude when flying low
and slow. Perhaps the writer has been kept alive, while
others have died, because of the good fortune associated
with his choices, and his body's reaction to unconscious
perceptions. I prefer to understand the reality of the
situation, and simply exercise extreme diligence whenever
flying low and slow. And \f2never\fP to make tight turns (in
either direction) under those circumstances.

Now that we understand that it is as safe to turn downwind
as upwind (that is, not very safe at all),
there is one final consideration worth
mentioning. Suppose you are desperately looking for an
emergency landing field and there are two in sight (marked A
and B on Figure 4). The wind will help you get to
field B, and hinder you getting to field A, even though
(with your "crabbing" taken into account) A will look
closer. (Of course you might then be faced with a downwind
landing, but each real situation needs its own responses.)

I apologise for the quality of the attached drawings, [not given in
the news posting anyway]
but I hope they help explain the associated text.

Yours sincerely


Greg Rose
AOPA member 34104
--
Greg Rose - assistant test pilot - Softway Pty Ltd for a little longer
PHONE: +61-2-698-2322 FAX: +61-2-699-9174 NET: gr...@softway.oz.au

Robert W Yoder

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Sep 20, 1990, 11:01:54 AM9/20/90
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In article <36...@softway.sw.oz.au>, gr...@softway.sw.oz.au (Greg Rose) writes:
[text deleted]

Thank you for an excellent explanation for what I believe to be the most major
misconception in aviation. I have saved your article so I can use it the
next time I get involved in this dumb argument.

I am a pilot (SEL & glider) and a skydiver, and it is incredible how many people
cannot understand the fallacy of this myth. It is somewhat understandable
among jumpers, (where the myth is manifested by a belief that a parachute will
'weathervane' into the wind), since most of them have had no formal education in
aviation principles, but it is inexcusable and possibly dangerous among pilots.

I spent a 5 month temporary assignment working for a fellow with whom I had a
running debate on this subject. He steadfastly insisted that an airplane would
automatically 'weathervane' into the wind when the pilot lined it up on final,
thereby automatically setting the crab angle for landing. I maintained that
years of experience had made the correction automatic for him through acquired
skill.

He could never explain to me how the airplane knew the relationship between the
air and ground while supported exclusively by the air, and he didn't have a good
explanation why an aircraft flying at 10 m/h above stall speed while traveling
against a 20 m/h wind, doesn't stall if it does a 180 while maintaining constant
speed.

Now he was a really great guy to work for, but he was also an electrical
engineer AND A CFI!!

--
Robert Yoder 306 Hawkins Graduate House, West Lafayette, IN 47906 (317)495-6845
Internet: ryo...@ecn.purdue.edu "Flame all you want, We'll post more."
UUCP: pur-ee!ryoder Apologies to Jay Leno & Doritos.
Bitnet: ryo...@ecn.purdue.edu@purccvm.bitnet

james.beckman

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Sep 21, 1990, 10:25:10 AM9/21/90
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In article <20...@amethyst.math.arizona.edu>, raymond@europa (Raymond Man) writes:
>
>Assuming an aircraft can complete a turn, either upwind or downwind, its GROUND
>speed will essentially be the same(Newton's Laws). So the relative wind will be
>stronger when upwind and the wing can generate more lift. That was Mr. Treloars
> point.
>

Oh goody, here comes the downwind turn fight again!! I thought it was dead
and buried, but no.

Let's take the above as gospel truth (ref. Bob 12:38) and consider that I
go out pottering along in my J3 Cub, stooging along at 40 mph indicated,
into the teeth of a 40 mph headwind. Since I find myself motionless
over the ground, I decide to turn around and go the other way. I do so,
which isn't difficult with 40 mph of wind over the wings. However,
applying the above hypothesis, I remain motionless over the ground!
There is, of course, still the matter of a 40 mph wind, now flowing
forward relative to my airplane. Thus we can connect to an even older
rec.aviation thread, and conclude that the Cub can indeed, FLY
BACKWARDS!! (And I suppose if I close the throttle even more, back
to maybe 35 mph indicated airspeed, I will fly backwards even FASTER!
So by maintaining minimum power settings, I will make maximum
progress toward my goal, flying backwards all the way. Picture the
surprise when I arrive in the pattern flying backwards!! On second
thought, maybe I'd better plan to make a straight-in approach.)

Jim Beckman AT&T BTL, Middletown, NJ att!mtqub!jeb

Teun Hendriks

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Sep 21, 1990, 1:10:23 PM9/21/90
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In article <20...@amethyst.math.arizona.edu> ray...@europa.UUCP(Raymond Man)writes:
.
.

>Assuming an aircraft can complete a turn, either upwind or downwind, its GROUND
>speed will essentially be the same(Newton's Laws). So the relative wind will be
>stronger when upwind and the wing can generate more lift. That was Mr.
>Treloar's point.

To shed some light on the controversy of "gaining airspeed" by
turning upwind, Newton's Laws of motion don't change when you switch from
a fixed reference frame (i.e. the ground) the a moving reference frame
(i.e. the airmass surrounding the aircraft), as long as the latter is
moving with a constant translational velocity. ( This constant velocity
factor is canceled out in the acceleration equations).


Vxground = Vair + Vxtas
Vyground = Vytas

So given that dVair/dt = 0, (the constant velocity assumption),
differentating these equations yield,the fact the accerelation (A) with
respect the ground and airmass reference frame are equal:

Axground = Axtas ( A = acceleration )
Ayground = Aytas

Concluding, the reasons for a change in airspeed are the same as for a change
in groundspeed.

But how to explain why it is that the groundspeed changes and not the
airspeed?

Well, a well executed turn is a turn in which (airspeed) accerelation
is normal to the (airspeed) velocity (no slip or crab), assuming that the
slight increase in drag while turning (because of extra lift required) can
be neglected. Thus:

Axtas*Vxtas + Aytas*Vytas = 0

Folowing from this the magnitude of the air velocity doesn't change (ie.
Vxtas*Vxtas + Vytas*Vytas = Vtas), only its direction changes.

Now using the above equalities, with quantities relative to the ground the
relation between velocity and becomes:

Axground*Vxground+Ayground*Vyground = Axground*Vair

This is why RELATIVE TO THE GROUND the acceleration is not normal the velocity!
Therefore the groundspeed will increase/decrease depending on the direction of
turn and the windvelocity.

If this seems counter-intuitive, think about this:

Take a ride in the subway,train etc. Walk in the subway, train, while it is
riding at constant speed. Make a circle. Are you thrown off balance or need to
grasp something, because the floor slides by?


Hope this helps!

-Teun
--
Teun Hendriks (914)-945-6408
Philips Laboratories
345 Scarborough Road
Briarcliff Manor, NY email: t...@philabs.philips.com

Glen Baker

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Sep 21, 1990, 2:46:11 PM9/21/90
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/ hpnmdla:rec.aviation / raymond@europa (Raymond Man) / 3:25 am Sep 21, 1990 /
In <37...@ut-emx.uucp> Robert Dorsett quoted me

>In article <20...@amethyst.math.arizona.edu> ray...@europa.UUCP(Raymond Man)writes:
.
.
>>>Assuming an aircraft can complete a turn, either upwind or downwind, its
>>>GROUND speed will essentially be the same(Newton's Laws). So the relative
>>> wind will be stronger when upwind and the wing can generate more lift.
>>>That was Mr. Treloar's point.

>and refuted my claim:

>>Nonsense. Assuming the indicated airspeed is held constant throughout the
>>turn, the groundspeed can vary greatly. If you fly into the wind, the
>>groundspeed will decrease. If you fly away from the wind, the grounspeed
>>will increase. Neither has the slightest effect on the airplane's ability
>>to fly.

>As usual, the error here is the assumption.
[much irrelevant text deleted]

>Before calling others' idiot theories and claiming one's facts, it is better
>to debate rationally according to the Laws of Physics first.

Before attempting to refute Mr. Dorsett's claims it would probably be best
to actually understand the laws that you wish to invoke. Mr. Dorsett is
entirely correct in his claims, and the "laws" of Physics will support him
to the hilt. As long as you are just *flying* (not attempting to navigate or
land) the wind direction has none, zero, effect on the ability of your
aircraft to generate lift. ALL that matters is the velocity of the airframe
relative to the airmass...the motion of the airmass relative to ANYTHING
else is COMPLETELY irrelevant.

I am appalled that anybody who wishes to invoke physics would, in the same
breath, claim that GROUND speed has any *aerodynamic* relevance whatsoever!

Mr. Dorsett presented some simple thought experiments that should easilly
prove the point, but if his examples were not obvious enough, consider this
one. Ever fly under the hood? Do you think you could *fly* the airplane
with only airspeed, altitude, and attitude indicators? Do you think that
given only those three instruments you could head the airplane into the
wind? No way. If Mr. Man is correct in his assumptions it would be a simple
matter of making a turn and watching the airspeed indicator to see what
happens. By his completely erroneous assumptions you should be able to
make a constant bank turn and watch the airspeed indicator rise as you head
"into" the wind and fall as you head "out".


I agree with Mr. Dorsett that Mr. Man's original statements were nonsense.

Attempting to use Newton to support those statements is nonsense on stilts.

Attempting to put those statements into play while flying an aircraft is
downright dangerous!


glen "spending my life in relative wind" baker

Andrew Koenig

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Sep 21, 1990, 3:10:33 PM9/21/90
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The original letter was of course dead wrong and the response correct:
airplanes do not know where the ground is going.

However, there is a psychological point that is worth mentioning:
pilots sometimes confuse groundspeed and airspeed, or heading and course.

As an example, here is a problem that was posed a while ago in
a meeting of our flying club: suppose you are coming into an
uncontrolled airport that has a direct crosswind (as shown by the
windsock). There is no preferred runway and no traffic in the pattern.
Would you be better landing with a left or a right crosswind?

There was much discussion on this matter with no real conclusion.

Later I tried some experiments and realized the answer: a right
crosswind is better in a left-hand traffic pattern. Similarly, a
left crosswind is better in a right-hand traffic pattern.

The reason is that it's better to have a headwind than a tailwind
on base leg. If you have a tailwind on base, the turn to final is
going to come sooner than you expected, so you'll have to turn more
steeply than you might have planned. Moreover, because your ground-
speed looks high, youl'll be tempted to slow down. This sets you up
for a classic stall-spin accident.

If you have a headwind on final, you may undershoot the turn. This
is no great problem, because the natural response is to add power and
decrease bank, both of which put you into a safer configuration.
Moreover, you'll have more time to think about what's happening.

In neither case does the airplane know what's happening on the ground.
The pilot, however, is a different matter entirely.
--
--Andrew Koenig
a...@europa.att.com

Paul Raveling

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Sep 21, 1990, 3:48:48 PM9/21/90
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In article <20...@amethyst.math.arizona.edu>, raymond@europa (Raymond
Man) writes:
>
>
> Neither indicated nor true airspeed will be constant in a normal turn
if there
> is significant wind velocity. Mr. Dorsett's examples were for straight
line
> steady flight.

Mr. Dorsett was right. Aircraft fly relative to the air mass
that surronds them. If an air mass is moving relative to the
ground, a turning plane still experiences exactly the same forces
and air velocities as if the air is still.

I've found a quote from the original article in a followup to
sci.aeronautics and agree with the early opinion that whoever
wrote "a sudden turn upwind could be a lifesaver if you're
critically low and slow" made a dangerous statement. Obviously
they didn't understand the physics of the situation.

The error in believing that turning upwind suddenly can increase
airspeed is in looking at the wrong reference frame. Take this
example:
^
15 kt Wind ----> | 55 kt airspeed
(relative to ground) | (relative to air)
|

Suppose you're flying directly crosswind at 55 knots, and the wind
is 15 knots relative to the ground. If you could instantaneously
yaw the aircraft 90 degrees to point upwind (and could do that
without stalling a wing!!!). Would you experience a 70 knot
forward airspeed? No way.

You'd experience a 0 knot forward airspeed and a 55 knot breeze
blowing across the airframe from right to left. Your ground speed
doesn't change in this instantaneous maneuver -- It still has
components of 55 knots and 15 knots.

> and how could I confuse airspeed with ground speed.

Sounds like one of those things left as an exercise for the
student in textbooks... Figure out how you confused them.
>
> Mr. Dorsett made some heavy charges:
>
> >I agree with Mr. Rose; it's unconscionable that pilots can be
certified not
> >knowing these basic facts. It's even worse when aviation-related
magazines
> >condone such irresponsible views. Especially idiot theories that
might give
> >novices the impression that they can recover from stalls by hard
turns into
> >the wind--which will invariably produce stall/spin accidents.

I agree 100%.

> Before calling others' idiot theories and claiming one's facts, it is
better

> to debate rationally according to the Laws of Physics first. While, as
I have
> said, the net can be a good place for such discussion, one can still
embarrass
> oneself needlessly.

That's good advice too. If you're not sure of how physics
applies, this is a great place to ask. If you are sure and are
wrong, you'll find out about it.


------------------
Paul Raveling
Rave...@Unify.com

Paul Stafford

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Sep 21, 1990, 6:13:49 PM9/21/90
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I can't believe this is being argued here, unless we have some flat earth folks
or perhaps some hold overs from ether advocates, or maybe folks who claimed
that the panama canal wouldn't work, because the oceans were at different
levels.

I must agree with Baker, Dorsett and others who have been arguing that
indicated airspeed is all that matters, and the groundspeed matters not a
whit in this context.

Do you mean to tell us that you cannot maintain a constant airspeed in a
turn without changing power? this is what your argument implies!
Can you determine wind direction under the hood? if so, you are quite special!
Lastly, lets go try this miracle maneuver by which a turn into the wind can
save you from stalling! But- please, lets try it from a safe altitude,
because it's going to be interesting..

If you would actually try these experiments, I'm sure you would then agree
with the pilots, and stop invoking LAWS OF PHYSICS that, while true, have no
bearing on the current problem.

p.s.- if you can't do experiment 1, don't go trying for a rating- the examiner
won't be pleased at the throttle activity!

Raymond Man

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Sep 20, 1990, 9:31:31 PM9/20/90
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In <36...@softway.sw.oz.au> Greg Rose published a letter to the editor of
a magazine and his own response.

I don't want to get into the fine points in both letter, but Mr. Rose' anger at
Mr. Treloar, the author of the first letter, may have stem from his misunderstanding
of one fundamental fact.

Assuming an aircraft can complete a turn, either upwind or downwind, its GROUND
speed will essentially be the same(Newton's Laws). So the relative wind will be
stronger when upwind and the wing can generate more lift. That was Mr. Treloar's
point.

Trying out arguments on the net first is a better idea because the potential
pit falls can come out faster and the flames here are usually more tolerable
(personal opinion here). I welcome others' comments ot flames here.

Just call me `Man'.
"And why take ye thought for " -- Matt. 6:28
ray...@jupiter.ame.arizona.edu

Robert Monsen

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Sep 20, 1990, 10:14:14 PM9/20/90
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[Regarding the "upwind turn" issue]
Try a thought experiment. Assume that you have a fog blanket under you
that is travelling along with the wind, just like you are, so you can't
see the ground. Can you tell which way the wind is blowing? (ignore shear
for now... and your VOR :-). Since you can't, then turning one way or
the other doesn't matter much from a stall-spin point of view. Your
component of motion with the wind can't be detected.

On the other hand, if you are trying to get back to the runway,
you are following a moving point on the fog blanket. If you turn in the
direction that the point is moving, then you will have a shorter distance
to go to get there. This is why glider pilots turn into the wind
when a rope breaks. Its a shorter distance, from the airframe's point
of view....

Bob Monsen
Orion Network Systems
Berkeley, California.

Robert Dorsett

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Sep 20, 1990, 10:58:08 PM9/20/90
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In article <20...@amethyst.math.arizona.edu> ray...@europa.UUCP (Raymond Man) writes:
>In <36...@softway.sw.oz.au> Greg Rose published a letter to the editor of
>a magazine and his own response.
>
>I don't want to get into the fine points in both letter, but Mr. Rose' anger at
>Mr. Treloar, the author of the first letter, may have stem from his
>misunderstanding of one fundamental fact.
>
>Assuming an aircraft can complete a turn, either upwind or downwind, its GROUND
>speed will essentially be the same(Newton's Laws). So the relative wind will be
>stronger when upwind and the wing can generate more lift. That was Mr.
>Treloar's point.

Nonsense. Assuming the indicated airspeed is held constant throughout the


turn, the groundspeed can vary greatly. If you fly into the wind, the
groundspeed will decrease. If you fly away from the wind, the grounspeed
will increase. Neither has the slightest effect on the airplane's ability
to fly.

If your thesis were correct, then an airplane with a groundspeed of, say,
10 knots would be expected to be in a deep stall, if its VS0 were 50 knots.
But what if the pilot's landing on a runway, into a headwind of 40 knots?
He would be indicating 50 knots at touchdown; THAT is the value that matters
when judging flying qualities, not the 10 knots the airplane would by flying
relative to the ground.

Groundspeed merely defines how fast the scenery goes by; thus, its primary
interest is in navigation, and during that nebulous interval when the air-
plane turns into a ground vehicle again.

I suspect you're confusing ground speed with true airspeed, but even then,
your basic premise is wrong.

I agree with Mr. Rose; it's unconscionable that pilots can be certified not
knowing these basic facts. It's even worse when aviation-related magazines
condone such irresponsible views. Especially idiot theories that might give
novices the impression that they can recover from stalls by hard turns into
the wind--which will invariably produce stall/spin accidents.

Rose presented a fairly good argument. If people still don't get it (or don't
believe it), read up on Langeweische: he puts it fifty different ways; one
of them's sure to stick. Or, better still, go fly an airplane in a stiff
wind, and try to reconcile the known slow-flight handling characteristics with
how the airplane flies through the air at normal indicated airspeeds...

If you (or anyone else) has read Treloar's theory in legitimate training
literature, I'd be interested in hearing where.

---
Robert Dorsett
Internet: r...@rascal.ics.utexas.edu
UUCP: ...cs.utexas.edu!rascal.ics.utexas.edu!rdd

Dirk Lummerzheim

unread,
Sep 21, 1990, 9:15:01 PM9/21/90
to
In article <1990Sep21.1...@jato.jpl.nasa.gov> he...@mars.jpl.nasa.gov (Vance Heron) writes:
>In article <20...@amethyst.math.arizona.edu> ray...@europa.UUCP (Raymond Man) writes:
>>
>>Assuming an aircraft can complete a turn,
>>either upwind or downwind, its GROUND
>>speed will essentially be the same(Newton's Laws).
>
>BZZZ - BAD ANSWER !!!
>
>You need to re-read Mr Rose's post again ... Why will the ground speed
>be the same ?? What does the airplane even know about the ground ??
>
>Try a thought experiment - what happens when a plane is "flying" into
>a headwind that matches its airspeed exactly ? (Try 100 KIAS)
>1) How fast is it flying ?
>2) What happens when it turns ? (or can it even turn at all)
>
>If that doesn't work, find a pilot & ask him to demonstarate it to you -
>have him fly at MCA and turn upwind & downwind ... no stall, no change
>in air speed, BIG change in ground speed (on a windy day).
>

Let me give it a try: in the idealized world of physicists (I know
what I am talking about... :-) ) we apply the laws of conservation
to an airplane. Take the thought experiment: The plane flies into the
wind with ground speed of 0. Its kinetic energy is therefore 0. We
neglect friction, assume constant altitude, no engine (if we don't
have friction, we don't need an engine either, right?). By some
magic the plane now gets turned around 180 degrees. What happens?
No force input ==> no change in energy, i.e. it is still at rest with
respect to the ground. But now the wind is blowing from behind.
So it flies backwards at 100 KIAS? Falls to the ground? Right, this
idealized plane would do that. And in this idealized world, I
would ALWAYS turn into the wind, if I was getting slow and low.

But in the real world there is friction, and an engine to overcome
it, and a wing to produce lift, and an engine to provide the force
to generate the lift, and lots more. And you can't just turn the plane
around on a dime.
Therefore, once you start turning the plane out of the wind, it will
accelerate (relative to the ground) and that mostly due to friction!
Sounds weird, doesn't it. But then, flying in 100 knots of wind is
weird, too. To put it into another way of looking at it: the plane
tends to go with the surrounding air. It sticks to it (due to
friction...). And for practical purpose, it is much easier to deal
with all of this in a reference frame that moves with the wind.
And once you do that, terms like upwind and downwind become
meaningless. And it is a "legal" frame of reference, it is conservative,
not accelerated, both Newton and Einstein agree for airplane speeds,
but this frame has much easier understandable and comprehendable physics:
it doesn't do harm to neglect the friction or to equate all friction
terms with the thrust of the engine. And now, if we look at the
energy conservation everything jives: the velocity is now measured
in this frame of reference, and to maintain kinetic energy, airspeed
is maintained, the plane keeps flying.

Of course, in the real reality, friction doesn't work instantaneously
and neglecting it in the frame that moves with the wind is an
idealization. But this is far closer to the real world than the
first Gedankenexperiment without friction in the airport frame of
reference.

Conclusion: in the real world you don't gain anything by turning
into the wind.
Dirk

Raymond Man

unread,
Sep 21, 1990, 6:25:35 AM9/21/90
to
In <37...@ut-emx.uucp> Robert Dorsett quoted me

>In article <20...@amethyst.math.arizona.edu> ray...@europa.UUCP(Raymond Man)writes:
.
.

>>Assuming an aircraft can complete a turn, either upwind or downwind, its GROUND
>>speed will essentially be the same(Newton's Laws). So the relative wind will be
>>stronger when upwind and the wing can generate more lift. That was Mr.
>>Treloar's point.

and refuted my claim:

>Nonsense. Assuming the indicated airspeed is held constant throughout the
>turn, the groundspeed can vary greatly. If you fly into the wind, the
>groundspeed will decrease. If you fly away from the wind, the grounspeed
>will increase. Neither has the slightest effect on the airplane's ability
>to fly.

As usual, the error here is the assumption. Mr. Dorsett is mixing two
different situations. For steady flight, when thrust balances drag, indeed
airspeed is constant, regardless of wether the aircraft is flying upwind
or downwind as observed by the ground. But a normal turn is a different
situation. Unless it is a very large radius turn and takes a long (I mean LONG,
in tens of minutes) time to complete, the slight increase in drag due to
flying upwind is not going to change the aircraft's inertial velocity a lot.
Since the turning force, a component of the lift, acts nearly normal to the
flight direction, it changes only the aircraft's velocity but not its
magnitude. So which ever direction the aircraft turns, as long as the time
to complete the turn is short enough, the final speed will essentially be
the same, as I have stated.

Neither indicated nor true airspeed will be constant in a normal turn if there
is significant wind velocity. Mr. Dorsett's examples were for straight line
steady flight.

Mr. Dorsett also wrote:

>I suspect you're confusing ground speed with true airspeed, but even then,
>your basic premise is wrong.

Since the difference between IAS and TAS is due to sensor position error,
compressibilty effect and change in atomspheric density only, none of which
enters into our discussion here, there is no need for their distinction here


and how could I confuse airspeed with ground speed.

Mr. Dorsett made some heavy charges:

>I agree with Mr. Rose; it's unconscionable that pilots can be certified not


>knowing these basic facts. It's even worse when aviation-related magazines
>condone such irresponsible views. Especially idiot theories that might give
>novices the impression that they can recover from stalls by hard turns into
>the wind--which will invariably produce stall/spin accidents.

Before calling others' idiot theories and claiming one's facts, it is better


to debate rationally according to the Laws of Physics first. While, as I have
said, the net can be a good place for such discussion, one can still embarrass
oneself needlessly.

James Wanderer

unread,
Sep 21, 1990, 9:43:04 PM9/21/90
to
In article <20...@amethyst.math.arizona.edu> ray...@europa.UUCP (Raymond Man) writes:
>Assuming an aircraft can complete a turn, either upwind or downwind, its GROUND
>speed will essentially be the same(Newton's Laws). So the relative wind will be
>stronger when upwind and the wing can generate more lift.

>(personal opinion here). I welcome others' comments ot flames here.

Ok, I'll try a flame. (We lurkers get so few chances at this).
I'll venture that the law we are refering to deals with change
in velocity (effect of inertia). However, the velocity of the plane
through the air (assumeing that the wind is constant - no gusts)
is not going to change due to any wind. The inertia is important
when wind speed changes. A gust from the front will increase
the air speed, but a lull will decrease it - so there is no
win relative to a head/tail wind that I can see here.

Disclaimer : I really know much more about networks than airplanes
Now I will resume my regular silence..

--
-----
Jim Wanderer - Department of Computer Science,
University of California at Los Angeles, Los Angeles, CA 90024
Internet: jim...@cs.ucla.edu Voice: (213) 826-3015

Steve Brecher

unread,
Sep 22, 1990, 12:10:42 AM9/22/90
to
What I understood ray...@europa.UUCP (Raymond Man) to be saying is that
due to Newton's third law changes in velocity (ground speed, or
"Newtonian speed" if you will) cannot be instantaneous. I suspect that
in <20...@amethyst.math.arizona.edu>,

> Assuming an aircraft can complete a turn, either upwind or downwind, its
> GROUND speed will essentially be the same (Newton's Laws).
he may have inadvertantly omitted a word such as "instantaneously" in
the first phrase of the sentence.

I think the issue is the time required for the wind to contribute its
effect on change in ground speed (velocity, a signed quantity) relative
to the time required to make the turn. If the former time is longer,
then for some interval after completing a turn to downwind the magnitude
of the ground speed is less than airspeed+windspeed, and hence the
airspeed is less than it was originally.

I am attempting here to clarify the issue at hand, rather than making
any substantive contribution.

--
bre...@btr.com (Steve Brecher)

Mike Ciholas

unread,
Sep 21, 1990, 11:38:40 AM9/21/90
to
In article <20...@amethyst.math.arizona.edu> (Raymond Man) writes:
>
>Neither indicated nor true airspeed will be constant in a normal turn if
>there is significant wind velocity. Mr. Dorsett's examples were for
>straight line steady flight.

AAAARRGGGHH! This is wrong, just dead wrong. A constant wind has
absolutely no affect on aircraft performance in a turn. In fact,
without a ground reference, it can be proven that a pilot cannot
detect the wind at all.

For example, over a solid deck at 5000 feet, you cannot see the
ground. Now perform your turns. You turn upwind, you turn downwind,
and the plane behaves the same. No sudden increase in lift when you
turn upwind at all. NONE. This may be the case even though the wind
at altitude could be 50 knots! Have you ever turned into the wind and
gotten an increase of 50 knots in your airspeed?

In fact, if you fly accurately, you turns should be made at a constant
airspeed. As you enter the turn, you may lose a little airspeed due
to the increased lift necessary, but once in the turn, your airspeed
should be constant.

GROUND SPEED HAS NO AFFECT ON AIRPLANE PERFORMANCE. ONLY AIRSPEED
AFFECTS AIRPLANE PERFORMANCE.

(except, of course, ground roll and heading/course corrections, but
thats another story altogther).

Again, the perpetuation of this bogus idea that turning into the wind
gets you more airspeed is VERY dangerous.

Mike Ciholas

email: mi...@lcs.mit.edu
snail: 303 Lowell St, #5 / Somerville, MA 02145
phone: (617) 623-3563
air: N1909C, a 1954 Cessna 170B

Andrew Koenig

unread,
Sep 22, 1990, 2:58:46 AM9/22/90
to
In article <20...@amethyst.math.arizona.edu>, raymond@europa (Raymond Man) writes:

> Neither indicated nor true airspeed will be constant in a normal turn if there
> is significant wind velocity. Mr. Dorsett's examples were for straight line
> steady flight.

> Before calling others' idiot theories and claiming one's facts, it is better


> to debate rationally according to the Laws of Physics first. While, as I have
> said, the net can be a good place for such discussion, one can still embarrass
> oneself needlessly.

I completely agree.

To avoid such embarrassment, you may wish to reconsider the first
paragraph quoted above.
--
--Andrew Koenig
a...@europa.att.com

Robert Dorsett

unread,
Sep 21, 1990, 11:37:10 AM9/21/90
to
>As usual, the error here is the assumption. Mr. Dorsett is mixing two
>different situations. For steady flight, when thrust balances drag, indeed
>airspeed is constant, regardless of wether the aircraft is flying upwind
>or downwind as observed by the ground.
^^^^^^^^^^^^^^^^^^^^^^

Again, as in email, the notion that the ground reference frame determines
flight performance seems to be pervasive in this discussion. I suggest
the ground reference frame is irrelevant.

What matters is the velocity of the airflow over the wings of the aircraft.
In a steady wind, or no wind at all, from the aircraft's perspective, the
wind simply IS NOT THERE. It DOES NOT EXIST. There cannot, therefore, be any
variation in the performance of the airplane due to the orientation of the
nose of the aircraft.


>in tens of minutes) time to complete, the slight increase in drag due to
>flying upwind is not going to change the aircraft's inertial velocity a lot.

There will be no change in drag, since flying "upwind" is a meaningless con-
cept. If an airplane is flying 100 kts TAS, at a given AOA, in a given
configuration, it will have a drag D. If it flies into a wind, in level
flight, its airspeed will STILL be 100 kts TAS; its GROUND SPEED will be
100 kts - WS, where wind- speed = WS. Its drag will still be D. If it flies
away from the wind, its GROUND SPEED will be 100 kts + WS. Its drag will still
be D. I have yet to see an equation of flight that is dependent on GROUND
SPEED. All ground speed is good for is to tell how fast the scenery's going
by.

You folks have to remember that as the airplane is flying around in the wind,
it is being carried along by the wind. The airplane does not "feel" the res-
istance of the wind like you do sitting on the ground. It cannot.


>Since the turning force, a component of the lift, acts nearly normal to the
>flight direction, it changes only the aircraft's velocity but not its
>magnitude. So which ever direction the aircraft turns, as long as the time
>to complete the turn is short enough, the final speed will essentially be
>the same, as I have stated.

No, you said the aircraft's GROUND SPEED will remain the same, which is not
correct, even by your own reasoning.


>Neither indicated nor true airspeed will be constant in a normal turn if there
>is significant wind velocity. Mr. Dorsett's examples were for straight line
>steady flight.

No, they were for turning flight, and the IAS and TAS will remain constant.
The airplane, once again, is being carried along by the wing. IAS and TAS
provide an indication of how fast the airplane is moving through the fluid;
there is no reason that they should change during a turn. Certainly not by
any *wind* effect.


>Before calling others' idiot theories and claiming one's facts, it is better
>to debate rationally according to the Laws of Physics first. While, as I have
>said, the net can be a good place for such discussion, one can still embarrass
>oneself needlessly.

Sometimes, all one needs to use the Laws of Physics (TM) is the right frame-
of reference. :-) It's understandable for land-lubbers (or sailors) to feel
a strong affiliation with the ground, and to choose frames of reference that
somehow include it, but from an aerodynamic standpoint, what matters is only
the wind and the airplane. The ground reference frame becomes pertinent
only at the instant in time that the airplane enters ground effect and touches
down, and, of course, for purposes of navigation.

Don Elton

unread,
Sep 22, 1990, 4:16:13 AM9/22/90
to
In-Reply-To: message from raymond@europa

Where did you get the idea that drag on a flying plane has anything at all to
do with wind direction? Next thing I suppose someone will tell us that a
flying plane's drag and airspeed are affected by the rotation of the earth as
well. The key to understanding the falacy of this downwind turn business is
to realize that one aloft, an aircraft is part of the wind and not something
that opposes any wind. Any opposition of wind is only important in reference
to the ground and is irrelevent to flight. Was there some authoritative
source you read that explained that there's something special about a downwind
turn (and don't tell me about laws of thermodynamics as your position in this
appears to demonstrate that you've misinterpreted them... I'd like to hear of
an authoritative aviation related source on this if one exists).

UUCP: [ ucsd nosc ] !crash!pro-carolina!delton MCI: delton
ARPA: crash!pro-carolina!del...@nosc.mil CIS: 72010,37
INET: del...@pro-carolina.cts.com AOL: delton

pro-carolina [300-2400 baud] 803-776-3936, login: register

Vance Heron

unread,
Sep 21, 1990, 1:52:38 PM9/21/90
to
In article <20...@amethyst.math.arizona.edu> ray...@europa.UUCP (Raymond Man) writes:
>
>Assuming an aircraft can complete a turn, either upwind or downwind, its GROUND
>speed will essentially be the same(Newton's Laws).

BZZZ - BAD ANSWER !!!

You need to re-read Mr Rose's post again ... Why will the ground speed
be the same ?? What does the airplane even know about the ground ??

Try a thought experiment - what happens when a plane is "flying" into
a headwind that matches its airspeed exactly ? (Try 100 KIAS)
1) How fast is it flying ?
2) What happens when it turns ? (or can it even turn at all)

If that doesn't work, find a pilot & ask him to demonstarate it to you -
have him fly at MCA and turn upwind & downwind ... no stall, no change
in air speed, BIG change in ground speed (on a windy day).

Vance
he...@mars.jpl.nasa.gov

Paul Raveling

unread,
Sep 21, 1990, 12:02:15 PM9/21/90
to
In article <20...@amethyst.math.arizona.edu>, raymond@europa (Raymond
Man) writes:
> In <36...@softway.sw.oz.au> Greg Rose published a letter to the editor
of
> a magazine and his own response.
...

>
> Assuming an aircraft can complete a turn, either upwind or downwind,
its GROUND
> speed will essentially be the same(Newton's Laws).

?????

This sounds a bit bizarre from just what I see above.
Unfortunately I didn't read the referenced articles and our
news server has already discarded them. Perhaps someone could
forward a copy to me by email.

In the presence of wind ground speed varies considerably
through turns without violating Newton's laws. In fact
simple Newtonian mechanics dictate this.


------------------
Paul Raveling
Rave...@Unify.com

Jim Morris

unread,
Sep 21, 1990, 12:00:00 PM9/21/90
to
In article <1990Sep20.1...@ecn.purdue.edu>, ryo...@ecn.purdue.edu (Robert W Yoder) writes:
> In article <36...@softway.sw.oz.au>, gr...@softway.sw.oz.au (Greg Rose) writes:
> [text deleted]
>
> I spent a 5 month temporary assignment working for a fellow with whom I had a
> running debate on this subject. He steadfastly insisted that an airplane would
> automatically 'weathervane' into the wind when the pilot lined it up on final,
> thereby automatically setting the crab angle for landing. I maintained that
> years of experience had made the correction automatic for him through acquired
> skill.

My instructor used to tell me the airplane would weathervane into the
wind on final. Later, when I read "Stick and Rudder" and first learned
that an airplane does not "feel" the wind, I thought at first that he
had been wrong.

However,...

Although an airplane cannot "feel" a steady wind, it can definitely
"feel" gusts. On final, in any kind of reasonably strong crosswind you
will normally get some gusting. At the instant of the gust, (a light)
airplane will tend to weathervane into the wind. I think this is the
effect to which my instructor had been referring.
--
Jim Morris ...!sequent,ogicse!verdix!simsol!jimm
Systems Simulation Solutions, 15455 NW Greenbrier Parkway #210
Beaverton, OR 97006 (503) 295-7658
Cessna 414A (N2689U)

Steven Philipson

unread,
Sep 22, 1990, 1:06:36 AM9/22/90
to

The discussion on the net has been accompanied by a lot of behind
the scenes e-mail. I received a request to explain a few things, and
a few people suggested that I post my response to the net. As is often
the case, my article is long, but hopefully it will clarify the issues
for those who are confused. And with no further ado....

--------------------------------------------------------------------


>> Groundspeed merely defines how fast the scenery goes by; thus, its primary

>I don't think that this is true. Newton's second law (Force = Mass
>times acceleration) is only true when refered to an "inertial
>reference frame". I think that straightforward application of the
>principle of conservation of energy also requires an inertial
>refernce frame. The moving airmass is not an inertial reference
>frame. The ground isn't either, but it's close enough for the problem
>at hand. The groundspeed is relevant, in the sense that it tells us
>the kinetic energy of the airplane.

The crux of the problem is contained in the above paragraph. The
airmass IS the inertial reference frame when we're trying to understand
principles of flight. The velocity and orientation of the aircraft
with respect to the airmass is what determines the performance of the
aircraft. The velocity of the aircraft with respect to the ground
is only important when one is trying to maneuver with respect to it
or when coming in contact with it. In that case, we use the ground
as a reference frame for determining such things as ground speed
and impact forces, but the aircraft's behavior still is determined
by its motion relative to the air. Things get a little complicated
when we look at wind/ground interactions, but I'll get into that in
more detail below.


>So, if I'm flying at 50 mph into a 50 mph head wind, I'm not going
>anywhere, and I have no kinetic energy.

Not correct. You have no velocity (or kinetic energy) with respect
to the ground, but you certainly do with respect to the airmass.
If you didn't, you wouldn't be able to generate lift!

Consider a very large balloon floating in this airmass. It has
no velocity relative to the airmass, e.g. an airspeed of zero. From
this balloon is hanging a 10 ton slab of steel. You'll probably
agree that you wouldn't want to fly into that slab. You have
considerable kinetic energy with respect to it. Now consider what
happens if the wind velocity or direction is different. There's
no difference in the kinetic energy that you have with respect to
the slab (or the airmass), but there would be a considerable
difference in your kinetic energy with respect to the ground.

> If I turn 180 degrees, so I
>am going 100 mph in the inertial reference frame, then I have the
>kinetic energy associated with going 100 mph.

Your velocity is now 100 mph relative to the ground, but still
50 mph relative to the airmass, and to the balloon and slab.

> Where does that energy
>come from? It comes from the wind pushing on my airplane. The thing
>is, the acceleration _cannot_ happen instantaneously. It takes time.
>The change in speed (actually momentum) is the integral of force over
>time, so an instantaneous change in speed would require an infinite
>force.

If the wind pushing on your airplane were causing it to accelerate,
then you'd continuously decelerate when flying into the wind and
accelerate when flying with the wind. Your airspeed remains constant
though -- the change in momentum is provided by the horizontal
component of lift produced by the wings, and thrust produced by the
engine equals the drag of the aircraft. The groundspeed is a vector
*sum* (and you will not continuously accelerate with respect to the
ground either) which indicates that you are moving with respect to the
airmass regardless of which way the airmass is moving.

>If my
>reasoning is correct, when we make a downwind turn too quickly, the
>airplane loses airspeed, because the wind doesn't have enough time to
>accelerate the aircraft. We can compensate by adding power, so that
>the engine adds energy in excess of that lost to drag. Similarly,
>when we make an upwind turn too quickly, the airplane gains airspeed,
>because the wind doesn't have enough time to decelerate the aircraft.
> [...]

>I never notice these effects. The airplanes that I fly seem to be
>light compared to the force that the wind exerts on them. I make
>gentle turns, so the wind has plenty of time to speed me up or slow me
>down. I rarely fly in winds that are greater than half the stall
>speed of the airplane that I'm flying, so any tendancy to descend in a
>downwind turn or climb in an upwind turn is small.

Your reasoning is not correct. Consider riding in a bus, and
flying a paper airplane in it while driving at 60 mph. The air
speed of the paper airplane is about 5 mph. If we trim the paper
airplane to fly in circles, it will fly the same whether the bus
is stopped or going 60 in any direction. The airmass inside the
bus is moving at high velocity with respect to the ground outside,
but the airplane is flying with respect to the airmass inside the
bus only.

Notice that the turns of the paper airplane are quite rapid
with respect to the ground, but no power changes are made, no
airspeed changes occur, and the airplane doesn't overspeed or
stall.

You never notice the effects you describe because they aren't
there. You could make the tightest turns possible in a 500 mph
wind or zero wind, and you'd not see any difference in aircraft
handling or performance, just as the airplane in the bus will
perform the same whether the bus is stopped or moving at high
speed. (The case of an accelerating airmass is dealt with below).

BTW, I often fly in winds that are greater than half my stall
speed, and occasionally in winds that are higher than my max
flight speed (the latter I've only done with model airplanes).
The airplane performs identically regardless of the wind, although
the ground track of circles in those strong winds is greatly
elongated downwind.


If the airmass itself accelerates, then we will see an effect on
the airplane. For instance, if a headwind suddenly increases, we'll
see the airspeed suddenly increase by the same amount, and the
airplane will respond accordingly -- the nose will pitch up, the
airplane will climb, but eventually the airspeed will return to
the pre-increase airspeed. The *airspeed* would end up the same
after the aircraft stabilized, but the *groundspeed* would be lower.
The transient effects of increase in airspeed and altitude continue
only as long as the airmass accelerates. Examples of this include
wind gusts, wind gradient (change of windspeed with altitude) and
wind shear (as produced by microburts).

> [...] we add power or descend in tight turns, and any variation in
>how much power or descent is required is probably quite small.

In the case of windshears and microbursts, the speed of the
airmass can change by a relatively small amount (tens of knots for
example) but the changes in power required to maintain airspeed
and glide angle (with respect to the ground) are quite noticeable,
and may even be in excess of full power of the aircraft.

>I suspect that we all agree that careful airspeed control,
>particularly when maneuvering close to the ground, is the key to
>avoiding the type of accidents that Mr. Treloar attributed to downwind
>turns.

Airspeed control is important, but it's only part of what's
necessary, and it alone will not keep you from having the kind of
downwind turn accident that started this discussion.

> I think that there may be an increased danger of losing
>control of airspeed when turning downwind near the ground. It's not
>because of physics, but because of confusing groundspeed and airspeed.

This is just about correct -- it is a factor of confusion, but
for pilots, it's not quite a confusion with groundspeed, but ground
track and the effect of bank angle on stall speed.

>Seeing the groundspeed increase upon turning downwind, a pilot
>(particularly a tired or inexperienced pilot) might raise the nose or
>reduce power, losing airspeed. Taken to an extreme, the plane stalls.

This is not typically how these low altitude turn accidents occur.
The typical accident occurs when a pilot sees his ground track for
the turn becoming wider than desired -- the aircraft flies through the
track that he's used to seeing with zero wind. So, the bank angle
is increased to increase the rate of turn. Of course, as the bank
angle increases, so does the load factor, and with it, the stall
speed. So, normal speed, normal track, yet we get an unexpected
stall in the pattern and maybe a few fatalities.

The problem is exacerbated by the effects of steep turns. Drag
goes up, so unless a pitch or power change is made, airspeed goes
down. That reduces the margin to wings level stall speed, and in
and of itself could reduce airspeed to the steep-bank stall speed.
The capper is that pilots who don't understand these phenomena are
hesitant to bank steeply while close to the ground, so they tend
to keep the bank shallow and unconsciously use rudder to increase
the rate of turn. This produces uncoordinated flight, which both
raises the stall speed AND causes the stall to turn into a spin.
Say good night, Gracie.

This misconception is common among radio control model aircraft
fliers. They have no airspeed indicators and fly purely by visual
reference. When we have very high winds some pilots still tend to
fly their downwind legs at close to their normal groundspeed, as
they are used to flying the airplane by how fast it appears to be
moving relative to themselves. Unfortunately, the airspeed is just
above stall. When they turn base, they find the ground track is
moving too far downwind, and they increase bank. Knowledgeable
fliers use pitch attitude instead of groundspeed, and they can
see what's coming... "hey, check out this guy -- he's too nose
high. 'Bet he'll stall it in the turn and claim radio failure.
Here we go... 3, 2, 1, ... " The airplane stalls and pilot yells
"I've been hit!" (i.e. radio interference). And a half dozen guys
shake their heads in dismay.

There's one more point that I must cover in a little more detail.
I've been talking about the airmass as having a constant velocity.
This is true at altitude, but not true close to the ground. The
aerodynamic drag of the ground causes the wind velocity to change
with altitude, usually decreasing as you get closer to the ground.
If you're descending into a decreasing headwind, you'll lose airspeed
as you descend. The effect is quite dramatic in sailplanes -- you're
quite aware of pushing the nose down to maintain speed.

This effect can really bite you if you make a low altitude steep
turn in an airplane with long wings. The lower wing will be in an
airmass that is moving much slower than the airmass at upper wing.
The lower wing may reach stall airspeed while the upper wing is well
above stall. The low wing will drop and recovery is impossible.
This is why we don't make steep banks in sailplanes while close to
the ground. The effect is most severe when turning into a headwind
that decreases with altitude. If we're making a turn away from the
wind (into a decreasing tailwind) the lower wing will see a higher
velocity and not tend to stall. Of course, you'd better not count
on this working every time, 'cause an error in judgment of the wind
gradient could ruin your whole day.

Steve
(the certified flying fanatic)
ste...@decwrl.dec.com

Andrew Koenig

unread,
Sep 22, 1990, 12:15:30 PM9/22/90
to
In article <1990Sep21.1...@jato.jpl.nasa.gov>, he...@mars.jpl.nasa.gov (Vance Heron) writes:

> If that doesn't work, find a pilot & ask him to demonstarate it to you -
> have him fly at MCA and turn upwind & downwind ... no stall, no change
> in air speed, BIG change in ground speed (on a windy day).

Yeah, I've gone up in a 150 on a breezy day; by slow-flying it with
full flaps I can obtain negative groundspeed...
--
--Andrew Koenig
a...@europa.att.com

Ray Swartz

unread,
Sep 21, 1990, 3:10:57 PM9/21/90
to
In article <20...@amethyst.math.arizona.edu> ray...@europa.UUCP (Raymond Man) writes:
>Assuming an aircraft can complete a turn, either upwind or downwind, its GROUND
>speed will essentially be the same(Newton's Laws). So the relative wind will be
>stronger when upwind and the wing can generate more lift. That was Mr. Treloar's
>point.


Not a chance. This is the same as saying that a rowboat's speed
with respect to the bank of a fast moving river will be the same whether
it's moving upstream or downstream. Utter hogwash.

The ground speed of an aircraft is the vector sum of it's
velocity with respect to the airmass (airspeed) and the velocity of the
airmass with respect to the ground (wind speed and direction). If one
is flying at 70 kts into 20 kt wind, one has a groundspeed of 50 kts,
while flying downwind would give you a ground speed of 90 kts. There is
a significant difference.

Another point: Newton's Laws have nothing to do with it. It is
a simple transformation of coordinate frames, in the Galilean form.


--
Raymond L. Swartz Jr. (r...@hepuxa.hep.uiuc.edu, rsw...@fnald.fnal.gov)
Private Pilot - Glider // Physics Grad Student // All around swell guy

'Everything in this book may be wrong' -- Richard Bach, ILLUSIONS

Ray Swartz

unread,
Sep 21, 1990, 4:00:05 PM9/21/90
to
In article <20...@amethyst.math.arizona.edu> ray...@europa.UUCP (Raymond Man) writes:
>As usual, the error here is the assumption. Mr. Dorsett is mixing two
>different situations. For steady flight, when thrust balances drag, indeed
>airspeed is constant, regardless of wether the aircraft is flying upwind
>or downwind as observed by the ground. But a normal turn is a different
>situation. Unless it is a very large radius turn and takes a long (I mean LONG,
>in tens of minutes) time to complete, the slight increase in drag due to
>flying upwind is not going to change the aircraft's inertial velocity a lot.
>Since the turning force, a component of the lift, acts nearly normal to the
>flight direction, it changes only the aircraft's velocity but not its
>magnitude. So which ever direction the aircraft turns, as long as the time
>to complete the turn is short enough, the final speed will essentially be
>the same, as I have stated.
>
>Neither indicated nor true airspeed will be constant in a normal turn if there
>is significant wind velocity. Mr. Dorsett's examples were for straight line
>steady flight.
>


No. No. No.
The airplane does not "sense" the ground at all, and is
independent of the ground in flight (assuming a high enough altitude so
that ground terrain does not cause any rough gusts).

The problem with your model here, is that you seem to be
assuming that a plane flying at a 90 degrees away from the wind
direction has a groundspeed independent of the wind. Let's look at your
example the correct way...


When one is about to make a proper turn (nice smooth turn in the
airmass, without changing the bank angle to correct for wind direction)
one is esentially making a turn around a point. As you have said, the
turning force acts normal to the flight direction, and one gets a nice
90 degree turn around that point in the airmass (assume that one started
"crosswind" initially, and turned upwind). Now one is heading upwind
with the same airspeed (relative to the airmass) and thus, the same
speed relative to that point one was turning around. BUT THAT POINT IS
MOVING DOWNWIND WITH THE AIRMASS. Therefore, your groundspeed is lower
than it was.

Now we could complicate the situation by including the changing
bank angle one uses to correct for the wind, but this is only done to
change the rate of the turn so that one has a nice groundtrack, and is
NOT done to adjust the airspeed. In other words, it is a complication
that does not affect the basic physics of the problem.

gregory.p.kochanski

unread,
Sep 21, 1990, 7:36:00 PM9/21/90
to
In article <20...@amethyst.math.arizona.edu>, raymond@europa (Raymond Man) writes:
>Assuming an aircraft can complete a turn, either upwind or downwind, its GROUND
>speed will essentially be the same(Newton's Laws). So the relative wind will be
>stronger when upwind and the wing can generate more lift.
>That was Mr. Treloar's point.

If the ground speed were the same, then you'd be right. The problem with your
argument quoted above is that Newton's Laws don't simply address this problem.
You have:
1) "an object (at rest, in motion) *with no forces acting upon it* will
stay (at rest, in the same state of motion)"
2) F=ma
Airplanes have many forces acting upon them, especially when turning,
so you can't use rule 1. Rule 2 will handle airplanes nicely,
but evaluating the forces isn't trivial.

What we can say is that all aerodynamic forces depend only on
v(airplane)-v(air), so as long as the wheels are up and you have only
aerodynamics, 'F' in 'F=ma' depends only on this difference.
The mass of the airplane is independent of everything.
So, we can use the equation and say that 'a' depends only on v(airplane)-v(air).

Now, acceleration is nicely additive: it's just the rate-of-change-of-speed,
and speeds add nicely (Walk at 5mph in a railroad car going 50,
and you end up going 55 mph).
Now, *if the wind is independent of time or position*
the acceleration of the air that the airplane flies in is zero --
it's just going straight at constant speed.
So, in this special case, we can write 'a' in 'F=ma' as 'a(airplane)-a(air)',
because 'a(air)' is zero.

So, everything depends ONLY on differences between the airplane and the air;
there is no reference to the groundspeed ANYWHERE, as long as the
air isn't accelerating.

Greg Kochanski gpk\@physics.att.com
AT&T Physics Research -- Physics for fun and profit

Raymond Man

unread,
Sep 21, 1990, 6:18:18 PM9/21/90
to
I was entirely wrong in my previous article, thanks for the responses
from the other netters.

Spencer Garrett

unread,
Sep 22, 1990, 4:50:07 PM9/22/90
to
In article <20...@amethyst.math.arizona.edu>, raymond@europa (Raymond Man) writes:
> Assuming an aircraft can complete a turn, either upwind or downwind, its GROUND
> speed will essentially be the same(Newton's Laws). So the relative wind will be
> stronger when upwind and the wing can generate more lift.

Now I understand why you've been giving me such a hard time about my attempts
to explain lift. You don't know *anything* about physics or aviation.
(Or perhaps all your flying experience has been on ground tether. Pull
the release knob sometime, Raymond, and learn how a real airplane flies.)
Folks, please remind me not to take this clown seriously in the future.

BTW, I've gotten half a dozen encouraging e-mail responses to my postings,
including two from folks at NASA. As amusing as my explanation may have
been, I'm sure we can do better. Would someone with a deeper understanding
of matters aerodynamic care to expand upon my Maximum Elbow Room paradigm?
(Or mail me some criticisms and suggestions and I'll try again.)

Spencer Garrett

unread,
Sep 22, 1990, 5:17:59 PM9/22/90
to

One way to make the irrelevance of ground speed clear is to note that it
is perfectly valid to view "wind speed" as motion of the ground under
a stationary air mass. The airplane is flying in that stationary air
mass, and doesn't care what the ground decides to do until it's time
to make contact with some specific spot on the ground. Then you have
to "chase" that spot wherever it goes the same way you have to chase
a kid when you're playing tag. In either case you'll crash and burn
if you try to use your target as your frame of reference (before you're
in firm contact with said target).

Raymond Man

unread,
Sep 23, 1990, 5:51:29 AM9/23/90
to
In <1990Sep22.2...@quick.com> Spencer Garrett wrote:
>Now I understand why you've been giving me such a hard time about my attempts
>to explain lift. You don't know *anything* about physics or aviation.
>(Or perhaps all your flying experience has been on ground tether. Pull
>the release knob sometime, Raymond, and learn how a real airplane flies.)
>Folks, please remind me not to take this clown seriously in the future.
>
>BTW, I've gotten half a dozen encouraging e-mail responses to my postings,
>including two from folks at NASA. As amusing as my explanation may have
>been, I'm sure we can do better. Would someone with a deeper understanding
>of matters aerodynamic care to expand upon my Maximum Elbow Room paradigm?

This is most unfortunate that technical discussions on the net can be
construed as and finally developed into personal attacks. Wether I know
*anything* about physics or aviation is not important. Wether the materials
presented by me make any sense is. I wrote nonsense, I was corrected and I
admits my wrongs.

Calling me names and discrediting me will not help a bit any "theory" found
on misconception. Better understanding can only be obtained through
rational argument and un-emotional thinking about the different sides of
the issue. While the mistake I made was inexcusably elementary, I am
glad that I came out of my clouded thinking through the application of the
above and not blinded by the emotion of my ego. I would like to think that
this shows that I am not limited in growth by my own mind.

Mark Kaplan

unread,
Sep 24, 1990, 3:42:39 PM9/24/90
to
In article <1990Sep21.1...@cbnewsj.att.com>, j...@cbnewsj.att.com (james.beckman) writes:
>
> Oh goody, here comes the downwind turn fight again!! I thought it was dead
> and buried, but no.


Kind of reminds me. It must have been 15 years ago that there was a
long running debate (close to a year long) in RCM (Radio control modeler)
about about model airplanes falling out of the sky when they turned
downwind on a breazy day?

I never had one fall out of the sky, though I witnessed many a crash
under this conditions. It was always a case of a shaky pilot flying at low
altitude, and getting suprised by the amazingly quick increase in ground
speed as the turn was completed. This often caught the pilot by surprise,
and the resulting panic ended with a loss of control, and the
inevitable "thud".

As I recall, numerous pilots who were also "real" pilots that flew
the "big ones" also wrote in with their expert opinions, which covered
both sides of the argument.

I can understand the RC pilot wrongly thinking something strange happened
because of perspective (from the ground). But it is scary that a
full scale pilot could have such a poor understanding of basic flight.

I guess it is true. "If you can drive a car, you can learn to fly".

Bruce Lowerre

unread,
Sep 24, 1990, 11:30:37 AM9/24/90
to
In article <1990Sep20.1...@ecn.purdue.edu>, ryo...@ecn.purdue.edu (Robert W Yoder) writes:
>
> Thank you for an excellent explanation for what I believe to be the most major
> misconception in aviation. I have saved your article so I can use it the
> next time I get involved in this dumb argument.

My explanation for the effect of wind on an aircraft in free flight is
that the wind is the movement of the ground beneath the airmass through
which the aircraft is flying. A wind of 360 degrees at 10 knots means
to the aircraft that the ground is moving north at 10 knots.

Dirk Lummerzheim

unread,
Sep 24, 1990, 5:24:53 PM9/24/90
to
In article <748...@hpnmdla.HP.COM> gl...@hpnmdla.HP.COM (Glen Baker) writes:
>
> Before attempting to refute Mr. Dorsett's claims it would probably be best
> to actually understand the laws that you wish to invoke. Mr. Dorsett is
> entirely correct in his claims, and the "laws" of Physics will support him
> to the hilt. As long as you are just *flying* (not attempting to navigate or
> land) the wind direction has none, zero, effect on the ability of your
> aircraft to generate lift. ALL that matters is the velocity of the airframe
> relative to the airmass...the motion of the airmass relative to ANYTHING
> else is COMPLETELY irrelevant.
>
Well, this statement isn't quite right either. And every pilot knows
about that, even if he hasn't experienced it.
GROUND SPEED has a significance for the aerodynamics of the plane.
There is no doubt about that. The laws of physics that I wish to
draw attention to are the conservation of momentum and energy. Both
depend on the velocity of the plane in an inertial frame. If you are
moving through an accelerated airmass, that airmass does not constitute
an inertial frame. But the ground always does!
The example that I am getting at is the effect of wind shear. The plane
keeps moving with the same GROUND SPEED, but the relative wind suddenly
changes (i.e. when you go through a microburst it might change from
headwind to tailwind). You can see that the plane "feels" the ground
speed here. (Please note that the concept of "absolute" velocity
is meaningless. Velocity is always relative. And if you don't like
the ground a relative point, add any constant velocity to it.)

This is not saying that you gain anything by turning the plane into
the wind (which was the starting point of this discussion). That
(directional) change of velocity is always so slow, that other forces
(friction, lift) which are applied during the turn have enough time
to accelerate the plane, such that the conservation laws are not
that easily applied, and one fares a lot better with just considering
the entire problem in the reference frame of the moving airmass.

Dirk

Robert Monsen

unread,
Sep 24, 1990, 12:57:14 PM9/24/90
to
I've just read the 15th article by someone who decided to show off how utterly
intelligent he was by beating up Raymond Man. Lets give it a break, ok?

Geoff, please post your netiquette article again, for those who might
not have gotten a chance to read it.

Thanks,


Bob Monsen
Orion Network Systems

Berkeley, Ca.

Jim Schinnerer

unread,
Sep 25, 1990, 11:50:07 AM9/25/90
to
It truely amazes me that this has gone on this long...


>GROUND SPEED has a significance for the aerodynamics of the plane.
>There is no doubt about that. The laws of physics that I wish to

This is just plain (plane) wrong. Even when taking off and landing,
the plane will fly according to aerodynamics. This is why we have airspeed
indicators and not speedometers.

>The example that I am getting at is the effect of wind shear. The plane
>keeps moving with the same GROUND SPEED, but the relative wind suddenly
>changes (i.e. when you go through a microburst it might change from
>headwind to tailwind). You can see that the plane "feels" the ground
>speed here. (Please note that the concept of "absolute" velocity

> Dirk

No, again. The airplane will not "feel" the ground until the crash.
In a shear condition, the airplane will feel that the airspeed has
changed, and will *aerodynamically* respond to that change.

Invoking physics in this argument is almost like statistics. Because of
the different reference frames, physics can tell you the wrong answer.

----------------------------------
Jim Schinnerer - PP-ASEL-IA
email - schi...@hpihoah.HP.COM
----------------------------------

Geoff Brown

unread,
Sep 25, 1990, 11:09:20 AM9/25/90
to
Dirk Lummerzheim writes:
>Therefore, once you start turning the plane out of the wind, it will
>accelerate (relative to the ground) and that mostly due to friction!

No, the acceleration comes from the horizontal component of the lift generated
by the banked wings (there was a similar discussion to this one on what caused
an aircraft to turn, but lets not get into that again, it applies whatever the
wind speed).


ASK6E

Geoff Brown Tel: (Home) +44 279 506927
(Work) +44 279 29531 Ext 2716
e-mail G.B.D...@stl.stc.co.uk

William C. Carton

unread,
Sep 24, 1990, 6:21:57 PM9/24/90
to
In article <1990Sep22.2...@quick.com> s...@quick.com (Spencer Garrett) writes:
>
>. . . . . . . . In either case you'll crash and burn

>if you try to use your target as your frame of reference (before you're
>in firm contact with said target).

Reminds me of an instructor who came from the midwest, ye olde lande
of strayte roades and otherwise featureless terrain. If he couldn't
find a suitable pylon and high enough winds, he would teach "turns
about a semi" [tractor-trailer].

Talk about moving frame of reference!!!! [I wouldn't do this with a
Cub, folks! Especially if the semi has a radar detector!]

The Roadie of Axiom Technology --- Bill Carton

Geoff Peck

unread,
Sep 25, 1990, 7:24:32 PM9/25/90
to
[This article can and will be posted to rec.aviation periodically, on an
unscheduled basis. It is likely that it has been slightly revised since
the last time you read it... Change bars in the left margin indicate
revisions, but it is probably worth your while to at least quickly read the
entire posting.]

I've been rather disappointed in the quality of articles on rec.aviation in
the last few weeks. It seems that people are not doing the one thing which
would make reading rec.aviation more palatable for all of us

**** THINK BEFORE ENGAGING THE KEYBOARD ****

My aplogies in advance for the length of this posting, but given the sorry
state of this group recently, I think that these things urgently need to be
said. This is important enough that I ask that posters print out these
thoughts and read them over a few times before they next hit that key...

Herewith are some specific guidelines, most of which have been culled
from previous postings by me and a few others:

(1) Only post if you have something *valuable* to add to the discussion.
"I don't know about X, but here's what I don't know about X" postings
are a waste of net bandwidth, not to mention the time of other readers.
Remember that when you post something to rec.aviation, you are causing an
| estimated 23,000 people to read what you wrote. If the appropriate
audience for what you are writing is considerably smaller (the poster to
whom you are responding, for example), please use e-mail instead.

To add some urgency to these comments, here are Brian Reid's statistics
(for the month of August 1990) on rec.aviation:

+-- Estimated total number of people who read the group, worldwide.
| +-- Actual number of readers in sampled population
| | +-- Propagation: how many sites receive this group at all
| | | +-- Recent traffic (messages per month)
| | | | +-- Recent traffic (kilobytes per month)
| | | | | +-- Crossposting percentage
| | | | | | +-- Cost ratio: $US/month/reader
| | | | | | | +-- Share: % of newsreaders
| | | | | | | | who read this group.
V V V V V V V V
| 23000 606 88% 798 1523.9 1% 0.10 2.4% rec.aviation

As Paul Kube commented,
When answering someone's posted question, if you find yourself typing
"I don't remember exactly, but..." or "I don't have the book here,
but I seem to remember...", STOP. Take some time (a day or two won't
hurt) and do a little research. Look things up or think them through
and get it right. Post a nice authoritative response that we can all
learn from.

If you've got a bona fide question, or don't understand something, or
sort of understand it, by all means, speak up in an interrogatory vein --
that's a great way of getting a discussion going.

(2) Don't post your version of something which has been commented upon and
resolved recently -- post only if you have something new and interesting
to add. Posting to show off only that you also know something is neither
required nor encouraged.

(3) Ensure that your posting will be of value to a *wide* audience. If
your posting is really only of value to a single individual, or a few
individuals on the net, use e-mail instead. If your posting is really
only of value to yourself, send it to /dev/null.

|(4) Sometimes, humorous threads are great. But, sometimes, the first posting
has been mildly amusing and we've gone *way* downhill from there. So, if
you're trying to be funny, please post a follow-up only if it's
| *screamingly* funny. Original funnies are usually no problem -- it's
| the follow-ups which are.

(5) There have been a number of interesting discussions on rec.aviation
recently in which there has been too much verbatim inclusion of previous
articles. Please, folks, remember that the readership of rec.aviation is
a relatively static and loyal community -- chances are that someone
reading your article will have read the original, and will quite possibly
even remember it. Thus, it's not necessary to include entire articles
(or even paragraphs) in your posting in many cases -- a simple summary,
like:

In article <12...@foo.bar.COM>, redb...@foo.bar.COM (The Ace Himself)
asked about using a recycled prefromulated framastat as a replacement
the infamous Continental engine in the Piper Malibu.

I think this sounds like a good idea, but you'd need to get an autogas
STC to make it work, and Piper doesn't seem to like autogas...

will usually suffice. If you do wish to quote, please, please, gentle
writers, trim those quotations down. You are welcome to use ellipsis
("...") to minimize the lines you're quoting.

(6) Please don't include signature lines from the original message in the
reply. Again: DO NOT QUOTE OTHER PEOPLE'S .signature LINES!

|(7) Please, folks, consult the FAR's (which, presumably, most U.S. readers
have) before asking simple questions. Frequently, there's a simple
answer. If the answer given in the FAR's is vague (or is contradicted by
other FAR's), then it's a fine topic for net discussion. Otherwise,
let's try not to waste bandwidth (*reader* bandwidth is really the
critical quantity here).

(8) Please speak and write in plain English. When one needs to use technical
terms, acronyms, or other words which might not be understood by all who
are listening or reading, the speaker or writer should define these terms
on first use. People who use unfamiliar terms to obfuscate their
statements are frequently demonstrating their lack of real knowledge or
| their insecurity.

In some places, such as Trade-a-Plane, acronymspeak may be warranted
because of space considerations and a supposedly knowledgeable
audience. In most other instances, acronymspeak is poor form...

|(9) If you use location identifiers (the 3- and 4-letter "abbreviations"
| for airports and weather-observing stations), PLEASE define them.

(10) Don't post silly "conversational" notes -- these are best left to
e-mail. Occasionally, these are fine, but when half of the daily
rec.aviation volume is consumed by these things, it's very, very
discouraging to the average reader (or even to me).

Basically, if you can't decide whether what you're sending is
appropriate to a posting or to e-mail, choose e-mail.

(11) Sci.military is probably the appropriate venue for military-aircraft
discussions. [from Robert Dorsett]

(12) Your subject line should be indicitave of the content of the article.
"Dumb question #1" or "There I was ..." is NOT a good subject line --
"Logging Flight Engineer Time in Piper Warriors" is a good one. If the
subject has changed since the subject line was originally written,
**replace it**. You might also consider deleting the list of referenced
articles if they are no longer relevant.

(13) If you are writing about more than one subject, please, please post
*separate* articles for each subject, with appropriate header lines.

(14) If you are responding to a group of articles on the same subject,
it is easiest for readers to read your comments if you make a
single, consolidated response.

(15) For those of us who selectively archive rec.aviation, it would help
tremendously if you would include something of the form "in article
<x...@foo.bar>, zar...@foo.bar (Mr. Science) wrote..." in the *body* of
any article which is a direct follow-up to someone else's article.

(16) Last, but not least:
Praise in public, criticize in private.
Rec.aviation is not a forum in which flame wars are well received.
However, it is very important that we maintain rec.aviation as a forum
in which constructive criticism can be given after-the-fact.
Real-world pilot experiences are important, and we can all learn from
the mistakes of others. When posting an original "experience",
expect some criticism and second-guessing. When commenting on someone
else's actions, do so gently, positively, constructively, and with
reason.

Again, what is required here is for the poster to exercise a bit of
judgement and take a bit of time before posting so that the thousands of
readers of this group don't have to take so much time reading it. I thank
you, thousands of times.

|Comments on this article are, of course, welcome.

Geoff
--
Geoff Peck, Apple Computer, Inc., MS 3PK, Cupertino, CA 95014 (408) 974-9303
ge...@apple.com {sun,nsc}!apple!geoff AppleLink: PECK.G #include <disclaimer.h>

Per Lindberg

unread,
Sep 25, 1990, 2:57:23 PM9/25/90
to
In article <11...@alice.UUCP> a...@alice.UUCP (Andrew Koenig) writes
about right vs left crosswind landings, and, given a choice, right is
preferred because it gives a "longer" base leg.

I can think of yet another reason for preferring right crosswind.
If you use the "crab" method, you get a better view of the runway
from the left seat.

--
Per Lindberg (The Mad Programmer) ! __!__
Front Capital Systems, Box 5727, ! _____(_)_____ Levelled, not straight
11487 Stockholm, Sweden. 8-6611510 ! ! ! !

Glen Baker

unread,
Sep 25, 1990, 10:46:46 AM9/25/90
to
lu...@hao.hao.ucar.edu Dirk Lummerzheim at High Alititude Observatory writes:

>/ hpnmdla:rec.aviation / lu...@hao.hao.ucar.edu (Dirk Lummerzheim) / 2:24 pm Sep 24, 1990 /


>In article <748...@hpnmdla.HP.COM> gl...@hpnmdla.HP.COM (Glen Baker) writes:

>> As long as you are just *flying* (not attempting to navigate or
>> land) the wind direction has none, zero, effect on the ability of your
>> aircraft to generate lift. ALL that matters is the velocity of the airframe
>> relative to the airmass...the motion of the airmass relative to ANYTHING
>> else is COMPLETELY irrelevant.

>Well, this statement isn't quite right either. And every pilot knows
>about that, even if he hasn't experienced it.

>GROUND SPEED has a significance for the aerodynamics of the plane.
>There is no doubt about that.

Sure there's doubt about it. In fact, it's not true!

>The laws of physics that I wish to

>draw attention to are the conservation of momentum and energy. Both
>depend on the velocity of the plane in an inertial frame. If you are
>moving through an accelerated airmass, that airmass does not constitute
>an inertial frame. But the ground always does!

See previous postings by other folks, the airmass DOES constitute an
inertial reference frame.

>The example that I am getting at is the effect of wind shear. The plane


>keeps moving with the same GROUND SPEED, but the relative wind suddenly
>changes (i.e. when you go through a microburst it might change from
>headwind to tailwind). You can see that the plane "feels" the ground
>speed here.

You're changing the problem! We were talking about a plane in a constant
velocity airmass (the velocity of the airmass being measured relative to
some arbitrary point on the ground). However, I submit that the plane
does not "feel" the groundspeed in the above argument, rather it "feels"
the delta (i.e. acceleration) of the relative wind.

>Velocity is always relative. And if you don't like

>the ground as a relative point, add any constant velocity to it.)

Fine. I'll add the velocity of the airmass over the ground. Now we can
get rid of the ground in this discussion!

>This is not saying that you gain anything by turning the plane into
>the wind (which was the starting point of this discussion). That
>(directional) change of velocity is always so slow, that other forces
>(friction, lift) which are applied during the turn have enough time
>to accelerate the plane, such that the conservation laws are not
>that easily applied, and one fares a lot better with just considering
>the entire problem in the reference frame of the moving airmass.

Accelerate the plane relative to what? Relative to the airmass ALL of the
plane's acceleraion is supplied by the turning forces (lift vector, yaw,
etc). The motion of the airmass relative to the ground is not a part
of the equation unless you are planning on having the ground and the
airplane mate.

There is nothing special about the ground as a reference plane! Consider
a large flatbed truck moving at 80 mph from east to west. Does the truck's
velocity have ANY effect on the airplane's ability to fly? Now try to
land on the truck....the velocity of the truck becomes *real* important!

O.K., I'll carry my reductio-ad-absurdum one step more. I mount the entire
state of california on a conveyor belt. Initially my conveyor belt is at
rest. I take off, climb to some high altitude, and then key my mike 4 times
which sets the ground into motion. Does my plane suddenly stop flying?

I submit that anybody who uses the ground as some magic reference point is
being exactly as silly as a person who uses my truck as a reference point.

..glen

Eric Myers

unread,
Sep 26, 1990, 1:20:57 PM9/26/90
to
I was going to e-mail this, but...

In article <45...@apple.Apple.COM> ge...@Apple.COM (Geoff Peck) writes:

>(16) Last, but not least:
> Praise in public, criticize in private.

Well done Geoff, thank you for doing this!
--
Eric Myers "If God had intended for man to fly
He would have given us the brains to build airplanes."
Center for Relativity, Department of Physics, University of Texas at Austin
my...@emx.utexas.edu | my...@utaphy.bitnet | my...@ut-emx.UUCP

lan...@css.enet.dec.com

unread,
Sep 26, 1990, 1:22:14 PM9/26/90
to

In article <20...@amethyst.math.arizona.edu>, raymond@europa (Raymond Man) writes...

>Since the turning force, a component of the lift, acts nearly normal to the
>flight direction, it changes only the aircraft's velocity but not its
>magnitude. So which ever direction the aircraft turns, as long as the time
>to complete the turn is short enough, the final speed will essentially be
>the same, as I have stated.
>
>Neither indicated nor true airspeed will be constant in a normal turn if there
>is significant wind velocity. Mr. Dorsett's examples were for straight line
>steady flight.
>

I know I should just leave this alone but . . .

Your first paragraph here is correct, but the "final speed" you talk
about is AIRSPEED, not ground speed. The airplane knows NOTHING of the
ground!

Airspeed WILL be constant in a turn (assuming you add a little power
to compensate for the "tilting" of the lift vector). At least, the
airpseed will be the same for a similar turn whether it is upwind or
downwind!

Any suggestion that the airplane can tell the relative speed of the
air over the ground is indeed nonsense!

chris

Joe Abley

unread,
Sep 27, 1990, 8:02:27 AM9/27/90
to

>You have:
>1) "an object (at rest, in motion) *with no forces acting upon it* will
> stay (at rest, in the same state of motion)"
>2) F=ma

I thought that was number 3? :-)

2 was about the corresponding equal & opposite reactions to all actions, I
thought.

Or maybe you thought that one wasn't important :-) :-)

Joe.

Steve Peltz

unread,
Sep 28, 1990, 5:54:12 AM9/28/90
to
I missed some of the original discussion on the downwind turns (but am pleased
to see that Raymond Man has learned something new), so maybe this was mentioned,
but the scary part about the myth of the upwind turn is that, usually, it is
exactly the opposite, due to the wind shear you will often find close to the
ground. Assuming you're descending in this case, turning into the wind will
lead you to a situation where your airspeed will decrease.

Last weekend we had some pretty good crosswinds, and I got to demonstrate how
you can hover a glider (at altitude, of course!), why a tailwind on base can
be most distressing, and some of the visual confusion caused by watching the
ground as you turn in a strong wind. I'll mention this discussion to my
students to make sure they never pick up this kind of mis-information.
--
Steve Peltz
Internet: pe...@cerl.uiuc.edu PLATO/NovaNET: peltz/s/cerl

Robert W Yoder

unread,
Sep 28, 1990, 7:50:11 PM9/28/90
to
[text deleted, you've all seen it before]

Allow me to beat this dead horse one more time. I am pleased that we have
made at least one admitted convert, but there seem to be a few non-believers
left, so I would like to address this to them.

The water/land relationship is a perfect analogy to the air/land relationship.
Try this as a thought problem, or try it for real:

A. Take a powerboat out on a swift river.
Point the boat into the current and shut off the engine. (balloon)
Does the current cause the boat to leave a wake?
Not unless you throw out the anchor. (tethered balloon)

B. Start the engine and get the boat up on plane going upstream, then back off
the throttle as much as possible without slipping out of plane. Now, make a
large radius turn, rolling out heading downstream. (constant bank turn)
Does the boat fall out of plane? (stall)
No.

C. Anchor the boat in chest deep water and jump in.
Stand on the bottom.(aircraft on runway)
Do you feel a current?
Of course.

D. Start slowly swimming. (you have just rotated)
Do you feel a current?
No. You are supported by the water and if you float motionless are
essentially part of it.

E. Close your eyes and swim in a circle.
Can you tell which way the current is flowing?
Not unless you get swept into a stationary object.

--
Robert Yoder 306 Hawkins Graduate House, West Lafayette, IN 47906 (317)495-6845
Internet: ryo...@ecn.purdue.edu "Flame all you want, We'll post more."
UUCP: pur-ee!ryoder Apologies to Jay Leno & Doritos.
Bitnet: ryo...@ecn.purdue.edu@purccvm.bitnet

David Michael Young

unread,
Sep 29, 1990, 10:45:38 AM9/29/90
to
Yet another tidbit to add to the stacks of articles on this subject. From
the August 1990 issue of FLYER (yes... I'm in the US, but it's a good
magazine!), page 48. "Accident Report: The potential dangers of downwind
turns:
" The Manual of the British Flight Centre describes the effects of downwind
turns as follows: 'Just as movement due to the speed of the aircraft is
more noticable at low level, so are the effects of the wind upon that
movement. When flying into wind and downwind, the change in groundspeed
can be seen quite clearly, giving the IMPRESSION of a change in airspeed.
[emphasis added... DMY] The temptation to alter the power setting or
raising the nose without reference to the airspeed indicator must be
resisted, as a reduction of speed when flying downwind, in the belief that
the airspeed has increased, could lead to an inadvertant stall.'

" The Royal Air Force Flying Instructors' Handbook suggests that
demonstrations of this effect should be made to show such turns may give
a pilot the false impression that the airplane is slipping into the turn
and that airspeed is increasing. Corrective action taken on these false
impressions, such as raising the nose or reducing the power, can lead to
a drop in airspeed."

While they do mention that raising the nose is a common response to these
false impressions of this type of turn, one thing is neglected bu the article
I quoted, AND THE NET AS WELL! Note that in a low-level, downwind turn there
is not only the impression of an increase of speed, but an impression of the
aircraft SLIPPING as well. This would indicate to me that in addition to
raising the nose/reducing the power, that the pilot might skid the airplane in
response to the perceived slip. Now we're not only approaching a stall, but
the aircraft is cross-controlled as well... inviting a spin.

Any comments??


=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
David Young voy...@cup.portal.com

Greg Rose

unread,
Oct 1, 1990, 7:35:23 PM10/1/90
to
Wow, what a lot of discussion was generated by this letter. I was out of
town and only say a subset of the followups. As usual, I've definitely
learned a few things.

Anyway, I saw wto statements I thought deserved further comment

One was:

>In article <20...@amethyst.math.arizona.edu> ray...@europa.UUCP(Raymond Man)writes:
>.
>.


>>Assuming an aircraft can complete a turn, either upwind or downwind, its GROUND
>>speed will essentially be the same(Newton's Laws). So the relative wind will be

>>stronger when upwind and the wing can generate more lift. That was Mr.
>>Treloar's point.

(An excellent demolition of this was given by:
In article <108...@philabs.Philips.Com> t...@briar.philips.com.UUCP (Teun Hendriks) writes:
>Teun Hendriks (914)-945-6408
with which I totally agree. However it's a more complicated chain of reasoning
than I like to use to non-technical pilots.)

The other statement came from a forgotten source,
but basically said that you needed an inertial frame of reference (correct)
and that the ground was but the air wasn't (not correct).

The statement I want to make is that the air mass you are flying in
*is* an inertial frame of reference; at least as good a one as the ground
is anyway. This is assuming smooth air flow, that is discounting turbulence
and wind shear and thunderstorms and microbursts and volcanos and ...

Teun Hendriks' argument essentially shows what a motion from one frame
of reference looks like in another, and points out that some things
(like angles) don't look the same any more.

Thanks for all the discussion.
--
Greg Rose - assistant test pilot - Softway Pty Ltd for a little longer
PHONE: +61-2-698-2322 FAX: +61-2-699-9174 NET: gr...@softway.oz.au

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