Plane Stopped in Midair

"DM" wrote

okay! This is clicking for me, even though I know it may just be another
way of saying "optical illusion". I read this page,

http://en.wikipedia.org/wiki/Parallax

and the concept makes sense to me (not to imply that most everyone
else's explanation didn't).

Debbie

Another thing to keep in mind, is that airplanes work by the laws of
physics. An airplane, be it large or small, can not have zero airspeed for
even an instant, or it will stop flying, and fall out of the air. Given
enough altitude, it can dive, regain airspeed, and start flying again, but
you would have seen that happen, and you did not.

What remains, is the fact that the jet you saw did not stop, and what you
perceived was due to your observation being in error, in some manner.

Have a good one!
--
Jim in NC


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Morgans wrote:

Another thing to keep in mind, is that airplanes work by the laws of
physics.

True enough.

An airplane, be it large or small, can not have zero airspeed for
even an instant, or it will stop flying, and fall out of the air.

Dave Morgan, Sharkey Ward, and a host of other Royal Air Force, Royal
Navy, United States Marine Corps, and Spanish Navy pilots would beg
to differ.

What remains, is the fact that the jet you saw did not stop, and what you
perceived was due to your observation being in error, in some manner.

Given that the reported wind speed that day was quite modest (*way*
short of tornado-grade), that is clearly true of the "cargo jet"
which Debbie described.

I guess it is conceivable that its cargo included a Pegasus engine,
but the aeroplane certainly wasn't being held aloft by one ...

"Robert Briggs" wrote

I guess it is conceivable that its cargo included a Pegasus engine,
but the aeroplane certainly wasn't being held aloft by one ...

OK, No zero airspeed, unless you are being held up by the thrust of the
engine, alone. That ought to cover Shawn Tucker, Harriers, and Super
Hornets. g
--
Jim in NC


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In article ,
says...

Dean Wilkinson wrote:

Its called parallax. Your motion, the motion of the plane, and the
background were in the right configuration so that the parallax made the
plane appear to be stationary when in fact it was not...



okay! This is clicking for me, even though I know it may just be another
way of saying "optical illusion". I read this page,



Honestly, I hate to bicker, especially on a subject that has been beaten into
the ground, but I believe this response in incorrect. I don't think this
sighting illusion has anything to do with parallax.

As pilots, all of us learn in our early VFR training, that in scanning the sky
for possible traffic conflicts, the most dngerous "targets" are those that
appear not to be moving. No apparent motion indicates that the target is
heading pretty much straight toward us. The only modification in the visual
profile of an object in this case is a gradual increase in size. This can be
*very* gradual while the target is still some distance away.

The large airplane fools our brain - we don't *expect* it to be so big,
compared with other landmarks, so we guess it is closer than it actually is.
When the profile doesn't increase appreciably in size, compared with how close
we "believe" it to be, our brain concludes it is stationary, or moving very
slowly.

It is a simple illusion, easily explained and understood - yet very beautiful
and impressive to behold!

G Faris

OK, so first off, here is a reference for you defining parallax:
http://www.phatnav.com/wiki/wiki.phtml?title=Parallax

Now, since when an observer is in motion, they expect to see objects
closer to them appear to move backwards with respect to objects in the
background (as they pass them) if they are stationary. If you are
observing a plane in flight, you expect it to move in its direction of
travel with respect to the background. If the plane is moving in the
same direction as your car, and the parallax motion of the plane
(backwards motion) equals the relative forward motion of the plane
with respect to the background, the plane will not move at all with
respect to the background and appear to "hang" in the air. This
effect is attributable to the phenomenon of parallax...

Do you still want to debate this subject?

Dean

(G Farris) wrote in message ...
In article ,
says...

Dean Wilkinson wrote:

Its called parallax. Your motion, the motion of the plane, and the
background were in the right configuration so that the parallax made the
plane appear to be stationary when in fact it was not...



okay! This is clicking for me, even though I know it may just be another
way of saying "optical illusion". I read this page,



Honestly, I hate to bicker, especially on a subject that has been beaten into
the ground, but I believe this response in incorrect. I don't think this
sighting illusion has anything to do with parallax.

As pilots, all of us learn in our early VFR training, that in scanning the sky
for possible traffic conflicts, the most dngerous "targets" are those that
appear not to be moving. No apparent motion indicates that the target is
heading pretty much straight toward us. The only modification in the visual
profile of an object in this case is a gradual increase in size. This can be
*very* gradual while the target is still some distance away.

The large airplane fools our brain - we don't *expect* it to be so big,
compared with other landmarks, so we guess it is closer than it actually is.
When the profile doesn't increase appreciably in size, compared with how close
we "believe" it to be, our brain concludes it is stationary, or moving very
slowly.

It is a simple illusion, easily explained and understood - yet very beautiful
and impressive to behold!

G Faris

In article ,
says...


Do you still want to debate this subject?


Not particularly.
I do maintain that you are mistaken about the application of parallax to
illusions related to apparent motion. Parallax, by definiton, requires two
distinct points of view. Thus, the distance between our two eyes would create
two slightly different views, and if we could measure it, we could use this
parallax to measure the distance to the object.

Nevertheless, parallax is not conventionally used to describe perceptions
related to motion. In the photogrammetric model, two views of the same subject
are taken and compared through a stereoscope to produce a pseudo-stereographic
image. Though the two images were taken over time, because of the movement of
the airplane, the information conveyed is considered to represent a geometric
offset, and not an expression of the airplane's movement. The same image could
have been created in the same instant by one, very large airplane, with a
camera at each extremity.

Parallax, is a term used to describe geometric differences in an observed
object from two differing viewpoints. It is not intended, nor is it
sufficient, to describe psychological illusions (expectations) or observations
or illusions related to motion (evolution of geometry over time) except
indirectly, when time is required to obtain two different views, as in the
photogrammetric example above. Definitions of parallax do not include time
constants - only geometric relationships.

Therefore, I maintain that the use of the term "parallax" to describe the
illusion discussed in this thread is imprecise at best.

G Faris

"Dean Wilkinson" wrote in message ...
Its called parallax. Your motion, the motion of the plane, and the
background were in the right configuration so that the parallax made the
plane appear to be stationary when in fact it was not...

Are you sure? Parallax is when you try to show a student how to make a
coordinated turn and you end up with the ball out to the left because
you aren't looking at the turn coordinator straight on.

-Robert, CFI

Parallax is the scientific term for relative displacement of objects
observed at different distances. Parallax is used often by astronomers to
ascertain the distance of objects within our own solar system, and even
nearby stars by observing them from different points in the earth's orbit
around the sun.

I the case of the turn coordinator, the reason the ball appears off center
when observed from the side is due to parallax. This does not mean that
this is the only instance in which the concept of parallax is applied. The
original poster described another circumstance that results from parallax;
i.e. a moving object appearing to be stationary when compared to the
background when the observer is also moving at a rate and in a direction
that allows the parallax to create the illusion that the airplane is
stationary when in fact it is not...

I just wanted to point out to the group that this is a well known phenomenon
that has a scientific term to describe it.

Dean Wilkinson

"Robert M. Gary" wrote in message
om...
"Dean Wilkinson" wrote in message
...
Its called parallax. Your motion, the motion of the plane, and the
background were in the right configuration so that the parallax made the
plane appear to be stationary when in fact it was not...

Are you sure? Parallax is when you try to show a student how to make a
coordinated turn and you end up with the ball out to the left because
you aren't looking at the turn coordinator straight on.

-Robert, CFI

Perhaps the bong you were holding obscured your view.....j/k

"DM" wrote in message
...
Yesterday I saw a cargo jet (a major air express company) come to what
seemed like a dead stop in midair as it was making its ascent. After about
20 - 30 seconds of hanging without dropping out of the sky, it continued
climbing and apparently did not crash. There's been nothing about it in
the local news but I've still been very concerned. Could someone here
explain how such a thing is possible?

Some details: the temperature was about 50 degrees F, the sky was mostly
clear, and the time was around 0645. I was traveling by car at about 40
MPH on a street that is parallel to a regular flight path. From this
street it's common to see 3 or 4 planes per minute either climbing or
descending; the airport is about a mile or two away from this particular
street.

As I was moving relatively slowly compared to the how fast the jet should
have been moving, I noticed that I was gaining on it. I quickly eyeballed
the area for tall buildings and other geographical reference points so I
could be sure that I had a good perspective and wasn't just "seeing
things". The object was either not moving or it was moving *very* slowly,
and it was not a helicopter. For a few seconds I was stopped at an
intersection looking at this hanging plane and at the people in the other
cars around me. No one else seemed to be paying any attention to it
besides me.

The main reason this bothered me so much is because had the plane fallen,
it would have landed less than a half mile from where I and about 30 other
running cars were, in addition to several warehouse-type buildings and
auto repair garages, plus a 6 or 8 lane freeway filled with morning
traffic. Since the plane had just taken off and was probably full of fuel,
and was still low enough for its markings to be readable from the ground,
the crash probably would have been extraordinarily disastrous.

I've done a lot of Googling to try to get an understanding of what I saw
and really haven't learned anything meaningful. I'm hoping someone here
can explain how a "regular" jet--versus a specialized military jet--can
apparently stop in midair and not drop from the sky. As a daily traveler
near a major metro airport, I'd really like to be reassured that this is
not a common occurrence.

Debbie

"WRE" (remove nospam) wrote in message
...
Perhaps the bong you were holding obscured your view.....j/k

Oh, that was funny, and useful, too. Not.
--
Jim in NC


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