The Impossibility of Flying Heavy Aircraft Without Training

Either that or I'm paying for fuel and
merely imagining I go to work :-)

Maybe you're imagining that you're getting something done.

g,d,rlh Jose
--
Money: what you need when you run out of brains.
for Email, make the obvious change in the address.

You're right that pressure itself is a momentum change, but that
doesn't appear to be what the momentum change advocates are, ah,
advocating.

They (and I too) are talking about local effects. It's fine to say no
=net= downard motion, of course there is no net downward motion - the
earth gets in the way, and the increasing pressure pushes the air back up.

Suppose the earth wasn't really there... we have an infinite field of
air in all directions, and no gravity. A airplane comes through. It
will (at the proper AOA) experience lift, and will accelerate upwards
(upwards being defined wrt the wings on a cessna, downwards being
defined wrt the wings on a piper). Newton requires that air be
accelerated downwards.

If we introduce plane gravity (that is, gravity that magically only
attracts airplanes and leaves air molecules alone), then this gravity
will pull the airplane down, and will prevent it from accelerating
upwards. However, air will still have to be accelerated downwards to
keep the airplane from succumbing to gravity. There will be localized
high pressure below the wing, and localized low pressure above the wing,
and there will be a vortex as the air rushes around the wingtips, but
the air that is accelerated downwards will not have anything to stop it
(except other air, which molecule by molecule accepts the transfer of
momentum).

If we let gravity work on the air molecules too, then there will also be
a gravitationally induced acceleration of air downwards, since there is
nothing to stop it. The air will be in free fall (and pretty soon the
airplane will not be able to keep up).

It's only when you put the earth itself in the picture that it all comes
together. With a hard surface below the air, (momentum from) molecules
that have been "thrown down" by the wing will get transferred to the
earth, and the air molecules will bounce back upwards again. THIS
causes the pressure that feeds the upwash (and helps keep the earth from
accelerating upwards towards the airplane). If we let the system
stabilize, the air molecules will pile up near the earth, and be sparser
further up.

However, the pressure change below the wing isn't downwash.

If there is a technical meaning to that word, I am not using it that way.

Jose
--
Money: what you need when you run out of brains.
for Email, make the obvious change in the address.

Remember, Time flies like an arrow...
Fruit Flies Like a Banana...


Al


"JJS" jschneider@remove socks cebridge.net wrote in message
...

"Jose" wrote in message
t...
snip
Feathers are not magical, they operate by strict scientific
principles and an insufficient amount of feathers won't even make a fruit
fly.

snip

Fruit flies have feathers?




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Al wrote:
Remember, Time flies like an arrow...
Fruit Flies Like a Banana...

Grouch Marx!

Bryan Martin wrote:
I don't know if it was the first time, but it happened once back in the
early '80s in, I think, Arizona. I believe it was a PSA DC-9. A disgruntled
ex-employee snuck a gun on board and forced his way into the cockpit and
shot the flight crew and then dove the plane into the ground. Of course in
that case the hijacker had a real weapon and about the only thing that might
have stopped him was someone else with a gun. Then of course there is the
case of the FedEx DC-10 where the crew barely managed to prevent a nut case
soon to be ex-employee from doing the same.

The 9-11 hijackers weren't even armed. No mater what anybody says, a box
cutter is not a weapon.

I am not disagreeing with the other stuff you wrote but a box cutter is
indeed a very dangerous weapon in the wrong hands and you could easily
kill a person with one swing, not to mention multiple attacks.
I wonder, if 911 was an inside job and nobody takes box cutters
seriously, why wouldn't the perpetrators plant other weapons in the
planes so the story would be more believable?

Jose wrote:
You're right that pressure itself is a momentum change, but that
doesn't appear to be what the momentum change advocates are, ah,
advocating.


Wrong. Momentum change is momentum. The time rate of momentum
change is Force. Neither is pressure.

They (and I too) are talking about local effects. It's fine to say no
=net= downard motion, of course there is no net downward motion - the
earth gets in the way, and the increasing pressure pushes the air back up.

Suppose the earth wasn't really there... we have an infinite field of
air in all directions, and no gravity. A airplane comes through. It
will (at the proper AOA) experience lift, and will accelerate upwards
(upwards being defined wrt the wings on a cessna, downwards being
defined wrt the wings on a piper). Newton requires that air be
accelerated downwards.

Why does Newton require that air be accelerated downwards?


If we introduce plane gravity (that is, gravity that magically only
attracts airplanes and leaves air molecules alone), then this gravity
will pull the airplane down, and will prevent it from accelerating
upwards. However, air will still have to be accelerated downwards to
keep the airplane from succumbing to gravity. There will be localized
high pressure below the wing, and localized low pressure above the wing,
and there will be a vortex as the air rushes around the wingtips, but
the air that is accelerated downwards will not have anything to stop it
(except other air, which molecule by molecule accepts the transfer of
momentum).

If we let gravity work on the air molecules too, then there will also be
a gravitationally induced acceleration of air downwards, since there is
nothing to stop it. The air will be in free fall (and pretty soon the
airplane will not be able to keep up).

It's only when you put the earth itself in the picture that it all comes
together. With a hard surface below the air, (momentum from) molecules
that have been "thrown down" by the wing will get transferred to the
earth, and the air molecules will bounce back upwards again. THIS
causes the pressure that feeds the upwash (and helps keep the earth from
accelerating upwards towards the airplane). If we let the system
stabilize, the air molecules will pile up near the earth, and be sparser
further up.

What color is the sky on your planet?


However, the pressure change below the wing isn't downwash.

If there is a technical meaning to that word, I am not using it that way.


OK.

--

FF

Wrong. Momentum change is momentum. The time rate of momentum
change is Force. Neither is pressure.

What I wrote (or intended to write) is that pressure is manifested by
many momentum changes. Each molecule that collides with a wall
transfers momentum to that wall, and the net force caused by all those
momentum changes (over time) manifests itself as pressure, (which is
force divided by area). The essential point is that pressure arises
from momentum transfer on a molecular level.

Why does Newton require that air be accelerated downwards?

To counterbalance the wing being accelerated upwards due to lift. The
wing is at an AOA which generates lift.

If we let the system
stabilize, the air molecules will pile up near the earth, and be sparser
further up.

What color is the sky on your planet?

Blue. Why? Do you not concur with the observation that on Earth the
air is denser near the ground, and less dense at higher altitudes?

Gravity is what holds the earth's atmosphere in place.

Jose
--
Money: what you need when you run out of brains.
for Email, make the obvious change in the address.

wrote:
Dylan Smith wrote:
On 2006-02-24, Greg Esres wrote:
There is a *net* downward momentum of air.

I have several aerodynamics books that say differently.


I guess that depends on what you mean by "net" downward movement. Air
does move downward from an airfoil. There is no difference between a
fan blade and wing.

Otherwise there is no lift.

If there is a pressure difference between the top and bottom, you will
have lift. Your airfoil is blisssfully unaware of the air with which
it has no contact.


Define 'contact' and 'aware.'

But air acts as a fluid. The airfoil certainly DOES have an effect on
air that it has no contact.
If you think there is no downward movement of air from an airfoil, stand
underneath a hovering helicopter some day. Or behind the propellor of a
plane - the prop is also an airfoil.

You might be able to get lift out of an airfoil in an enclosed tube with
no downward movement of the air, but that won't happen in the real
world.


In the real world airplanes have flown with pressure sensors
on the wings, confirming lift from the Bernojuli effect in actual
flight.


In the real world there are many photographs of huge canyons carved in
layers of cloud and smoke as airplanes fly over them, as well as
photographs of ripples and spray in water below them. The downward
deflection of air is caused by the low pressure area above the wing, so
of course the Bernoulli effect is confirmed. The downward flow of air
is predicted by Bernoulli.

This does NOT disprove the notion that there is localized downward
flow from some parts of the aircraft. However, there is no NET flow
of air down or up from airplane wings or helicopter blades. Otherwise,

ambient pressure at ground level would steadily increase as more
and more aircraft pushed the air down...

No it would not, once the aircraft was out of ground effect. The
downward flow dissipates rapidly after the aircraft has passed.
Otherwise you could say that all the air is being sucked out of the
space above airplanes and nothing is moving in to replace it, so that
eventually everything above heavily travelled altitudes will become a
vacuum. Are you saying that a fan will eventually increase the ambient
pressure on one side of the room and leave a vacuum on the other side?
It would make half of my living room kind of uncomfortable, wouldn't
it? Air moves in from the sides and quickly equalizes the air pressure.

Bryan Martin wrote:


Bryan,

Ex-nay!

Or wander over to alt.politics to feed the loons...

I brought up this whole Bernouli mess just to get
rid of these guys.

It worked quite well.

Now don't screw it up again!

Richard

Richard Lamb wrote:
Bryan Martin wrote:


Bryan,

Ex-nay!

Or wander over to alt.politics to feed the loons...

I brought up this whole Bernouli mess just to get
rid of these guys.

It worked quite well.

Now don't screw it up again!

Richard

Isn't a Bernouli what an Arab wears?

Dan, U.S. Air Force, retired