The Impossibility of Flying Heavy Aircraft Without Training

Jose wrote:
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.

IOW, the causual basis for macroscopic gas laws is governed
by statistical mechanics. Agreed.


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.


I think you are refering to Newton's third law, often stated as: "For
every
action there is an equal and opposite reaction."

For an aircraft in level flight, the upwards acceleration due to lift
is
counterbalanced by the downward acceleration due to gravity.
This satisfies Newton's third law.

For a wing in level flight, the vertical component of momentum is
zero. Therefor, conservation of momentum requires that the vertical
componenet of momentum of the air also be zero. The wing imparts
as much upward momentum to the air as it does downward momentum.

The pressure differential through the wing, from bottom to top,
integrated
over the wing area, provides an upward force for a wing in level
flight.

The downwash behind the aircraft, which is counterbalanced by a more
diffuse upwash around it, is real but not relevent to the issue of
lift.

--

FF

Two years ago when I was in college I used to read science journals for
fun... One particular, just published within the past two years
(spring '04?) caught me.

It discussed the Bernuolli theory of flight- and (if I recall) quite
conclusively proved that one of the _fundamental_ assumptions of the
Bernuolli theory- that air that travels path over the top of the wing
is flowing appreciably faster than air that flows over the bottom- is
simply incorrect in a compressible fluid....

Obviously, you should take this with a grain of salt because A- this is
my first post on this board and B- I can't remember either the journal
or the exact date... but take it for what its worth

On Wed, 1 Mar 2006 at 22:43:58 in message
.com,
" wrote:

It discussed the Bernuolli theory of flight- and (if I recall) quite
conclusively proved that one of the _fundamental_ assumptions of the
Bernuolli theory- that air that travels path over the top of the wing
is flowing appreciably faster than air that flows over the bottom- is
simply incorrect in a compressible fluid....

There is not really a Bernoulli theory of lift. Bernoulli's theory
shows the relationship between the velocity and pressure of fluid flow
when energy is not added or removed and the flow is subsonic. It is a
very simple theory which is correct for much of the time. It quite
accurately, at lower speeds, represents the velocity and pressures
between streamlines.

The air does flow faster over the top than the bottom and for the lower
subsonic region air behaves very closely to being incompressible.
Generally pressure changes are transmitted at the velocity of sound.

At high subsonic and of course at supersonic speed the effect of
compressibility cannot be ignored.

Shock waves form, first on places like the top surface of the wing where
the air first reaches the velocity of sound. As the speed rises they
become bigger and move towards the leading and trailing edges. Above
Mach one the air does not detect the approaching aircraft! :-)

I have just read a few more messages in this thread and discussing lift
in this general way without maths and without using at least simple
physics and slowly developing the methods is almost futile.

What's it matter about lift as long as the aircraft fly? !!!!!
--
David CL Francis

David CL Francis wrote:

The air does flow faster over the top than the bottom and for the lower
subsonic region air behaves very closely to being incompressible.
Generally pressure changes are transmitted at the velocity of sound.


I hate to be a spoil sport (or dullard?), but...

the (stationary) air does WHAT (as the wing passes by)???


))

On Fri, 3 Mar 2006 at 02:27:02 in message
. net, Richard Lamb
wrote:
I hate to be a spoil sport (or dullard?), but...

the (stationary) air does WHAT (as the wing passes by)???

The nature of things is such that the situation does not change if you
change the frame of reference. It is normal in doing calculations to
start with a frame of reference based on the aircraft. If you follow the
aircraft then the air is going past it.

The presence of the wing changes the air flowing past the aircraft in
the same way as if you consider the aircraft passing through the air.
The 'stationary' air as you call it has its local velocity and direction
changed by the aircraft.
--
David CL Francis

David CL Francis wrote:
On Fri, 3 Mar 2006 at 02:27:02 in message
. net, Richard Lamb
wrote:

I hate to be a spoil sport (or dullard?), but...

the (stationary) air does WHAT (as the wing passes by)???


The nature of things is such that the situation does not change if you
change the frame of reference. It is normal in doing calculations to
start with a frame of reference based on the aircraft. If you follow the
aircraft then the air is going past it.

The presence of the wing changes the air flowing past the aircraft in
the same way as if you consider the aircraft passing through the air.
The 'stationary' air as you call it has its local velocity and direction
changed by the aircraft.

Yeahbut...

A handy frame of reference is - handy.

But it can be very misleading.....

Richard Lamb wrote:
David CL Francis wrote:

On Fri, 3 Mar 2006 at 02:27:02 in message
. net, Richard Lamb
wrote:

I hate to be a spoil sport (or dullard?), but...

the (stationary) air does WHAT (as the wing passes by)???



The nature of things is such that the situation does not change if you
change the frame of reference. It is normal in doing calculations to
start with a frame of reference based on the aircraft. If you follow
the aircraft then the air is going past it.

The presence of the wing changes the air flowing past the aircraft in
the same way as if you consider the aircraft passing through the air.
The 'stationary' air as you call it has its local velocity and
direction changed by the aircraft.


Yeahbut...

A handy frame of reference is - handy.

But it can be very misleading.....


For instance?

If the air is moving, we expect a lower pressure. Nod to Bernoulli.

But the air would also be moving along the bottom side of the wing also?

And what would that do to the pressure under the wing?

And if the pressure under the wing is below ambient....

On Fri, 03 Mar 2006 01:27:46 +0000, David CL Francis wrote:

Above
Mach one the air does not detect the approaching aircraft! :-)

If it did, what would happen?

On Fri, 3 Mar 2006 at 15:21:11 in message
, "00:00:00Hg"
wrote:
On Fri, 03 Mar 2006 01:27:46 +0000, David CL Francis wrote:

Above
Mach one the air does not detect the approaching aircraft! :-)

If it did, what would happen?

The whole point is that disturbances in the air are propagated at or
near the velocity of sound. It follows that at supersonic speeds
nothing happens to the air until it reaches the supersonic aircraft, or
vice versa.
--
David CL Francis