The Impossibility of Flying Heavy Aircraft Without Training

I'd really like to see what the bigger brains can make of it.

I think you did fine. I will take issue with:

The air below presses against the earth. As I've said before, that one is
so obvious (that we stop looking?).

The air does press on the earth, and this is "where the momentum goes",
which is a big question in one of the poster's minds. No earth, nothing
to press against, and the momentum just keeps on going down. It is
(thus) not true that there is no local momentum transfer. That is one
of the points I was making. There is of course no global momentum
transfer once all parts of the closed system are taken into account.

Jose
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Could we reduce the crossposting? I think one newsgroup is more
than sufficient. You chose, and I'll follow.

Absent protest, from just after "now" on, I'll reply and post this
thread only to r.a.piloting.

Jose
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I think you'll agree:

1) It is useful to have a specific term for each variable that needs
to be considered.

2) It is condusing to have two or more words for the same variable.

This is true. I think that in this case we should use more basic terms
(such as "local downflow velocity") for the velocity of the downflow
near the wing.

The upper
atmosphere =is= (slightly) depleted by the flight.

And then returns to ambient pressure, at a slightly higher
termperature, after the wing has passed, right?

Wrong. It does not return to ambient pressure (regardless of
temperature) until the airplane lands.

Jose
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After the airplane lands how does the air return to ambient
pressure, at a higher temperature, (or at any rate at a not
lower temperature) without any upward flow?

After the airplane lands, the downflow from the wings ends. The
downflow from the wings is what keeps the air above rarified, and the
air below squished. Once that is removed, the pressure below can
relieve itself by having the air flow upwards a sufficient amount.

AT THAT POINT, the air has returned to its normal pressure distribution,
(albeit slightly warmer). But so long as the airplane is flying, it has
to be supported by the air, and the pressure below the wing is greater
than (and the pressure above the wing is less than) it would have been
absent the continuous downflow induced by the wing in flight.

Similarly, for a fan in a closed room, the air pressure on one side of
the room will remain higher than the pressure on the other side, until
the fan is turned off. Then the air will spring back.

And similarly, when you sit on a chair, it deforms slightly. When you
get up, it springs back (unless you broke it!)

Jose
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When you say a)"rarified" and b)"squished" do you mean...

Ultimately I mean lower pressure and higher pressure. Specifically I am
talking about the extra pressure that is distributed across the entire
earth's surface while the airplane is flying. Two things are being
disscussed here - what keeps the airplane up, and what is ultimately
supporting it; I was addressing the latter.

By "flow upwards a sufficient amount" I presume you are
thinking about some sort of density variation changing the
volume of air below the wing that is later released?

Yes, though I am not talking just about the air immediately below the
wing, but of all the air that is pressing against the earth. The thing
that prevented this (net) upward flow while the plane is flying is the
continued downflow from the wing. Once that stops, the air can spring back.

This concept of "springing back" implies both pressure and
density changes. While it's true that gas does change
volume and density when pressure is applied, when studying
the phenomenon of lift at subsonic speeds, we usually ignore
the density changes.

Yes, and that is a good approximation for some analyses. It does leave
something out though, and sometimes the thing that is left out is the
answer being sought. When I jump up, I push the earth down. This can
usually be ignored, but it is necessary to complete the analysis of all
the forces and their conservation. On a larger scale (the moon orbiting
the earth) it becomes significant.

If this is a discussion about lift ( I apologize if it's not
:-) and not just a pure discussion of the physics of a
compressible gas, it's not clear to me why you would want to
consider compressible effects. It's pretty universally
agreed we can ignore them.

The contention is that there is =no= net downflow. That contention is
not true (although it's "pretty close"). The difference between "no"
and "almost no" is what holds the airplane up.

But of course, it doesn't have to compress (at least not in
theory) and when you sit on a rock, the extent to which it
does compress is so small we can easily ignore it.

Well, actually it does have to compress. That's the source of the
force. Even a rock compresses. We can ignore it for most practical
purposes, but not when you are asking where the force comes from. And
that is the question being addressed.

Jose
--
Money: what you need when you run out of brains.
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On Tue, 7 Mar 2006 at 05:06:54 in message
, Jose
wrote:

Momentum is always conserved. If you see momentum disappearing, you
are not looking at the whole system. In the case of the land vehicle
propelled by a fan, the air blown back acquires momentum in one
direction, exactly balanced by the momentum that the vehicle acquires,
plus the (rotational) momentum (due to wheel friction) that the earth
acquires.

From a Physics book:

A jet of water merges from a hose pipe of a cross sectional area 5x
10^-3 m^2 and strikes a wall at right angles. Calculate the force on the
wall assuming the water is brought to rest and does not rebound.
(Density of water = 1.0 x 10^3 Kg m^-3)

After explaining the simple calculation which gives a force of 45N it
goes on to say; " in practice the horizontal momentum of the water is
seldom completely destroyed and so the answer is only approximate."

~~~~~~~~~~
Any changes to the entire earth as a result are insignificant. Closed
systems are adequate for most practical purposes.

All these arguments about the 'total system' are irrelevant to
considering the kind of problem we have here. As in many problems you do
not need to include the whole universe to get practical and accurate
answers.

In the same way including discussions about molecular velocities beating
on the sides of the aircraft is a mere distraction. At normal altitudes
these effects are negligible compared to the consideration of the air as
an incompressible fluid.

Some aircraft can maintain a 7g turn banked at the appropriate angle.
(81.8 degrees approximately). What happens to that 7 times pressure on
the earth now? The same calculations will give accurate figures of lift
only slightly affected by the small difference of speed between the two
wings and the circular path. The earth hardly comes into it for
accuracy except that it is gravity that is being balanced.

--
David CL Francis

I don't think leaving out density changes leaves out very
much that's relevant to lift, but I have to admit to not
being sure exactly what question this discussion is trying
to answer, so I'll hold off any comment.

The question was whether Bernoulli (did I get it right?) supersedes
Newton. I maintain that, while Bernoulli's equations are very useful,
they obscure something Newtonian about the source of the force. If you
have lower pressure above and higher pressure below, you get lift. But
you can't get to that condition without throwing air down. Consider a
"wing" with a flat bottom. Define the JAOA to be the angle the =bottom=
makes with the airflow. In this case the JAOA is zero. The top of the
wing is an arc. The air has to go a longer way around the top of the
arc, so the conventional Bernoulli argument would be that there is lower
pressure above for this reason alone.

I don't think that kind of wing, in that configuration, would generate
any lift. If it does, there will be downwash. Increase the JAOA and
you certainly get downwash, and you will also get lift. The two will
balance.

The argument of "no net downwash" has to do with whether the air comes
back up. It does not (completely) until the airplane lands.

I will grant that, once the plane is flying over the earth, there will
be no net accumulation of air below - the attempt at accumulating the
air will be counteracted by the increased pressure (which is also
causing the upwash ahead). But enough air will already have been
accumulated below (and will remain accumulated below until the end of
the flight) so that that increased pressure will support the aircraft.

In the (silly) configuration where there is no earth, and no gravity,
there will be no pressure accumulation. The high pressure below will,
in addition to pushing air ahead up, will continue to push air below
down. This flow will dissipate, but not disappear. The aircraft cannot
be supported by the earth (like sitting on a rock), so it has to be
supported by downward thrust (downflow). As far as the wing is
concerned, this is what happens anyway. The wing sees rising air,
flings it down, and keeps going.

Taken in its entirety, I'd say the [no net downflow idea] is false.
Pressure differences hold the plane up. Force holds the plane up.
Neither *requires* any net downflow.

Yes. But to get the plane flying there is some net downflow; enough to
increase the global pressure by (weight of airplane)/(area of earth).
When the plane lands, there is a net upflow to release this pressure.

Globally, because of the earth's surface, there is no net downflow
during steady state flight. (I believe that's the point you want me to
concede - I do concede that point). However, this is because of the
earth's surface. Locally, the wing is changing the vertical velocity of
the air it encounters. Locally, the wing is throwing air down. This
has the consequence (which the wing doesn't care about) that air rises
up to meet it, because the air density is mostly unchanged. But, that
tiny density change caused by the tiny pressure change is what
ultimately enforces the "no net downwash" because of the earth's surface.

So if I understand this, you are saying that if air were
incompressible, there would be no lift?

No. If air were truely incompressible, there would be no downflow at
all; the entire earth would be pushed away just as it does when I jump.

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

A jet of water merges from a hose pipe of a cross sectional area 5x 10^-3 m^2 and strikes a wall at right angles. Calculate the force on the wall assuming the water is brought to rest and does not rebound. (Density of water = 1.0 x 10^3 Kg m^-3)

After explaining the simple calculation which gives a force of 45N it goes on to say; " in practice the horizontal momentum of the water is seldom completely destroyed and so the answer is only approximate."

Is this a US book? This is why the US lags in science.

The original question is ok (after all, in physics we use cylindrical
cows, frictionless surfaces, and point masses). But the comment at the
end is very misleading. The momentum is never "destroyed". It is
actually transferred to the wall, and thus to the earth. What they are
probably trying to say is that there is usually some rebound of the
water, and it sprays all over the place rather than becoming embedded
like machine gun bullets in sand... which would have been a better example.

Any changes to the entire earth as a result are insignificant.

Depends whether you are trying to understand the fundamental physics or
just trying to calculate an answer.

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

Jose wrote:
I don't think leaving out density changes leaves out very
much that's relevant to lift, but I have to admit to not
being sure exactly what question this discussion is trying
to answer, so I'll hold off any comment.

The question was whether Bernoulli (did I get it right?) supersedes
Newton.

Maybe I mised something becuase I did not see that particular question
posed.


I maintain that, while Bernoulli's equations are very useful,
they obscure something Newtonian about the source of the force. If you
have lower pressure above and higher pressure below, you get lift. But
you can't get to that condition without throwing air down.

No, you can throw it horizontally.

...
Globally, because of the earth's surface, there is no net downflow
during steady state flight. (I believe that's the point you want me to
concede - I do concede that point).

Dunno about him, but that has been my point.
....

So if I understand this, you are saying that if air were
incompressible, there would be no lift?

No. If air were truely incompressible, there would be no downflow at
all; the entire earth would be pushed away just as it does when I jump.


If the Earth is pushed away, wouldn't that stretch out
the air molecules between the plane and the Earth decreasing
the pressure below the wing?

--

FF

No, you can throw it horizontally.

.... where it throws the air in the way down to get it out of the way.
Why down? Because the wing is going up, and below is where the (new)
room is.

If the Earth is pushed away, wouldn't that stretch out
the air molecules between the plane and the Earth decreasing
the pressure below the wing?

Not if the air is truly incompressible. There would be more air
molecules between the wing and the earth because the wing is up there
and not down here.

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