The Impossibility of Flying Heavy Aircraft Without Training

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.

cjcampbell wrote:
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.

For a fan in open air the momentum of the air moving through the
fan is equal and opposite to the momentum of the air moving around
the fan to replace the air removed from the front of the fan. There is
no net momentum change in the air. For ducted flow that returns the
air to the front fo the fan, the net momentum is also zero. Net flow
and
net momentum through any closed loop is zero--else the 'loop'
is not 'closed'.

Followjng a wing in level flight, the downward momentum of the
air in the downwash is equal and opposite to the upward momentum
of the air to either side that moves up to replace the air that washes
down. There is no net momentum change in the air.

...

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,

Cool! Got any links to some? How about pictures of airplanes
flying just below the ceiling?

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.

'Dissipation' is flow. If you include that dissipation into your
sum, there is no net flow. Otherwise, as stated above, the
ambient pressure at ground level would steadily increase and,
as you note below, the pressure higher up woudl steadily drop.

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.

Precisely my point. The downwash hypothesis sucks. It _is_
quite intuitive, it makes a lot of sense, but nature is not bound
by intuition or common sense.

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?

Are you saying that if there is net flow from one side of the room
to the other the pressure of both sides will stay the same?

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.

Precisely. There is no net flow.

--

FF

'Dissipation' is flow. If you include that dissipation into your
sum, there is no net flow. Otherwise, as stated above, the
ambient pressure at ground level would steadily increase and,
as you note below, the pressure higher up woudl steadily drop.

But the ambient pressure at ground level =does= increase, by an amount
equal to the weight of the airplane (divided by the surface area of the
earth). This remains true for as long as the airplane is being
"supported" by the air.

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

wrote:

In the real world there are many photographs of huge canyons carved in
layers of cloud and smoke as airplanes fly over them,

Cool! Got any links to some? How about pictures of airplanes
flying just below the ceiling?


The most famous photo is one that you can see on this site he
http://adamone.rchomepage.com/index4.htm
He has the caption wrong, though. The Citation never flew through the
cloud, only over it.

Pictures of airplanes flying just below the ceiling are an interesting
idea, but I have not seen any. Usually the ceiling is fairly ill
defined and ragged anyway.

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

Yes.

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.

Yes.

For a wing in level flight, the vertical component of momentum is
zero.

No.

That is, on a microscopic scale, no. The wing is constantly
freefalling, then being bounced back up by impact with air molecules.
Averaged over all the molecules, yes, the net is zero (the wing flies)
but on a microscopic scale, the wing is in constant brownian motion.
This implies momentum transfer, and following the momentum on a
microscopic scale is instructive.

The wing imparts
as much upward momentum to the air as it does downward momentum.

This is where I disagree. Upward momentum gets imparted, but not
(directly) by the wing. Rather, it is imparted by the ground, mediated
through other air molecules. Of course this wouldn't happen if the wing
didn't pass through and throw the air down to begin with, but the ground
is what ultimately imparts the upwards 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.

That's the shortcut. Where does this pressure differential come from -
that is the question.

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.

I disagree here too. It's important in seeing the entire picture.

The wing is ultimately being supported by the ground, the same way
somebody standing on a stool is ultimately supported by the ground.

Well, ok, a slightly different way, but only in detail.

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

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

Yes.

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.

Yes.

For a wing in level flight, the vertical component of momentum is
zero.

No.

Please show us your arithmetic. Suppose a 1500 lb airplane is
flying horizontally at 120 mph at 5000 feet above MSL. What
are the vertical and horizontal components of the momentum
of that aircraft?


That is, on a microscopic scale, no. The wing is constantly
freefalling, then being bounced back up by impact with air molecules.
Averaged over all the molecules, yes, the net is zero (the wing flies)
but on a microscopic scale, the wing is in constant brownian motion.
This implies momentum transfer, and following the momentum on a
microscopic scale is instructive.


OK, show us your arithmetic.

The wing imparts
as much upward momentum to the air as it does downward momentum.

This is where I disagree. Upward momentum gets imparted, but not
(directly) by the wing. Rather, it is imparted by the ground, mediated
through other air molecules.

The ground is stationary. How does the stationary ground impart
momentum to anything?

Of course this wouldn't happen if the wing
didn't pass through and throw the air down to begin with, but the ground
is what ultimately imparts the upwards 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.

That's the shortcut. Where does this pressure differential come from -

Bernouli effect.

that is the question.

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.

I disagree here too. It's important in seeing the entire picture.

Well, yes it is part of the entire picture. Its just not relevent to
the
issue of lift, which is only part of the picture.

--

FF

OK, show us your arithmetic.

First, do you agree that air is made of individual molecules separated
by a lot of space compared to the size of the molecules themselves?
Then do you accept that a wing is in freefall during all the (very brief
but very numerous) time in between molecular collisions? (If not, what
holds it up when it is not in contact with any air molecules?)

If so, then during the time it is in freefall, it acquires a downward
velocity. Small, no doubt, but nonzero. The next molecular impact
pushes it back up. On the average they will sum to a net zero vertical
motion. Is this the arithmetic you want to see?

The ground is stationary. How does the stationary ground impart
momentum to anything?

The ground is not stationary. Like the wing, the ground is jiggling
around in brownian motion. Such motion is greatly overwhelmed in
quantity by other things, but it is nonzero. Gravity pulls the ground
towards the airplane just as strongly as it pulls the airplane towards
the ground. This is the same effect as the one that gives high tides on
the side of the earth that is away from the moon.

Where does this pressure differential come from -

Bernouli effect.

That's the shortcut. Where does the Bernoulli effect come from - on a
molecular level? That's what I'm addressing. The Bernoulli effect is a
shortcut for doing the calculation in bulk (where it makes the most
sense if you want a numerical answer) but it all comes from molecular
collisions.

Its just not relevent to the
issue of lift, which is only part of the picture.

We disagree here. Both explanations are true as far as they go, but it
is important to see just how far they go (or don't go). The Beruoulli
effect does not explain, for example, how the earth ultimately supports
the aircraft, nor how the upwash starts (for example, suppose there were
a vertical column of vacuum separated from the air by a very strong
piece of cellophane. A wing travels through the vacuum and penetrates
this cellophane. The air behind the cellophane does not rise up to meet
the wing - it has no idea there's a wing coming. Once the wing has
entered the air, that rising motion will start, but why?

That's the question to which I am applying my molecular model.

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

Jose wrote:
fredfighter wrote:

Please show us your arithmetic. Suppose a 1500 lb airplane is
flying horizontally at 120 mph at 5000 feet above MSL. What
are the vertical and horizontal components of the momentum
of that aircraft?

That is, on a microscopic scale, no. The wing is constantly
freefalling, then being bounced back up by impact with air molecules.
Averaged over all the molecules, yes, the net is zero (the wing flies)
but on a microscopic scale, the wing is in constant brownian motion.
This implies momentum transfer, and following the momentum on a
microscopic scale is instructive.

OK, show us your arithmetic.

First, do you agree that air is made of individual molecules separated
by a lot of space compared to the size of the molecules themselves?

Yes.

Then do you accept that a wing is in freefall during all the (very brief
but very numerous) time in between molecular collisions? (If not, what
holds it up when it is not in contact with any air molecules?)

Yes.


If so, then during the time it is in freefall, it acquires a downward
velocity. Small, no doubt, but nonzero.

Sometimes it does, sometimes it does not. I'll allow as the vertical
component of velocity decreases during that time, for a positive up
coordinate system and a plane in (macroscopic) level flight.

Do you agree that in each collision momentum is transferred to the
air molecule that is equal and opposite to the momentum transferred
to the wing?

The next molecular impact
pushes it back up. On the average they will sum to a net zero vertical
motion. Is this the arithmetic you want to see?

No, I want you to calculate the horizontal and vertical componenets
of momentum for the example I gave, or any other reasonable example
of a fixed wing airplane in horizontal flight.

...

That's the shortcut. Where does the Bernoulli effect come from - on a
molecular level? That's what I'm addressing. The Bernoulli effect is a
shortcut for doing the calculation in bulk (where it makes the most
sense if you want a numerical answer) but it all comes from molecular
collisions.

I agreed quite some time ago that the theoretical basis for
macroscopic gas laws is to be found in statistical mechanics.

On a macroscopic level, the vertical component of momentum of the
wing is zero. Therefor on a macroscopic level, the sum of the
momenta transferred to the air molecules, integrated over all of
the air molecules must also be zero by Newton's third law.

Right?

For an airplane in straight level flight there is no net momentum
transfer in the vertical direction, between the air and the airplane,
just like there is no net vertical force acting on the airplane.

--

FF

If so, then during the time it is in freefall, it acquires a downward
velocity. Small, no doubt, but nonzero.

Sometimes it does, sometimes it does not. I'll allow as the vertical
component of velocity decreases during that time, for a positive up
coordinate system and a plane in (macroscopic) level flight.

Ok. (I was sloppy - it doesn't "acquire a downward velocity", it really
"endures a downward acceleration", which depending on the initial
vertical velocity may or may not end up with the plane going downward.)
So we are saying the same thing here.

Do you agree that in each collision momentum is transferred to the
air molecule that is equal and opposite to the momentum transferred
to the wing?

Yes I do. This is what I call "throwing the air down". That downward
momentum will remain with the air (dissipated across many other
molecules as it keeps colliding, but never disappearing) until it is
transferred to the earth, which has been accelerating upwards in the
same fashion.

I agreed quite some time ago that the theoretical basis for
macroscopic gas laws is to be found in statistical mechanics.

Ok.

On a macroscopic level, the vertical component of momentum of the
wing is zero.

Yes.

Therefor on a macroscopic level, the sum of the
momenta transferred to the air molecules, integrated over all of
the air molecules must also be zero by Newton's third law.

Right?

Only in a nonaccelerated frame. We are dealing with an accelerated
frame. Consider a rocketship hovering over the moon. The (macroscopic)
vertical component of its momentum is zero also. However it has to
continually throw down rocket exhaust to stay there. So, without
looking at the rest of the picture, your conclusion about momentum is
flawed.

In the case of the wing, the momentum is transferred a few times... once
when the wing hits the air molecule (throwing the air down), again when
that molecule hits the earth and bounces back (throwing the earth away
from the wing), and then again when that air molecule (or its proxy)
hits the wing on the way back up.

Think about a person sitting on a stool. No momentum transfer (or so it
would seem). But then think about a person supporting himself by
dribbling a basketball. There is a lot of momentum transfer, but no
=net= change. The reason there is no net change is that the basketball
keeps pushing the earth away too.

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

Jose wrote:

fredfighter wrote:

Please show us your arithmetic. Suppose a 1500 lb airplane is
flying horizontally at 120 mph at 5000 feet above MSL. What
are the vertical and horizontal components of the momentum
of that aircraft?
....

If so, then during the time it is in freefall, it acquires a downward
velocity. Small, no doubt, but nonzero.

Sometimes it does, sometimes it does not. I'll allow as the vertical
component of velocity decreases during that time, for a positive up
coordinate system and a plane in (macroscopic) level flight.

Ok. (I was sloppy - it doesn't "acquire a downward velocity", it really
"endures a downward acceleration", which depending on the initial
vertical velocity may or may not end up with the plane going downward.)

Right, but don't forget that the downward acceleration is constant
without regard to the velocity of the aircraft.

So we are saying the same thing here.

Do you agree that in each collision momentum is transferred to the
air molecule that is equal and opposite to the momentum transferred
to the wing?

Yes I do. This is what I call "throwi ng the air down". That downward
momentum will remain with the air (dissipated across many other
molecules as it keeps colliding, but never disappearing) until it is
transferred to the earth, which has been accelerating upwards in the
same fashion.

Do you agree that the net momentum transfered to the Earth by the
air molecules is equal and opposite to the net momentum transferred
to the wing by the air molecules?

Do you agree, therefor that there is no net momentum transfered to
the air?


I agreed quite some time ago that the theoretical basis for
macroscopic gas laws is to be found in statistical mechanics.

Ok.

On a macroscopic level, the vertical component of momentum of the
wing is zero.

Yes.

Therefor on a macroscopic level, the sum of the
momenta transferred to the air molecules, integrated over all of
the air molecules must also be zero by Newton's third law.

Right?

Only in a nonaccelerated frame. We are dealing with an accelerated
frame. Consider a rocketship hovering over the moon. The (macroscopic)
vertical component of its momentum is zero also. However it has to
continually throw down rocket exhaust to stay there.

Instead, let's consider a wing in level flight.

So, without
looking at the rest of the picture, your conclusion about momentum is
flawed.

In the case of the wing, the momentum is transferred a few times... once
when the wing hits the air molecule (throwing the air down), again when
that molecule hits the earth and bounces back (throwing the earth away
from the wing),

At which ponit the Earth throws the air molecule back up so that the
net momemtum transferred to the air molecule is zero (averaged over
the entire atmosphere)

and then again when that air molecule (or its proxy)
hits the wing on the way back up.

Which again transferes an equal and opposite momentum to the
molecule which again is transferrred to the Earth leaving no net
transfer
of momentum to the air.


Think about a person sitting on a stool. No momentum transfer (or so it
would seem). But then think about a person supporting himself by
dribbling a basketball. There is a lot of momentum transfer, but no
=net= change. The reason there is no net change is that the basketball
keeps pushing the earth away too.

And there is no net transfer of momentum to the basketball. This is
clear as the average velocity of the basketball is zero, even though
the average speed is non-zero.

Think of the example we had earlier of a piston supported by air
pressure in a cylinder. The momenta transferred by air molecules
to the piston is equal and opposite to the momenta transfered by the
air molecules to the bottom of the cylinder. There is no net transfer
of momentum to the air.

--

FF