The Impossibility of Flying Heavy Aircraft Without Training

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

I have several aerodynamics books that say differently.

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.

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.

--
Dylan Smith, Port St Mary, Isle of Man
Flying: http://www.dylansmith.net
Oolite-Linux: an Elite tribute: http://oolite-linux.berlios.de
Frontier Elite Universe: http://www.alioth.net

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.

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.

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.

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...

--

FF

On 2006-02-27, wrote:
In the real world airplanes have flown with pressure sensors
on the wings, confirming lift from the Bernojuli effect in actual
flight.

Bernoulli's equations and Newton's are not mutually exclusive or somehow
additive - they are just looking at different aspects of the same thing
and both explain 100% of lift.

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...

But this seems a bit irrelevant. When I go to and from work in my car,
there is no net movement by my car either, since when I get home I park
it in the same place. But in the discussion of whether my car got me to
work or not this is irrelevant. Either that or I'm paying for fuel and
merely imagining I go to work :-)

--
Dylan Smith, Port St Mary, Isle of Man
Flying: http://www.dylansmith.net
Oolite-Linux: an Elite tribute: http://oolite-linux.berlios.de
Frontier Elite Universe: http://www.alioth.net

Dylan Smith wrote:
On 2006-02-27, wrote:
In the real world airplanes have flown with pressure sensors
on the wings, confirming lift from the Bernojuli effect in actual
flight.

Bernoulli's equations and Newton's are not mutually exclusive or somehow
additive - they are just looking at different aspects of the same thing
and both explain 100% of lift.

I'm not sure what hyou mean by this. One supposes that Bernouli's
equaitons are derived using Newtoniam mechanics and the
ideal gas law (it has been a long time since I took fluid mechanics
but don't see that there is anything else to work with). For that
matter
the ideal gas law can (probably) be derived using Newtonian mechanics.

Is that what you meant?


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...

But this seems a bit irrelevant. When I go to and from work in my car,
there is no net movement by my car either, since when I get home I park
it in the same place. But in the discussion of whether my car got me to
work or not this is irrelevant. Either that or I'm paying for fuel and
merely imagining I go to work .

Precisely. Flow is irrelevent.

--

FF

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.

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.