The Impossibility of Flying Heavy Aircraft Without Training

But considering each molecule separately, there certainly =is=
momentum change at each collsision.

True, because that's what pressure is. Ok, let's say we drop the
temperature down to absolute zero. Now there's NO momentum change.
:-) Ok, while true, we're getting away from the conditions under
which lift is generated, so it's unrealistic.

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. Now, after thinking about it for a while, I have no idea
what they mean. The idea of taking a mass of air and "throwing it
down" makes a nice, intutive image, but I can't quantify it.

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

And I suspect that "pulling up" part is a little harder to
visualize. What, for instance, is it pulling AGAINST?

The reason it's hard to visualize is that Bryan is correct; there is
no "suction force." If the pressure above and below the wing are
ambient, there is no net force because the two forces cancel. If you
lower the pressure above, the ambient pressure below the wing pushs
the wing upwards.

While a "suction force" is a useful fiction, like centrifugal force,
it can confuse an analysis if its origin is not understood.


Stalls:
At some value of high alpha, the low pressure ABOVE the wing exceeds
the shear value (viscosity) of air, and the flow "tears" loose.

Stalls happen because the air flow over the top of the wing run into
an increasing pressure along the back half of the wing, which slows
down the air. At some point, the air will move backwards The flow is
said to be "separated" at this point.

The "suction force" concept might be confusing the issue here too.

As we get down to within a wing span or so of the ground, and the
higher pressure on the bottom side actually does come into contact
with the ground, said high pressure area under the wing gets trapped
and is noticibly stronger.

Perhaps, but that suggests that Ground Effect is an increase in lift,
when in reality it's an decrease in drag.

Richard Lamb wrote:
Bryan Martin wrote:
The low pressure above the wing never pulls up on the wing at all, it just
doesn't push down as hard as the high pressure below so the net force is up.
Suction is not the opposite of pressure, suction doesn't really exist at
all, it is merely a term used to describe a lower pressure.


I'm going to gently disagree, Bryan.

If the pressure above the wing is below ambient, it sucks...

In a episode of _Star Trek:The Next Generation_ Riker, Data and
others are exploring a delelict. Riker, after listiening to a voice
recording
says that evidently one of the crew opened an airlock door and was
sucked outside into space. Data said, "Blown out". Riker asked,
"'What?". Data Replied "Blown out. They were blown out. It is a
common mistake."

Data was right.

--

FF

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

Greg Esres wrote:
The disk is constantly transferring momentum to the air below it,
which is transferring it right back after bouncing off of the floor.


There is no momentum change here because there is no *net* force on
either the air or the disk. The molecules next to the disk have a
pressure equal to the weight of the disk below it and the actual disk
above it. There is no net force and thus no momentum change.

Pressure never equals weight for the same reason that voltage
never equals power.

To be precise, the upward force the disk is the difference
in the pressure below the disk less the pressure above, multiplied
by the area of the disk. The downward force is the weight of
the disk which is the product of the mass of the disk times
the local acceleartion due to gravity. The disk stops moving when
the two forces are equal.


Momentum = mass * velocity, and the vertical velocity of the air and
the disk are zero.

Force is defined as the time rate of change of momentum. Therefor
when the disk is neither accelerating nor changing mass there is
no force acting on it.

So what holds it up? ;-)

--

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
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To be precise, the upward force the disk is the difference in the
pressure below the disk less the pressure above,

You're correct; equating pressure with force is a convenience to
simply discussions; it's a bit sloppy.

Force is defined as the time rate of change of momentum. Therefor
when the disk is neither accelerating nor changing mass there is
no force acting on it.

That's not a definition, it's an equation. *If* you have a change in
momentum, it's proportional to the NET forces acting on it.

Highflyer wrote:

...

No. wingtip vortices are caused by downwash. Infinite wings don't have
wingtip vortices because they don't have ends, not because they don't have
downwash.


Wingtip vortices are caused by the pressure differential between the
lower and upper wing surface. The air moves from the lower to the
upper, around the wingtip. Wingtip vortices reduce lift because
they cause the pressure below the wingtip to be lower and above the
wingtip to be higher, than at points further inbound.

Conservation of angular momemtum causes the air to continue swirling
after the aircraft has passed. Once the pressure changes caused by
the passage of the aircraft have died out and the pressure restored to

ambient the net downward flow from the entire aircraft is equal to net
upward flow from the entire aircraft, provided the aircraft is in level
flight.
If the aircraft is climbing or diving the only net flow is that cuased
by the
displacement of the volume of the aircraft.

Those last two sentences are key.

Infinite wings lack wingtip vortices because they lack wingtips.

--

FF

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

Greg Esres wrote:
To be precise, the upward force the disk is the difference in the
pressure below the disk less the pressure above,

You're correct; equating pressure with force is a convenience to
simply discussions; it's a bit sloppy.

Force is defined as the time rate of change of momentum. ...

That's not a definition, it's an equation.

Again, to be precise, that is the equation which defines force IF
you work in a aystem in which mass and acceleration are previously
defined. In physics, definitions usually are equations.

--

FF