The Impossibility of Flying Heavy Aircraft Without Training

Jose wrote:
What is the net momentum change when the airplane falls to the ground?

The vertical compenent first rises from zero to Vt * M where Vt is the
terminal velocity of the falling aircraft and M is the mass of the
falling
aircraft. Then the vertical component of momentum RAPIDLY drops
to zero again after the aircraft contacts the ground.

Well, actually, only sorta. The momentum of the airplane is equal to
the momentum of the earth, except in sign. Net is zero. The center of
mass of the earth/airplane does not move.

Leave the earth out of it and just look at the aircraft, and you are
correct. And to keep an airplane up, in view of this acceleration, an
opposite acceleration needs to be applied. Air must be thrown down with
sufficient (net) force to counteract gravity's attempt to accelerate the
wing downwards.

No. You can also generate an upward force on an airplane by
creating low pressure over the upper surface of the wing while
the pressure below the wing remains at ambient. I dunno if
there are any airfoils that leave the air below the wing exactly
the same as ambient, but if there were, it would fly. There is
no NEED to throw anything downward.

--

FF

You can also generate an upward force on an airplane by
creating low pressure over the upper surface of the wing while
the pressure below the wing remains at ambient. I dunno if
there are any airfoils that leave the air below the wing exactly
the same as ambient, but if there were, it would fly. There is
no NEED to throw anything downward.

I suppose a wing that gobbled up air molecules from the top of the wing
and beamed them into outer space would do the trick. Another way would
be to supercool the top surface, and let the general gas law reduce the
pressure above. But doing either one, air above the air above the wing
would rush down, as the air below the wing pushes the wing up into that
same space. The two will collide, or the wing will have passed by then.
In the latter case, downward momentum has been imparted to the air
above the air above the wing, which gets dissipated as I argued for
conventional wings.

Jose
--
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Jose wrote:
You can also generate an upward force on an airplane by
creating low pressure over the upper surface of the wing while
the pressure below the wing remains at ambient. I dunno if
there are any airfoils that leave the air below the wing exactly
the same as ambient, but if there were, it would fly. There is
no NEED to throw anything downward.

I suppose a wing that gobbled up air molecules from the top of the wing
and beamed them into outer space would do the trick.

Or blew them out the rear for thrust.

Another way would
be to supercool the top surface, and let the general gas law reduce the
pressure above. But doing either one, air above the air above the wing
would rush down, as the air below the wing pushes the wing up into that
same space. The two will collide, or the wing will have passed by then.
In the latter case, downward momentum has been imparted to the air
above the air above the wing, which gets dissipated as I argued for
conventional wings.


Yes and that is what a conventional wing does. It creates lower
pressure above the wing so that the ambient or near ambient
pressure below the wing pushes up on the wing creating lift.

The air from above that low pressure region begins moving down
into that region but doesn't get there until after the wing has
passed. Downwash occurs, as you describe in the paragraph
above. It is a consequence of lift, not the cause. In fact the
energy put into the air by the downwash phenomenum is wasted.
A more efficient wing will produce less downwash than a less efficient
one, for the same lift.

--

FF

not the cause.

Jose
--
Money: what you need when you run out of brains.
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Downwash occurs, as you describe in the paragraph
above. It is a consequence of lift, not the cause.

We perhaps disagree merely on the idea of which "causes" the other. How
do you figure that lift causes downwash? Lift doesn't happen unless the
air above is rarified. The air above is not rarified until some of the
molecules are gotten rid of somehow. The process of getting rid of
those molecules is just a newtonian process (which lends itself to
certain bulk equations).

They are different ways of looking at the same thing, depending on which
aspect you want to concentrate on.

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

Jose wrote:
Downwash occurs, as you describe in the paragraph
above. It is a consequence of lift, not the cause.

We perhaps disagree merely on the idea of which "causes" the other. How
do you figure that lift causes downwash? Lift doesn't happen unless the
air above is rarified. The air above is not rarified until some of the
molecules are gotten rid of somehow.

No.

The process of getting rid of
those molecules is just a newtonian process (which lends itself to
certain bulk equations).

They aren't 'gotten rid of' they are accelerated which causes them to
be spaced farther apart -- thus lowering the pressure.


They are different ways of looking at the same thing, depending on which
aspect you want to concentrate on.


The lift is a result of the pressure difference between the lower and
upper surfaces of the wing. The downwash is the result of the momentum
of the air above the rarefied region created by the wing moving
downward.

The downrushing air starts it s downwash above the wing and does
not pass the wing in the vertical direction until after he wing has
passed.
It does not contribute to lift.

It is not really caused by lift (my mistake), it is caused by the same
phenomenum that causes lift.

--

FF

They aren't 'gotten rid of' they are accelerated which causes them to
be spaced farther apart -- thus lowering the pressure.

Accelerating them gets rid of them in the sense I mean, but I suppose I
was sloppy there. In any case, to be accelerated, they need to go
somewhere. The standard explanation is that there is a longer path up
top. The reason there is a longer path is that the air is bent
downwards. If you bend plywood (concave down), the top sheet is
stretched and the bottom sheet is compressed. Same with the air.

When the air is bent downwards, the air is accelerated downwards. This
causes downwash. Air accelerated downwards by the wing requires (by
Newton) the wing to be accelerated upwards (counteracting in this case
the acceleration due to gravity). It does so in a manner that also fits
Bernoulli's equations.

The lift is a result of the pressure difference between the lower and
upper surfaces of the wing. The downwash is the result of the momentum
of the air above the rarefied region created by the wing moving
downward.

And the pressure difference is sustained by the wing continually
imparting momentum (indirectly by creating the pressure differential) to
the air above the rarified region.

The downrushing air starts it s downwash above the wing and does
not pass the wing in the vertical direction until after he wing has
passed.

Matters not. It is another way to look at lift.

[The downrushing air] is not really caused by lift (my mistake),
it is caused by the same phenomenum that causes lift.

Fair enough. What this says is that both ways of looking at it are
valid. Bernoulli is easier to calculate, Newton is easier to conceptualize.

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

Jose wrote:
They aren't 'gotten rid of' they are accelerated which causes them to
be spaced farther apart -- thus lowering the pressure.

Accelerating them gets rid of them in the sense I mean, but I suppose I
was sloppy there. In any case, to be accelerated, they need to go
somewhere. The standard explanation is that there is a longer path up
top. The reason there is a longer path is that the air is bent
downwards. If you bend plywood (concave down), the top sheet is
stretched and the bottom sheet is compressed. Same with the air.

There is a longer path along the top because the wing is convex up.


When the air is bent downwards, the air is accelerated downwards. This
causes downwash.

Not until after it passes the high point in the airfoil. Befor it
gets there,
it is accelerated upwards.

Air accelerated downwards by the wing requires (by
Newton) the wing to be accelerated upwards (counteracting in this case
the acceleration due to gravity). It does so in a manner that also fits
Bernoulli's equations.

When the air reaches the trailing edge it is back to where it started.
But in the meantime air above it has begun to flow down. After the
wing has passed the momentum of _that_ downflow carries the air
down past the altitude of the wing. But that is after the wing has
passed. The downflow is -art of what happens as the air in the wake
of the airplane is restored to equilibrium.


The lift is a result of the pressure difference between the lower and
upper surfaces of the wing. The downwash is the result of the momentum
of the air above the rarefied region created by the wing moving
downward.

And the pressure difference is sustained by the wing continually
imparting momentum (indirectly by creating the pressure differential) to
the air above the rarified region.

Regardless, the lift is a result of the pressure differential between
the upper and lower wing surfaces.


The downrushing air starts it s downwash above the wing and does
not pass the wing in the vertical direction until after he wing has
passed.

Matters not. It is another way to look at lift.

No, it is a way of looking at downrushing air that has never
contacted the wing.


[The downrushing air] is not really caused by lift (my mistake),
it is caused by the same phenomenum that causes lift.

Fair enough. What this says is that both ways of looking at it are
valid. Bernoulli is easier to calculate, Newton is easier to conceptualize.


No. That says that the downrushing air and lift are both caused by the
same phenomenum.

--

FF

On Sat, 4 Mar 2006 at 05:38:33 in message
.com,
wrote:

A more efficient wing will produce less downwash than a less efficient
one, for the same lift.

Yes but it still has to provide the exact same amount of rate of change
of momentum. It tends to move a bigger mass of air slower but at the
same momentum change.
--
David CL Francis

David CL Francis wrote:
On Sat, 4 Mar 2006 at 05:38:33 in message
.com,
wrote:

A more efficient wing will produce less downwash than a less efficient
one, for the same lift.

Yes but it still has to provide the exact same amount of rate of change
of momentum. It tends to move a bigger mass of air slower but at the
same momentum change.

An infinite wing has no induced downwash.

How is it supported?

--

FF

An infinite wing has no induced downwash.

How is it supported?

It has downwash. There is upflow in front of the wing, and downwash
behind the wing. There is more downwash than upflow, this counteracts
mg of the wing.

There is no vortex at the wingtips (no wingtips) but there is a vortex
in the direction of flight.

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