The Impossibility of Flying Heavy Aircraft Without Training

On Fri, 03 Mar 2006 19:05:18 -0500, Morgans wrote:


"00:00:00Hg" wrote

I thought resistance was useless.

Nah, it's "resistance is fruitile."
g

I think I'll have an apple and see if
eating it will reveal the secrets of
gravity as I gaze at the Moon.

"00:00:00Hg" wrote

I thought resistance was useless.

Nah, it's "resistance is fruitile."
g
--
Jim in NC

"00:00:00Hg" wrote

I think I'll have an apple and see if
eating it will reveal the secrets of
gravity as I gaze at the Moon.

Yeah, but can you tell me the horizontal and vertical components of it's
momentum?

I was thinking apple, but I need two; I'll have a pear, instead. g
--
Jim in NC

On Fri, 03 Mar 2006 21:06:30 -0500, Morgans wrote:

I think I'll have an apple and see if
eating it will reveal the secrets of
gravity as I gaze at the Moon.

Yeah, but can you tell me the horizontal and vertical components of it's
momentum?

Not any more, I'll have to pick another.

Not the Moon... it has no stem.

You are looking here at the basic question of how does the
starting vortex form.

No, I'm also looking at how it is maintained.

You have staked out the position
that a ground is required for the vortex to form.

No, I've staked out a position that the ground is required for there to
be no net momentum change. The ground is ultimately what the air (given
downward momentum) bounces against, either for real or by proxy.
Granted this is not what provides lift, but it does provide the ultimate
support when the wheels themselves leave the ground.

Do we agree or disagree that the "wave" i.e.
starting vortex, however it got started can continue in the
absence of the ground?

We agree. I do not see however how it can continue in the absence of
energy, and I still maintain that in order to cancel out mv^2/2 of the
wing (which otherwise would be falling), there has to be (locally) an
equal mv^2/2 which the air acquires, and spreads out over the surface of
the earth (where it bounces off, keeping the earth away). Like a
dribbler who supports himself by dribbling, there is lots of momentum
transfer (to the ball, back and forth), which, while it nets to zero,
only does so because of the earth. IF there were no earth, the ball
would never bounce back.

That is not the same as what you seem to think I am maintaining:

that without a ground an infinite
wing would require a constant input of infinite energy to
accelerate the air and give it momentum (and kinetic energy)
for the uncanceled downwash.

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

An object moving through air doesn't cause any significant compression
(change in volume) of the air until its speed gets close to the speed of
sound.

Is there not a (slight) pressure increase in front of any object,
especially a blunt one, moving through the air? (if not, what causes
the air to get out of the way, and what causes the breezes as it goes past?)

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

The distinction is that a compressible fluid (commonly called gas)
undergoes a volume change proportionate to the pressure change

Well, when an object passes through the air, does it not compress the
air in front of it (and rarefy the air behind it)? This is how speakers
work. Those are all pressure changes.

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

In open air the volume of air moving around the fan is larger,
but moving at a lower speed than the air moving through the
fan so that the momenta of the flow in either direction is equal
magnitude and opposite in direction to the flow in the other
direction.

Seems to me "almost equal" would make more sense, otherwise an airplane
propeller would not work. A propeller throws air backwards (alabeit
imperfectly); the airplane moves forwards in response.

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

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. All this air piles up against the earth (just a little
bit, but enough that, when multplied over the earth's surface it adds up
to the weight of the aircraft)

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

Jose wrote:
The hovering spacecraft has zero horizontal and vertical momentum.
It has weight, directed downwards. The engine accelerates
mass downward producing an upward force equal in magnitude
and opposite in direction to the weight of the spacecraft. This
imparts an acceleration to the spacecraft equal in magnitude and
opposite in direction from the local acceleration due to gravity.

The flying wing has some horizontal momentum which is secondary here,

How much?

and zero vertical momentum.
It also has weight, directed downwards. The wing accelerates
mass downward (mass it finds in the air molecules) producing
an upward force equal in magnitude and opposite in direction to
the weight of the wing (and its presumably attached aircraft. It does
so by finding air in front of it, flinging it downwards and forwards
(which causes the air in front to try to get out of the way by rising).
In the steady state, one can measure high pressure below and low
pressure above, but this is just the macroscopic manifestation of the
greater number of molecular collisions below, and the lesser number of
collisions above. That's what pressure is - we have both agreed on this.

The greater number of collisions below
imparts an acceleration to the aircraft equal in magnitude and
opposite in direction from the local acceleration due to gravity.

I agree that lift is a force, exerted on the aircraft by the air,
which in steady level flight is equal in magnitude and opposite
in direction to the weight of the aircraft. Energy is 'pumped'
into the air by the plane. There is no need for a net momentum
exchange between the airplane and the air in order for
energy to be exchanged or for forces to be applied.
Indeed, in those last two paragraphs above, you make
no mention of momentum.

BTW, I was wrong to invoke conservation of momentum.
Momentum is conserved in elastic collisions, like the
collision between a cue ball and the eight ball. Momentum
is not conserved in inelastic collisions, like the collision
between a cue ball and a nerf ball.

Roll the airplane into a 90 degree bank. The weight is
now orthogonal to the lift. As teh airplane falls, it
banks even though there is no Earth 'under' the
belly. Why?

--

FF