The Impossibility of Flying Heavy Aircraft Without Training

On Fri, 3 Mar 2006 at 21:10:20 in message
.com,
wrote:

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.

Wrong there is a need. The pressure change you postulate would itself
have an effect on the air and cause it to change direction.

Helicopters are the easiest things to consider. They expend an awful lot
of power just to hang there. Where does all that energy go? It goes into
changing the momentum of a large amount of air downwards. The rate of
change of momentum then being equal to the weight of the helicopter.
--
David CL Francis

On Fri, 3 Mar 2006 at 05:30:06 in message
.com,
wrote:

Newton had three laws of motion, you're ignoring the first.
Is there a net change inmomentum of the fan? If not,
how can there be a net change of momentum of the air?

I am ignoring nothing. The above statement is wrong. You agree below
that energy is put into the air. In the case of a fan that energy goes
into increasing the velocity of the air. The rate of change of momentum
(mass flow times velocity increase) produces forces that increase the
momentum of the air. Energy changes momentum. Momentum destroyed turns
back into energy.

This argument is hung up on the idea that the air returns to a steady
state eventually - which it does! But not quite back to where it was
because of losses Nevertheless energy is lost and replaced by the
engines of the aircraft.

There no question that energy is put into the air. There is
no net change in momentum, of the air. otherwise all the
air would pile up on one side of the fan and there would
be a vacuum on the inlet side. Air moving through the
fan in one direction is offset by air moving around the fan
in the other direction.

The air slows down and looses energy and momentum far away from the
aircraft - so what? Any small drop in pressure at the fan also reaches
back and develops flow some way in front of the fan. For lift purposes
it does not matter much. The air may or may not make its way back to the
inlet again, some of it will.

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.

Except for losses that occur due to friction and eddies that float away
to dissipate themselves elsewhere. But the answer must be still be so
what? Momentum is not conserved because energy has been added. Are you
saying that a land vehicle with a horizontal fan to drive it along rails
will not accelerate and move? Will the vehicle not build up momentum
because of this?
--
David CL Francis

On Fri, 3 Mar 2006 at 04:46:18 in message
.com,
wrote:

The momentum of an airplane in level flight at
constant speed is constant. Conservation of
momentum REQUIRES that there is no net
change in themomentum of the air. There is
momentum exchanged between the airplane
and the air. But there is no NET momentum
change in the air.

You are perhaps neglecting the fact that in momentum exchanges energy is
generally lost.

Your statement is true in a way. If you consider a lump of air striking
something and being defected then it is analogous to the impact between
two billiard balls. If they strike at an angle then momentum is
conserved but it is redistributed and energy is generally lost.

The point you are making makes no difference to the outcome. The energy
lost has to be exactly balanced by the energy input to the system by the
engines or by potential energy. The energy is what is needed to
continually add to the downward momentum imparted. The local change in
momentum is provided by the aircraft descending exchanging potential
energy for kinetic or by having a source of power.
--
David CL Francis

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

Alan Baker wrote:
In article .com,
wrote:

....




Well then if the downflow is NOT balanced by upflow why doesn't
the upper atmosphere run out of air?

Because the air contacts the earth and *stops* moving downward.


Could you define downflow?

--

FF

David CL Francis wrote:
On Fri, 3 Mar 2006 at 02:27:02 in message
. net, Richard Lamb
wrote:

I hate to be a spoil sport (or dullard?), but...

the (stationary) air does WHAT (as the wing passes by)???


The nature of things is such that the situation does not change if you
change the frame of reference. It is normal in doing calculations to
start with a frame of reference based on the aircraft. If you follow the
aircraft then the air is going past it.

The presence of the wing changes the air flowing past the aircraft in
the same way as if you consider the aircraft passing through the air.
The 'stationary' air as you call it has its local velocity and direction
changed by the aircraft.

Yeahbut...

A handy frame of reference is - handy.

But it can be very misleading.....

Richard Lamb wrote:
David CL Francis wrote:

On Fri, 3 Mar 2006 at 02:27:02 in message
. net, Richard Lamb
wrote:

I hate to be a spoil sport (or dullard?), but...

the (stationary) air does WHAT (as the wing passes by)???



The nature of things is such that the situation does not change if you
change the frame of reference. It is normal in doing calculations to
start with a frame of reference based on the aircraft. If you follow
the aircraft then the air is going past it.

The presence of the wing changes the air flowing past the aircraft in
the same way as if you consider the aircraft passing through the air.
The 'stationary' air as you call it has its local velocity and
direction changed by the aircraft.


Yeahbut...

A handy frame of reference is - handy.

But it can be very misleading.....


For instance?

If the air is moving, we expect a lower pressure. Nod to Bernoulli.

But the air would also be moving along the bottom side of the wing also?

And what would that do to the pressure under the wing?

And if the pressure under the wing is below ambient....

Momentum is not conserved because energy has been added. Are you saying that a land vehicle with a horizontal fan to drive it along rails will not accelerate and move? Will the vehicle not build up momentum because of this?

Momentum is always conserved. If you see momentum disappearing, you are
not looking at the whole system. In the case of the land vehicle
propelled by a fan, the air blown back acquires momentum in one
direction, exactly balanced by the momentum that the vehicle acquires,
plus the (rotational) momentum (due to wheel friction) that the earth
acquires.

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

In article .com,
wrote:

Alan Baker wrote:
In article .com,
wrote:

...




Well then if the downflow is NOT balanced by upflow why doesn't
the upper atmosphere run out of air?

Because the air contacts the earth and *stops* moving downward.


Could you define downflow?

Sure.

The aircraft passes through and air moves downward. As it moves its
motion is dissipated into more and more air moving less and less, but
eventually the momentum that was transferred to it is transferred back
to the earth.

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."

Alan Baker wrote:
In article .com,
wrote:

Alan Baker wrote:
In article .com,
wrote:

...




Well then if the downflow is NOT balanced by upflow why doesn't
the upper atmosphere run out of air?

Because the air contacts the earth and *stops* moving downward.


Could you define downflow?

Sure.

The aircraft passes through and air moves downward. As it moves its
motion is dissipated into more and more air moving less and less, but
eventually the momentum that was transferred to it is transferred back
to the earth.

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."