The Impossibility of Flying Heavy Aircraft Without Training

David CL Francis wrote:
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.

Well I'm sorry to see that I an not the only one who was confused
on this issue. In Newtonian dynamics, energy is always conserved,
mass is always conserved, and momentum is always conserved.

When the momentum of a body changes, then energy is converted
from one form to another, but the momentum of a body can only
change by being transferred to another body. Momentum,
like energy and mass, is never destroyed.

I'm not fond of 'unit analysis' but consider that the units of energy
and the units of momentum when reduced to fundamental units,
are different. Conversion between the two is impossible.


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.

Yes, the airplane puts energy into the air. But in the closed system
that consists of the airplane and the atmosphere, or the fan and
the air in the room, momentum is conserved, just as mass and
energy are.


... 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.

If only some of it does, then mass is not conserved. ALL of it,
or rather an equivalent amount of displaced mass makes it
back to the inlet of the fan. In order to make it back, it has
velocity. for a rather slow fan in a rather small room the velocity
through the fan may be ten times the average velocity of the
air moving around the fan in the opposite direction. That's
OK, but conservation of momentum requires that ten times
the mass be moving in that opposite direction at one tenth the
'fan' velocity and a moments consideration should convince you
that this also conserves mass.

In te case of the aircraft, the fan is moving through the air so that
when the air (or rather an equivalent displaced mass of air) returns
to the inlet, the inlet has move on.


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.

No. The turbulance dissipates energy, (that is to say it converts to
heat) not momentum.

Momentum is always conserved.

--

FF

wrote:
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.


I was hoping for a mathematical defintion, rather than a description
of the process. That would minimize my opportunity to draw an
incorrect inference. In this regaerd, a mathematical definiton
would be best.

I infer from your description the definition: "downflow is a flow
of air from the airplane toward the ground". That removes a
potential abiguity, whether downflow was a flow of momentum
through the air, (like a pressure wave) or a flow of mass.
Is that how you define downflow, as a flow of air molecules
(with mass) toward the ground?

Can you state a mathematical definition of downflow?

Fred Thomas' in _Fundamentals of Sailplane Design_ defines
the freestream velocity, then states the relationship between
that, the local velocity near the airfoil and the induced downwash
as a vector sum and goes on to show how this produces an
effective angle of attack less than the geometric angle of attack.
But he does not present a separate mathematical defintion of
induced downwash or the local velocity of the air near the wing
so the vector sum above does not serve (within the context of
his discussion) to define either term.

But it is clear that the induced downwash is a velocity, not a
a massflow. Yes, it is mass that has that velocity but the
parameter _induced downwash_ is a velocity.

So, can we agree to the definition of downflow as a flow
of air toward the ground and define the induced downwash
as the velocity of that air near the wing?

Meanwhile:

Earlier I wrote:

The point is that downflow is a consequence of,
not the cause of lift, and it is balanced by
upflow, (albeit a more diffuse flow) otherwise
the upper atmosphere would run out of air.
[and later corrected this to: downflow is a
consequence of the same phenomenum that
produces lift, not the cause of lift]

You replied:

No. It is balanced by the downflow eventually
transferring its momentum back to the earth.

So I asked:

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

Your response:

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

Earlier you corrected me regarding conservation of
momentum. Now, consider conservation of mass.

That the downflow stops upon contacting the Earth does
NOT explain why the upper atmosphere is not depleted
of air.

Plainly if air flows to the Earth and *stops* there as
you wrote, it has displaced other air which flowed
up to replace it, right?

--

FF

T o d d P a t t i s t wrote:

David CL Francis wrote:


The nature of things is such that ....


I've been following along (more or less :-) and chose
David's post to jump in again, since, from experience I have
great trust in any analysis by David.

This thread, however, seems to wander all over the place.
It looks like one participant will make one set of
assumptions, then another will assume something different.

I see the following discussions going on:

1) A pure thread related purely to lift and Bernoulli. In
this thread, the subject matter is maximally simplified by
a) using the standard Bernoulli assumptions, inviscid flow,
incompressible, subsonic, etc., b) ignoring parasitic drag
c) using 2-D flow (or equivalently infinite wing)
assumptions and looking at steady state conditions. This
gets to the heart of upwash and downwash.

2) The same as 1) above, but looking at 3-D flow. Now we
have induced drag and the wing/fan produces a net motion of
the air as it passes through. Much of this discussion seems
focused on issues relating to closed systems (rooms, earth
with ground, etc.) and what happens to the air, how big a
system should be looked at, etc.

3) The same as 2) above, but with viscosity added so that
the air ultimately stops its motion and heats up due to
viscous losses.

Quite honestly, for most of the posts here, I can't figure
out what assumptions lie behind the comments.

Todd,
Your #2 and #3 is where I wanted to go with thus mess,
Thankee.

The down wash, being transferred from air near the wing to air
far away from the wing...

Air is quite springy stuff.

The energy transfer is spread over an increasing area (or volume)
and quickly reduced in magnitude - to the point where it is no
longer detectable (without invoking Steven Hawking).

For all practical purposes, that would seem to indicate that the
"down flow" would not reach the ground before being dissipated into
the larger air mass. (not arguing against the eventual contact with
the entire surface of the planet. But that doesn't help us understand
basic aerodynamics!)

Only when the wing is close to the ground is the down wash detectable
because it hasn't had time (or room) to be absorbed/dissipated.

Now, while the above is obviously not true in the molecular sense,
it may help us understand the practical parts better.

Also, add #4?

While we have been concentrating on the pressure side (bottom) of the
wing, it's the upper surface that has the greater influence here.

The only way I see of increasing pressure on the bottom surface is to
increase speed (or density?).

But the top side is where the pressure is reduced.
And there are a lot of factors that effect that part.

Thickness of the camber line is a big one.
Deeper camber tends to cause a lower pressure on top - hence more lift
for a given surface and speed.

This is most often accomplished by deploying flaps.
True they go down into the stream on the bottom side - and probably do
to some degree - invoke some impact lift (pressure) on the bottom.

But the curvature of the airfoil has increased also - and the camber
line has deepened - and the apparent angle of attack has increased.

These factors further decrease the lowered pressure field on top of the
wing - WAY more than any useful increase in pressure below it.

Lastly (for now), if we are indeed pressing down on the air below the
wing, we are also Pulling Down on the air above it...

The air below presses against the earth. As I've said before, that one is
so obvious (that we stop looking?).

But I think the low pressure field Above the wing is also pulling down
on the atmosphere above it.

While air pressure decreases with altitude me may think that the field
above the wing dissipates quicker. Maybe true, BUT - the pressure field
Above the wing is of much higher magnitude - so maybe not.

Well, so much for my silly idea.
I don't know how to analyze that one mathematically.

I'd really like to see what the bigger brains can make of it.

Richard

Could we reduce the crossposting? I think one newsgroup is more
than sufficient. You chose, and I'll follow.

It's being posted to piloting, homebuilt, and student. We could easily
lose homebuilt. Should we lose student?

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

But in the closed system
that consists of the airplane and the atmosphere, or the fan and
the air in the room, momentum is conserved, just as mass and
energy are.

Except that that's not a closed system. You need the earth to close the
system. No earth but phantom gravity, and to conserve momentum, the air
will continue to downflow, which is what would happen.

If only some of it does, then mass is not conserved. ALL of it,
or rather an equivalent amount of displaced mass makes it
back to the inlet of the fan.

No, mass can be conserved by having some of it pile up. This is in fact
what happens. The pressure on one side of the room goes up. Guess why.

In te case of the aircraft, the fan is moving through the air so that
when the air (or rather an equivalent displaced mass of air) returns
to the inlet, the inlet has move on.

In the case of the aircraft (propeller), the air does not return to the
inlet. It keeps on being blown back, since there is no wall to stop it.
The momentum stays with the air and the earth (which starts to spin a
little one way) until the airplane lands, and pushes the earth the other
way. Momentum is always conserved.

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

On Mon, 06 Mar 2006 17:56:42 -0800, fredfighter wrote:

Could you define downflow?

It happens to geese as they exceede the speed of down.

So, can we agree to the definition of downflow as a flow
of air toward the ground and define the induced downwash
as the velocity of that air near the wing?

I don't think this is a useful definition. Downflow and downwash are
the actual movement of something, not merely the velocity of that movement.

Plainly if air flows to the Earth and *stops* there as
you wrote, it has displaced other air which flowed
up to replace it, right?

Only if pressure is constant. (at constant temperature). However,
pressure does not remain constant. The pressure below the wing (and
thus against the earth) increases due to the extra molecules that have
been thrown down. Those molecules came from above the wing. The upper
atmosphere =is= (slightly) depleted by the flight.

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

I'd really like to see what the bigger brains can make of it.

I think you did fine. I will take issue with:

The air below presses against the earth. As I've said before, that one is
so obvious (that we stop looking?).

The air does press on the earth, and this is "where the momentum goes",
which is a big question in one of the poster's minds. No earth, nothing
to press against, and the momentum just keeps on going down. It is
(thus) not true that there is no local momentum transfer. That is one
of the points I was making. There is of course no global momentum
transfer once all parts of the closed system are taken into account.

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

Could we reduce the crossposting? I think one newsgroup is more
than sufficient. You chose, and I'll follow.

Absent protest, from just after "now" on, I'll reply and post this
thread only to r.a.piloting.

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

On Tue, 7 Mar 2006 at 05:06:54 in message
, Jose
wrote:

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.

From a Physics book:

A jet of water merges from a hose pipe of a cross sectional area 5x
10^-3 m^2 and strikes a wall at right angles. Calculate the force on the
wall assuming the water is brought to rest and does not rebound.
(Density of water = 1.0 x 10^3 Kg m^-3)

After explaining the simple calculation which gives a force of 45N it
goes on to say; " in practice the horizontal momentum of the water is
seldom completely destroyed and so the answer is only approximate."

~~~~~~~~~~
Any changes to the entire earth as a result are insignificant. Closed
systems are adequate for most practical purposes.

All these arguments about the 'total system' are irrelevant to
considering the kind of problem we have here. As in many problems you do
not need to include the whole universe to get practical and accurate
answers.

In the same way including discussions about molecular velocities beating
on the sides of the aircraft is a mere distraction. At normal altitudes
these effects are negligible compared to the consideration of the air as
an incompressible fluid.

Some aircraft can maintain a 7g turn banked at the appropriate angle.
(81.8 degrees approximately). What happens to that 7 times pressure on
the earth now? The same calculations will give accurate figures of lift
only slightly affected by the small difference of speed between the two
wings and the circular path. The earth hardly comes into it for
accuracy except that it is gravity that is being balanced.

--
David CL Francis