The earth pulls down on the plane...

In article ,
cavelamb wrote:

Alan Baker wrote:
In article ,
cavelamb wrote:

Alan Baker wrote:
In article ,
cavelamb wrote:

It's all trolling noise, and, to my mind, began with an incorrect
assertion
that one can not "pull" on air.



http://www.northsailsod.com/articles/article6-1.html
Air cannot pull on a surface. Any pressure at all means the air is
*pushing* on the surface.

That remains the reality, regardless of a website that uses less than
precise language.

You didn't even look.

I did look, and I even read.

It shows a diagram of "pressure" with the arrows pointed *away* from
from the sail. Pressure doesn't act away from a surface.



Alan, It's ALL about suction.

And suction is actually higher pressure pushing something towards a
region of lower pressure.

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg

(Since this was partially written a couple days ago, I decided to
complete it and post it anyway even though I'm sure some were hoping the
the discussion had died. :-))

Alan Baker wrote:
Jim Logajan wrote:
To that question I say: "Nothing in the physics rules it out." The
math appears to allow a solution wherein the fluid moves such that
the *forces*, *momentum*, and *mass flows* are *all* conserved _long
before_ any downwash strikes the earth.

Sorry, but you're wrong.

Tsk. Here's the math that shows there are solutions wherein momentum is
conserved yet yields zero average mass flow. It uses two conservation
laws (one of them being the first derivative of conservation of
momentum):

A toy helicopter is hanging from a line in a sealed room initially at
rest, and is released from the line and engine started at the same time
so it remains stationary. The law of conservation of mass requires that
the net flow of mass through the surface of any volume in an
incompressible fluid must be zero. For this problem the volume of the
room bounded by the walls above the plane made by helicopter blades and
said plane is used for one of the needed constraints.

The vertical forces through the plane made by the blades is described by
two areas: the area made by the blades (call it A1, in m^2) and the
remaining area of that same plane (call it A2, in m^2; the area from the
blade disk to the walls):

(1) F1 = M1d*V1
(2) F2 = M2d*V2

Whe
F1: Force of downwash through A1, in N. Positive number.
F2: Force of any upward flow through A2, in N. Positive number.
M1d: Average mass flow, in kg/s, of through A1. Positive number.
M2d: Average mass flow, in kg/s, of through A2. Positive number.
V1: Average velocity, in m/s, of flow through A1. Positive number.
V2: Average velocity, in m/s, of flow through A2. Positive number.

Since the chosen volume remains completely filled with fluid at all times
due to the constraints on it, its center of mass never moves, indicating
zero net force. The helicopter never moves either, so the force the
helicopter exerts on that volume, the force any upflow exerts on it, and
the force of gravity on the helicopter must all sum to zero:

(3) Fz - F1 + F2 = 0
Whe
Fz: Gravitational force, in N, on helicopter. Positive number.

Substituting (1) and (2) into (3):

(4) Fz - M1d*V1 + M2d*V2 = 0

(5) M1d - M2d = 0
{Conservation of mass; that is, the net flow of mass through the
surface of any volume in an incompressible fluid must be zero. So
the net flow into our chosen volume must be zero.}

Solving for V2 in terms of M1d, V1, and Fz (exercise left for the reader;
see also note [1])) yields:

(6) V2 = V1 - Fz/M1d {for F1 Fz, i.e. V1 Fz/M1d}

Now an example:

Given a downward gravity force of
Fz = 10 N
with a downwash of
V1 = 6 m/s
M1d = 2 kg/s
the equations yield the following average upward flow values:
V2 = 1 m/s
M2d = 2 kg/s

Plugging those back into equations (4) and (5) indicates they are valid
solutions. Forces balance and momentum is conserved. Mass flow is
conserved (no sinks, no sources.) So the SOLID PART part of the earth
DOES NOT have to move up to insure conservation of momentum because part
of the FLUID PART of the earth has already moved up and done that job.

[1] The M1d and M2d variables actually contain V1 and V2 respectively;
e.g. M1d = rho*A1*V1 where rho = density of the fluid in kg/m^3 and A1 =
area in m^2 that V1 is measured through.

The only way to get momentum is to have mass in motion. That it is
greater and greater mass as the system evolves means it is moving more
and more slowly, but it is still moving until it can transfer its
momentum to something else.

You've contradicted yourself. You are now saying there is a reduction in
velocity of the downwash. That can only happen if a force is acting
upward on it. That upward force, that you inadvertently overlook, would
be due to the air itself. It has inertia and transmits the pressure
exerted on it to the ground much as a solid object would - at the speed
of sound in that material.

Here's what really happens: the instant the blades started to move the
first tiny bit of fluid downward, conservation of mass in a fluid
required an equal tiny bit of mass of the fluid to flow upward. The fluid
flow starts as a tiny closed circular-like flow and grows into a large
circular-like flow. Whatever their shape and size, the flows form closed
circuits.

Meanwhile, the fluid masses that are forced to move cause pressure waves
to move out at the speed of sound (which is infinite in a perfectly
incompressible flow) and it is those pressure changes that will appear
elsewhere, such as the earth's surface. The earth accelerates upward
until the downward pressure balances the upward gravitational pull
yielding net zero force: in the idealized case of a perfectly
incompressible fluid, the instantaneous rise in pressure at the surface
yields a net force just balancing the pull of the aircraft's mass and the
earth doesn't move upward at all.

I can't adjust my thinking to your way of thinking until you explain
which directions the fluid is flowing in my hypothetical case to my
satisfaction.

Your trying to obfuscate and I'm not buying it.

You obfuscate - I clarify: I've just presented a simplified mathematical
model (more than you've presented) and a better conceptual model for
incompressible flow as it relates to flight than yours.

You've already admitted ignorance of fluid mechanics when you said the
law of conservation of masses was only relevant to chemistry - so I
should really be charging you tuition.

Net force up on plane: net force down on air.

That's correct.

Net force down on air: net momentum down.

Transmitted at the speed of sound, not at the speed of the downwash.

Net momentum down: net velocity down.

Transmitted at the speed of sound, not at the speed of the downwash.

In article ,
Jim Logajan wrote:

(Since this was partially written a couple days ago, I decided to
complete it and post it anyway even though I'm sure some were hoping the
the discussion had died. :-))

Alan Baker wrote:
Jim Logajan wrote:
To that question I say: "Nothing in the physics rules it out." The
math appears to allow a solution wherein the fluid moves such that
the *forces*, *momentum*, and *mass flows* are *all* conserved _long
before_ any downwash strikes the earth.

Sorry, but you're wrong.

Tsk. Here's the math that shows there are solutions wherein momentum is
conserved yet yields zero average mass flow. It uses two conservation
laws (one of them being the first derivative of conservation of
momentum):

Where to begin? There is so much nonsense in this post.


A toy helicopter is hanging from a line in a sealed room initially at
rest, and is released from the line and engine started at the same time
so it remains stationary. The law of conservation of mass requires that
the net flow of mass through the surface of any volume in an
incompressible fluid must be zero. For this problem the volume of the
room bounded by the walls above the plane made by helicopter blades and
said plane is used for one of the needed constraints.

Well right off the bat, air is not an incompressible fluid.


The vertical forces through the plane made by the blades is described by
two areas: the area made by the blades (call it A1, in m^2) and the
remaining area of that same plane (call it A2, in m^2; the area from the
blade disk to the walls):

(1) F1 = M1d*V1
(2) F2 = M2d*V2

Whe
F1: Force of downwash through A1, in N. Positive number.
F2: Force of any upward flow through A2, in N. Positive number.
M1d: Average mass flow, in kg/s, of through A1. Positive number.
M2d: Average mass flow, in kg/s, of through A2. Positive number.
V1: Average velocity, in m/s, of flow through A1. Positive number.
V2: Average velocity, in m/s, of flow through A2. Positive number.

Since the chosen volume remains completely filled with fluid at all times
due to the constraints on it, its center of mass never moves, indicating
zero net force. The helicopter never moves either, so the force the
helicopter exerts on that volume, the force any upflow exerts on it, and
the force of gravity on the helicopter must all sum to zero:

The centre of mass of the notional volume never moves, no. The fluid
most definitely moves. You could say exactly the same thing about fluid
moving through a pipe. Would you claim that that fluid had no momementum?



(3) Fz - F1 + F2 = 0
Whe
Fz: Gravitational force, in N, on helicopter. Positive number.

Substituting (1) and (2) into (3):

(4) Fz - M1d*V1 + M2d*V2 = 0

(5) M1d - M2d = 0
{Conservation of mass; that is, the net flow of mass through the
surface of any volume in an incompressible fluid must be zero. So
the net flow into our chosen volume must be zero.}

Solving for V2 in terms of M1d, V1, and Fz (exercise left for the reader;
see also note [1])) yields:

(6) V2 = V1 - Fz/M1d {for F1 Fz, i.e. V1 Fz/M1d}

Now an example:

Given a downward gravity force of
Fz = 10 N
with a downwash of
V1 = 6 m/s
M1d = 2 kg/s
the equations yield the following average upward flow values:
V2 = 1 m/s
M2d = 2 kg/s

Plugging those back into equations (4) and (5) indicates they are valid
solutions. Forces balance and momentum is conserved. Mass flow is
conserved (no sinks, no sources.) So the SOLID PART part of the earth
DOES NOT have to move up to insure conservation of momentum because part
of the FLUID PART of the earth has already moved up and done that job.

I'm sorry, but you're wrong. Remove the atmosphere for a moment, the
force of gravity means that the helicopter exerts a force upward on the
earth equal to its own weight.


[1] The M1d and M2d variables actually contain V1 and V2 respectively;
e.g. M1d = rho*A1*V1 where rho = density of the fluid in kg/m^3 and A1 =
area in m^2 that V1 is measured through.

The only way to get momentum is to have mass in motion. That it is
greater and greater mass as the system evolves means it is moving more
and more slowly, but it is still moving until it can transfer its
momentum to something else.

You've contradicted yourself. You are now saying there is a reduction in
velocity of the downwash. That can only happen if a force is acting
upward on it. That upward force, that you inadvertently overlook, would
be due to the air itself. It has inertia and transmits the pressure
exerted on it to the ground much as a solid object would - at the speed
of sound in that material.

There is a force acting upward on it as the flow of downward moving air
encounters air that is not yet moving. But the force interaction between
the moving air and the non-moving air ensures that the total momentum is
unchanged. IOW, more air moving more slowly.



Here's what really happens: the instant the blades started to move the
first tiny bit of fluid downward, conservation of mass in a fluid
required an equal tiny bit of mass of the fluid to flow upward. The fluid
flow starts as a tiny closed circular-like flow and grows into a large
circular-like flow. Whatever their shape and size, the flows form closed
circuits.

No. Your conservation law means that when a bit of fluid exits a volume,
an equal amount must enter (we'll ignore for the moment that the law
actually only applies to incompressible fluids), it says nothing about
it having to be traveling in the opposite direction to that which exits.


Meanwhile, the fluid masses that are forced to move cause pressure waves
to move out at the speed of sound (which is infinite in a perfectly
incompressible flow)

Which, of course, air is not.

and it is those pressure changes that will appear
elsewhere, such as the earth's surface. The earth accelerates upward
until the downward pressure balances the upward gravitational pull
yielding net zero force: in the idealized case of a perfectly
incompressible fluid, the instantaneous rise in pressure at the surface
yields a net force just balancing the pull of the aircraft's mass and the
earth doesn't move upward at all.

Waves cannot carry momentum continuously.


I can't adjust my thinking to your way of thinking until you explain
which directions the fluid is flowing in my hypothetical case to my
satisfaction.

Your trying to obfuscate and I'm not buying it.

You obfuscate - I clarify: I've just presented a simplified mathematical
model (more than you've presented) and a better conceptual model for
incompressible flow as it relates to flight than yours.

You've already admitted ignorance of fluid mechanics when you said the
law of conservation of masses was only relevant to chemistry - so I
should really be charging you tuition.

Net force up on plane: net force down on air.

That's correct.

Net force down on air: net momentum down.

Transmitted at the speed of sound, not at the speed of the downwash.

Find me a single reference that agrees with you...


Net momentum down: net velocity down.

Transmitted at the speed of sound, not at the speed of the downwash.

Complete and utter nonsense.

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg

Alan Baker wrote:
Jim Logajan wrote:
Alan Baker wrote:
Net force down on air: net momentum down.
Transmitted at the speed of sound, not at the speed of the downwash.
Find me a single reference that agrees with you...

Since we have made differing claims that are actually testable by simple
experiments, why not use them to determine the real truth? Nothing else
would be as definitive at resolving the claims than an experiment simple
enough that anyone could run independently, so anyone having any doubts is
not slaved to trust the claims of others. The methodology would also be
open to independent analysis and critique. I.E. we use good old scientific
method.

I can think of a possible experiment that uses a large box (cardboard may
suffice) hanging from a ceiling from four vertical strings (or wires or
ropes) so that it can swing back and forth with two opposite ends that can
be opened and closed.

A small fan can be taped in place inside the center of the box and some
tests run - first with the ends opened (to measure the thrust of the fan
using some simple trig on displacement and known weight of the fan+box, and
thence compute the average wind speed of the fan) and then with the ends
closed. Based on how much the closed box swings when the fan is turned on,
one should be able to determine which of our theories is correct.

If your claim is correct, the box will swing more than what my claim
predicts (because the wind from the fan travels slower than the speed of
sound, so if you are correct the fan would push the box farther along in
its swing before the force of that wind hits the interior wall of the box.)
The approximate amount of swing (which could be recorded several simple
ways) should be predictable for both our theories based on several factors
that should be measurable using common equipment like a ruler and weight
scale (though a bathroom scale might not be precise enough to measure the
box+fan weight, though such scales are fairly common.)

So if you think the above is reasonable, or you need more insight into what
I am proposing before you commit, I am willing to flesh out the proposed
experiment in more detail.

In article ,
Jim Logajan wrote:

Alan Baker wrote:
Jim Logajan wrote:
Alan Baker wrote:
Net force down on air: net momentum down.
Transmitted at the speed of sound, not at the speed of the downwash.
Find me a single reference that agrees with you...

Since we have made differing claims that are actually testable by simple
experiments, why not use them to determine the real truth? Nothing else
would be as definitive at resolving the claims than an experiment simple
enough that anyone could run independently, so anyone having any doubts is
not slaved to trust the claims of others. The methodology would also be
open to independent analysis and critique. I.E. we use good old scientific
method.

It seems to me you're making a tacit admission that you can find no
supporting reference, and I note also that you've snipped by factual
refutations of your earlier "theory" about how air moves.

Why is this such a problem for you?

A fan on a table moves air, doesn't it? You can feel a column of moving
air coming from it, right?

Well once that air is moving it has momentum and all you can do with
that momentum is transfer it to something else.

If the air slows down as it move further away from the fan, it can't be
because the momentum just disappears. Momentum is always conserved.
ALWAYS. Therefore, if the air is moving more slowly, more air -- more
mass -- must be moving.

Period.

snipped deflection by proposed "experiment"

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg