Backwash Causes Lift?

On Oct 3, 9:26 pm, TheSmokingGnu
wrote:
Le Chaud Lapin wrote:
You can have lift of an object with no Bernoulli. It's simple vector
addition.

1. You are talking about nature's abhorrence of a vacuum.
2. Vacuum abhorrence is not lift.
3. Airplanes do not generate lift as a result of vacuum abhorrence.
----------
Conclusion: you are not talking about how aircraft generate lift.

Yes, I am. It's a combination of many things taking place at once.
Vacuum generation by the forward motion of the wing is one of them.

QED.

-LCL-

Le Chaud Lapin wrote:
Yes, I am. It's a combination of many things taking place at once.
Vacuum generation by the forward motion of the wing is one of them.

Then why do wings generate lift at negative AOA? Surely the immense
vacuum pressures generated would immediately pull any flying craft
desperately into the Earth the moment the wing crossed that threshold
(say, in a descent).

My goodness, it's a good thing you got on here to tell us all this;
imagine all those airliners going overhead that have been doing it wrong
all this time, actually descending to a destination. They ought very
well to know that they could never do such a thing because the vacuum
pressures won't allow it!

TheSmokingGnu

On Oct 3, 9:26 pm, TheSmokingGnu
wrote:
Le Chaud Lapin wrote:
Yes, I am. It's a combination of many things taking place at once.
Vacuum generation by the forward motion of the wing is one of them.

And the other is the displacement of air downward.


Then why do wings generate lift at negative AOA? Surely the immense
vacuum pressures generated would immediately pull any flying craft
desperately into the Earth the moment the wing crossed that threshold
(say, in a descent).

Must be careful not to mix attitude up with angle of attack. The
path of the wing's chord line through the air determines AOA. If the
airplane is pointed downward a bit so that the chord line is down 2°
with respect to the horizon, and the descent path of the airplane is
3°, the AOA is still 1°. An airplane in a steep climb, with its nose
up 20°, does not have a 20° AOA. Its flight path is upward at maybe
10° so that its AOA is only 10°.

Some wings (thick, heavily cambered wings) will generate lift
at up to -4° AOA. The bottom surface of the wing is not the chord
line; that's the line between the leading and trailing edges. The
bottom surface might be angled downward even more in level flight. the
old Champ was a good example: the bottom surface was angled quite
visibly down in level cruise flight, but the chord line was still at a
positive degree or two.

Admittedly there are instructors who don't understand this
stuff well at all and think they know more than they do. I'm still
learning 34 years after starting to fly. I'm old enough now to realize
how little I knew when I thought I knew it all, and to know that I'll
now never have a good handle on it all. Too little time and too many
other responsibilities. But private pilots need to have the basics,
because that's all they have time for and because they'll kill
themselves without them. I'm appalled when I see a pilot do a low-and-
over and yank back hard for the vertical zoom. They have no idea how
close they come to an accelerated stall doing that. Those that manage
to get the stall don't live to avoid the same mistake again, and the
accident report gives a bland, uninteresting and uninformative "pilot
lost control in the climb after the low pass." They don't give the
real reason: the pilot did not understand AOA, never did, and thought
he was safe because the airplane's speed was well above the stall
speed.


Dan

On Oct 3, 10:48 pm, wrote:
On Oct 3, 9:26 pm, TheSmokingGnu

wrote:
Le Chaud Lapin wrote:
Yes, I am. It's a combination of many things taking place at once.
Vacuum generation by the forward motion of the wing is one of them.

And the other is the displacement of air downward.

By the bottom part of the wing, right?

Remember, when Newton was talking about the whole action/reaction
thing, he did not say you could arbitrarily define the sources of
forces. He was talking specifically about two objects, A and B, A
generating a force on B, and B generating a reciprocal force on A.

If you have compression under a wing do to extended flaps and laminar
friction of airflow, for example, then the lower surface of the wing
forces air downward, and the air beneath the lower surface forces wing
upward.

If you have have downwash above the wing, the downwash has to be due
to a force acting to move the air downward. I've argued that it is
effectively normal atmospheric pressure, acting against what
effectively becomes a partial vacuum generated by the forward
displacement of the wing, above the wing. Newton did not say that you
could arbitarily say, "Oh, there is some air moving downward, I'll
just pick a convenient reason arbitrarily."

Another way to look at this is to imagine a "level" wing with heavy
flap extension. Have an "air gorilla" move the entire wing forward, in
an abrupt motion, not given air anytime to redistributed. If this is
done, there will be compression beneath the wing, strong at the
boundary of compression, or in the flap pocket. Behind the win (above
it, but behind flaps), there will be ....

a huge void!

Now, if air is suddenly allowed to flow, yes indeed, there would be
downwash above the wing into the void, but the wing itself will not be
causing this downwash. It will be the pressure surrounding the void
causing the downwash. Since the source of movement of this air is not
the wing but the air above it, Newton's law cannot be used will-nilly
to say thay that there was some kind of action, so this must be the
reaction. You have to attribute the forces to their sources.

In this case, someone said Newtons law had to be use under penalty of
death, it would be simple:

Take a thin layer or air right at the boundary between the void and
ambient air. If another thin layer of ambient air pushes against this
thin layer, the thin layer will will push back against the ambient
thin layer. This is reciprocity of forces. The reason that the first
thin layer "loses" the pushing battle is because there nothing to
oppose the first thin layer as it moves into the void. The molecules
of the second thin layer has its friends to contend with. After the
first thin layer has moved into the void, those molecules can no
longer participate in pushing at the ambient air (because they have
assumed new position in space - neither air nor people are
telekinetic) and thus we get air flow.

Of course, there are not layers, but a distribution of momentum of the
particles, but this is close enough.

So in summary, downwash cause by high ambient pressure confronting a
void must not be used to contribute to lifting force of the wing. One
_can_ say that the pressure under the wing see no opposing force in
the relative void above the wing does result in net upward force.

-Le Chaud Lapin-

Le Chaud Lapin writes:

By the bottom part of the wing, right?

Any displacement of mass downward will produce a matching upward forced. You
could generate lift of a sort by launching rocks off the wing, but you'd soon
run out of rocks so that's not very practical. But there's plenty of air
mass, so if you can find a way to divert it, you can generate lift.

If you have compression under a wing do to extended flaps and laminar
friction of airflow, for example, then the lower surface of the wing
forces air downward, and the air beneath the lower surface forces wing
upward.

In reality, the high pressure effects below the airfoil are almost
insignificant. The lift is generated mostly by the diversion of air flowing
over the wing downward.

Why is this all so important?

Interested readers might enjoy looking at alphatrainers.com for a
discussion of lift. Mx's assertion that lift is mostly the result of
downwash flies (pardon the pun) in the face of 'center of lift'
analysis which in effect is that point on the wing where if for
balance considerations the integrated upward forces were concentrated
they could be considered to be operating at a point. If downwash, the
center of action of which is somewhat aft of the following edge of the
wing, was the major contributer of lift, one would expect the center
of lift to be in that area -- aft of the wing. It's not.

But what do I know, I'm just a psychologist -- with a minor in
physics.

Tina writes:

Interested readers might enjoy looking at alphatrainers.com for a
discussion of lift. Mx's assertion that lift is mostly the result of
downwash flies (pardon the pun) in the face of 'center of lift'
analysis which in effect is that point on the wing where if for
balance considerations the integrated upward forces were concentrated
they could be considered to be operating at a point. If downwash, the
center of action of which is somewhat aft of the following edge of the
wing, was the major contributer of lift, one would expect the center
of lift to be in that area -- aft of the wing. It's not.

I don't understand how you reached this conclusion. It's a bit like saying
that all of the planet Earth must be massless except for a dimensionless point
at its center, since that is where the center of gravity is.

But what do I know, I'm just a psychologist -- with a minor in
physics.

Knowledge is more important than credentials.

TheSmokingGnu wrote:


Then why do wings generate lift at negative AOA? Surely the immense
vacuum pressures generated would immediately pull any flying craft
desperately into the Earth the moment the wing crossed that threshold
(say, in a descent).

Think "Relative Wind". Then rethink negative AOA.

TheSmokingGnu writes:

Then why do wings generate lift at negative AOA?

They don't. That's a very common misconception, even among pilots.

The effective AOA is always positive when the wing is generating lift.

On Oct 4, 6:27 am, Mxsmanic wrote:
TheSmokingGnu writes:
Then why do wings generate lift at negative AOA?

They don't. That's a very common misconception, even among pilots.

The effective AOA is always positive when the wing is generating lift.

Once again, thou knowest not of what thou speakest. I
just told you, in apost not long ago, that some airfoils will generate
lift at up to -4° AOA. Here's a graph that shows lift being generated
on some anonymous airfoil at -5°:
http://en.wikipedia.org/wiki/Lift_coefficient
Bernoulli at work. Newton, too, because there's downwash being
generated.

Dan