Backwash Causes Lift?

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.

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

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.

writes:

Once again, thou knowest not of what thou speakest.

Several others from the treehouse club here have told you the same thing.

No positive AOA, no lift. Learning this is extremely useful to pilots.

I just told you, in apost not long ago, that some airfoils will generate
lift at up to -4° AOA.

No airfoil generates lift at a negative angle of attack.

Here's a graph that shows lift being generated
on some anonymous airfoil at -5°:
http://en.wikipedia.org/wiki/Lift_coefficient

Would you like me to draw a graph that shows something different and upload it
in place of that one?

wrote:
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


Don't bother. Believe me it's not worth it. You can talk this guy to
death and all he will do is count you as another response.

Angle of attack as we all know (with one exception it seems :-) can be
both positive or negative.
In fact, in high performance jets with a fuselage loaded IYMP, entering
a coupled spin after a departure, it's extremely disorienting if the
aircraft goes through PSG and stabilizes in an inverted spin mode where
yaw is opposite to roll. g is a bad indicator as with a fuselage loaded
IYMP you can get negative g either erect or inverted.
The way we deal with this is through instrument interpretation rather
than trying to eyeball what's happening, which can be next to impossible.
The AOA indicator in the aircraft has a positive and negative side. If
the AOA is stabilized at some value on the negative side, and the
airspeed is stabilized at some mean low value, the spin is inverted. The
turn needle will show spin direction either way.

Again, don't waste your time.

--
Dudley Henriques

Mr Dudley, once again you are confused. Most define angle of attack as
the chord line of a wing, and of course with that definition it can be
negative and still generate lift. Mr Mx chooses a different way of
defining it. It is some angle such that when it goes negative the
airfoil can generate no lift. Do you remember the disbarred former
president Clinton saying something about "It depends on what 'is'
means"?

In Mx's case, words change meaning so that he is NEVER wrong. It must
be an interesting version of English he teaches.

But he does offer amusement for some of us on otherwise humorless
days, doesn't he?

Tina wrote:
Mr Dudley, once again you are confused. Most define angle of attack as
the chord line of a wing, and of course with that definition it can be
negative and still generate lift. Mr Mx chooses a different way of
defining it. It is some angle such that when it goes negative the
airfoil can generate no lift. Do you remember the disbarred former
president Clinton saying something about "It depends on what 'is'
means"?

In Mx's case, words change meaning so that he is NEVER wrong. It must
be an interesting version of English he teaches.

But he does offer amusement for some of us on otherwise humorless
days, doesn't he?



AOA actually can be defined relative to any given reference datum, but
normally it's considered in the industry as being the angle formed
between the chord line of the wing and the relative wind as you have
correctly stated.

Quite frankly, I read what Mxemanic writes on occasion and can't figure
out how he can be so close to getting it right and still manage to get
it wrong. He's amazing, and an interesting study if nothing else.
It's too bad he's taken this path on these groups. I've always felt he
has a genuine interest in things aviation and would like to contribute,
but he seems to be such a jerk that he gets in his own way.

--
Dudley Henriques

On Oct 4, 6:39 am, Tina wrote:
Still waiting for the conservation of momentum derivation. My husband,
also trained as an engineer, casually remarked he didn't think you
could get from Newton's First Law to the that confirms my memory,
but we are both willing to have that belief rebutted.

My apologies for broaching the subject. Frankly, I would rather save
it for the physicists.

He also pointed out that how a CFI might explain how a VOR works would
not satisfy an engineer. For that matter, the physics of flight as
explained to a student pilot would not satisfy someone who might be
interested in designing, as opposed to flying, an airplane, but I
don't think the manuals you are looking at are in error.

If the manuals are in error, then they are in error. If the manual
issues a disclaimer, saying something, like, "this is not really what
is happening, but this will suffice for us..." that would be ok.
That's not what's happening. The manual mentions things like
Bernoulli, Newtons laws of motion. It even uses vector notation for a
few of the formula's. When one gets that close to the merchandise,
they need to purchase it.

I would point
out that each field has its own language, and you denying the
conventions used in aviation -- drag, lift and so on -- demonstrates
an unbecoming trait for a student, and even a worse one for an
employee. You may want to rething that attitude if you use it in real
life.

There is a difference between convention and errononeous information.
I never discounted drag, lift, or so on...I discounted the
explanations given some of my flight education materials. If it's
wrong, it's wrong. If someone reading it gets comfort from thinking
they understand, or whatever, that's fine for that person. But the
writers of those manuals should know that their audience is broad, and
should not publish erroneous information (after they know that it is
erroneous). There is gross difference between explaining something in
simple terms and being correct, versus explaining something in
moderately-difficult terms, and being incorrect.
I could probably explain VOR to a 10-year-old, without ever mentioning
things like counters, angular frequency, anisotropic radiation,
frequency bands, sub-carriers, convolution, etc....and my explanation
would still be correct.

"When you drink from a straw, there is no suction force."

-Le Chaud Lapin-

Tina wrote:
Still waiting for the conservation of momentum derivation. My husband,
also trained as an engineer, casually remarked he didn't think you
could get from Newton's First Law to the that confirms my memory,
but we are both willing to have that belief rebutted.

You can't get conservation of momentum from F = m*a (or vice versa) since
the latter is not a statement about anything being conserved. But you can
get "For every force there is an equal opposite force" from conservation
of momentum, and vice versa, with a small number of assumptions. You can
use derivatives to derive one way and integration to derive the other.
Here are several conservation laws that share a common derivation,
starting with:

Center of Mass is Conserved
---------------------------
Center of mass of a closed system of particles of mass m1, m2, m3, ... mn
must remain fixed for all time, which with a suitable selection of
coordinate origins may be stated mathematically as:

(a) m1*x1 + m2*x2 + m3*x3 + ... = 0
m1*y1 + m2*y2 + m3*y3 + ... = 0
m1*z1 + m2*z2 + m3*z3 + ... = 0

Note that this doesn't say that, for example, x1 can't vary with time. It
only says that if it does then m1, m2, m3, x2 or x3 or other masses or
positions must somehow change so the left hand side still remains zero.

Momentum is Conserved
---------------------
If position with respect to time is continuous (no discontinuities; e.g.
no jumps) then we can take the time derivative of the above, yielding:

(b) m1*dx1/dt + m2*dx2/dt + m3*dx3/dt + ... = 0
(And so on for the other coordinate axis.)

This is of course just the conservation of momentum equations because
dx1/dt = Vx1, a velocity. Note that d(m1*x1)/dt would have been more
appropriate if the mass of particles varies with time.

Force is Conserved
------------------
Given the continuity assumption above, then we can keep taking time
derivatives of the above, yielding the next conservation statement:

(c) m1*d^2(x1)/dt^2 + m2*d^2(x2)/dt^2 + m3*d^2(x3)/dt^2 + ... = 0
(And so on for the other coordinate axis.)

This is of course just the old equal and opposite action statement in
mathematical form because d^2(x1)/dt^2 = ax1, an acceleration.

So if one claims any _one_ of the above conservation laws exists then the
other two appear to follow with only a small set of (presumably
reasonable) assumptions.