Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Judgement on opposing airfoil views pt. 2

On Fri, 27 Aug 1999, Timothy Folkerts wrote:

I still keep coming back to the bird in the cage, or a helicopter in a
hanger.

The bird steps off the perch and starts to hover (must be a humming bird).
It pushes one puff or air down, followed by another, and the bird finds
itself in equilibrium. But soon the downdraft will set up a circulation
pattern - up along the sides and back down the middle. So now the bird is
flying in a downdraft (which it created itself), which means it needs to
flap harder, which sets up a bigger downdraft ....

AHA! That's the problem! There's a flaw in the above description because
you're imagining that the cage is so small that its walls interact with
the air near the bird and "focus" the downdraft back onto the bird. In a
similar fashion, we could show that if we put a tight box around the
rotating blades of a helicopter, the lifting would vanish. Therefore
instead imagine that a tiny fly is hovering in the center of a huge
plexiglas tank having a size of about 10 feet on a side. Put the whole
tank on scales, and the scales detects the weight of the fly whether it
hovers or not.

The fly throws a stream of air downwards. It spreads and mixes
turbulently. It never quite arrives at the ground, but it spreads, slows
by viscous contact with the ground, and eventually all of the air within a
few feet of the sides of the tank is moving upwards, and a broad region in
the center of the tank is moving slightly downwards. (The tank is no
longer "focusing" the downwash back onto the fly.) The fly must flap very
slightly harder to counter this slight increase. Because the fly is
flapping only *slightly* harder, the next round of increase in upwards
velocity of the air in the tank will be that much smaller. The
speed-increases fall rapidly to zero.


(And all this time, a scale under the cage will read less than the total
weight! The bird an cage may be stationary, but the air is getting
compressed below the bird, so the c.m is still falling, so the normal force
from the scale doesn't have to hold the whole weight.)

Ideally the air need not be compressed below the bird. (I didn't really
grasp this point earlier myself). When a solid surface deflects an air
stream, the air is not compressed, even though the solid surface feels a
significant pressure. The air pushes upon the solid surface, and the
solid surface experiences pressure, but then the solid surface pushes upon
the air, *then the air deflects as a result.* Instead of having its
pressure increased, the air just gets accelerated and its direction of
flow is changed.


Where does it stop? I think the answer is in the pressure. Eventually the
air below the bird gets compressed by all that air getting pushed down. I
suppose you could think of it as flying in ground effect, since the bird
gets enough additional lift from the air molecules that are bouncing back
up to overcome the downward force from the downdraft.

If the bird is so close to the bottom of the cage that back-pressure
becomes an issue then that is ground effect flight. But ground-effect
flight is very different than normal flight. To solve the problem, think
of that tiny fly in that large tank. If the whole tank was on scales, we
would see the weight of the fly even when it was hovering.


(It is only at this point that equilibrium is truly achieved, and the scale
under the cage will truly read the correct weight.)


AHA! moment (I think). Even level flight is an intrinsically
non-equilibrium state and any explanation of flight must at least recognize
this.

Yeah! It's like a hovering rocket. It's a dynamical process.


If I might be so bold as to go way out on a limb... Bill's model
corresponds best to the bird just jumping off the perch. The downward puff
holds the bird up, but it is not a truly equilibrium state. (Of course in
real life, the plane simply flies to a new "cage", i.e. an undisturbed
parcel of air and continues its non-equilibrium state.)

Or use a bigger cage which has lots of undisturbed parcels.

John's model more
accurately looks at the bird in true equilibrium, where only the
interaction of the air with the ground can ultimately hold the plane and
the air off the ground. (But other than ground effect, this is not really
close to the situation of normal flight.)


Right, and that is the central flaw in John's reasoning: it only explains
ground effect flight. John's model cannot explain why airplanes fly in
general. He is convinced that upwash causes lift and downwash causes
lift, but this violates Newton's laws. But he does not see this. Nex he
is saying that the Anderson/Eberhardt paper is wrong, and he is suggesting
to outside webmasters that it be removed from their lists of links. When
I see that occur, the kid gloves come off. In any situation it would be a
violation of scientific ethics.

In my opinion John Denker is steadfastedly and absolutely refusing to
accept that any flaws could exist in his own reasoning, yet he sees that
there is a big problem *somewhere.* He mistakenly decides that
Anderson/Eberhardt have the problem. He so deeply believes that this must
be true that he is moved to try to put a stop to the growing popularity of
their paper.

What will John do if he ever perceives the flaws in his arguments? I'd
certainly hate to be in his shoes. It will mean that his actions against
Anderson/Eberhardt were unjustified, and that his long-running refusal to
think through his own reasoning has caused him to do a Very Bad Thing.
Things like this have destroyed lesser people in the past. Yet when you
come right down to it, they have nobody but themselves to blame.


((((((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://www.amasci.com
EE/programmer/sci-exhibits science projects, tesla, weird science
Seattle, WA 206-781-3320 freenrg-L taoshum-L vortex-L webhead-L