Starve.Org White Noise Usenet Archive: "Re: How airplanes fly (long)"

In article <24rio3$...@agate.berkeley.edu> isaac...@chaos.berkeley.edu (Isaac Kuo) writes:

>In article <1993Aug16.144516.17...@cas.org>, <sd...@cas.org> wrote:
>>In article <24j93b$...@agate.berkeley.edu> isaac...@chaos.berkeley.edu (Isaac Kuo) writes:
>>>You say that the downward deflection of air is an insiginificant portion
>>>of the lift. Then consider this "helicopter:"
>
>>>The rotor is a pair of level airfoils, designed to fly in level flight
>>>at zero angle of attack. Now to insure that none of the lift generated
>>>comes from the downward deflection of air, the rest of the rotor disk
>>>is "filled in" by a thin flat circular membrane. A cross section would
>>>look like this:
>>> ,----___
>>>------<________>--------------
>
>>> ^airfoil ^membrane
>
>>>The question is this: Now that the effect of downward deflection is
>>>nullified, can this rotor hover? Can this rotor generate any lift?
>
>>>The answer is _no_. The only "lift" this rotor can generate at any
>>>speed is the buoyancy from its displacement (now _that's_ an insignificant
>>>portion). As the other person responding to this discussion
>>>mentionned, from a "systems" point of view, the lift must come,
>>>directly or indirectly, from the downward deflection of air.
>
>>If you think about your modification of your original position
>>to the point we are at now, you did acknowledge that Bernoulli
>>did work after all, and you then said that the downdraft is
>>initiated by a downward movement of air towards the low pressure
>>area *over* the wing.
>
>My original position is unchanged. You may have interpreted some weird
>extras--like an imaginary claim that there's no pressure loss over the
>wings, or that the downward deflection of air results in a strong downward
>wind--but the truth is that I never said any such things.
>
The original position I refer to is that taken when you were posting
in rec.arts.sf.movies. I acknowledge that I interpreted your statement
that the Bernoulli principle is only applicable to "closed pipes" to
mean that there could not be a low pressure region generated over a
wing. If that was not your intent when you were ridiculing my position
that a low pressure region formed over the wing is responsible for
most lift production, I apologize.

As for your statements regarding downward deflection of air...
You say in the same post in which you make this comment that, unless
all the air set in motion, whether above or below the wing,
can move below the wing, there is no lift. At least that is the only
way I can interpret your statement that a membrane in that plane will
cancel out lift. The statement is reproduced below, and no doubt you
will enlighten me on an alternate interpretation, but it seems to me
that I am justified in reading this as you stating that a downdraft
(in the sense of air being pushed/pulled - your choice of verb - below
the wing plane) - with a momentum equal to that required to produce the
lift is required for flight. Yet, in another post to another poster,
you acknowledge that there are several effects which could cause
the downward moving air to be stopped above the wing without affecting
lift. You are willing to agree with others that the requirements of
momentum balance don't require air to move below the wing, but you
won't agree to that with me. Odd.

>> By your own logic, then, you can see this
>>should generate some lift, since the membrane would not affect the
>>downward movement of air *above* the airfoil and since the membrane
>>would not be hit by all the downward moving air.
>
>By my own logic, this rotor does not generate any lift. That membrane
>is there for one and only one reason--to prevent any downward movement
>of air in the area immediately around the airfoil. The membrane needn't
>be bombarded by every air molecule directly in order to stop its downward
>motion. It's effects are transmitted through the atmosphere by pressure.
>
>> It would be
>>a weak lift, I'll grant. (This presumes that your rotor is sufficiently
>>small that you get the same equilibration at the trailing edge that
>>you would get absent the membrane, and that by "zero angle of attack"
>>you actually mean the angle at which, absent the membrane, there is
>>no deflection of the trailing edge flow lines.)
>
>By zero angle of attack, I mean that the trailing edge is directly
>horizontal to the leading edge--in absence of a hard edge, this is taken
>to be the point separating the airflows going above and below the wing.
>Thus, the only effect of the membrane is to prevent any downwash and
>"straighten out" the flow lines. This naturally presses down on the
>membrane; by your argument the amount is insignificant so the rotor will
>still produce a lot of lift.

I said "a weak lift" - you translated this to a lot of lift. Perhaps
the whole problem here is that we use words differently - I say small,
you read large; you write large, meaning small, but I read it as large.
If I read your series of posts with this in mind, they make a lot more
sense.

Downward moving air reaching the membrane would cancel out the effects of
upward momentum transfer imparted to the wing by the effects of the low
pressure area adjacent the upper wing. It would not cancel out the lift due
to the pressure difference, or the lift due to momentum transfer from
downward moving air that does not impact the membrane or the rotor
(You've agreed that all the downward moving air in real systems
doesn't move below the air foil plane).

Note that I am presuming here that the membrane, in addition to being
incredibly thin, is also rigid, since with a flexible membrane you would
have to account for the effects on the membrane of air from below the
wing trying to "fold over" into the low pressure region above the wing.
>
>By my argument, it won't produce any lift at all (discounting the edge
>effects).
>
>>As for this being a good model of a fixed wing:
>
>>Again, you are overlooking several points. The most obvious is
>>that the airfoils on a helicopter *rotate.* The tips will move
>>more rapidly than points further inside the airfoil (all points
>>have the same angular velocity so for greater radius you have
>>a greater equivalent linear velocity). For a fixed wing, all
>>points on the airfoil move at the same velocity, creating a
>>uniform "relative wind velocity" across the whole of the wing.
>>This fundamental difference between fixed and rotary wing
>>craft explains a lot of their individual characteristics.
>
>As another pointed out, there is no difference in the principles
>behind which a helicoptor wing and airplane wing work. That
>"fundamental difference" goes a long way toward explaining why
>helicopter rotors and airplane wings have different overall
>shapes, but the principles are identical. Anyway, most helicopter lift
>comes from the outside third of the rotors, and the relative
>wind is pretty constant there.
^^^^^^ ^^^^^^^^
(More evidence for my theory of different understandings of
the meaning of magnitudes between us.)

Didn't say there was a difference in the working principles.
Did say there was a difference in the characteristics.
Hint: where does most of the downwash for a hovering helicopter
originate?

>
>>When you get back to Berkeley, read up on tip effects. Then
>>read up on rotating airfoils. This will give you a greater
>
>Please. I understand helicopter control and aerodynamics.
>But the hypothetical rotor I am talking about has little to
>do with real helicopters since it can't be used on a real
>helicopter.

If you understand helicopter aerodynamics, you will recognize
that your hypothetical rotor would still produce a tip
vortex downdraft, even if everything else canceled...

>--
> /["o"]|8 Isaac Kuo (isaac...@math.berkeley.edu)
> ,^-----^==_ "... his most ambitious record breaking escapade to date,
>/___________\ because Richard Branson is attempting to go 24 hours
>\=\>-----</=/ without publicity." Mel Smith, Smith & Jones

******************************************************************************
Renegade academician. They're a dangerous breed when they go feral,
academics are...a chemist, too.
-(James P. Blaylock in "Lord Kelvin's Machine")

My organization hasn't agreed with any of my opinions so far, and
I doubt they'll start now.

Stanley Roberts