Date: Wed, 30 Dec 1998 16:11:21 -0800 (PST)
Message-Id: <l03102800b2afb821c925@[209.142.17.164]>
In-Reply-To: <199812230916.BAA23423@mail.inreach.com>
To: "Pinhole Listserv" <pinhole@exploratorium.edu>
From: Ronald Wong <ronwong@inreach.com>
Subject: Re: Flying & wing shapes
Re: Flying & wing shapes - Pinhole Digest #110 - 12/23/, "BenP" said:
>One factor not discussed in the original transmission, or the response
>about wings, is the "angle of attack" of the wing. The angle of attack is
>the angle between a line which traces the airflow past the plane (the
>relative wind), and a line drawn from the leading edge to the trailing
>edge of the wing (the wing's Chordline). An aircraft's wings do not
>necessarily travel through the air in a level orientation. For example,
>if the leading edge of the wing is higher than the trailing edge, this
>results in an angle of attack greater than zero.
>
The reason the "angle of attack" was not brought up in my response to the
original question was due to the fact that Bernoulli's Principle does not
explain the flight characteristics of a wing in even the simplest case,
when the angle of attack is zero, let alone the more typical cases as was
ably described by "BenP".
The most common explanation using Bernoulli's Principle involves an
asymmetrical wing where the bottom is flat and the top is convex. The
curvature of the top is greater near the front and tapers back towards the
trailing edge. The wing is usually oriented so that the bottom is
horizontal as is its motion through the air (i.e. zero angle of attack) .
To understand how the principle is applied, think of the following (better
yet, do it): Fold a towel in half and place it on a table. Place your
right hand down on the crease so that the towel will stay in place. Place
your left hand, palm down (this represents the lower, flat surface of the
"wing"), within the folded towel with the thumb up against the crease (or
as close to it as possible). As you move your left hand to the left, the
lower part of the towel will remain in place while the upper part will move
over the top of your hand (the convex surface) and come back down TO ITS
ORIGINAL POSITION on the bottom half of the towel.
The fact that the part that moves over the top of the wing ends up back at
its original position relative to the bottom part is important to this
explanation. It means that the time of passage is the SAME from front to
back no matter which path was taken. Since the part that traveled from the
front to the back over the top of the wing traveled a greater distance
during this time, it must have moved faster than the part that passed under
the wing. It's this difference in speed that allows the disciples of
Bernoulli's Principle to explain lift. The difference in speed means a
difference in air pressure on the wings. The greater speed above the wing
relative to the bottom means less air pressure on the top relative to the
bottom - Bernoulli's Principle - and, as a result, lift takes place.
You can usually tell when Bernoulli's Principle is being offered to explain
how airplanes fly by simply looking at the diagram that accompanies the
explanation. A series of horizontal lines are drawn . The cross section of
a wing is placed in the middle of the pattern. The horizontal lines below
the wing remain horizontal while the others are displaced so that they pass
over the top of the wing and, once past the trailing edge of the wing,
assume a horizontal position once again.
When the technology improved enough to where scientists and engineers could
actually observe the motion of the air molecules over the surface of a wing
with greater precision, they discovered two things:
1. The time it takes for the air molecules to pass OVER the wing is NOT the
same as the time for the air molecules to pass UNDERNEATH the wing. It's
longer.
2. There is a net downward deflection of the air as a result of the wing's
passage through the air.
So much for the Bernoulli's Principle as an explanation for flight (don't
blame Bernoulli for this one, he died long before mankind had come up with
winged flight).
Under these circumstances it's far simpler to invoke Newton's third law as
an explanation. In fact, one of the "gotcha's" of the day is to go through
current physics and physical science text books to see if the air flow past
the trailing edge of a wing is drawn sloping downwards. I've come across
articles where the author went to some length to explain the inadequacies
of Bernoulli's Principle as it applied to flight and then offered a sketch
where the air flow after passing over the wing was the same before.
------------------------------------------
>Even if a wing is symmetrical relative to the surface formed by the
>infinite number of chordlines along the length of the wing, it may not be
>symmetrical with respect to the relative wind. <snip>...Also, because of
>the shifted stagnation point, there is an area of relatively high pressure
>below the leading edge of the wing, adding to the lift.
>
All of this is true, but the discussion gets far more entertaining when you
introduced all of these other factors. The role of vortices, laminar flow,
adhesion, and viscosity become major factors in the discussion. Despite
these other factors, the issue of time and relative speed remain the same.
The bottom line still remains that Newton's third law is a simpler
explanation than the principle of Bernoulli. The effect of the air passing
over the wing's surfaces leads to a net downward displacement of the air by
the wing. If the wing deflects the air downwards, then the air must deflect
the wing upwards.
---------------------------------------------
>As the angle of attack increases, the ratio of the upper surface area :
>lower surface area will increase. A craft with wings designed for
>inverted flight (as 'symmetrical' ones likely are) ...
>
It's true that planes designed for acrobatic maneuvers have symmetrical
wings but, as "BenP"'s discussion would lead you to believe, any wing -
symmetrical or not - will allow one to fly inverted if the angle of attack
is correct for it's shape.
Enthusiasts of radio control airplanes are well aware of this. Many have
built models with traditional asymmetrical wings (like a classic Piper Cub)
and have flown them inverted with no trouble at all despite the fact that
the flat surface of the wing is uppermost and the curved surface is facing
downwards.
Nature supplies the most interesting example of this phenomena. The upper
part of a bird's wing is convex and the underside concave. Amazingly
enough, some species, known for their aerial acrobatics, can fly inverted
for some distance and do so apparently just for the fun of it even though
the elements of their "wing" that make such flight possible are now the
complement of what they would normally be.
----------------------------------------
>In sum, <snip>... Therefore I don't believe the theories of Bernoulli, et
>al. are brought into question by the existence of 'symmetrical' wings.
>
The theories of Bernoulli aren't in question. It's just the application of
one of his theories to flight that has been found wanting in the last
thirty years or so.
Newton's third law. It's simple, short, consistent with the observations
and will leave everyone happy.
ron