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To: |
WM James Davis <> |
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Subject: |
Re: Flight dynamics. |
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From: |
Robert BERRY <> |
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Date: |
Mon, 15 Sep 1997 17:49:47 -0700 |
Dear James,
Stick with the short answer, its right. But .... consider an albatross
gliding into wind
L = Cl * 0.5 * rho * Vsquared * S
where my editor lacks super and subscript and greek characters and
L = Lift
Cl = coefficient of lift
rho = air density
V = relative velocity of air flow and aerofoil
S = surface area.
The coefficient of lift is a dirty little number that is specific to the
aerofoil
and the angle of attack. The angle of attack is the angle between ( say ) the
chord
of the aerofoil and the relative wind. Lift increases with the angle of attack
to a
certain limit. At this limit laminar flow departs, the wing stalls and lift
decays
to very low levels. You can get an appreciation of some of this by hanging your
hand
out of a moving car window.
In sustained level flight lift equals weight. If your Cessna 172 pilot wants to
fly
slowly he reduces propellor speed and slowly pulls back on the control yoke to
adopt
a greater angle of attack. Normal cruise is about 100 knots, taking advantage
of the
increased lift from increased angle of attack, level flight can be sustained at
about 45 knots. Now you're dead right that airspeed is important because if its
diminished any further your Cessna falls out of the sky. This is fine if you're
a
long way up because you point it downhill, push on the throttle, pick up speed
and
sort it all out.
Lets not exclude the primaries from consideration, let us rather accept the
fact
that the aerofoil section varies from root to tip. Each section will have a
different coefficient of lift and may even be at a different angle of attack
and
contributes a different amount of lift - so what?
As I hinted in epistle 1 there are ways to explain the pressure distribution
around
the wing but these are the result of the air striking the wing at a certain
speed
and angle - regard these as side effects, along with induced drag ( which I
have
avoided addressing ). Air is deflected down, the wing and its owner are
deflected
up. In flapping flight air is also deflected back, the bird is deflected
forward.
Newton!
Wing tip vortices cause drag for the producer and contribute nothing to lift.
They
are reduced in ground effect which Pelicans are masters of exploiting.
I recommend "Mechanics of Flight" by A. C. Kermode, I have the 9th ed. Longman
Scientific. I have never seen a decent account of flight in an Ornithology Text
but
would be pleased to know of any.
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