Re: Archie a non-flyer? (was:Re: origin of bats/reply 2 to TMK)

[jrc <jrccea@bellsouth.net>]

----- Original Message ----- 
From: "Roberto Takata" <rmtakata@gmail.com>
To: "jrc" <jrccea@bellsouth.net>
Sent: Monday, June 30, 2008 10:46 AM
Subject: Re: Archie a non-flyer? (was:Re: origin of bats/reply 2 to TMK)


> > Why not?  As you know, the flow on the 'leading' side of the airfoil and 
> > aft
> > of the stagnation line (lower surface when developing upward lift) 
> > remains
> > fairly close to the surface of the airfoil and moves fairly directly aft
> > along the airfoil -- while the flow on the 'trailing' side of the airfoil
> > (upper surface when developing upward lift), reverses direction, goes 
> > around
> > the leading edge, moves aft along the outer perimeter of the seperation
> > bubble, thereby forming a cambered path that increases velocity and 
> > lowers
> > pressure on that surface.
>
> It can't be mechanical cause because velocity changes implies energy
> exchange (in this case, energy input). There is no reason to a longer
> path per se results in increased speed. (If so, the runner in the
> outer track in 400 m track running would be in advantage, and must be
> placed afterward and not forward other competitors.)

Again, no one ever said that a longer path results in increased speed.  I 
suggest that you read McCormick's text  in Aerodynamics, Aeronautics, and 
Flight Mechanics, Chapter 3, pages 66 through 69 (in the first edition) as 
one typical explanation of the acceleration creating lift on a flat plate 
that is lifting upward.  I quote from page 66, "In Figure 3.4b, observe that 
there is one streamline that divides the flow that passes over the plate 
from that below.  Along this "dividing streamline," the flow comes to rest 
at the stagnation point, where it joins perpendicular to the lower surface 
of the plate near the leading edge.  As the flow progresses forward along 
this line, it is unable to adhere to the surface around the sharp leading 
edge and seperates from the plate.  However, it is turned backward by the 
main flow and reattaches to the upper surface a short distance from the 
leading edge.  The resulting nonsymmetrical flow pattern causes the fluid 
particles to accelerate over the upper surface and decelerate over the lower 
surface.  Hence, from Bernoulli's equation, there is a decrease in air 
pressure above the plate and an increase below it.  This pressure difference 
acting on the airfoil produces a lift".

This is the process I have been describing to you throughout our exchange, 
and it works for both flat plates and cambered airfoils, thin or thick.  If 
the fluid dynamics text that you use to teach your students says anything 
much different from this, I suggest that you switch texts :-)

All the best,
JimC