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




----- 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