Re: Physics of Flight: inferring the pressure

[John Denker <jsd@MONMOUTH.COM>]

At 11:55 AM 8/12/99 -0400, Michael Edmiston wrote in part:

> once you have a particular
> airfoil in a wind tunnel at some angle of attack and some overall air
> velocity, why measure velocity differences and calculate lift when you
> can directly measure lift (and drag) from your airfoil support
> mechanism?

An excellent question.   The answer is that the support mechanism measures
the lift and drag of the wing as a whole.  That is OK for a start, but for
research purposes one typically wants to know which parts of the wing are
contributing more or less than their share.  To do that you want the
pressure field everywhere.  It may well be expedient to measure the
velocity field and then infer the pressure field.

> to use [Bernoulli's principle] you have to know the velocities.

Yes indeed.

> These velocities are best measured in a wind tunnel.

What is "best" depends on circumstances.  A wind tunnel is a fine way, but
not the only way.  There are ways of calculating the velocities
analytically.  This is easy for simple shapes like a cylinder, and
moderately easy for a wide class of shapes (including airfoil shapes) that
can be cobbled up using conformal transformations.