Re: Air resistance

[brian whatcott <inet@intellisys.net>]

At 12:00 12/10/97 -0400, you wrote:

>   It is true that a Phys-L thread should not degenerate into a dialogue.
>   If you think that a thread is interesting please indicate so, perhaps
>   in a form of a short sentence...                   Ludwik Kowalski
Before there was a NASA there was a NACA.
Before there was a NACA there was an ACA.
 It pleases me to compare the experiments reported in 
"Applied Aerodynamics" Bairstow, Longmans (1920)*
and the current Phys-l thread.

(* This has references to ACA papers and pieces in 
Phil Tran Roy Soc. and National Physics Lab (NPL))

At that time, scaling laws were still being developed.
 It is unfortunate that data are provided there only for smooth and 
stranded wire, struts and airships of various fineness ratios as 
measured in the 'air channels' of the time, rather than the spheres 
which would be of great interest.

It seems to me that this topic has great intrinsic interest, great 
historical value, and great didactic potential.

 It will be helpful to some fraction of the souls exposed to this material
to realise that the air resistance of an object does not remain a simple
function of cross-section, velocity, and air density.

 It will be helpful to realise that a machine that averages multiple
readings in some way can act to falsify the law relating those readings if
the law is not of an expected type: linear ramp, or power law...

 It is most helpful to see that well-intentioned people can come to widely
different estimates of the error bounds on their numerical conclusions.

I could wish that there were more people like me, not unwilling to expose
the great depths of their ignorance from time to time in an effort to
contribute something to a thread.

Above all, I would like to see a high-school teacher tape up a cardboard
'wind-channel' with a domestic fan to drive it, a variac (or light-dimmer
switch) to modulate the airspeed, a ball suspended by a knife blade in
 the flow - and a force gauge like a spring balance or some such to 
measure the air resistance over a 3 or 4:1 speed range....

The Wrights' device is still available for inspection, as I recall...

Sincerely,

brian whatcott <inet@intellisys.net>
Altus OK