Bill,
Firstly, us Brits are entirely conversant with imperial units, and you'll
find that probably most people over 40 (and plenty of us under that age)
prefer them. Bit of a sore point with some of the population (a "metric
martyr" grocer was prosecuted for pricing his fruit in lb and not kg!) but
don't get me going on this issue... (but thanks for translating
"plexiglass" to "perspex"!!!)
Being an engineer with an extremely vague grasp of aerodynamics, I am not
that surprised at all that 12" makes a difference to the effectiveness of
your screen.
As for the height, this is bound to matter. I have no idea what the optimum
height is not to catch the rearwards flowing breeze, but I guess you want
the screen to be close to that, and probably at least an inch or two above
the top of your head. Maybe I should get one of the more fluid dymanics
conversant engineers in my office to do a computer model of it... Or you
could just hold your hand at different heights above your head while driving
at speed, to try and detect where the strong airflow starts. You may get
some funny looks from other road users!
Richard Gosling
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