I think you are missing something here Bill. There are
two kinds of blowers on the market. Centrifugal belt or gear driven
blowers that don't make much boost from idle and roots or positive
displacement types that do. The problem is the centrifugal blower
adheres to the square law. If you gear it to not exceed max RPM there
is little or no boost at idle. Way back in the old days (30's) Paxton built
a variable speed belt drive centrifugal blower to overcome this problem.
Originally used on mining trucks in the Andes. It was sold
up until
recently and may still be available.

Paul Lamar

Paul, I don't think I'm missing anything as I am aware of the Paxton. 
The centrifugal super charger was a means to an end. It was very compromised 
at lower speeds in general. I don't think it is a "bad idea" since there 
are many aircraft that used it. These aircraft usually had power to burn, 
(like the P51), but to overcome the typical centrifugal super charger problems 
they had a two speed gearbox adding even more complication. I know this is 
a SIMPLIFICATION as there are many instances where somebody got wise and 
made good choices. The point I was trying to make is that the mechanical 
super charger is GENERALLY best for producing power at low rpm, while 
the turbo is better for our aircraft uses when the operating RPM 
is well known before hand.
Bill Jepson

All that is true but you are not making a distinction between a gear
driven centrifugal supercharger and positive displacement a roots type blower.
The roots blower is linear while the centrifugal super charger
is square law.

Paul Lamar
 
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