You are right about no rootes blowers on aircraft engines but they allow for 
a simpler and cheaper engine. McCulloch (yes, the chain saw people) several 
years ago had a 2 cycle aircraft diesel in the "X" layout and it uses a 
rootes blower. It is on display at the Pima Air Museum in Tuscon. I suspect 
any new engine will be designed for cheap production and cheap operation. I 
am back from a fantastic trip.
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