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Date: Tue, 18 Dec 2007 15:41:35 -0500
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Supercharging
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wrjjrs@aol.com wrote:
> Ernest, No that isn't true. I agree that any supercharging method 
> certainly adds complication. The MAJOR difference is that a 
> SUPERcharger, rather than a turbo requires additional careful 
> engineering to enable the DRIVE for the blower to survive. A turbo 
> must be mounted itself, but the drive is simple, even if the plumbing 
> isn't.
> Cheers,
> Bill
>


Ed Anderson wrote:
> Depending on the compressor type used on the supercharger your 
> efficiency can drop down to below 50% (some of the older roots type 
> expellers) , the centrifugal compressors (which can be on super or 
> turbochargers) are generally higher efficiency (can approach 75%)  in 
> converting input energy into high pressure air. So a bit more efficient.

I'm going to sidestep Ed's argument about using energy before or after 
it has been converted to mechanical energy, because just thinking about 
it is making my head hurt.  Instead, I'll move on to the type of blower 
and it's mounting.  I spoke about this at the Rotary Roundup a couple of 
years ago.  Tom is just about through with machining the parts, and I'm 
about through building my fuselage, so I should have real hardware to 
test with "Real Soon Now"(TM).

The rotary is unique among aircraft engines, in that we have 6000 shaft 
RPM to work with.  Normally, that is considered a liability, in that it 
requires an extra gearbox to drop the RPM to a range usable by a 
propeller;  but, supercharging is a case where I believe we can make 
some lemonade. 

6000RPM is about the speed that commercial leaf blowers use.  The 
blowers just happen to be about the same size as an engine housing.  A 
conservative blower will easily deliver 600 CFM at 150MPH.  We don't 
usually want a LOT of boost in an airplane, since that will impact 
reliability, but how much is a LOT?  The typical supercharger is heavy, 
requiring some major engineering to produce a reliable mount and drive 
system.  A turbo carries its own list of issues, not the least of which 
is blowing the intake air across a hot exhaust component.  A blower, 
mounted as an integral part of the flywheel and enclosed by the bell 
housing, would sidestep most of the problems.  It won't deliver a LOT of 
boost, but neither will it require pop-off valves, waste-gates, 
intercoolers or a heavy mounting system.  I will consider it a win, if I 
can get 200Hp at 7000RPM.

Richard Sohn asked me at the Round-up about how would I know if it 
works.  I had to think about that one.  The answer is that I will 
provide for an alternate air intake (at least temporarily).  I will do a 
pull test, while running the engine with air being pulled through the 
blower and through the alternate air intake.  The blower will have to be 
shuttered in the second test to insure it doesn't rob energy from the 
system.  That won't give me raw horsepower, but it will give me a solid 
ratio of power difference between the two.

Any ideas on what I should do, not do, or do differently?  I'm still 
technically in the planning stages, so I would appreciate any and all input.