F. Marc de Piolenc wrote:

Paul wrote:

Is there something incorrect with the data I posted for two example
turbofan engines?  Could you please post your data (diameter, rpm,
etc) that shows all current commercial turbofans have subsonic tip
speeds in their fan stages?

Not data per se - just that all the design literature I have on gas
turbine design and gas turbine installations in aircraft emphasizes the
need to keep flow into the first compressor stage subsonic. Inlets in
supersonic military 'planes are designed to "shock down" the flow to
achieve this end. Add to that the fact that even turbomachinery
purpose-built for supercritical flow sacrifices some efficiency, and
that the literature on supercritical turbomachines is still considered
"cutting edge," and there you have it.

If turbomachinery does operate supersonic these days, why not have
positive camber on the turbofan shrouds? You'd get an immediate
improvement in mass flow, hence propulsive efficiency. I can't think of
any good reason for diffusing the fan flow other than controlling speed.

Marc de Piolenc
Mass Flow

I think we are getting things a little confused here. The fan tips
are what we are concerned about and the sudden drag increase as 
they approach mach one. 

The compressor on the other hand  being much smaller in
diameter and higher RPM may or may not be running with supersonic tips.
It will benefit from higher pressure however. 

As Richard said a diffuser duct causes an increase in 
pressure AS WELL as a reduction in flow velocity. 
Since the fan tips in a turbofan are running at supersonic 
speeds with zero forward velocity anyway a duffuser duct
will not help much with the reduction in tip mach number at speed.
Hence the pressure increase for the compressor  is the main 
reason for the diffusing duct.

For our application it Seems to me running the fan 
tips at a higher (possibly supersonic) speed will result 
in a wider effective speed range for the ducted fan. 

As Perry's design stands now I suspect the engine will bog, 
possible as low as 5000 RPM. This means it will be short 
of its design goal of 6000 RPM and 180 HP. This will be 
caused by using the thick Clark Y wood airfoil I suspect. 

I think a thinner supersonic airfoil is indicated and may allow
the engine to rev higher hence generate more HP. I also think
it can be done with carbon fiber.

Paul Lamar
 
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