Richard White wrote:

Richard White wrote:

Here are some numbers for the GE90 (as used on a
Boeing 777, for  example).  With a fan diameter of 123 inches
and a fan rpm of 2485 producing 105,000 pounds of static
thrust at sea level, the fan blade tip speed is
2485/60 * 123/12 * Pi = 1334 ft/sec which is about mach 1.2,
which as the company advertises, is slower than other typical
high bypass engine's fan stages.

F. Marc de Piolenc wrote:

Are you taking into account the deceleration in the fan's inlet
diffuser? I believe that all current commercial turbofans have
subsonic compressor staging.

Paul wrote:

I don't think Richard is. I think it is purely a geometrical
calculation. To say that is the mach number is mis-leading as
mach number is usually thought of in terms of local air flow.
As someone pointed out we don't really know the local airflow.

Jumping back into the conversation, Richard wrote:

Paul is right about my geometric calculation, as I assumed
local airflow is zero knots.  I guess I should have been more
clear that the numbers I gave above are for an engine on a
test stand at sea level.  I don't believe that the airflow can be
decelerated any slower than standing still.
[he says, trying to inject a little humor here]

The whole point of my original post was to dispell the myth(?)
that turbofan inlet diffuser ducts are designed to keep the tips
of the fan blades subsonic.  The tips are supersonic regardless,
even standing still on the ramp.

But no need to take anyone's word for it, let's check out the data.
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?

Changing topic slightly, NASA and General Electric experimented
with an unducted fan (on a 727) which had supersonic tip speeds,
but I don't remember where I put the data for that one.  Does anyone
remember that?  This kind of thing may not seem relevent to an
airplane cruising at only 200 mph, but it does demonstrate that
supersonic tip speeds are not an absolute no-no (just ask some of
the formula one racers at Reno).

Happy Times Upon You,
Richard.

We believe you Richard :-)

Your data checks. Robin brought home the 777 manual and I have
it right here. :-) Besides that Mark Wrathall said the same thing.
I am just surprised that's all. 

Got any suggestions for airfoil coords and thickness?

The formula one information proves that the rate of increase in 
net thrust HP  with RPM is greater than the rate of decrease in 
prop efficiency. Prop diameter and RPM are everything! Damn the tip
speeds. Full steam ahead.

The question is what happens to the supersonic prop MPG? :-)

Paul Lamar
 
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