matt weber wrote:
>
> >
> >Boeing experimented with advanced turboprot designs from Allison and GE
> >back in the early-to-mid 80's, with their proposed 7J7. Recall that
> >when that plane was first mooted, oil prices peaked at $90/barrel in
> >today's dollars (measuring inflation over 25 years is not an exact
> >science, needless to say, but $90/barrel is probably as good an
> >estimate as any.) As oil prices eased, Boeing killed the 7J7.
> >
> >The big problem with the 7J7 was noise -- two rows of contrarotating
> >blades hanging out in the breeze are going to make a lot of noise.
> >This was a problem not just in the area surrounding the air****t, but
> >for people inside the plane and even people under planes at cruise (as
> >the planes reached high mach numbers in cruise, the noise levels went
> >up tremendously.) I don't think that there's been anything done in the
> >last 20 years that will make the engines much quieter.
>
> The noise problem wasn't the engine per se, the problem was the
> contra-rotating props in the 'Prop-Fan' or Unducted Fan were
> supersonic by a comfortable margin. A supersonic prop is kind of a
> fact of life if you want to cruise much above 400mph.
>
> The tips on the fan of many large turbo fans are supersonic as well,
> about M1.3 at takeoff power, but since they are inside a duct, you can
> do a lot to capture the shockwave/noise inside the duct, which is
> exactly what they do. That isn't an option without the duct however.
Correct. The compromise is to make higher bypass ratio engines. This
leads, inevitably, to larger cowl diameters and longer landing gear. I'm
wondering if, at some point, the trend toward larger engines will make
the manufacturers revisit high wing designs. These would create more
room under the wing to hang big engines.
One other consideration in unducted fan vs ducted fan engines is blade
containment. Fan shrouds help quite a bit in containing broken blade
fragments that might otherwise hit the fuselage.
--
Paul Hovnanian mailto:Paul@[EMAIL PROTECTED]
gnu, will travel.
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