>
>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.
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