Pulse jet active sound attentuation

"Thad Beier" wrote in message
...
Jay wrote:
In an earlier post I read somewhere that the pulse jet is a device
that converts fuel into noise. Someone else had suggested firing 2
engines 180 degrees out of phase with each other to cancel the sound.
If the 2 engines were stacked one over the other, then you might
actually be able to cancel a lot of the sound at least in a circle
around the aircraft in the plane of the earths surface. Below the
aircraft it would be just as loud but twice the frequency, but
standing on the ramp 50 yards from the aircraft, it might actually be
made to cancel somewhat. What do you guys think?

The pulsejet sounds is not a sine wave, or even close, so
two 180-out-of-phase signals will not cancel out.

Interestingly, there is a PDE-powered LongEZ in the current
issue of Aviation Week. It's completely astonishing.

PDEs are Pulse Detonation Engines. Basically you get a long
pipe, fill it with an explosive mixture of fuel and air, and
then detonate it at one end. You get compression ratios
of about 40:1 (for good efficiency) and extremely rapid burning,
for low NOx production. But, they're really loud. Ridiculously
loud. Someone described the sound of it as "somebody ripping
the sky apart"

Anyway, the LongEZ powerplant is basically a regular flat
airplane engine with really long cylinders open at the bottom.
It uses the normal valves and camshafts to let the fuel-air
mixture into the cylinder (although both valves are used as
intake values, as the exhaust is out the bottom. The plane
has four pipes projecting back about four feet. There's no
description in the magazine article about how they're driving
the camshafts, I would guess it would be an electric motor
and batteries, and these will be very short flights.

The article in Aviation Week shows a number of other
PDE engines. There's an amazing engine that is basically
a 'revolver', sort of a rotary or Gatling Gun PDE. The tubes
sequentially rotate into firing position.

The article goes on to say that there could be large (like 15-20%)
increases in gas turbine efficiency if you could use a PDE to
replace the compressor and combustor with a PDE. Aviation
engineers would sell their grandmothers back several generations
for a 5% increase in efficiency...20% is shocking.

There are, of course, practical problems in the way, but the promise
is there.

thad

Reminds me of a story about some bored roughnecks in West Texas. Seems they
rigged up a length of 3" drill pipe so they could fill it with just the
right mixture of oxygen and acetylene and then set it off with a spark plug.
It was said the muzzle flash and report resembled a 5" Navy gun. I'd bet
that had 40:1 compression.

Bill D