Happy-flier From Canada, joined Dec 1999, 299 posts, RR: 0 Posted (15 years 10 months 4 weeks 1 day 22 hours ago) and read 8000 times:
As many of us know from photographs and perhaps from memory, the JT3 and JT4 turbojets that powered the first model DC-8 and 707 were equipped with specialized "sound suppressor" nozzles. Thinking about this fact today, I find it very odd that these devices, in use 40 years ago, could have substantially reduced the noise of first generation pure-jet engines. But did they? Does anyone know what effectiveness those original sound suppressors achieved on the original JT3 and JT4 turbojets?
May the wind be always at your back . . . except during takeoff & landing.
ExPratt From United States of America, joined Mar 2000, 311 posts, RR: 0
Reply 1, posted (15 years 10 months 3 weeks 4 days 21 hours ago) and read 7829 times:
The 707s with the JT3Cs used the "Buck Rodgers tube" suppressor, which was about 15 - 20 tubes from the rear of the engine. It earned its name because it looked similiar to the back of Buck Rodgers' spaceship. The DC-8s had a mixer and ejector setup. The mixer was typical for the mixers that are currently in use. The ejectors were controlled from the cockpit with a switch on the center overhead console and would be extended for takeoff and landing. I believe an amber light over the engine gages would flash when the ejectors were in transit and illuminate when the ejectors were deployed. I don't think they worked very well. After 100 db, deafening is still deafening.
Air2gxs From , joined Dec 1969, posts, RR:
Reply 3, posted (11 years 2 months 3 weeks 5 days 21 hours ago) and read 7777 times:
Basically sound supression is a function of mixing the cold, lower velocity fan air with hotter, faster core air. It appears that the shear area (where hot, fast air meets cold slow air) is a major component in sound production. By causing these air flows to mix at the exhaust it creates an area where the sound is reduced. Somebobdy out there can probably get into the technicals aspects better.
I've never heard of water injection being used in sound supression. It was used in the past to augment the power of straight jets and older fans by lowering the density of the air through evaporative cooling, thus allowing more bang for the buck. It was normally used at tae-off.
AeroWeanie From United States of America, joined Dec 2004, 1612 posts, RR: 51
Reply 5, posted (11 years 2 months 3 weeks 5 days 19 hours ago) and read 7763 times:
Jet engines dump their exhaust at a speed higher than the flight speed. This difference causes shear, which is a major noise source. This is particularly bad when the airspeed is low (take off or landing) and the thrust is high. The best way to reduce shear is to mix the exhaust and freestream air. The amount of mixing is a function of how much edge length the exhaust nozzle has. Dividing a simple circular exhaust nozzles into two circular nozzles will increase the edge length, increasing mixing and decreasing noise. Continue this to 15-20 nozzles increases the mixing further, decreasing noise further. Most hushkits add mixers which mix the core and bypass air and then the exhaust and freestream air, all to reduce noise. These mixers are corrugated, but function just like the old "Buck Rodgers tube" suppressors.
N243NW From United States of America, joined Jul 2003, 1658 posts, RR: 19
Reply 9, posted (11 years 2 months 2 weeks 5 days 11 hours ago) and read 7528 times:
Quoting AeroWeanie (reply 5): Jet engines dump their exhaust at a speed higher than the flight speed. This difference causes shear, which is a major noise source. This is particularly bad when the airspeed is low (take off or landing) and the thrust is high.
So if I am not mistaken then, does this mean that as the aircraft gains speed during the takeoff roll, the difference in speed between the exhaust and the ambient air becomes less, and the exhaust actually gets quieter?
Of course, I'm assuming this loss in exhaust noise is probably negligable, though, since airframe turbulence and similar noises get louder as the plane accelerates, and as a result the whole plane doesn't make any less noise.