ExPratt From United States of America, joined Mar 2000, 311 posts, RR: 0
Reply 2, posted (15 years 8 months 1 week 4 days 9 hours ago) and read 2609 times:
JT8D hush kits consist of a mixer, inlet guide vane spacer case, No. 1 disk and hub, accoustic treatment of the fan ducts, and tubes.
The mixer is installed just behind the turbine exhaust case. The mixer breaks up the core engine flow and mixes it with the fan flow. The high velocity core engine exhaust flow is the primary component of jet noise.
The inlet guide vane (IGV) spacer case moves the IGV case about 4 inches forward. This moves the wakes of the air going through the IGVs forward of the fan blades. The fan blades cutting through those wakes is another major component of jet noise.
The new No. 1 disk and hub has a longer nose piece for the No. 1 bearing journal to account for the IGV spacer case and the repositioned No. 1 bearing support.
The fan ducts have panels on the interior surfaces that have holes drilled in them. The holes are intended to absorb specfic sound frequencies of the compressor and combustor noise.
The tubes are the oil and antiice tubes that had to be replaced because of the respaced inlet case.
Because the 727, 737, and DC9 airplanes were certificated engines that generated specific levels of thrust, the hush kits could not and did not effect engine performance.
JETPILOT From United States of America, joined May 1999, 3130 posts, RR: 27
Reply 3, posted (15 years 8 months 1 week 4 days 2 hours ago) and read 2569 times:
That was a very informative post.
But it's only a piece of the pie.
Some hushkits go about the same business in a different way.
Raisbeck modifies flap settings, derates the engines, and modifies the cascade reversers to achieve the same effect.
Raisbeck covers a portion of the reverser cascades with sheetmatal to reduce noise during reverse operations. The trade off is a much reduced braking.
Raisbeck modifies the flap settings and limits the 727 to a flap 25 landing configuration requiring less power on approach but higher airspeeds. This extends the landing roll.
Raisbeck derates the engines and lowers the aircrafts MGTOW utilizing a lower EPR setting for TO. The efffect is a much lower usefull load, and less than stellar climb performance.
Raisbeck also offers an external exhaust gas mixer (cookie cutter). This increases drag and fuel burn.
So hush kits come in different forms. There is the right way, and then there is the Raisbeck way. But the Raisbeck is considerably cheaper. It is a good modification for planes with little useful life left.