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Why No Buzz Saw Sound, When C-17s Take Off?

Wed May 04, 2011 9:23 pm

Maybe it is me, but I notest that when C-17s take off, they do not produce a buzz saw sound. I know P&W 2040 powered 757 has a strong buzz saw sound at take off, so I know it not the engine type. DC-8 has four CFM 56-2 that produce a strong buzz saw sound, so it not the number of engines. When a C-17 take off, it has a high pitch thrust sound, so why are they missing the buzz saw sound?
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RE: Why No Buzz Saw Sound, When C-17s Take Off?

Thu May 05, 2011 2:20 am

When I flew them, you could hear the buzz saw sound from the cockpit. The times I remember it occurring the most were mainly during really high power (MAX power setting) takeoffs; maybe DRT (derated thrust) takeoffs produced the same sound, but not too often from what I recall. The thing I remember the most about a C-17 takeoff, especially when viewed from the ground, was the ridiculously loud roar. They had about the same intensity as a 707 but without the fans' sonic screeching.
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RE: Why No Buzz Saw Sound, When C-17s Take Off?

Thu May 05, 2011 5:41 am

Just a guess but, attempt to lower the heat signature so they aren't easy targets for missiles maybe?
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RE: Why No Buzz Saw Sound, When C-17s Take Off?

Thu May 05, 2011 3:30 pm

I certainly hear it often when they depart out of CHS. It may be a little more muted than the 757, but it has more to do with engine power settings and nacelle design than anything else. I don't know if the military version of the P&W 2040 has any differences in the diameter of the fan, shape of the blades, etc., but that would make a big difference as well.
I got my head checked--by a jumbo jet
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RE: Why No Buzz Saw Sound, When C-17s Take Off?

Thu May 05, 2011 6:07 pm

IIRC the buzz saw sound is - and I stand to be corrected by any of my more learned brethren on here - the pressure waves caused by the tips of the fanblades on the front disk going supersonic. The buzzsaw sound only happens at high thrust settings ie: takeoff.

So why would a C17 not create the buzzsaw noise? No idea - it has four PW2000s, right? Heard them go buzzing many times on PW-powered 757s so it cant be the engine.

Perhaps a derated takeoff, or maybe some degree of acoustic or thermal shielding on the cowlings and exhaust that the 757 doesnt have?
What do you mean you dont have any bourbon? Do you know how far it is to Houston? What kind of airline is this???

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