Topic Author
Posts: 132
Joined: Tue May 23, 2000 9:46 am

Bypass Significance... XNV...

Mon Sep 25, 2000 12:17 am

XNV, that was a great topic. Thanks to everyone for the explanations, I learned quite a bit! Now I have a related question- someone stated that the GE TF39 on the C-5 was substantially louder than other H-BP engines, and I have to agree. The C-5 has the loudest, shrillest, most annoying fan thrum of any engine I have ever heard. Why are they so much louder than comparable engines such as the JT-9D and CF-6???
Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

RE: Bypass Significance... XNV...

Mon Sep 25, 2000 2:10 am

I'm not an engineer, but as ex-military flier I can say that military transport aircraft design does not normally take into account..... noise.
*NO CARRIER* -- A Naval Aviator's worst nightmare!

RE: Bypass Significance... XNV...

Mon Sep 25, 2000 2:16 am

I'm not sure, but I will agree with you on that. I spent a lot of time around C-5's this summer in Turkey and got to know that sound all too well. It was the only heavy that you could hear idling from across the base. No matter where you are on base you can hear that big bastard idling. It sounds like heavy truck tires at highway speeds. I would assume that the technology of the time did not incorporate measures to reduce noise. This engine was developed in the mid-1960's and the USAF was not in the least bit concerned about noise then. They just wanted the airplane built. Anybody got any technical reason why it is so much louder?
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Joined: Tue Aug 08, 2000 10:34 am

RE: Bypass Significance... XNV...

Mon Sep 25, 2000 3:46 am

I don't know why the high volume sound. I own a building where I restore cars in Kentucky. C-5s and occasionally a 747 with the Space Shuttle on it fly overhead. I easily can tell which one is flying over.

However, I don't work with them and am seldom near them. I have long thought the C-5's were the prettiest sounding aircraft flying. But again I am not close to them. They have a two tone whistle that sounds pretty. I guess up close they must be loud.

Posts: 130
Joined: Tue Nov 23, 1999 11:02 am

RE: Bypass Significance... XNV...

Mon Sep 25, 2000 5:08 pm

The military CF-6 (TF-39) does not have any sound suppression in the inlet. It also has Two (read it 2) fan stages. The front one is slightly smaller than the rear one. The turbine is also unencumbered by any acoustic treatment and produces a very high whine. I ran engines on C-5s for years and it does have a sound of its own.
The "Fan Growl" is present with all fan engines from the tinny little growl of a Garrett 731 on a Cessna Citation to the deep basso of a GE-90 on a 777.
Posts: 817
Joined: Tue Aug 08, 2000 10:34 am

RE: Airgypsy

Wed Sep 27, 2000 6:34 am

Actually I know of two turbofans that have NO growl. They are both GE engines.

One is the aft-fan engine used in the older French Falcon. The other is the aft-fan engine used in the Convair 990 (CJ-805-23).



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