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TF39 A CF6?

Wed Nov 29, 2000 1:47 pm

Is a TF39 military american aero engine the same as a civil CF6 engine? I know that the TF39 has an amazing 8:1 bypass ratio, but no CF6 has such a ratio.

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Joined: Wed Jun 30, 1999 9:23 am

RE: TF39 A CF6?

Wed Nov 29, 2000 9:58 pm

Actually the TF39 is the mother of all the CF6 series of engines. The TF39 shares its HPC core with the first model CF6-6 used on the DC-10-10. The main differences between the engines were that the TF39 has a 100inch 1-1/2 stage fan and a 6 stage LPC and the CF6-6 has a 86.4inch single stage fan and a 5 stage LPC. It is interesting that they made the same amount of thrust at 41,000lbt. You are correct about the differences in the BPR's. While the TF39 has an 8:1 BPR the CF6-6 started life with a BPR of 6:1.

There are of course further developments of the CF6 series of engines and the subsequent models have significant changes from the TF39 parentage. The CF6-50 was the next major step in the line. The main changes being a reduction of HPC stages from 16 to fourteen, a reduction in LPC stages from 5 to 4, and the addition of 2 more stages on the LPC aft of the fan. The fan included repitched blades and the combustor was redesigned as well. All of these changes added up to an engine with a 4.4:1 BPR higher PR's and lots more thrust taking the CF6-6's 41,000 to over 50,000 on the CF6-50C2.

As the CF6 engine family has been developed it moved even further from its TF39 roots. The next step in the development was the CF6-80A with a basically unchanged fan, and another combustor redesign. There was also another significant change in the design with the engine having the accessories mounted on the core instead of the fan case. This was something interesting because the TF39 used on the C5 aircraft had the accessories on the core from the start while all of the CF6 models up to the -80A had theirs mounted on the fan case. You can find the CF6-80A's on airbus 310's and early 767 models. It is almost indistinguishable from the next-to-latest version of the CF6; the -80C2. The CF6-80C2 improves on the -80A with a 93inch fan, vs 86.4 for the -80A, and an additonal LPC stage behind the fan, bringing the total to 4, a fifth stage on the LPT, higher turbine temperatures and new materials to bring thrust to over 61,000lbt with a BPR of 5.4:1 for 767's, 747's md-11's, and the newest Airbus A310/A300's.

The latest growth stage, the -80E1 has given the venerable basic CF6-80C2 a 96.13inch fan vs 93in, higher temperatures, some internal structure and aerodynaminc changes to produce up to 71,500lbt with a BPR of 5.8:1 for the A330.

It is quite interesting that the decendants of the TF39 have all maintained the same basic core design, even down to some of the dimensions and geometry of the HPC. Its hard to belive when your looking at a brilliant modern-day CF6-80E that it still posseses basic design elements that first ran 34 years ago.

I hope that my little rant gives you some useful information.


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RE: TF39 A CF6?

Thu Nov 30, 2000 1:05 am

I love your rants, very nicely written. But just to add my 2 cents. In addition to what you wrote, another major difference between the CF6-6 and -50 is the -6 doesn't incorporate VBV's (variable bleed valves) while the CF6-50 and subsequent models do. Of course all CF6's have VSV's (variable stator vanes).
Also, The CF6-80A has a fan case gearbox, I wasn't sure if you wrote it did or didn't.  
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RE: TF39 A CF6?

Fri Dec 01, 2000 2:06 pm

Why in the world did Lockheed continue with the low tech low thrust TF39 on the C-5B that was introduced in 1985 when they could have used a much higher thrust & more modern CF6-50C2 such as the engine on the KC-10. Even the early 80's vintage CF6-80A made 48,000lbs. The TF39 on both the 1960's C5A & the 1980's C5B both create 41,000lbs, which I could understand in 1968, that was alot, but when the C5B came out, 41K is pretty low.

Lastly, Lockheed put the GE engine on the C5, why then did'nt lockeed put the TF39 on the tristar? It made the same thrust that the RB.211-22B made & GE already had a proven engine running. I know that the rolls royce engine delayed the tristar due to serious financial troubles with RR

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Joined: Wed Jun 30, 1999 9:23 am

RE: Tarantine

Fri Dec 01, 2000 10:13 pm


I would not neccessarily call the TF39 low tech. For its day it was a revolutionary engine, and it was purpose-built for the military with a suitibly rugged design. As with most military applications change is slow in coming. It is much better in the military pervue, to develop equipment into a proven design than to contiuously overhaul equipment with completely new designs even if proven elsewhere.

As you may already know, a CF6-80 powered C5 is already in the works but the military adaptation will come at the end of a long evaluative process. A good example of the scope of the process is the TF33 application on the Boeing B-52. The current engines used on this aircraft are early developments of the JT3D used on 707, a 40 year old design. Now with the B52 nearing it's 50th anniversary, the USAF is contemplating a reeinging with R-R 535's currently used on 757 and the TU-204. So after 50 years the B-52 design will have recieved only 2 major upgrades in engines, replacing the JT3 turbojets with the updated JT3-based TF33 turbofans, and the to-be-seen Rolls Royce upgrade from the TF33.

It is interesting that you also mention the RB.211 application on the Tristar. At the time there was a lot of wrangling going on with the development of engines for the Tristar, DC-10, and developments of the 747. Lockheed wanted an exclusive manufacturer for the engines on tristar and Rolls-Royce sealed the deal by offering the required thrust at a nice fixed price. But the engine was delayed not only by the financial difficulties of Rolls-Royce but also by the fact that there was serious difficulty in meeting the promised specification. Not until a financial take-over by the UK government and an operational take-over by Ken Keith did were the troubles overcome.

GE made a bid for the Tristar with the CF6 as well, but was beaten by R-R. They instead got on the DC-10 and were much more succesful there than was Rolls-Royce with the Tristar in the sense that they sold many more engines. And why no TF39 on the Tristar? For that application it was too big, too thirsty and lacked potential for thrust growth with its odd, but durable fan design.

There I go again



RE: TF39 A CF6?

Sun Dec 03, 2000 1:25 am

Lockheed is going to re-engine the C-5A and B models with the GE CF6 to replace the TF39 and will be redesignated the C-5M. As far as the B-52H models, they will be re-engined with a RR RB211 series used on the 757 that will be licensed built by Allison.
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Joined: Tue Nov 23, 1999 11:02 am

RE: TF39 A CF6?

Sun Dec 10, 2000 2:52 pm

The TF-39 is a flat rated engine. 41K to 18K ft. So, it may be rated low but it lasts a good long time.
Mooney did the same thing with one of their aircraft. Put in a real big engine and then rate it real low at sea level. No need to turbo or sweat the density altitude. Same deal with the TF39. Big motor, low rating.

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