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Why Was The Bypass Fan On The Ge Cj805 On The Aft End Of The Engine?
| May 20, 2009 | Posted by HTBP under How to Bypass |
In the late 1950's General developed a turbofan derivative of the successful J-79 afterburning turbojet (military version) . This engine was used in the Convair 990 Coronado which until the B-747 and the later Concorde was the fastest commercial aircraft flying. The CJ 805 was unique in that the bypass fan was placed aft of the compressor section. Why was this and why has it not been repeated?
Can't tell you why it hasn't been replace but it's not the only aft bypass fan engine. They also added an aft fan to the CJ610 that was on the early Lear jets and also used with an afterburner in the F5 and T38 Talon super sonic trainer. With the addition of the fan the GE engine became known as the CF 700 and was used on the Falcon 20 and the Saber 75 executive jets. Not very efficient engines though for a two spool fan engine but GE made allot of money by using the original core of the CJ 610.
Interesting. I'd thought the CJ610's and the CF700's were the only aft fan dual bypass engines ever made.
I have no idea why they were designed like this, but I have some ideas. I know the CF700 low bypass turbine fan's are a little more resistant to FOD damage, albiet not super fuel efficient engines. Maybe they sacrificed some fuel efficiency for durability?
Another theory is the noise generated by aft fan engines is possibly higher than forward fans, as both the commercial version smaller engines I know of are pretty loud.
The reason for this is simplicity and lower cost due to the use of an existing design. The CF700 and 805 were fanned upgrades to existing single spool turbojet engines. These are not true "dual spool" engines. Reason is this: There is only one spool in a turbojet, therefore, there is no low pressure turbine section to spin a fan on the compressor side of the engine. A fan can be added to the back side of the existing turbine section that uses the left-over exhaust to drive a fan at a slower pace. This is called a free floating fan. It keeps the manufacturer from having to completely re-engineer the rotating assembly to include two shafts because the low pressure turbine does not have to drive a fan using an individual shaft through the high pressure turbine and compressor sections. It is all accomplished at the aft end of the turbine, the fan disk itself. It really is not a good design. As an example, the Falcon 20 re-engined with TFE731-5BR engines (a forward fan design) has an advertised 75% better fuel economy over the same aircraft with CF700 engines. It was cheap for GE and also allowed them to sell the engines at a lower price at a time when turbofans weren't that common and therefore, there wasn't as much competition, but it's just not a design that's competitive from an efficiency standpoint today. It was at that time, a good way for GE to cheaply add thrust and efficiency to an existing turbojet engine.
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