Re: CV-990 Aft Fan

Date:         17 Oct 99 23:29:44 
From:         "Robert Wright" <>
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>Jeff Taylor wrote:
>> I would like information or possibly some pictures of the CV990 Aft
>> Fan Engine. This would be the General Electric CJ805-23. I have read
>> that it was a civlian J-79 from the F4 Phantom and other Military
>> Aircraft.
>Not a J-79, but definitely a J-79 derivative. Actually, it seems like
>quite a clever idea- definitely a quicker way to concoct a turbofan
>from an existing turbojet than adding a spool. However, I assume it was
>probably considerably less efficient or had some other deficiency
>compared to "front-fan" engines. The CV-880's engines were closer to

    I think the aft-fan arrangement would have given rise to considerable
interference drag due to its proximity with the lower surface of the wing.
Also, the airflow coming along the outer surface of the core would be
somewhat "dirtier" than if it hit the fan directly after sitting around at
35,000 feet, minding its own business, blissfully unaware of the monstrous
set of airfoils bearing down on it.

    With a front fan, you also get the added benefit that the fan blades can
sling rain, hail, gravel, etc. towards the outside, possibly missing the
core and reducing damage to the high compressor and turbines, or cutting
down on the amount of water being sucked into the combustor.  I don't think
that effect was really appreciated until well into the 1980's, however.


p.s. "The Road to the 707", by William Cook, has some discussion of the GE
aft-fan/Convair arrangement and some of the problems it encountered, along
with one profile drawing of the PW JT3D, GE aft fan, and JT3C (the turbojet
precursor to the JT3D).