B744 APU

[NGjockey]

Does anyone have some information about the history of the PW901A APU installed in the B744? According to Pratt&Whitney it is derived from an engine used in "popular Business Jets". Which popular business jets had that engine, and what was it called then?

[pattern_is_full]

The PW901A uses a centrifugal compressor. Rather unusual in turbofans.

That would make the obvious suspects the P&WC JT15D series (Cessna Citation I/II and others) or the PW600 series (Citation Mustang, Eclipse 400/500, Phenom)

The scale and weight would suggest the JT15D.

[tdracer]

Originally Posted by pattern_is_full

The PW901A uses a centrifugal compressor. Rather unusual in turbofans.

Centrifugal compressors are quite common on small turbine engines for a couple simple reasons. First off - with a small turbine engine and a ~30-1 compressor ratio - with an axial compressor you end up with such small parts in the high stages you need a watchmaker to put the thing together. Which leads directly to the second reason - centrifugal compressors - especially small ones - are cheaper to produce (usually by quite a bit). Although centrifugal compressors increase the frontal area of the engine, you need a large volume for the combustor (pressure loss through a combustor is proportional to the local Mach number squared, so you want to slow the airflow way down - less than 0.1). As a result on smaller turbine engines there isn't much of a penalty.

[pattern_is_full]

All true. But what percent of all turbofans on aircraft post-1960 are centrifugal? If < 20% I would call that "unusual."

[NGjockey]

@pattern_is_full: Thank you for the answer!