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In another question, an answer mentions that the P-3 Orion uses a gearbox driven compressor for cabin pressurization, even though the aircraft uses T-56 turbine engines that supply bleed air in other designs. (How are turboprops and other propeller planes pressurized?)

Brief research suggests that the "EDC" exists, but I can not find any rationale for it. (http://blueaero.com/wp-content/uploads/2016/11/Heico_P3_EDC_Repair.pdf)

Does using gearbox driven compressors help in some way with the mission, possibly in dealing with the environment? Is it an anachronism/carryover from the L-188 Elektra? Is it not actually gearbox driven, but somehow an exchanger for bleed air?

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    $\begingroup$ Bleeding air from compressor reduces air available for combustion and also reduces compression ratio, so it impacts both the engine output and thermal efficiency. You can think of bleed air as the reverse of a supercharger. If engine performance is tight but weight budget is OK, then it make sense to take some power from the output shaft and drive a separate compressor. $\endgroup$ Commented Apr 30, 2020 at 2:52
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    $\begingroup$ @user3528438 In other words, taking bleed air costs more power in lost combustion than the power which is extracted? $\endgroup$ Commented Apr 30, 2020 at 2:54
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    $\begingroup$ Even if it cost no power in lost combustion, you are still stealing power from the HP shaft after all, that alone costs as much power as an external compressor. But in reality lost combustion is many times that. $\endgroup$ Commented Apr 30, 2020 at 3:04
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    $\begingroup$ This is only one possible rationale. P-3 may have other reasons that leads to this design. Also some more info here: aviation.stackexchange.com/questions/17030/… $\endgroup$ Commented Apr 30, 2020 at 3:12
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    $\begingroup$ US Navy P-3s commonly operated with 2 or 3 engines shut down to conserve fuel - may have something to do with it - a single active engine would have to provide 4 times as much bleed air as it would with all active, and 2 active engines would need to each provide twice as much. $\endgroup$
    – stretch
    Commented Jun 4, 2020 at 14:48

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The T-56 engine, like all turboprops, is much more efficient at converting fuel into rotational energy than compressing air. It takes approximately 50-100 shaft horsepower to run an EDC, depending on altitude and how cold of a temperature you select. Bleeding a small amount of air off the engine to run engine anti-ice costs you a 9% loss of SHP, which in flight, would be anywhere from 150-250 SHP. I don't know the exact volume of bleed air you would need to run a pressurization system, but it is definitely more than the engine-anti ice system. I'm making that assumption base on the size of the ducting. The anti-ice tubing is much smaller than the EDC tubing, or the main bleed air manifold for that matter.

All in all, I think it's due to efficiency. The EDC is more efficient for that motor, but is more complicated and introduces more possible malfunctions. A P-3 requires a LOT of cold air to keep the computers from overheating, more than a C-130 (same motor) for sure. I'm not sure if the C-130's back in the day also had EDC's, but they use bleed air nowadays. So the designers went with whatever can move the most air. I don't have my Orion Service Digests handy, but I bet there's a more thorough answer in there. I'll check whenever I go back to work.

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Retired P-3 Flight Engineer here.

The EDC's were only on the inboard engines, #2 & 3. The C model and later variants required a fair amount of cooling and the cabin was quite cool for the crew.

Someone mentioned shutting down engines for fuel conservation, while true typically #1 engine was shutdown and if conditions and weight allowed #4 engine could also be loitered. #1 didn't have a generator or other accessories while #4 only had a generator.

Taking bleed air from the engine robbed horsepower, increased temperature in the combustion chambers and increased fuel consumption which was a major concern. EDC were reliable and supplied all the air needed for both pressurization and cabin heat or air conditioning.

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    $\begingroup$ Welcome to aviation.stackexchange! $\endgroup$
    – DeltaLima
    Commented Aug 3, 2022 at 7:40
  • $\begingroup$ The P3 is such a cool aircraft, thanks for sharing the first hand experience! $\endgroup$ Commented Sep 8, 2022 at 10:58
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From memory, the use of air derived directly off an engine compressor assembly was prohibited for the use of air-conditioning (IIRC, it had to do with the possibility of oil leakage/fume contamination from seals within the engine core assembly of the turbine engines) in passenger aircraft - back in the days of the Electra. The Orion simply carried this over from the Electra days. The C130 didn't have these restrictions, as it was designed primarily as a cargo aircraft, not a passenger type aircraft, although plenty of passengers - myself included have flown in both C130 & P3 aircraft.

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    $\begingroup$ I think this answer provides a very good reason for this design decision, however you need to provided a source for this statement, otherwise the answer will not be upvoted... $\endgroup$
    – U_flow
    Commented Jul 20, 2022 at 14:42

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