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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. $\endgroup$ – user3528438 Apr 30 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? $\endgroup$ – trognanders Apr 30 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. $\endgroup$ – user3528438 Apr 30 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/… $\endgroup$ – user3528438 Apr 30 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 Jun 4 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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