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I know how most turbofan jets are pressurized: enter image description here

But it can't be the same with turboprops, as I don't understand how propellers can take in the air. Yet, planes like Q400s can fly above 8000', all the way up and higher than FL220! Can somebody explain how this is possible?

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    $\begingroup$ Where did you get the idea turboprops can't take in air? A turboprop is a type of turbine engine. $\endgroup$
    – GdD
    Commented Apr 28, 2020 at 16:05
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    $\begingroup$ It is exactly the same for turboprops. $\endgroup$
    – user3528438
    Commented Apr 28, 2020 at 16:57
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    $\begingroup$ Are you thinking of pressurized non-turboprop (non-turbine-driven) piston propeller planes? In which case, does this QA help? aviation.stackexchange.com/questions/38115/… $\endgroup$
    – Dai
    Commented Apr 29, 2020 at 0:20
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    $\begingroup$ You know... a turbofan engine is a turboshaft engine that drives a fan. They don't take bleed air off the fan, they take it off the turboshaft's compressor stages. A turboprop engine is just a turboshaft engine that drives a prop. Same difference, really, just one looks cool. $\endgroup$
    – Harper - Reinstate Monica
    Commented Apr 29, 2020 at 1:06
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    $\begingroup$ And even some jets (google "787 no bleed air") now use plain old electric air compressors, instead of turbine bleed air. Apparently it's cheaper, but some passengers believe it's better for air quality, too, $\endgroup$
    – CCTO
    Commented Apr 29, 2020 at 4:05

4 Answers 4

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Turboprops are actually turbine engines. They can produce bleed air just like turbine engines (e.g. turbofan). The bleed air can be used directly to pressurize the cabin, or it can drive another turbo compressor to pressurize fresh air from the outside:

enter image description here

Modern aircraft with supercharged piston engines simply use bleed air from a) the main engine's compressor or

enter image description here

b) the main engine's turbo charger

enter image description here

Another solution is to drive dedicated air compressors to pressurize the cabin. Typically this may be Roots blowers or centrifugal fans. They are mechanically driven from the main engine(s).

enter image description here

Image source and more information on this topic: www.aircraftsystemstech.com

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A turboprop plane can be pressurized in the same way a turbofan plane can: via bleed air from the compressor stage of the turbine engine. A turboprop and turbofan are not that different actually. You have a turbine engine core that powers the big fan at the front or the big propeller via a gearbox (although the turbofan still gets some of its thrust from the engine core, but the turboprop does not):

Turboprop vs. Turbojet
(image source)

On the Dash-8 Q400 for example, the engines provide bleed air for pressurization:

The aeroplane is pressurized by engine bleed air supplied to and distributed by the air-conditioning system. Pressure is maintained and controlled by the cabin pressure control system which governs the rate of outflow from the pressurized areas (Figure 12.2-10) of the aeroplane. An aft outflow valve primarly controls the outflow of air, and is assisted by two safety valves.

(Dash-8 Q400 FCOM - Air Conditioning and Pressurization 12.2.2.1)

Air entering at the engine inlet is directed rearard and compressed (Figure 12.23-1). Two compressors carry out compression for combustion and bleed extraction purposes. Air is first ducted to the low-pressure (NL) axial compressor and then to the high pressure (NH) centrifical compressor where it undergoes a seconds stage of compression.

(Dash-8 Q400 FCOM - Power Plant 12.23.3.1)

The following image shows the bleed port in the PW150A engine as used on the Dash-8 Q400:

PW150A
(image source)

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    $\begingroup$ I cannot believe that a document as serious as an aircraft accident report has been produced using Comic Sans. $\endgroup$
    – Moss
    Commented Apr 30, 2020 at 1:25
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I was a P-3 Flight Engineer and the Allison T-56 engines had engine driven compressor's mounted on the propeller gearboxes on the inboard engines that allowed for a 30'000 ft. Ceiling.

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    $\begingroup$ This directly answers the question presented. First-hand stories and knowledge are valuable. $\endgroup$
    – le3th4x0rbot
    Commented Apr 30, 2020 at 0:04
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    $\begingroup$ With a decade of flying Herks with T-56's, I've never heard of a compressor mounted to the gearbox. What advantage over bleed air does that provide to be worth the extra equipment? $\endgroup$
    – Ralph J ♦
    Commented Apr 30, 2020 at 1:30
  • $\begingroup$ I am actually pretty curious about this now as well. These guys mention repairing an EDC (engine-driven compressor) for the P-3. blueaero.com/p-3 $\endgroup$
    – le3th4x0rbot
    Commented Apr 30, 2020 at 2:39
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    $\begingroup$ I added a specific question if you would like to elaborate: aviation.stackexchange.com/questions/77696/… $\endgroup$
    – le3th4x0rbot
    Commented Apr 30, 2020 at 2:46
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Piston powered aircraft can also be pressurized, the models that are have turbo-superchargers, an exhaust driven turbine that pressurises the air going into the cylinders. Some of the air from the turbos goes to the passenger cabin. The level of pressure is controlled by outflow valves just like jet powered aircraft.

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