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Can you control the cabin temperature if the cabin is unpressurized? What if the "cabin" isn't really that big, like a fighter aircraft?

For example, could I cruise in comfort at 20,000 ft in an unpressurized cockpit with an oxygen mask and heater?

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Yes, you could. The smaller cabin is easier to heat because of its size.

If the engine is big enough to carry you to 20,000 ft, it produces plenty of waste heat for cabin heating. When the Russians experimented with docking fighters to bombers to extend the range of those fighters, the fighter pilots complained bitterly about the freezing temperatures in their cockpits once their engine was switched off. The first prototype of the FW-190 had no armament and a shorter fuselage, so the firewall was immediately ahead of the cockpit. The test pilot said flying it felt like having his feet roasted at a fireplace because the engine heat radiated into the cockpit.

Piston-engined aircraft allow to mix pre-heated and unheated air for cabin ventilation. The heat is either from a heat exchanger or just from piping the air near the engine. Jets and turbine-powered helicopters use bleed air which has been heated in the compressor as the source of cabin heat.

This is different in gliders, because there is no heat source and when flying at high altitude it can be really cold. You can wear thick clothing, but another problem is frosting of the canopy from the inside caused by the moisture in the pilot's breath. I found this more annoying than the cold temperature when flying in freezing air.

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    $\begingroup$ Interesting note about waste heat from the engines. I guess I'll route a small air duct over the engines and into the cabin! $\endgroup$ – techSultan Nov 14 '16 at 23:23
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If the cabin is unpressurised, the temperature could be controlled to a limited extent- this is usually done in a number of helicopters.

You can certainly cruise at 20k feet without a pressurized cabin - WWII pilots regularly did this with supplemental oxygen and heated clothing- though I'd hardly call it comfortable.

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    $\begingroup$ If WWII pilots could cruise in the cold, I think our customers can do it too with today's lighter, warmer clothing. To give you some context, I'm in a senior capstone team designing a light attack aircraft with an RFP unit cost limit of $5 million, so I'm trying to make the aircraft as bare-bones as possible! $\endgroup$ – techSultan Nov 14 '16 at 23:26

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