Hello,

Im an engineering student working on designing a non-pressurized search and rescue aircraft. I have been assigned to work on the aircraft's environmental control system (ECS), specifically the temperature control of the cabin, and am having trouble getting a grasp on what features of the aircraft's air conditioning machine would not be included since it is not a pressurized vessel. I have been reading quite a lot on air-cycle machines and inverse-brayton cycles and am wondering as to how the engine bleed air for this non-pressurized aircraft is cooled and delivered to the cabin. I hate to say this but I am kind of in a hurry if anyone can help me this is quite urgent. Thank you in advance to anyone who can help me out.

I'd wager it's probably not much different. Just no outflow valve. The whole cabin is a big outflow valve.


Just like one of those air conditioned Cessna 172s that can only hold two passengers and fuel.

Hello Woodreau, thank you for your reply, I appreciate it very much. So if I understand you correctly, first of all their definitely IS an air cycle machine in non-pressurized aircraft and it would consist of the usual 2 heat exchangers, a turbine, and a compressor. And the bleed air is cooled in a heat exchanger with RAM air. Just for the record we are only looking to heat the cabin at our cruising altitude of 13,000 ft.

Look at a Cessna 172 or Air conditioned GA aircraft for the EVS for a non pressurized aircraft. No need to reinvent the wheel.

If it is a piston aircraft? why use bleed air?
Ram air heated by the motor should be fine.

For heating and cooling a non-pressurized turbine aircraft look at the Beech 99 and the Dornier 228

The real differences between pressurized and non is the airframe and the size of the ECS system. In a non pressurized aircraft the fuselage is not robust enough for the pressure cycles and there is no pressurization controller to control the outflow valve. You would still have a overpressure dump valve and the ECS would be downsized to only provide conditioned air and not the volume you would need not pressurize the vessel.

Thinking back thirty years to my Be99 days, I recall there was a electric heater and a VCS air conditioning system that were mostly used on the ground. Yes, it was basically an automotive system mounted on one of the engines. It was a real pain to keep working. Beech had tons of problems with compressor mounts and the seals on the entire system were always going bad on our old Be99.

Hello all, I thank you very much for your replies. I have made a decision as to the design implementation of our aircraft, feedback is more than welcome. I have decided to go with the final design of heating RAM air by the motor and pumping it into the cabin. Avionics will be cooled by free stream air flowing through a vent. Does this make sense and is this applicable? I am also aware of heating the cabin by having a combustion heater, but decided it was a more complex and power consuming design.

Are you actually going to be rescuing folks with the aircraft? Or just search? If you are actually picking folks up in a rescue situation then being able to control the cabin temperature is a bit more important to help with hypothermia. You may want to go with a more complex/reliable system. If you are just heating the cabin for crew comfort during a search operation, then it can be secondary.
I work on EMS helicopters, and if our heater or air conditioner is broken we are essentially out of service since they are not allowed to carry patients. It can be MEL'd, but in reality does no good for operation reasons.

Hello dragon, I appreciate your response. The aircraft our team is designing is a fixed-wing search and rescue aircraft. To my knowledge the aircraft will only conduct the searches and in return dispatching the choppers to do the rescuing. Please stop me if my information is incorrect.