Ram Air - Pneumatics - Airframes & Aircraft Systems #37

Summary

This lesson delves into the pneumatic systems of aircraft, highlighting how light aircraft utilize ram air and heating methods like exhaust gas or combustion heaters for cabin comfort. It explains the role of blowers in supplying air for pressurization and conditioning in larger aircraft, and the use of Roots blowers for creating a comfortable cabin environment. The importance of safety features in combustion heaters and the operation of various valves for temperature control are also discussed.

Takeaways

• 🛫 Most aircraft have a pneumatic system, with large modern airliners using a high-volume, low-pressure system that supplies air from engine compressors.
• 🚁 Light aircraft may use ram air, which is dynamic air pressure created by the aircraft's motion, to operate gyroscopic instruments.
• 🔥 Ram air can be heated by engine exhaust gases or a combustion heater to heat the cockpit and cabin, and to demist the windscreen.
• 🌬️ Aircraft without engine compressor air, like large pressurized piston engine aircraft, use blowers driven by the engine to supply air to air conditioning and pressurization systems.
• 🛬️ Older turboprop aircraft, such as the Fokker F27, use high-pressure pneumatic systems for operating landing gear and brakes.
• 🌡️ A typical light aircraft system using an exhaust gas heater includes hot windscreen misters and a fresh air blower for use on the ground.
• 🔧 Muff or exhaust muff allows ram air to come into close contact with the hot exhaust pipe to provide hot air for heating the cabin and demisting the windscreen.
• 🔥 Combustion heaters, which use the same fuel as the aircraft's engines, are standalone devices that burn a fuel-air mixture within a combustion chamber.
• 🌬️ Cabin air supply in some aircraft is provided by blowers driven through the engine accessory gearbox or by turbo compressors driven by bleed air.
• 🌡️ Roots blowers, a type of positive-displacement blower, are used in some systems to supply air, but they can cause a thumping noise in the air-conditioning ducts.

Q & A

What is the primary source of air for the pneumatic system in large modern airliners?

The primary source of air for the pneumatic system in large modern airliners is from the engine compressors, supplying a bleed air manifold with high-volume low-pressure air.

How do light aircraft typically operate their gyroscopic instruments?

Light aircraft typically operate their gyroscopic instruments using ram air, which is a dynamic air pressure created by the aircraft's motion.

What are the alternative sources of heating for the cockpit and cabin in light aircraft?

Alternative sources of heating for the cockpit and cabin in light aircraft include ram air heated by engine exhaust gases or by a combustion heater.

What is a muff or exhaust muff, and how does it contribute to heating the cabin in aircraft?

A muff or exhaust muff is a close-fitting cowl around the exhaust pipe that allows ram air to come into close contact with the hot exhaust pipe, providing hot air for heating the cabin and for misting the windscreen.

How does a combustion heater function in an aircraft?

A combustion heater functions by burning a fuel-air mixture within its combustion chamber. Air for combustion is supplied by a combustion blower, and the fuel is supplied via a solenoid-operated fuel valve. The flame is lit by an ignition unit, controlled by duct temperature sensors.

What safety devices are typically included in a combustion heater system in aircraft?

Safety devices in a combustion heater system include an automatic shutoff if the outlet air temperature becomes too hot, fire protection in case of combustion chamber failure, and an automatically operated fire extinguisher in some systems.

What type of blowers are used in aircraft to supply air to the air conditioning and pressurization systems when engine compressor air is not available?

Blowers driven by the engine, such as those in the engine accessory gearbox or turbo compressors driven by bleed air, are used to supply air to the air conditioning and pressurization systems when engine compressor air is not available.

What is a roots blower, and how does it function in an aircraft's cabin conditioning system?

A roots blower is a type of positive-displacement blower that consists of two lobes which mesh together and are rotated by the engine gearbox. It pulls in air through the inlet and expels it through the outlet, producing a small pressure increase but large flow rates.

Why is a silencer used in the cabin conditioning system with a roots blower?

A silencer is used to smooth out the airflow and reduce noise caused by the pulsing effect of the roots blower, as the air comes out in short pulses due to its design.

How is the air temperature in the cabin maintained at a comfortable level in aircraft with a pneumatic system?

The air temperature is maintained by mixing hot and cold air supplies in varying proportions. A variable restrictor, known as a choke valve, can be progressively closed to increase the pressure and temperature of the air leaving the blower.

What is the role of a mass flow controller in the pneumatic system of an aircraft?

A mass flow controller signals a spill valve to vent excess airflow to the atmosphere when too much air is supplied, especially at high engine speeds and low altitudes, ensuring the required mass flow of air under all operating conditions.