A Ram Air Turbine, often called a RAT, is a small emergency turbine that deploys into the airstream when an aircraft loses normal power. It acts like a compact wind turbine, using the aircraft’s forward motion to spin blades and drive a generator or hydraulic pump. This matters because large aircraft rely on electrical and hydraulic systems for flight controls, instruments, and communication.
The RAT provides a last-resort source of power that can help pilots keep control during serious failures.
Key Facts
- A Ram Air Turbine converts airflow energy into mechanical rotation during an emergency.
- Dynamic pressure is q = 1/2 rho v^2, where rho is air density and v is airspeed.
- Ideal wind power through a turbine area is P = 1/2 rho A v^3.
- Actual RAT power is lower than ideal because of efficiency losses: Pout = eta Pin.
- RAT output increases strongly with airspeed because available airflow power depends on v^3.
- A RAT may power an electric generator, a hydraulic pump, or both depending on aircraft design.
Vocabulary
- Ram Air Turbine
- A deployable emergency turbine that uses the aircraft’s forward motion through air to produce hydraulic or electrical power.
- Dynamic Pressure
- The pressure associated with moving air, calculated as q = 1/2 rho v^2.
- Hydraulic Power
- Power transmitted by pressurized fluid to move aircraft components such as control surfaces or landing gear.
- Generator
- A machine that converts mechanical rotation into electrical energy.
- Airspeed
- The speed of an aircraft relative to the surrounding air.
Common Mistakes to Avoid
- Thinking the RAT is a normal engine, which is wrong because it does not burn fuel or produce thrust for flight.
- Assuming the RAT works while the aircraft is parked, which is wrong because it needs strong airflow from forward motion to spin effectively.
- Forgetting that power depends on v^3, which is wrong because doubling airspeed can greatly increase available turbine power.
- Treating RAT power as unlimited, which is wrong because it only supplies essential emergency systems and has efficiency and size limits.
Practice Questions
- 1 A RAT has a swept area of 0.40 m^2 and the aircraft is moving at 90 m/s through air with density 1.0 kg/m^3. Estimate the ideal airflow power using P = 1/2 rho A v^3.
- 2 If the ideal airflow power into a RAT is 120 kW and its overall efficiency is 35 percent, what useful output power does it provide?
- 3 Explain why a Ram Air Turbine is useful after engine failure but cannot replace the full electrical and hydraulic power normally supplied by the aircraft engines.