Thrust reversers are devices on jet engines that help slow an aircraft after it touches down on the runway. They do not make the plane fly backward, but they redirect some engine airflow forward to create a braking effect. This reduces the workload on the wheel brakes and helps the aircraft slow safely, especially on wet or slippery runways.
Pilots use thrust reversers as part of a landing system that also includes spoilers and brakes.
Key Facts
- Forward thrust pushes an aircraft forward, while reverse thrust creates a force opposite the aircraft motion.
- Net stopping force = wheel braking force + spoiler drag + reverse thrust.
- A turbofan thrust reverser usually redirects bypass air, not the hot core exhaust.
- Impulse relation: F = Δp/Δt, so changing the direction of airflow changes the force on the engine.
- Spoilers reduce lift and increase drag, helping the wheels press harder on the runway for better braking.
- Kinetic energy before stopping is KE = 1/2 mv^2, so higher landing speed greatly increases stopping energy.
Vocabulary
- Thrust reverser
- A device on a jet engine that redirects airflow forward to help slow the aircraft after landing.
- Turbofan engine
- A jet engine that uses a large fan to move a large amount of bypass air around the engine core.
- Bypass air
- Air pushed by the fan that flows around the engine core and produces much of the thrust in a modern airliner.
- Spoiler
- A movable panel on the wing that rises after landing to reduce lift and increase drag.
- Wheel brake
- A braking system in the aircraft wheels that uses friction to convert motion energy into heat.
Common Mistakes to Avoid
- Thinking thrust reversers make the airplane go backward. They mainly help slow the aircraft by producing a force opposite its motion after touchdown.
- Ignoring the role of wheel brakes. Thrust reversers assist stopping, but the wheel brakes usually provide much of the stopping force.
- Forgetting that spoilers help braking. Spoilers reduce lift so more of the airplane's weight presses on the wheels, improving tire grip and brake effectiveness.
- Assuming reverse thrust is used in the air during normal flight. On airliners, thrust reversers are normally deployed only after landing because using them in flight would be dangerous.
Practice Questions
- 1 An aircraft has 40,000 N of wheel braking force, 12,000 N of spoiler drag, and 18,000 N of reverse thrust. What is the total stopping force?
- 2 A 70,000 kg aircraft lands at 70 m/s. Using KE = 1/2 mv^2, calculate its kinetic energy just after touchdown.
- 3 Explain why spoilers make wheel brakes more effective after landing, even though spoilers are mounted on the wings.