Definition
A turbine that extracts energy from a flow of hot gas using a combination of two principles along the length of each blade: impulse action near the blade root, where the gas strikes the blade and pushes it like a paddle, and reaction action toward the blade tip, where the blade is shaped like a small airfoil so that gas accelerating across it produces a forward force. Most turbine engines used in aviation use this combined design because it gives even loading along the blade and higher overall efficiency than a pure impulse or pure reaction turbine.
Plain English
A turbine wheel whose blades are shaped so that the inner part of each blade is pushed by the hot gas hitting it, while the outer part is pulled along by the gas flowing across its curved shape. The two effects work together to spin the wheel.
Context Anchor
Seen in turbine engine theory and maintenance discussions, especially when learning how a gas turbine turns hot gas into shaft power.
Derivation
Impulse comes from Latin 'impellere,' meaning to push against. Reaction here refers to Newton's third law -- every action produces an equal and opposite reaction. The name simply tells you the blade uses both effects at once: a push at the root and a reaction force across the curved tip.
Why Pilots Care
Knowing that a turbine blade works on two principles along its length helps explain why turbine sections are sensitive to damage, heat, and contamination. A nick or erosion on the blade tip changes its airfoil shape and reduces the reaction part of the energy extraction, which shows up as lost performance and higher operating temperatures.
Intuition Check
Do not read “impulse-reaction” as two separate turbines. It describes one turbine design that uses both impulse force and reaction force to turn.
Example Sentence 1
The hot section inspection revealed erosion on the impulse-reaction turbine blades, which had reduced engine efficiency over time.
Example Sentence 2
During overhaul the technician checked the impulse-reaction turbine blades for cracks and erosion.