Definition
A positive-displacement air pump that uses two intermeshing lobed rotors turning in opposite directions inside a close-fitting housing to trap a fixed volume of air on each rotation and push it from the inlet side to the outlet side. It does not compress the air internally; compression occurs when the trapped air is delivered into a higher-pressure outlet system.
Plain English
A pump with two spinning, lobed rotors that scoop fixed pockets of air from one side and push them out the other side. It moves air rather than squeezing it down inside the pump itself.
Context Anchor
Seen in aircraft system discussions where compressed air is supplied for equipment such as older pressurization, pneumatic, or engine-related air systems.
Derivation
Named after the Roots brothers, Philander and Francis Roots, who patented the design in 1860 in Indiana. They originally invented it as a water pump, but it turned out to work much better moving air, and the name stuck.
Why Pilots Care
Knowing it is a positive-displacement pump explains its behavior: output volume is tied directly to rotor speed, not to system demand. If the system needs less air, a relief valve or bypass must handle the excess, because the pump will keep delivering the same volume per turn.
Analogy
Think of two interlocking gears scooping water from a bucket on one side and dumping it into a tank on the other side. Each turn moves the same amount, no matter how full the tank gets.
Intuition Check
“Roots” does not mean plant roots or the root cause of a problem here. It is the name of a compressor design that moves air with two rotating lobed parts.
Example Sentence 1
The aircraft's cabin air supply uses a Roots-type air compressor driven off the accessory section of the engine.
Example Sentence 2
A Roots-type air compressor on the engine provided steady boost for takeoff performance at high density altitude.