Definition
The lower of two impeller speed settings on a two-speed supercharger, used at lower altitudes to provide moderate manifold pressure boost without overcompressing the induction air. The supercharger's gear drive turns the impeller at a reduced ratio, supplying enough additional air to maintain rated power up to a designed critical altitude before the pilot must shift to high blower.
Plain English
It is the slower setting on a supercharger that has two speeds. At lower altitudes the engine doesn't need as much help, so the supercharger spins its impeller at the lower ratio. As the airplane climbs higher and the air thins out, the pilot switches to the faster setting to keep the engine making full power.
Context Anchor
Seen in discussions of supercharged piston engines, especially when describing how the pilot or system selects between lower and higher supercharger settings as altitude changes.
Derivation
"Blower" is an old aviation term for a supercharger, named for the way it blows compressed air into the engine's intake. "Low" refers to the lower of two gear ratios driving the impeller. The phrase reflects the era when supercharged engines were common and pilots spoke of selecting low or high blower the way they would select a gear.
Why Pilots Care
Selecting the correct setting prevents engine damage from overboosting at low altitudes while still providing needed power.
Intuition Check
Low blower does not mean the supercharger is weak or mounted low on the airplane. It means the lower-speed supercharger setting.
Example Sentence 1
During takeoff and climb at lower altitudes, the pilot kept the supercharger in low blower and only shifted to high blower as the airplane passed through its critical altitude.
Example Sentence 2
As the aircraft reached the critical altitude the pilot shifted from low blower to high blower to maintain manifold pressure.