Definition
A supercharger fitted with a two-speed gear drive that allows the pilot to select either a low-blower or high-blower setting, changing the speed at which the impeller is driven by the engine. The low setting is used for takeoff and lower altitudes; the high setting is engaged at higher altitudes where the thinner air requires the impeller to spin faster to maintain manifold pressure.
Plain English
A supercharger with two gear settings. The pilot picks a slower setting for low altitudes and a faster setting for high altitudes, so the engine can keep getting the air pressure it needs as the air outside gets thinner.
Context Anchor
Seen in discussions of high-performance piston engines and how they keep engine power from dropping off as the airplane climbs.
Derivation
“Supercharger” combines “super,” meaning above or extra, with “charge,” an older engine word for the air mixture supplied to the cylinders. “Two-speed” tells you the compressor can be driven at two different speeds, not that the airplane has two flying speeds.
Why Pilots Care
It provides sea-level power up to a higher critical altitude while avoiding overboost at lower altitudes.
Analogy
Think of it like a two-speed bicycle gearbox. Low gear works fine on flat ground, but as the climb gets steeper you shift into a different gear to keep the wheels turning at the right speed.
Intuition Check
Do not read “two-speed” as two aircraft speeds. Here it means two drive speeds for the supercharger itself: a lower speed and a higher speed.
Example Sentence 1
Climbing through the published shift altitude, the pilot moved the supercharger control from low blower to high blower to maintain manifold pressure.
Example Sentence 2
On the takeoff checklist the crew confirmed the two-speed supercharger was still in low gear for maximum low-altitude power.