Piston Engines - Performance and Power Augmentation — Page 147, Lesson 193

Right, let's get into the heart of how we actually control a supercharged piston engine from the cockpit. We're moving from the hardware—the supercharger itself—to the levers that manage it and the performance limits that come with it. First, the throttle lever. In a supercharged engine, we don't call it just a throttle; it's effectively a boost selection lever. Its position, within the limits imposed by full throttle height, determines the boost pressure delivered by the supercharger. So, together with the propeller control lever, it sets the engine's power output. The throttle controls the manifold pressure; the propeller lever controls the rpm. Now, the propeller pitch control lever—the rpm lever. Modern practice fits a variable pitch propeller, where the blade angles can be adjusted in flight between fine and coarse limits. That adjustment changes the load on the engine, which increases or decreases the rotational speed. So that's your rpm control. Here's the key operational concept: the action of the throttle in the internally supercharged engine. At sea level, the throttle valve must be partially open—what we call choked—to restrict manifold pressure and prevent excessive cylinder pressure. As the aircraft climbs, the air gets thinner, so the throttle must be progressively opened, either manually or automatically, to maintain that manifold pressure. Eventually, you reach a height where the throttle is fully open. That height is the Full Throttle Height, or FTH. Above FTH, power falls off just like a normally aspirated engine, because you can't open the throttle any further. Now, here's a subtle but critical point. Since the supercharger's effect depends on the impeller's rotational speed, each power setting has a different Full Throttle Height, depending on the engine speed and manifold pressure used. The Full Throttle Height at rated power settings is specifically called the Rated Altitude. Let me define those terms precisely. Rated Power, or Maximum Continuous Power (MCP), is the maximum power at which continuous operation is permitted. Take-off Power, and sometimes Climb Power, may have a time limitation imposed on their use. So, at Rated rpm and at Rated Boost—that's the rated manifold pressure—the height achieved is the Rated Altitude. It's a full throttle height, but only when you have both Rated rpm and Rated Boost set, meaning rated power. So, to tie it together: the throttle selects boost, the propeller lever selects rpm, and together they define your power setting. Each power setting has its own Full Throttle Height, and the specific one for rated power is the Rated Altitude. Above that, you're back to normal aspiration fall-off. That's the performance envelope of the supercharged engine.