Piston Engines - Performance and Power Augmentation — Page 147, Lesson 195

Let's pick this up right where the summary leaves off, because the first thing I want you to grasp is the relationship between Rated Power, Full Throttle Height, and the two levers you're holding in the climb. Think of Full Throttle Height as the altitude at which the throttle valve is fully open—wide open—and you can no longer maintain your selected boost by opening it further. Above that height, the manifold pressure will start to fall off. Now, the summary gives you two contrasting effects. First: if you climb at less than Rated Power by keeping the Rated rpm but selecting less than Rated Boost, you increase the Full Throttle Height. Second: if you climb at less than Rated Power by keeping the Rated Boost but selecting less than Rated rpm, you decrease the Full Throttle Height. Why the difference? It comes down to the compressor. Remember, it's the size and the rotational speed of a Centrifugal—or Radial—Compressor that determines its output. If you reduce the rpm, the compressor spins slower, so its output drops. To make up for that reduced compressor output, you have to open the throttle valve more to get the same boost. So in the climb, with low rpm, the throttle opens more quickly to compensate for the slower impeller speed, and you reach the full-open position sooner—hence a lower Full Throttle Height. Conversely, if you keep the rpm up but select a low boost, the compressor is still spinning fast, producing plenty of pressure. You don't need to open the throttle as much, so it opens more slowly in the climb, and you can keep climbing before the valve reaches fully open—hence a higher Full Throttle Height. Now, here's a key link: the propeller control lever can be said to be an engine speed control. And because the impeller is geared to the crankshaft, any change in engine speed results in a corresponding change in the speed of rotation of the impeller. So when you move that propeller lever, you're directly changing how fast the supercharger impeller spins. That brings us to the Automatic Boost Control—the ABC unit. Let me explain why it exists. The supercharger is designed to maintain a given pressure at altitude. To do that, the impeller must be driven at a high speed, because at altitude the atmospheric pressure has dropped considerably. But here's the problem: at low altitudes, where the air is more dense, that same high-speed supercharger produces too much pressure. If you let that pressure run unchecked, you'd get severe detonation and mechanical stresses from excessively high combustion pressure. So the delivery pressure must be restricted by only partially opening the throttle valve. As the aircraft climbs, the air thins out, and the throttle valve must be progressively opened further to maintain a constant boost pressure. Now, imagine the pilot having to constantly fiddle with the throttle lever during every climb and descent—that's a heavy workload and easy to get wrong. So to relieve the pilot of that responsibility, the boost pressure is kept constant automatically by the Automatic Boost Control unit, which is generally attached to the carburettor. So the ABC unit's job is to sense the boost pressure and automatically adjust the throttle position to hold it steady, so you don't have to. That's the core of what we're looking at here—the automatic regulation that keeps your manifold pressure constant as you change altitude.