After the First World War supercharger development concentrated on gear-driven centrifugal compressors and turbochargers. NACA was almost alone in the pursuit of the Roots blower.
Gear-Driven Centrifugal Blowers
A centrifugal compressor consists of an impeller and a diffuser housed in a helical casing, or scroll. The diffuser, sometimes called the stator, occupies the annular space between the impeller and scroll. Passages created by the diffuser vanes open wider as they approach the discharge throat. Vanes on the impeller wheel are arranged radially and may be straight or curved. The use of curved vanes came relatively late in the period and improved efficiency.
Air enters at the impeller hub, rotates with the impeller and, under the influence of centrifugal force, moves outward in a path defined by the impeller vanes. Upon contact with the diffuser, the air expands and slows, converting much of its kinetic energy into static pressure.
Because the impeller cannot be allowed to make physical contact with the shroud, there is always some leakage between the vane tips and the scroll. The seal consists of air, an elastic medium. At low rotational speeds the impeller merely flays about delivering little or no output. As tip velocity increases, the air seal becomes more positive and the compressor begins to pump. Unlike Roots blowers that move the same volume of air per revolution, centrifugal compressors are dynamic machines, whose output increases as the square of speed—double the speed and the output theoretically quadruples.
Work on aircraft superchargers began in Europe around 1915 with the intent of normalizing output at high altitudes. Initially experiments were carried out with a variety of pumps, but within a year or so the French settled on the Rateau turbocharger. The Royal Aircraft Factoryconcentrated on gear-driven centrifugal pumps until about 1917, then turned to turbochargers and, in the years immediately after the war, resumed work on geared compressors. J.E. Ellor, who would become the leading British authority on supercharging, directed these efforts. German researchers concentrated on direct-drive superchargers until the Treaty of Versailles put a temporary halt on aircraft research. When research efforts resumed in the 1930s, the focus was on geared centrifugal compressors. Only one German airplane of WW2 was turbocharged.
In order provide even moderate levels of boost, centrifugal impellers had to be geared up to turn at five or six times crankshaft speed (Fig. 1). At these speeds, the inertia of the impeller and its drive gears was about equal to that of the propeller. When coupled to a torsionally flexible crankshaft, immense stresses were put on the drive train. Gear teeth stripped during acceleration (most pronounced when the engine started) and deceleration.
In 1916, Sam D. Heron observed the flight test of a supercharged BE2. As Heron later put it for publication, “The observer sat forward with his feet under the fuel tank and over the supercharger’s gear drive. The gears were quite inadequate and the pinion failed in flight, producing showers of sparks and a feeling of distinct concern.” Since Heron was not the most equitable of men and his immediate reaction was probably less restrained.
The Royal Aircraft Factory tried various expedients to reduce or at least dampen forces acting on the gears. Experiments were made with lightweight, sheet-steel rotors and various sorts of cushion drives. One of these drive mechanisms, a centrifugal slip clutch designed by Heron and F.M Green, appears to have solved the problem (Fig. 2). The clutch was used on the Armstrong-Siddeley Jaguar of 1926, the first supercharged radial engine to achieve operational status. About the same time, a modified version of the clutch appeared on the Bristol Jupiter.