Page 12 - August 2015 Volume 9, Number 8

Ask the Expert
A Supercharger
on a King Air?!
by Tom Clements
Why, yes indeed! All straight 90s, A90s and B90s use a single supercharger, driven by the left engine, as the source of air inflow for cabin pressurization. The PT6A-6 and PT6A-20 engines that these models used had lower compression ratios and delivered less air than later members of the PT6A family, yet in 1971 when the B90 ceased production and the C90 took its place, Beech did indeed use bleed air, tapped equally from both engines – the same old PT6A-20s – as the pressurization air source. So if sufficient air had always been available, why was it not used on the first three King Air models?
After all, not only does bleed air have the advantage of redundant, left and right supplies, but also it is so much warmer than the air from the supercharger that it becomes the primary in-flight heat source for the cabin. This allows the Jet-A-burning combustion heater (Janitrol heater) to be replaced by a much simpler and more reliable electric heater. With these advantages in mind, again we wonder why Beech ever made King Airs with superchargers.
The answer to this enigma is that Beech had always planned a piston-engine-powered “King Air,” and no bleed air was available from that type of powerplant. By going with the supercharger, a common system could be fitted to both models. The decision was made to proceed with the turbine version first. The piston- powered one, the Queen Air 88, did not make its appearance until a couple of years later. The 88 was never a hot-seller and less than 50 were made during its brief four-year production run. (Prospective customers had been spoiled with the much better performance of the turbine version, the venerable King Air. The 88 earned the not-so-affectionate nickname “Lead Sled” among the factory pilots.)
The PT6A-6 on the original 90, as well as the IGSO- 540 on the Model 88, both used the same method for driving the supercharger: The engine had a hydraulic pump that circulated fluid to a hydraulic motor that spun the supercharger. This cumbersome arrangement was replaced on the A90 and B90 models by a much simpler mechanical drive gearbox attached to the left
10 • KING AIR MAGAZINE
engine’s accessory case. I doubt that any 90s remain that have not been upgraded to the mechanical drive.
So what exactly is this supercharger? Wikipedia states, “The Roots type supercharger or Roots blower is a positive displacement lobe pump which operates by pumping a fluid with a pair of meshing lobes not unlike a set of stretched gears. Fluid is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust. It is frequently used as a supercharger in engines, where it is driven directly from the engine’s crankshaft via a belt, chain, or gears.
It is named after the American inventors and brothers Philander and Francis Marion Roots, founders of the Roots Blower Company of Connersville, Indiana, who first patented the basic design in 1860 as an air pump for use in blast furnaces and other industrial applications. In 1900, Gottlieb Daimler included a Roots-style supercharger in a patented engine design, making the Roots-type supercharger the oldest of the various designs now available. Roots blowers are commonly referred to as air blowers or PD (positive displacement) blowers, and can be commonly
called “huffers” when used with the gasoline- burning engines in hotrod customized cars.”
Wow! A design from 1860 still doing service as an air supply for some pressurized airplanes! Google search it if you’d like and you can find some fascinating drawings and even videos of the blower in action.
Figure 1 shows a two- lobe rotor model which is used in the King Air. The pump body has external fins to help dissipate the heat caused by compression. I do not have

A Roots blower with two-lobed rotors. Most real Roots blowers’ rotors have three or four lobes. Key:
1 Rotary vane 1 2 Pump Body
3 Rotary vane 2
a Intake
b Pumping
c Forced air or air-fuel mixture into intake manifold
Figure 1: An example of a two-lobe rotor model as is used in the King Air.
OCTOBER 2015