Viscount Technical Details
Introduction
The Rolls-Royce Spirit of Ecstasy
The magic of a name: Rolls-Royce. It is still a byword in automobile engineering, and their aero
engines have been equally famous since the first half of the Twentieth century. The Dart is the
name of a smooth-flowing, fast-running English river, and Rolls-Royce named their first and at
the time their most unique engine after it - unique for two reasons, firstly because it was the
first turbine in the world to drive a civil aircraft, and secondly on account of its striking
simplicity.
The operating cycle of the propeller turbine (turbo-prop) follows that of the well-known jet
engine. A large volume of air is sucked in at the front of the engine, compressed and blown out
behind the compressor into combustion chambers where fuel is continuously sprayed in and burnt.
This releases enormous heat energy, with the result that incandescent columns of gas roar out of
the chambers, past the turbine blades which drive the compressor, and escape through the tail
pipe into the atmosphere as a high energy jet-stream.
This is the noise you hear when a jet flies over, the roar coming from its turbines. Its power
is in the form of thrust, derived from the 1,000 mile-per-hour acceleration it has imparted to
the air between entering and leaving the engine, like the recoil of a gun as it fires a shell.
In just the same way, the thrust of a propeller derives from the acceleration of the air it pumps
through it, only of course there's much less acceleration, less noise but correspondingly more
air. The noise you hear from a Dart engine is the high pitched whine of the low pressure compressor
rotating assembly. The reason for the noise is that the tips of the rotating parts are virtually
travelling at supersonic speed.
The efficiency of either of these 'propulsive pumps' is highest when the speed of the air leaving
it is the same as that of the aircraft itself, or in other words, when the slipstream is not
moving relative to the atmosphere and there's no wasted energy. It can never quite happen, but
that is the theory, and when you have grasped it you can see why a jet is inefficient at the
slow-to-medium flight speed suited to the propeller, and why jet automobiles will never be built.
On the other hand, there are practical limits to the velocity which a propeller can impart to
its slipstream - the blade tips soon reach the speed of sound and then areodynamics are apt to
go haywire - and so it in turn is unsuitable for very high-speed aircraft.
Propellers were the logical choice when it came to deciding how to power the Viscount
But since speed costs money, commercial jets engines are mostly confined to long-range airliners,
and so propellers were the logical choice when it came to deciding how to power the Viscount,
which was designed for economic cruising speeds over medium ranges. It was known that a propeller's
economy might be combined with the smooth-running and compact simplicity of a turbojet, if the
latter's turbine were made big enough not only to turn the compressor, but to harness the
remaining jet energy for driving a propeller.
