Rolls-Royce Dart
Part 2
Dynamometer testing commenced on July 13 1946 and test runs in a hangar using a slave propeller
on July 7 1947.
Development did not go smoothly, as when the RDa1, as it was now known, was initially weighed it
was found to be overweight at over 1100 Lbs against a target weight of 850 Lbs. The oppositions
Mamba weighed in at a mere 707 Lbs, so Rolls-Royce had to rethink the whole design, if they were
to remain competitive.
The Rolls-Royce Dart on a TCA - Trans-Canada Airlines Viscount
Photo © Jim Bruce
Power runs also highlighted that the engine was only developing about 600 shaft horsepower, way
short of the target of 1000 shaft horsepower. Problems with gas and air leakage were experienced
and on some tests the turbines were cooled using compressed air from an external source. Starting
was also problematical at times and often resulted in a welding torch being inserted into an
igniter access port in order to ignite the fuel vapour. Clearly these problems demanded a lot of
changes from the design team to get back on track both in terms of weight and power.
The engine consisted of two stages of centrifugal compressor and two stages of turbine coupled
together to make up a single shaft arrangement driving a reduction gearbox.
Weight savings were made by re-designing the reduction gearbox with helical gearteeth in place
of the spur gear arrangement and incorporating the oil tank within a cast air intake casing.
Magnesium was also utilised instead of aluminium for all the casing castings, diffusers and
compressor impellers but after only 100 hours of test running at 350 shaft horsepower numerous
stress cracks were found which resulted in the majority of components reverting back to
aluminium, although the reduction gearbox and auxiliary gearbox casings survived in magnesium.
One of the benefits of the integral oil tank was that the hot oil helped to keep the air intake
from icing up under certain flight conditions.
An RDa1 was installed in the nose of an Avro Lancaster bomber (NG465) and first flew October 10
1947 carrying out the first flight trials of this new engine. During later flights a spray grid
was installed in front of the propeller to allow icing trials to be conducted.
Whilst the power development programme was continuing to reach the 1000 shaft horsepower target,
Rolls-Royce received a new requirement from the Ministry of Supply to produce a new variant of
the Dart for the proposed Vickers-Armstrongs Viceroy ( later re-named Viscount) with an increase
in power to 1400 shaft horsepower. The increase in power proposed actually helped Rolls-Royce as
the power to weight ratio improved.
A reduction in air and gas leaks coupled with a change in internal components to improve the
airflow and thermal power efficiency improved things dramatically. Over 100 shaft horsepower was
gained in the re-design of the combustion chambers to stop all the joints leaking air at the
front and gas at the rear. The compressor module was subjected to a major re-design of the throat
areas, static and rotating vane angles and profiles in order to deliver a higher mass flow of air
to the combustions and turbines. The turbine itself came in for some major changes to improve the
aerodynamics and to make the nozzle box and inner cone more rigid which cut down on the number of
gas leaks from the discharge nozzles.
The end result achieved in various stages of development and involving numerous tests and the
charting of the improvements gained, resulted in an engine achieving 1250 shaft horsepower and
designated as an RDa4 and 1400 shaft horsepower designated as an RDa3 for the Viscount. There
is no reference to the RDa2 designation, which presumably was allocated to the aborted Balliol
project.
