Rolls-Royce Dart
Part 3
During early 1948 two RDa1 engines were installed in a Vickers Wellington bomber (LN715) for
continuation of the flight trials. This was the first aircraft to fly exclusively powered by
the Dart.
In 1948 the Viscount prototype (G-AHRF) was fitted with Mark 503 RDa4 which produced 1250 shaft
horsepower, 150 horsepower below the original requirement. Naturally the prototype could cope
with these low power ratings by virtue of its light operating weights. After the Mark 504 RDa3
engines were installed the true payload capability of the aircraft could be explored under more
normal operating conditions. During the first ten months a total of 750 hours flying was carried
out, giving Rolls-Royce a lot of data to work with.
An Airspeed Ambassador was also fitted with Dart engines, and was another aircraft type
operated by BEA at this time. A water spray rig was fitted to the forward port fuselage to
allow continuation of the icing trials, which resulted in several engine modifications to
prevent the hazardous buildup of ice.
During 1949 the RDa3 was installed in a Douglas DC3 to allow Rolls-Royce to carry out more
intensive flight trials. This aircraft mainly operated from the Hucknall Flight Test
Establishment.
Rolls-Royce Darts fitted BEA DC-3 G-ALXN
A further two DC3 were modified (G-ALXN and G-AMDB) and started operations with BEA throughout
Europe in early 1951 as freighters, giving them their first experience of the operational
issues that they would later encounter with the Viscount. Over a period of two years these
aircraft accumulated a total of 3870 flying hours without an unplanned removal. All this helped
to convince the Air Registration Board (now CAA) that this engine had a future in the powering
of modern aircraft. These aircraft operated up to a ceiling limit of 25,000 feet which often
caused comment from air traffic control who found it hard to believe that a DC3 could be flying
so high. No doubt the flight crew played on this and had some fun in their well insulated
cockpit as they sucked away on their oxygen masks.
Production Viscounts, which were much larger than the prototype, were originally fitted with
either the Mark 505 RDa3 engines or the Mark 506 RDa3 engines producing up to 1400 shaft
horsepower, which became the standard engine on the BEA 700 series fleet.
The next major change to the Dart resulted in the RDa6 family with a substantially re-designed
engine producing 1535 horsepower for the Viscount 800 series. The Mark 510 was adopted by BEA
and others and some chose the Mark 515 variant. All this coincided with another increase in the
size and weight of the Viscount. A lot of RDa3 Viscounts were upgraded to the RDa6 variant which
changed the profile of the engine cowlings from parallel to a more pronounced bulge, noticeable
in many period photographs.
The engine was further refined to produce up to 1835 shaft horsepower by totally re-designing
the turbine module to incorporate three stages of turbine instead of two and an upgrade to the
reduction gearbox, compressor and combustion chambers. The maximum engine speed was also
increased from 14500 RPM to 15000 RPM. This resulted in the RDa7 installed in the Viscount as
the Mark 520 at 1630 shaft horsepower, Mark 525 at 1730 shaft horsepower and Mark 530 at 1835
shaft horsepower. This family of engines allowed the Viscount to grow and evolve into a very
useful commuter aircraft with subsequent worldwide sales.
