Engine Description
Engine
The heart of the engine is a miniature turbojet designed specifically to produce a gas flow to drive a 2
nd
turbine to generate a shaft drive to power small model aircraft. It has a single stage billet machined
centrifugal compressor and single stage cast Inconel axial flow turbine. The engine is fitted with an
internal ceramic glowplug which enables the engine to initiate combustion directly on liquid fuel after
which further fuel is gradually introduced into the main part of the combustion chamber to provide
combustion heat to operate the engine.
A high speed brushless electric starter motor fitted with a clutch mechanism, provides drive to the rotor
all the way up to 60,000rpm idle for starting the engine. The starting sequence is controlled by an
electronic system fitted to the engine (ECU) which initiates the start sequence and controls the
parameters of the engine within design limits.
The engine rotor shaft is supported by ceramic bearings, lubricated by a small percentage bleed off the
pressurized main fuel supply, which is added a small percentage of oil for this purpose. The rotor discs
are separately balanced and then 2-stage dynamically balanced on assembly to the engine. Disturbing
the rotor will lose this delicate balance and the engine will need to be returned to a service unit for
rebalancing.
2
nd
Stage
The 2
nd
stage is the part which converts the fast gas flow from the engine to a shaft drive to power an
external load. It has three major parts, the interstage vane, the power turbine and the spider. The
interstage and spider are high temperature stainless and the power turbine is of Inconel and is blown
round at high speed by the gas flow which is angled in direction by the interstage vanes. The spider
holds the power turbine bearing in the centre.
The power turbine is mounted on a shaft with substantial ceramic bearings and drives the gearbox to
which it is connected. The orientation and reduction in the gearbox is dependant on the application. The
heli unit uses a right angle single stage spiral bevel reduction, the turboprop uses a horizontal 2 stage
spur gear reduction. All gears are fully hardened for long life. The 2
nd
part of the turboprop gear reduction
is available with two ratios to facilitate large or small reduction ratios for different prop drive regimes.
The gearbox of all versions use a magnetic pickup to read output shaft rpm. This signal is connected to
the ECU via the Aux input on the Hub and used to monitor shaft speed conditions and detect potential
anomalies which either require a limiting function, a constant speed function or some protection function
when a fault condition is shown. These conditions and adjustments which can be made to them are
described in more detail later.
Lubrication for the 2
nd
stage and gearbox is taken from a bleed off the main fuel supply, plus a small air
pressure feed to maintain a positive pressure in the gearbox. These are mixed and output via the 2
nd
4mm swivel fitting on left side of the engine. A small pipe conveys this mixture to an injection point on the
gearbox.
Never run the engine without this pipe properly in position as 2
nd
stage bearings and gears will be
severely damaged within a short time (30secs) running.
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