Wren 44 TurboProp Users Manual. Copyright Wren Turbines Ltd February 2008 Page 6
General description of the Wren 44 two-shaft drive system
The Wren 44 TurboProp is the worlds smallest commercial 2-shaft turbo-prop engine. It is
designed for use in miniature aircraft applications in place of an I/C engine. It is generally suited
to aircraft up to 25kg (55lb) in all up weight and will replace I/C engines of around 80cc (4.8cu
inch). The engine runs on standard kerosene and is supplied in full auto-start configuration.
What is a two-shaft system?
The two shaft system means there are two independant shafts running within the unit. The first
shaft is contained within the engine end of the unit and rotates at very high speed (up to
195,000rpm) with just a small compressor wheel and turbine attached at each end. This engine
end of the unit generates a flow of gas at high pressure and volume, and its operation is exactly
as a small gas turbine engine. It fulfills the function of what we call a
gas generator
. If a nozzle is
attached to the outlet of the engine it imparts a slight squeezing of the gas into a high velocity jet
for producing jet thrust and is the configuration for a thrust engine. For a gas generator version of
the engine, instead of squeezing the gas through a nozzle it is redirected by a vane assembly to
turn a 2nd turbine wheel mounted on the 2nd stage shaft. This is driven round in the gas stream
and this rotation drives the input shaft to the gearbox and onwards to the propeller. This 2nd
turbine is much larger in diameter than the 1st stage and correspondingly runs much slower - up
to
only
90,000rpm - still far higher than any 2-stroke or electric motor could achieve, but at a high
torque level. The energy given up by the gas driving the 2nd stage turbine drastically reduces the
speed of the exhaust gas with the result than only a small residual thrust remains from the
exhaust outlets.
What happens if I stall the propeller?
When operating from long grass or in a nose-over situation that causes the propeller to stall, the
gas generator will continue to function normally with little ill-effects. On releasing the propeller
from its stalled form it will spin back up to it's normal running speed. This should be born in mind
when retrieving the model from the long grass or nose-over situation - for you or your helper to
keep well clear of the propeller whenever the gas generator is running.
What sort of gearbox is required?
The modest rpm levels generated by the 2nd stage turbine (by gas turbine standards) enable an
suitable reduction to be contained in a small gearbox, the ratio of which is chosen to suit the
operational needs of the load driven. In the case of the 44 TurboProp the reduction is 9:1 and this
gives a propeller shaft speed range of 6,000-9,000rpm. The 2nd turbine has a wide operating rpm
range and may be slowed with high load or allowed to speed up with low load without upsetting
the 1st stage, therefore the choice of propeller is not at all critical, providing it presents enough
load for the system (see warning below). The main criteria for propeller choice being the type of
plane the unit is fitted to (scale, aerobatic, sports etc).
What are other two shaft examples?
Other Wren applications that use the same 2-shaft system are the Wren 44 Marine variant that
has a 2.3:1 reduction gearbox for an output speed of 25,000-40,000rpm and the Wren 44 Heli
unit with a reduction of 4:1 and output speed range of 12,000-20,000rpm.
WARNING - it is most important that there must always be some load on the output shaft
as otherwise the 2nd stage turbine will be running unrestrained and may easily speed up
beyond it's safe running speed, even when the gas generator is running at only a modest
rpm. This means the unit should never be run without a suitable propeller fitted.
