James Hill, Chairman of the Gas Turbine Builders Association (GTBA), spoke to the Hawker association on 13th July.
James’s main career was international commodity trading in heavy fuels but now is commercial property management. However, he has a long standing interest in aviation and mechanical engineering and his enthusiasm for model helicopters led him to model gas turbines. Excelling in metal work and technical drawing at school he developed these skills in model engineering and gas turbine design.
Model gas turbine development was started in the UK by Gerry Jackman with a group of five engineers in the 1970s. A working self-sustaining engine was running in 1975 and giving useful thrust in 1982. The world’s first gas turbine powered model aircraft, the twin boom “Barjay”, flew from Abingdon, Oxfordshire, in 1983 on 9 lb of thrust from a 4 lb engine, 4 ins in diameter and 14 ins long, running on propane at 85,000 rpm.
The GTBA was formed from the Gas Turbine Builders Contact Group in
1995 and within two years had 1000 members. Designs were improved due
to the centralised information in the GTBA being available to all
members. A modern gas turbine gives 14 lb of thrust from a 2 lb engine,
still 4 ins in diameter but only 6 ins long, running on kerosene at
A number of members went on to start their own model gas turbine and component manufacturing businesses.
Thermodynamically a model gas turbine is the same as a full size
engine but because air doesn’t scale it’s not possible simply to scale
down the components. Also, the operating envelope and requirements are
different. Compared to a full size engine the model runs at slow speeds
at low altitude for short durations with high thrust being the aim and
with fuel efficiency a low priority.
Design choices are as for full scale (centrifugal or axial compressors with radial inflow or axial turbines) but usually models have had centrifugal compressors and axial turbines (like Whittle), but now radial inflow turbines are favoured for reasons of robustness. All-axial engines would have lower frontal areas but would be complex. However, hybrid (axial plus centrifugal) compressors are finding favour by giving increased compression ratios without too much complexity.
James gave some typical figures for a model engine. Compressor: inlet air speed 300 m/sec, diffuser air speed 70 m/sec, compression ratio 2.5:1, temp rise 150 deg C. Combustion: vaporising system at 2,000 deg C. Turbine: entry temp 730 deg C. Jet pipe: air speed 450 m/sec at 580 deg C. Air flow through engine 6 cu ft per second.
The high operating rpm make bearing integrity and rotor balance
critical. Bearings were normal steel ball and race type but now hybrid
bearings with ceramic balls and steel races are used at up to 160,000
rpm! Dynamic balance is achieved by balancing in stages (shaft, shaft
plus compressor, shaft plus turbine, complete) using piezo quartz
sensors, strobes and test weights in a computerised rig.
Model gas turbines, both turbojet and turbo shaft, are used to power
fixed and rotary winged aircraft, boats, trains and road vehicles.
Furthermore, development continues with a turbofan already running.
James had brought along examples of beautifully made exquisite examples of miniature engineering: components, rotors and complete engines, for Members to handle and examine and, even better, set up his engine running test stand outside.
Fortunately the weather was good. The engine was instrumented with all the basic pressure, temperature and rpm parameters being displayed as traces on a lap-top screen. The engine was started by spinning it up to its 35,000 rpm idle speed with an electric motor where pre-heated propane was used to start ignition after which kerosene was fed in for continuous running during which EGT and RPM were carefully monitored. Those of us who had participated in aero engine ground runs were in familiar territory.