Model Aircraft Gas Turbine Engines

 


Miniature gas turbine engines for model aircraft propulsion can be
purchased now from a number of manufacturers or in fact built at home
if you have access to precision engineering facilities.

Model turbo-jet engines consist of a centrifugal compressor wheel driven from
a single stage turbine. Sometimes turbo-charger compressor wheels
are used or they are manufactured specially. Combustion chambers are
either through flow (from the front of the engine to the back) or reverse
flow where the combustion gases reverse and flow from back to front
and then turn around. The reverse flow tends to shorten the length of
the rotating shaft needed to connect the compressor to the turbine wheel.
A reverse flow combustor also provides a longer burning length.

Cobra Turbo-Jet Engine

Model gas turbine engines use either radial inflow turbine wheels (Compressor wheel in reverse) usually originating from
turbo-chargers or axial flow turbine wheels which are specially made.

Model engines operate on liquid fuel or propane gas or both during starting. Liquid fueled engines quite cleverly use
very small vaporizing burners to remove the need for a high pressure pump/atomizer system.


Home built model gas turbine engines

The Cobra Turbo-Jet Engine

The Cobra is a high performance miniature gas turbine designed for the propulsion of large model aircraft and other unmanned airborne vehicles. It can also be used as a demonstration gas turbine for research and educational use.

The Cobra was designed by David James and manufactured
by his company James Engineering Turbines Ltd. The engine
was originally conceived as a ground based design and build
exercise but has proved so successful that it has been flown
in several model aircraft. The engine is capable of a class-leading 30 pounds of thrust and an exceptional power to
weight ratio, it weighs only 6.2 pounds.


Prototype Cobra installed in a Supermarine Swift

The engine is based around the rotating assembly of a turbo-charger which makes for an efficient and durable unit.
A centrifugal compressor is driven from a radial-inflow turbine wheel, these components are interconnected by a shaft
supported by spring loaded angular contact ball races. The combustion chamber is of an anular reverse-flow design.
The Cobra is fueled by propane gas, the gas is ignited by a glow plug and starting is achieved using high pressure
air impinging on the compressor wheel.

J.E.T. Ltd has developed a microprocessor controlled automatic start system for the engine which allows for a single push-button start cycle. Once started a second microprocessor engine control unit (ECU) looks after the cobra's speed and exhaust temperature. The engine speed is detected by an opto sensor mounted at the air intake and the exhaust temperature by a standard "K" type thermo-couple mounted in the jet-pipe. The ECU is designed to interface with a standard radio-control
throttle demand signal.

I have been lucky enough to witness this fine engine operating on a number of occasions both on the ground in a test-rig
and in the air. It's a beautiful unit and runs very smoothly thanks to it's dynamically balanced rotating assembly, you can
put your finger on top of the unit and you can hardly feel a thing. At idle it makes a very sweet whistle but at full chat
there is quite a roar and the jet efflux can be felt at quite a distance. For me personally model aircraft have always been let
down by the awful buzzing noise which they make, but to see and hear this engine flying in a Supermarine Swift is quite
something, at a distance you cannot tell the scale of the aircraft, it looks and sounds absolutely real!

One advantage of gas turbine powered models is the very low vibration levels experienced, this has allowed us to operate a video camera in the nose of an aircraft and even down-link it to the ground via a microwave link. This is something that is difficult to achieve with even the most accurately balanced piston engine and propeller.

2004 The Cobra engine is no longer in production but it remains a fine example of a minature turbo-jet engine.

Cobra Facts and Figures
Thrust................................................... ...163N
Weight.....................................................3.1 Kg
Pressure ratio at max rpm.........................3.0
Max rpm.................................................105,000 rpm
Idle speed................................................30,000 rpm
Max exhaust gas temp..............................640 degrees C
Mass flow................................................0.31 kg/s
Specific Fuel Consumption (Propane).......0.8 Kg/N/Hr
Lubrication..............................................Total loss system, Aeroshell 390 pressurised from compressor bleed.
length including jet pipe............................444 mm
Maximum width.......................................197 mm



The Worlds Smallest Jet Aircraft Powered by two Cobra Engines

Two J.E.T. Ltd Cobra turbo-jet engines have been
successfully installed in a French built Cri-Cri
aircraft. The Cobra powered plane is the World's
smallest jet .


Link to other Model Jet Stuff

Gas Turbine Builders Association
Wren Turbines
SWB Turbines
Heward Microjets
AMT Jets


Home

Page Updated May 2007