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Gas Turbine Builders Association a |
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Frequently asked questions |
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Design |
| Small gas turbines are not scaled down
versions of full-size engines. For several reasons, any attempt to do
so will almost certainly fail. Jack Jackman pioeneered the original approach of a small turbine engine with the first ever engine to successfully fly. Kurt Schreckling brought the small turbine within reach of the amateur with his renowned FD3/64. He carried out the theoretical considerations and came to the conclusion that a simple radial compressor and axial turbine wheel with a single annular combustion chamber would produce the best results. His views have been confirmed by the rapid progress in refining the design and extracting more power from the same basic size. Spreadsheets have been developed by a number of people based on the Formulas in the Schreckling and Thomas Kamps books that model the processes that go on in the engine. The GTBA has also commissioned burst analysis of the turbine wheel during its evolution. Modern miniature turbine engines look surprisingly similar to their early predecessors , yet their power output and reliability have improved enormously. Many of these cutting edge developments were achieved from within the GTBA membership. |
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How does it work? |
| In very simple terms, a turbine engine is a heat engine. It works on the principle of drawing in air from atmosphere and heating it. This air is then ejected out the Thrust nozzle. Being at a greatly increased temperature, the air is in an expanded state creating a much higher velocity out, than when it came in. Air is taken in from the intake at the front of the engine and compressed by the Compressor. The compressor discharge characteristics primarily defines the overall performance of the engine. The compressed air is fed to the combustion chamber through holes of precise size and location to enable complete combustion to take place, followed by cooling the super-heated combustion gasses to a level where it can safely enter the Nozzle Guide Vanes NGV. |
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Combustion |
| The Combustion chamber is the most difficult part to design. The combustion process only uses about 15% of the compressed air delivered by the compressor. The flame has to burn as near to the front of the chamber as possible in a rolling doughnut shape. It has to complete its combustion before the gasses reach the rear of the chamber, so they can be mixed with the remaining 85% of cool compressed air entering through holes near the mid section of both the inner and outer walls of the combustion chamber. As the gas burns at about 2,000 Deg C it is amazing that such a short combustion chamber can burn the gas and cool it to 750 Deg C before the NGV. This enables the use of stainless steels for the blades provided the revs are kept down.
Normal practice now is for the turbine wheels to be made of a super-alloy, typically Inconel 713. |
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Bearings |
| There are some significant axial forces acting on the turbine rotor but almost no radial force, providing balance is good. Ever since the early engines, ball races have found to be the most suitable bearing type. These days almost every engine uses angular contact ball races with ceramic balls in steel races and no retaining cages. These are known as Full Compliment Angular Contact Ceramic Bearings and are typically rated to reliably run at 120,000 rpm.
A small percentage of the compressed air from the compressor is allowed to pass through the bearing housing (Shaft Tunnel) which then exits at the turbine end, its purpose is to carry an oil mist to the bearings, cooling and lubricating them as it passes. Early engines had a separate oil feed powered by compressor pressure, Modern designs incorporate the oil with the fuel and a very small proportion of the fuel feed is bled to the bearings. Very low flows of oil are better than a constant flow as the drops can cause the ball to skid in the raceways. |
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Fuel |
| Most small gas turbines use Kerosene as a liquid fuel, however an initial supply of Propane is needed to heat the engine before starting on liquid fuel. Some early engines ran on Propane only, however, Kerosene is a good choice of fuel as it is not very volatile, cheap and readily available. Fuel is delivered to the engine by a metal geared rotary pump similar to the pump used for a car windscreen washer. When installed in the plane they work off no more than 7.2volts (6 cells). Commercial, purpose made pumps are now in general use. |
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Fuel tanks and consumption |
| The consumption will vary from engine to engine and on throttle setting of the engine. It is reasonable to expect that 2 litres will last 4-5 mins to give time to land under power. The engine must not allow air to enter the fuel system or the engine will cut. Fuel tanks are usually made from nylon/plastic containers and need a Felt Clunk filter to remove particles that might otherwise block the tiny injector needles. |
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Starting the engine |
| All engines must have the rotor spun at considerable speed to start them. There are three regular methods used for starting. In the early days it was common to use a high volume fan (like a hairdryer) held on the air intake. Alternatively a compressed air source, such as a Scuba tank, was used to spin the rotor by an air impingement jet on the compressor wheel. More commonly modern engines use an electric motor start.
The heating gas is lit and the starter motor keeps the engine turning. The liquid fuel is slowly fed in until the engine speed picks up and the engine self sustains. The turbine can get hot when starting if too much fuel is fed in too quickly. With practice starting is simple and un-dramatic, there should be no flames visible either during starting or when the engine is running. |
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Controlling the speed |
| This is achieved by the use of an electronic speed controller that controls the electric fuel pump motor. When bench testing it is more convenient to control the speed by a hand operated controller such as a minidrill speed controller rather than use a transmitter receiver and speed controller. When mounted in a plane a motor speed controller is used or if available a fully programmed ECU. Over-speed should be controlled by a combination of battery power and pump pressure so that whatever happens to the receiver or transmitter the supply of fuel to the engine is restricted. Under-speed can be controlled via a pressure switch that stops the engine if pressure drops below a pre-determined point. Modern ECU's (Engine Control Units) do all of these things and more! |
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What is a sensible rev limit? |
| It is customary to design the engine so that the peripheral tip speed of the compressor wheel runs at around 0.8 MACH. This determines the maximum RPM of the engine. There are other considerations regarding mechanical stress of rotating components and in the Kamps book there is detail on how to calculate the critical speed for the shaft. The rotation speed will depend on the material of the rotating components, the thickness of the shaft as well as the weight of the compressor and turbine and their positions. In practice, engines with a 316 or 310 stainless steel turbine should have the revs restricted to about 75,000 rpm. Above this speed the material will expand and cause it to bind on the housing. The Kamps engine is limited to about 103,000 rpm and the new Schreckling (now developed and named the KJ66) at 120,000+rpm. The higher the revs the more need there is for the engine to be well balanced.
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How do I balance the engine? |
| In the past, Simple static balancing has proved to be adequate. but with modern, higher revving engines, especially those fitted with ceramic bearings, dynamic balancing has taken over. The GTBA archives have constructional details on how to build quite simple dynamic balancing machines with an amazing sensitivity. |
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How do I make the turbine wheel? |
| The early wheels are made from sheet that is slit and twisted to the correct angle and profiled by hand mini-grinder. Better wheels can be ground from thick discs. This is time consuming but will give better results particularly with Inconel. Cast Inconel wheels are now standard in modern engines. |
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Are the engines noisy? |
| If you stand close to a gas turbine you will find the noise level greater than a piston engine. However the noise emitted is at a higher frequency so that in the air, the noise is dissipated
more quickly making it much less obtrusive than a piston engine. Indeed if a piston engine and a turbine are flying together it is difficult to hear the turbine at all. |
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How do I weld stainless steel? |
| This is not a skill everyone can acquire. Thin stainless can be spot-welded which is suitable for the combustion chamber and case. The NGV assembly will require MIG or TIG welding. TIG sets are expensive and it may be necessary to find a specialist welder used to welding stainless steel to do the welding for you. Modern engines employ a casting for the NGV which is more stable during natural heat cycling. |
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What instrumentation will I need? |
| At the very least when testing you will need a Pressure gauge that can measure up to 1 bar and a temperature gauge that can measure up to 1000deg C. kitchen scales will provide thrust measurements. A rev counter can be made from the design in the newsletter. It is helpful also to be able to measure the fuel pump pressure but this needs a scale up to 3 bar. All modern ECU's are capable of performing these tasks with the exception of the pressure gauge reading. |
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What engine should I make? |
| This is the most often asked question and the most difficult to answer! Do you want to build an engine to fly? Do you want reliability? Do you want to build everything yourself? are all factors in deciding which engine to build. It's possible at this current time, and with more than 10 years of hind-sight that we can: (1) If it's your first turbine engine project or if you want an engine capable of flight, then have a look at the engine page on this site and choose the KJ66 or newer design. You will
then have a very good chance of completing a successful engine in your home workshop. |
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How much will it cost? |
| This will depend on how able you are to get scraps from you local welders/machine shop! The Schreckling FD3/64 is very cheap to build and would cost probably no more than £100. The other designs that have turbochargers and cast wheels will be more expensive - perhaps £300- 400. |