How it all began

The Gas Turbine story, from Full-size to Miniature.

It was Frank Whittle, a British pilot, who, in 1930, designed and patented the worlds first turbojet engine.

With private financial support but no government interest or funding, he began construction of his first engine in 1935.

This engine, which had a single-stage centrifugal compressor coupled to a single-stage turbine, known as "the Whittle Unit" (WU), was successfully bench tested on the 12th April 1937.   When the first run was made, onlookers ran for cover amidst a "noise like an air raid siren". However, the principle had been proven.

It was interesting to see the ministry's view;   "In its present state, and even considering the improvements possible when adopting the higher temperatures, proposed for the immediate future, the gas turbine engine could hardly be considered a feasible application to airplanes, mainly because of the difficulty in complying with the stringent weight requirements imposed by aeronautics. The present internal combustion engine equipment, used in airplanes, weighs about 1.1 pounds per horsepower and to approach such a figure, with a gas turbine, seems beyond the realms of possibility with existing materials."

Whittle would later say   "Good thing I was too stupid to know this!"

Incredibly, no-one thought it important enough to keep this project within the UK. Details were freely published in many countries, including Germany, where research on jet engines began just a year later, whilst Whittle's vein attempts to drum up interest within British industry got nowhere - Sadly, an all too familiar experience for many British designers and inventors, even today.

Whittle was eventually forced into handing over the development of the gas turbine to Rolls Royce, the prominent aero engine manufacturer of the time. Whittle was reputed to have commented, when he handed the design over, that "This is the world’s simplest engine". The story goes that the response, from the receiving Rolls Royce team, was "We’ll soon design that out of it...…".     It is probably fair to say that they did.

After giving away the plans for the jet engine to the Americans, at the end of the war, the new Labour government also gave Rolls-Royce jet engines to the Russians, resulting in the quick development of the MiG-15 fighter aircraft.

Dr Hans von Ohain. conceived his theory of jet propulsion in 1933, the idea of a continuous cycle combustion engine design (similar in concept to that of Sir Frank Whittle).    The Whittle design having been published to the world at large just three years earlier, the Germans must have viewed this as "manna from heaven".

Von Ohain was not alone in imagining the jet engine, there were other engineers around the world pondering the exact same challenge. Whittle himself said that it was never a case of "IF" but "WHEN" the jet engine would finally be developed.

Von Ohain’s development engines had a centrifugal compressors, similar to Frank Whittle’s design, but he incorporated an axial flow inlet stator. Von Ohain was granted a patent for his turbojet engine in 1936. Von Ohain then joined the Heinkel Company, working for Ernst Heinkel and specialised in advanced engines. A successful bench test of his first engine was accomplished in September 1937, just months after Whittle had successfully tested his first engine.

As gas turbine powered jet aircraft started to appear in the skies, aeromodellers, keen to emulate the sleek new airframes, were only able to power their new models with either a propeller, detracting from the scale appearance, or the newly developing alternative, ducted fans. Whilst at least ducted fans were hidden inside the model, giving a jet like appearance, they were noisy, often remarked to as sounding like "an angry bee". They also had to have a larger nozzle area, to develop the required thrust to give these models a reasonable level of performance, which again detracting from the overall scale appearance.

The desire to build a miniature gas turbine for model propulsion quickly followed the development of the full-size turbine engines. It therefore comes as no surprise to learn that the modellers of the day looked at the first production engines for inspiration.

The first Goblin engines (right), had a single sided centrifugal compressor, which set the modellers on the right track of simplification for miniaturisation. However, they assumed that at least three flame tubes were the minimum possible, as it was considered that a single annular combustor would overheat the shaft tunnel.

Insights into how a miniature turbojet could be developed produced some pretty wild ideas, probably based on the general idea of how the model-size internal combustion engine had been developed compared to the full-size.

The main problem for model internal combustion engines had been foreseen, by the sceptics, as the impossibility to scale down the carburettor and internal jets. Once all the rules were ignored and someone actually tried it, it became obvious that an extreme simplification, using a single jet and spray bar worked very well, at least for full throttle use.

The fuel metering system for any proposed miniature Turbojet brought the sceptics back into the limelight but this time they were probably right, at least by reading what the general consensus of informed opinion was at the time. A kind of carburettor was visualised, where the needle valve would be operated in conjunction with rpm. As is now known, it was simply a matter of regulating the amount of fuel injected into the combustion chamber, the engine then automatically balances its rpm to the amount of fuel available. Fuel options had also already been determined as being anything from petrol to coal dust!!! In the end, liquid petroleum gas (LPG) and finally paraffin (kerosene) were to become the fuel of choice for full-size and model alike.

The book "Model Jet Reaction Engines", published in 1948, had a statement saying that a miniature turbojet engine might be developed by the time this book gets to print.    I don't know what this guy was on, because many years passed before a miniature gas turbine was eventually developed.

It was Jerry Jackman who assembled a small team of engineers in the UK with the aim of developing a working model gas turbine engine. In 1975, they succeeded in getting their engine to self sustain but were unable to make it throttle up or down.

It took another eight years to overcome the many problems, experienced by the team, before they succeeded in creating an engine that had a low enough operating temperature whilst developing a high enough thrust to fit into and power a model aircraft.

It was a chance remark to Jerry Jackman that turned out to be the inspiration to finally resolve a challenge that had eluded modellers worldwide. A colleague happened to ask him why no-one had been able to make a working gas turbine for model flight.    Why not indeed?

Legend has it that this chance encounter took place at his local hostelry and a challenge laid down, with a wager of £1, that Jerry would not succeed. Sadly, history does not recount whether the bet was eventually satisfied but certainly the basis of the wager was won by Jerry.

Jerry had an engineering background, access to some machinery and lots of pluck. The Mark 1 engine went through a number of modifications, some successful, others not so. After much frustration and disappointment, his dream was realised, when the world’s first miniature gas turbine engine proved itself in flight, as it powered "Barjay" off the Greenham Common Airbase runway on 20th March 1983.

Jerry (left), with his team, holding Barjay.       

Success had not come easily and, with each setback, Jerry realised he needed help. He therefore set about selecting people to enhance the chance of success for the project. Luckily for Jerry, his local model flying club was rich will talent and he was able to pick fellow club members to join him on the journey. All enthusiastic and experienced aeromodellers, they took on the challenge, with Jerry, with great relish.

Each member of the team brought specialist knowledge and skill to the project. Barry Belcher was the aerodynamicist. He improved the turbine airfoil section, significantly increasing the engine's performance. He was also responsible for designing the flying test bed, Barjay, which turned out to be so successful.

A catastrophic turbine failure, which destroyed a good part of the engine, enhanced the realisation of the significant forces present in these engines and the need to calculate the stresses involved and to design to withstand them. Ray Carter joined the team, primarily to bringing his design experience to the project but he also brought specialist welding techniques, metal pressing and he also prepared all the mechanical drawings of the engine.

Persistent bearing problems, exacerbated by shaft resonance, lead to the recruitment of David Sitch to the team. With a background in specialist bearings, David was able to provide advice on bearing selection, the best location for the bearings on the shaft and also the significant beefing up of the shaft itself, from the original ¼ inch diameter up to a full 1 inch, to overcome the resonance issues.

The final member of the team was Chris White, who concentrated on the thermodynamic design of the engine and was constantly seeking solutions to improve the engine's thermal efficiency, which translated into improvements in the thrust developed by the engine.

The Turbine wheel, stumbling block of many before, called for skill in design, machining and metallurgy, to enable it to survive 85,000 rpm at around 650 degrees centigrade. The engine was designed for 97,000 rpm but the team decided, for safety, to never run above 90% of the maximum power setting. The centrifugal force on the wheel at these high rpms, not to mentioned the loss in tensile strength at such high temperatures, meant having to use Nimonic Alloys, which maintain high strength at elevated temperature. The downside of these high nickel alloys is that they are incredibly hard to machine.

Jerry realised that the turbine had to be made as a complete bladed disk "Blisk" after unsuccessfully, and disastrously, making a turbine with individual blades attached to a hub.

Whilst Nickel Alloys are very difficult to machine with conventional tooling, it can very easily be machined using spark erosion, a process known as Electro Discharge Machining or EDM. This machining method can achieve very smooth and accurate surface finishes but it is a very slow process.

Commercial EDM machines were extremely expensive to buy and even outsourcing to a specialist company, for one off components, was not financially viable. Jerry therefore set about designing and building his own EDM machine, which he used to make his turbine wheels. However, as mentioned, the spark erosion process is slow, so it would typically take Jerry about a week (150 hours +) to spark erode each turbine wheel. The process has to be undertaken submerged in a dielectric, typically paraffin, the fumes from which are unpleasant. Jerry found that adding perfume to the dielectric made the odour slightly more bearable around the house, whilst each wheel was being spark eroded.

It is striking to see the significant resemblance between the rotor of Jerry's model engine (above right) and the rotor parts of the a full size engine of the period (right).

The compressor also called for close tolerance milling to precise co-ordinates which were, in themselves, the result of significant in-depth analysis and calculation. Nevertheless, Jerry was able to make good use of his trusty Myford Super 7 model engineering lathe and a small hobby mill to create his very impressive centrifugal compressor.

Jerry clearly had a good understanding of his workshop equipment, as machining something as complex as a centrifugal compressor will have been a significant challenge without CNC.

After only modest improvements in performance, using the spark eroded turbines, Ray Carter, though his business contacts at Deritend foundry in Birmingham, a specialist in Inconel casting for the aerospace industry, was able to arrange to have subsequent turbines cast in Inconel. This move, from spark erosion to cast turbine wheels, was to eventually to prove decisive in advancing the overall performance of the engine.

Interestingly, the Mark 2 engine, which was never completed or tested, made use of a commercially available turbocharger compressor wheel. So, even back in the 1980’s, this pioneering development engine was not dissimilar to the rotor arrangements of today.

All the time the engine was being tested on a test stand, it was fuelled using gaseous propane, the flow of which was controlled by a relatively simple needle valve. However, due to space constraints and the need to carry sufficient fuel to enable a reasonable duration of flight, it was decided to switch to liquid propane, stored in a light weight pressure tank, located under the engine bay.

Unfortunately, the original needle valve, used to control gaseous propane, was unable to control the flow of liquid propane sufficiently reliably. It was also found that, on colder days, flames would be burning in the turbine, because the liquid propane was not vaporising quickly enough to burn completely inside the combustion chamber.

Development was required to achieve a smooth and steady throttle control, when fuelled with liquid propane. The main change was the addition of a vaporiser, similar to a primus stove, where the heat of the running engine would vaporise the liquid propane, as it passed through a tube coiled around the engine exhaust, turning the liquid fuel to its gaseous form before it entered the combustion chamber.

Further changes had to be made to the needle valve throttle mechanism, because the original needle valve arrangements could not smoothly control the higher pressure of the vaporised propane in the same smooth way it could with the cold gaseous propane used on the test bench.

Above all, as Jerry was the first to admit, the development of the T.S.T. (Thatchham small turbines) Mk1 engine was only possible through the recruitment of the team of highly qualified specialists, who gave their time and skills freely, to meet the challenge.

Barjay, the flying test bed for the engine, was an impressive looking aircraft and would not look out of place amongst today’s jet models. It had an impressively moulded fibreglass central fuselage. The wings, twin boom and tail plane were balsa construction covered with tissue and dope and finished in an eye catching bright red paint. It was fitted with trike undercarriage with a steerable nose wheel that was also fitted with a brake. Barjay has a wingspan of 78 inches (192 cm) and a length of 71 inches (180 cm) and weighed an impressive 16 lbs (7.26 kg).

The date of the first flight was originally scheduled for later on in 1983. The purpose of the tests on 20th March was to undertake fast taxi trials. The engine had been mechanically limited to a maximum of 60,000 rpm, which was significantly lower than the 85,000 rpm full power capability. Nevertheless, even 60,000 rpm represented a power to weight ratio, of the model, greater than that of Concorde at full power.

During the taxi test runs, the aircraft surprisingly quickly accelerated to a speed sufficient to become airborne. Jerry had a split second to make a decision, throttle down and remain on the runway, or let it keep going. With pluck, he kept the throttle open and Barjay took to the skies. Barjay did two full circuits with a total duration of just three minutes, before landing safely back on the runway. Jerry was reported to have said later "It flies like a dream". Only a small handful of people were lucky enough to witness the first flight, primarily because it was supposed to be just a ground testing session. What a thrill it must have been to be there.

For each member of the team, it had been a rewarding and educational experience, to produce an engine that weighed 3 3/4 lbs (1.7 kg) yet developed thrust in excess of 9 lbs (4 kg) at 85,000 rpm.

All test flights were open to the public, being advertised in the Radio Modeller magazine, so the team's personal pleasure could be shared. Even at Abingdon in June, when the turbine chose to spectacularly disintegrate! Such setbacks were accepted with grace, as part of the essential experience of development.

Subsequent to that event, and despite many hours, not to mention costs, to repair the engine and get it operational again, it transpired that the revised blade angles of the replacement turbine were wrong, so yet another new turbine pattern had to be made and subsequently cast, before the aircraft would be ready to fly again. Sadly, it never took to the skies again.

Barjay with the Mk1 engine completed just 6 flights in total but it very much proved the concept that model gas turbine power was feasible.

It would be almost another 10 years before the eventual start of the model jet age, with engines being widely developed by hobbyists and commerce alike.

It was Kurt Schreckling's book "Gas Turbine Engines for Model Aircraft", first published in German in 1992 and then in English in 1994, featuring the plans for the FD3 home build model gas turbine, that sparked the formation of the Gas Turbine Builders Association in 1995. The GTBA was instrumental in accelerating the subsequent rapid development of model gas turbines, by the combined cooperation and ingenuity of its worldwide membership, an ethos that still continues today.

These days, little thought is given to the complexity of what is actually going on inside one of these amazing little engines. The electronic control system now manage all the complexity that Jerry and the subsequent early pioneers suffered, having to trying to manage their engines pretty much by feel and sound. These days these engines are pretty much plug and play, a far cry from those early pioneering days.

This little engine has a rotor diameter of just 33 mm (1.3 inches). Maximum thrust is achieved at an impressive quarter of a million (250,000) rpm yet this engine was made by a GTBA member in his home model engineering workshop.

This is a picture of Jerry Jackman's original Mk1 engine, that powered the world's first genuine gas turbine powered model aircraft, Barjay, into the history books. It looked, sounded and smelt like its full-size cousins and was the inspiration for us all to follow. We, the World’s aeromodellers, owe Jerry and his team a debt of gratitude for their amazing pioneering work.