Sunday, December 19, 2010

Scaled ambition Sea Harrier

Ewald Schuster saw his first Hawker Siddeley Harrier at age nine and was so captivated by the machine that he devoted 22 of the next 34 years of his life to creating a fully functioning radio control scale model of the vertical take-off aircraft.
This coming spring, on a dry lake bed in the high California desert, the research laboratory technician at the University of Southern California (USC) will get to find out if the all-composite third-generation Harrier he built with the help a few dozen engineering students will fly, or more importantly, whether it will transition.
While jet-powered radio-controlled (RC) models are becoming the rage in the hobbyist world, no-one as of yet has developed a properly functioning scale Harrier, meaning one that can perform the transition between hover and forward flight and vice versa using four articulated mid-body nozzles and attitude control puffers on the wings, nose and tail.

Ewald Schuster
© Ewald Schuster
An earlier version of of Ewald Schuster's Harrier used a turbojet engine (pictured) but the latest has a homebuilt turbofan
Simple in concept, the mechanisation is difficult in practice, requiring Schuster, among other feats, to develop another first for the RC world: a miniature turbofan engine similar to the Rolls-Royce Pegasus that powers the Harrier. With a turbofan, relatively cool bypass air can directly power the forward nozzles and piping for the puffer reaction control system on the nose, the wings and the tail can be lighter weight compared to the temperature-resistant materials needed for the rear articulated thruster nozzles.
Such lessons took time, knowledge and patience to learn, however. Along the way, Schuster educated himself in the art of mechanical and aerospace engineering, creating his personal brand of mathematics largely based on observation and driven by pure motivation in an end product - the Harrier. "I started flying RC at age nine," says Schuster. "That's when I first saw the Harrier. I said, 'I have to do that'."
To date, he says no-one in the RC world has done what he hopes to do in the spring. "There's a Harrier in England and one in Taiwan, but none that transition," he says. He built his first model, a 1/10 scale Harrier that used a 4,000mm3 (0.25in3) ducted fan powered by a piston motor, at age 14. "It wouldn't lift off the grass," he says. At age 18, he built number two, a 4kg (9lb), 1/8 scale Harrier with a 14,900mm3 nitro-powered ducted fan. "I took it out, went to full throttle, and it lifted itself," he says. But there was no extra power left for transitioning to forward flight, or for putting additional needed equipment onboard. "That was an eight-year project," he adds.
At 29, Schuster began his third Harrier, a 1/6 scale model, that would accurately reflect the thrust-vectoring characteristics of the real thing. "After the second one not having enough power, I had reasons to go bigger," he says. "Bigger flies better, and efficiencies get better." He knew he needed an engine to mimic the Pegasus engine for the Harrier, a unique turbofan that has a duct around the fan output to power the forward nozzles and attitude control functions and separate rear duct to capture the output of the hot gasses to power the rear nozzles.
Like the Pegasus, his would also have counter-rotating shafts to the reduce angular momentum that otherwise might cause attitude disturbances in turns from gyroscopic cross-coupling. The problem was, no-one on the RC market, then or today, had a miniature turbofan engine. So Schuster set out to build his own.
"Originally I bought a JPX [off the shelf] RC turbine engine," he says. "It had poor power-to-weight ratio, so I took it apart and redid everything, using titanium on the hot side and magnesium on the cold side. I learned a lot about balancing and the basic things about a jet engine, like vibration and bearing pre-load."
Schuster never actually flew the resulting engine, but got it running. "At that time, I got serious about building a bigger one for the Harrier," he says, adding, "there was nothing to copy or go off of. I was experimenting - one step at a time."
He began with an outline of the Pegasus engine selected the internal architecture to meet his thrust needs - a fan, a centrifugal compressor from an automobile turbocharger (the only off-the-shelf component in the engine), an inconel combustion chamber and two inconel turbine wheels, the forward one on a common shaft with the compressor (high spool) and the rear, or power turbine, on a "low spool" with the fan.
The fan output is sleeved to send bleed air to the forward articulating nozzles and attitude control puffers; the power turbine outlet sleeved to power the rear articulating nozzles. "I kind of learned how to do more with math as time went on by studying the flow through several engines," he says. "Particularly the 'open areas'."
A key relationship he observed over the eight years of development time was turbine temperature versus turbine rotor sizing. "On the first engine, the power turbine was too small and the whole back of the engine was glowing red hot," he says. "The power turbine needed to be larger to relieve the pressure. Too big, and the temperature is too cool; Too small, and it's too hot. I lucked out on the second build and hit the right size. Because of the size difference, the engine needs a 3:1 bypass ratio to provide attitude control functions that the full-size Pegasus does with a 1.2 bypass ratio.
Schuster says about 650°C (1,200°F) behind the HPT is about right, which makes the combustors run at about 2,000°C. The 1/6th scale Pegasus engine produces 40lb (0.178kN) of thrust on a fuel flow of 0.53litres (0.14USgal)/min, about half the fuel flow of an equivalent turbojet version of the engine, he says.
Life expectancy for the engine? Schuster says that's a relative term. He's run more than 380 litres of fuel through the engine and has seen no degradation when he takes it apart. "If temps are kept at reasonable levels, it's hard to tell what the limit is," he says. "People generally crash their aircraft way before the bearings or engine components wear."
Like other miniature turbine makers, Schuster uses ceramic ball bearings and a 5% oil mixture in the fuel for lubrication. His "high spool" runs at 100,000rpm at full speed while the fan turns at 32,000rpm. He's built an engine that runs faster. Schuster has a 2.5in diameter turbojet engine that idles at 60,000rpm and runs full speed at 240,000rpm.

Ewald Schuster & University of Southern California students
 © Ewald Schuster
Schuster (centre) works with students from the University of Southern California
Yet another engine, which he thinks will have an application as a handheld generator, idles at 150,000rpm and runs full speed at 500,000rpm, still in development. He also notes that there's currently no burst protection from all the spinning machinery. "Don't stand beside it," he says. While Schuster developed the turbofan engine on his own and several prototype non-hovering Harriers, the first one powered by an off-the-shelf JetCat P120 turbojet, USC provided him with merit research students who have been gaining hands on experience in research, machining and carbon fibre projects for the third generation Harrier construction. The help came quite by accident.
Schuster had previously worked in the special effects arena for Hollywood, putting his talents to work on movies like Titanic, Galaxy Quest, and Small Solders in the late 1990s when it seemed to come to an abrupt end due to computers taking over in special effects.
"I came to USC asking a professor for advice about some of my engines," he says. Impressed, the school gave him a job in the fabrication shop where he worked for five years.
After a stint away, Schuster came back in 2004 and has since put the creative talents of several dozens of students to build, hover and test the Harrier. Contributions include control system development, flight characterisation studies and composites work on the airframe. The group has successfully hovered the vehicle on a tether with a substitute electric motor, though as with any first flight, dozens of details remain open.
"How do we prepare for flying? We have to go out there and risk one," says Schuster. "We thought about testing it in a wind tunnel or on top of a truck, but those options were too complicated." Other options include adding a ballistic airframe parachute, an option that Schuster says would add about 1.4kg to the 9.5kg unfuelled aircraft. Schuster says he may take it up "really high" and try to transition to a hover, the second most difficult task as going into forward flight from hover. A successful first flight may be bittersweet for Schuster.
"When it's done, it's going to be a weird feeling," Schuster, now 43, admits. "My whole life has been in this." Along with finishing a book about the experience, Schuster plans to hit the international RC show circuit with the Harrier, and if the stars align, also put the aircraft to work in a movie.

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