It is the always the most eye-catching designs that receive the most attention. A rotorcraft featuring an entirely new architecture is sure to gain more publicity than a new helicopter that looks, well, pretty much like any other helicopter. It may have details that lift it over previous designs but if, to the casual observer, nothing radical has changed, it is likely to be overlooked. That truism applies even more strongly to engine upgrades, where there is little visual clue to the modification.
And this is the case at Airbus Helicopters. Of its current research and development programmes, the most arresting are its high-speed compound rotorcraft (the dreadfully-named LifeRCraft) and its participation in the wider City Airbus project, an electrically-powered, vertical take-off and landing aircraft designed as an urban run-around. Both are currently at the concept stage, represented solely by futuristic-looking digital renderings.
They have, as you might expect, gathered acres of publicity in both the technical and popular media. However, while such concept projects vital to a company’s long-term future, they do nothing to address nearer-term challenges or simple issues like operational cost.
Meanwhile, with very little fanfare, Airbus Helicopters has been working on a research programme that it believes could deliver a reduction in fuel consumption of more than 40% to operators of light single-engined helicopters.
Using an H120 light single as a flying test-bed, the manufacturer has replaced the helicopter’s stock Safran Helicopter Engines Arrius 2F turboshaft with a specially-designed high-compression, kerosene-fuelled piston. Conducted under the auspices of the EU’s Clean Sky environmental research programme, flight-tests of the modified helicopter began in November 2015 and ran to July 2016. Further flight evaluations are due this year as the airframer and its partners, Austro Engine and Teos Powertrain Engineering, look to refine the design, taking it from technology readiness level (TRL) 6 all the way to serial production.
Piston-engined helicopters are not new, of course, and date back to the earliest rotorcraft designs; Robinson and others continue to manufacture hundreds of them every year.
But, explains project head Alexandre Gierczynski, the typical piston design cannot be scaled up sufficiently to replace a turbine engine, due to power and weight constraints. The engine utilised on the demonstrator aircraft is a V8, featuring a machined – rather than cast – aluminium engine block to save weight, and a dry-sump system to ensure constant lubrication.
The closest comparison, he says, is to engines used by cars in the Le Mans 24 hour race – in fact, partner Teos builds V6, V8 and V12 engines for endurance racing and powered the 2009 overall Le Mans winner, a Peugeot 908 diesel. As Gierczynski points out, the design is not a great deal more complex than a standard V8 automotive engine: “The piston architecture is the same,” he says.
“The design of the engine is classical," he says. "There are some secrets, that I will not give you, in order to get light weight but it is [mostly] a standard four-stroke engine.” It features a high-pressure, common-rail fuel injection with four valves per cylinder. “It is nothing very original,” he says.
However, used in a helicopter, it operates at lower specific power in order to “achieve the reliability you need from an aeronautical application". “Le Mans is 24 hours, our time between overhaul is 2,000 hours,” he says.
Of course, the task was not as easy as simply transplanting an automotive engine into an aircraft. Multiple modifications were required to ensure compatibility, Gierczynski admits. Some issues, like excessive vibration, were easily cured – but others required more innovative solutions.
Take the issue of cooling. As Gierczynski says, a turbine engine sheds a lot of unwanted heat via its exhaust gas, something a piston engine cannot replicate. A car partly uses the airflow from forward motion for an element of cooling, but a helicopter’s highest power demand comes in the hover “where there is no dynamic air flow to benefit from”.
Instead, an engine-driven fan, located to the rear of the powerplant, drives air through the engine to dissipate the heat. Tests were carried out last summer in temperatures of up to 31°C (88°F) with no adverse effects, says Gierczynski.
Another problem is the way a piston engine delivers power. Unlike a turbine, where combustion is continuous, a four-stroke design produces torque unevenly, and, says Gierczynski, “the rotor is not capable of seeing these high power variations”. Uncorrected, it would “damage either the engine or the gearbox”.
To overcome the issue TEOS has come up with a means of damping these torque oscillations, so the engine now has “the same characteristics as a turboshaft”.
Other developments included a specially designed dual-channel full authority digital engine control system, vital for maintaining a constant rotor speed, and changes to the avionics to display accurately the new engine parameters.
Although the engine itself is twice as heavy as the turboshaft it replaces, the significant reduction in fuel consumption means the helicopter needs to carry less fuel, resulting in weight neutrality, he says.
The weight factor rules out any application on twins, however: “Twice the additional mass would not be compensated for by the improved fuel consumption.”
The first phase of testing accumulated only around 10h of flying time, although three engines also amassed a combined 600h on bench tests. However, Gierczynski says the flights so far have “been sufficient to demonstrate the technical points we wanted to evaluate”, with “very good” results so far. “Basically we comply with all the criteria that we wanted to test, and had even better results than expected.”
For instance, the team aimed to cut fuel consumption by 30%, but achieved 42%. That figure could be reduced further, explains Gierczynski, but by no more than 2-3%. “If we can gain it then of course we will, but we won’t spend a lot of effort to get that 2% as we have already achieved 42%.”
Power output was also “consistent and better than a turboshaft in hot and high conditions”, he says, with that power maintained at 2,500m and ISA+20°. Simpler maintenance also contributes to a reduction in direct operating costs of 30%.
The next phase of flight testing will help to refine the design, as Airbus Helicopters pushes towards serial applications. Gierczynski declines to give away much detail on how and when it will take it to market, but concedes that “around 2020” would be the likely timeframe.
In addition, there is no indication as to which aircraft could feature the engine. Airbus Helicopters makes three light singles – the 1,720t (3,780lb) H120, 2,250kg H125 and 2,500kg H130 – and Gierczynski says it would be applicable to all three. There is room for growth to take account of higher power requirements, too. The basic architecture of the high-compression engine is scalable – both up and down – from a V2 at one end, up to a V16 at the other. However, Gierczynski believes a V12 is the logical limit for rotary applications.
But he declines to reveal if either the H125 or H130 are in its plans. “We are thinking, but there is nothing concrete today. This is an application for all light single helicopters,” he says.
The engine may not have the sex appeal of the company’s more high-profile programmes, but light helicopters could be in for a revolution all of their own.
Source: FlightGlobal.com
