High Hopes - UAV Programmes

Mike Richardson finds that by taking the right attitude to altitude, a brand new UAV engine has been launched to enable high performance improvements over normally aspirated offerings of equivalent altitudes. A new Medium Altitude Long Endurance (MALE) UAV engine has just been unveiled by Plextek. At just 13kg, the UAV engine design weighs up to 30% less than current solutions in its class and uses the type of supercharger normally found on manned aircraft to maintain engine power at higher altitude. The engine is also designed to fit into the streamlined fuselage of a 10inch (254mm) diameter tube which is said to be ideal for MALE UAVs.
Traditionally, MALE UAVs engines have required a larger engine to cope with power loss with altitude. As the UAV flies higher, the air gets thinner and with less oxygen, the result is that some engines can lose up to 50% of their power. However, Plextek's supercharger solution is claimed to overcome this problem.
According to Plextek Defence Group's senior project consultant, Dr Mike Roberts, no other engine in this class can boast the same level of performance with such a low weight, as he explains how the company set about approaching and solving the original design goals for the MALE UAV engine project.
“We set ourselves quite a tough challenge to start with by looking at the previous UAV propulsion systems we'd been considering on previous projects and asking ourselves the question where this kind of solution might be required to operate,” he begins. “Military and defence operations currently operate in some of the most hostile environments and difficult terrains imaginable, and are areas typically about 5,000ft above sea level, whilst the mountains around them are perhaps another 10,000ft. In addition, temperatures can reach 50ºC to 60ºC on the surface, so we soon realised we needed a solution that could operate at the equivalent altitudes of 20,000ft as the norm.
“In addition, there are often winds whipping across the top of mountain ridge lines, so even at 20,000ft the UAV engine will need a reasonably high level of performance just to maintain its gradual ascent. We set ourselves the challenge of designing an engine that could climb to 15,000 to 20,000ft above sea level under the hottest and harshest conditions.”
Less weight, more fuel
To meet the MoD's demands for a UAV that can stay airborne for as long as possible, Plextek's design team needed to consider the different ways of reducing the engine weight because any savings here can then be turned into fuel – and more fuel means more time in the air!
“We set out to design the engine for the smallest fuselage possible with the goal of ensuring everything would sit inside a 10inch diameter circle,” he continues. “Within this space, the supercharger operates at 250,000rpm from an engine that runs in the region of about 8,000rpm, so it's a step up ratio of around 30:1 via an ‘inline' gearbox inside the supercharger. The major hurdle was in getting the engine inside the package whilst ensuring it still worked reliably.”
Dr Roberts adds that the design team turned to the emerging supercharger solutions found in the automotive market and particularly with Japanese car manufacturers who, for tax reasons, are trying to squeeze higher engine performances from the latest crop of small cars.
“These car manufacturers have been developing superchargers that are targeted at small engines about the same size as those of our customers involved with UAVs,” notes Dr Roberts. “We began finding solutions and making use of the emerging developments in this sector. It hadn't been used in UAV applications because it wasn't a cost effective solution - until now.”
Plextek's Defence Group was set up as part of a strategic initiative to help raise its visibility in the mil/aero supply chain and strengthen existing relationships with the MoD and the wider defence community. The Group plays a key role in communicating the potential application of Plextek's commercial expertise in the MoD environment.
“We've been developing this UAV engine in conjunction with a large UK defence Prime's UAV programme,” concludes Dr Roberts. “Plextek is an ‘ideas factory' and we hope to become more involved with UAV research and development programmes, such as flight trials or demonstrations, in the near future.”
  www.plextek.com