The test flight of the Eurocopter X³ technology demonstrator (First unveiled in February 2009) has shown another company‘s vision of the helicopter of the future. With the traditional ‘Penny Farthing’ layout beginning to reach its upper technological limit, alternative layouts and composite designs will need to be thoroughly investigated in the near future.
The X³ (or X Cube Eurocopter parlance and suitably registered F-ZXXX) flew on 6 September and is truly a composite helicopter, not just in the sense of its layout. It utilises an AS365N3 Dauphin fuselage coupled with a modified EC175 transmission, in addition to the EC 155 main rotor. Many other components of the aircraft were also lifted from the Eurocopter spare parts bin, including the EC145.
The missions envisaged by Eurocopter for the X³ include oil and gas exploration and support, passenger transport, search and rescue and of course, military. Although not just simply a ‘Poor man’s V-22 Osprey’, the stub wings and propellers offer significantly higher forward speeds and range.
At high speeds the disc load on the main rotor is off-loaded by lift from the stub wings, this helps reduce the drag from the main rotor by allowing it to rotate at a slower speed, in an attempt to keep the blade tips sub-sonic. The stub wings also off-set retreating blade stall, inherent in a helicopter with a high forward speed.
The engines for the aircraft have been described as coming from the NH90, by this we can guess they are Rolls Royce RTM322’s. The drive to the two wing-mounted rotors is provided by two small turbo-prop engines, although the promotional video suggested transmission take-off drives. From this, it could be construed that there is an element of redundancy with direct drive from the main transmission in the event of engine failure, necessary if the props also provide yaw and anti-torque control. From the video, it appears that the two drive propellers rotate in opposite directions, the port propeller clockwise (when viewed from the front), and the starboard propeller counter-clockwise.
Initial viewing suggests that the X³ is a very logical layout and an ideal solution to the problems of high-speed rotary performance. However, from a military perspective, the design does have a number of limitations.
Firstly, the additional engines for the drive props would introduce an additional large component into the maintenance and logistics chain, its also one more thing to go wrong or get hit by enemy fire, despite the suggestion of mechanical redundancy.
Secondly, admittedly this is purely a technology demonstrator, the forward drive props in their present position do risk decapitating and dismembering deplaning personnel. This risk would undoubtedly be reduced if an aircraft like the NH90 was adapted, however, the anhedral design of the wings may still pose an inherent problem. Although these are necessary to keep the propellers and main rotors at a safe distance from each other in the event of blade flapping in high winds or heavy landings.
Ducting the props or converting them to a pusher configuration may mitigate some of these issues. Some years ago Dowty trialled ducted turbofans on a Britten Norman Islander.
Thirdly, from a military perspective, the stub wings are an ideal place to hang additional stores, such as weapons and drop tanks. Spinning a large propeller at the front would preclude the aircraft from firing rockets or missiles unless they were fuselage-mounted.
As an alternative design, the tail-mounted Vectored Thrust Ducted Propeller (VTDP) demonstrated on Piasecki’s modified Sikorsky Seahawk, offers a safer and less complex solution. It would allow use of the stub wings for the stores and put the danger of a rotating propeller where soldiers expect it to be, at the end of the tail at the back.
Sikorsky has also recently shown an advanced technology demonstrator, the X2.
The single-engined fly-by-wire aircraft features coaxial rotors and a pusher propeller that Sikorsky believes will revolutionize the helicopter world with cruise speeds of up to 250 kts, some 100 kts faster than current production helicopters. “This isn’t an aeroplane we are training to hover. It’s a helicopter that will go very, very fast,” said Sikorsky CEO Jeff Pino. “I think it will get to 260 kts.” (The helicopter world speed record is held by a Westland Lynx at 216.45 kts).
It is good to see companies exploring different avenues to try and resolve current design issues and limitations. For the X³ I would give Eurocopter a ten out of ten for ingenuity and courage.
The question is, are we going to see something as innovative as this or the Sikorsky X2 from our very own AgustaWestland?