Elytron 2S: The out-of-the-box tiltrotor concept
Some people are never satisfied. You give them a plane, and they say it can’t hover. You give them a helicopter, and they say it can’t fly very high or fast. Looking to combine the advantages of both fixed-wing aircraft and helicopters, Elytron Aircraft LLC of Mountain View, California is developing the Elytron 2S, a small tiltrotor aircraft that uses a box wing configuration and is aimed at the civilian market.
Airplanes and helicopters do well in their respective aeronautical niches. Airplanes can fly high, fast, far, and carry heavy loads, while helicopters can hover, fly backwards, land and take off vertically, and maneuver in very tight spaces. The tricky thing is when jobs arise where requirements overlap; like an airplane that can land vertically, or a helicopter than can fly at high altitude.
Though there have been some aggressive development projects in recent years, such as the Bell V-280, the AugustaWestland Project Zero and AW609, the only operational manned, prop-driven aircraft that manages this sort of hybrid flight is the V-22 Osprey tiltrotor aircraft.
In service with the US Marine Corps and the air forces of the US, Japan and Israel, the military tiltrotor spent many years in controversial development and, to this day, no civilian version has taken off. This is unfortunate because early visions of tiltrotor aircraft saw them employed as air taxis, short haul transports, and in other civilian applications.
The Elytron 2S is a new take on tiltrotor configuration that incorporates an unconventional wing design to create a hybrid aircraft with greater performance – especially in the difficult transition between vertical and horizontal flight. One of a family of aircraft developed by the Elytron company over the past decade, the 2S is intended to combine the features of a helicopter and a fixed-wing aircraft, with the company seeing applications for emergency medical services, search and rescue, air taxis, and oil exploration.
Comparing the 2S to the Osprey, the most obvious difference is its wing configuration, known as Prandtl's box wing design. It looks exotic and futuristic, but it’s actually a variation on a wing concept that dates back to the earliest days of aviation when it was used by the pioneer flyer Louis Bleriot in 1906. It has also been used more recently by Lockheed Martin for one of its concept jets.
2S’s design uses two pairs of wings. One pair is set forward on the fuselage and is swept back. A second pair is set on top of the tail and swept forward. The tips of the fore and aft wings are linked by winglets, forming a skewed box. With this layout the forces are distributed through the box structure, making it stronger and lighter. It also provides a high aspect ratio for greater lift of about 15 percent over conventional wings.
This results in greater fuel efficiency and less weight. In addition, the box wing is more stable, has better glide ratios and is less prone to stalling. In all, this makes Prandtl's box wing very suitable for short take-off and landing craft. The stall factor is particularly important because it helps keep the 2S in the air during the vertical to horizontal flight transition.
So far, this is unusual, but not unprecedented. According to Elytron, the novel bit about the 2S is the extra pair of rotating wings mounted amidships, which contain the motors and props. The rotary wings, called "proprotors," are powered by two 90-degree gearboxes and are mounted on a single, centrally-mounted tilt wing that travels through 100 degrees and has four control surfaces.
Elytron says that this arrangement inside the box wing obviates interference with the proprotors’ thrust, while the winglets protect against rotor strikes. Because the proprotors can drop to the horizontal for forward flight, the blades don’t have the disadvantage of helicopter rotors where the retreating side of the rotors slows the craft and introduces unwanted torque. The design of the proprotors is much less complicated than a helicopter rotor with no need to angle or feather the rotors, so the linkages are simpler and their fewer parts are spread out through the wing.
According to Elytron, the result of all this is low drag at high speed in a craft that can fly two to three times the speed of an equivalent helicopter and, if the V-22 is an example, it will have a higher operating ceiling. It’s also safer because its glide ratio means that the rotors don’t need to autorotate during an emergency landing. If that doesn't work. Elytron says it will install a ballistic parachute.
The Elytron 2S is a two-seater technology demonstrator built using fast prototyping with carbon composite materials. The company says that it plans to install a 450 bhp turbocharged race engine in the 2S with the goal of conducting flight tests next year. If successful, the next phase will include building a seven-seater version for commercial use. In the meantime, the 2S is on display at the Oshkosh Airshow through August 3.