DARPA’s VTOL X-Plane

By Maren Leed, Melodie Ha 

JUL 30, 2014 

http://csis.org/publication/darpas-vtol-x-plane

For decades, helicopters and other rotorcraft have been essential components to countless military operations; they can maneuver in any direction, hover, and land on almost any flat surface. Despite the technological advancements made in Vertical Take Off and Landing (VTOL) over the years, rotorcraft remain limited in terms of speed, operating at 195mph or slower. Faster VTOL aircraft could not only shorten mission times and increase the potential for operational success, but also reduce vulnerability to enemy attack. However, previous attempts at new VTOL designs have been unable to increase top speed without sacrificing range, efficiency, useful payload, and simplicity of design. 

Defense Advanced Research Projects Agency’s (DARPA) VTOL X-Plane program aims to design a plane to overcome these challenges. Through a hybrid of fixed-wing and rotary-wing technologies, DARPA is looking at a plane that can efficiently take off and hover like a helicopter and fly at high-speeds like an aircraft. This project is currently exploring unmanned aircraft, but the technology can apply to manned aircraft as well.

DARPA is looking to challenge the aviation industry and innovative engineers to create a hybrid aircraft with specific requirements in the following four areas: 1) enhancing speed up to 460mph 2) increasing hover efficiency from 60% to at least 75%, 3) obtaining a cruise lift-to-drag ratio of at least 10, and 4) maintaining the ability to carry a useful load of at least 40% of the vehicle’s gross weight of 10,000 to 12,000lbs.

This $130 million project will be in the works for 52 months, from 2013 to 2018, and consists of three phases. The four companies that DARPA has selected to proceed with Phase 1, (developing a preliminary concept design for the aircraft) are Boeing, Karem Aircraft, Aurora Flight Sciences, and Sikorsky Innovations.

Among the four contenders, Boeing is the only company to have built and tested a flying subscale model of the aircraft, branded the Phantom Swift X-Plane. The full-size Phantom Swift will measure at 50ft from wingtip to wingtip, 44ft from nose to tail, and will weigh in at 12,015lbs. The Phantom Swift includes two large fans inside the fuselage for vertical lift, with two smaller tilt-wing fans for additional lift and hovering capability. With the use of duct-fan technology, the Phantom Swift can maintain high-speed flight up to 460mph without sacrificing the ability to hover efficiently. During high-speed cruise, the lift fans are shut down and the doors are closed for greater aerodynamic performance, and forward propulsion is provided by the wingtip thrusters. The demonstrator will be powered by a conventional General Electric CT7-8 engine, but Boeing plans on incorporating an all-electric drive in the long term as soon as the technology becomes feasible.

Karem Aircraft’s design incorporates a tilt-rotor concept and a slender bodied fuselage with a high aspect-ratio gull wing. The outer sections of the wings each have two large propellers that can rotate 90 degrees.

Sikorsky Innovations, teamed up with Lockheed Martin’s Skunk Works, is refining its Unmanned Rotor Blown Wing concept for their X-Plane design. The Rotor Blown Wing is a unique combination of fixed-wing aerodynamics and advanced rotor control, which will provide a low complexity configuration that is capable of meeting DARPA’s program requirements.

Aurora Flight Sciences’ LightningStrike X-Plane is the only aircraft that has yet to reveal its design. The company has mentioned achieving overall high efficiency for this program by integrating a propulsion system into the air vehicle’s aerodynamic design. Aurora has been a pioneer in ducted-fan and hybrid-propulsion aircraft.

DARPA will select one design and award one company a contract to proceed to Phase 2 (development, construction, and integration) by 2015 and Phase 3 (conducting flight test demonstrations) by 2017-2018. The goal will be for a first vertical takeoff and landing X-Plane within 47 months. Ultimately, U.S. military leaders would like to use the technology developed by DARPA’s VTOL X-Plane program to design an aircraft to replace the V-22 Osprey tiltrotor. 

Maren Leed is senior adviser with the Harold Brown Chair in Defense Policy Studies at the Center for Strategic and International Studies (CSIS) in Washington D.C. Melodie Ha is an intern with the Harold Brown Chair in Defense Policy Studies .