How to Make a Vertical-Takeoff Plane That Doesn’t Suck
A Q&A with DARPA's Dr. Ashish Bagai
In early March, the U.S. Defense Advanced Research Projects Agency announced it was awarding a contract to Virginia-based Aurora Flight Sciences to develop a small robotic demonstrator aircraft as part of phase two of DARPA’s Vertical Takeoff and Landing X-Plane initiative.
“For decades, aircraft designers seeking to improve vertical takeoff and landing capabilities have endured a substantial set of interrelated challenges,” DARPA explained. “Dozens of attempts have been made to increase top speed without sacrificing range, efficiency or the ability to do useful work, with each effort struggling or failing in one way or another.”
The aim of VTOL Phase 2 is “to overcome these challenges through innovative cross-pollination between fixed-wing and rotary-wing technologies and by developing and integrating novel subsystems to enable radical improvements in vertical and cruising flight capabilities.”
Aurora’s design embeds many small rotors inside a thick wing and rotates the entire wing — rather than just the engines — in order to transition between vertical and horizontal flight. I interviewed Dr. Ashish Bagai, a program manager in DARPA’s Tactical Technology Office, about VTOL Phase 2, its potential and its challenges.
Q: Did DARPA take into account safety considerations when picking the Aurora design — specifically, problems with previous VTOL/tiltrotor aircraft (i.e., the V-22), including vortex ring state, autorotation and brownout?
A: Safety is always a concern and the DARPA program had specific guidance on addressing a variety of considerations, primarily pertaining to executing a flight test program of a completely new type of aircraft as safely as possible with the greatest likelihood of preserving the aircraft. Different and many more criteria become applicable to production aircraft.
Vortex ring state is primarily an issue with relatively low-disk loading air vehicles that descend into their own wake. For high-disk loading machines with high-wake velocities that are more jet-like, VRS is less likely to be an issue. Rather, wake-induced instabilities, when hovering in proximity to the ground, for instance, are more likely to upset the aircraft. These types of issues are recognized and are to be addressed via active control systems that are part of the developmental effort.
Additionally, other design attributes of the aircraft propulsive system are being designed to help control such issues. Autorotation with hybrid designs such as this is not physically possible, and other means of controlled landings in the event of full power-out would need to be developed and verified. The configuration, as it stands, serves to demonstrate key technology areas that are being developed and matured to support vertical flight capabilities. It is not intended to be a prototype for a production vehicle.
Brownout is also a consideration, but is induced differently for ducted-fan designs than for traditional rotorcraft, where tip vortex roll-up results in large recirculating flow fields around the descending helicopter. Jet like flow-fields produced by the current design will, no doubt, entrain dust and loose material. Whether this produces a degraded visual environment that hinders the operations of the aircraft will depend on a number of factors, including the depth of the deflected jet stream or the existence of persistent vortical structures. The DARPA program provided guidance to the performers to recognize the downwash signature and devise ways to manage it. The Aurora concept includes such considerations.
As an experimental aircraft, these are the types of challenges that such a program is intended to encounter. There are other significant risks — like performance of the integrated wing and thrusters, electric flight, tilting wings, over actuated control systems — all critically important to enabling new technologies for new capabilities. We’re not out to design or develop a re-run of an existing type of aircraft, and we can’t expressly mitigate every anticipated issue for production applications at this stage.
We expect that discoveries will be made. We do our best to ensure they aren’t “fatal.”
Q: Does DARPA intend the VTOL Phase 2 X-plane to be scalable so that it can be made larger (and manned) in order to meet possible future military requirements?
A: Yes, the program is working on building an aircraft that is up to 12,000 pounds in weight. This is in the size category of manned aircraft. The present aircraft will not be manned, but the technologies are scalable with the intent of applying them to manned and unmanned systems that may be developed in the future to meet military requirements.
Q: Is DARPA collaborating or coordinating with the Army — specifically in regard to the Army’s Future Vertical Life program — as both DARPA and the Army are now separately developing VTOL rotorcraft/aircraft. Really, what I’m asking is … is there a duplication of effort here?
A: The DARPA program, especially the selected work, is independent of the Army program and (completely) non-duplicative. It informs the FVL program roadmap that includes a spectrum of aircraft sizes and capability sets. As such, the DARPA Phase 1 effort has presented its technology efforts and details of the designs and sub-systems to the Army … The FVL office — [the] council of colonels and executive steering group — have also been kept informed.
The DARPA program is focused on developing new technologies to create new trade spaces that can be applied to creating new capabilities and performance attributes. Not long ago, the vertical flight community was struggling to rebuff the sentiment that rotary-wing flight technologies were mature and stagnant. The variety of concepts under consideration by DARPA, as well as the Army, are so varied they prove otherwise.