U.S. Navy technology may allow in-flight conversion from helicopter to fixed wing.
Researchers at the U.S. Naval Research Laboratory are developing unmanned aircraft technology that will allow the conversion from a vertical take-off and landing system to a fixed-wing craft during in-flight operation. The conversion capability will provide the take-off and landing flexibility of a helicopter with the longer range, higher speeds and lower wear and tear of an airplane.
The technology demonstrator is referred to as the Stop-Rotor Rotary Wing Aircraft. It is capable of cruising at about 100 knots, weighs less than 100 pounds and can carry a 25-pound intelligence, surveillance and reconnaissance (ISR) or electronic warfare payload, such as the Expendable, Mobile Anti-submarine warfare Training Target (EMATT). “We decided to do a demonstration vehicle that could carry an EMATT. It’s like a little submarine that can generate sonar signals, and it’s for training anti-submarine warfare operators,” explains Steven Tayman, an aerospace engineer at the Naval Research Laboratory. “It’s a neat payload.”
The unmanned aerial vehicle (UAV) includes a removable payload bay that is about 12 inches wide, 38 inches long and six inches deep with “bomb bay doors” for dropping payloads, such as sonobuoys. “You could use a UAV to deploy a sonobuoy field, which would be pretty exciting,” Tayman says. “There’s really no limit to the payload other than volume.”
He adds that other options include electro-optic systems or electronic warfare decoys for spoofing missiles. “You could use the speed and range to deploy from a ship or remote area without a runway and drop sensors on the ground for monitoring what’s going on. That would be pretty easy,” he asserts.
With the vertical take-off and landing capability, the system could be launched from ships or any flat area without the bulky launch or recovery systems needed for some UAVs, Tayman notes. But converting to fixed wing mode also offers advantages, including longer range and higher speed. “One advantage of flying at higher speeds is being able to penetrate winds. Let’s say you run into a 40 or 50 mile-per-hour wind, which is not that unusual, especially at altitude—you’re not really going to go anywhere if you have a 60 or 70 mile-per-hour aircraft,” he says.
Tayman adds that some UAVs are designed for lower speeds because it makes launch and recovery easier, whereas the Stop-Rotor Rotary Wing Aircraft flies at speeds comparable to general aviation aircraft. “When you’re flying at a general aviation speed, you can get some place. But if you’re flying at 60 knots, that’s more like an ultralight aircraft, which is more dependent on wind for your ability to get from point A to point B,” he adds.
As a technology demonstrator, the system has not yet reached the prototype stage. Research lab personnel are searching for partners to help fund further development. If everything falls into place, Tayman says he could see the system being fielded in three to five years.
The aircraft flew autonomously in November of last year and is undergoing some modifications, including software improvements and airframe stiffening near the rotor area. Officials intend to hold another demonstration flight in June, but are probably not yet ready to demonstrate the in-flight conversion. “The flight mode conversion will depend on our progress with autopilot development. That’s obviously a big milestone. The technical risk is quite high, so we want to make sure we have everything as far along as we can get to improve the chances that it will work the very first time,” Tayman states. That’s probably further down the road. Things would have to go super well for us to do that in June.”
Even further into the future lies the possibility of adding a turbine propulsion system rather than the current battery electric propulsion. “If you convert to airplane mode at relatively low altitude and then had a turbine propulsion system, which performs at high altitude, you could then climb in airplane mode to high cruising altitude, more like a conventional turboprop aircraft. And you wouldn’t be limited on your ability to fly at high, subsonic numbers. Conceivably, that’s in the future,” Tayman explains.
However, using the same technology for larger aircraft—especially manned aircraft—is likely not in the future because during the in-flight conversion, the aircraft will essentially be in free fall. For obvious reasons, the timing and the duration of that free fall need to be precise and short. The unmanned system is designed to convert in one second, which Tayman says is critical to avoid falling too far or winding up at a bad flight path angle. “It might be more suitable to smaller vehicles and to UAVs in particular. The free fall might not be acceptable to a manned operation,” Tayman offers. “The UAV doesn’t care. The one-second free fall is nothing, just a bump in the road for a UAV, but for a manned operation, whether or not that would be acceptable is hard to say.”