Ducted-fan VTOL unmanned aircraft have a lot to offer - the ability to take off, land and fly in confined urban spaces being one. But quiet they are not, at least those that are big enough to carry a useful payload for a useful time.
Now small firm D-Star Engineering has received what appears to be the first contract (for $4.8 million) awarded under the Intelligence Advanced Research Projects Agency's (IARPA) Great Horned Owl (GHO) program to develop a new class of quiet small unmanned aircraft.
Quiet... ...Not (Photos via wikipedia)
The GHO program is managed by Sam Wilson, who has a distinguished history of involvement in V/STOL development. He managed DARPA's Micro Air Vehicle and Organic Air Vehicle program, which led to Honeywell's RQ-16 T-Hawk ducted-fan (above) - an under-20lb VTOL MAV that is deployed (not quietly) in Afghanistan to help IED disposal units. Wilson later joined Avid Aerospace, where he continued to work on ducted-fan MAVs.
Sound is the number one signature that gives away the presence and location of small UAVs, says IARPA, and Phase 1 of GHO is focused on reducing the noise produced by power sources and propulsors. A separately competed Phase 2 will build the technology into a flight vehicle.
Although battery-powered UAVs are quiet, they don't have the payload or endurance required, IARPA says, so GHO is focused on hybrid turbine-electric propulsion systems that run on readily available automotive fuels - gasoline or diesel. The idea is to turn fuel into electricity and electricity into thrust.
A turbine driving a generator (above) would produce electricity to power the propulsor directly, eliminating noise-producing gearboxes. Adding batteries would allow the UAV to fly for up to 30min in ultra-quiet mode, with the turbine engine shut down, after which the engine would restart in flight to power the vehicle and recharge the batteries.
The sound goal is to stay in under the "60 phon" curve - phon curves trace the perceived loudness as noise frequency increases - meaning that no emission should exceed a sound pressure level of 100 dB at any frequency up to 100kHz. In Phase 1, IARPA will measure the noise produced by an uninstalled propulsion system at a distance of 10m, over a 15min run and a frequent range of 10Hz to 15kHz.
For Phase 1, IARPA is looking for a power system that can produce more than 4kW with a power loading of better than 0.4kW/lb at a fuel consumption lower than 1.1lb/kWh. The agency's eventual goal is more than 10kW of power with a better than 0.6kW/lb power loading and less than 0.8lb/kWh fuel consumption. The noise goal for the eventual vehicle is to stay below the 50 phon curve.