Micro UAVs have been proved to be feasible, but larger mini UAVs may be a better way forward

Graham Warwick/WASHINGTON DC

There is nothing like doing something to help you decide you don't want to do it after all. And after proving that a 150mm (6in) "micro" unmanned air vehicle (MAV) can perform a useful military mission, the US Defense Advanced Research Projects Agency (DARPA) has decided it does not want a vehicle quite that small.

So move over MAV and make room for the "mini" unmanned air vehicle (UAV): after a successful four-year MAV technology demonstration, the US Defense Advanced Research Projects Agency (DARPA) is launching a mini UAV initiative, called the Organic Air Vehicle (OAV) programme. "MAV was a technology programme; OAV is about the system," says programme manager Sam Wilson.

The OAV project is targeted at providing an airborne sensor platform for the Future Combat Systems (FCS) programme, another DARPA effort aimed at developing concepts for a family of lightweight, manned and robotic fighting vehicles to enter service with the US Army in the 2015 timeframe.

Under the three-year OAV programme, contractors will each test a family of vertical take-off and landing (VTOL) air vehicles scalable in size from 230mm to over 610mm. "They will look at what is the right size," Wilson says.

The decision to look at larger vehicles was one result of the MAV programme. Another was the decision to focus on a VTOL vehicle. "We need something that can "perch and stare"- land, look, take-off and relocate," Wilson says. In fact, the OAV is less a UAV than a "relocatable unmanned ground sensor", he says. "We are building binoculars that can see round corners - eyes that are not attached to the soldier's head."

DARPA's MAV programme ended last year having demonstrated that a micro UAV could perform a useful mission. The progress achieved is illustrated by AeroVironment's Black Widow. The 150mm fixed-wing vehicle first flew for just 2min carrying no payload. By the end of the programme, the battery-powered vehicle could fly for 30min and downlink live video from a colour camera. Equipped with an autopilot, the MAV weighed just 81g (3oz), including an 8g payload, and could fly up to 1.8km (1nm) from the ground station and reach altitudes close to 770ft (235m).

"This was proof of what we set out to do," says Wilson. Meanwhile, MAV contractor Lockheed Martin Sanders (now part of BAE Systems) flew a series of progressively smaller UAVs under its MicroStar programme. Starting with a 610mm fixed-wing vehicle, the company eventually flew a 190mm UAV and a 380mm MicroStar demonstrating waypoint navigation using an onboard global positioning system (GPS) receiver.

Fixed-wing limits

"Fixed-wing vehicles have a lot of limitations," Wilson says. "Every landing is a crash. Luckily the vehicles are light and don't get damaged." The MAV programme also looked at a variety of vehicles able to take-off and land vertically. "In the next phase we need hover capability. It needs more energy, but it pays back."

Under the MAV programme, US firm Lutronics flew a hovering MAV which featured a fixed-pitch propeller with a duct for protection and vanes in the slipstream for control. The 150mm, 441g vehicle flew for 40min carrying a camera but could not fly faster than 18kt (33km/h) "because they couldn't hold the nose down", Wilson says. "It was great at hovering, but not forward flight."

One vehicle, however, set the stage for the OAV programme. It is the "lift augmented ducted fan" (LADF) MAV developed by Micro Craft and harks back to a "ring wing" UAV tested in the 1980s by General Dynamics' Convair division. Under the MAV programme, the company flew a camera-carrying, 230mm-diameter MAV that could take-off and land vertically, hover, yet reach speeds exceeding 50kt in forward flight. "The centre of gravity is near the centre of lift, so the vehicle can fly horizontally," Wilson says.

Micro Craft's MAV was relatively heavy, weighing in at 1.4kg (3lb) including two cameras and 0.2kg of fuel. The tiny internal combustion engine produced 4.3lb-thrust (0.02kN), propelling the vehicle, after transition to forward flight, to a dash speed of 53kt.

DARPA uses the 230mm LADF UAV to illustrate its concept for the OAV, although contractors are welcome to propose other ducted designs. The duct's primary purpose is to protect the UAV in collisions with obstacles, but it also improves hover performance. In vertical flight, the duct provides a 50% increase in lift over an unducted fan, Wilson says.

A ducted fan offers other advantages: "It has great adverse weather characteristics," says Wilson. There is no 'weather-vaning' - the tendency to turn into the wind - and because the vehicle gets its lift from the fan thrust, the design promises to be less sensitive to gusts. Icing can be overcome by directing engine exhaust into the duct to warm the airflow. "We want this tofly when no-one else is able to."

The OAV programme will look at enhancements to the design. Where Micro Craft's MAV was tele-operated, the OAV will have to operate fully autonomously. "Telepresence is not the way to go for small unit operations. There are no spare soldiers," says Wilson.

Relocatable sensor

Fundamentally a relocatable sensor, the OAV could be deployed ahead of advancing forces to monitor road intersections or bridge crossings. The vehicle would park and watch, sending back images only when it detected movement. "There is no need for streaming video. One frame per second is enough for a soldier to see things move," Wilson says. The OAV would relocate as the force moves.

One requirement of the FCS programme is that the OAV be able to operate with robotic ground vehicles. In this role, it could act as an "untethered mast", looking down to spot "negative obstacles" such as ditches that might pose an unseen hazard to the unmanned fighting vehicle.

As its name suggests, the OAV will be organic to the FCS and will have to "live with" the ground vehicles, so a small engine burning the same JP8 fuel as the vehicles has to be developed. Most of the MAVs used nitromethanol engines which are "very sensitive" and unsuitable for combat use.

One product of the MAV programme which is likely to find its way into the OAV is the thermo-electric generator. This mounts on the engine exhaust and uses the heat flow across a solid-state device to generate electricity. By the end of the MAV programme such devices had developed to the point where they could convert 7.5% of the waste heat into electricity. "There is real pay-off from integrating this with the OAV," says Wilson.

DARPA plans to kick off the OAV programme in February with the award of contracts to at least two teams. Wilson says flight testing should begin "almost right away". Each team will fly the same vehicle design in more than one size. "The aim of the next phase is to pick a size - 6in may be too small, but 2ft may be too big."

Source: Flight International