The T-Wing : A VTOL UAV for Defense and Civilian Applications

This paper describes progress made on the T-Wing tail-sitter UAV programme currently being undertaken via a collaborative research agreement between Sonacom Pty Ltd and the University of Sydney. This vehicle is being developed in response to a perceived requirement for a more flexible surveillance and remote sensing platform than is currently available. Missions for such a platform include coastal surveillance, defence intelligence gathering and environmental monitoring. The use of an unmanned air-vehicle (UAV) with a vertical takeoff and landing (VTOL) capability that can still enjoy efficient horizontal flight promises significant advantages over other vehicles for such missions. One immediate advantage is the potential to operate from small patrol craft and frigates equipped with helipads. In this role such a vehicle could be used for maritime surveillance; sonobuoy or other store deployment; communication relay; convoy protection; and support for ground and helicopter operations. The programme currently being undertaken involves building a 50-lb fully autonomous VTOL tail-sitter UAV to demonstrate successful operation near the ground in windy conditions and to perform the transition maneuvers between vertical and horizontal flight. This will then allow the development of a full-size prototype vehicle, (The “Mirli”) to be undertaken as a prelude to commercial production. The Need for a Tail-Sitter UAV Defence Applications Although conflicts over the last 20 years have demonstrated the importance of UAV systems in facilitating real-time intelligence gathering, it is clear that most current systems still do not possess the operational flexibility that is desired by force commanders. One of the reasons for this is that most UAVs have adopted relatively conventional aircraft configurations. This leads directly to operational limitations because it either necessitates take-off and landing from large fixed runways; or the use of specialized launch and recovery methods such catapults, rockets, nets, parachutes and airbags. One potential solution to these operational difficulties is a tail-sitter VTOL UAV. Such a vehicle has few operational requirements other than a small clear area for take-off and landing. While other VTOL concepts share this operational advantage over conventional vehicles the tail-sitter has some other unique benefits. In comparison to helicopters, a tailsitter vehicle does not suffer the same performance penalties in terms of dash-speed, range and endurance because it spends the majority of its mission in a more efficient airplane flight mode. The only other VTOL concepts that combine vertical and horizontal flight are the tiltrotor and tilt-wing, however, both involve significant extra mechanical complexity in comparison to the tail-sitter vehicle, which has fixed wings and nacelles. A further simplification can be made in comparison to other VTOL designs by the use of prop-wash over wing and fin mounted control surfaces to effect control during vertical flight, thus obviating the need for cyclic rotor control. For naval forces, a tail-sitter VTOL UAV has enormous potential as an aircraft that can be deployed from small ships and used for long-range reconnaissance and surveillance; over† Department of Aeronautical Engineering, University of Sydney ‡ Sonacom Pty Ltd the-horizon detection of low-flying missiles and aircraft; deployment of remote acoustic sensors; and as a platform for aerial support and communications. The vehicle could also be used in anti-submarine activities and anti-surface operations and is ideal for battlefield monitoring over both sea and land. The obvious benefit in comparison to a conventional UAV is the operational flexibility provided by the vertical launch and recovery of the vehicle. The US Navy and Marine Corps who anticipate spending approximately US$350m on their VTUAV program have clearly recognized this fact.  A Typical Naval UAV Mission: Monitoring Acoustic Sensors For ground based forces a tail-sitter vehicle is also attractive because it allows UAV systems to be quickly deployed from small cleared areas with a minimum of support equipment. This makes the UAVs less vulnerable to attacks on fixed bases without the need to set-up catapult launchers or recovery nets. It is envisaged that ground forces would mainly use small VTOL UAVs as reconnaissance and communication relay platforms. Civilian Applications Besides the defence requirements, there are also many civilian applications for which a VTOL UAV is admirably suited. Coastal surveillance to protect national borders from illegal immigrants and illicit drugs is clearly an area where such vehicles could be used. The VTOL characteristics in this role are an advantage, as they allow such vehicles to be based in remote areas without the fixed infrastructure of airstrips, or to be operated from small coastal patrol vessels. Further applications are also to be found in mineral exploration and environmental monitoring in remote locations. While conventional vehicles could of course accomplish such tasks their effectiveness may be limited if forced to operate from bases a long way from the area of interest.  A Historical Perspective Although tail-sitter vehicles have been investigated over the last 50 years as a means to combine the operational advantages of vertical flight enjoyed by helicopters with the better horizontal flight attributes of conventional airplanes, no successful tail-sitter vehicles have ever been produced. One of the primary reasons for this is that tail-sitters such as the Convair XF-Y1 and Lockheed XF-V1  experimental vehicles of the 1950s proved to be very difficult to pilot during vertical flight and the transition maneuvers.  Convair XF-Y1 and Lockheed XF-V1 Tail-Sitter Aircraft. 2 With the advent of modern computing technology and improvements in sensor reliability, capability and cost it is now possible to overcome these piloting disadvantages by transitioning the concept to that of an unmanned vehicle. With the pilot replaced by modern control systems it should be possible to realise the original promise of the tail-sitter configuration. The tail-sitter aircraft considered in this paper differs substantially from its earlier counterparts and is most similar in configuration to the Boeing Heliwing vehicle of the early 1990s. This vehicle had a 1450-lb maximum takeoff weight (MTOW) with a 200-lb payload, 5-hour endurance and 180 kts maximum speed and used twin rotors powered by a single 240 SHP turbine engine. A picture of the Heliwing is shown in   Boeing Heliwing Vehicle