The Design of UAV Systems
Armament 2 Hellfire missiles
4.3 Close-range/Battlefield Aircraft
4.3.1 Non-VTOL Aircraft Systems
Characteristics of Aircraft Types 59
The amount of engine power installed per unit of aircraft mass is similar for all the aircraft with piston engines, irrespective of their being HTOL or VTOL aircraft. The gas-turbine-powered aircraft, i.e. Firescout and Sea Eagle have more power installed, partly because both use a higher disc loading (especially in the case of the Sea Eagle) but also because the turbine engines deliver more power for their mass.
With the exception of the tilt-rotor Sea Eagle and the Seagull, all types have a similar cruise speed of about 200 km/hr. The Sea Eagle has twice the cruise speed of the others, as is expected and has power to match. The actual speed of the Seagull is not confirmed, but it may well be slower than the other aircraft since it is the only one which is configured to accommodate an optional single pilot, making it less compact and having greater aerodynamic drag than the more dedicated UAV.
With the exception of the Ranger, all the HTOL aircraft offer longer flight endurance than the VTOL aircraft. This may be due as much to the difference in their operating roles as to their fuel efficiencies. The radii of action of all aircraft are not dissimilar and may be determined more by similar communications limitations as for any other reason.
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but suffice it to say here that the need to achieve a difficult compromise is presented to the system designer.
Too long a ramp is ungainly and difficult to transport, but too short a ramp length requires a high value of acceleration be imposed, not only upon the aircraft, but upon its often delicate and expensive payloads and sensor systems. The ruggedisation of airframe and payloads can add considerably to their mass and cost.
This compromise can be ameliorated to some extent by reducing the aircraft minimum flight speed needed to reliably sustain flight as the aircraft leaves the end of the ramp. This requires either an increase of wing area or wing flaps, both of these not only adding to aircraft weight and cost, but increasing the aircraft aerodynamic drag, power required and fuel consumption in cruise flight.
The problem, of course, does not end with the design problems of the launch. The aircraft, now airborne, must be recovered at the completion of its mission. But there is no convenient runway awaiting its return, nor is it feasible to align it to decelerate back along the ramp. Two alternative recovery methods are generally employed.
The most ubiquitous is the deployment of a parachute from the aircraft and, to cushion its impact on landing, an airbag is deployed. Both of these sub-systems, together with their operating mechanisms, must be carried within the airframe, further adding to its mass, cost and volume.
Non-VTOL systems are represented here by the IAI Pioneer, BAE Systems Phoenix, the smaller Qinetiq/Cranfield Observer and Boeing/Insitu Scan Eagle UAV systems.
The IAI Pioneer continues with the ubiquitous pusher-propeller, twin-boom configuration which is the most popular for the medium and close-range UAV systems. A three-view drawing of the Pioneer UAV is shown in Figure 4.16 as representative of this configuration.
As with the medium-range aircraft described in Section 4.2.1, the configuration offers a compact fuselage with the option of alternative payloads and electronics in the nose, aft-mounted engine and pusher propeller which distances the power-plant and its ignition system from the electronics and provides an uninterrupted view forwards for the payload. The two booms provide some protection for personnel from the propeller. For recovery, a parachute can be mounted above the fuel tank and aircraft centre of mass. The main challenge in the structure of the configuration is to achieve sufficient stiffness in the twin booms to prevent torsional and vertical oscillation of the empennage.
Figure 4.16 Pioneer three-view drawing
Characteristics of Aircraft Types 61
All-Up-Mass 36kg Wing span 2.42m Wing area 1.73m2 Engine power 5.25kW Wing loading 184N/m2 Span loading 120N/m Cruise speed 125km/hr Loiter speed 110km/hr Mission radius 25km Endurance 2 hours
All-Up-Mass 177kg Wing span 5.5m Wing Area 3.48m2 Engine power 19kW Wing loading 500N/m2 Span loading 316N/m Cruise speed 158km/hr Loiter speed 126km/hr Mission Radius 50km Endurance 4 hours Figure 4.17 Close-range UAV systems: Observer and Phoenix (Reproduced by permission of Cranfield Aerospace Ltd)
Close-range systems adopting other airframe configurations are shown in Figures 4.17 and 4.18.
The Phoenix system started operation with the British Army about 1990 after an extended development phase. The prime contractor of the system was GEC Avionics who put the emphasis on the aircraft carrying a separate payload and avionics pod slung beneath the fuselage under the aircraft centre of mass. This presumably was the reason for installing the power-plant, unusually, at the front of the aircraft in order to achieve a longitudinal balance. The system saw extensive service in the Balkans and Gulf, but is no longer in service.
The Observer offers a simpler and more rugged airframe, tailored to improve its spatial stability in air turbulence by designing it, as far as is possible, to have neutral aerodynamic stability and stabilising it
All-Up-Mass 18kg Wing span 3.10m
Wing area 0.62m2 Engine power (23cc) 1.1kW Maximum speed 120km/hr
Cruise speed 90km/hr Endurance 15 hours Interchangeable payloads:-Optical & IR video, Mini SAR
Figure 4.18 Boeing/Insitu Scan Eagle. Source: Boeing-Insitu Inc; Cloud Cap Technologies; Insitu
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electronically in space coordinates. It is fitted with an ingenious surveillance payload which uses three miniature TV cameras which look at increasing elevations along the major axis of the aircraft to provide contiguous coverage. This achieves a large image footprint with high resolution and is claimed to reduce operator workload to a minimum. More details of this system are presented in Reference 4.3.
The Scan Eagle system uses an innovative sky-hook recovery method, but this adds a further vehicle and equipment to the system. However, the system has been successfully introduced into service with several military and naval operators, including the US Army, US Navy, Australian, Canadian and Singaporean Forces. The system has amassed more than 200 000 operating hours within its first five years of deployment.
The ‘flying wing’ configuration was presumably chosen in view of the demands of the recovery system as an empennage might have fouled the sky-hook. For more details see Chapter 12.