Lockheed Martin Desert Hawk III
4.7 Novel Hybrid Aircraft Configurations
As remarked in the previous chapter, for operational reasons, the ideal aircraft is one which can take off and land vertically, yet fly at high speeds. This benefits the whole system in that less infrastructure is required compared with systems in which the aircraft is launched from a runway or ramp.
Helicopter types of rotary wing aircraft are the most efficient in hover flight but, as already explained, have limits to their forward flight ability.
For many years, attempts have been made to produce aircraft which can perform well in both flight regimes. Inevitably the results are compromises where the aircraft are less efficient in both regimes compared with the ‘specialist’ hover (helicopter) or cruise flight (high wing-loaded fixed-wing) aircraft.
Hence the emergence of tilt-rotor and tilt-wing aircraft types. The search for the ideal aircraft continues and is made easier to achieve if no provision has to be made for aircrew to be accommodated or to function. Three different approaches which are aimed at achieving this ‘El Dorado’ are shown in Figures 4.27 and 4.28.
The Sky Tote
This is essentially a tilt-wing-body aircraft. A configuration similar to this was built, in prototype form, for VTOL fighter aircraft in the 1960s by Convair and Lockheed of the USA. However, both projects were abandoned when it was found how difficult it was for a pilot to land the aircraft whilst lying on his back with his feet in the air.
AeroVironment Inc. of the USA has a prototype Sky Tote UAV under development. Unlike the Convair and Lockheed prototypes which used a delta wing of low aspect ratio, it uses a main wing of relatively high aspect ratio and tail surfaces. At the time of writing the aircraft is undergoing hover tests and very little specification data have yet been released. It will be of interest to see how it fares during the transition
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AeroVironment “Sky Tote”
Honeywell T-Hawk Ducted-Fan MAV
All-up-Mass 110kg Wing Span 2.4m Powerplant One i.c.engine ? kW Predicted
Performance:-Speed 370km/hr Endurance 1.5hr Range Unknown Payload Mass 23kg
All-up-Mass Approx. 8 kg without fuel Duct Diameter 0.33m Powerplant One i.c.engine 3.38(?)kW Speed 74km/hr Range up to 10km Endurance 50 minutes Payload Optical & I.R. Sensors
Figure 4.27 Novel UAV systems 1 (Reproduced by permission of AeroVironment Inc and Honeywell)
mode. One problem which may be presented by the configuration could be interference by the rotors of any forward-looking imaging sensors.
Honeywell Ducted-fan MAV
Although designated a micro-AV, one would have thought that it more appropriately comes within the mini-UAV category by nature of the aircraft mass which is likely to be increased before it is qualified
Selex Damselfly This aircraft is in early development
and little data on it have yet been released other than the intake duct
is about 1 metre in diameter.
An internal fan, powered by either an electric motor or an internal
combustion engine, provides an air flow to four nozzles. These can swivel through 90 degrees to provide vertical or horizontal thrust.
It is hoped that the aircraft will Achieve a forward speed of 150 knots.
Figure 4.28 Novel UAV systems 2. Source: Selex
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for service. It is creditable that the back-packed system was exposed to a military environment in Iraq so early in its development.
Lessons have been learned already to indicate that it was underpowered, the flight endurance was inadequate and that vibration was causing a sensor problem. These short-comings are not unusual and are to be expected in a relatively novel design, and their discovery is better addressed earlier than later in the programme.
The new engine in development is stated to produce 3.38 kW. A simple estimate indicates that the power required to hover out of ground effect in SL ISA conditions must be of order 2.6 kW. The margin of 0.78 kW may be barely adequate to allow for engine power reduction with increases in ambient temperature and altitude and engine wear with usage in addition to the extra power required in manoeuvres. Those unused to VTOL aircraft operation, for example, seem often to be unaware of the margin of power to be instantly applied in the pull-up from a vertical descent. This is even more critical for small aircraft and ducted systems in particular since they gain little benefit from ground effect.
The designers/developers task in this brave programme will inevitably be centred on mass reduction.
Fortunately technological development is on their side.
Jet-lift Aircraft
A model of the Selex S&AS Company’s Damselfly is shown in Figure 4.28. This uses the jet-lift principle, in this case employing four separately directable nozzles which, presumably, achieve both lift and control functions. The Company’s claims for the product of having ‘the hover capability of a helicopter’ and
‘outstanding wind-gust resistance’ remain nonvalidated at the time of publication, with little supporting evidence available in the public domain.
The ultra high jet velocity of the configuration and long ducting must surely result in a large demand for power in a size regime where engines are not known for their frugality. Flight at low speeds must be very expensive in fuel consumption and, with a large wing, vulnerable to gusts, making it a challenge to perform effectively at low speeds in the urban canyons.
The author looks forward to being proven wrong, in which event Selex will have achieved a winner.
However, the claim of 150 kt cruise speed hardly justifies the expense of the ambitious new development since a hover-efficient helicopter, let alone a tilt-rotor machine, can achieve that speed.
The model is an 8.75%
dynamically scaled version of the Boeing Company’s
proposed airliner of a radically new configuration
Figure 4.29 Blended wing-body model
74 Unmanned Aircraft Systems