1 An evaluation of the historical issues associated with achieving non-helicopter V/STOL capability and the search for the flying car B Saeeda and G. B. Gratton Brunel Flight Safety Laboratory School of Engineering and Design Brunel University, Uxbridge, UK This document is produced from the final Microsoft Word file submitted to Aeronautical Journal of the Royal Aeronautical Society, which led to publication of the final paper – the words and pictures are correct therefore, but the formatting is not identical to that in the Journal. The full citation for this paper (no.3470) is: B Saeed & GB Gratton, An evaluation of the historical issues associated with achieving non-helicopter V-STOL capability and the search for the flying car, AeroJ Vol.114 No.1152 pp91-102 (Feb 2010) a Corresponding author 2 ABSTRACT VTO vertical take-off Combined Vertical and Short Take-Off and Landing, VTOL vertical take-off and landing or ‘V/STOL’ capability has been of great demand and W gross weight (kN) interest in the field of aeronautics since the creation of ρ air density at sea level ( 1.225kg / m3 ) the aircraft. V/STOL capability is a targeted capability Area ratio between fan and duct exhaust for many projected or prototype future aircraft. Past V/STOL aircraft are reviewed and analysed with µ dry friction coefficient Propulsive efficiency regard to their performance parameters. This research p has found two embedded categories in this class of aircraft based on their propulsion systems, i.e. jet and non-jet propulsion, and highlights the significant performance differences between them. In light of historical experience the performance of a relatively 1 INTRODUCTION new class of aircraft, the flying cars, has been evaluated. V/STOL refers to Vertical or Short Take-Off and Landing capability, an aircraft that can perform either vertical or short take-off or landing is said to inherit NOMENCLATURE V/STOL capability e.g. BAE Harrier. The term V/STOL is composed of two other VTOL, vertical take-off and landing, and STOL, short take-off and a Acceleration ms/ 2 landing. An aircraft with insufficient vertical thrust C drag coefficient D may attempt a short take-off and vertical landing upon CG centre of gravity reducing weight from fuel consumption, this class of C Lift coefficient L aircraft is specifically designated by STOVL. C maximum lift coefficient Lmax CTOL Conventional take-off and landing V/STOL capability cuts the need for long runways and d Fan diameter (m) reduces the time to achieve horizontal flight: g 2 Gravitational acceleration ms/ conventional jet aircraft land and take-off with speeds ks Suckdown factor of, around, 80 to 120 m/s and may require runways up L Lift (N) to 3,500m in length in some cases – this is an LOGE Lift generated by out of ground effect expensive infrastructure problem that V/STOL has Jet fountain lift potential to solve. Lf Ls Suckdown lift Interest in V/STOL flight probably arose when early Mass flow rate AU (kg/s) attempts at powered flight tried emulate the behaviour m of birds; however, no early man made machine, based NTSB National Transport Safety Board heavily on birds ever achieved controlled flight. It was P Engine power (watts) not then recognised that the short and vertical take-off S 2 wing surface area ( m ) capability of birds is in large measure made possible 2 Sd Propeller disk area ()m by their low wing-loading, which is a natural result of S0 Take-off ground roll (m) their small size. Figure 1 depicts a chart of wing STOL short take-off and landing loading and flight speed for a variety of birds and it T Total Thrust (N) can be seen that the birds with relatively low wing- Thrust generated by a ducted propellor loading and forward velocity are actually VTOL Tdp capable, such as the hummingbird, blackbird and barn Thrust required for VTOL Treq swallow, other birds being either V/STOL or TR reversed engine thrust conventional take-off and landing (CTOL) capable. Sir UAV unmanned aerial vehicle George Cayley, who was the first to recognise the V/STOL vertical/short take-off and landing importance of distinguishing lift from thrust and in V Cruise velocity (m/s) particular to recognise the fact that for level flight, the Vmax maximum cruise velocity required thrust is one or two orders of magnitude less (1) Vref approach speed than the required lift . vc Rate of climb or climb velocity 3 Figure 1, Take-off and landing characteristics of birds [data adapted from2,3,4,5]. In order to understand this phenomenon further, consider lift and drag curves for a typical wing section as shown in Figure 2. The lift is always larger in magnitude than the drag at typical flight conditions, which typically correspond to10CCLD / 15 , and hence the thrust required to overcome the drag is less than the lift generated. Early vertical flight – in helicopters - was achieved by an aerodynamic, or rotary wing, solution, rather than a pure propulsive solution. The following equation best describes the relationship between the forces acting on an aircraft in level flight. Thrust Drag Lift Weight Jet propulsion achieves Vertical Take Off, or VTO by working against an aircraft’s weight due to gravity directly, whereas the rotary wing solution does work initially against drag (profile and induced) and hence benefits from the phenomenon of lift being much larger than drag. 4 Figure 2, Typical NACA wing-section characteristics [taken VTO Propulsion Strategy Aircraft Model from (6)]. Transcendental 1 Tilt Shaft/Rotor Model 1G Regardless of mechanism, the development of Bell XV-3 2 Curtiss-Wright X- 3 V/STOL capability has also inevitably been reliant 100 Tilt Prop upon the availability of suitable power plants. In Curtiss-Wright X- 4 particular, the gas turbine engine, with its high thrust : 19 weight ratio, made possible the eventual development Doak 16 VZ-4 5 of aircraft with static thrust to gross weight ratios Tilt Duct Bell-X22A 6 greater than one – a prerequisite for VTOL capability. Nord 500 Cadet 7 As the speeds of aircraft continue to increase, the Vertol 76 VZ-2 8 power plant requirements for V/STOL operation and Hiller X-18 9 forward flight performance become compatible. Tilt Wing LTV-Hiller Ryan 10 Furthermore, at the same time, above Mach 1 the XC-142 Canadair CL-84 thrust required is nearly equal to or exceeds the gross 11 Dynavert weight of the aircraft at level flight – coinciding some Bell XV-15 12 aspects of the design solutions for V/STOL and Same Propulsion System for Hover Tilt Rotor Bell Boeing V-22 13 supersonic aeroplanes. and Forward Flight Osprey Tilt Jet Bell 65 14 The most prolific V/STOL capable aircraft, so far, is Robertson VTOL 15 Deflected Ryan 92 VZ-3 clearly the helicopter; however in level flight the 16 Slipstream Vertiplane helicopter is inefficient compared to a typical fixed- Fairchild 224 VZ-5 17 wing aeroplane, with speed and range only between a Bell X-14 18 half or one third (approximately) that of the aeroplane. Hawker P.1127 19 Also, due at-least in part to their greater complexity, Kestrel helicopters demonstrate poorer safety than Vectored Thrust Yakovlev Yak-36 20 (7) conventional aeroplanes : with for example light BAe Harrier 21 conventional aeroplanes suffering a fatal accident rate Boeing X-32 22 of 11.7/million flying hours, versus 33.5/million flying Lockheed XFV-1 23 Convair XFV-1 hours for small helicopter. The same complexity also 24 contributes to a greater cost: for example at time of Tail Sitters Pogo writing the typical hire cost of a Robinson R44 Ryan X-13 Vertijet 25 SNECMA C450 26 helicopter in the UK is £400/hr or to purchase the Short SC.1 27 aircraft would cost £100,000-£200,000, whilst a Lift Separate Power + Dassault Balzac V 28 Cessna C172 aeroplane, which has similar payload and Plant for Hover Cruise Dassault Mirage III- cruise performance capability, can be rented for about 29 £150/hr or purchased for about £30,000-£100,000 – V EWR VJ101C 30 costs around 30-40% of the cost of the helicopter. Dornier Do 31 31 Lift Lockheed XV-4B 32 + The search for V/STOL capability has provoked Combined Power Lift/Cruise VFW VAK 191B 33 Plant for Hover research into embedding VTOL capability of a Yakovlev Yak-38 34 helicopter into a conventional fixed-wing aeroplane. Yakovlev Yak-141 35 McDonnell XV-1 36 However, this has rarely been achieved. The authors Tip Jets have identified 45 fixed-wing aircraft which have Fairey Rotodyne 37 Lockheed XV-4A 38 attempted to combine V/STOL capability of the Ejector helicopter with high forward flight speed of a Rockwell XFV-12A 39 Vanguard conventional aircraft. Of these 45, only four: the BAe 40 Omniplane Harrier, Yak-38, Bell-Boeing V-22 and Lockheed Augmented Power Fan GE-Ryan XV-5A 41 Plant for Hover Lockheed Martin Martin X35 Joint Strike Fighter have ventured much 42 beyond the prototype stage. Table 1 below presents X-35 Kamov Ka-22 43 these 45 aircraft arranged according to their propulsion Rotor Piasecki 16H-1 44 systems. Lockheed AH-56 45 Table 1, V/STOL aircraft arranged according to their propulsion systems. 5 This table is dominated by aircraft whose designs For cruise-dominated VTOL aircraft – such as may be attempt to use the same power system for both VTOL- designed for transport purposes, a more severe lift and propulsion: including the BAe Harrier and the problem involves thrust matching.