The Farnborough F1: a modern concept

Citation

(2000), "The Farnborough F1: a modern concept", Aircraft Engineering and Aerospace Technology, Vol. 72 No. 1. https://doi.org/10.1108/aeat.2000.12772aaf.001

The Farnborough F1: a modern concept

Keywords Aircraft, Project, Design, Air transport

In Richard Noble's opinion, despite the introduction of faster aircraft, flying from one destination to another takes longer today than it did 25 years ago. The answer he believes lies in smaller airfields. He considers that these are the keys that will unlock the present jam in business travel. The ingredients that will allow them to do so are either in place or will soon be a reality he states. Internet booking will allow "one click" desktop booking of both the air and ground segments of journeys. Computerized scheduling will allow several small aircraft operators to collaborate efficiently to provide a seamless service. Precision GPS (satellite navigation) approach systems will open up most small airfields to instrument landings without requiring expensive ground equipment. Electronic filing of flight plans and payment of landing fees will minimise ground time. Improved avionics will reduce the pilot workload and give pilots improved situational awareness. The gap between the capabilities of small aircraft and airliner avionics has narrowed considerably and in the near future will disappear.

Richard Noble considers that the availability of the above will provide conditions under which a distributed air travel network will out compete the current "hub-bound" system over shorter journeys. However, he is also convinced that this will be possible only with an aircraft designed specifically for this role. It is his intention that aircraft will be the "hub-busting" F1 from the recently launched company, Farnborough Aircraft.

The Farnborough F1 concept is of an aircraft designed to set new standards in executive travel, featuring point to point travel at the speed of a congested airliner. The aircraft is optimized as a high performance business executive mover, cruising at high speeds and operating off small uncongested airfields. It thus saves travel to and from the terminals at both ends, as well as the wasted check in and terminal time. Not to mention delays caused by lost slots. All of us, having boarded and settled into our seats, have groaned on hearing the captain announce that the aircraft will sit on the runway for an hour or so until a slot is available. This may well be a frustrating experience for the holidaymaker, but for the businessman it is a costly and possibly disastrous delay.

The design targets

Richard Noble explains that the design targets for the F1 stem directly from the mission. The aircraft must:

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  be safe when flown by pilots of all abilities;

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  be operable from all but the smallest of strips, paved or unpaved;

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  provide generous accommodation for up to five passengers;

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  cruise at speeds comparable to airliner short haul block speeds (>300kts);

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  have a cockpit layout compatible with modern glass screen avionics;

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  have seat-mile costs lower than the lost opportunity costs of current short-haul business air travel; and

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  have a minimal noise footprint to allow operation from airstrips whose neighbours have become accustomed to low disturbance levels from current low levels of activity.

F1 design philosophy

Farnborough Aircraft is dedicating itself to designing a high technology aircraft, the Farnborough F1, to meet the above requirements and in so doing it hopes to revitalise the British light aircraft industry.

In designing the F1, Farnborough Aircraft reports that it has applied the following philosophy:

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  Focused design: the aircraft is highly optimized for its core role, ruthlessly avoiding the temptation to allow "mission creep".

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  Application of appropriate technologies: technologies that improve the aircraft's ability to fulfil its role, such as carbon composites, computational fluid dynamics and laminar flow are being applied to the full.

The company also informs us that the temptation to apply fashionable technologies such as pusher propellers, canards, or forward-swept wings that do not improve the aircraft's ability to fulfil its role have been resisted.

F1 design features

The F1's design team, headed by Ralph Hooper, has incorporated the following features into the aircraft's design:

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  All carbon composite construction: saving 200lb in weight.

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  Laminar flow wing: giving low drag at high speed and high lift at low speed.

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  Mature Pratt & Whitney PT6A turboprop engine: combining proven reliability with low noise.

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  Large cabin cross-section: larger than the other aircraft in its class.

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  Thirty per cent chord Fowler flaps: giving slow stall and approach speeds.

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  Powerful ailerons and full depth rudder: giving positive safe control at low speeds.

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  Short takeoff and landing ability: takeoff to 50ft in under 500m.

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  High rate of climb and a quiet engine installation: minimising noise footprint.

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  Unprecedented cruise-to-stall speed ratio: 330 knot cruise/59 knot stall.

Aerodynamics

The Farnborough F1's aerodynamic design is thought to be one of the keys to its "hub-busting" performance. Ron Ayers, the project's consultant aerodynamicist, believes that to fulfil its role the aircraft must combine a very high cruise speed with low stall and approach speeds. It is significantly more difficult to develop an aircraft to fly both fast and slow than just to fly fast and requires an ultra-efficient wing design. This has been achieved by the application of two key technologies:

1. 1.

   computational fluid dynamics; and

2. 2.

   laminar flow aerofoils.

The spectacular rate of increase of computer power has allowed the designers of the F1 to employ computer power that only a few years ago would have been limited to companies like Boeing and Airbus. Ayers also considers that the aerodynamic design of the aircraft is the direct result of the use of the latest in computational fluid dynamics (CFD) software that simulates the airflow around aerofoils, wings or whole aircraft. The amount of wind tunnel testing required has been dramatically reduced by the use of this "virtual wind tunnel". The ability to visualize the flow directly has transformed the design process giving rapid feedback on the effect of design changes. The development of the F1's ultra-efficient wings would have been impossible without this ability to analyse quickly the aircraft's aerodynamics using CFD.

Laminar flow aerofoils

Air flows over a surface in one of two distinct modes, laminar or turbulent. In laminar flow, the air close to the surface (boundary layer) flows in thin layers, passing smoothly past each other with virtually no mixing. In turbulent flow, as the name implies, the boundary layer air flows over the surface in a chaotic fashion with mixing between air close to and further from the surface. At typical flight speeds, the extra energy dissipated in this turbulent mixing results in the skin friction, and therefore the drag on the aircraft, being approximately three times that of laminar flow.

Flow over a wing begins as laminar at its leading edge but becomes unstable, changing to the higher drag turbulent state. In conventional wings, this transition takes place close to the wing's leading edge. Stabilization of the laminar flow so as to delay transition would give low drag over more of the wing surface and has therefore long been an aim in aerofoil design.

The wing surfaces of conventional aluminium aircraft exhibit considerable waviness and have rough surface features such as rivet heads. These imperfections trigger the instability that leads to transition. It is therefore impractical to maintain laminar flow over wings of conventional aluminium construction. With composites it is possible to manufacture exceptionally smooth wing surfaces. Without the surface imperfections, the position of transition is then determined solely by the distribution of air pressure over the surface. The aerofoil shape can therefore be designed to provide a stabilising pressure distribution. Modern composite construction techniques have therefore freed designers to develop "laminar aerofoil" profiles that maximise the extent of laminar flow.

It is believed that existing laminar aerofoil profiles give low drag at cruise speeds, but these are not capable of generating the high lift at low speeds required to achieve the F1's low stall speed. Farnborough Aircraft have therefore carried out an in-house development programme using CFD to design a family of aerofoils optimized specifically for the F1. The resulting aerofoils are said to be capable of generating an exceptionally high low speed lift in conjunction with a Fowler flap and maintain extensive laminar flow at cruise speeds.

Farnborough Aircraft have given the aerofoil family the designation HLLF (high lift laminar flow). The exceptional performance claimed of the HLLF family is said to be apparent from comparison of the HLLF29140 aerofoil with one of the most advanced of current aerofoils, the NASA LS0413. The HLLF and LS aerofoils develop the same flapped lift but the cruise drag of the HLLF aerofoil is reported to be 75 per cent of that of the LS aerofoil. The HLLF aerofoils are also said to be designed to be more tolerant to surface contamination than other laminar aerofoils.

The HLLF aerofoil's combination of high flapped lift and high speed laminar flow are believed to be the basis for the F1's performance.

Composite structure - construction method and implications

The primary structure of the Farnborough F1 is manufactured from a composite material consisting of carbon fibres set in a toughened epoxy matrix. This material is moulded into the required shapes, which are then glued (bonded) together. The company explains that this method of construction was selected for a variety of reasons:

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  The high strength and stiffness (per unit weight) of carbon/epoxy, compared with the more usual aluminium alloys, means that it expects to show reductions in weight, for given components, of about 10-17 per cent. This will give it a reduction in aircraft weight of around 200lb (equivalent to one passenger), compared with an aluminium design.

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  The outstanding performance claimed of the F1 is driven largely by achieving laminar airflow over the wings. To obtain this, structural designer, Dr William Brooks, points out that it needs very accurately shaped wings with smooth surfaces (i.e. no rivets or panel joins). This is said to be possible with moulds taken from CNC machined patterns.

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  Carbon/epoxy is extremely resistant to fatigue and corrosion. This will, it is thought, give a long operational life for the airframe, even given the anticipated high utilisation levels, and high relative number of takeoffs and landings to hours flown. The airframe maintenance should be largely limited to repairing impact or scratch damage.

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  The airframe, we are informed, is to be constructed from a small number of large components. These components will be subsequently bonded together. This, it is said, dramatically reduces the number of assembly operations required, compared with an aluminium airframe. According to Farnborough Aircraft the number of assembly man hours will be reduced, fewer highly skilled fitters will be required and, for licensed production, the production learning curve for licensees will be eased. In addition to this, the moulded components will be interchangeable between airframes, so there will be less need for fitting during the assembly process.

The company lists the key features of structural configuration as follows:

1. 1.

   Wing configuration:

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     mounted to fuselage with removable links and location spigots;

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     carbon fibre/epoxy primary structure, with the following aluminium alloy exceptions: undercarriage pick-up, wing/fuselage joint fittings, flap tracks and Aileron attachments;

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     continuous inboard twin box spars and structural leading edge, giving high structural redundancy;

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     integral fuel tank;

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     top and bottom skins stiffened with bonded top hat stringers, designed to carry loads above limit in a post-buckled condition. Single skin internal ribs.

2. 2.

   Fuselage:

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     moulded in two halves with a vertical split;

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     sandwich construction with carbon skins and honeycomb core, minimising the need for internal frames and stringers;

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     dished separate rear pressure bulkhead; and

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     floor and seating supported by two longitudinal underfloor keel members.

3. 3.

   Tailplane:

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     one-piece with continuous carry-through box across fuselage;

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     sandwich skins with minimal ribs; and

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     bolted to fuselage via front and rear spars.

4. 4.

   Fin:

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     twin spar, bolted to fuselage; and

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     sandwich skins.

Noise levels

As explained, the Farnborough F1 aims to unlock the current "hub-locked" air transport system by flying into the thousands of small airfields that are inaccessible to other high speed aircraft. To allow operation from these airstrips, whose neighbours have often become accustomed to low disturbance levels from the current low levels of activity, it will have a minimal noise footprint. The company believes that the following noise-reducing features will allow the F1 to set new standards in neighbour-friendliness:

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  Turboprop engines produce much lower take off noise levels than turbofan engines of equivalent power. Low revving propeller noise is principally a function of rotational speed. Farnborough Aircraft have chosen a low 1,700rpm propeller speed, minimising noise.

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  The harshest, most obtrusive noise from a gas turbine engine is generated by its compressor. By using a reverse flow engine it has been possible to bury the engine's compressor deep within the air intake, thereby suppressing this noise source.

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  Noise reaching the ground reduces rapidly as the altitude of the aircraft increases. The Farnborough F1 is claimed to have an exceptional rate of climb, ensuring the aircraft will be at significant altitude even as it reaches the airfield perimeter. Similarly, the aircraft is being designed to fly steep landing approaches. Combined with the aircraft's low noise emissions, its steep approach and climb will, it is believed, result in a minimal noise footprint.

Whilst the Farnborough F1 is suitable for a number of applications including medical evacuation, the team states that it has been very determined to avoid mission creep. This they believe can result in a product that, whilst broadly capable of a wide range of commercial applications, does none spectacularly well. The Farnborough F1 is optimized as a high performance business executive mover, cruising at high speeds and operating off small uncongested airfields.

For those of us that love flying, but hate airports, e-mailing a flight from a nearby airstrip could soon be a reality.

The development of Richard Noble's Farnborough F1 aircraft can be followed on his Web site: http://www.Farnborough-Aircraft.com