Patent Description:
It is desirable to decrease aircraft drag, to reduce fuel burn and increase range. One method of reducing drag is to provide aircraft having "tip propellers". An example experimental aircraft using such a propulsion system is the Chance-Vought XF5U. Such aircraft produce reduced induced drag (i.e. drag due to lift). Such propulsions systems also provide for increased propeller efficiency. However, such designs are compromised by several considerations. For example, by providing tip propulsors at the wing tips, a relatively large yaw force and / or roll force is generated when one of the tip propulsors fails. This requires either a large tail to counteract such forces, or requires a relatively short span wing, to reduce the forces. Such a short span wing implies a low aspect ratio, which increases induced drag, thereby removing much of the benefit of such systems. Furthermore, the large weight of such systems far from the aircraft centreline implies a large structural weight penalty. The present disclosure seeks to provide an aircraft and propulsion system that overcomes these problems.

United States patent <CIT> discloses an aircraft wing design that creates a bell shaped span load, which results in a negative induced drag (induced thrust) on the outer portion of the wing; such a design obviates the need for rudder control of an aircraft.

United states patent application <CIT> discloses methods for upper and lower surface boundary layers to cancel their opposing vorticity upon shedding from the trailing edge, thereby eliminating wake vorticty, induced drag and associated noise. This requires wing-rotor-propeller or fan blades with a platform designed for uniform bound circulation and with boundary layer control near the tip. In addition this requires special techniques to counter span-wise pressure gradients, such as tip circulation control blowing or an upwind small propeller or wind turbine on each tip. These techniques can eliminate wake vorticity with its induced drag, noise, flying on the backside of the power curve and the option for asymmetric loading by pneumatic means to eliminate need for cyclic pitch control or conventional ailerons.

According to the invention there is provided an aircraft comprising; a port wing and a starboard wing, each having a spanwise lift distribution extending from a root to a tip, the lift distribution defining an inboard region defining a positive lift contribution, an outboard region defining a negative lift contribution, and an intermediate region defining a neutral lift contribution, the intermediate region being spaced from the tip and from the root;
a propulsion system comprising a propulsor mounted to the port wing and a propulsor mounted to the starboard wing; wherein each wing mounted propulsor is located aft of a trailing edge of the corresponding wing; wherein the wing mounted propulsor has a rotational axis positioned substantially at a span of the wing where a value of δLift/δSpan is at a maximum magnitude for the span of the wing, and wherein the propeller mounted on the port wing rotates in a clockwise direction when viewed from the aft of the aircraft, and the propeller mounted on the starboard wing rotates in an anti-clockwise direction when viewed from the aft of the aircraft.

Advantageously, it has been found that this positioning of the wing mounted propulsor in combination with the above lift distribution allows for reduced drag and increased propeller efficiency. Without wishing to be constrained by theory, this is thought to be a result of wing vortices generated at a span position corresponding to a maximum value of the first differential of the lift with respect to span resulting in a reduced advance ratio of the propulsor, leading to a higher propeller efficiency. Further features and advantages are provided by the disclosed arrangement, as set out below.

The wing mounted propulsor may comprise one of an open rotor propulsor such as a propeller, and a ducted propulsor such as a fan.

The wing mounted propulsor may be located at the intermediate region of the span of the wing. For some lift distributions, this position will correspond to the point of maximum magnitude of δLift/δSpan.

The wing mounted propulsor is located aft of a trailing edge of the wing. Where the wing mounted propulsor is located aft of the trailing edge of the wing, the downwash generated upstream of the wing is reduced, thereby reducing drag.

The wing may comprise an aspect ratio greater than <NUM>. The wing may comprise an aspect ratio greater than <NUM>, and may comprise an aspect ratio greater than <NUM> and may comprise an aspect ratio greater than <NUM> and less than <NUM>.

The wing mounted propulsors are relatively inboard compared to previous aircraft. This reduces the yaw generated by the propulsors in the event of a propulsor failure. This in turn allows for an increase in the span of the wing, since the wing span is no longer limited by the placement of the wing mounted open rotor propulsors, which would have been provided at the wing tips in previous designs. This higher span wing further reduces lift induced drag, in view of the lower span loading implied by a larger span, and so reduces fuel use and increases range.

The propulsion system may comprise one or more further propulsors. The one or more further propulsors may be located inboard of the intermediate region of the wing. By providing further propulsors, the propulsive requirements of the aircraft can be met by dedicated propulsors, while the wing mounted propulsors can be sized and optimised to the requirements of drag reduction, rather than thrust production. Furthermore, by placing the further propulsors inboard of the intermediate region of the wing, the weight of the propulsors provides wing bending moment relief, which counteracts the wing bending moment produced by the positive lift generated by the inboard region.

The wing may comprise a folding mechanism configured to fold upward and stow the outboard region of the wing, wherein the folding mechanism. The folding mechanism may define a pivot point at or outboard of the position of the wing mounted propulsor and / or the intermediate region of the wing. Advantageously, a large aspect ratio wing can be provided, without requiring excessive clearance during ground operations. The provision of the pivot point outboard of the wing mounted propulsor reduces the weight that must be raised by the wing folding mechanism. The provision of the pivot point outboard of the neutral point ensures that the folding portion of the wing is urged toward the deployed position by aerodynamic forces in flight, thereby increasing safety.

The propulsion system may comprise one or more internal combustion engines, and may comprise a gas turbine engine. The or each internal combustion engine may be configured to drive one or more propulsor.

The propulsion system may comprise an electric motor configured to drive the wing mounted propulsor.

The aircraft propulsion system may comprise a parallel hybrid propulsion system, comprising the internal combustion engine which is mechanically coupled to at least one wing mounted propulsor. Where the aircraft propulsion system comprises a parallel hybrid system, the system may comprise an electric generator coupled to the internal combustion engine and configured to provide electrical power to the electric motor.

Alternatively, the aircraft propulsion system may comprise a series hybrid propulsion system, in which the internal combustion engine is coupled to an electric generator configured to provide electrical power to a propulsor motor coupled to at least one wing mounted propulsor, and wherein the internal combustion engine is not mechanically coupled to a propulsor.

As a further alternative, the aircraft propulsion system may comprise an electric propulsion system, wherein the propulsion system comprises one or more electric storage devices, such as a fuel cell, capacitor or chemical cell configured to provide electrical power to the electric motor.

With reference to <FIG>, a plan view of an aircraft <NUM> is shown. The aircraft <NUM> comprises a fuselage <NUM>, wings <NUM>, tail <NUM> and a propulsion system comprising a pair of open rotor wing mounted propellers <NUM>. Each propeller <NUM> is mechanically driven by a respective internal combustion engine in the form of a gas turbine engine <NUM>.

The wing <NUM> has a generally elliptical planform (though other planforms could be employed), and is shaped to provide a lift distribution as shown in <FIG>. As will be understood by the skilled person, the lift distribution is the lift coefficient produced by the wing under given conditions such as angle of attack of the aerofoil, the Reynolds number of the flow, and its Mach number, as a function of the span of the wing. As is well known, the lift coefficient CL is given as: <MAT>.

Where L is the lift force, S is the relevant surface area, and Q is the fluid dynamic pressure, in turn linked to the fluid density and flow speed.

In the present disclosure, the wings <NUM> have a "bell-shaped" lift distribution, similar to that described in<NPL>. Such wings are thought to have benefits in terms of overall aircraft drag, taking into account the structural weight of the wing. As will be understood by the skilled person, the lift coefficient can be modified by adjusting various parameters. For instance, lift coefficient can be increased by increasing the camber of the wing section, and reduced by reducing the camber. Different aerofoil sections can be used. Similarly, increasing the chord will typically increase lift coefficient. Increasing local angle of attack can also increase lift coefficient, and so "twist" can be applied to the wing to control local angle of attack. Any one or a combination of these techniques can be applied to provide a desired local section lift coefficient.

The wing <NUM> of the aircraft <NUM> of the present disclosure comprises several distinct regions. A root <NUM> of the aircraft wing <NUM> is defined at an inboard position of the wing <NUM>, adjacent the fuselage <NUM>. Similarly, a tip <NUM> is defined at the extreme outboard end of the span of the wing <NUM>.

Referring to <FIG>, a positive lift region <NUM> is defined at an inboard region of the wing <NUM>. In the positive lift region, the wing <NUM> is arranged to provide lift in an upward direction, as conventionally defined, at neutral or positive angles of attack. It will be understood that the lift provided varies as a function of span, with more lift being generated at the root <NUM> than at positions closer to the tip <NUM>. Outboard of the positive lift region <NUM>, adjacent the tip <NUM>, is a negative lift region <NUM>. In this region, a downforce is generated at a positive or neutral angle of attack. Between these two regions is an intermediate neutral region <NUM>. The neutral region <NUM> provides little or no lift or downforce. In some cases, the neutral lift region <NUM> may comprise a single point on the span of the wing <NUM>, at the point at which the positive and negative lift regions <NUM>, <NUM> meet.

Typically, the positive lift region <NUM> occupies the majority of the span of the wing <NUM>, and may extend between <NUM> and <NUM>% of the span of the wing from the root <NUM>. In one example, the positive lift region extends approximately <NUM>% of the span of the wing <NUM> from the root <NUM>. Similarly, the negative lift region occupies between <NUM> and <NUM>% of the span of the wing from the tip <NUM>. As can be seen, the lift generated by the positive lift portion <NUM> is greater than the downforce generated by the negative lift portion <NUM>, such that the wing <NUM> produces net upward lift. Typically, in total, the positive lift portion <NUM> produces between <NUM> and <NUM> times as much lift as the negative lift portion <NUM> produces downforce.

The section lift coefficient of the negative lift portion <NUM> varies across the span, with a maximum at a central region, and minima at the tip <NUM> and adjacent the neutral lift region <NUM>.

Referring now to <FIG>, the continuous line shows the lift distribution, while the dotted line shows the first differential with respect to span δLift/δSpan of the lift distribution. As can be seen, the wing mounted propellers <NUM> are positioned such that their rotational axis X is coincident with, or adjacent, the neutral lift region <NUM>, and with the position of maximum magnitude (positive or negative) of δLift/δSpan, which in this particular case coincide. Each propeller <NUM> is positioned behind a trailing edge <NUM> of the wing <NUM>. The propeller <NUM> positioned on the port wing rotates in a clockwise direction when viewed from the aft of the aircraft, and the propeller <NUM> positioned on the starboard wing rotates in an anti-clockwise direction when viewed from the aft of the aircraft.

Without wishing to be limited by theory, this is believed to have the effect of increasing propeller efficiency, as outlined below.

As will be understood, as they generate lift, wings generate vortices. In traditional wing designs, these vortices are generated at the tips of the wings, with a vortex emanating from the port wing and rotating in a counter-clockwise direction when viewed from the aft of the wing, and another vortex emanating from the starboard wing and rotating in a clockwise direction when viewed from the aft of the wing. However, in the case of wing having a negative lift region at the tip, vortices will instead be generated at the neutral lift portion, where the positive and negative lift portions meet. By placing a propeller <NUM> in this region, with each propeller configured to rotate in a direction opposite to the direction of rotation of the local vortex, the propeller interacts with the vortex. This results in an increase in propeller efficiency.

Ordinarily, propellers generate thrust having a circumferential component, which imparts swirl on the airflow, rather than thrust. This effectively reduces propeller efficiency. The wing vortices turn this swirling flow in the direction of the aircraft longitudinal axis, thereby converting this swirl to thrust, and increasing propulsive efficiency. In other words, the advance ratio of the propellers is reduced, leading to higher propeller efficiency.

This effect can be managed by controlling the swirl imparted on the airflow by the propeller <NUM>. As will be understood, propeller swirl can be controlled by controlling one or more of propeller diameter, blade chord, pitch and helix angle. The propeller <NUM> may be controlled such that the vortex is substantially eliminated at different flight conditions, by controlling blade pitch in flight. Additionally, this arrangement may allow for a wider design space, as designs that would cause a large degree of swirl would normally be rejected as inefficient - in this arrangement, these inefficiencies are eliminated. For example, the propeller may have a twist that is designed to exploit the incoming vortex.

In contrast to prior aircraft, the wing mounted propellers <NUM> of the present disclosure are provided relatively inboard along the span of the wing <NUM>, rather than at the tips, as would be conventional. This gives the designer several opportunities. For example, the wing mounted propellers <NUM> of the present disclosure may be provided at between <NUM>% and <NUM>% of the span of the wing from the root, or between <NUM>% and <NUM>% of the span of the wing, and preferably at about <NUM>% of the span of the wing. In other words, the rotational axis X of the wing mounted propellers <NUM> are preferably located within <NUM>% of the span of the neutral lift point, and preferably within <NUM>% of the span of the neutral lift point.

Since the wing mounted propellers <NUM> are not located at the tips, the span of the wing <NUM> (and so its aspect ratio) can be increased, without risking excessive yaw in the event of a propeller or engine failure. This increased aspect ratio will reduce induced drag, since induced drag is known to be related to aspect ratio, in accordance with the following formula: <MAT>.

Where CDi is the drag due to induced drag, e is the Oswald efficiency, and AR is the aspect ratio. In the present case, it is thought that an aspect ratio of between <NUM> and <NUM> can be achieved, without causing excessive yaw in the event of an engine failure, even where the aircraft is a twin propeller aircraft.

Similarly, the designer is able to choose a vertical tail having a low tail volume coefficient, in view of the relatively low yaw moment caused by one engine being inoperative. Indeed, in some cases, elimination of the vertical tail may be possible, in view of the elimination of adverse yaw in aircraft having this lift distribution.

Further advantages are provided. In view of the downforce / upwash generated by the wing at the tip, and the weight of the propeller <NUM> inboard, significant wing bending moment relief is provided. This allows the structural weight to be reduced, which can be traded for lower overall aircraft weight, or further increased aspect ratio. In either case, fuel use is reduced, and range increased.

Another further advantage of the reduced vortex is the reduction in turbulent aircraft wake. This may allow for closer aircraft spacing, which may in turn allow for close formation flying for the purposes of aircraft fuel reduction, or shorter spacing for aircraft taking off and landing, which may reduce congestion at airports.

<FIG> shows a second aircraft <NUM>. The second aircraft <NUM> is similar to the aircraft <NUM>, and also comprises a wing <NUM> having the lift distribution shown in <FIG>, having positive lift <NUM>, negative lift <NUM> and neutral <NUM> regions, and having wing mounted propellers <NUM> provided at the neutral lift region <NUM>. However, there are also various differences.

The propulsion system of the aircraft <NUM> comprises further propulsors <NUM> provided inboard of the wing mounted propellers <NUM>, and also inboard of the neutral lift region <NUM>. The further propulsors <NUM> are also in the form of propellers in this embodiment, though they could take different forms, such as ducted propulsors. Similarly, the further propulsors <NUM> are provided upstream of the leading edge of the wing <NUM>, but could be positioned in different locations, or could be attached to the fuselage.

By providing further propulsors <NUM> inboard of the wing mounted propulsors <NUM>, several advantages are realised. Firstly, only a proportion of the thrust required by the aircraft must be generated by the wing mounted propulsors <NUM>. This reduces their required size and weight, and also reduces the consequence of the failure of such a device. Consequently, yaw in the event of an engine failure is again reduced, which gives rise to the advantages outlined above. Secondly, additional wing bending moment relief is provided, since the weight of the further propulsors counteracts the bending caused by the lift generated by the wing in the positive lift region <NUM>. Since the negative lift region <NUM> does not contribute to lift, no bending moment relief is provided by providing engines in these locations.

In order to take further advantage of this bending moment relief, additional structure and equipment may be installed in the wing inboard of the neutral lift region. For instance, fuel tanks <NUM> may be provided at this location.

Additional features are provided of the aircraft <NUM>. The aircraft comprises pivotable wings, an outboard portion <NUM> of which can be pivoted between a deployed position (as shown in <FIG>) and a stowed position (as shown in <FIG>). In the stowed position, the effective span of the aircraft wings is reduced, improving ground handling. For instance, gate size requirements are reduced. Consequently, wing span can again be increased, without facing ground handling constraints.

A pivot point <NUM> for the wing pivoting system is located outboard of the wing mounted propeller <NUM>, and possibly also outboard of the neutral lift region <NUM>. Consequently, the weight of the pivotable portion of the wing is relatively low, thereby reducing the weight and power requirements for the wing folding actuators. Furthermore, since the outboard portion <NUM> is coincident with the negative lift portion <NUM> of the wing <NUM>, aerodynamic forces urge the portion <NUM> to the deployed position in flight, thereby increasing safety.

The propulsion system of the aircraft <NUM> also differs from that of the aircraft <NUM> in other ways.

<FIG> shows the propulsion system of the aircraft <NUM> in more detail. The propulsion system includes a pair of internal combustion engines in the form of gas turbine engines <NUM>. Each gas turbine engine <NUM> comprises, in axial flow series, a propulsor in the form of a propeller <NUM>, a compressor <NUM>, combustion equipment <NUM> and a turbine <NUM>.

The gas turbine engine <NUM> works in the conventional manner so that air is accelerated by the propeller <NUM> to produce two air flows: a first core air flow into the compressor <NUM> and a second air flow which bypasses the compressor <NUM> to provide propulsive thrust. The core air flows through the compressor <NUM> where it is compressed, before delivering that air to the combustion equipment <NUM>, where it is mixed with fuel and the mixture combusted. Fuel is supplied to the combustors <NUM> via fuel injectors (not shown). The resultant hot combustion products then expand through, and thereby drive the turbine <NUM> before being exhausted through a nozzle to provide additional propulsive thrust. The turbine <NUM> drives the compressor <NUM> and propeller <NUM>, by a suitable interconnecting shaft. An electrical generator <NUM> is provided, which is coupled to the gas turbine engine shaft. Consequently, rotation of the shaft generates propulsive thrust via the propeller <NUM>, and also generates electrical power.

Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g. three) and/or an alternative number of compressors and/or turbines. Further, the engine may comprise a gearbox provided in the drive train from a turbine to a compressor and/or fan. The propulsion system <NUM> further comprises one or more electrical machines driving at one or more propulsors. In particular, the system <NUM> comprises a pair of electric motors <NUM>, which drive a respective wing mounted propulsor <NUM>. The motor <NUM> is of a conventional type, such as an induction or permanent magnet electric machine.

The electric motor <NUM> is optionally coupled to an energy storage device (<NUM>) in the form of one or more of a chemical battery, fuel cell, and capacitor, which provides the electric motor <NUM> with electrical power during operation.

An alternative propulsion system for the aircraft <NUM> is shown in <FIG>. In this case, a single gas turbine engine <NUM> is provided, which is similar the gas turbine engines <NUM>. The gas turbine engine <NUM> comprises an electric generator <NUM>, which is in turn coupled to electric motors <NUM>, which drive a respective propulsor <NUM>, <NUM>. In this case, all of the propulsors are driven by electric motors, and so the system can be described as being a series hybrid.

<FIG> shows an alternative aircraft <NUM> not in accordance with the present invention. The aircraft again comprises wings <NUM> and a fuselage. The wings <NUM> in this instance have a rearward swept planform, in which the leading edge at the tip is located aft of the leading edge at the root. This both reduces drag at high transonic speeds, and allows for a further reduction of vertical tail volume, in view of increased longitudinal stability. Otherwise, the wing <NUM> is similar to the wing <NUM>, having the lift distribution shown in <FIG>.

In this case, wing mounted propulsors <NUM> are provided at the neutral lift point, with further propellers <NUM> provided inboard. The wing mounted propulsors <NUM> are in the form of ducted propulsors (fans), and are provided upstream of the leading edge of the wing, to reduce upwash generated forward of the wing. Again, this will be expected to reduce drag, and increase propulsive efficiency.

The aircraft <NUM> is an electric aircraft, comprising an energy storage unit in the form of a fuel cell <NUM>. The fuel cell provides electric power to motors <NUM>. The fuel cell could be replaced or supplemented by other electric power sources, such as chemical batteries, capacitors.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein.

Claim 1:
An aircraft (<NUM>) comprising;
a port wing (<NUM>) and a starboard wing (<NUM>), each having a spanwise lift distribution extending from a root (<NUM>) to a tip (<NUM>), the lift distribution defining an inboard region (<NUM>) defining a positive lift contribution, an outboard region (<NUM>) defining a negative lift contribution, and an intermediate region (<NUM>, <NUM>) defining a neutral lift contribution, the intermediate region (<NUM>, <NUM>) being spaced from the tip (<NUM>) and from the root (<NUM>);
a propulsion system comprising a port propulsor (<NUM>) mounted to the port wing and a starboard propulsor (<NUM>) mounted to the starboard wing (<NUM>);
wherein each wing mounted propulsor (<NUM>) is located aft of a trailing edge (<NUM>) of the corresponding wing (<NUM>);
wherein
each wing mounted propulsor (<NUM>) has a rotational axis (x) positioned substantially at a span of the wing (<NUM>, <NUM>, <NUM>, <NUM>) where a value of δLift/δSpan is at a maximum magnitude for the span of the wing (<NUM>, <NUM>, <NUM>, <NUM>), and wherein the propeller (<NUM>) mounted on the port wing (<NUM>) rotates in a clockwise direction when viewed from the aft of the aircraft, and the propeller (<NUM>) mounted on the starboard wing (<NUM>) rotates in an anti-clockwise direction when viewed from the aft of the aircraft.