GB1593393A - Aircraft - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO AIRCRAFT (71) I, SECRETARY OF STATE FOR DEFENCE, LONDON, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to aircraft, and is concerned with relaxing their longitudinal stability requirements.

Conventionally, aircraft have always been designed with positive longitudinal stability, so that should the aircraft attitude be disturbed by atmospheric perturbations the aerodynamic forces acting on the aircraft are such as to tend to return it to its previous attitude. The degree of longitudinal stability has always been a compromise, as, in general, the more inherently stable the aircraft the lower its manoeuvrability, (that is, the rapidity with which the aircraft's attitude is altered deliberately by control inputs). The tendency is for passenger carrying aircrat, such as commercial airliners, to be more stable than military aircraft such as fighters.

Recent developments in electronics have made it possible to consider aircraft having reduced longitudinal stability, and even in some cases negative stability. This has been achieved by using systems which sense non-demanded changes in aircraft attitude and automatically apply control movements to correct the change in attitude. Because failure of the control system would mean that an aircraft having reduced longitudinal stability would be difficult or impossible to control, presenting difficulties in airworthiness certification, such systems have so far been implemented only for military aircraft.

However, as experience of their use increases, it is probable that commercial aircraft having reduced longitudinal stability will be designed.

In an aircraft having reduced longitudinal stability the centre of gravity is closer to the centre of lift of the main aerodynamic lifting surface than is the case in a conventional aircraft. In practice this means that the main aerodynamic lifting surface (wing) is further forward on the fuselage than is a conventional aircraft. Modern large commercial aircraft invariably have undercarriages of the type wherein two main undercarriage units are situated behind the centre of gravity and one undercarriage unit is (or two units are) situated forward of the centre of gravity, these units being arranged on a tricycle formation. Ground handling stability considerations mean that the rear units are most conveniently situated in the aircraft wings, whilst the forward unit or units are situated in the front fuselage.As the wings are moved relatively further forward on the fuselage the rear undercarriage units must be positioned further back relative to the wings to maintain a sufficient loading on the forward unit for adequate ground handling characteristics. This could require extra strenthening to the wing structure, or even rearward extending pods or separate sponsons for housing the rear undercarriage units, resulting in weight and cost penalties which would offset some of the potential advantages of relaxing the stability requirements. Additionally, one of the advantages of reduced stability is that the size (and hence the weight and cost) of the longitudinal stabiliser and its control surfaces (tailplane and elevator) can be reduced.The dimensions of an aircraft's undercarriage are designed to ensure that during the take-off run the angle of attack of the aircraft (and in particular the angle of the wing) is such that drag is minimised. During the take-off run whilst the wheels of all undercarriage units are in contact with the runway the wings will be generating little or no lift. As take-off speed is approached a downward force is applied to the longitudin al controls to rotate the aircraft to a position wherc the wings are at an angle of attack sufficient to provide take-off lift. Especially in the case of an aircraft having reduced stability the requirement to provide the rotational force for take-off can result in the longitudinal stabiliser and its control surfaces being much larger than would otherwise be necessary for control in other phases of the flight regime.

During the landing run it is desirable for the aircraft to gencrate high aerodynamic drag and brake power, augmented in most cases by reversed thrust from the engines, so that landing can be effccted in a minimum runway length. Unfortunatcly the conventional tricycle undercarriage is designed to minimise drag of the basic aircraft configuration to allow rapid acceleration for take off, and in consequence flaps, spoilers and the like must be deployed to enhance the aircraft drag during the landing run; main wheel braking cannot be applied to full effect as the effect of main wheel braking is to transfer load from main-wheels to nose wheel with a consequent reduction in brake power; and reversed thrust cannot be used to full effect throughout the landing run as the effect is to unload the nose wheel, and a specified nose wheel loading must be maintained for effective steering.In practice, drag devices and reversed thrust are best used during the early phase of the landing run and main-wheel braking in the terminal phase of the landing run.

The present invention provides an aircraft having a configuration which avoids the above problems.

According to the prcsent invention an aircraft has its centre of gravity so positioned that in trimmed flight conditions and at take off positive lift is provided both by main aerodynamic lift surfaces and by a longitudinal stabiliser, and having an undercarriage system including a pair of units forward of the aircraft centre of gravity and a unit aft of the centre of gravity, the units being movable between retracted and extended positions, the unit aft of the centre of gravity including a leg which, when the unit is extended, is movable between, and cap ablc of supporting a load at, at least a first position corresponding to the aircraft's designed angle of attack for the take-off run and a second position corresponding to the aircraft's designed angle of attack for takeoff, and means for moving the leg between these positions in a controllcd manner while carrying the design loads.

Preferably the leg is movable also to a third position corresponding to the aircraft's designed angle of attack for landing touch down.

One embodiment of the invention, and some of the potential advantages of the invention, will now be described, by way of example only, with reference to the drawings accompanying the provisional specification, in which:- Figure I is an elevation of an aircraft according to the invention on its take-off run, Figure 2 is an elevation of the aircraft shown in Figure 1 immediately prior to take-off, Figure 3 is an elevation of an aircraft according to the invention with underwing engines, and Figure 4 is a plan of the aircraft shown in Figure 3.

An aircraft shown generally at 10 in Figures 1, 2, 3 and 4 has a fuselage 11 on which are mounted wings 12, tailplane and elevators 13, fin 14 and engines 15. The aircraft 10 has an undercarriage system including two units 16 mounted in the wings 12 forward of a centre of gravity G, and a unit 17 in the fuselage 11 aft of the centre of gravity G. The unit 17 aft of the centre of gravity has a leg 18 movable between a first position, as shown in Figure 1, which maintains the aircraft at an angle of attack compatible with minimum drag during a take-off run, a second position, as shown in Figure 2 and at 18a in Figure 3 at which the wing 12 is at a designed angle of attack for take-off, and a third position, as shown at 18b in Figure 3, at which the wing 12 is at the designed angle of attack for landing.

The units 16, 17 are movable between retracted positions within the wings 12 and fuselage 11 when the aircraft 10 is airborne, and extended positions for use in landing and on the ground.

In use the leg 18 is at its first position (as shown in Figure 1) prior to and during the take-off run. As the aircraft approaches its designed take-off speed the leg 18 is moved to the second position (as shown in Figure 2) rotating the aircraft 10 so that the wing 12 is at an angle of attack whereby a lift, shown as L1 in Figure 2, is supplied which allows the aircraft to take-off. The design of the aerodynamic surfaces, wings 12 and tailplane 13, is such that the centre of pressure of the wing (indicated by the arrow L1) is forward of the centre of gravity G, so that a positive lift increment as shown by the arrow L2 in Figure 2, is supplied by the stabiliser and its control surfaces 13.

At landing touch down the leg 18 is positioned to ensure an angle of attack such that there is an adequate reduction of aerodynamic lift to maintain contact of the wheels with the runway while providing a high aerodynamic drag to minimise the landing run. The leg position may also seek to optimise brake power and reversed thrust for maximum retardation of the aircraft, taking advantage of the change of engine thrust line with position of leg 18. Figure 3 shows an example of a position 18b of leg 18 such that the reversed thrust of wing mounted engines provides a thrust component that adds to the load on the aircraft wheels as an aid to braking and positive contact with the runway. At the end of the landing run and during the taxiing phase the leg can be extended to the position shown at 18 in Figure 3 for passenger comfort and to enhance the pilot's view.

Movement of the leg 18 to the positions appropriate to the different phases of the take-off and landing runs may be initiated by movement of a pilot's control or may be initiated automatically. Automatic control might be control by an airspeed sensor and might also be designed to take account of aircraft weight, centre of gravity position and attitude by the use of appropriate sensors in the units 16 and 17.

Using the invention there is no requirement for the aircraft stabilising and longitudinal control unit 13 to rotate the aircraft for take-off, which allows the size of this unit to be kept to a minimum. Also the fact that the unit 13 supplies positive lift at the take-off and trimmed flight conditions means that less lift need be supplied by the wings 12, allowing their size to be reduced. Both these factors result in a decrease in total aircraft weight. An additional advantage for an aircraft, of the type shown in Figures 1 and 2, wherein the engines 15 are mounted at the rear of the fuselage 11, is that the weight of the engines 15 can be substantially supported by the undercarriage unit 17 during ground handling, reducing the stresses normally experienced by the rear fuselage of similar aircrat having nose wheel undercarriages. This is yet another weight reducing feature.It will be realised that the above described type of undercarriage is completely different from earlier types of "tail wheel" configurations, and that the take-off procedure is entirely different.

With conventional tail wheel aircraft the take-off procedure requires the aircraft to be accelerated in a high drag configuration until sufficient tail lift is generated to raise the tail so that drag is minimised for a further phase of acceleration, after which the aircraft can be rotated to take-off attitude as for a nose wheel undercarriage type aircraft. In the landing configuration a single compromise must be made in tail wheel position to accommodate the differing considerations of passenger appeal, pilot's view, high landing run drag and lift shedding at touch down to maintain positive contact of the wheels with the runway.

The tail wheel undercarriage configuration described here as compared with the earlier type of tail wheel configurations requires shorter take-off and landing runs, provides better pilot's vision and passenger appeal, and easier control in the landing phase in relation to reducing lift at touch down and avoiding bounce landings.

WHAT I CLAIM IS: 1. An aircraft having its centre of gravity so positioned that in trimmed flight conditions and at take off positive lift is provided both by main aerodynamic lift surfaces and by a longitudinal stabiliser, and having an undercarriage system which includes a pair of units forward of the centre of gravity and a unit aft of the centre of gravity the units being movable between retracted and extended positions, the unit aft of the centre of gravity including a leg which, when the unit is extended, is movable between, and capable of supporting a load at, at least a first position corresponding to the aircraft's designed angle of attack for the take off run and a second position corresponding to the aircraft's designed angle of attack for take off, and means for moving the leg between these positions in a controlled manner while carrying the design loads.

2. An aircraft as claimed in claim 1 wherein the leg is movable also to a third position corresponding to the aircraft's designed angle of attack for landing touch down.

3. An aircraft substantially as herein described with reference to the drawings accompanying the provisional specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. thrust line with position of leg 18. Figure 3 shows an example of a position 18b of leg 18 such that the reversed thrust of wing mounted engines provides a thrust component that adds to the load on the aircraft wheels as an aid to braking and positive contact with the runway. At the end of the landing run and during the taxiing phase the leg can be extended to the position shown at 18 in Figure 3 for passenger comfort and to enhance the pilot's view. Movement of the leg 18 to the positions appropriate to the different phases of the take-off and landing runs may be initiated by movement of a pilot's control or may be initiated automatically. Automatic control might be control by an airspeed sensor and might also be designed to take account of aircraft weight, centre of gravity position and attitude by the use of appropriate sensors in the units 16 and 17. Using the invention there is no requirement for the aircraft stabilising and longitudinal control unit 13 to rotate the aircraft for take-off, which allows the size of this unit to be kept to a minimum. Also the fact that the unit 13 supplies positive lift at the take-off and trimmed flight conditions means that less lift need be supplied by the wings 12, allowing their size to be reduced. Both these factors result in a decrease in total aircraft weight. An additional advantage for an aircraft, of the type shown in Figures 1 and 2, wherein the engines 15 are mounted at the rear of the fuselage 11, is that the weight of the engines 15 can be substantially supported by the undercarriage unit 17 during ground handling, reducing the stresses normally experienced by the rear fuselage of similar aircrat having nose wheel undercarriages. This is yet another weight reducing feature.It will be realised that the above described type of undercarriage is completely different from earlier types of "tail wheel" configurations, and that the take-off procedure is entirely different. With conventional tail wheel aircraft the take-off procedure requires the aircraft to be accelerated in a high drag configuration until sufficient tail lift is generated to raise the tail so that drag is minimised for a further phase of acceleration, after which the aircraft can be rotated to take-off attitude as for a nose wheel undercarriage type aircraft. In the landing configuration a single compromise must be made in tail wheel position to accommodate the differing considerations of passenger appeal, pilot's view, high landing run drag and lift shedding at touch down to maintain positive contact of the wheels with the runway. The tail wheel undercarriage configuration described here as compared with the earlier type of tail wheel configurations requires shorter take-off and landing runs, provides better pilot's vision and passenger appeal, and easier control in the landing phase in relation to reducing lift at touch down and avoiding bounce landings. WHAT I CLAIM IS:

1. An aircraft having its centre of gravity so positioned that in trimmed flight conditions and at take off positive lift is provided both by main aerodynamic lift surfaces and by a longitudinal stabiliser, and having an undercarriage system which includes a pair of units forward of the centre of gravity and a unit aft of the centre of gravity the units being movable between retracted and extended positions, the unit aft of the centre of gravity including a leg which, when the unit is extended, is movable between, and capable of supporting a load at, at least a first position corresponding to the aircraft's designed angle of attack for the take off run and a second position corresponding to the aircraft's designed angle of attack for take off, and means for moving the leg between these positions in a controlled manner while carrying the design loads.

2. An aircraft as claimed in claim 1 wherein the leg is movable also to a third position corresponding to the aircraft's designed angle of attack for landing touch down.

3. An aircraft substantially as herein described with reference to the drawings accompanying the provisional specification.