Flight Control System

The present invention relates to a flight control system for an aircraft.

Considering an aircraft, equipped with an engine, it is well known that there exists a maximum descent glide slope possible in a constant airspeed steady state flight. This will depend on the lift, drag and thrust produced by the aircraft in such a flight condition. The problem is that some operations require capability of performing a steady flight in a steeper descent path, although, depending on the conditions mentioned above, a given aircraft may lack this capability at a certain relatively low airspeed. In these cases, the most commonly used solution is to introduce devices, mechanisms and methods to produce more drag in the aircraft, in order permit a steeper descent trajectory. In the prior art, many proposed solutions utilise drag devices, e.g. airbrakes, flight spoilers, dive brakes and others.

An object of the present invention is to provide a flight control system which can achieve a relatively steep descent (using drag devices such as flight spoilers for example) at relatively low speeds.

It is well known in aeronautics engineering that drag devices such as flight spoilers are capable of increasing the maximum descent ratio. Consequently, prior art arrangements have used such devices in order to increase the angle of the glide path. Nevertheless, this sort of solution may have drawbacks because of undesirable collateral effects caused by using such devices, as follows:

i) For a given angle of attack, it is possible that drag devices such as spoilers reduce the lift produced by the aircraft, thus reducing significantly the margins to the stall condition. Consequently, there is a concern regarding the speed margins that are required for a safe flight operation. It is possible that speed enhancements will be necessary in order to regain the same margins as before, jeopardizing the performance of aircraft approaches.

ii) It is desirable for the aircraft to carry the same payload; thus the total lift must be maintained in the steep descent configuration, regardless

of the fact that drag devices such as spoilers might kill some lift. In these cases, other parts of the aircraft will be loaded more to compensate for this fraction of killed lift, occasionally producing local overloading problems.

iii) Deterioration of flying qualities, especially during steep approaches, flare and landing manoeuvres.

iv) Exposure to high touch down sink rate and tail strike in steep approaches and landings.

In view of the above, some attempts have been made in the prior art to incorporate some control of these surfaces, to reduce some of the effects mentioned above. The prior art includes the following:

US 3589648 by Gorhan and. Richter proposes a device in which the spoilers are biased to a predetermined position, and are commanded around this position depending on a signal received from an autopilot. The autopilot produces those signals based on a comparison between a reference trajectory, which is desirable; and the real trajectory sensed by aircraft systems. Hence, this patent discloses a command to the spoiler to correct the flight trajectory using a feedback signal, augmenting the aircraft response. In US 3738594 to Donovan and Wood, a mechanism is proposed in which spoilers are also biased to a predetermined position. Further, the aircraft pitch command is connected to the spoilers through a "washout" or viscous damping mechanism, in such a way that a quicker pitch command produces a higher amplitude spoiler movement around the biased position, after which the inputs to the spoiler actuators are gradually returned to the original bias position.

US 3774869 to Harmon discloses a mechanism for connecting the throttle to the spoilers and dive brakes (this surface being located under the wing). In these cases, the inventors created a mechanism to coordinate the spoiler handle with thrust lever commands, i.e., the drag produced by the spoiler augments the thrust changes commanded by the pilot. Similar systems are presented in US 3850388 to Olcott et al., and US 4489907 to Ziegelmeyer.

In the arangement disclosed in WO 2006/016070 Al, in the names of Boissenin and Rosay, spoilers are also deployed to a biased predetermined position. In this invention, the spoilers remain fixed in this position up to certain altitude, whence they are automatically and gradually commanded to a more faired position. Further, in this invention, hyper lifting devices are commanded in coordination with spoiler closure. There is no spoiler modulation based on pilot longitudinal commands.

In all of the above disclosures, some control is incorporated to attain an improvement of flying qualities.

In one aspect the invention provides a flight control system for an aircraft having a drag device for increasing the angle of the descent path, the flight control system comprising processing means for monitoring a plurality of flight parameters associated with a safe operating envelope of the aircraft, and control means coupled to the drag device and arranged to keep the aircraft within the safe operating envelope by reducing the drag of the drag device in response to an output of the processing means.

By monitoring the flight parameters and automatically closing the drag devices if the operative envelope of the function is exceeded, it is ensured that the stall and structure limit margins as found in the configuration with the drag devices faired are maintained, without any requirement for a physical modification of the airframe.

In another aspect the invention provides a flight control system for an aircraft, the flight control system comprising processing means for monitoring a. plurality of flight parameters, wherein said processing means is arranged to generate a Boolean function of a plurality of logical inputs associated with the respective flight parameters and to generate an output signal, the flight control system being arranged to control at least one longitudinal flight control surface in response to said output signal and to a pilot or autopilot longitudinal command signal.

In a preferred embodiment the above flight control system commands drag devices such as a set of flight spoiler pairs, keeping or enhancing the flying qualities and manoeuvreability by modulating the drag devices as a function of longitudinal control, further providing an innovative protection to keep the same margins and structural loading as found in a conventional flight control system.

In this new system, as found in the prior art, the spoilers are preferably biased to a predetermined position, although a new command mechanism is provided, The position of the spoilers is preferably computed as a function of longitudinal pilot (or autopilot) command.

In a preferred embodiment the system provides a spoiler command which creates extra drag as required in steep descents, approaches and landings. Beyond creating extra drag, the flight control system preferably provides a spoiler augmentation command to enhance the longitudinal control of the aircraft, which is a modulation based on a pilot longitudinal command, used in conjunction with the elevators command.

The system preferably includes a logic module that receives a set of signals, and its task is to decide when the system is engaged, and thus, when the spoilers are deployed. The logic module optionally also receives a signal from a cockpit switch used to allow the pilot to override (i.e. engage or disengage) the system.

The function of the set of signals received by logic module is preferably twofold:

i) to identify the state of the flight; and

ii) to check the aircraft configuration,

and decide if the system is operating within its operational envelope.

In response to the above, an automatic envelope exceeding disengagement may be triggered. This feature provides structural protection, stall protection, and reduces the pilot workload when changing the aircraft configuration.

Furthermore, in preferred embodiments the longitudinal control surface sensitivities to their command signals are modified to give better flying qualities when flying with the new system engaged. When engaging or disengaging the function, i.e., the spoilers commanded to the biased position, an automatic longitudinal command is sent to elevators or to horizontal stabilizer in order to provide an automatic balancing of longitudinal moments due to spoiler pitch moment.

The invention also extends to an aircraft including the above-defined flight control systems.

Further preferred features are defined in the dependent claims.

A preferred embodiment of the invention is described below by way of example only with reference to Figures 1 to 6 of the accompanying drawings, wherein:

Figure 1 is a schematic plan view of one aircraft - a civil transporter turbo- fan;

Figure 2 is a schematic block diagram of the preferred embodiment of the flight control system of the present invention, showing the basic architecture of the system;

Figure 3 is a schematic diagram of the function of the software of the above flight control system, showing how the pilot command is transformed to a spoiler command under the control of a logic module;

Figure 4 is a diagram showing the details of the logic module, which enables the spoiler command based on a series of sensor inputs;

Figure 5 is a diagrammatic illustration of the longitudinal control command of the system, and

Figure 6 is a diagrammatic illustration of the longitudinal trim command of the system.

The preferred embodiment of the invention is a system used in an aircraft equipped with longitudinal control, and a drag device, such as . spoilers, dive brakes, airbrakes, etc. Figure 1 shows an example: a twin turbo-fan engined civilian transporter aircraft, with a wing (13) equipped with flight spoilers (12) installed in its upper surface.

Referring to Figure 2, the aircraft is equipped with a pilot switch 1 used to allow the pilot to command the engagement or disengagement of the present system. The term pilot switch encompasses a plurality of devices used in the aeronautic industry to permit the pilot to turn ON or OFF a system, comprising push buttons, levers, rotating buttons, and others. This flight control system receives the input position signals from the pilot longitudinal controller 2 command (p), and from pilot throttle lever 3 (δj). The term pilot controller encompasses a plurality of devices used in aeronautics industry to allow the interface with the human pilot, e.g. columns, mini-columns, sticks, side-sticks and all others. The term pilot throttle lever encompasses a plurality of devices used in aeronautics to permit pilot interaction with the aircraft engine plant, which produces the thrust.

Further, the system receives signals from a set of sensors, namely: angle of attack 18 (δ), airspeed 19 (u), the flap position 20 (δ _F ), gear position 21 (δ _G ) and the in on-ground/in-air status 22 (& _AIR )-

The information flows via a parallel data bus 5. All these data, i.e. pilot commands, and sensor outputs, are sent to a programmable microprocessor (4). Then, microprocessor 4 is able to compute a spoiler command 6 based on the input data received. This command is sent to a means for actuating a flight control surface 7, which comprises a control unit able to command the spoiler surfaces to the commanded position. Consequently, the spoiler surfaces are deployed according to the command computed by microrocessor 4.

Figure 3 details the program coded in microprocessor 4, presenting a signal flow schematic. This way, the system comprises a controller to spoiler function 8, a logic module 9 and a logical signal switch 10.

The logic module 9 receives a set of signals that defines its operational envelope and state of flight, which comprise: the angle of attack 18 (δ), airspeed 19 (u), pilot throttle lever position 23 (δ _τ ), the hyper-lifting devices (e.g. flap and/or slats) position 20 (δp), landing gear position 21 (δ _G ) and on- ground/in-air status 22 (b _A ικ)- These variables, as well as the engagement pilot switch signal 24 (S) are all used in the logic module which calculates a Boolean signal function engaged status 25 (bj), which is TRUE if the system is engaged and operating, or FALSE if it is not engaged.

The airspeed in aviation is a parameter computed based on airdata sensors, such as impact pressure or dynamic pressure. The speed is computed based on those data, or equivalently those raw parameters may be used directly instead of airspeed. Additionally, the logic module 9 may be responsive to a stall warning signal indicative of the stall warning system margin being exceeded. The stall warning system margin may be as defined in FAR/JAR 25.207 "Stall Warning" for example.

The logic module 9 is detailed in the schematic of Figure 4. The logic module comprises the following logical interlocks: the angle of attack (δ) shall be lower than a maximum angle permitted (5 _UM ) (26), the airspeed shall be lower than M _{M AX} speed (27) and greater than uum speed limits (28), and the aircraft shall be in air (6 _AIR is TRUE) (33).

Further, the logic module comprises interlocks 29, 31 and 32 to identify the aircraft configuration, using respectively the parameters of pilot throttle angle (δ _τ ), hyper-lifting devices (flap and/or slat) position (δ _F ) and the gear position (δ _G ). Basically, the logic module 9 compares the current aircraft configuration with that permitted to the system to be operative, and defined a priori.

Thus if all the above interlocks are TRUE then the aircraft is safely within the flight control system envelope.

The switch signal (S) toggles the state of the Boolean signal function engaged status (bf) according to the logic function 30: i) if bf is FALSE, and the parameter S indicates that the switch is pressed, and if all other interlocks are TRUE, bf toggles to TRUE, ii) if bf is TRUE and the parameter S indicates that the switch is pressed, Zy toggles to FALSE. Even if the switch status S is TRUE, the logic shown in Figure 4 prevents flying outside the flight control system envelope.

The logic switch 10 in Figure 3 is used to select whether the command augmentation to spoiler is enabled: if the system is engaged (bf= TRUE), the spoiler augmentation command from 8 is activated, otherwise, if disengaged (bf = FALSE), the spoiler augmentation command is deactivated. Hence, when engaged (bf= TRUE), the pilot or autopilot longitudinal command is linked to the spoilers through the function fφ) in 8, i.e., the spoiler will deploy fill in δ _{s =} f(p), augmenting the aircraft longitudinal control.

In this way, the system commands a set of spoilers (which may comprise any number of flight spoiler pairs (or other drag devices)) selected a priori to be enabled to perform the required function. Therefore the system shown in the Figure 2 can command any and all suitable spoilers.

This embodiment thus provides a spoiler command modulated by the longitudinal command of aircraft, via a spoiler augmentation command.

The preferred embodiment of the flight control system changes the "gearing" ie the inputoutput slope of the control function of the pilot longitudinal controller to the longitudinal flight control surface; for instance, the elevator (15). A new gearing function 35 is used instead of another 34 used in the clean configuration without the drag device. The command is switched by a logic switch 36, which is activated by the function engaged status (bf) (25).

Referring to Figure 6, the flight control system includes an automatic longitudinal trim command offset to the pitch him device. When the system engaged status b _f = TRUE, an automatic pitch trim offset 37 is sent via a pitch trim command, to reduce the pilot workload during the function engagement.

When disengaging, an opposite direction trim occurs because of the offset is removed.

The term pitch trim device refers to the devices responsible to balance the longitudinal moments and neutralizing the pilot/autopilot longitudinal command/force. The most common pitch trim devices are canards, trim tabs and horizontal stabilizer. For instance, in the twin-engine transporter of Figure 1, the pitch trim device is performed by a movable horizontal stabilizer surface 17.

The logical switches 10, 36 and 38 have a fading in time and a fading out time, and the output signal is the result of an interpolation of both input signals during the fading time, such that the transition between the states of the function engaged does not cause any discontinuous transient.

Considering a flight control surface command, the term augmentation command is used in aviation to refer to an additive signal summed with all other pre-existing commands, such that, other commands may be added to this augmentation command performing other desirable functions. For instance, the spoilers may perform other functions as roll spoilers, speed brake, ground spoilers and others.