’s Falcon 7X is the first purpose-built business aircraft with a digital flight control system, a technology intended to ease pilot workload, provide flight envelope protection and enable designers to reduce drag and improve fuel efficiency.
Design of the system capitalizes on Dassault’s three-plus decades of building Mach 2-class fighters with fly-by-wire (FBW) flight controls. The company believes its military aircraft design experience gave it FBW design capabilities for the Falcon 7X that are not available to other civil aircraft manufacturers.
Both the Mirage 2000 andfighters, for instance, are designed to be aerodynamically unstable to enhance their maneuverability. These aircraft are virtually impossible to fly without the aid of their flight-control computers, but with FBW they exhibit docile handling characteristics in all flight regimes. They also provide carefree handling characteristics, including angle-of-attack and sideslip limiting, flight-path vector (FPV) hold and overstress protection.
Similar to Dassault’s military aircraft, the Falcon 7X’s digital flight control system provides handling ease, including automatic trim and flight-path vector hold, plus low- and high-speed flight-envelope protection.
For low-speed protection, the system will not allow angle of attack to exceed 14 deg. in the clean configuration or 17 deg. with slats and flaps extended without commanding a nose-down pitch. At the other edge of the envelope, while the Falcon 7X aerodynamically and structurally was engineered for Mach 0.97 and 430-kt.-indicated-airspeed (KIAS) demonstrated dive speeds, its digital flight control system prevents the aircraft from being flown faster than Mach 0.94 and 405 KIAS.
In addition, maximum nose-up pitch angle is limited to +35 deg. and nose-down pitch is limited to -28 deg. There is no maximum bank angle limit, but the aircraft has an artificial roll stability function that levels the wings at bank angles less than 6 deg., holds bank angle between 6-35 deg. and reduces angle of bank to 35 deg. in steeper turns.
The system has a three-axis stability augmentation function that makes the aircraft easier to fly and enabled Dassault to reduce the empennage size by 20%, thereby reducing trim drag. The center of gravity envelope also is comparatively broad, ranging from 19.5-38.5% mean aerodynamic throughout most of the weight envelope.
Compared with the Mirage 2000 or Rafale, the Falcon 7X has inherent aerodynamic stability and is not designed to be flown above Mach 1. Dassault determined it only needed digital flight controls with 50-millisecond response time, but designed Falcon 7X’s system with the same 12.5-millisecond response time as the Rafale’s FBW system because there was no advantage to slowing it down to conventional air transport levels.
Dassault did not need to use the Rafale’s 100-deg.-per-sec. control surface actuators on the Falcon 7X because of its passenger transport mission. Instead, engineers elected to use slower actuators with lower hydraulic power requirements, thereby saving flight control and hydraulic system weight.
The Falcon 7X, however, has considerably greater control-system redundancy than Dassault’s fighters. The 1970s-vintage FBW system in the Mirage 2000 has four single-channel analog computers that command the control surfaces. The quad-redundant system has built-in monitoring between each of the computers, enabling the system to vote to exclude one of four or one of three errant computers.
The Rafale has three single-channel digital computers and one dual-channel analog computer that vote on control position commands, so one errant computer can be disqualified. If the digital processors cannot be used, the Rafale’s analog computer provides degraded back-up control of the surfaces on the wing and tail, but not the canards.
The Falcon 7X has three dual-channel main flight control computers [MFCC] that host normal, alternative and direct flight control laws. It also has three single-channel secondary flight control computers [SFCC] that host direct flight control laws. Only one of the six MFCCs or SFCCs is needed to fly the airplane. All six computers send flight-control positioning commands to four actuator control and monitoring units [ACMU] that essentially are fly-by-wire control-signal quality assurance inspectors. The ACMUs also receive feedback from the electro-hydraulic control surface actuators, thereby ensuring they move where they are commanded.
That architecture provides the 10-9 critical level redundancy required for flight control systems. But Dassault also installed a back-up analog computer that provides pitch control by means of pitch trim commands and roll spoiler control via rudder pedal inputs. This enables the flight crew to control the aircraft long enough to reset the MFCCs and SFCCs if all six units were to fail, actually providing 10-10 probability of failure. Dassault claims this level of redundancy is higher than that available with conventional hydro-mechanical controls.
But what does all this mean to pilots?
In early March, Dassault Falcon Jet invited Aviation Week to its U.S. headquarters at Teterboro (N.J.) Airport to fly Falcon 7X and evaluate its digital flight control system and other new technologies. I strapped into the left seat of Serial No. 48, Dassault Falcon Jet’s current demonstrator, accompanied by senior demonstration pilot Ken Dromgold.
The basic operating weight of the aircraft was 36,713 lb. With two passengers, extra gear and 10,000 lb. of fuel, the ramp weight was 48,000 lb., only 68.5% of maximum takeoff weight. Using the slats/flaps 2 setting for takeoff, our V speeds were 100 KIAS for V1, 106 KIAS for rotation, 113 KIAS for V2, 133 KIAS for slat/flap retraction and 167 KIAS for one-engine-inoperative en-route climb. Computed takeoff field length was 2,900 ft.
After starting the three engines, pre-taxi protocols included flight-control-system built-in test checks with the parking brake set, not unlike the Rafale’s after-start checks. There also were flight control input, electrical bus tie, back-up hydraulic pump, electronic circuit breaker, anti-ice and air-brake checks. The auxiliary power unit usually is shut down prior to taxi because it is not approved for inflight operation.
At our comparatively light weight, little thrust was needed to begin taxi. The Falcon 7X’s nosewheel steering [NWS], similar to that on the Mirage 2000 and Rafale, is controlled through the rudder pedals. This is the first Falcon without a NWS steering tiller. The steer-by-wire system’s steering authority is inversely proportional to speed, with up to 60 deg. of authority for tight maneuvering. The brake-by-wire carbon brakes subjectively felt more sensitive than other Falcons we’ve flown.
Once cleared for takeoff on Teterboro’s Runway 1, we pushed up the thrust levers. With more than 19,000 lb. of thrust, the lightly loaded aircraft accelerated briskly. Rotation force was pleasantly light, even less than that required in a Falcon with conventional flight controls. Only 16 lb. of pull is needed to achieve full deflection of the side stick. Control response is geometrically proportional to side-stick movement, facilitating small, precise corrections. Compared with an, the Falcon 7X’s side stick requires much less effort, but it is less twitchy than the side stick of the Rafale.
The key to flying a FBW aircraft with side-stick controls is to let go when attitude changes are not needed. As with the Rafale, a flight-path symbol [FPS] is the primary reference for controlling the aircraft. We adjusted pitch attitude so that the FPS followed the flight director command bars and called “Clean the wing” at 133 KIAS, prompting Dromgold to retract the slats and flaps. We turned to a heading of 40 deg. to comply with the departure procedure and then adjusted attitude to put the FPS on the horizon line in order to level off the aircraft at 2,000 ft.
The Falcon 7X’s FBW automatic trim and artificial stability functions kept the FPV on the horizon line with no need for inputs to the side stick. The fly-by-wire system hosts conventional autopilot inner-loop functions, and pilots make control-stick steering inputs to the FBW system rather than directly moving control surfaces. The flight director provides top-level guidance functions and sends commands to the FBW system. This eliminates the need for a conventional autopilot.
During the climb to FL 450, Dromgold demonstrated the aircraft’s left- and right-side control input summing function. If both pilots simultaneously manipulate the side sticks, each control vibrates, a “dual input” aural alert sounds, and the inputs are algebraically summed to the flight-control computers. In case of pilot incapacitation, the other pilot can depress a priority push button to deactivate the cross-side controls.
Once level at FL 450, we asked for an altitude block beneath us and proceeded to push down the nose to accelerate the aircraft beyond its 0.90 maximum operating Mach number (Mmo). We maintained maximum continuous thrust and kept pushing the side stick forward beyond Mmo. At Mach 0.938, with the side stick against the forward stop, the FBW system’s high-speed envelope protection started to reduce pitch attitude to prevent the aircraft from flying any faster.
Conversely, later we slowed the aircraft in level flight with power off and attempted to induce a stall. As with previous Falcons, the outboard slats automatically extended at high angles of attack to increase roll control authority. But unlike Falcons with hydro-mechanical controls, it was not possible to pull back until the aircraft reached aerodynamic stall. In addition, had the air brakes been extended, they would have automatically retracted at high angle of attack.
The Falcon 7X’s low-speed envelope protection eased down the nose at 14-deg. angle of attack, or about 1.5 deg. less than maximum lift coefficient in the clean configuration. The system allows up to a 17-deg. angle of attack with slats and flaps extended, but this too is less than the stalling angle of attack.
Steep turns were especially easy in the aircraft. Slight side-stick pressure into the turn is required to hold the aircraft at a 45-deg.-bank angle because of the maximum 35-deg.-bank-angle artificial stability function. But keeping the FPS on the horizon line, we could almost freeze altitude while flying at 250 KIAS.
We then proceeded to Stewart International Airport, Newburgh, N.Y., to fly the ILS Runway 27 approach and one visual landing pattern. We elected to use slat/flaps 2 for landing, a takeoff configuration, rather than full flaps because we did not want to change configuration during the touch-and-go. We set 126 KIAS as the reference speed (VREF) based on that configuration and a 44,600-lb. landing weight. Computed landing distance was 2,312 ft.
The Falcon 7X’s light side-stick control pressures made it easy to fly a mild right wing down/top rudder approach to runway centerline. The long-travel, trailing-link landing gear provided a soft landing. At touchdown, an automatic de-rotation function fed in nose-down elevator to cancel the FBW’s attitude-hold function. The result is a natural speed-stability feel that makes the aircraft easy to fly.
Returning to Teterboro, we landed on Runway 6 at a weight of 41,000 lb., using slats and full flaps. VREF was 115 KIAS and landing distance was 2,280 ft. Again, touchdown was soft, but the wheel brakes were a little sensitive. The single-thrust reverser on the center engine provides some deceleration, but was not as effective as other aircraft with reversers on all engines.
Our conclusions? The Falcon 7X is the nicest flying Falcon yet produced by Dassault, doubtlessly because its digital flight control system has roots in the Mach 2 Mirage 2000 and Rafale FBW systems. It clearly delivers on its promises of reduced pilot workload, greater passenger comfort and enhanced safety margins.
|Wing Loading||90.7 lb./sq. ft.|
|Power Loading||3.59 lb./lbf.|
|Noise (EPNdB)||83.7/90.3/92.6 EPNdB.|
|Length||76.1 ft./23.2 meters|
|Height||26.1 ft./8.0 meters|
|Span||86.0 ft./26.2 meters|
|Length||46.0 ft./14.0 meters|
|Height||6.2 ft/1.9 meters|
|Width (maximum)||7.7 ft./2.3 meters|
|Width (floor)||6.3 ft./1.9 meters|
|Output/Flat Rating OATC||6,405 lb. ea/ISA+18.4C|
|Max Ramp||70,200 lb./31,843 kg.|
|Max Takeoff||70,000 lb./31,752 kg.|
|Max Landing||62,400 lb./28,304 kg.|
|Zero Fuel||41,000 lb./18,597 kg.|
|BOW||36,700 lb./16,647 kg.|
|Max Payload||4,300 lb./1,950 kg.|
|Useful Load||33,500 lb./15,196 kg.|
|Executive Payload||1,600 lb./726 kg.|
|Max Fuel||31,940 lb./14,488 kg.|
|Payload with Max Fuel||1,560 lb./708 kg.|
|Fuel with Max Payload||29,200 lb./13,245 kg.|
|Fuel with Executive Payload||31,900 lb./14,470 kg.|
|Time to FL 370||18 min.|
|FAR 25 OEI rate||615 fpm.|
|FAR 25 OEI gradient||280 ft./nm.|
|Certificated||51,000 ft./15,545 meters|
|All-Engine Service||41,360 ft./12,607 meters|
|Engine-Out Service||31,560 ft./9,620 meters|
|Sea Level Cabin||29,200 ft./8,900 meters|