Structure, Aerodynamics and Systems
Gulfstream's customers dictated the terms of the G650's design. Overwhelmingly, they favored a go-slow approach toward embracing new technologies, especially composite airframe construction. Accordingly, the G650's primary airframe structure, including the fuselage pressure vessel and wings, is made of high-strength aluminum alloys, augmented with steel, stainless steel and titanium, similar to previous Gulfstream aircraft. However, more composites are used in the G650 than in earlier Gulfstreams, including the horizontal stabilizer, elevators and rudder, plus floorboards, rear pressure bulkhead, engine nacelles, winglets and various fairings, among other secondary structures.
Fuselage frames are evenly spaced at 17.5-in. intervals, wider than that on previous Gulfstreams. The design change makes room for longer-spaced cabin windows. That also increases legroom in the seating groups because seat placement is based upon window spacing. The windows also are larger, visually complementing the larger fuselage cross-section.
Structural efficiency also is improved. While the aircraft has a larger fuselage cross-section than the G550, it has about the same weight per linear foot, plus it has considerably higher pressurization. But green aircraft empty weight goes up by more than 5,000 lb. because of the new aircraft's 3.3-ft.-longer fuselage, larger wing, engines and empennage, among other factors.
Aerodynamically, the G650 leaps ahead of previous Gulfstream large-body jets. The nose has been completely recontoured to reduce shock-wave drag. The windshields and side windows are much larger than on legacy large-cabin Gulfstreams, greatly improving outward visibility.
As with its previous large-cabin models, Gulfstream elected not to fit the G650 with leading edge slats because its engineers believed the additional weight and complexity did not offset the improvement in high angle-of-attack (AOA) lift. Instead they fitted the aircraft with a 1,283-sq.-ft. wing that results in the lowest wing loading of any ultra-long-range business aircraft. This results in acceptably low takeoff and landing speeds.
The wing also has 33 deg. of sweep at quarter chord to reduce Mach-induced drag. Large, outwardly canted, highly swept, blended radius winglets will help reduce wingtip vortices, an important design feature considering the wing's modest 7.73:1 aspect ratio. The airfoil thus is optimized for Mach 0.855 cruise, which is Mach 0.055 higher than the G550's airfoil. It also generates 8% more peak lift at cruise speed. The G650 is the first large-cabin Gulfstream to have an area ruled aft fuselage to reduce high-speed interference drag between the engine nacelles and fuselage. That's a drag cheating technique long used byand on their large-cabin aircraft to lower the local Mach number in that section of the airframe.
The result is an aircraft that can cruise 7,000 nm at Mach 0.85+ and 6,000 nm at Mach 0.90. It also consumes less fuel while cruising at Mach 0.85 or better than any other large-cabin aircraft in current production. It can fly nonstop from San Diego to Sydney, Chicago to Cape Town or even Hamburg to Hawaii at long-range cruise. The G650 is the first purpose-built business jet that can fly from New York to Tokyo, Seoul or Beijing in well above ISA conditions, even with less than optimum routing and ATC climb clearances.
Most G550 systems are carried over but have been updated to improve redundancy, simplicity and reliability. The electrical system, for instance, has many more backups because of the aircraft's digital fly-by-wire (FBW) control system and electrically powered backup flight control hydraulic system.
To start, two engine-driven 40 KVA constant-speed generators and a 40 KVA APU generator supply 115/200 VAC, three-phase, 400 Hz current mainly for motors, heaters and battery chargers. A 15 KVA ram air turbine has been fitted to the aircraft to provide a fourth source of emergency AC power.
Most of the 115 VAC is converted to 28 VDC by five transformer rectifier boxes for use by other equipment. The two battery chargers also can function as transformer rectifiers under certain conditions. Two 28 VDC, 53 AH nicad main batteries supply power for APU starting, operating the auxiliary hydraulic pump, and left and right essential buses if no other DC power is available. There are two 24 VDC, 9 AH nicad emergency batteries, plus uninterruptible power supply and backup flight control hydraulic power pack emergency batteries. Notably, Gulfstream initially announced that the G650 would use lithium-ion batteries, but the development team abandoned that plan in late 2011 when the candidate batteries failed to meet performance requirements. Nicads add about 150 lb. to empty aircraft weight, compared with lithium-ion batteries.
There are comparatively few mechanical circuit breakers in the overhead panel and in the left and right electrical equipment racks just aft of the cockpit. Most AC and DC circuits are controlled by electronic circuit breakers hosted by the secondary power distribution system (SPDS), accessible by means of the MCDUs in the center console. The SPDS eliminates 3 mi. of wiring, 400 mechanical circuit breakers and 300 lb. of system weight.
All interior and exterior lights are LEDs, with the exception of the high intensity discharge (HID) xenon landing lights. Gulfstream developed a proprietary landing light pulsing system for the HID bulbs that makes the aircraft easier to spot by other aircraft and birds.
All fuel is carried in the left and right wet wing tanks. A heated fuel recirculation system prevents fuel gelling during long, high-altitude missions. DC-powered main and alternate pumps in each wing provide positive fuel pressure to the engine-driven pumps. The single-point pressure refueling system has a refuel quantity preselect feature that may be controlled by either the standby multifunction controller in the glareshield or external ground service control panel near the right wing leading edge. Improved fuel distribution and balancing slashes refueling time to 26 min., a 19-min. improvement compared with the G550's upload time. The aircraft also may be refueled by means of overwing gravity ports, but doing so reduces total refuel quantity by 550 lb.
Gulfstream engineered a five-channel FBW flight control system for the G650 having left- and right-side, dual-channel flight control computers, plus a backup flight control unit, for the aircraft. FBW saves 100 lb. of weight and makes it much easier to rig flight control surfaces, and offers more redundancy than a conventional hydromechanical flight control system. FBW also eliminates control cables, pulleys and linkages in the aft equipment bay, greatly improving preflight access to other systems. The G650 thus becomes the first Gulfstream to have fully powered flight controls rather than boosted controls.
Electrical power sources include the two main generators, the APU and the RAT, along with two main, two emergency, singleand single backup flight control batteries. With five control channels and 10 power sources, there's actually a great deal more redundancy in the flight control system than with traditional hydromechanical control links.
Conventional control yokes and rudder pedals in the cockpit, supplied by, have five signal transducers that send commands to the four flight control computer (FCC) channels and the backup flight control unit (BFCU) built by Canada. Any one of the five channels can command movement of all flight control actuators. The two FCCs or BFCU, in turn, send command information out to remote electronic units, furnished by Parker Aerospace, that command the movements of each flight control actuator.
Each FCC has one active and monitoring channel. It's as though two airplanes were flying in tight formation with one pilot flying and one pilot monitoring aboard each airplane. Thus these four pilots in the two airplanes and all four channels in the two FCCs are different, but they all have to agree on a course of action. The BFCU, though, “flies” solo. It's only there in case the other two FCCs are completely unavailable.
There are four primary flight control modes, three of which are hosted by the dual FCCs. Normal mode, using inertial and air data inputs in addition to pilot commands, provides flight envelope protection, maneuver and dynamic rudder load alleviation, and stability augmentation, including a speed stability function. Stability augmentation includes automatic compensation for pitching moments induced by changes in flap, landing gear or speed brake configuration. Gust load alleviation, thrust asymmetry compensation and P-Beta yaw/roll coupling compensation are not built into the control laws.
Direct law, rather than more complex pitch rate command law, is used for takeoff and landing. Up and away, the primary high-level pitch control law is Nz, or vertical acceleration, command with “U” speed stability, meaning the pilot has to trim pitch for speed changes. Theand 787 and also use similar FBW speed stability control laws.
Certain sensor or communications malfunctions cause the FCCs to revert to alternate law mode. Most high-level FCC control law functions are retained, but envelope protection and the autopilot are not available. In addition, stall warning stick shaker is activated at a lower angle of attack. And Vmo, Mmo and crosswind limits are reduced.
If so many sensors and/or malfunctions make alternate law unavailable, then the FCCs revert to direct law. All high-level control law functions are lost, including automatic actuation of the speed brakes and ground spoilers. Control inputs from the pilots to the yoke, rudder, speed brake handle and trim switches directly result in control surface movement.
Finally, if all functionality of both FCCs were to be lost, there is a backup flight control computer that provides complete control, but all high-level control law functions are lost.
The G650 is the first large-cabin Gulfstream to have a fully trimmable horizontal stabilizer. Inputs to the normal or backup pitch trim switches send signals to command movement of the horizontal stab. Pitch trim commands, though, may be sent to the elevator during certain phases of flight. The horizontal stabilizer then is automatically trimmed by the FCCs to relieve the load on the elevator.
The roll trim switch on the center console repositions the neutral point of the control wheel, commanding movement of the ailerons and spoilers. However, moving the rudder trim switch only commands movement of the control surface, not the rudder pedals. An “auto center” button in the console provides a convenient way to center the rudder, such as on short final.
The aircraft has simple, four-position, slotted, trailing edge Fowler flaps. Multifunction spoilers, with three panels on each wing, provide roll augmentation, air brake and ground spoiler functions.
The G650 retains the dual hydraulic system architecture of its predecessors, although it has FBW flight controls with no mechanical links between the cockpit and the control surfaces. Most aircraft with fully powered flight controls, be they traditional hydromechanical or FBW systems, require three separate hydraulic systems in order to provide the required redundancy.
Gulfstream's innovative approach, which enabled the G650 to retain the dual 3,000-psi hydraulic circuit design, involves using remotely located, DC-powered packs in hybrid flight control actuators. Seven of the 16 flight control actuators can function either as conventional electrohydraulic actuators or as DC electrically powered actuators in the event of the loss of normal left or right hydraulic pressure. Gulfstream calls these components electric backup hydrostatic actuators, EBHAs for short. The EBHAs can power the ailerons, elevators and rudder, along with the outboard spoilers, thereby providing the redundancy, but not the weight or complexity, of a third hydraulic system.
There'stype certification pre–cedence for the dual hydraulic, electric power pack backup design approach. uses a similar architecture on its super-jumbo and military airlifter, but the electrohydraulic actuators work full-time instead of just being backups.
Aboard the G650, both left and right 3,000-psi hydraulic systems are powered by engine-driven pumps most of the time. The left side does most of the heavy lifting, including powering part of the flight controls, as well as the landing gear, flaps, nosewheel steering and inboard brakes, plus on-side thrust reverser. If the left engine-driven pump is inoperative, a DC auxiliary pump or a right-to-left power transfer unit also can power the left side. The right hydraulic system powers part of the flight controls, along with the outboard brakes and on-side thrust reverser.
The main landing gear employs a trailing link design for smooth touchdowns. The brake-by-wire system has both brake temperature and tire pressure monitoring through the EICAS. However, there is no auto-brake function. The nose gear has a digital steer-by-wire with up to 7 deg. of authority through the rudder pedals and up to 80 deg. of authority with the tiller, depending upon groundspeed. If the tiller sensor fails, nosewheel steering authority through the rudders increases to 16 deg. Notably, the nosewheel steering links do not have to be disconnected for towing.
Bleed air is used for wing leading edge and engine cowl anti-ice protection, cabin pressurization and air-conditioning, along with main engine starting and for sucking ambient air through the total air temperature probe. The APU, an external cart or the cross-side engine can supply air for starting a main engine. The APU supplies more bleed air than both engines at idle thrust, so it's advisable to turn off the bleed air from the main engines and use the APU to supply the twin air-cycle machine packs during ground operations in warm weather. In addition, engine ground idle rpm is reduced with the bleeds off, thus there's less residual thrust and reduced need to use the brakes while taxiing.
The interior has three-zone temperature control, with one thermostat in the cockpit and two at opposite ends of the passenger cabin. Chilled air also is used to cool the forward electrical equipment racks. Maximum pressurization is 10.69 psid, so typical cabin altitudes range from 3,000 ft. at FL 410 to 4,850 ft. at FL 510.
The G650 has a fully MSG-3 maintenance friendly design with 600-hr. basic inspection intervals.