nflight Report: Gulfstream G-IV

PILOT REPORT
I
nflight Report:
Gulfstream G-IV
A new engine, advanced avionics and the strong
desire to reduce an unexpected drag rise in the G-III
motivated Gulfstream to create the impressive G-IV.
By JOHN W. OLCOTT
January 1987 Document # 2904, 5 pages
Aircraft designers quest for perfection, yet live with
compromise and occasional surprises. The products of
their labors exemplify the fact that the best efforts of talented
men and women do not always produce exactly
what was predicted, in spite of the marvels of our computer
age.
Such was the lesson reconfirmed when the Gulfstream
III took to the skies seven years ago. Although the G-III
is an impressive and capable performer, it experiences
more aerodynamic drag than its designers predicted-so
much additional drag, in fact, that the aircraft was in
jeopardy of falling short of its guaranteed range performance.
Wind tunnel studies and analytical calculations
failed to predict the aircraft’s drag rise at higher Mach
numbers, thereby causing Gulfstream’s performance
engineers to overestimate specific range.
Had it not been for another, more fortunate example
of the innate imperfection of design, the G-III would
have missed its range goal by more than the five percent
leeway allowed. But because the aircraft’s wing
was capable of carrying more fuel than was predicted,
thus compensating for the unexpected drag, the aircraft
just did meet its range guarantees.
Designers, however, are unwilling to rely on serendipity,
no matter how much they appreciate being the
recipient of its largess. Thus when the Gulfstream IV
was conceived, a closer look at high-speed drag was
uppermost on the list of design priorities.
Had it not been for the unexpected drag of the G-III,
the G-IV might well have been less grand than it has
now emerged. Originally, Gulfstream’s plan was to reengine
the G-III with Rolls-Royce Tay turbofans and
stretch the aircraft’s nose a modest and somewhat uninspiring
two feet. But with wind tunnel and flight test
results clearly demonstrating that something could be
done to improve performance, Gulfstream accepted the
challenge to make a better wing for the new aircraft.
NEW TESTS
Based upon flight tests of the G-III as well as correlations
between wind tunnel and flight data on both the
G-II and G-III, Gulfstream engineers suspected that the
original wind tunnel studies of the G-III were inaccurate
at high Mach numbers, possibly because insufficient
allowances were made for the characteristics of the particular
tunnel that Gulfstream hired.
Rather than debate the vendor who conducted those
tests, however, Gulfstream contracted with a facility in
Bedford, England to evaluate the G-III and develop the
basis for creating a more efficient G-IV. Over 800
hours of tunnel tests were conducted, focusing largely
on how best to mount the larger Tay engines on the
slender Gulfstream fuselage.
The tunnel tests revealed a need to move the Tay
engines back 18 inches and up 10 degrees on the fuselage.
This new location was required to place the
engines farther away from the wing, thereby reducing
the drag that occurs when air at high speed becomes
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blocked as it attempts to flow through the three-sided
channel formed by the wing, fuselage and engine
nacelle.
In addition to encountering an occasional surprise,
another reality of aircraft design is the interdependence
of each element in the vehicle. Rarely can one change
in a fundamental area- such as engine placement-be
accomplished without its effect being felt somewhere
else in the design. In this case, moving the Tay engines
farther aft created a balance problem that necessitated
another one-foot stretch in the fuselage forward of the
wing. That added pounds to the G-IV, which caused the
engineers to look for ways to reduce weight through
redesign.
The wing seemed to be a logical place to start. By
moving from a wing structure predicated on fail-safe criteria
to one based upon the more modern concept of
damage tolerance, weight could be reduced. A new
wing structure also would allow the engineers a relatively
free hand with airfoil re-contouring to tackle the
drag creep at high Mach numbers that beset the G-III.
Using the inputs from the tunnel tests conducted in
Bedford and applying new computer programs aimed
at improved predictions of lift and drag, particularly at
higher speeds, Gulfstream engineers determined that
the outboard portions of the G-IV wing required a new
shape. In spite of their new location further aft on the
fuselage, the Tay engines (which are noticeably wider
in diameter than the G-II and G-III’s Spey powerplants)
were still interfering with the air flow near the inboard
portions of the wing, thereby causing the outer portions
of the wing to produce an excessive portion of the total
lift needed to support the aircraft’s weight. Thus the
span-wise distribution of lift was not efficiently spread
along the length of the wing, and drag continued to be
higher than desired.
To solve this problem, Gulfstream engineers extended
the leading edge of the outer two-thirds of the wing by
about 7.5 inches and twisted the wing so that the outer
portion is drooped by about a degree. They also
changed the leading-edge contour to achieve the characteristics
of a supercritical airfoil along the modified
portion of the wing. In addition to distributing a greater
portion of the lift inboard, thereby allowing the wing to
produce lift more efficiently by lessening induced drag,
the new shape reduced the effective sweep angle of the
wing outboard, which reduced the drag resulting from
shock waves that form at high subsonic Mach numbers.
The net result was five percent less drag at high
speed and the ability to achieve a normal cruise at
0.80 Mach. The new wing design is so good, in fact,
that the G-IV can cruise at 0.85 Mach and has an
MMO of 0.88.
Not all the wing changes were aerodynamic, however.
The G-IV wing is structurally quite different than its
predecessor. The three upper wing panels employed on
the G-III have been replaced with a single upper skin,
and all access panels have been located on the lower
wing surface. The new wing has 23 fewer ribs and 70
percent fewer internal fasteners. In total, 30-percent
fewer parts are needed to construct the G-IV wing, compared
with its predecessor. Furthermore, the new airfoil
weighs 870 pounds less than the G-III wing, has 30-
percent fewer structural parts, costs less to build and
can accommodate 1,000 pounds more fuel.
NEW AIRCRAFT THROUGHOUT
The wing is not the only item that has been redesigned.
While the G-IV’s fuselage is essentially that of a G-III
with a three-foot plug just aft of the cabin entrance, its
aft pressure bulkhead has been redesigned and constructed
of high-strength, carbon composite material that
enables the surface to be flat, not conical like the G-III’s.
Consequently, compared with the G-III, the G-IV’s pressurized
interior contains 4.5 feet more usable space-3.0
feet from the stretch and 1.5 feet from the redesigned
pressure bulkhead.
Lighter weight, high-strength steel is used in the landing
gear, and Permaswage fittings have replaced the
conventional joints used in the hydraulic system in order
to save weight and reduce the likelihood of leaks.
Unlike the G-III’s hydraulics, the G-IV system operates at
a constant pressure of 3,000 psi, rather than switching
between 3,000 and 1,500 psi.
The G-IV also employs a “brake-by-wire” system built
by Goodyear, and a “steer-by-wire” system built by
Dowty. Instead of the pilot’s brake pedals and nosewheel
steering handle being directly linked to the
appropriate hydraulic valve, each is connected to its
own transducer, which sends an electrical signal to the
brake valves or nosewheel steering unit, which receive
their muscle from hydraulic pressure. These signals are
processed to achieve the desired response, thereby providing
the pilot with excellent control over braking and
steering.
The most obvious and outwardly impressive changes
that distinguish the G-IV from the G-III (and their competitors)
are the Rolls-Royce Tay engines and the Sperry
electronic flight instrument system.
Designed around the core of the Spey RB.183-555
engine (which powers the Fokker F28) rather than the
RB.183-511-8 used in the G-III, the Tay incorporates
fan technology that was developed in conjunction with
the RB.211 series of turbofans that powers a number of
wide-body airliners.
Because the Spey has accumulated well over 30 million
flight hours in its illustrious career, the Tay starts life
with a 7,000-hour time between overhaul and a 3,500-
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hour hot-section inspection interval.
Operators of the G-IV, therefore, will
enjoy the same overhaul periods that
operators of the G-II and G-III do.
The Tay, however, is neither as
thirsty nor as noisy as the Spey. Flatrated
at 12,420 pounds of thrust
(which it can maintain up to 35
degrees C) and capable of delivering
a specific fuel consumption at 0.80
Mach that is 16 percent less than that
of the Spey installed in the G-III, the
Tay provides the G-IV with lots of
brawn and a more reasonable
appetite for calories. Since the G-IV
has a maximum takeoff weight of
71,700 pounds (2,000 pounds more
than the G-III’s), its power loading is
about the same as its predecessor’s in
spite of the greater thrust of the Tay.
Hot-and-high performance is better
due to the Tay’s ability to produce
rated power in temperatures considerably
above ISA.
Unlike the Spey, which produces a
fair share of noise as it converts Jet A
into thrust, the Tay is impressively
quiet. Sideline noise during a takeoff
at maximum gross weight is 88.8
EPNdB, takeoff noise at the same
weight is only 79.1 EPNdB, and
approach noise at the G-IV’s maximum
landing weight of 58,500
pounds is 91.0 EPNdB. Those figures
are well below FAR Part 36, Stage 3
requirements, which are 94, 89 and
98 EPNdB, respectively. In terms of
sound measured on the A scale (dBA
is the yardstick many municipalities
use to set their noise curfews), the G-
IV enjoys a takeoff reading of 67 dBA
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and an approach reading of 83 dBA. These figures,
incidentally, are below Washington National Airport’s
nighttime limits, which are 72 dBA for takeoff and 85
dBA for approach.
During B/CA’s visit to Savannah, home of Gulfstream
Aerospace, we listened as the G-IV made several takeoffs,
flybys and approaches. We were impressed. With
our editorial offices located at Westchester County Airport,
home of more G-IIs and G-IIIs than any other airport
in the United States, we are very familiar with the
sound of the Spey engine. The Tay is noticeably quieter.
Another important although lesser-known requirement
for engines deals with air pollution. At idle
power, where emissions are measured, the Tay is sufficiently
efficient to be well within the limits set by
SFAR Part 27. The powerplant also satisfies the standards
set for smoke.
Both the improved efficiency and the lower sound levels
associated with the Tay are due in part to the
engine’s fan technology, such as the use of wide-chord
blades made of titanium, and its three-to-one bypass
ratio. The improved emissions are the result of more
complete combustion and higher thermal efficiency,
also due to the use of higher levels of technology than
that employed in the Spey.
ALMOST STAR WARS
As if the G-IV’s new wing, improved structure,
stretched fuselage and new engines weren’t enough
to capture the hearts of chief pilots as well as chief
executives, the aircraft’s flight deck is something that
cannot be resisted. Consisting of six eight-inch by
eight-inch, full-color cathode ray tubes (CRTs), the EFIS
designed and manufactured by Sperry Flight Systems
is nothing short of fantastic.
Sperry has introduced an advanced symbology
specifically for the G-IV, and some familiarization is
required before a pilot can appreciate fully its presentation.
But adopting to the wealth of information that
Sperry provides comes quickly, and the small effort
required to grasp the value of these presentations is
worthwhile.
Although each of the six CRTs that are spread across
the instrument panel is identical, the outermost two are
the primary flight displays (one for the pilot; the other
for the copilot). Moving inboard, the next two are the
pilot’s and copilot’s navigational displays, and the
remaining two constitute the engine instrument and
crew alerting system (EICAS). These central CRTs are
mounted one on top of the other; the top tube is used to
present engine data, and the bottom tube provides
warning messages and information related to the aircraft’s
systems.
Flight management also is provided by Sperry, via
Specifications Gulfstream G-IV
(Preliminary)
B/CA EQUIPPED PRICE $15,800,000 (1986 dollars)
SEATS 2+14/19
ENGINE
Model 2 RR Tay Mk 610-8
Power
12,420 lbs ea.
TBO 7,000
DESIGN WEIGHTS (lb/kg)
Max ramp 70,200/31,843
Max takeoff 69,700/31,616
Max landing 58,500/26,536
Zero-fuel 44,000/19,958
BOW (B/CA equipped) 39,100/17,736
Max payload 4,900/2,223
Useful load 31,100/14,107
Max usable fuel 29,300/13,290
Payload (max fuel) 1,800/816
Fuel (max payload) 26,200/11,884
LOADING
Wing (lb/ft 2 ) 73.3
Power (lb/hp) 2.8
PSI 9.5
LIMIT SPEEDS (KCAS)
MMO 0.850
VMO 340
VFE (app) 220
V2 144
VREF 116
PERFORMANCE (SL, ISA, MGTOW)
BFL (ft/m) 5,100/1,554
BFL, 5,000 ft. ISA 20 0 C (ft/m) 7,150/2,179
Climb (fpm/mpm)
All-engine 4,070/1,241
Engine-out 1,080/329
Certificated ceiling (ft/m) 45,000/13,716
All-engine
service ceiling (ft/m) 43,800/13,350
Engine-out
service ceiling (ft/m) 30,730/9 367
Part 121 landing
distance (ft/m) 2,670/814
NBAA VFR range (ft/m) 4,746/1,447
NBAA IFR range (ft/m) 4,371/1,332
dual FMZ-800 FMSes that can accomplish both lateral
and horizontal navigation as well as performance computations
with impressive clarity. Both plan and elevation
views of the aircraft’s progress will be presented on the
aircraft’s navigational displays (the CRTs just inboard of
the primary flight displays) once all the software is certificated,
and an autothrottle is to be installed within the
next year, thereby completing the elements needed for
complete performance management.
The FMSes receive data from dual Honeywell laser
inertial-reference systems as well as from Collins navigational
receivers. (Collins also provides the communications
radios.) Provisions have been made to accept
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inputs from all conceivable sensors, such as global positioning
satellites, VLF/Omega stations and microwave
landing systems.
Pilot input to the EFIS, EICAS and much of the FMS is
by means of a display controller mounted just below the
glareshield, directly in front of the pilots. Each airman
has within easy reach and in his direct line of sight a
small, rectangular CRT that depicts the selected modes
of EFIS operation and presentation. A wealth of information
can be presented there, including performance
speeds such as V1, VR, V2, etc.
FLYING THE G-IV
Upon entering the cockpit of the G-IV, a pilot’s first
impression may well be that something is missing. The
instrument panel is bleak and lifeless...until the electrical
power is turned on. Then the panel erupts in color and
impressive arrays of meaningful presentations as the six
CRTs become active.
Programming the FMS for flight and entering performance
airspeeds into the EFIS is accomplished easily.
While the Honeywell LASEREF II was aligning, which
took six minutes, we programmed into the display controller
the aircraft’s V-speeds for our demonstration
weight of 57,940 pounds. Consequently, these performance
speeds were annunciated on the vertical tape
that depicts airspeed on the left-hand edge of the primary
flight display.
The starting process for the Tays is easy as the Sperry
EICAS tracks turbine gas temperature (TGT) and other
engine parameters. Had a start value been exceeded,
the EICAS needle for the appropriate indicator would
have turned red to provide a vivid warning of the undesired
condition.
In spite of its size, the G-IV taxies with the grace of a
typical business jet. The Tays respond nicely to small
power requests, and their idle speed is sufficiently low
so that there was no need to rely heavily on the aircraft’s
carbon brakes to assure a comfortable taxiing
speed. Dowty’s steer-by-wire system provides excellent
harmony between inputs by either the nosewheel-steering
wheel to the pilot’s left (for sharp turns) or by the
rudder pedals (for up to seven degrees of nosewheel
deflection). The brake-by-wire system also functions
smoothly and effectively.
Acceleration to our V, speed of 104 KIAS, through
VR of 124 and on to a V2 of 138 KIAS, was quickunder
20 seconds. The aircraft tracked very well in
spite of a slight crosswind from the left. The stick forces
required for rotation were comfortable, and the aircraft’s
initial rate of climb was impressive, as one might
expect at our considerably reduced takeoff weight.
Despite some ATC delays, the climb to our initial
cruise altitude was 22 minutes and 40 seconds from
brake release. The unrestricted climb between FL 220
and FL 450, which is the aircraft’s intended ceiling,
required only 12 minutes and 54 seconds. At altitude,
where the temperature was -64 degrees C, we easily
achieved a cruise Mach number of 0.80 on a total fuel
flow of 2,560 pph, which yielded a specific range of
0.176. Increasing the fuel flow to 3,250 pph, we
obtained a cruise of 0.85 Mach with an aircraft weight
of 54,845 pounds. Specific range for that condition
was 0.147. (Gulfstream says that production Tays will
produce six percent more thrust above FL 400 than the
engines on the aircraft that we were flying.)
The G-IV’s handling qualities during the climb and at
cruise were typically Gulfstream. In fact, a design goal
was to make the G-IV fly like the G-III, and that objective
has been achieved. The aircraft exhibits good pitch
response in both its short- and long-period modes; its
lateral/directional characteristics are good, with low
adverse yaw and a damped Dutch roll. Stall protection
is provided by a stick shaker and stick pusher, as in the
G-III, even though the G-IV’s new wing provides about
one degree more stall margin than does the G-III’s.
High-speed buffet, when excited by a steep turn at altitude,
produced an easily manageable shutter that is
most apparent in the ailerons.
In the approach configuration, the G-IV exhibits very
good speed stability and tracks heading nicely. Small
changes in heading, such as those a pilot would make
if hand-flying an ILS approach, can be made with ease.
The control forces needed to flare the G-IV are comfortably
low, and ground handling is good right from the
moment of touchdown .
(Incidentally, the EFIS presentation seems to reduce
the workload normally associated with precision tracking
of airspeed and heading, although some time is
needed to adopt to the somewhat unfamiliar orientation
of airspeed, attitude and altitude on the primary flight
display.)
Truly, the Gulfstream is an example of the quest for
perfection in a business aircraft. It has impressive performance,
futuristic-and very effective-flight instrumentation,
modern systems and good handling qualities.
Corporate pilots and corporate executives will find considerable
pleasure in the G-IV well into the 21st century.
B/CA
COPYRIGHT 1995 THE MCGRAW-HILL COMPANIES, INC. ALL RIGHTS RESERVED