Quest for Performance: The Evolution of Modern Aircraft
Chapter 14: Business Jet Aircraft
Representative Aircraft Types
[466] Eight representative business jet aircraft are briefly described below and are illustrated in figures 14.2 to 14.10. Information on the many different models of business jet aircraft now available may be found in references such as reference 144 and the various issues of Jane's All The World's Aircraft.
Lockheed JetStar
The first of the dedicated business jets, the Lockheed JetStar, completed its maiden flight in September 1957. Initial development of the....

 ground view of Lockheed Jetstar
[467] Figure 14.2 - Four-engine Lockheed JetStar. [Peter C. Boisseau]

...aircraft was undertaken as a private venture. The first two prototypes were equipped with two Bristol Siddeley Orpheus turbojet engines. The aircraft was later entered in an Air Force competition for a small four-engine utility and personal transport, and in this version was equipped with four Pratt & Whitney JT-12-8 turbojet engines of 3300 pounds thrust each. The JetStar won the Air Force competition and in that service is known as the C-140. The aircraft is shown in figure 14.2. Production of the original JetStar ended in 1973; however, an improved version known as the JetStar II, powered with four Garrett TFE 731 turbofan engines of 3700 pounds thrust each, was offered by Lockheed in 1976. Production of the type ended late in 1978, at which time a total of 160 JetStar aircraft had been built.

The JetStar, with a gross weight of 42 500 pounds, is one of the heaviest of the business jets. A typical cabin configuration accommodates 8 to 10 passengers; a range of 2415 miles is possible with a payload of 3500 pounds. Takeoff and landing field lengths are 4700 and 3550 feet, respectively. These field lengths are based on climb and descent over a 50-foot obstacle, however, and are not to be compared with the FAR field lengths given in table VII for transport aircraft. Maximum cruising speed is 567 miles per hour at 21 000 feet, which corresponds to a Mach number of 0.80.
Wing of the JetStar is characterized by a 30° sweepback angle, an aspect ratio of 5.3, and airfoil section thickness ratios that vary from 12 percent at the root to 9 percent at the tip. An instant recognition feature of the aircraft is provided by the large external fuel tank located at the midspan position of' each wing. Unlike most business jet aircraft, the high-lift system of' the JetStar is relatively complicated and consists of [468] a double-slotted trailing-edge flap and a leading-edge flap. Lateral control is provided by ailerons without the assistance of' spoilers, and a speed brake is located on the underside of' the fuselage. The longitudinal trim system is unusual in that the stabilizer is fixed to the fin, which pivots to change the stabilizer angle. An indication of' this pivoting action is provided in figure 14.2 by the apparently unpainted portion of the lower part of the fin. All controls are power operated.
Gates Learjet 24B and 55
The prototype Learjet model 23 made its first flight in October 1963 and may be considered as the progenitor of a whole family of Gates Learjet business aircraft of different gross weight, passenger capacity, and range. All the aircraft, however, are of the same basic configuration. The data in table VIII are for the Gates Learjet model 24B, shown in figure 14.3. Deliveries of model 24 began in 1966, and model 24B was certified in December 1968. Over 1000 aircraft of all versions had been built by the end of 1980, and several models are in production at this time.
The Learjet model 24B is one of the smaller business jets with a gross weight of 13 300 pounds and a cabin configured to accommodate a maximum of six passengers. The aircraft has a range of 1271 miles with a maximum payload of 1910 pounds; and with full fuel tanks and a reduced payload, the maximum range achievable is 2041 miles. The maximum cruising speed is 534 miles per hour, and the cost-economical speed is 508 miles per hour. Both of these speeds are at 41 000 feet; the corresponding Mach numbers are 0.81 and 0.77. The Learjet model 24B is equipped with two General Electric CJ610-4 turbojet....

ground view of a Learjet 24B
 Figure 14.3 - Gates Learjet 24B. [Peter C. Boisseau]

[469]....engines of 2850 Pounds thrust each. These engines, together with the low gross weight, give a high takeoff thrust-to-weight ratio of 0.43. This value of' the thrust-to-weight ratio is much higher than any of those given in table VII for transport aircraft and is about the same as that of the well-known North American F-86D fighter of the 1950's. (See chapter 11.) As can be seen from the table, the high thrust-to-weight ratio, coupled with a wing loading of only 57.4 pounds per square foot, gives an outstanding short takeoff capability.

Wing-planform shape of all Gates Learjet aircraft is characterized by a small sweepback angle of 13, together with a straight trailing edge. Shape of the wing can be seen in the view of a Gates Learjet 25C shown in figure 14.4. Wing airfoil-section thickness ratio is 10.9 percent. The high-lift system employed on the wing is simple and consists only of a single-slotted trailing-edge flap; no leading-edge devices are used. Ailerons, rudder, and elevators are manually actuated; spoilers for increasing drag and reducing lift are located ahead of the flaps and are power actuated. Longitudinal trim is achieved by varying the incidence of the stabilizer. Possible deep-stall problems (see chapter 10) associated with the T-tail are avoided by the use of a combined stick....

ground view of a Learjet 25C
Figure 14.4 - Gates Learjet 25C showing wing-planform shape. [mfr]

[470]...shaker/pusher. As in the case of the JetStar, part of' the fuel load of the Gates Learjet 24B is carried in external fuel tanks; the tanks on the Gates LearJet, however, are located at the wingtip instead of' the midspan position employed on the JetStar.

Latest version of the Gates Learjet to be offered is the model 55, which is depicted In figure 14.5. Comparison of the data in table VIII between this model and model 24B shows that model 55 is larger, heavier, more commodious, and has a much longer range. Not shown by the data in table VIII is the cabin size of the Gates Learjet, which for model 55 is about a foot larger in both width and height than for earlier versions of the aircraft. Power in the Gates Learjet 55 is supplied by two AiResearch TFE 731-3 turbofan engines of bypass ratio 2.79 and thrust of 3650 pounds each. The lower specific fuel consumption of these engines as compared with that of the turbojets employed on model 24B is no doubt partly responsible for the increased range capability of the new aircraft. The large nacelles required to accommodate the turbofan engines are clearly evident in figure 14.5.
Perhaps the most noteworthy recognition feature of the Gates Learjet 55 is the small winglike vertical surfaces located at each wingtip. These tip devices are a modern development (by Dr. Richard T. ....

aerial view of a Learjet 55
 Figure 14.5 - Gales Learjet model 55. [mfr] [Original photo was in color, Chris Gamble, html editor]

[471] Whitcomb of the NASA Langley Research Center) of an old concept that is intended to trick the flow over the wing into behaving as though the wing span, and thus aspect ratio, is greater than is actually the case. "Winglet" is the popular name for one of these tip devices. The use of winglets causes a reduction in induced drag and a consequent increase of a few percent in maximum lift-drag ratio; some improvement in climb performance is also attributed to their use. At least two other new aircraft employ winglets.
Not evident in figure 14.5 is the increased wing span of model 55 as compared with earlier models of the Gates Learjet. The corresponding aspect ratio of the new aircraft is 7.3, as compared with 5.4 of model 24B. Improved airfoil sections are incorporated in the wing of model 55, as are fences and other flow-control devices designed to improve stalling characteristics. Control and high-lift systems are essentially the same as described for model 24B.
The Gates Learjet 56 is similar to the 55 but has a larger fuel capacity and longer range, coupled with a somewhat reduced passenger capacity. Detailed descriptions of the models 55 and 56 as well as others in the Gates Learjet series of aircraft may be found in reference 130.
Dassault-Breguet Falcon 20
The Falcon 20 is one of a series of business jets manufactured by the French firm of Dassault-Breguet. The aircraft, with a gross weight of 28 660 pounds, lies in a weight class about midway between the JetStar and the Gates Learjet 24B. Power is supplied by two General Electric CF700 aft fan engines of 4315 pounds thrust each and bypass ratio 1.9. The Falcon 20 is used extensively in the United States and is frequently referred to as the Fan-jet Falcon in this country. First flight of the aircraft equipped with the General Electric engines took place in July 1964. A Falcon 20 is shown in figure 14.6.
The aircraft has a maximum payload capability of 3320 pounds and features a cabin that can accommodate 8 to 10 passengers. With a reduced payload of 1600 pounds, the aircraft has a range of 2 220 miles. Maximum cruising speed is 535 miles per hour at 25 000 feet, and cost-economical speed is 466 miles per hour at 40 000 feet. The corresponding Mach numbers are 0.77 and 0.70, respectively. The data in table VIII indicate about the same landing and takeoff field lengths for the Falcon 20 as for the JetStar.
Configuration of the Falcon 20 is characterized by a wing of 30° sweepback angle, an aspect ratio of 6.5, and airfoil-section thickness....

 ground view of a Falcon 20
[472] Figure 14.6 - Dassault-Breguet Falcon 20 business jet with aft-fan General Electric engines. [Peter C. Boisseau]
....ratios that vary from 10 percent at the root to 8 percent at the tip. Figure 14.6 shows a large flow-control fence on top of the wing part way between the root and tip. A leading-edge flap, similar to an unslotted slat, is employed inboard of the fence and a conventional slat is utilized outboard. A single-slotted trailing-edge flap completes the high-lift system. Lateral control is provided by ailerons alone. Spoilers located ahead of the flaps are deployed symmetrically to increase the drag for braking and rapid descent and are not part of the lateral control system. Longitudinal control is provided by elevators, and trim is maintained with an electrically driven stabilizer. With the exception of the stabilizer, all the movable surfaces are hydraulically actuated.
The Falcon 20 and its derivatives continue in production. In addition to use as an executive transport, the aircraft is also available in a cargo version. The latest in the Falcon series, the Falcon 50, is equipped with three engines located in a manner similar to that of the Boeing 727.
Gulfstream Aerospace Gulfstream II
An examination of the data in table VIII indicates that the Gulfstream II, shown in figure 14.7, is heavier in weight, larger in size, faster in speed, and longer in range than any of the other business aircraft. For example, the gross weight of the Gulfstream II is 62 500...

 ground view of a Gulfstream 2 with boarding lader extended
[473] Figure 14.7 - Gulfstream Aerospace Gulfstream II. [Peter C. Boisseau]

....pounds, nearly five times that of the Gates Learjet 24B, and the wing area is about three-and-a-half times that of the smaller aircraft. The Gulfstrearm II has an intercontinental range capability and a maximum cruising speed of 588 miles per hour, or Mach number of 0.85, at an altitude of 25 000 feet. The aircraft, frequently referred to as the G-II, was developed as a jet-powered successor to the highly successful turboprop-powered Gulfstream I. First flight of the G-II took place in October 1966, and a total of 256 units were manufactured before production ended in 1979. The more advanced Gulfstrearn III became available in late 1980.
The Gulfstream II is a low-wing configuration incorporating a Ttall and a wing of aspect ratio 6.0, 25 sweepback, and airfoil sections varying in thickness ratio from 12 to 8.5 percent. Power is supplied by two Rolls-Royce Spey MK 511-8 turbofan engines of 11 400 pounds thrust each. These engines are equipped with a five-stage fan and have a bypass ratio of 0.64; target-type thrust reversers are employed. The high-lift system consists of single-slotted trailing-edge flaps. Lateral control is provided by a combination of ailerons and spoilers. The spoilers may also be deployed symmetrically to increase drag and reduce lift. Elevators are used for longitudinal control, and trim is [474] accomplished with a variable incidence stabilizer. All controls are hydraulically actuated.
The passenger cabin of the Gulf'stream II is usually configured for 10 to 14 passengers (maximum capacity of' 19 passengers). The range-payload data given in the references are incomplete but indicate a range of 3881 miles with maximum fuel. Takeoff and landing distances are in the same class as those of the Lockheed JetStar.
As compared with the Gulfstream II, the Gulfstream III is slightly larger and heavier and has a greater range. The appearance of the new aircraft, however, differs little from that of the Gulfstream II. Winglets similar to those employed on the Gates Learjet 55 provide the primary identification feature of the Gulfstream III.
Cessna Citation
The Cessna Citation I and II are small business Jet transports in the same weight class as the Gates Learjet. Low first costs, economy of operation, safety, and viceless handling characteristics were among the design objectives. In order to provide wide operational flexibility, the aircraft was designed to take off and land from most fields used by light and medium twin-engine propeller-driven aircraft, and from unpaved runways. First flight took place in September 1969, and the aircraft was certified in September 1971. That the Citation I and II have been widely accepted is clearly demonstrated by the more than 1000 aircraft that have been produced; the type is still in production and will likely continue to find a significant share of the business jet market for a number of years. Although the Citation I and II are similar in appearance, the Citation II is somewhat larger and heavier than the Citation I and has a longer range capability. A Citation I in flight is shown in figure 14.8, and data are given in table VIII for the Citation II.
The Citation II has an unswept wing, an aspect ratio of 8.3, and airfoil-section thickness ratios that vary from 14 percent at the root to 12 percent at the tip. The horizontal tall is located near the root of the vertical fin and incorporates a small amount of dihedral to reduce immersion in the jet exhaust. To improve directional stability, the vertical tall has a relatively large dorsal fin together with a small ventral fin. Power is supplied by two Pratt & Whitney JTI 5D-4 turbofan engines of 2500 pounds thrust each and bypass ratio of 3.3.
The high-lift system on both versions of the Citation consists of a single-slotted trailing-edge flap; no leading-edge devices are employed. Spoilers located on the upper wing surface ahead of the flap are used as air brakes and are not part of the lateral control system, which....

Citation 1 in flight
[475] Figure 14.8 - Cessna Citation I. [mfr]

....utilizes only ailerons. Longitudinal control is by elevators, and trim is obtained by an electrically operated trim tab on the elevator. All controls re manually operated.

The Citation II has a gross weight of 13 300 pounds and a cabin hat is usually configured to carry from 6 to 10 passengers. With six passengers, the aircraft has a range of 1969 miles. Normal cruising speed is 443 miles per hour, which corresponds to a Mach number of .64 at
25 000 feet; maximum operating speed is 485 miles per hour at 25 000 feet, which gives a Mach number of 0.70. The stalling speed of 94 miles per hour is the lowest of any of the aircraft listed in table VIII. This low stalling speed is obtained with a relatively simple highlift system because the wing loading is only about 41 pounds per square foot. This low wing loading together with the high thrust-to-weight ratio of 0.38 are responsible for the short takeoff distance given in the table.
To complement the highly successful Citation I and II line of business jets, Cessna is now producing an entirely new aircraft of higher performance, the Citation III. First flight of this aircraft took place in May 1979, and first deliveries were scheduled for 1983. As can be seen from Figure 14.9, the Citation III bears no resemblance to the earlier Citation I or Citation II. Instead of a straight wing, the new aircraft has a 25 sweptback wing of 9.11 aspect ratio. Incorporated in the wing are....

Citation 3 in flight
[476] Figure 14.9 Cessna Citation III. [mfr]
....NASA supercritical-type airfoil sections that vary in thickness ratio from 16 percent at the root to 12 percent at the tip. The high-lift system consists of trailing-edge slotted flaps; lateral control is provided by spoilers and small ailerons. In contrast to earlier Citations, the Citation III employs a T-tail. The aircraft is powered by two AiResearch TFE 731-3 turbofan engines of 3650 pounds thrust each and 2.79 bypass ratio. To assist in braking on landing rollout, the engines are equipped with hydraulically actuated thrust reversers.
An examination of the data in table VIII shows that the gross weight of the Citation III is in the same class as that of the Gates Learjet 55. The aircraft has a maximum passenger capacity of 13 and is capable of carrying 6 passengers (plus 2 pilots) over a nonstop United States coast-to-coast range of 2875 miles. Maximum cruising speed is 539 miles per hour at 33 000 feet (M = 0.8 1), and maximum certified cruising altitude is 51 000 feet. Stalling speed is a relatively low 104 miles per hour and results from a combination of low wing loading, good high-lift flaps, and high maximum-lift characteristics of the blunt leading-edge supercritical airfoil sections.
Indeed, the Citation III appears to be a worthy stablemate to the highly successful Citation I and II aircraft.
[477] MBB HFB 320 Hansa
The German MBB HFB 320 Hansa is included in this brief overview of business jet aircraft because of its interesting and unique configuration. It features a sweptforward wing, as shown in figure 14.10. Design of this unusual aircraft was begun in March 1961, and first flight took place in April 1964. Production of the aircraft began in 1966 and continued until approximately 50 units were manufactured.
The desirability of an unobstructed cabin floor and some of the means for achieving this objective are discussed at the beginning of this chapter. The wing of the Hansa is mounted near the middle of the fuselage (in the vertical sense), and the wing carry-through structure is located behind the passenger cabin. In order to place the wing aerodynamic center in the desired position relative to the aircraft center of gravity, 15 of forward sweep is incorporated in the wing. Fuel tanks are mounted at each wingtip; the small horizontal surfaces seen at the rear end of the tanks help to stabilize the wing-tank system against divergence. (See the section on swept wings in chapter 10.) The landing gear retracts into blisters located on the fuselage at the wing root, and the empennage incorporates a horizontal tall mounted at the top of the vertical surface in the T position. Power is supplied by two General Electric CJ610-1 turbojet engines of 2850 pounds of thrust each.
In contrast to a sweptback wing, which stalls initially at the tip, a wing with forward sweep stalls first at the root. Initial stall at the wing root can produce pitch-up just as does tip stall on a sweptback wing.

MBB HFB 320 on the ground
Figure 14. 10 - View of MBB HFB 320 Hansa showing unusual sweptforward wings. [Flt. Intl.]
[478] To alleviate this problem, an inboard leading-edge slat and a large upper surface fence located at about mid semispan are used for stall control on the Hansa. The high-lift system utilizes these devices as well as a trailing-edge double-slotted flap. Upper and lower surface spoilers are deployed symmetrically for the purpose of increasing drag and decreasing lift, and ailerons are used for lateral control. Trim about all three axes is provided by tabs on the ailerons, elevators, and rudder; the horizontal stabilizer is not adjustable.
Gross weight of the Hansa is 18 740 pounds, and the aircraft can carry a maximum payload of 2650 pounds for a distance of 949 miles; with full fuel tanks and a reduced payload of 1760 pounds, the range is 1668 miles. The cabin is usually configured for nine passengers. Maximum cruising speed is 509 miles per hour at 26 000 feet, which corresponds to a Mach number of 0.74. Landing and takeoff field lengths are comparable with those of the Falcon 20.
A configuration layout incorporating a sweptforward wing would seem to offer interesting possibilities for the business jet aircraft. The reason for the short production life of the Hansa is not known. Perhaps the configuration concepts employed in this aircraft will be examined again at some time in the future. The divergence problem of the sweptforward wing may be alleviated without a significant weight penalty by the use of composite materials that permit a degree of control over wing torsional stiffness not possible with conventional metal structures.