ch14-2

Quest for Performance: The Evolution of Modern Aircraft


Part II: THE JET AGE


Chapter 14: Business Jet Aircraft


Configuration Features



[464] Most business Jet aircraft are of low-wing design and have engines mounted at the aft end of the fuselage. Except for one three-engine and one four-engine design, all are powered with two engines. Both pure jets and turbofan engines are used. Most of the modern aircraft produced today have turbofan engines; some, of these are repowered versions of aircraft that originally appeared with turbojet engines. The wings of most of the aircraft have a modest amount of sweepback, although one business jet described below has a sweptforward wing.

Like any aircraft, the size and performance of business jets vary with the function for which the aircraft has been designed. Aircraft are available that vary in gross weight from about 11 000 to 65 000 pounds. Cruising speeds lie in the range from 0.7 to 0.85 Mach number. Ranges vary from intercontinental values to as low as 1150 miles. Many of the new aircraft being produced have at least nonstop transcontinental capability. The number of passengers that can be accommodated, even on aircraft of the same design, varies widely depending on the interior cabin arrangements. Aircraft can be found with the capability of carrying from 5 to 19 passengers.

Most corporate aircraft are expected to operate from a wide variety of airports. The landing and takeoff field lengths they require are accordingly shorter than those for the larger transport aircraft. The desired landing and takeoff field lengths of business jets, as compared with transports, are usually obtained through a combination of low wing loading and high thrust-to-weight ratio, together with a relatively simple high-lift system. A simple slotted trailing-edge flap frequently constitutes the entire high-lift system.

The small size of many business jets imposes certain design constraints not encountered in large transport aircraft. One dimension that cannot be scaled as the size of an aircraft is reduced is the size of the human body that occupies the cabin. This essentially invariant dimension is usually a predominant factor in determining the fuselage diameter. A small fuselage diameter is desirable in order to reduce weight and to maintain as low a value of the ratio of wetted area to wing area as possible. Accordingly, only the very large business jets have a cabin diameter sufficiently large to accommodate a person standing in an upright position. Figure 14.1 shows the cabin size of three business aircraft relative to a 6-foot-tall person. Some of the smaller aircraft are essentially sit-down vehicles in much the same sense as an automobile. Some feature a cabin diameter that permits limited mobility in a....




[465] Figure 14.1 - Cabin interior of three business jet aircraft.


....stooped posture. A cabin floor free of obstructions is a desirable feature intended to reduce the possibility of a passenger tripping or falling. Such a floor design requires that the wing carry-through structure be either beneath or behind the cabin. There are disadvantages to both arrangements. An increase in fuselage diameter results from passing the wing structure entirely beneath the floor; whereas, placing the wing behind the cabin may result in a center of gravity that is farther forward than desired. Placement of the wing carry-through structure behind the cabin combined with the use of a sweptforward wing offers a means for overcoming the disadvantages of the other two methods of [466] achieving an unobstructed cabin floor. The German Hansa jet described below utilized this design concept.

Two other size-related design factors are worth mentioning. The short distance between the ground and the bottom of the wing precludes the use of the under-wing- engine mounting and is largely responsible for the aft-engine location employed on all current business jet aircraft. Two alternative arrangements suggest themselves: (1) a high wing location with engines mounted beneath the wing or (2) a low wing configuration in which the engines are mounted on top of the wing. So far, neither of these arrangements has been utilized on a business jet, although one small transport aircraft (the VFW Fokker 614) has been produced that employs the over-wing-engine arrangement. In most cases, the aft-engine arrangement used on business jets forces the horizontal tail to the tip or part way up the vertical tail. Possible problems associated with a high horizontal-tail location are discussed in chapter 10. Finally, the small size of the business jet results in a Reynolds number¹ that is much lower than the Reynolds number characteristic of transport aircraft. That portion of the drag coefficient attributable to skin friction is accordingly higher for the small aircraft. For example, if all the dimensions of a small business jet are assumed to be one-fifth those of a large jumbo jet, the skin-friction drag coefficient of the small aircraft will be about 30 percent higher than that of the jumbo aircraft. For this reason and because the ratio of wetted area to wing area may be higher than that of many larger aircraft, the maximum lift-drag ratios characteristic of business jet aircraft tend to be lower than those of the large transports.

¹ The Reynolds number is a nondimensional quantity expressing the ratio of inertia to viscous forces in the fluid flow. Reductions in aircraft size and speed as well as increases in flight altitude cause a reduction in Reynolds number. In most practical cases, a reduction in Reynolds number causes an increase in skin-friction drag.