Teor´ıa de perturbaciones

The commonest ways of ‘stretching’ requirements are to increase the payload, or the fuel load, both of which increase the all-up weight. Increased payload involves stretched fuselages: it is significant that few transport aeroplanes designed in recent years have remained unmodified for long in their original form. Increased fuel loads involve either overload tanks — hung beneath wings and fuselage — or wings of larger area.

Increased weight with unchanged wing area increases the wing loading: which may be thought of as a pressure applied to the supporting air by the wing surfaces. There is a relationship between wing loading and dynamic pressure, 0.5ρV2, or 0.7pM2 (as given in Eqns (1-5) and (1-8)) that must be maintained if the aeroplane is to continue flying at the most efficient angle of attack to the air. In the next chapter we shall examine the aerodynamic picture to see the way in which the aerodynamic forces vary with speed and attitude to the air. If the wing loading is increased, then it must be met with increased dynamic pressure, and this can only be done by a reduction of height, for to increase speed at the same height involves running the engines off-design and, hence, inefficiently.

The effect of increased wing loading is to shrink the boundaries of Fig. 2.1 towards the centre, by increasing the stalling speed, decreasing the ceiling and decreasing the maximum and cruising speeds. Increased stalling speeds result in higher landing speeds and greater kinetic energies to be absorbed by the wheels and brakes. For example, an aeroplane weighing 17 short tons (34,000 lb) touching down at 165k represents about 41,000,000 ft-lb of kinetic energy: enough to kick a 5-ton elephant 4,000ft straight up into the air. If the aircraft could be landed at 110k, then the reduced energy would be only 18,000,000 ft-lb: the saving being equal to the heat potential to melt 100 lb of steel.

If, on the other hand, it is possible to increase the wing area at some stage in the design, then the wing loading may be maintained or even reduced. Of course, the increased wetted area being moved through the

air may well result in a decreased overall lift/drag, but this is offset by reduced landing speeds when most of the fuel has gone. Unfortunately, stretching requirements tends to result in weight increasing faster than wing area, as initially indicated in Fig. 4.10, which shows the growth of the medium and long-range versions of the Aerospatiale/ BAe Concorde from November 1962 to May 1964. The initial wing loading increased from 60 lb/ft2 to 79 lb/ft2: a growth of more than 30%. It must be remembered, however, that the growth was brought about by the building-in of more advanced features and advantages.

Fig. 4.10 Diagram showing the way the Anglo—French Aerospatiale/BAe Concorde was reported to have grown in weight and size since it was first announced as a firm project. The aeroplane grew 63% heavier and 14ft longer in a little more than 18 months. More than 25 years later it is still in operation.

4.3.1 Physically stretching the basic aeroplane

The disadvantages of overloading a basic aeroplane by modifying it to carry more fuel within its existing envelope, or by attempting to increase productivity (payload times block speed, Eqn (3-1a)), by shortening the seat pitch and leg space of passengers, can be severe. A favored alternative is to insert volume-increasing structural ‘plugs’, confined as far as possible but not exclusively within the existing frontal area. Where this cannot be done, then changes which increase the area and span of wings and tail surfaces, without decreasing their aspect ratios, can be just as beneficial, even though frontal area is somewhat increased.

The reason for maintaining aspect ratio as far as possible is that the lift-dependent or induced drag of a wing, DL, is directly proportional to the span-loading: the total weight of aeroplane carried per unit length of

wing span (W/b). The lower the weight (hence mass) borne per unit length of wingspan, then the slower the lift-generating downwash and waste of fuel spent on giving the air mass a downwards momentum. The other way of looking at it is that the induced drag coefficient CDL and therefore DL are inversely proportional to the

aspect ratio, A (see Eqns 6-5) and then (5-11)). The higher is A, the lower is DL.

An example is that of the changes made by the European Consortium, Airbus Industrie, to its A340-300 airliner, to counter the transatlantic challenge posed by the US Boeing 777-300. Figure 4.11 shows that the basic European aeroplane is enlarged and re-winged to produce two new variants. The first, the A340-500, is stretched by means of structural ‘plugs’ or inserts, turning it into an ultra long-haul version of the basic

A340-300 (designed for 295 passengers). The manufacturer claims that the plugs increase design cruising speed, in spite of modest increases in frontal area and wetted-area of skin. The modifications improve speed flexibility, making the specific air range (Eqn 4-9)) less sensitive when the aircraft is flown off-design. The much increased gross weight is said to accommodate enough fuel to extend the range from Europe to as far as the west coast of the USA. The second, A340-600 variant, has cabin capacity increased by means of even larger plugs to carry 25% more passengers.

(picture)

Plate 4.2 (a) and (b) The European Airbus Industrie Consortium response to the Trans-Atlantic challenge from Boeing (Plate 4-3(a) and (b)). Note the winglets at the tips and the flap-track fairings at the wing trailing edges. Both sets of devices are shaped to generate vortices which weaken, by diffusion, the powerful trailing vortex system shed by the wings, which is the origin of lift-induced (or lift-dependent) drag. The lift/drag ratio is

improved and with it range (see also Appendix C). (picture)

Plate 4-3 (a) and (b) showing the Boeing challenge to European competition in the form of the stretched 747- 600X (now understood to have been abandoned); and the first of the 777-200 and 777-300 family. A major problem with all large transport aircraft is fast and controlled passenger evacuation in the event of an emergency (see also Appendix C).

Fig. 4.11 Airbus A340/Boeing 777-300 comparison. (Courtesy of Flight International, 9—15 October 1996.) The same structural philosophy applies to wings and tail surfaces. A tapered wing-box, inserted to widen the chord without increasing wing thickness, increases wing volume while making the section

aerodynamically finer. Both area and fuel volume are improved within the ‘wet-wing’ (a structure which acts as a sealed aerofoil-shaped fuel tank). The longer span and widened chord counter an increase in wing loading, while maintaining aspect ratio which, as we have just seen, is proportional to span squared/area. The ratio of lift/drag is the powerful aerodynamic contribution to range-flying ability (see Eqns (4-11), and (4-11a and d) in Section 6.4).

Enlarging a fuselage and a wing increases their weights, and thus the total mass and moments of inertia, these in turn generally force increases in the size of the tail control and stabilizing surfaces. The larger the aircraft the more powerful the flywheel effect about the axes through its centre of gravity. Increased inertia makes the aircraft more sluggish in response to control. When it is rotating about its CG, then like a heavier flywheel the motion takes more stopping, so that greater control authority and larger tailplane and fin areas are needed to damp and oppose the motion.

The act of stretching an aircraft inevitably increases the weight. There is often increased risk of a tail-strike on take-off and landing. Undercarriage units must often be strengthened if they and their supporting structures have insufficient reserve strength. There may also be a need to increase the number of wheels, as in the case of the A340, which has them mounted on a centre-gear, beneath the centre-section. If stretching has markedly increased the gross weight, then re-engining may also be necessary.

Increased weight without alteration of structural strength is equivalent to flying at increased normal acceleration, in that smaller margins are left within the maneuver envelope. Maneuvering has to be carried out more gently and smaller accelerations must be applied if strain and failure are to be avoided. Most aircraft are now fitted with cockpit accelerometers that measure the normal acceleration in flight. Most heavy aeroplanes are also fitted with V—g recorders which maintain a continuous count of acceleration levels exceeded on every flight.