De los principios y valores éticos generales

alloy. In contrast to conventional aircraft, the report suggests that ground loads would be a more critical design condition than in flight, due to the distribution of structural and payload weight across the wing span. A structural weight fraction of 27% MTOW is estimated;

however, Lee (1961) comments that the cabin weight estimate may be optimistic.

A low cruise drag is achieved using slit suction: the resulting low profile drag permits a large wing area without a parasite drag penalty, and hence low span loading and induced drag.

Laminarised tip fins increase the effective aspect ratio, without any costs in terms of parasite drag. Consequently, a vortex induced drag factor of 0.943 is assumed. To avoid turbulence contamination downstream of areas not available for laminarisation, such as the cockpit windshield and doors, ‘clean-up’ slots are utilised, which enable a new laminar boundary layer to begin downstream.

The powerplant installation consists of six (very) low bypass-ratio Rolls-Royce RB.163 engines. One group of engines receive sucked boundary layer flow, whilst another group receive air from the clean-up slots, supplemented with contributions from ram intake. This scheme was selected as it had an advantage in terms of first cost benefit over a separate suction pump system. As prefigured by Lachmann (1955), Lee (1961) identifies takeoff thrust as the most significant engine design feature.

Lee (1961) gives the overall, effective, cruise L/D at 33. With a cruise lift coefficient of 0.17, a vortex-induced drag factor of 0.943, and an aspect ratio of 2.7, the induced drag coefficient CD,i is 0.0032, and therefore the profile drag coefficient (including pump drag) C_D0 is 0.0020. Consequently, the low value for aspect ratio has compromised the aircraft’s aerodynamic efficiency. On the basis of the Handley Page cruise and climb performance studies, which assume suction availability for altitudes above 15,000 ft, the estimated fuel burn of the HP117 is around 18 g/pax.km.

2.4. SUMMARY AND RESEARCH PROPOSAL

documented by Lee (1961), to find a reasonably detailed conceptual design study with this information. The HP117 has an estimated specific fuel burn of around 18.0 g/pax.km. This figure is only marginally better than that for the forthcoming B787 aircraft, which is forecast to consume 19.6 g/pax.km¹. In other words, although the Handley Page design would have been a major improvement on aircraft of the 1960s, it falls short of the performance suggested by Greener by Design. New studies, assuming present-day technology, are therefore required to substantiate (or challenge) the Greener by Design estimate. The overall objective of this thesis is to produce one such study.

We have seen that the basic architecture of a practical LFC suction system is well es-tablished. However, there is still little general guidance available on the important design parameters and their interactions. Furthermore, the requirements imposed on the system by a passenger-carrying LFW are likely to differ significantly from the conventional context, as revealed by the Handley Page study. As a consequence of the viscous drag reduction, the induced drag was also small, which in turn required a wing with low span loading, operat-ing at a lift coefficient below conventional values. Finally, the freedom to optimise section geometry was restricted by passenger accommodation requirements, which in turn lead to thick aerofoil sections, and moment balance constraints. These changes therefore reduce the applicability of the past LFC studies to the configuration of interest here.

The HP117 study started from a perceived market need, which then defined a mission (i.e.

range, speed, payload) and, eventually, the aircraft itself (as is normal in aircraft design). In contrast, it is more productive to reverse this approach for our LFW; specifically, start from the premise that an LFW is to be designed, with an emphasis above all on fuel efficiency, and then assess its capabilities once the design is complete. This means that the approach departs in some respects from a conventional methodology; by so doing, it ensures that the resulting LFW will not be unnecessarily constrained by current practice. With this philosophy, a high-level exploration of the design space can be performed to identify a specification for the key configuration and mission parameters of the proposed LFW. On the basis of this specification, a detailed conceptual design study can be undertaken. To quantify the direct benefits associated with a flying-wing configuration utilising LFC, it is necessary to compare against the performance of a conventional baseline-turbulent aircraft with the same mission and propulsion characteristics.

It should be acknowledged that, while LFC is proven in principle, its operational use remains subject to practical difficulties. Surface finish requirements are demanding, and en-vironmental contamination is a major concern. Of interest here, however, is the fuel-efficiency potential of the LFW. This can be assessed independently of operational issues. Indeed,

con-1http://www.greencarcongress.com/2007/07/boeing-rolls-ou.html, January 2012

cern over the latter seems unnecessary until it is clear that the LFW holds sufficient promise to justify its radical nature.

There is obvious merit in performing simpler studies ahead of more complex ones. There-fore, extensive and systematic optimisation procedures shall be eschewed in favour of reasoned design choices, and a configuration that is amenable to analysis by simplified methods shall be insisted upon. It is thus unlikely that the resulting concept aircraft will represent the best possible LFW, but it will provide a useful first indication of what may be expected of the type. Equally, any critical restrictions in the design tools will become apparent, allowing future development efforts to be efficiently targeted.

Chapter 3

Design Methodology

In this chapter, an overview of the conceptual design approach for conventional aircraft is given, and compared against that envisaged for a LFW aircraft. Then, the numerical tools and design methodologies employed in this study are discussed.

3.1 General Approach

The conventional approach to an aircraft conceptual design problem begins with the definition of a mission specification. Then, a suitable donor aircraft is usually identified, and is used to base initial estimates of target weight, surface areas, etc. The design approach, following e.g. Raymer (1999), is then as follows:

(1) target gross weight specified;

(2) cruise C_L estimated, thereby fixing cruise altitude;

(3) tailplane geometry defined for balance and stability;

(4) detailed drag assessment in the en-route configuration provides an estimate of the lift-to-drag ratio;

(5) aircraft structural weight estimated;

(6) engine sized and fuel weight to complete mission estimated;

(7) MTOW evaluated and, if higher than initial target specification, process repeated from (2) with revised MTOW.

In contrast, with an emphasis above all on fuel efficiency, the design process for the LFW is reversed. The design methodology, shown schematically in Fig. 3.1, is broken down into five

main parts: 1) 3-D airframe generation, 2) aircraft performance analysis, 3) suction-system technology integration, 4) propulsion system design, and 5) structures and weights. (However, as is evident from the figure, the design process is not linear.) Once a viable aircraft has been identified, the mission can be specified. For each design area, it is helpful to consider what are the main inputs and outputs, what are the constraints, and what must initially be assumed. These shall be discussed further as the conceptual design evolves in Chaps. 5 and 6. Before this, an overview of the technical approach is provided in the following section;

note, the suction-system design approach is presented in Chap. 4.

Aircraft Performance

Propulsion Planform

Low-speed

Suction-System Technology

Integration

Range

Fuel burn Pax

Cabin

Thrust

Mission fuel SFC

Spars

3-D Airframe Generation

Structures and Weights

Loading

Cruise C.G. travel

Power Volume

Engine weight Suction pump weight

Wing

Aerofoil section

Airframe

geometry Lift distribution

Suction distribution Control surface area

Aero.

efficiency Allowable weight

Pax

Mission

Unit Re, CL, Mach No.

Max. thickness, chord, span, sweep

Unit Re, Mach No.

Mach No., altitude, allowable weight Cabin width

Figure 3.1: Schematic diagram of the design process envisaged for the LFW.