Egbert Torenbeek

Cruise performance and range prediction reconsidered

Abstract A unified analytical treatment of the cruise performance of subsonic transport aircraft is derived, valid for gas turbine powerplant installations: turboprop, turbojet and turbofan powered aircraft. Different from the classical treatment the present article deals with compressibility effects on the aerodynamic characteristics. Analytical criteria are derived for optimum cruise lift coefficient and Mach number, with and without constraints on the altitude and engine rating. A simple alternative to the Breguet range equation is presented which applies to several practical cruising flight techniques: flight at constant altitude and Mach number and stepped cruise/climb. A practical non-iterative procedure for computing mission and reserve fuel loads in the preliminary design stage is proposed.

Design for performance

After laying down the broad outlines of the general arrangement and finalizing the design of the fuselage, the designer’s next step will be to decide on the type of engine to be installed and the size of the wing. Both have a direct effec* on performance and operating costs.

Airplane weight and balance

The sensitivity of airplane performance and operating economy to the empty weight is discussed and the value of weight-saving is demonstrated.

Evaluation and presentation of a preliminary design

The presentation of a preliminary design comprises layout drawings, a summary of the principal characteristics and geometry, and the results of a preliminary estimate of performance, flight characteristics and operating costs.

The undercarriage layout

The basic requirements for the design of an undercarriage are that it must be capable of absorbing a certain amount of energy, both vertically and horizontally, and that during taxying, liftoff and touchdown no other part of the aircraft will touch the ground. No instabilities must occur, particularly during maximum braking effort, crosswind landings and high-speed taxying. In addition, the undercarriage characteristics must be adapted to the load-carrying capacity of the airfields from which the aircraft is intended to operate. This chapter indicates how these requirements can be translated into an acceptable initial choice of the undercarriage layout, without going into the details of its structural design.

Preliminary tailplane design

The aerodynamic design of the tailplane is based on many specific requirements regarding its functions, which are to provide equilibrium in steady flight (trim), to ensure that this condition is stable and that disturbances are well damped, and to generate aerodynamic forces for maneuvering the aircraft. The control forces involved must be acceptable to pilots, with the airplane both in trimmed and out-of-trim conditions.

The general arrangement

A sound choice of the general arrangement of a new aircraft design should be based on a proper investigation into and interpretation of the transport function and a translation of the most pertinent requirements into a suitable positioning of the major parts in relation to each other. The result of this synthetic exercise is of decisive importance to the success of the aircraft to be built. However, no clear-cut design procedure can be followed and the task of devising the configuration is therefore a highly challenging one to the resourceful designer.

History of Aviation

The discovery of hydrogen gas in the 18th century led to the invention of the hydrogen balloon, at almost exactly the same time that the Montgolfier brothers rediscovered the hot-air balloon and began manned flights.[1] Various theories in mechanics by physicists during the same period of time, notably fluid dynamics and Newton’s laws of motion, led to the foundation of modern aerodynamics, most notably by Sir George Cayley.

An appreciation of subsonic engine technology

This survey presents some background information which is required when an engine has to be chosen for a new subsonic aircraft design.

Analysis of aerodynamic and operational characteristics

The object of the design synthesis process dealt with in the previous chapters is to achieve the goals laid down in the design specification. The first cycle of the iterative design process will be concluded with an analysis of the operational characteristics for the purpose of investigating to what extent the design requirements have been met. Some general comments on the prediction of aerodynamic characteristics are made in this chapter. Definitions and subdivisions of the drag according to several schemes are discussed. The choice of operational limit speeds and the determination of n-V diagrams are then briefly reviewed. A procedure to analyze the flight profile, reserve fuel quantity and payload-range characteristics is given, followed by some general aspects of climb and field performance.