Aircraft Performance

13.1 Overview

This chapter assesses whether the aircraft being configured, thus far, meets the FAR and customer requirements given in the form of specifications. Coursework fol­lows linearly from the mock market survey (see Chapter 2). Specification require­ments addressed in this chapter include aircraft performance to meet the (1) TOFL, (2) LFL, (3) initial rate of climb, (4) maximum speed at initial cruise (especially for civil aircraft design), and (5) payload range. Chapter 16 computes the aircraft DOC, which should follow the aircraft performance estimation.

Aircraft performance is a subject that aeronautical schools offer as a separate course. Therefore, to substantiate the FAR and customer requirements, this chap­ter addresses only what is required – that is, the related governing equations and computational examples associated with the five substantiation parameters listed previously. Substantiation of the payload range requires integrated performances of climb and descent that show fuel consumed, distance covered, and time taken for the flight segments. Integrated climb and descent performances are not specification requirements at this stage; therefore, their detailed computational examples are not provided. Instead, the final results in graphical form carry out the payload-range estimation. It is suggested that readers refer to appropriate textbooks for details on this topic. The turboprop example is not worked out but there is sufficient informa­tion to compute it similarly.

The remainder of the book after this chapter (except Chapter 16) presents infor­mation that aircraft designers should know and apply to their configurations. These topics may comprise the coursework of a second term following the finalized con­ceptual study in the first term. The discussion in Section 13.7 is useful to readers.

13.1.1 What Is to Be Learned?

This chapter covers the following topics:

Section 13.2: Preliminary information on aircraft performance

Section 13.3: Engine performance graphs

Section 13.4: Pertinent aircraft performance equations

Section 13.5: Performance equations to substantiate Bizjet aircraft capabilities

Section 13.6: Performance equations to substantiate AJT aircraft capabilities

Section 13.7: Discussion in summary form

13.1.2 Coursework Content

Readers perform the following steps for their design projects:

Step 1: Generate the appropriate engine performance graphs from the non­dimensional graphs provided in Chapter 10.

Step 2: Using the engine thrust thus obtained, compute the aircraft perfor­mances of the sized Bizjet and AJT as coursework exercises. (The instructor’s assistance may be required to compute integrated climb, descent, and specific-range performances.)

Step 3: If aircraft performance requirements are not met, then iterate the aircraft-configuration, sizing, and engine-matching exercises until they are. The spreadsheet method is helpful for the iterations.

13.2 Introduction

The final outcome of any design is to substantiate the performance it is intended to do. In the conceptual design phase, aircraft performance substantiation must be con­ducted mainly for those critical areas specified by the FAR and customer require­ments; a full aircraft performance estimation is conducted subsequently (it is beyond the scope of this book). All worked-out aircraft performance estimations (i. e., Bizjet and AJT) use the standard day. Non-ISA-day performance computations are calculated in the same way using non-ISA-day data.

The sizing exercises in Chapter 11 demonstrate a rapid-performance method to generate relationships between wing-loading (W/Sw) and thrust-loading (Tsls/W) to obtain the sizing point that simultaneously satisfies the requirements of the TOFL and LFL, initial rate of climb capability, and maximum speed at initial cruise. The aircraft-sizing point gives the installed, maximum sea-level takeoff static thrust, TSLSjnSTalled, of the matched engines. Chapter 10 presents the generic, uninstalled-engine performances of rubberized engines in nondimensional form, from which the installed-engine performances are obtained.

This chapter develops available engine performance in terms of installed thrust and fuel-flow rates at various speeds and altitudes at the power settings of takeoff, maximum climb, and maximum cruise ratings at standard day, matched for the sized aircraft under study. Applying the installed-engine data, the chapter continues with more accurate computations of aircraft performance to substantiate requirements of the TOFL and LFL, initial rate of climb, maximum speed at initial cruise, and payload range. At this point, it may be necessary to revise the aircraft configura­tion if performance capabilities are not met. If the aircraft performance indicates a shortfall (or an excess) in meeting the requirements, the design is iterated for improvement. In coursework, normally one iteration is sufficient.

Finally, at the end of the design stage, the aircraft should be flight-tested over the full flight envelope, including various safety issues, to demonstrate compliance.