Aircraft Drag

9.1 Overview

An important task in aircraft design is to make the best possible estimation of all the different types of drag associated with aircraft aerodynamics. Commercial aircraft design is sensitive to the DOC, which is aircraft-drag-dependent. Just one count of drag (i. e., CD = 0.0001) could account for several million U. S. dollars in operating cost over the lifespan of a small fleet of midsized aircraft. This will become increas­ingly important with the increasing trend in fuel costs. Accurate estimation of the different types of drag remains a central theme. (Equally important are other ways to reduce DOC as described in Section 2.1; these are discussed in Chapter 17.)

For a century, a massive effort has been made to understand and estimate drag, and the work is still continuing. Possibly some of the best work on aircraft drag in English is compiled by NACA/NASA, RAE, AGARD, ESDU, DATCOM, Royal Aeronautical Society (RAeS), AIAA, and others. These publications indicate that the drag phenomena are still not fully understood and that the way to estimate aircraft drag is by using semi-empirical relations. CFD (see Chapter 14) is gaining ground but it is still some way from supplanting the proven semi-empirical relations. In the case of work on excrescence drag, efforts are lagging.

The 2D-surface skin friction drag, elliptically loaded induced drag, and wave drag can be accurately estimated – together, they comprise most of the total aircraft drag. The problem arises when estimating drag generated by the 3D effects of the aircraft body, interference effects, and excrescence effects. In general, there is a tendency to underestimate aircraft drag.

Accurate assessments of aircraft mass, drag, and thrust are crucial in the air­craft performance estimation. The also contribute to aircraft stability and control analyses.

Sections 3.2, 3.3, 3.12, and 3.16 define the basic elements of drag. This chapter outlines the considerations and methodology to estimate aircraft drag using worked – out examples.

9.1.1 What Is to Be Learned?

This chapter covers the following topics:

Introduction to aircraft drag Parasite drag

Aircraft drag breakdown structure Theoretical background of aircraft drag Subsonic aircraft drag estimation methodology Methodology to estimate minimum parasite drag (Copmin) Semi-empirical relations to estimate CDpmin Excrescence drag

Summary of aircraft parasite drag (CDpmin)

Methodology to estimate ACDp Methodology to estimate subsonic wave drag Summary of total aircraft drag Low-speed aircraft drag at takeoff and landing Drag of propeller-driven aircraft Military aircraft drag

Empirical methodology for supersonic drag estimation Bizjet example – civil aircraft Military aircraft example

9.1.2 Coursework Content

The coursework task continues linearly. Readers will carry out aircraft component drag estimation and obtain the total aircraft drag.

9.2 Introduction

The drag of an aircraft depends on its shape and speed, which are design-dependent, as well as on the properties of air, which are nature-dependent. Drag is a complex phenomenon arising from several sources, such as the viscous effects that result in skin friction and pressure differences as well as the induced flow field of the lifting surfaces and compressibility effects (see Sections 3.12 and 3.16).

The aircraft drag estimate starts with the isolated aircraft components (e. g., wing and fuselage). Each component of the aircraft generates drag largely dictated by its shape. Total aircraft drag is obtained by summing the drag of all components plus their interference effects when the components are combined. The drag of two isolated bodies increases when they are brought together due to the interference of their flow fields.

The Re has a deciding role in determining the associated skin-friction coeffi­cient, CF, over the affected surface and the type, extent, and steadiness of the bound­ary layer (which affects parasite drag) on it. Boundary-layer separation increases drag and is undesirable; separation should be minimized.

A major difficulty arises in assessing drag of small items attached to an aircraft surface such as instruments (e. g., pitot and vanes), ducts (e. g., cooling), blisters,
and necessary gaps to accommodate moving surfaces. In addition, there are the unavoidable discrete surface roughness from mismatches (at assembly joints) and imperfections, perceived as defects, that result from limitations in the manufactur­ing processes. Together, from both manufacturing and nonmanufacturing origins, they are collectively termed excrescence drag.

The review in Section 2.6 makes clear that accurate total aircraft drag estimation is not possible using analytical or CFD methods. Schmidt of Dornier in the AGARD 256 is categorical about the inability of CFD, analytical methods, or even wind- tunnel model-testing to estimate drag. CFD is steadily improving and can predict wing-wave drag (CDw) accurately but not total aircraft drag – most of the errors are due to the smaller excrescence effects, interference effects, and other parasitic effects. Industrial practices employ semi-empirical relations (with CFD) validated against wind-tunnel and flight tests and are generally proprietary information. Most of the industrial drag data are not available in the public domain.

The methodology given in this chapter is a modified and somewhat simplified version of standard industrial practices ([1], [3], and [7]). The method is validated by comparing its results with the known drag of existing operational aircraft.

The design criterion for today’s commercial high-subsonic jet transport aircraft is that the effects of separation and local shocks are minimized (i. e., compressibility drag almost equal to zero) at the LRC (before the onset of wave drag) condition. At HSC, a twenty-count drag increase is allowed, reaching Mcrit, due to local shocks (i. e., transonic flow) covering small areas of the aircraft. Modern streamlined shapes maintain low separation at Mcrit; therefore, such effects are small at HSC. The dif­ference in the Mach number at HSC and LRC for subsonic aircraft is small – on the order of Mach 0.05 to Mach 0.075. Typically, estimation of the drag coefficient at LRC is sufficient because it has a higher Cf, which gives conservative values at HSC when ACDw is added. The LRC condition is by far the longest segment in the mis­sion profile; the industry standard practice at the conceptual study phase uses the LRC drag polar for all parts of the mission profile (e. g., climb and descent). The Re at the LRC provides a conservative estimate of drag at the climb and descent seg­ments. At takeoff and landing, the undercarriage and high-lift device drags must be added. In the next phase of the project, more detailed drag estimation is carried out.

Supersonic aircraft operate over a wider speed range: The difference between maximum aircraft speed and Mcrit is on the order of Mach 1.0 to Mach 1.2. There­fore, estimation of C0pmm is required at three speeds: (1) at a speed before the onset of wave drag, (2) at Mcrit, and (3) at maximum speed (e. g., Mach 2.0).

It is difficult for the industry to absorb drag-prediction errors of more than 5% (the goal is to ensure errors of <3%) for civil aircraft; overestimating is better than underestimating. Practitioners are advised to be generous in allocating drag – it is easy to miss a few of the many sources of drag, as shown in the worked – out examples in this chapter. Underestimated drag causes considerable design and management problems; failure to meet customer specifications is expensive for any industry. Subsonic aircraft drag prediction has advanced to the extent that most aeronautical establishments are confident in predicting drag with adequate accu­racy. Military aircraft shapes are more complex; therefore, it is possible that predic­tions will be less accurate.

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