# spl4 - Design Variable Concepts 4 Mar 06 Lab 4 Lecture...

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Unformatted text preview: Design Variable Concepts 4 Mar 06 Lab 4 Lecture Notes Nomenclature I W aircraft weight S reference area (wing area) b wing span c average wing chord c r root wing chord c t tip wing chord λ taper ratio (= c t /c r ) AR wing aspect ratio o wing root bending inertia E Young’s modulus M C C η η P ρ air density elec electric power m electric motor eﬃciency p overall propeller eﬃciency L lift coeﬃcient D drag coeﬃcient c d wing profile drag coeﬃcient CDA 0 drag area of non-wing components δ tip deﬂection o wing root bending moment Design Space Design Variables are numbers whose values can be freely varied by the designer to define a designed object. As a very simple example, consider a rectangular wing with a pre-defined airfoil. It can be defined by deciding on the values of the following two design variables: { b, c } (1) Placing these variables along orthogonal axes defines a design space , or set of all possible design options. Each point in the design space corresponds to a chosen design, as illustrated in Figure 1. c 3 2 1 b 4 8 12 Figure 1: Two-variable design space of a rectangular wing. 1 Variable Set Choice Frequently, an alternative variable set can be defined in terms of the starting variable set. For example, we can define the same design space using the variable set { AR , S } (2) with the following relations translating between the two alternative variable sets: AR = b/c b = √ S × AR radicalBig (3) S = bc c = S/AR Figure 2 compares the two design spaces. Other variable sets are possible for this case, such as { AR, b } , { S, b } , etc. The best variable set is usually the one which gives the simplest or clearest means to evaluate the objective function , or performance of the design, so that the best point in the design space can be selected. For example, a possible design objective for the wing will be to minimize ﬂight power for an electric aircraft. Therefore, the objective...
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## This note was uploaded on 01/28/2012 for the course AERO 16.01 taught by Professor Markdrela during the Fall '05 term at MIT.

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spl4 - Design Variable Concepts 4 Mar 06 Lab 4 Lecture...

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