Page 166 - Aeronautical Engineer Data Book

138 Aeronautical Engineer’s Data Book
9.3 Aircraft design studies
Aircraft design studies are a detailed and itera­
tive procedure involving a variety of theoretical
and empirical equations and complex paramet­
ric studies. Although aircraft specifications are
built around the basic requirements of payload,
range and performance, the design process also
involves meeting overall criteria on, for
example, operating cost and take-off weights.
The problems come from the interdepen­
dency of all the variables involved. In particu­
lar, the dependency relationships between wing
area, engine thrust and take-off weight are so
complex that it is often necessary to start by
looking at existing aircraft designs, to get a first
impression of the practicality of a proposed
design. A design study can be thought of as
consisting of two parts: the initial ‘first approx­
imations’ methodology, followed by ‘paramet­
ric estimate’ stages. In practice, the processes
are more iterative than purely sequential. Table
9.3 shows the basic steps for the initial ‘first
approximations’ methodology, along with some
general rules of thumb.
Figure 9.4 shows the basis of the following
stage, in which the results of the initial
estimates are used as a basis for three alterna­
tives for wing area. The process is then
repeated by estimating three values for take-off

Wing estimate Wing estimate
Wing estimate
area S 1
area S 2 area S 3

Different engine possibilities/combinations
Choose suitable take-off mass

Calculate
performance
criteria
Fig. 9.4 A typical ‘parametric’ estimate stage