Airframe and Powerplant Scaling via Constraint Analysis

The roadmap (if that is not too ambitious a word) offered in the previous section will have, we hope, delivered the reader to a point where a clearly articulated design brief is in place and a conceptual layout has been derived from it. This is now a sketch that makes the major components (enclosures, main lifting surfaces, powerplants, and major structural members) and their connectivities clear, as well as perhaps containing a primitive shape definition of each. What this proto-aircraft lacks most conspicuously now is dimensions. This is the right time to return to the performance constraints introduced in Section 10.1.3 and convert them into numbers that scale the layout to provide a geometry.

The Role of Constraint Analysis

With a topology and rough shape (nondimensional design sketch) in hand, this must now be scaled to provide a first-order iteration toward the final geometry of the vehicle. The computation of this starting point of the preliminary design process is commonly achieved via a constraint analysis procedure, which is the subject of this section.

The scale of the airframe has to be considered in conjunction with the scale (performance) of the propulsion system. To use a somewhat crude intuitive example, if a target climb rate can be achieved with a certain combination of wing area and engine thrust (or power), switching to a more powerful powerplant might allow us to reduce the wing area.[1]

The goal is to determine the feasible region of the wing loading (W/S) versus thrust to weight ratio (T/W) space; the starting design point of the design process will have to live in this region. Two key design decisions can be made once we know the boundaries of this region: the first-order selection of a wing area, and the choice of a powerplant (sometimes we will start with a given powerplant, in which case constraint analysis will simply tell us the feasible wing area range).

  • [1] Of course, both of these changes can have an impact on the overall weight, which may also affect the climb performance - aircraft design is riddled with such complicated webs of interactions.
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