Lesson 1 · 35 min

Inside the Conceptual Design Process

Module 1 ended with the design wheel: requirements, concept, analysis, sizing, and around again. This lesson opens the wheel up into the steps a conceptual designer actually takes, shows where each one is covered in this course, and explains why the few months of conceptual design decide most of what an aircraft will cost.

Learning objectives

List the steps of conceptual design in order, from requirements to a sized baseline, and explain where the loops are.
Distinguish rubber-engine sizing from fixed-engine sizing, and compute the thrust a rubber engine must give.
Describe the conceptual, preliminary and detail design phases, and the reviews that close them.
Explain why design freedom falls and committed cost rises fastest in the earliest phase.

From the wheel to a process

The design wheel says that design goes round in circles. It does not say where to start, or what to do first. There is a usual order in which a conceptual designer works, and Figure 1.1 lays it out as a flowchart. Notice two things about it. First, the aircraft is sized twice: once from a sketch and historical data, before anything is drawn properly, and again after the layout has been analyzed. Second, the arrows going back up are as important as the ones going down.

Figure 1.1 The conceptual design process, after Raymer. Select a step (click it, or use Tab and Enter) to read what happens there and where this course covers it. The dashed arrow is the main loop: if the analyzed design does not meet the requirements, or a trade study finds a better one, the layout is revised and analyzed again.

The first pass through the top of the chart can take a single afternoon: a rough sketch, a weight estimate from historical fractions (Module 1, Lesson 7), and a guess at the wing and engine size. The value of the quick pass is that it shows at once whether the requirements are sensible. A requirement that leads to an absurd gross weight is better discovered on the first day than in the third month.

Rubber engines and fixed engines

Early in conceptual design, the engine is not yet chosen. The answer is the rubber engine: the designer assumes an engine of the right type that can be scaled to exactly the thrust (or power) the aircraft needs, with its weight, size and fuel consumption scaled with it. Sizing then finds the wing area and the engine size together, so that every requirement is just met.

Later, or whenever an existing engine must be used, the designer switches to fixed-engine sizing: the thrust is given, and the question becomes what the aircraft can do with it. Usually the fixed engine is a little too big or too small, so one requirement is exceeded and the design is no longer exactly optimal. The trade study then asks whether a new engine is worth its development cost.

Thrust the rubber engine must give

\[ T_{\text{total}} = \left(\frac{T}{W}\right)\colW{\Wo}\,g, \qquad T_{\text{engine}} = \frac{T_{\text{total}}}{N} \]

\(\TW\) is the takeoff thrust-to-weight ratio from the sizing (Module 5 shows where it comes from), \(\Wo\) in kg, \(N\) the number of engines.

Example 1.1 — Rubber engine or fixed engine?

A first sizing of a twin-engine transport gives \(\Wo = 72\,000\ \text{kg}\) and \(\TW = 0.30\). (a) What takeoff thrust must each rubber engine give? (b) Two existing engines are available: engine X with \(120\ \text{kN}\) and engine Y with \(98\ \text{kN}\) each. What \(\TW\) does each give, and what would you report?

Show solution

\[ T_{\text{total}} = 0.30(72\,000)(9.81) = 211\,900\ \text{N}, \qquad T_{\text{engine}} = \frac{211\,900}{2} = 105.9\ \text{kN} \] \[ \text{X: } \frac{T}{W} = \frac{2(120\,000)}{72\,000(9.81)} = 0.340, \qquad \text{Y: } \frac{T}{W} = \frac{2(98\,000)}{72\,000(9.81)} = 0.277 \]

Engine Y is about 8% short of the thrust the requirements need: some requirement (probably field length or one-engine-out climb, Lesson 4) would not be met. Engine X has 13% more thrust than needed, so it meets every requirement, but its extra weight, drag and cost would make the aircraft heavier than the rubber-engine design. A rubber engine would be X scaled to \(105.9/120 = 0.88\) of its thrust. The report to the program: X works with a weight penalty; a derivative of X with about 12% less thrust would be ideal.

Phases, reviews and the baseline

Conceptual design ends when the team has a baseline: one configuration, sized and analyzed well enough that the company (or the customer) can decide whether to go on. The phases that follow, and the reviews that close them, are summarized below. The review names are those of systems engineering practice; companies use their own names, but the idea of a formal gate between phases is universal.

The phases of an aircraft program
Phase	Question answered	Typical team and tools	Ends with
Requirements	What does the customer need, and what will the rules allow?	Market analysis, customer discussions, operations analysis	A requirements document; a System Requirements Review
Conceptual design	What will it look like, weigh and cost, and can it meet the requirements?	A small team; statistical and simple analytical methods; many configurations, quick trades	A sized baseline, often a proposal; the launch or go-ahead decision
Preliminary design	Will the baseline really work, in depth?	Specialist groups in every discipline; wind-tunnel tests, finite elements, CFD; the outer shape lofted precisely	A frozen configuration; a Preliminary Design Review
Detail design	How is every part made, joined and inspected?	Hundreds to thousands of engineers; drawings or 3D models of every part, tooling, production planning	Released design; a Critical Design Review; first flight and certification testing

Large civil programs usually take five years or more from launch to the first delivery. The conceptual phase before launch can be short, a few months, or can continue for years as a company studies a market and waits for the right engine or the right customer.

Freedom, knowledge and committed cost

At the start of a program the designers know very little about their aircraft, but they can change anything. By the time they know a great deal, almost nothing can be changed cheaply. Meanwhile, the decisions taken early (the configuration, the wing loading, the engine, the materials) fix most of the cost of building and operating the aircraft long before that money is actually spent. Figure 1.2 shows the usual picture.

Figure 1.2 Design freedom, knowledge about the design, cost committed and cost spent through a program. The curves are illustrative: their shape follows the figures common in systems engineering texts, not data from a particular program. Move through the program to read the values.

The lesson for a conceptual designer is not that early decisions are final; it is that they are expensive to reverse. Changing a wing loading in the second week costs an afternoon of calculation. Changing it after the wing is in production can cost the program. That is why so much effort goes into requirements and trade studies (Lessons 2 to 7), and why even a quick first sizing is worth doing carefully.

Check your understanding

Key takeaways

Conceptual design runs from requirements through an initial sketch and a first sizing, a layout, analysis in every discipline, and sizing with trade studies, looping back until a baseline meets the requirements.
A rubber engine is scaled to exactly the thrust needed, \(T = (\TW)\,\Wo g\); a fixed engine is a real engine, and the aircraft is then sized around it.
Phases end at formal reviews: requirements, conceptual (baseline and go-ahead), preliminary (frozen configuration), detail (every part released).
Most of the cost is committed in the earliest phases, when knowledge is lowest and changes are cheapest.
Next, Lesson 2 looks at the start of the chart: where requirements come from.