Phases of aircraft design and mission requirements

Why it matters

Every aircraft starts as a list of requirements: carry this payload, this far, this fast, from runways this short, at an acceptable cost. How those requirements are written, and how the design moves from a sketch to a certified machine, decides whether the project succeeds. Most of the life-cycle cost of an aircraft is committed by decisions made in the first, cheapest phase of design, so getting the early phase right matters more than anything that follows.

Key ideas

Requirements first. A design starts with a specification (also called the requirements document or RFP — request for proposal). It states:

• Payload — passengers, baggage, cargo or weapons, with their masses.
• Range or endurance — the design mission distance or time, plus fuel reserves required by the rules.
• Speeds and altitudes — cruise Mach number and altitude, maximum speed, ceiling.
• Field performance — take-off and landing field lengths at a stated airport elevation and temperature.
• Climb and manoeuvre — rate of climb, one-engine-inoperative climb gradient, sustained turn load factor.
• Certification basis — for example CS-25 / FAR Part 25 for large transport aircraft, CS-23 for small aeroplanes, or a military standard.
• Cost, maintenance and environmental targets — noise, emissions, operating cost per seat-km.

The mission profile. Requirements are turned into a mission profile: a sequence of segments such as engine start and taxi, take-off, climb, cruise, loiter, descent, landing, plus a reserve segment (diversion to an alternate airport and holding). Each segment burns fuel and the total sets the fuel weight, which drives the take-off weight. A transport has a simple profile; a fighter may have combat and dash segments.

The three phases of design.

1. Conceptual design — answers "what will it look like and roughly how big is it?" Configuration sketches, first weight estimate from historical trends, wing loading and thrust-to-weight selection, constraint diagram, initial sizing, rough cost. Many alternatives are examined quickly; the configuration is fluid.
2. Preliminary design — the configuration is frozen and the major disciplines go deeper: wind-tunnel testing and CFD, structural layout and finite-element models, control-law design, systems layout, detailed weight and balance. The output is a design good enough to commit the company to a launch decision, with performance guarantees.
3. Detailed design — every part is designed for manufacture: drawings or 3-D models, tooling, fasteners, systems routing, test articles. Ground and flight testing then lead to certification.

The process is iterative, not linear. A change found in preliminary design (say, the wing is heavier than estimated) feeds back to sizing, which changes fuel, which changes the wing again. This is called the design spiral.

Trade studies. A trade study varies one design parameter (aspect ratio, cruise Mach, engine bypass ratio, number of seats) and re-sizes the aircraft for each value, plotting a figure of merit such as take-off weight, fuel burn or cost. The optimum is rarely at an extreme; it is a compromise between, for example, induced drag (favours high aspect ratio) and wing weight (favours low aspect ratio).

Figures of merit. Designers judge alternatives by take-off gross weight, empty weight, fuel burn per seat, direct operating cost and, increasingly, emissions. Lower take-off weight is usually a good proxy for lower cost.

Formulas

The mission's central performance relation is the Breguet range equation for a jet in cruise at constant L/D and V:

R = (V / c) · (L/D) · ln(Wi / Wf)

• R — range, m
• V — true airspeed in cruise, m/s
• c — thrust-specific fuel consumption on a weight basis (weight of fuel per unit time per unit thrust), 1/s. A value quoted in 1/h must be divided by 3600.
• L/D — lift-to-drag ratio, dimensionless
• Wi, Wf — weight at start and end of cruise, N (only the ratio matters, so kg also works)

If TSFC is given on a mass basis, c_m in kg/(N·s), use c = g · c_m.

Applies to steady level cruise with constant V, L/D and c (cruise-climb). Rearranged, it gives the cruise weight fraction:

Wf / Wi = exp(−R · c / (V · L/D))

For a propeller aircraft the corresponding form is R = (η_p / c_p) · (L/D) · ln(Wi / Wf) with c_p the power-specific fuel consumption on a weight basis (1/m, i.e. N of fuel per J of shaft work) and η_p the propeller efficiency.

Level-flight lift balance, used throughout the mission:

L = W = ½ · ρ · V² · S · C_L

• ρ — air density, kg/m³; S — wing reference area, m²; C_L — lift coefficient.

Worked examples

Example 1 (standard): range from a mission fuel fraction. Given: V = 230 m/s, L/D = 16, c = 0.6 h⁻¹, cruise-end weight 85% of cruise-start weight.

1. Convert c: c = 0.6 / 3600 = 1.667 × 10⁻⁴ s⁻¹.
2. V / c = 230 / 1.667 × 10⁻⁴ = 1.38 × 10⁶ m.
3. Wi / Wf = 1 / 0.85 = 1.1765, ln(1.1765) = 0.1625.
4. R = (V / c) · (L/D) · ln(Wi/Wf) = 1.38 × 10⁶ × 16 × 0.1625 = 3.588 × 10⁶ m.
5. R ≈ 3590 km.

Example 2 (GATE level): fuel needed for a specified mission. A requirement asks for a 5000 km cruise. The design cruises at V = 240 m/s with L/D = 17 and c = 0.55 h⁻¹. The mass at the start of cruise is 60 000 kg. Find the cruise fuel mass.

1. c = 0.55 / 3600 = 1.528 × 10⁻⁴ s⁻¹.
2. Exponent: R · c / (V · L/D) = 5.0 × 10⁶ × 1.528 × 10⁻⁴ / (240 × 17) = 764 / 4080 = 0.1872.
3. Wf / Wi = exp(−0.1872) = 0.8293.
4. Fuel fraction burned = 1 − 0.8293 = 0.1707.
5. Fuel mass = 0.1707 × 60 000 kg = ≈ 10 240 kg of cruise fuel (taxi, climb, descent and reserves come on top).

Notice how the requirement (5000 km) turns directly into a weight fraction — this is how the mission drives sizing in the next topic.

Common mistakes

• Using c in h⁻¹ with V in m/s: the range comes out wrong by a factor of 3600.
• Using a mass-based TSFC (kg/(N·s)) without multiplying by g.
• Dropping L/D from the Breguet equation — range is proportional to L/D.
• Writing ln(Wf/Wi), which gives a negative range.
• Treating the phases as a one-way sequence; design is iterative and the conceptual estimates are revised.
• Forgetting reserve fuel and the non-cruise segments when converting range into fuel.
• Confusing a requirement (what the customer needs) with a design choice (how you meet it).

For GATE AE

GATE treats design mostly through performance: expect Breguet range and endurance numericals, cruise weight fractions, lift-equals-weight problems and unit conversions of fuel consumption. Conceptual questions ask which activities belong to which design phase. Practise converting TSFC units, rearranging the range equation for the weight ratio, and checking whether an answer is physically sensible (a transport's cruise fuel fraction is typically 0.1–0.4).

Quick check

1. Name the three phases of aircraft design in order.
2. Which phase freezes the configuration?
3. A jet has c = 0.5 h⁻¹. What is c in s⁻¹?
4. If L/D doubles with everything else the same, what happens to the Breguet range?
5. What is a trade study?

Answers: 1. Conceptual, preliminary, detailed. 2. Preliminary design (the configuration is frozen at its start). 3. 1.389 × 10⁻⁴ s⁻¹. 4. It doubles. 5. Varying one design parameter, re-sizing the aircraft for each value and comparing a figure of merit to find the best compromise.