Operational Requirements

Every aircraft has a certain operational environment, including aspects of flight and ground operations, that it is designed to serve in throughout its lifetime. For example the operational requirements of a fighter jet are much more strenuous than those of a commercial airliner. The flight regime is broadly defined by the range of different flight speeds and altitudes called the flight envelope. Within this range lies the so-called design point, which is the operational environment in which the aircraft is expected to spend most of its time in. An example plot of a typical flight envelope is shown in Figure 1. The outline of the envelope defines the limit of performance for a specific aircraft configuration. The left edge defines the minimum speed required to keep the aircraft flying at a certain altitude. The small dip in the curve at around Mach 1.0 denotes the increase in drag caused by small supersonic pockets close to the leading edge of the airfoil. Supersonic flow is inherently terminated by a shock wave that causes an increase in fluid pressure. At speeds around Mach 1.0 these shockwaves are still located on the airfoil surface and therefore exacerbate the adverse pressure gradient across the suction surface, leading to premature boundary layer separation and higher pressure drag. The top of the curve marks the region where the minimum level speed is equal to the maximum speed that can be sustained by the aircraft’s engine and structural capability. The declining curve on the right indicates the envelope where speed is limited first by the power of the engines and then by the weight of the aircraft i.e. as the aircraft speed increases so do the loads on the airframe and therefore the material required (mass) to sustain these loads. The flight envelope in Figure 1 can be drawn for any aircraft and will be different depending on the unique role e.g. commercial transport, freight, fighter, bomber etc. Today the different roles of aircraft are no longer as clear-cut since aircraft are expected to fulfil multiple roles (e.g. freight and commercial transport) for economic reasons.

Figure 1. Flight envelope of supersonic aircraft (1)

The operating environment influences the overall shape of the aircraft which can broadly be broken down into three design segments: aerodynamic shape of wings, fuselage and controlling surfaces; the choice of propulsion; and the structural layout. Naturally, for a given design point and payload there will be conflicting requirements and optimal solutions for each area individually. However, an important point to realise is that an aircraft design will only be successful if these three design factors are dealt with concurrently i.e. the optimal compromise must be found.

The commutative property is valid for the above equation i.e. the shape of the aircraft is defined by the operational requirements, similarly the shape given to an aircraft restricts the functions that the aeroplane is capable of. This means that in the early parts of the design process the engineers need to be aware what variables can be fixed and where flexibility can be maintained to limit limitations if the design environment changes.

This picture is often complicated by a additional demands that have nothing to do with the flight envelope. Thus under the given flight envelope the engineers deal with added issues of economic requirements, manufacturability, passenger ergonomics and safety, airfield requirements, environmental and noise regulation. For example, an airline operator wishes to maximise profit on each flight and therefore a major incentive for commercial aircraft manufacturers’ is to cater to this need and not the goal of engineering state-of-the-art technology. Freight and travel airlines are in the business of making money from the payload they transport from A to B. The higher the profit per kg of payload carrier the better for the airline. In this respect the dry mass of the aircraft is of critical importance for profitability. The lower the dry mass of the airline the more payload can be carried over a certain distance for a given amount of fuel. Thus not only is the fuel efficiency improved (lower costs) but the revenue is also increased by carrying more goods. This is one of the reasons why lightweight composite materials are such a big driver for future aircraft design.

References

(1) Stinton, D. The Anatomy of the Airplane. 2nd Edition. Blackwell Science Ltd. (1998).