Monday, July 20, 2015

Aircraft Performance - Part 1: The Design Crossroads

The following is the first in a five-part series tracing how aircraft performance is calculated, and its relationship to the mission requirements for the aircraft.  This combination of metrics for performance and mission requirements go hand-in-hand throughout the early phases of the airplane design process.
Performance encompasses a broad series of design requirements, from take-off and landing distances, to cruise and maximum speeds, to turn rate and acceleration.  All of these metrics describe how an airplane will fly.

The mission requirements, on the other hand, define how far the airplane is expected to fly, and with what payload.  Both a 737 and a 747, for example, might have many performance metrics that are similar, from take-off distance to cruise speed.  Their range and payload requirements, however, are radically different.  Where these two sets of requirements meet - both the performance metrics and the mission requirements - will define how large the airplane will need to be.

Early in the airplane design process, during conceptual and even into preliminary design, the airplane's performance requirements will typically be portrayed in the form of a constraint diagram.  This diagram will plot the thrust loading, or thrust-to-weight ratio on the vertical axis, and the airplane's wing loading or the weight divided by the wing area on the horizontal axis.  On this diagram - for a given airplane configuration - the developer will plot lines that define the thrust loading and wing loading combinations that meet the minimum performance objectives laid out for that aircraft.  Collectively, these lines will identify the boundary for the solution space, defining all possible thrust loading and wing loading combinations that meet all of these performance objectives.

Within this solution space, the developer will typically down-select to the configuration with the minimum thrust-to-weight ratio, and the highest wing loading (minimum wing area) that satisfies all of the design criteria.  This is because the customer will seldom pay extra for additional performance that they did not require.  A larger engine or a larger wing would require additional raw materials to produce, driving up the unit cost with no appreciable increase in what the customer is willing to pay.  Moreover, if forced to choose between an airplane with a higher thrust-to-weight ratio (larger engine), and one with a lower wing loading (larger wing), the developer will usually gravitate towards the solution with the larger wing.  Pound-per-pound, adding wing area will be far less expensive than adding a larger engine.

Finally, the resulting design combination - the airplane's configuration, thrust-to-weight ratio, and wing loading - will be exercised in a simulated mission analysis to determine the size of the resulting aircraft.  As we will see in the following segments in coming weeks, this design effort will be an iterative process.

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