Sunday, November 1, 2015

Alternative Fighter Performance Metrics

In follow-up to my previous series of posts regarding Aircraft Performance, I will be focusing on some of the modern fighter performance metrics - which began to emerge in the latter 1980s - and which may be less familiar to many outside of the aircraft performance community.

An example E-M Diagram, comparing between the F-4 and A-4.
To recap, in the early years of jet aviation, the majority of fighter performance measures used to evaluate designs were single design-point metrics.  Things like maximum speed, or time-to-altitude measures.  This changed during the 1960s with the emergence of Energy-Maneuverability Theory.  E-M diagrams, plotting isocontours of specific excess power against the speed and turn rate of each aircraft, allowed for the entire flight envelope to be graphically portrayed, and compared.  This was part of a revolution in how jet fighters were evaluated and designed.

In the latter 1980s, however, a new generation of computers made possible a new breed of fighter agility and performance measures - measures which combined different elements of the airplane's flight characteristics into a single metric.  Measures such as Combat Cycle Time (CCT), Dynamic Speed Turn (DST) plots, and Relative Energy State (RES) emerged as alternative figures of merit - and, according to their proponents, superior figures of merit.

Some insight into what these new measures bring to the table can be gleaned by examining the calculation of one such metric: Combat Cycle Time (CCT).  Combat Cycle Time was first proposed in 1988 by B.F. Timrat.  CCT represents the time that it takes for an aircraft to perform a turn (usually 180-degrees), and recover the energy lost in making that turn.  This can be subdivided into steps as follows:
Optimum trajectory analysis can reduce CCT by over 20-percent.
Early CCT studies assumed, erroneously, that each aircraft being analyzed would exercise the full limits of its "doghouse plot".  In practice, however, the minimum time to turn and recover energy is often found at somewhat lower turn rates that more effectively conserve energy.  An optimum trajectory analysis is therefore needed to draw a true comparison between different designs.

An example of one such comparison is provided in a study undertaken by Ryan and Downing (1995).  This study compared the performance of the F/A-18, X-29 and X-31 in a simulated 180-degree turn.  The starting conditions were fixed to Mach 0.8 and 15,000 ft altitude for each aircraft, and each aircraft was restricted to no more than a 2,500 ft change in altitude.  To make the comparison more consistent, each aircraft was restricted to a maximum 7g load factor, and optimum trajectory analysis was applied to ensure that each aircraft achieved a minimum CCT.
Comparing between the three aircraft, the shortest CCT was achieved by the F/A-18, the airplane which also had the highest instantaneous turn rate.  The next shortest CCT, however, was achieved by the X-31, the airplane with the lowest instantaneous turn rate.  What this implies is that CCT, like many of the other modern performance measures, represents a combined maneuver metric, incorporating elements of both turn rate and acceleration which could not be directly portrayed by any of the more simple, legacy metrics.

Despite the obvious utility of such metric, the modern maneuver metrics have not superseded their more simple predecessors.  A large part of this is due to the complexity and computational cost of these metrics.  Whereas single design point evaluations of time-to-altitude or maximum turn rates are relatively straightforward and inexpensive to calculate, the modern fighter agility metrics require a far more complex computational model that simulates the transient behavior of the aircraft across a range of possible maneuver and loading conditions.
Modern agility metrics provide therefore provide insight into the combined effect of the components that go into a maneuver, but do so at an added computational cost.  These metrics are also restricted to individual evaluations against a single maneuver, assuming that each individual pilot would choose to perform the same equivalent maneuver even when flying different aircraft.  This is in contrast to E-M Diagrams, which plot the excess energy of each aircraft but make assumptions regarding how each pilot would choose to utilize that energy.  The modern fighter agility metrics therefore compliment, but not replace, traditional performance measures

In summation, the underlying utility of each performance metric – relative to its computational cost – is a subjective measure.  More detailed metrics will require more complex and accurate computational models to calculate.  Single design-point metrics are still among the easiest to calculate – and still provide an inexpensive first insight into aircraft performance.  E-M Diagrams still provide a comprehensive view of performance – across the entire flight envelope – without pre-supposing what form of maneuver each pilot or each aircraft might undertake.  They will therefore continue to be an invaluable tool for many years to come.  More modern agility metrics offer additional insight into the combined effects of different maneuver components, but require significantly more computational investment to undertake.  And in the end, fully featured, piloted simulations will remain the gold-standard for assessing the merits of new technologies and alternative designs.  They will continue to offer the most complete assessment tool available, short of building and flying each individual aircraft.

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