Monday, August 10, 2015

Aircraft Performance - Part 4 - Energy Maneuverability

Energy-Maneuverability theory arose during the latter 1960s as a means to evaluate air combat capabilities in a more holistic sense.  Prior to this, fighter jet requirements had been specified on the basis of a few discrete performance parameters - parameters which often had little to do with the success or failure of an aircraft in a combat setting.  Criteria such as maximum speed, maximum altitude, or time to altitude were not uncommon.

Energy-Maneuverability was the brainchild of then Major John Boyd (USAF) and Thomas P. Christie, a mathematician and civilian consultant to the Air Force.  E-M theory provided a means to portray aircraft capability - and even relative capability comparing two different designs - by utilizing isoplots of specific excess power, plotted on a graph portraying speed or Mach number on the one hand, and turn rate on the other.

In a typical E-M diagram, we will see the airplane's speed or Mach number plotted on the horizontal axis, and the turn rate plotted on the vertical axis.  Within this plot, will be portrayed the maneuver envelope for the airplane.  The left-hand bound will typically designate the lift-limit of the aircraft - as defined by its stall characteristics.  The upper bound will designate the airplane's maximum load factor or g-limit.  And the right-hand bound will be defined by a dynamic pressure limit that usually designates either an airframe limitation, or an engine limitation.  Where the lift limit and the g-limit converge, will be the airplane's maximum instantaneous turn rate.  Note that the speed or Mach number where this occurs will be identified as the "corner speed" of the aircraft.
Within this envelope will be plotted a series of isocontours for specific excess power.  This is the amount of energy that the airplane has available to either accelerate or climb.  The contour where specific excess power equals zero will define the maximum sustained turn rate for the aircraft.  This is a contour - not a single value.  It will change depending on altitude and Mach number.  Above this maximum sustained turn rate, specific excess power will be negative - meaning that the airplane will loose either speed or altitude when it flies in this regime.  Below this line, specific excess power will be positive - meaning that the airplane can accelerate or climb.

If we combine the E-M diagrams for different aircraft, we can draw a direct comparison for how each aircraft would fare in a dissimilar air combat exercise.  The example shown here is from published sources, comparing between the F-4J Phantom and the A-4M Skyhawk.  Note that at the top of the diagram, it describes the fuel, payload and weight of each aircraft, as well as the altitude at which this comparison was made.  We can see that the limit load factor or g-limit for each aircraft is very similar.  As expected, however, the F-4 has a higher maximum speed.  Conversely, we can see that the lift-limit, or stall speed for the A-4 is significantly lower than for the F-4.

The zero-specific excess power isocontour identifies the maximum sustained turn envelope for each aircraft.  If we draw a line connecting the equivalent isocontours for the two aircraft, we can begin to understand where the pilot of aircraft would prefer to engage his opponent.  The A-4 pilot will seek to draw his opponent into a low-speed turning engagement, where his aircraft will have the upper hand.  The Phantom pilot, on the other hand, would be advised to retain his speed advantage and to engage his opponent on the lower right side of the envelope, where he will have a specific excess power advantage.

There are of course a number of other factors which can effect the outcome of an engagement.  Control authority is one such example.  During the Korean War, on paper, the MiG-15 should have had a clear advantage in a turning engagement over its U.S. F-86 Sabre counterpart.  However, the MiG of that day was still employing a mechanical control system featuring push-rods and pulleys, whereas the F-86 was the first fighter to employ a hydraulically boosted control system.  This meant that the MiG pilot had to exert tremendous force at higher g-loadings to control his aircraft.  This was further exacerbated by the excessive flexibility of the MiG wing structure, which could lead to aileron reversal at high g-loading - resulting in loss of control.  Other examples of the central role of control authority can be seen in the decision to increase the size of the tail surface between the YF-16 and later F-16 production aircraft.  A similar phenomena occurred between the YF-22 and the F-22 production fighter.

Weapons systems can also play an important role in the outcome of an engagement.  During the Falklands War in 1982, the British armed forces were at a distinct advantage in part due to the all-aspect heat-seeking missiles at their disposal.  The Argentine Air Force, in contrast, had to rely on older generation missiles that required the pilot to maneuver before he could fire the missile, so that it could home in on the hot exhaust of his opponent's engine.  Similarly, the advent of high-off-boresight heat seeking missiles, in combination with a helmet mounted sight, potentially placed U.S. aircraft at a distinct disadvantage vis-a-vis their Soviet counterparts during visual range engagements by the latter 1980s.  This "missile gap" was only recognized after the fall of the Berlin Wall, when U.S. aircraft had the opportunity to interact with former East German Luftwaffe fighters.  The advantage that this technology provided to the East German MiG-29s was overwhelming - leading the United States to develop the AIM-9X together with a helmet-mounted sight produced to close capability gap.

And of course, modern sensors have similarly altered the conduct of war, with the introduction of AWACS being the most noteworthy example of this impact.

Nonetheless, pilot training remains essential, even today.  Despite all of the electronic wizardry, the pilot remains the chief tactician behind any aerial engagement.  It is up to the pilot to select the strategy that will place the enemy within the lethal envelope of his own weapons, without unduly exposing himself to his adversary's weapons.  This is a role that no machine can substitute for.

Finally, there will be some that will question whether energy maneuverability, and maneuverability in general, are still relevant in an age of beyond visual range (BVR) missiles.  As demonstrated in actual air combat experience, however, the majority of air-to-air engagements - even in a missile age - will still occur within visual range.

A study commissioned by the U.S. Air Force in 1986, combining data on all known U.S. and foreign missile kills, found that only four kills could actually be ascribed to BVR missiles fired from beyond visual range.  That's 4 out of 407 successful missile kills.  Even those kills made by radar-guided, BVR missiles it turned out, were predominantly made within visual range.

In the decades since, sensor technology has continued to advance.  During the 1991 Gulf War, 16 out of 38 allied kills were made from beyond visual range.  However, this still means that the majority of air-to-air kills, 58% to be precise, occurred within visual range.  Similarly, in the post-Gulf War era, only 3 out of 11 successful kills by U.S. aircraft were made from beyond visual range.

 Typical BVR engagement profile (Herbst 1983)
Studies into the dynamics of air combat have also affirmed that even in BVR engagements, "careful power management" is essential to maximizing a fighter's kill-to-loss ratio.  Maintaining an energy advantage therefore remains as essential today as it was when E-M theory was first developed in the 1960s.