Fighter Combat

APPENDIX 409Outside the zero-Ps lines are negative PS values, which indicate the rate atwhich energy (altitude and/or airspeed) will be lost while pulling that loadfactor at a given combination of speed and altitude. Inside the zero-P senvelope are positive PS values, which give the rate at which energy may begained (by a climb and/or acceleration) under those conditions. The conditionsunder which the given load factor can just be sustained are thosewhich lie directly on the respective zero-P s lines. Once the sustained-Gcapability of a fighter is known at a given speed, its sustained turn rate andradius under those conditions may be determined mathematically or byFigure A-2.Note that for this supersonic fighter the zero-P s curve has two peaks. Athigh altitudes and fairly low G levels (less than about 5 G for this aircraft),the highest peak, and consequently the greatest sustained G, is found inthe supersonic speed range. At low altitudes, where sustained-G levels arehigher, however, the maximum sustained G is usually achieved at speedsnear the subsonic peak. Obviously, subsonic fighters would generategreatest sustained G in the subsonic region at all altitudes.Sustained G, however, is generally of less value in air combat than thecorresponding turn rate and radius are. Because of the interaction of airspeedand G, best sustained turn rate is generally achieved at a speedslightly below that for maximum sustained G at a given altitude. Forsupersonic fighters this speed is almost always near the subsonic-Ps peak,even at high-altitude/low-G conditions, since supersonic speeds greatlyreduce turn rate. Because of the very great sensitivity of turn radius toairspeed, minimum sustained turn radius is normally achieved at fairlyslow airspeed (generally 1.4 to 1.5 times power-on stall speed for jets),considerably slower than for best sustained turn rate.Speed control is quite important for prolonged sustained turn performance.Pulling too great a load factor will cause speed to bleed off below theoptimum value, resulting in reduced sustained turn. This lost speed can beregained only by relaxing G (further reducing turn performance) until theaircraft accelerates back to the desired speed, or by diving, which allowsgravity to provide the needed acceleration.The sustained-G capability of a fighter is proportional to its ratio ofthrust to weight (T/W) at its particular altitude and airspeed. High T/W isanalogous to low "power loading" (weight/horsepower) for prop-poweredfighters. Just as important for sustained-G performance, however, is thelifting efficiency of the wing-airframe combination of the fighter, which ismeasured by the lift-to-drag ratio (L/D) at sustained-G levels. Therefore arelatively low-powered fighter with high L/D may possess a greater sustained-Gcapability. For two fighters with roughly the same sustained-Gperformance, the one that achieves its optimum sustained turn capabilityat the lower airspeed will have better sustained turn rate and radius. Thissuperior low-airspeed performance is generally achieved by designing alarger wing for a given aircraft weight, which results in lower "wingloading" (aircraft weight/wing area), or by providing greater L/D for thewing by use of slots, slats, flaps, etc.The fighter pilot can optimize his sustained turn performance by con-