1. Why High AoA is Important for Combat Aircraft ?
1.1. Long History of “Turn vs Speed” - One Compromise : Go to High AoA
During the Falklands and Gulf war, missiles reclaim their position in the aerial combat via improving their reliance and range. IR seekers of the WVR missiles expanded their angular range from direct 6’o clock to all-aspect-angle of the enemy fighters while BVR missiles got own active sensors to achieve independence from the ‘shooter’ aircraft. More than that, after collapse of the Soviet Union, Off-boresight performance of the R-73 shocked Western counter-part and stimulated development of the advanced WVR missiles including ASRAAM, IRIS-T, AIM-9X, Python-4, and MICA-IR. These missiles even can track targets beyond shoulder of the pilots during combat maneuver, and has lock-on-after-launch (LOAL) capability to handle unexpected situations.
The first generation of the active seekers on the BVR missiles was very large and complex system like old-times 1000 lb weight AIM-54. At the late 80’s, time of AIM-120s, the size of the seeker can be fit into the size of standard medium range missiles, and radar of the fighters became free from fully-guidance-task. Combination of modern BVR and WVR missiles is now standard armament of the aircraft for aerial combat. In a long term, this trend is obvious since advent of the missiles, maybe lessons and learns from the Vietnam war just delayed it.
Fig. 1.9 Advance of WVR missiles in their range/angle
Fig. 1.10 Change of AIM-7 and 120 via advance of electric technology
Other Important decisive change in the picture of the aerial combat is ‘Stealth’ performance.
Evolution of the radar and missile did not provide good aspect on the aircraft; big SAMs for regional defense evolved as small and easily portable missiles without harming their fatality. Record of the loss of IAF during the middle east wars and interruption of the SAMs on the air missions during the Gulf war showed that radars on the air and ground platform left very small room for the fighters. F-117, one of the revolutionary aircraft in the aviation history, showed how stealth aircraft neutralized complicated air-defense system consisting of latest generation of SAMs and jet fighters. Actually, before F-117, a lot of special purposed aircrafts such as U-2 and SR-71 already considered ‘stealth’ performance as one of the important parameters however penalty of stealth performance for these early aircrafts, cost and performance degradation, make application of stealth performance impractical. After advent of F-117, considering stealth performance for the tactical aircraft became reality, and finally there was one more added consideration for performance inventory of the jet fighters.
Fig. 1.11 Stealth fighter examples (Left: F-117, Right: F-22)
The latest situation changes after 70’s gave fighter designers headache due to a lot of considerations. Contrary to the past fact that 40’s war-birds just chased speed, just in 40 years, designers should balance many goals to make successful jet fighter although recent fighters pursue advanced missile, stealth, and situation awareness performance rather than classical speed and maneuver performances. The important turning point of the weighting and balancing of these parameters was ATF project for USAF at late 80’s. This project was the first project for the strongest country which can chase every known goals of the list. Because lots of performance goals can be followed, contenders from many companies, Lockheed, Boeing, Northrop, McDonnell, Rockwell, Grumman, and General Dynamics, proposed many concepts emphasizing their own important performance parameters. After initial filtering out, it was obvious that USAF abandoned ‘All-in’ concept from companies, such as speed one (super-cruiser from Boeing and Rockwell), maneuverability one (X-29 style from Grumman) are out of competition. The concept aircrafts having possibility that lack of one of parameter on the list were also eliminated from competitions; it showed that US did not want to repeat their mistake of 60’s.
Fig. 1.12 ATF initial concepts; Boeing, McDonnell, Rockwell, Grumman, and General Dynamics
The two best contenders of the projects were YF-22 and 23, brain-child of Lockheed and Northrop respectively. The first-one, YF-22, is result of the subtle balancing of the all known parameters of the fighter design; speed, maneuverability, sensors, armament, and stealth capabilities. In order to achieve the ‘Best’ for whole areas of the performance, a lot of advanced technologies were used, thrust vectoring (TVC) with most powerful engine at that time, S-shaped duct, internal weapon bays, complicated combination and scheduling of control surfaces at high AoA, angled-body-surfaces for stealth, and active phased array radar (AESA). None of these were whole brand new technologies however combination of these for one aircraft was the first attempt for Lockheed Martin.
Northrop’s YF-23 was also well balanced aircraft among contenders. However compared to the Lockheed’s YF-22, they did not installed TVC on their tails while aircraft had longer-slimed and stealthier fuselage, at least it seemed. Like ATB competition held at just before ATF, Northrop’s aircraft seemed to have more potential for further growth. Although Northrop sacrificed super-maneuverability using TVC, they had wider body and wing for better maneuverability at conventional AoA region than past fighter and even F-22. Also this wider and long body and wing help strategic maneuverability, always one of the important for the US since WWII, and super-cruise capability in given thrust condition. Actually, in the past competitions in the US, most requirement for the fighter was well defined, and most contenders had similar performance even they had different geometry. However, in the ATF, companies arbitrary pursued their own goals in different geometry, and it seemed fight among concepts which one is the most appropriate for the future aerial combat.
Fig. 1.13 YF-22 and YF-23
Still, there are many argues about choice of USAF; someone insists YF-23 is better for the tommorow’s environment. Anyway USAF chose YF-22 as F-22 for the USAF inventory, and which decide fate of two companies and affect other adversary countries of the US. This decision showed USAF did not want any lack-of-performance for one possible parameter, maybe except cost. It looked conservative, and still nightmare of Vietnam war alive. Except F-35, designed mainly for general-day-one attacker, most recent fighters of other countries, Pak-FA, J-20, X-3 of Japan, KF-X, Rafale, Eurofighter should care about the whole parameters which they can completely care or not.
6th generation fighter, F/A-XX for USN, is still ambitious about their concepts like initial phase of the ATF; directed energy weapon (DEW) might be added as a new member of performance inventory. If DEW is attached at the aircraft, aircraft become heavier to support massive power requirement of laser, and do not need to have good maneuverability due to its unlimited bullet speed, deep-magazine and fast-tracking gymbal of the turret. If future air-warfare goes in that way, fighters will follows fate of ship-of-the-line, and UCAV or similar one substitute current fighter’s position in the aerial combat. Speculation of the future aerial combat is very interesting one for me, however it is not focus of current article and nothing is certain at 2017. Further discussion for the future air combat will be discussed at Chap. 4 of this. Before DEW is matured for small-sized UCAV which possibly neutralize advantages of maneuverability of the aircraft, balancing of the fighter-design-parameters is important task for the current age fighters and the future-substitute of the ship-of-the-line.
Fig 1.14 Concept art of 6th generation fighter and DEW
It has been a long story that these parameters are important for the fighter design. Situation-awareness payloads, speed, acceleration, stealthy exterior design, and maneuverability should compromise each other to make ‘appropriate balance’ for the good fighter. When the aircraft is sized for their mission, thrust and weight including fuel and situation awareness are decided to meet its purposes. After thrust and rough size of the aircraft is decided, balancing of the speed, acceleration and turning performance is the issue. It is nature law of the aerodynamics that higher lift for the turning performance induces drag, proportional to the square of the lift. So, many engineers have found ways to achieve higher lift in the given drag penalty; RSS, relaxed static stability, flap scheduling, and expanding AoA capability of the aircraft were proposed to obtain this goal. The RSS with fly-by-wire system, common solution for the fighter since F-16, could enhance lift via reducing trim drag and a good solution for the sustained turn and cruise efficiency. Flap scheduling for the leading and trailing edge of the wing could modify the camber of the airfoils to obtain higher lift coefficient. These two solution is good solution for the given AoA, however the effect of two subtle solutions are limited in the point of view for the total lift.
The expanding AoA region is more radical solution for the jet fighter especially for instantaneous turn which become important as lethality of the missile is increased and one or two chances are only available for the fighters. Indeed, in order to ‘truly’ enhance the combat effectiveness of the jet fighter without harming speed relative performances, jet fighter should enter the higher AoA region to shoot their missiles or evade attack of the enemy aircraft.
Fig 1.15 High AoA turn of the jet fighter
In the next 1.1.2, I’ll discuss about high AoA of jet fighter more specifically.