1. High AoA Aerodynamics for Combat Aircraft : 1.1.1.A

1. Why High AoA is Important for Combat Aircraft ?

1.1. Long History of “Turn vs Speed” - One Compromise : Go to High AoA

 PART A

 “Speed is life”, maybe the most famous rule for the fighter aircraft, and this rule shows its influence from WWI to modern air warfare. Advantage of the speed gives pilots initiative of the engagement among ‘leave’, ‘engage’, ‘pursuit’ and maybe other choices. The advantage could divide result of engagement result as live or death, and accumulation of exchange ratio is critical even for strategic level. One of the dramatic example of ‘speed-dominance’ is American and German-war-bird in WWII. Those two-country-made aircraft such as Bf 109, Fw 190, P-38, P-47, F6F, F4U, and P-51 used energy-fighting tactics to fight against other-country-made fighters effectively, and they are famous for their performance during the war.

Fig. 1.1. Top: Fw 190 uses vertical tactics based on energy advantage, typical style of Fw 190 

Bottom : Energy advantage of aircraft is not only useful for initiative of the engagement, but for chasing enemy’s 6’o clock

Fig. 1.2. Top: F6F of USN and A6M is good example of showing “speed is life”, about 30~50km/h difference of maximum speed determine their destiny

Bottom : Extreme long range and top class maximum speed made P-51 as ultimate fighter

 This prerequisite was based on the ‘gun-weaponry’ of the classic air-warfare of which weapon has limited range and angle to shoot against enemy aircraft. Most people easily thought that this limited range and angle of the gun-weaponry change dog-fight into turn-dominant-stage, however the limited effective range of the gun system help speed-advantage aircraft to win easily. For example, the most advanced gun on board cannot achieve kill beyond for more than 2 nm except stationary targets, and the safe-distance is easily obtained by energy-tactics fighters. Turn-based fighters without having enough speed to catch up energy-tactics fighters can only aim their guns on one energy-tactics fighters while the other energy-tactics fighters can pursuit and shoot easily the turn-based fighters. Most of the aerial combat has been form of ‘many-versus-many’ and this is why energy-tactics could dominate gun-weaponry air-warfare.

 (If you have more interest about turn-vs-energy tactics, lots of other papers or articles were already written and discussed about this topic.)

 Dominance of the speed-priority was peaked at just before start of the Vietnam war, however reason for the emphasis on the speed became different after WWII. At the time of end of the WWII and start of the Cold-war age, advent of early form of the missile and fear from the high-speed nuclear bombers changed air-warfare permanently. For a while, most fighters became interceptor to block the bombers rather than pure fighter role, which means that fighters carried more missiles with higher maximum speed. Due to the limited performance of the early jet engines and radars (for missile guidance), aircraft should have big-slender body to reduce drag and carry big electronics. This kinds of evolution naturally aggravated “maneuverability” of the fighters in fighters-to-fighters

Fig. 1.3. Top speed of 50~60’s jets, F-86, F-100, and MiG-21 can be shown, and their trend is remarkable (Design for Air Combat, R Whitford)

 While the fighters became bomber-interceptors, acceleration and minimum turning performance of the fighter were sacrificed for their primary target performances. Doctrines for ‘long-range-penetrating-fighters for tactical nuclear strike’ and ‘missile-replacing-dog-fighting’ made situation even worse, and fighters like F-105, F-111 (long-range-penetrating), and F-4 (missile-replacing-dog-fighting) had been produced to meet these needs. Only few light fighters at that age kept reasonable maneuverability not because of opposite ideas but for other reasons such as keeping reasonable prices (F-5 series), and point intercepting performance (Mirage III family and MiG-17/19).

Fig. 1.4. One of the famous and typical example of long-range-penetrating-fighters for tactical nuclear strike; suffering lack of air-to-air capability

 Dictatorship of the speed and missiles went to end because of many reasons at end of 60’s

(1) Vietnam war proved WVR dog fighting still occurs (missiles cannot do everything)

(2) Fear of high-speed nuclear bombers changed to fear of SLBM/ICBM

(3) Growth of jet engines and electronics makes room for fighter design

 Lessons and Learned from (1) is very famous for aviation history, and I do not need to repeat about this. Reason (2) is not famous as (1) but important, because fighters were free from ‘strategic-intercepting-work’. While small air-to-air missiles proved its immature performance in theater of south-east Asia, big missiles for strategic nuclear strike widen its proportion in ‘Global-Level’ weapon inventory. Advent of surface to air missile, SAM, also instigated fall of high-speed nuclear bombers from crown of the strategic strike mission. This is the reason why XB-70, B-1A, T-4 is cancelled, and high rank officers in the air-forces threw their sole-purpose-bomber-interceptors like old shoe. Only aircraft like Tornado ADV, F-14, and MiG-25/31 survived in UK, US, and Russia as dedicated interceptor where intercept mission is still important (UK strongly emphasized intercepting performance on Eurofighter replacing Tornado ADV even at the current age).

 At the early stage of the Cold war, as I mentioned earlier, integration of big radar and powerful jet engine itself was a big task for fighter designers, and most of the volume and system should be satisfied to support the two big items. Reason (3) changed situation dramatically. Radar of 70’s which can detect enemy from few tens of nm and guide missiles weighted much less and were more powerful than that of 60’s. Increase of engine thrust in given volume helped to reduce fuel volume in the jet aircraft, and even more, advent of the turbo-fan engine improved fuel-economy of the fighter to reasonable level.

Fig. 1.5. Owing to advance of electronic technology, capability of the radar is increased without penalty of weight (trend of weight is not shown) (Fundamental of Fighter Design, R Whitford)

Fig. 1.6. Increase of pressure ratio, specific thrust, and T/W of jet engines (Fundamental of Fighter Design, R Whitford)

 Owing to the situation changes and technology advances, fighters changed their enemy from bombers and ground target to ‘other fighters’. As proved in the Vietnam war, fighting in the supersonic region for more than Mach 1.4 is ‘unlikely-happen’ between fighters due to its limited size of fuel tank and external stores. At the age of interceptors, from 50 to 60’s, because jet fighters already expanded their maximum speed from subsonic to Mach 2, the designers looked forward for more practical performance parameters such as acceleration, turning performance, rate of climbs, and etc, called “maneuverability”. 

 American teen series, F-14/15/16/18, and Soviet originated MiG-29 and Su-27 were born in this trend of post-Vietnam war, and still enjoyed their heydays (although part of reason came from end of the Cold war). Post-Vietnam war jet fighters were result of delicate balancing among speed, acceleration, and turning performances; T/W, W/S of the jet were carefully selected to meet their complex requirements and to overcome possible adversary in the sky.

Fig. 1.7. T/W, W/S of various jet fighters, noticeable change of trend after 70’s jets (Design for Air Combat, R Whitford)

Fig. 1.8. Turning capability comparison of F-16 (70’s) and F-4 (60's)

Continue-in Part. B