1. High AoA Aerodynamics for Combat Aircraft : 1.1.2

1. Why High AoA is Important for Combat Aircraft ?

1.2. Turn w/o harming speed performance using higher AoA

 As discussed in the Chap 1.1, jet-fighter go to high AoA region to achieve higher turning performance without increasing wing area (drag). As you can see in Fig. 1.16, lift coefficient is proportional to AoA while drag is square of AoA or the lift coefficient. There are three methods, increasing wing area, lift device, and expanding AoA region, for lift increment of the aircraft; each method has their own pros and cons for jet fighter application.

Fig 1.16 Lift and drag curve of typical jet fighter

Fig. 1.17 Sizing of jet fighter (W/S, T/W)

 Most simple one of the three is increasing wing area without chaning wing planform or aerofoils however, its impact on design criteria is significant than the others as shown in Fig. 1.17. Subsonic maneuverability at high alitude better did the increased wing area make while wing become prone to weight increase, loads problem at high dynamic pressure condition, and drag at high speed. Weight increase is very obvious via increment of wing area because more spars, ribs, skins, and actuators are needed. Loads problem is also clear that longer arm of the wing compel reinforced structure at the wing root to mitigate deflection of the wing. This phenomenon goes severe at low altitude with higher air density which forces higher dynamic pressure on the wing, and this is why low altitude penetrating aircraft (TSR.2 as shown in Fig. 1.18) has relatively small wing area than similar class aircrafts (Some low penetrators such as Tornado, F-111, and Su-24 choose variable swept wing to adapt complex mission profile; they should have good flight characteristics at both high and low altitude cruise).

 Contribution of the jet fighter drag is simulataneously changed at its speed and alitutude. At the high altitude and subsonic condition, aircraft faces low dynamic pressure and requires higher AoA to lift own weight (survivability from AAA, cruise efficiency of engine made jet fighter go to high altitude). However as you saw in Fig. 1.16, higher AoA generates more induced drag, and designers try to avoid that kind of situation. Low altitude or supersonic condition with higher dynamic pressure condition, everything has changed for not only structural load problem as described in the previous paragraph also for the drag. In the high dynamic pressure condition, required lift coefficient for cruise become small and contribution of the induced drag is negligible. At that time drag of the aircarft shape and area is truly important, but unfortunately, larger wing area provides more skin-friction and form drag which give negative impact on the acceleration and maximum speed performance. Indeed, increasing wing area for the lift increment is sensitive issue for the aircraft designers which cannot easily be chosen without sacrificing other performance parameters.

Fig. 1.18 TSR.2 of British; smaller wing helps this aircraft to be a better low altitude penetrator, however complex jet flap and higher AoA is required for its take-off and landing.

Fig. 1.19 Tornado, F-111, Su-24; As customers of these aircrafts requires long range (high altitude cruise with low induced drag) and low altitude penetration capability (smaller drag at higher dynamic pressure region), designers select variable swept wing to response to the complex mission profile. Usage of variable wing is now rare as low altitude intruder tactics become legacy of the past.

Fig. 1.20 Breakdown of the drag structure (Sadraey M., Aircraft Performance Analysis, 2009)

Fig. 1.21 U-2; most famous and dramatic example of the high altitude cruiser wing design. Russian M-4 and B-52 also chose high aspect ratio wing to increase cruise efficiency at the dedicated altitude. These aircraft do not have any serious consideration on acceleration and maximum speed requirement like jet fighters, and could choose large-cruise-effecient type wing.

 Adding lift devices on the wing is effective way than changing wing size in that side effect of the first method would be mitigated. This active method is similar to compromise between maximum lift and cruise or maxmimum speed condition; leading and trailing edge of the aerofoil is intentionally changed to achieve wanted lift and drag characteristics. Good reference is provided by Hoerner for the lift and drag change effect of the flap, and numerous flap devices had been developed for not only fixed wing aircraft but for cars and rotary wing aircraft (Very detailed discussions on the lift devices take large volume of talk and I recommended to read Hoerner’s work). If you have any chance to watch air-show or its video on You-Tube, you can figure out that leading edge and trailing edge devices are changed simultaneously during the maneuver of the aircraft as shown in Fig. 1.22.

LE devices helps aircraft to adjust AoA for their wings to generate maximum L/D in various AoA conditions, so, many of fighter jets choose LE flaps or slat which also delay flow separation problem at high AoA limiting maximum lift of the aircraft. In general, slat type LE devices are more effective than simple LE flap because slat type can provide fresh air on the upper surface of aerofoil which suppress flow separation, and carefully designed slat accelerates of the fresh air to increase lift. This is why the slat type is popular for airliners which requires very large lift at take-off and landing conditions; however, this type requires relatively more complex mechanism for operation than simple LE flaps. Indeed, for jet fighters, the slat type is chosen when higher lift coefficient is required (F-14 or other naval fighters) or planform is delta which is prone to flow separation and need higher lift increment via LE device.

 Compare to LE devices, TE flap is optional for the turn performance because it has clearer pros and cons than the LE ones. TE flap could generate massive lift at given AoA condition, which leads to induced drag. In the point of view for the lift, TE flap scheduling is effective way to increase lift, however, it can harm L/D performance of the fighter. So, If the jet turns with its TE flap, it can turn quickly than without it, but it loses most of its speed. This is why TE flap is optional for the turn performance enhancement as shown in Fig. 1.22. Only F/A-18 series, F-22A, and F-2A uses its TE flap during the turn while others did not. When the designers of each aircraft scheduling the flap deflection, balance between instantaneous and sustained turn affect decision of the usage of the TE flaps. As discussed in earlier, use of the TE flap could enhance the instantaneous turn performance, however, un-wanted induced drag gave negative impact on the sustained turn performance. Possible compromising method for this situation is that TE flap is deflected only when the jet certainly loses its energy. This method probably be used by the designers who should optimize their jet however, unfortunately, effectiveness of the most TE flap is gradually decreased at the high AoA region. So, the jet fighter like F-16, and Su-35 did not choose to use TE flap during the turn; designers gave more weight on sustaining energy rather than instantaneous turn.

 As a summary of the adding lift devices on the jet fighter, compare to the previous simple method, increase of the main wing area, the adding lift devices enhance the turn performance without harming speed related performance, and be loved by many aeronautical engineers who should satisfy their customers. Complex flap system could provide attractive increment of lift coefficient however this level of the system costs complexity and structural weakness. And engineers should notice that effectiveness of the flap is not guaranteed at the various flow conditions where the jet fighter should encounter.

** Additionally, F-22A deflects its aileron in the counter direction of the TE flap, and it probably related to the suppression of the wing-tip flow separation. Role of this kinds of technique will be discussed in the high AoA part.

Fig. 1.22 Various examples (F/A-18A, F/A-18F, F-22A, Su-35, F-2A, F-16, Eurofighter, Mirage 2000) of LE and TE device scheduling. F/A-18A, F/A-18F, F-22A, and F-2A uses their TE and LE flaps to maximize their turn performance while others uses only LE flaps. Details of reason is described in the text.

Fig. 1.23 Various flap types (LE and TE)

 Last method we discuss here is making jet fighter enter the high AoA. Fig. 1.16 naturally reveals aircraft can achieve higher lift at high AoA. ‘Using high AoA region for turn’ shows similar characteristics of using TE flap; it induces more lift and costs drag to lose its speed. In F-16, designers limited its max AoA due to its deep-stall characteristic, on the other hands, F-16 can maintain its speed during the turn due to the effort of this limiter. In the old days of ‘dog fighting era’, balance of the ‘sustain’ and ‘instantaneous’ capability of the F-16 deserves praise, however, advent of the modern short range missiles does not give any rooms for ‘sustain’ turn. Few quicker instantaneous turn could determine jet fighter’s fate. Maybe this kind of trend change is a reason why USAF choose F-35 as replacement of F-16 which cannot maintain higher turn rate for a long time than its predecessor. They also thought capability related to the energy can be covered by advantage of sensors and stealth, and sustaining turn capability will become legacy of the old school due to the fatality of short range missiles and future-possible-laser-turret which will neutralize any advantage of traditional fighters. Compare to the additional lift devices on the wing and increase of wing area, entering high AoA region to achieve better turning performance does not require any additional mechanical system, weight, and degradation of other performance parameters, and amount of the obtained lift is much larger than that of lift devices. Indeed, due to the effort of growth of the computational fluid dynamics (CFD) and advanced flight control system, AoA limit of jet fighter is relaxed for most 4.5^th and 5^th generation fighters.

 In the previous paragraph, there is no reason to hesitate using higher AoA region for turn, however, every fighter cannot enter high AoA region. Why? Aeronautical risk holds back of the aircraft to enter the high AoA. Aircraft including jet fighters have various wing surfaces with deflectable part to maintain stability in air-flow perturbation. Two directional perturbation of the aircraft are change of AoA and Side-slip angle of the flow which can drive aircraft in departure status (un-controllable status of the aircraft which should be avoided). Vertical and horizontal tails stabilize aircraft in longitudinal, directional and lateral directions, however, effectiveness of the tails is decreased as AoA is increased. Details of the high AoA aerodynamics will be discussed in Chap. 2, however, brief reasons for HARD things in the high AoA is shown in next paragraph.

 At the high AoA, aircraft fall into very-different dynamics; main reason for this change is wake from main wing and body as shown in Fig. 1.24. As AoA is increased, projected area from the aircraft is increased, and dramatic flow field change occurs such as vortex generation, vortex breakdown, flow bubble, and its separation. These kinds of non-linear aerodynamics behavior make aircraft un-predictable and dangerous. Except, aircraft having canard or similar control surfaces, horizontal and vertical tails is submerged in the downstream of this ‘non-linear’, and it results that aircraft can lose effectiveness of the tails at high AoA. If the tails lose their effectiveness, problem is not just ‘lose control’; AoA and side-slip angles can be diverged and lead to fatal spin which took lots of lives of pilots at the early time of the aviation. Although the aircraft does not fall into that kind of deadly event, problems like wing-rock or buffet still should be considered in high AoA to provide appropriate handling quality for the pilots.

As a summary of the entering high AoA, it does not require any sacrifice of other performance or additional mechanical system. Although usefulness of the method is limited for instantaneous turn, the instantaneous turn performance is emphasized in modern air-to-air combat. Only obstacle for the entering high AoA is un-predictable characteristics of flow separation which leads to fatal accident of the aircraft. However, modern control strategy and CFD help to expand AoA limit of the jet fighters without taking risky flight tests. Indeed, high AoA aerodynamics is important for achieving better fighting capability of the fighter, and I will deeply discuss about high AoA aerodynamics at the next Chap. 2.

Fig. 1.24 Wake problem via high AoA wake of main wing