2. High AoA aerodynamics for fighter design
2.2 Solutions for high AoA flow around the jet fighters
PART B. Attachment on the wing
In Part A, we found impact of main-wing, VT, HT design, and its control surface on high AoA characteristics. Part B investigates impact of smaller devices on the wings like vortex generators, dog-tooth, notch. Due to the effort of recent engineering technique and reducing RCS requirement, usage of these devices are restrained in recent days. So, this part only reviews the past usage case of the devices without considering current and future perspective.
Array of vortex generators (VG) were usually attached for cold war age jet fighters as shown in Fig. 2.51. Most of these aircraft are subsonic which could generate significant amount of drag in supersonic conditions. Various types of VG exist as shown in Fig. 2.52, single, multi-vane, and plow type, and multi-vane is divided to co or counter rotating flow type. Plow type is not usual for its drag penalty, and co-rotating type is usual for jet fighters while both types are usual for light passenger aircraft. In general, effectiveness of counter rotating one is stronger than co-rotating one however, flow around counter rotating VG could be compressible at high transonic speed. This is why co-rotating one is usual for aircraft.
High speed aircraft usually adopt wing-fence type devices to control flow; span-wise flow inducing high lift loading on wing tip and leading to stall in small AoA. Rather than array of small sized VG, wing fence is also effective for high AoA flow and generate less drag in supersonic condition. Thus, as shown in Fig. 2.53, early supersonic jets prefer wing fence to mitigate high AoA flow problem in swept or delta wing. These fences were also acted as structural stiffener, and pylons were also attached to use weapon below the wing. Dog-tooth and notch are also famous options for the high AoA flow solution for jet fighters; this discontinuity on the wing makes additional vortex, not separated in certain high AoA for stability. More precise impact of these devices on the longitudinal stability of the aircraft is summarized (Fig. 2.54) for AJS-37 Viggen in Fig. 2.55.
Fig. 2.51 VG array of wing; Harrier, A-4 Skyhawk, Javelin, and T-45 Goshawk
Fig. 2.52 VG array type; single, multi-vane (co- and counter rotating), plow
Fig. 2.53 Wing fence of wing; Su-24 Fencer, Mig-17, Su-22, YF-102
Fig. 2.54 Various options for high AoA flow control on wing
Fig. 2.55 Various options and impact for high AoA flow control on wing (AJS-37 case) [1]
PART C. Design of Fuselage or Nose
In Part A and B, various design options related to wing itself were investigated; some of options are still active. However, recent trend is generating vortex from start of body to control flow around the body without using past solutions causing un-wanted drag or RCS increase. Design for mission performance, mostly done in moderate AoA, is done for wing-planform design, then, precise vortex control design is performed for wing-fuselage OML design.
One of the typical example of the approach is adding strake between fuselage and body. Northrop have heavily investigated effect of this devices to optimize turn performance of the light weight fighter since early 60’s. Finally, F-5 and F/A-18 series, shown in Fig. 2.56, widely adopted strake as LERX, and they could achieve their fame from high AoA dog-fight capability. After success of the F-5, many 4th generation fighter designer did not hesitate to use LERX on their jet fighters as shown in Fig. 2.57, F-16, Rafale, Su-27, and MiG-29. These aircraft adopted LERX as high vortex lift generator and suppression of flow separation via high energy vortex. Position of the VT or VTs are sophisticatedly defined to maximize controllability of the jets. Reference studies [2, 3] and other numerous articles provided LERX gives higher lift and L/D ratio than non-LERX case while it gives slight higher drag and pitching moment (Fig. 2.58, and 59).
Fig. 2.56 Famous example of adopting LERX, F-5E and F/A-18C, and they could achieve high level of maneuverability at their time
Fig. 2.57 Other example of adopting LERX, F-16, Rafale, Su-27, MiG-29
Fig. 2.58 Lift enhancement via LERX addition is represented. Additional LERX gives extra lift for moderate and high AoA, however, LERX induced vortex makes buffet-occurring-AoA lower than no-LERX case [2]
Fig. 2.59 Lift, drag, and pitching moment coefficient change via LERX for segmented LEF. LERX provided higher lift and drag with higher L/D ratio while positive pitching moment occurs [3]
Shape of the nose is also important for vortex distribution around the fighter. As described in previous paragraph, slight condition change of nose induces huge difference in vortex separation point in F/A-18. Chambers [10] studied effect of various nose shape for jet fighters, and discover that elliptical shape provided better vortex distribution and stability. In that case, when elliptical nose faces side slip flow at high AoA, it could generate lift for opposite direction of the side slip flow to mitigate high AoA directional instability. More than the nose shape, Jet fighter like Gripen attached small VG pair on the nose to generate effective vortex expected to be stabilize the fighter at high AoA as shown in Fig. 2.60. Recent trend of design emphasizing low RCS value and effort of CFD technique make mission of vortex generation for delicate design of chine-nose and body lines as shown in Fig. 2.61. Chine-nose generates massive amount of vortex with low RCS characteristics, and this is why most recent jet fighter choose that shape of nose. Complex shape of F-35’s OML is result of compromise between internal layout of components and vortex generation. Many discontinuities of the F-35’s OML as shown in Fig. 2.61 become start point of the vortex wrapping whole aircraft. This kinds of vortex layout do not only help lift also help high AoA stability via suppressing abrupt fall of huge vortex usual in conventional clean body-wing shape.
Fig. 2.60 Various nose shape for high AoA stability test [10], and vortex generator on Gripen nose
Fig. 2.61 Design optimization effort via CFD [11, 12] which represents vortex generation via chine-nose [11, 12] and delicate design of body-lining of F-35 [12]
* Reference
[1] Karling, K., 1986, Aerodynamics of Aircraft 37 – Part 1 : General Characteristics at Low Speed, NASA TM-88403
[2] Ray, E. J., et al., 1972, Maneuver and Buffet Characteristics of Fighter Aircraft, NASA TN D-7131
[3] Henderson, W. P., 1978, Effects of Wing Leading Edge Flap Deflections on Subsonic Longitudinal Aerodynamic Characteristics of a Wing-Fuselage Configuration with a 44 deg Swept Wing, NASA Technical Paper 1351
[4] Sisk, T. R., and Matheny, N. W., 1980, Precision Controllability of the YF-17 Airplane, NASA Technical Paper 1677
[5] Monahan, R. C., and Friend, E. L., 1973, Effects of Flaps on Buffet Characteristics and Wing-Rock Onset of an F-8C Airplane at Subsonic and Transonic Speeds. NASA TM X-2873
[6] Kraus, W., 1980, Delta Canard Configuration at High Angle of Attack, ICAS80-13.1, 12th ICAS Congress
[7] Mason, W. H., Some High Alpha and Handling Qualities Aerodynamics, Configuration Aerodynamics Class
[8] Ray, E. J., et al., 1972, Maneuver and Buffet Characteristics of Fighter Aircraft, NASA TN D-7131
19730017272_LEF_wing
[9] Dean J. P., et al., High Resolution CFD Simulation of Maneuvering Aircraft Using the CREATE-AV / Kestrel Solver, AIAA 2011-1109, 49th AIAA Aerospace Science Meeting, AIAA-2011-1109
[10] Chambers, J. R., 1986, High Angle of Attack Aerodynamics: Lessons Learned, AIAA 86, AIAA 4th Applied Aerodynamics Conference
[11] Jeans, T. J., et al., 2008, Aerodynamic Analysis of a Generic Fighter with a Chine Fuselage/Delta Wing Configuration Using Delayed Detached Eddy Simulation, AIAA 2008-6228, 26th AIAA Applied Aerodynamics Conference
[12] Wooden, P. A., et al., CFD Prediction of Wing Pressure Distributions on the F-35 at Angles of Attack for Transonic Maneuvers, 25th AIAA Applied Aerodynamics Conference
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