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Some High Alpha and Handling Qualities Aerodynamics Some High Alpha and Handling Qualities Aerodynamics W.H. Mason Configuration Aerodynamics Class Slide 1 4/19/18 Issues in Hi-α Aero • General Aviation – Prevention/recovery from spins – Beware: Tail Damping Power Factor (TDPF) » Sometimes advocated for use in design » Has been shown to be inaccurate! • Fighters – Resistance to “departure” from controlled flight – Carefree hi-α air combat maneuvering » Fuselage pointing » Velocity vector rolls » Supermaneuverability » etc. • Transports – Control/prevention of pitchup and deep stall (the DC-9 case study) Slide 2 4/19/18 High Angle of Attack • Aerodynamics are nonlinear - A lot of flow separation - Complicated component interactions (vortices) - Depends heavily on WT data for analysis - This means critical conditions outside linear aero range • Motion is highly dynamic - often need unsteady aero • Keys issues (from a fighter designer’s perspective): - adequate nose down pitching moment to recover - roll rate at high alpha - departure avoidance - adequate yaw control power - the role of thrust vectoring Slide 3 4/19/18 The Hi-α Story Longitudinal Typical unstable modern fighter – the envelope is the key thing to look at + Max nose up moment Pinch often around α ~ 30°-40° Cm 0° Cm* α 90° Max nose down moment - Minimum Cm* allowable is an open question Issue of including credit for thrust vectoring Suggested nose down req’t: Pitch accel in 1st sec: -0.25 rad/sec2 Slide 4 4/19/18 Example: F-16 An α-limiter was Found in flight, and put in the control also in tests at the system to prevent NASA Full Scale pilots from going Tunnel, other above 28° tunnels didn’t find this, and so the design was made with this “issue” NASA TP-1538, Dec. 1979 (horizontal tail later increased in size 25%) Slide 5 4/19/18 The Hi-α Story Directional + Stable Forebody Effect Cnβ Vertical Tail V-Tail in wake 0 0° α 90° Config w/o V-Tail Directional problem also often around 30° - Unstable Slide 6 4/19/18 0.0100 Example: F-5 WT data: NASA TN D-7716 F-5 Full Config., 0.0050 Vertical Tail On Cnβ Above 30°, the Forebody alone directional 0.0000 stability comes entirely from Full Config., the forebody! Tail Off -0.0050 -10° 0° 10° 20° 30° 40° 50° α, deg. Chine forebodies are even more effective! Sketch from NASA TN D-7716, 1974 by Grafton, Chambers and Coe Slide 7 4/19/18 Forebody Vortices. A CFD Calculation At “high” angles of attack, vortices form over the forebody, producing additional forces, and often interacting with the rest of the airplane flowfield I’m looking for a better, color, figure F-5A forebody, α = 40°, β = 5° NASA CR 4465, Aug. 1992 Slide 8 4/19/18 Illustration of Forebody Vortices and a “Fix” • Laser Light Sheet Video of the RFC forebody • And Flow Asymmetries: - an amazing story - solve with nose strakes X-29 forebody showing nose strakes Taken at the USAF Museum, WPAFB, Ohio Slide 9 4/19/18 Rudder Lock and the B-17 Vertical Tail Mod Originally Dorsal fin added! Darrol Stinton, Flying Qualities and Flight Testing of the Airplane, AIAA, 1996 Slide 10 4/19/18 Vertical Tail Shaping II: “Subtle” Tail Changes SAE Paper 556, 1951 by Willis Hawkins Note that yaw is the negative of sideslip, thus the negative slope Also note good use of labels to identify effects Slide 11 4/19/18 Who’s on first? A true story from the Grumman STAC Airplane program The Stability and Control Group Plane See Hendrickson, et al, AIAA Paper 1978-1452 for the story The Aerodynamics group plane Slide 12 4/19/18 Some High Alpha and Handling Qualities Aerodynamics Again W.H. Mason Configuration Aerodynamics Class Slide 13 4/19/18 The Hi-α Story Lateral + Drooping LE devices helps control severity “Unstable” Clβ 0 α 0° 90° Dihedral Effect “Stable” (also due to sweep) Flow Separates on wing - Slide 14 4/19/18 Example: F-4 NASA TN D-7131, July 1973 Slide 15 4/19/18 Control NASA TP 1538, Dec. 1979 Effectiveness = 30° Change with alpha δr C Example: Δ n F-16 Lateral/ = -20° Directional δa δ = -5° Control d Conventional aero = -20° control surfaces lose δa δd= -5° effectiveness at high ΔCl angles of attack Subscripts: r – rudder δ = 30° a – aileron r d – differential horizontal tail α, deg Slide 16 4/19/18 A way to get yaw: differential canard Grumman STAC Differential deflection of the canard creates differential pressures on the side of the forebody, and hence yawing moment Lapins, Martorella, Klein, Meyer and Sturm, NASA CR 3738, Nov. 1983 See also, Re and Capone, NASA TN D-8510, July 1977 Slide 17 4/19/18 Example: the Evolution of the SCAMP • How and Why the modified F-16 supercruiser SCAMP became the F-16XL Original Concept The Final Airplane Dave Miller & Schemensky, “Design Study Results of a Supersonic Cruise Fighter Wing” AIAA Paper 1979-62, Jan. 1979 Talty and Caughlin, Journal of Aircraft Vol. 25, No. 3, March 1988, p. 206-215. Slide 18 4/19/18 Low Speed Tests Mandated Planform Mods Baseline low speed Grafton, NASA TM 85776, May 1984 configuration (note crank) The “Apex” mod The TE extension mod One more not shown: a “Wing fence” Slide 19 4/19/18 The F-16XL Original All mods combined Note “Wing Fence” called a sensor fairing WT data: Grafton, NASA TM 85776, May 1984 Slide 20 4/19/18 Hi-α Flight Mechanics I • Departure – Aircraft “departs” from controlled flight – May develop into a spin • Wing drop – Uncommanded motion: asymmetric wing stall » Typical spin entry for straight wing GA airplanes, a wing roll-of – Became famous again as the F-18E Abrupt Wing Stall Problem • Wing rock – “Damping” characteristics generate wing roll oscillations • Nose slice – Yawing moments from airplane (mainly forebody) greater then rudder power – May develop into a yaw-type entry into a spin » Swept wing fighter, with totally different inertial characteristics • So-called “fuselage heavy”, as oppose to “wing heavy” light planes • Tumbling – Typically associated with flying wings Slide 21 4/19/18 Hi-α Flight Mechanics II • Becomes closely integrated with flight control system • Requires extensive ground and flight simulation From Johnston & Heffley (STI), “Investigation of High-AOA Flying Qualities Criteria and Design Guides,” AFWAL-TR-81-3108, Dec. 1981. Determinants of High a Flying Qualities Approach Example Aerodynamics must provide an all around solution with respect to performance, Aerodynamic stability and controllability F-5 Dominance FCS may give fine tuning, but is not required for basic stability and control Aerodynamic design favors performance Balanced Aerodynamics with some regions of undesirable flying F-15 and FCS qualities. FCS necessary to enhance or provide acceptable stability and control Aerodynamic design favors performance FCS Dominance FCS to provide good flying qualities up to F-16 maximum usable lift coefficient Slide 22 4/19/18 The problem: configurations have changed from WWII The fuselage is longer and heavier, the wings relatively smaller Figure from Joe Chambers, “High-Angle-of-Attack Aerodynamics: Lessons Learned,” AIAA Paper 1986-1774 Slide 23 4/19/18 Departure Criteria I • Really need dynamic data and piloted simulation – Needs to be connected to flight mechanics analysis – Simulation math model requires massive amount of data from WT(s) • Issue: can we do anything w/o dynamic data? • Some rough approximations to get started (note inertia ratio): – Open loop: (actually a very rough approximation): Cnβ dynamic > 0 C C cos I I C sin nβ, dyn = nβ α − ( z x ) lβ α » Several forms depending on coordinate system, this is essentially body axis (actually principal axis) - see Calico, Journal of Aircraft, Vol. 16, No. 12, Dec. 1979, pp 895-896. » Comes from the lateral-directional equations of motion linearized about steady, rectangular flight, and an examination of the resulting stability quartic (ignoring rate derivatives) – see Greer, NASA TN D-6993, 1972. This is essentially a Dutch Roll Approximation Note: the classic F-16 study is “Simulator Study of Stall/Post-Stall Characteristics of a Fighter Plane With Relaxed Longitudinal Static Stability,” by Luat Nguyen, et al, NASA TP 1538, Dec. 1979 (pdf avail) Slide 24 4/19/18 Departure Criteria II • Closed loop: LCDP, the lateral control departure parameter > 0 $ C + G C ' LCDP = C − C nδa ARI nδr nβ lβ & C G C ) % lδa + ARI lδr ( • Where GARI is the ratio of rudder to aileron deflection for an airplane with an aileron-rudder interconnect (ARI) • When yaw due to aileron gets large, commanded roll produces a motion in the opposite direction! • See AFWAL-TR-81-3108 for examples C Note: the value (and drawback) of n β , dy n and LCDP are that they are based on static aero coefficients Damping derivatives: • Cnr < 0 (yaw damping), but not too negative • Clp < 0 (roll damping), to prevent wing rock Slide 25 4/19/18 F-16 Hi-α characteristics NASA TP-1538, Dec. 1979 Slide 26 4/19/18 F-16 LCDP and augmented LCDP NASA TP-1538, Dec. 1979 Slide 27 4/19/18 Departure Map Integrated Bihrle-Weissman Chart 0.010 Highly departure and 0.005 High directional spin resistant instability, Spin resistant, LCDP little data objectionable roll reversals 0.000 F can induce departure and E post stall gyrations Strong departure, roll B -0.005 reversals and spin C tendencies Weak spin tendency, strong roll reversal -0.010 results in control induced Strong departure, departure roll reversals and spin tendencies -0.015 -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 Cnβ dyn E - Weak spin tendency, moderate departure and roll reversals, affected by secondary factors F - Weak departure and spin resistance, no roll reversals, heavily influenced by secondary factors Slide 28 4/19/18 Ways to get aero data, including dynamic
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