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Aero Engineering 315

Aero Engineering 315. Lesson 38 Lateral/Directional Static Stability. Important Safety Tip… . Glider project due next lesson! Turn in paper copies of Cover sheet (name and documentation) Glider Design Project questions Spreadsheet (color version – or highlight which, if any, blocks are RED)

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Aero Engineering 315

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  1. Aero Engineering 315 Lesson 38Lateral/Directional Static Stability

  2. Important Safety Tip… • Glider project due next lesson! • Turn in paper copies of • Cover sheet (name and documentation) • Glider Design Project questions • Spreadsheet (color version – or highlight which, if any, blocks are RED) • Design AND chart pages • Keep the rest of the info stuff for the fly off portion • Remember it is INDIVIDUAL EFFORT ONLY • Glider need not be constructed until flyoffs (beginning Monday, 10 May) Glider Design Spreadsheet My Glider Design

  3. B-2—Lat/Dir Stability Challenge

  4. Lat/Dir Stability Objectives • Define CNband recognize directional stability • Understand the contributions of the vertical tail, wing and fuselage to directional stability • Define CLband recognize lateral stability • Understand the contributions of the vertical tail, geometric dihedral, wing sweep, and wing placement to lateral stability • Understand coupling effects of directional and lateral stability

  5. Sideslip Angle (b) b is the angle between V and the aircraft’s x-axis. V b Positive b occurs when the aircraft is flying with “wind in the right ear”

  6. +b V + Ncg x y Directional Static Stability (Weathercock Stability) Yaw Moment Coefficient: Dir.-Stat. Stability Derivative: slope of CN,cg vs. b curve For a stable system what sign should this have? (+) positive

  7. CN,cg b ZERO YAW MOMENT AT ZERO b Directional Static Stability (Weathercock Stability) Positive slopeindicates DIRECTIONAL STATIC STABILITY - a positive b generates a positive (restoring) moment - a negative b generates a negative (restoring) moment

  8. +b V + Ncg Lv Top View x y Vertical Tail Contribution to Directional Static Stability • Design Considerations (main contributor) • Vertical tail aft of c.g. is stabilizing • To increase directional stability • Vertical tail further aft • Vertical tail bigger (or add another) • Increase tail lift curve slope (increase ARvt and/or increase evt) • Ventral fin

  9. +b V Lw/b - Ncg - Ncg Top View x y Wing/Body Contribution to Directional Static Stability Design Considerations - Fuselage area forward of the cg is directionally destabilizing - That’s why aircraft have tails!

  10. Tail Wing/body Individual Component Contributions to Directional Static Stability CN,cg Aircraft b

  11. Directional Static Stability at High Mach TAIL LESS EFFECTIVE WITH INCREASING SUPERSONIC MACH CN,cg Tail Aircraft b Wing/body TOTAL ACFT CAN BECOME UNSTABLE AT HIGH MACH (EX.: F-15 & F-16, XB-70)

  12. THE F-117A IS WEAK IN DIRECTIONAL STABILITY • HIGH PEAKED CANOPY • SMALL RELATIVE VERTICAL STABILIZATION • WEAK YAW STABILITY TO 0.6 MACH (300 KNOTS) • ABOVE 0.6 MACH - UNSTABLE, AND INSTABILITY INCREASES AS MACH INCREASES • AT WEAPONS EMPLOYMENT MACH, AIRCRAFT IS VERY UNSTABLE • YAW INSTABILITY EXAGGERATED BY WEAPON BAY DOORS THE YAW AXIS. . . • THE F-117A IS WEAK IN NEUTRAL DIRECTIONAL STABILITY • HIGH PEAKED CANOPY • SMALL RELATIVE VERTICAL STABILIZATION

  13. V y y z z Top View Lateral Static Stability (Dihedral Effect) Rolling Moment Coefficient: Lat.-Stat. Stability Derivative: slope of CL’,cg vs. b curve V y +L’cg Rear View

  14. CL’,cg < 0 Zero roll moment at zero b b Lateral Static Stability (Dihedral Effect) Negative slope indicates lateral static stability - Positive b generates a negative (restoring) moment - Negative b generates a positive (restoring) moment

  15. Vertical Tail Contribution to Lateral Static Stability Design Considerations - Vertical tail above c.g. is stabilizing - To increase lateral stability: V • Vertical tail taller • Vertical tail bigger • Increase tail lift curve slope (increase ARvt and/or increase evt) y -L z Rear View

  16. Wing Sweep Contribution to Lateral Static Stability x + b V V y z y Rear View More lift Less lift Positive wing sweep is stabilizing Top View

  17. Wing Position Contribution to Lateral Static Stability Rear Views High wing placement is laterally stabilizing V High Wing Mid wing placement is neutral Mid Wing V Low wing placement is laterally destabilizing V Low Wing

  18. z -L +L + y y - V V z z y Geometric Dihedral Contribution to Lateral Static Stability V Rear Views destabilizing Anhedral is stabilizing Positive dihedral is Paper airplane example…

  19. < 0 Coupling of Directional and Lateral Static Stability Modes Directional Static Stability: Lateral Static Stability: Good aircraft design provides enough stability for both as well as a reasonable balance between the two: = bad “spiral mode” (>2/3) = bad “dutch roll” (<1/3) • Too much directional static stability (compared to lateral) • Too much lateral static stability (compared to directional)

  20. Summary • Sideslip • Directional Static Stability • Vertical Tail • Wing/Body • Lateral Static Stability • Vertical Tail • Geometric Dihedral • Wing Sweep • Wing Position (high/low) • Lateral/Directional Coupling

  21. Next Lesson (39)… • Prior to class • Read dynamic modes of motion handout • In class • Discuss dynamic modes and glider construction

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