1. Aero Engineering 315 Lesson 38 Lateral/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 C N  and recognize directional stability  Understand the contributions of the vertical tail, wing and fuselage to directional stability  Define C L  and 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 ()  is the angle between V  and the aircraft’s x-axis. Positive  occurs when the aircraft is flying with “wind in the right ear” VV 

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

7. Positive slope indicates DIRECTIONAL STATIC STABILITY - a positive  generates a positive (restoring) moment - a negative  generates a negative (restoring) moment C N,cg  ZERO YAW MOMENT AT ZERO  Directional Static Stability (Weathercock Stability)

8. Vertical Tail Contribution to Directional Static Stability Design Considerations (main contributor)  Vertical tail aft of c.g. is stabilizing  To increase directional stability Top View x y + N cg  LvLv VV  Vertical tail further aft  Vertical tail bigger (or add another)  Increase tail lift curve slope (increase AR vt and/or increase e vt )  Ventral fin

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

10. Individual Component Contributions to Directional Static Stability C N,cg  Tail Aircraft Wing/body

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

12.  THE F-117A IS WEAK IN NEUTRAL DIRECTIONAL STABILITY HIGH PEAKED CANOPY SMALL RELATIVE VERTICAL STABILIZATION  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...

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

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

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

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

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

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

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

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