Vertical tail design and Spiral Stability

Vertical tail designCentral of lateral area Inertial roll coupling Locating the CLA Vertical tail aspect ratio Dorsal fins All –moving horizontal T-tailsRudder power Rudder aileron effect Spiral stability Levels of spiral stability Lateral and directional coupling Balance of forcesSpiral stability margin

Verticlal Tail Design

Wing dihedralFuselageLanding gear side areasCG location Tail areaRudder

CLA (spiral stable)-25% of TMA aft CG-a line through CLA & CG horizontal-the wing joining the front CLA &rear CLA sloped

upward to the front -

Inertial roll coupling

Aerodynamic axis & inertia axis aligned –no inertial coupling.

Rotation abt aerodynamic axis will create CF.

CFs produce pitching moment

Dihedral

sweepback 2 to 3 deg. equivalent to 1deg. Dihedral High wing-2Mid wing -3 Low wing -4

Locate CLA Profile of designSettle to 25% of TMA to get good VTLeft right lateral surface due to dihedral

-large VT area with no dihedral angle leads to sideslipLarge tail resist slip

Vertical aspect ratio -ARv=1.55*Bv^2/Sv

Dorsal fin

-Increase fuselage stability at high speed angle

-Reduce VT stalling

Rudder power

-Area 30% of VT

-Rudder aileron effect –reduce aileron efffect ( forced yaw)

-Rudder area center above horizontal tail line through CG will act as aileron

Spiral Stability

-Degree of spiral stability -Spirally stable -neutrally spirally stable-spirally unstable-angle of bank slowly increases ,speed gradually increases-for sailplanes good SS, sport model moderate , aerobatic models neutral

Lateral & directional coupling

SS requires a balance between lateral and directional forcesDutch roll--large dihedral angle with small VT lateral forces are high wagging coupled with a slight side to side roll

Balance of forces –Lateral &directional forcesLS-Dihedral , angle of sweep back , Cl , AoAForward sweepback- destabilizes , increases yaw-VT area prevents yaw-advantage -wing stall at root first

DS-

-Dorsal fin-to overcome a lack of VT effectiveness flaps extended & high side slip

-Sweepback- aids DS

- Drag moment reduces yaw

Aileron eliminate aileron induced adverse yaw

-SSM 25% of VTMA

-CG moved aft of the wing SSM VTMA decrease