Chapter 24: Directional and Lateral Stability Flashcards
β
Positive or negative
Sideslip angle β
RAF from right = β > 0
RAF from left = β < 0
Yawing Moment Coefficient Equation
CN = N / ( q * S * b )
w/ S surface area of fin
b fin cord
Directional Static Stability
What does it mean
AC turns into RAF
β > 0 (wind from right), then Cn > 0 (AC turns right)
Trim Point
What is?
Point around which sum of total yawing moments = 0
CN vs AoAa Graph
Stable, unstable, sweeback, why is sweeback good
Stable: upgoing slope
Unstable: downgoing slope
Sweepback: stable around 0 AoA, neutral then unstable at stall in negative and positive AoA
Sweepback has high AoAs: good for high crosswind
CN
Factors (4)
CG forward
Fin surface area
IAS
β
Ventral Fin
Affect directional stability, high speed vs low speed
Good for directional
Cruise: hidden by fuselage, less drag
Low speeds: more flow, more stability
Directional Stability
(De)stabilizing elements (2), why
Fuselage: destabilizing
Wings: stabilizing (wing in sidesleep sees more air -> more L, more D -> AC yaws into sideslip)
Lateral β, CL’
Positive or Negative, explain
Roll right -> Right sideslip β > 0
Roll left -> Cl’ < 0
Lateral Stability
(De)stabilizing elements (4), why
Dihedral: down wing sees higher AoA
Highwing: upwash on downwing, downwash in upwing
Sweepback: wing on sideslip side sees more air -> L
High Keel Surface: elements above CG sees more air from sideslip -> restoring moment
Spiral Instability
Strong / weak stability? Description
Strong directional, weak lateral
β -> yaws into β -> more L on outer wing -> more β
Dutch Roll
Strong / weak stability? Description, solution?
Strong lateral, weak directional
β > 0 -> rolls left (more L on right wing) -> yaws right -> rolls right
Yaw damper critical
