Ch 22 - Intro To Stability And Control Flashcards
Stability definition
The aircrafts natural ability to look after itself / stay put
Effected by; control inputs and external inputs
More stable = less controllability
Two types of stability
Static - the initial response to disturbance
Dynamic - the subsequent response to the disturbance
Types of Static Stability
Neutral Static - gets disturbed and then stays where it is
Positive Static (stable)- Inclination to return to its original position
Negative Static (unstable) - Inclination to move further away from its original position
Types of Dynamic Stability
Neutral Dynamic - Continues to stay where it was displaced to
Positive Dynamic; Aperiodic - straight back to original position, Periodic - oscillates but eventually back to original position
Negative Dynamic; Aperiodic - Deviates further away from original position, Periodic - Oscillates getting further and further away from original position
Axis of Stability
Longitudinal Stability - Stability in the pitch (about the lateral axis)
Lateral Stability - Stability in the roll (about the longitudinal axis)
Directional Stability - Stability in the yaw (about the normal axis)
Aerodynamic Centre
Is a fixed point that has been calculated so that the size of the resultant moment stays (roughly) the same for all angles of attack (independent of AoA)
Can also be called the neutral point
Usually 23-27% of the MAC (depends on camber)
At high speed, the CP and AC will move aft to approx. 50% MAC
Moments and Coefficients
Longitudinal Stability (Pitch) (M) coefficient = Cm +VE = UP (Around lateral axis)
Lateral Stability (Roll) (Ł) Coefficient = Cł +VE = Right (around the longitudinal axis)
Directional Stability (Yaw) (N) Coefficient = Cn +VE = Yaw Right (around the normal axis)
Longitudinal Static Stability
Around the lateral axis
Principle contributes is the Tail plane deflection
Size of moment and therefore amount of static stability depends on; Tailplane area and the distance between the CG and the Tailplane
Forward CG = increased longitudinal Stability
Directional Static Stability
Principle contributor is the Fin (vertical stabiliser)
Increases with larger fin area and longer arm to the CG
Forward CG = increased Directional Static Stability
Secondary contributor is the amount of fuselage behind the CG; more fuselage will give the Aircraft (AC) a natural weathercocking ability
Lateral Static Stability
Primary contributor is the wing.
Increases with; Dihedral (and the dihedral effect), sweep back, high mounted wing, and a high keel surface with low CG
Unlikely to have all as it would become too stable
Dihedral on lateral stability
Wings that are canted upwards (tips higher than roots)
The more dihedral the wings are, the more lateral stability it will have. Lower wing produces more lift due to a higher angle of attack meaning that the AC has a natural inclination to roll back to its original position
Sweepback on lateral stability
Increased sweepback increases the lateral Stability because the initial roll is shortly followed by side slip. The lower wing then has a larger effective wingspan relative to the air and so, produces more lift = restoring moment
High wing on lateral stability
The higher the wing, the more lateral stability the AC has. When bank is applied, the down going wing produces more lift due to (CP moving onto down going wing) increased upwash giving the wing a restoring moment.
Keel Area on Lateral and Directional Stability
Keel area: The surface area of the side of the AC
Dorsal Fins; increase lateral and directional stability
Ventral Fins; reduce lateral stability (lower CG) but increase directional ability
Stick Free or Stick Fixed
Stick Free; Decreased AC stability as the controls will compound the external input
Stick Fixed (either powered or held); increases AC stability as the controls wont move unless input by the pilot reducing the effect of the external input
