Directional And Lateral Stability Flashcards
Directional Stability Is
Yaw about the normal axis
Lateral Stability Is
Roll around longitudinal axis
+ Static directional is
Initial tendency to return to S+L after disturbance in yaw (weathercock)
Sideslip Angle
Angle between longitudinal axis and the relative air flow
Positive directional stability the yawing moment N is
N is positive
Negative Directional Stability the yawing moment N is
N is negative
What does positive directional stability look like on CN/B graph
Positive
+CN and +B
-CN and -B
Negative
+CN and -B
-CN and +B
The Fin On Directional Stability
Strongest effect on directional stability
Change in RAF causes sideslip angle
Creates AOA with the fin to produce sideway lift force
Yawing moment towards the RAF
Factors that increase fins restoring moment
CM is greatest when CG is further forward
When fin surface area is larger
Higher IAS
High sideslip angles
Aspect Ratio on Fin
High aspect ratio bad - more cl produced less fin AOA +B angle
Low aspect ratio ideal - higher fin AOA +B for CL
Fin Sweepback on directional stability
Sweepback reduces side force created however essential for high speed flight by delaying onset of shockwaves
Increases directional stability with increase sweep back angle/effective span more lift and induced drag to pull nose into wind
Dorsal Fin on directional stability
+ effect
Decreases aspect ratio
Increases stalling angle and improves directional stability
Adds to fin swept back
Ventral Fin on Directional Stability
Increases fin area
Increases directional stability but reduces lateral stability
Effective and high AOA when protruding into airflow
Strakes on directional stability
Long fins with little vertical extent on lower fuselage
2 fitted to improve directional stability at slow speed
Swept wings do what to directional stability
Increase directional stability
Wing sweep changes effective span more lift and induced drag on wing into wind
Pull nose into wind (yaw)
Fuselage on directional stability
Negative directional stability
Fuselage presents itself at aoa aerodynamic force away from RAF
AC is usually ahead of CG on fuselage therefore aerodynamic force is created
High wing on directional stability
Hits fuselage to lift wing up in roll
Strong Vs weak lateral stability
Weak = slow recovery or no recovery from roll
Too strong = hard to control (bank to make heading changes)
What is Bank Angle
Angle at which ac is inclined about its longitudinal axis in respect to horizontal.
Greater the bank angle the larger the sideslip angle
+ roll moments and - roll moments directions
+ to the right
- to the left
From behind
CL/B Graph - Lateral Stability
Stable:
+CL+ -B
-CL + +B
Side slip + gives us a roll in the opposite direction
Unstable:
+CL + +B
-CL + -B
Roll in one direction = yaw in the same direction
Dihedral effect
Collective term for positive static lateral stability
Dihedral on lateral stability
Greater +B for side slip angle
Larger AOA on low wing
Greater lift on down wing which creates stabilising moment
Anhedral on lateral stability
Less lateral stability
Less lift produced on lower wing during sideslip
Sweepback on lateral stability
Sideslip causes lower wing to have greater effective span
Generates more lift than upper wing
Restores in balance by rolling towards equilibrium
Lateral Stability and High Wings
Positive lateral stability
Wings AC vertically displaced from CG restoring couple creates
Increased upwash and angle of attack into wind low wing creating restoring moment
Keel Surface on lateral stability
Surface area above CG
Restoring moment
Dorsal fin raises the CP so increases dorsal effect
Ventral fins will reduce
Spiral Instability
Too much directional stability weak lateral stability
Further roll caused by yaw
Dutch Roll
Laterally stable by weak directional stability
Oscillation of roll/yaw/sideslop
Prevention to Dutch roll
Yaw damper - Moves rudder to prevent
Fly at lower levels where dynamic stability is greater
Greater IAS for TAS gives smaller slip angle
Increase in altitude does what to lateral stability
Dynamic stability reduces with pressure altitude as increased TAS for same IAS resulting in smaller sideslip angle
Static lateral stability remains the same
Dynamic lateral stability decreases
Design features that increase static lateral stability
High Wing
Sweep Back
Large and High vertical fins (dorsal)
High keel
Dihedral
Sideslip manoeuvre
Aileron input in one direction and rudder input in the opposite direction
Dutch roll can be caused by
Nature of shock waves on winds at high Mach numbers
Static Lateral Stability should not be too large as
Too much aileron deflection required in a crosswind landing
Static directional stability definition
The tendency of an aeroplane to recovery from a skid without control input from pilot
Periodic
Dutch roll
A periodic
Spiral dive
Sideslip angle is
Angle of the airplanes longitudinal axis (plane of symmetry) from the relative airflow (speed vector)
