Directional And Lateral Stability

Question 1

Directional Stability Is

Answer 1

Yaw about the normal axis

Question 2

Lateral Stability Is

Answer 2

Roll around longitudinal axis

Question 3

+ Static directional is

Answer 3

Initial tendency to return to S+L after disturbance in yaw (weathercock)

Question 4

Sideslip Angle

Answer 4

Angle between longitudinal axis and the relative air flow

Question 5

Positive directional stability the yawing moment N is

Answer 5

N is positive

Question 6

Negative Directional Stability the yawing moment N is

Answer 6

N is negative

Question 7

What does positive directional stability look like on CN/B graph

Answer 7

Positive
+CN and +B
-CN and -B

Negative
+CN and -B
-CN and +B

Question 8

The Fin On Directional Stability

Answer 8

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

Question 9

Factors that increase fins restoring moment

Answer 9

CM is greatest when CG is further forward
When fin surface area is larger
Higher IAS
High sideslip angles

Question 10

Aspect Ratio on Fin

Answer 10

High aspect ratio bad - more cl produced less fin AOA +B angle
Low aspect ratio ideal - higher fin AOA +B for CL

Question 11

Fin Sweepback on directional stability

Answer 11

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

Question 12

Dorsal Fin on directional stability

Answer 12

+ effect
Decreases aspect ratio
Increases stalling angle and improves directional stability
Adds to fin swept back

Question 13

Ventral Fin on Directional Stability

Answer 13

Increases fin area
Increases directional stability but reduces lateral stability
Effective and high AOA when protruding into airflow

Question 14

Strakes on directional stability

Answer 14

Long fins with little vertical extent on lower fuselage
2 fitted to improve directional stability at slow speed

Question 15

Swept wings do what to directional stability

Answer 15

Increase directional stability
Wing sweep changes effective span more lift and induced drag on wing into wind
Pull nose into wind (yaw)

Question 16

Fuselage on directional stability

Answer 16

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

Question 17

High wing on directional stability

Answer 17

Hits fuselage to lift wing up in roll

Question 18

Strong Vs weak lateral stability

Answer 18

Weak = slow recovery or no recovery from roll
Too strong = hard to control (bank to make heading changes)

Question 19

What is Bank Angle

Answer 19

Angle at which ac is inclined about its longitudinal axis in respect to horizontal.

Greater the bank angle the larger the sideslip angle

Question 20

+ roll moments and - roll moments directions

Answer 20

+ to the right
- to the left

From behind

Question 21

CL/B Graph - Lateral Stability

Answer 21

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

Question 22

Dihedral effect

Answer 22

Collective term for positive static lateral stability

Question 23

Dihedral on lateral stability

Answer 23

Greater +B for side slip angle
Larger AOA on low wing
Greater lift on down wing which creates stabilising moment

Question 24

Anhedral on lateral stability

Answer 24

Less lateral stability
Less lift produced on lower wing during sideslip

Question 25

Sweepback on lateral stability

Answer 25

Sideslip causes lower wing to have greater effective span
Generates more lift than upper wing
Restores in balance by rolling towards equilibrium

Question 26

Lateral Stability and High Wings

Answer 26

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

Question 27

Keel Surface on lateral stability

Answer 27

Surface area above CG
Restoring moment
Dorsal fin raises the CP so increases dorsal effect
Ventral fins will reduce

Question 28

Spiral Instability

Answer 28

Too much directional stability weak lateral stability
Further roll caused by yaw

Question 29

Dutch Roll

Answer 29

Laterally stable by weak directional stability
Oscillation of roll/yaw/sideslop

Question 30

Prevention to Dutch roll

Answer 30

Yaw damper - Moves rudder to prevent
Fly at lower levels where dynamic stability is greater
Greater IAS for TAS gives smaller slip angle

Question 31

Increase in altitude does what to lateral stability

Answer 31

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

Question 32

Design features that increase static lateral stability

Answer 32

High Wing
Sweep Back
Large and High vertical fins (dorsal)
High keel
Dihedral

Question 33

Sideslip manoeuvre

Answer 33

Aileron input in one direction and rudder input in the opposite direction

Question 34

Dutch roll can be caused by

Answer 34

Nature of shock waves on winds at high Mach numbers

Question 35

Static Lateral Stability should not be too large as

Answer 35

Too much aileron deflection required in a crosswind landing

Question 36

Static directional stability definition

Answer 36

The tendency of an aeroplane to recovery from a skid without control input from pilot

Question 37

Periodic

Answer 37

Dutch roll

Question 38

A periodic

Answer 38

Spiral dive

Question 39

Sideslip angle is

Answer 39

Angle of the airplanes longitudinal axis (plane of symmetry) from the relative airflow (speed vector)