STABILITY and CONTROL

Question 1

Stability is the tendency of ……………

Answer 1

Stability is the tendency of an aircraft to return to a steady state of flight without any help from
the pilot, after being disturbed by an external force.

Question 2

What is a state of equilibrium (trim) ?

Answer 2

An aircraft is in a state of equilibrium (trim) when the sum of all forces is zero and the sum of
all moments is zero; there are no accelerations and the aircraft will continue in steady flight.
If equilibrium is disturbed by a gust, or deflection of the controls, the aircraft will experience
accelerations due to an unbalance of moments or forces.

Question 3

Static Stability

Answer 3

The type of static stability an aircraft possesses is defined by its initial tendency, following the
removal of some disturbing force.
– Positive static stability (or static stability) exists if an aircraft is disturbed from equilibrium
and has the tendency to return to equilibrium.
–Neutral static stability exists if an aircraft is subject to a disturbance and has neither the tendency to return nor the tendency to continue in the displacement direction.
– Negative static stability (or static instability) exists if an aircraft has a tendency to continue in the direction of disturbance.
–The term “static” is applied to this form of stability since any resulting motion is not considered.
Only the initial tendency to return to equilibrium is considered in static stability.

Question 4

AEROPLANE REFERENCE AXES

Answer 4

–In order to visualise the forces and moments on the aircraft, it is necessary to establish a set of reference axes passing through the centre of gravity.
–The longitudinal axis passes through the CG from nose to tail. A moment about this axis is a rolling moment, L, a roll to the right is a positive rolling moment.
–The normal axis passes Avertically@ through the CG at 90° to the longitudinal axis. A moment
about the normal axis is a yawing moment, N, and a positive yawing moment would yaw the
aircraft to the right.
–The lateral axis is a line passing through the CG, parallel to a line passing through the wing
tips. A moment about the lateral axis is a pitching moment, M, and a positive pitching moment
is nose up.

Question 5

Thus, lateral stability is about the ………… axis (rolling), directional stability is about the ……….. axis (yawing) and longitudinal stability is about the ………….. axis (pitching)

Answer 5

Thus, lateral stability is about the longitudinal axis (rolling), directional stability is about the normal axis (yawing) and longitudinal stability is about the lateral axis (pitching)

Question 6

Static longitudinal stability

Answer 6

–An aircraft will exhibit static longitudinal stability if it tends to return towards the trim angle of attack when displaced by a gust OR a control input.
–It is essential that an aircraft has positive static longitudinal stability. If it is
stable, an aeroplane is safe and easy to fly since it seeks and tends to maintain a
trimmed condition of flight. It also follows that control deflections and control
“feel” (stick force) must be logical, both in direction and magnitude.

Question 7

What is Neutral stability ?

Answer 7

–If the aircraft is neutrally stable, it tends to remain at any displacement to which it is disturbed.
– Neutral static longitudinal stability usually defines the lower limit of aeroplane stability since it is the boundary between stability and instability. The aeroplane with neutral static stability may be excessively responsive to controls and the aircraft has no tendency to return to trim following a disturbance - generally this would not be acceptable.

Question 8

When is an aircraft longitudinally unstable ?

Answer 8

–The aircraft which is unstable will continue to pitch in the disturbed direction until the
displacement is resisted by opposing control forces.
–The aeroplane with negative static longitudinal stability is inherently divergent from any intended trim condition. If it is at all possible to fly the aircraft, it cannot be trimmed and illogical control forces and deflections are required to provide equilibrium with a change of attitude and airspeed - clearly, this would be totally unacceptable.

Question 9

What should be the pitching moment about the AC for an aircraft to be stable ?

Answer 9

–It should be noted that the pitching moment about the AC is negative (nose down) and that this negative (nose down) pitching moment about the AC does not change with changes in angle of attack.
–The pitching moment about the AC remains constant as the angle of attack is increased because
the magnitude of the lift force increases but acts through a smaller arm due to the CP moving
forward. It is only at the AC (25% chord) that this will occur. If a point in front of, or to the rear
of the AC were considered, the pitching moment would change with angle of attack.
–For the study of stability we will consider the lift to act at the AC. The AC is a stationary point
located at the 25% chord, only when the airflow is subsonic.

Question 11

When a wing is considered alone, is it statically stable ?

Answer 11

A wing considered alone is statically unstable, because the AC is in front of the CG, Figure 10.6.
A vertical gust will momentarily increase the angle of attack and increase lift (ΔL), which, when
multiplied by arm ‘x’, will generate a positive (nose up) pitching moment about the CG. This
will tend to increase the angle of attack further, an unstable pitching moment. The wing on its
own would rotate nose up about the CG. Page 247

Question 12

What is a neutral point ?

Answer 12

–The position of the CG when the sum of the changes in the tail moment and wing moment
caused by the gust is zero, is known as the neutral point.
–If you consider the CG moving rearwards from a position of static longitudinal stability:-
» the tail arm ‘y’ will decrease and the wing arm ‘x’ will increase; consequently,
» the (negative) tail moment will decrease and the (positive) wing moment will increase,
»Eventually the CG will reach a position at which the tail moment is the same as the wing moment. If a vertical gust were to displace the aircraft nose up, the sum of the moments will be zero and there will be no angular acceleration about the CG to return the aircraft towards its original position of equilibrium.
»Because there is no resultant moment, either nose up or nose down, the aircraft will remain in
its new position of equilibrium; the aircraft will have neutral static longitudinal stability.

Question 13

What is static margin ?

Answer 13

–The distance between the neutral point and the aft CG limit gives the required minimum static stability margin.
–If the CG is positioned just forward of the neutral point, the tail moment will be slightly greater
than the wing moment (arm ‘y’ increased and arm ‘x’ decreased). A vertical gust which increases
the angle of attack will generate a small nose down angular acceleration about the CG, which
will gently return the aircraft towards its original position of trim (equilibrium).
–The further forward the CG, the greater the nose down angular acceleration about the CG - the
greater the degree of static longitudinal stability.

Question 14

–Stability is the tendency of an aircraft to …………………, after being disturbed by an external force, without any help from the pilot.
–There are two broad categories of stability; ………………
–An aircraft is in a state of equilibrium (trim) when ……………………..
–The type of static stability an aircraft possesses is defined by …………………….

Answer 14

–Stability is the tendency of an aircraft to return to a steady state of flight, after being disturbed
by an external force, without any help from the pilot.
–There are two broad categories of stability; static and dynamic.
–An aircraft is in a state of equilibrium (trim) when the sum of all forces is zero and the sum of
all moments is zero.
–The type of static stability an aircraft possesses is defined by its initial tendency, following the
removal of some disturbing force.

Question 15

–The longitudinal axis passes through …………….
–The normal axis passes “vertically” through ………………….
–The lateral axis is a line passing through the CG, parallel to ………………….
–The three reference axes all pass through the ………………….

Answer 15

–The longitudinal axis passes through the CG from nose to tail.
–The normal axis passes “vertically” through the CG at 90° to the longitudinal axis.
–The lateral axis is a line passing through the CG, parallel to a line passing through the wing tips.
–The three reference axes all pass through the centre of gravity.

Question 16

–Lateral stability involves motion about the ………………….
–Longitudinal stability involves motion about the …………………….
–Directional stability involves motion about the …………………………
–We consider the changes in magnitude of lift force due to changes in angle of attack, acting through a stationary point; ………………………

Answer 16

–Lateral stability involves motion about the longitudinal axis (rolling).
–Longitudinal stability involves motion about the lateral axis (pitching).
–Directional stability involves motion about the normal axis (yawing).
–We consider the changes in magnitude of lift force due to changes in angle of attack, acting through a stationary point; the aerodynamic centre.

Question 17

–The aerodynamic centre (AC) is located at the …………. chord position.
–The negative pitching moment about the AC ……………………………………….
–A wing on its own is …………………… because the ……………………….. because
–An upward vertical gust will momentarily ………. the angle of attack of the wing. The increased lift force magnitude acting through the AC ………………………………………….This is an ………………. pitching moment.
–The tailplane is positioned to generate a ……………………………..

Answer 17

–The aerodynamic centre (AC) is located at the 25% chord position.
–The negative pitching moment about the AC remains constant at normal angles of attack.
–A wing on its own is statically unstable because the AC is in front of the CG.
–An upward vertical gust will momentarily increase the angle of attack of the wing. The increased lift force magnitude acting through the AC will increase the positive pitching moment about the CG. This is an unstable pitching moment.
–The tailplane is positioned to generate a stabilising pitching moment about the aircraft CG.

Question 18

–If the tail moment is greater than the wing moment the sum of the moments ………….. and the resultant …………… moment will give an angular acceleration about the CG.
–The greater the tail moment relative to the wing moment, the greater ………………………
–The tail moment is increased by moving the aircraft CG …………., which increases the ………. and decreases the …………………

Answer 18

–If the tail moment is greater than the wing moment the sum of the moments will not be zero and the resultant nose down moment will give an angular acceleration about the CG.
–The greater the tail moment relative to the wing moment, the greater the rate of return towards the original equilibrium position.
–The tail moment is increased by moving the aircraft CG forwards, which increases the tail arm and decreases the wing arm.

Question 19

–If the nose down (negative) tail moment is ……….. than the nose up (positive) wing moment, the aircraft will have ………………….
–The position of the CG ……………………………………. is known as the neutral point.
–The further forward the CG, the greater the ………………. angular acceleration about the CG - the ………….. the degree of static longitudinal stability.

Answer 19

–If the nose down (negative) tail moment is greater than the nose up (positive) wing moment, the aircraft will have static longitudinal stability.
–The position of the CG when changes in the sum of the tail moment and wing moment due to a disturbance is zero, is known as the neutral point.
–The further forward the CG, the greater the nose down angular acceleration about the CG - the greater the degree of static longitudinal stability.

Question 20

–The distance the CG is forward of the ………. will give a measure of the …………..; this distance is called the static margin.
–The greater the static margin, the greater the ……………………..
–The aft CG limit will be positioned …………………..
–The distance between …………………………… gives the required minimum static stability margin.
–An aircraft is said to be trimmed if all ………………………

Answer 20

–The distance the CG is forward of the neutral point will give a measure of the static longitudinal stability; this distance is called the static margin.
–The greater the static margin, the greater the static longitudinal stability.
–The aft CG limit will be positioned some distance forward of the neutral point.
–The distance between the aft CG limit and the neutral point gives the required minimum static stability margin.
–An aircraft is said to be trimmed if all moments in pitch, roll, and yaw are equal to zero.

Question 21

Trim (equilibrium) is the function of the controls and may be accomplished by:-

Answer 21

Trim (equilibrium) is the function of the controls and may be accomplished by:-
a) pilot effort
b) trim tabs,
c) moving fuel between the wing tanks and an aft located trim tank, or
d) bias of a surface actuator (powered flying controls).

Question 22

–The term controllability refers to the …………………
–A high degree of stability tends to reduce ……………………
–The stable tendency of an aircraft ……………… from trim equally, whether by pilot effort on the controls (stick force) or gusts.
–If the CG moves ……….., static longitudinal stability increases and controllability ……….(stick force …….).
–If the CG moves …….., static longitudinal stability ………….. and controllability increases (stick force ………..).

Answer 22

–The term controllability refers to the ability of the aircraft to respond to control surface displacement and achieve the desired condition of flight.
–A high degree of stability tends to reduce the controllability of the aircraft.
–The stable tendency of an aircraft resists displacement from trim equally, whether by pilot effort on the controls (stick force) or gusts.
–If the CG moves forward, static longitudinal stability increases and controllability decreases(stick force increases).
–If the CG moves aft, static longitudinal stability decreases and controllability increases (stick force decreases).

Question 23

–With the CG on the forward limit, static longitudinal stability is ……, controllability is …… and stick force is ………
–With the CG on the aft limit, static longitudinal stability is ….., controllability is …….. and stick force is …….
–The aft CG limit is set to ensure a ……………………
–The fwd CG limit is set to ensure a ………………. under the worst circumstance.

Answer 23

–With the CG on the forward limit, static longitudinal stability is greatest, controllability is least and stick force is high.
–With the CG on the aft limit, static longitudinal stability is least, controllability is greatest and stick force is low.
–The aft CG limit is set to ensure a minimum degree of static longitudinal stability.
–The fwd CG limit is set to ensure a minimum degree of controllability under the worst circumstance.

Question 24

–Positive static longitudinal stability is indicated by a ……… slope of ………….. The degree of static longitudinal stability is indicated by the ………..
–The net pitching moment about the lateral axis is due to …………….
–In most cases, the contribution of the fuselage and nacelles is ………….

Answer 24

–Positive static longitudinal stability is indicated by a negative slope of CM versus CL. The degree of static longitudinal stability is indicated by the slope of the curve.
–The net pitching moment about the lateral axis is due to the contribution of each of the component
surfaces acting in their appropriate flow fields.
–In most cases, the contribution of the fuselage and nacelles is destabilising.

Question 25

Noticeable changes in static stability can occur at high CL (low speed) if:

Answer 25

Noticeable changes in static stability can occur at high CL (low speed) if: a) the aeroplane has sweepback, b) there is a large contribution of ‘power effect’, or c) there are significant changes in downwash at the horizontal tail,

Question 26

--The horizontal tail usually provides the ................... --Downwash ............. static longitudinal stability. --If the thrust line is below the CG, a thrust increase will produce a ................ moment and the effect is ...........

Answer 26

--The horizontal tail usually provides the greatest stabilising influence of all the components of the aeroplane. (Para. 10.9). --Downwash decreases static longitudinal stability. --If the thrust line is below the CG, a thrust increase will produce a positive or nose up moment and the effect is destabilizing.

Question 27

--High lift devices tend to ...... downwash at the tail and reduce the ......, both of which are ............ --An increase in TAS, for a given pitching velocity, .......... aerodynamic damping. --The aeroplane with positive manoeuvring stability should demonstrate a steady ....... in stick force with increase in load factor or “g”.

Answer 27

--High lift devices tend to increase downwash at the tail and reduce the dynamic pressure at the tail, both of which are destabilizing. --An increase in TAS, for a given pitching velocity, decreases aerodynamic damping. --The aeroplane with positive manoeuvring stability should demonstrate a steady increase in stick force with increase in load factor or “g”.

Question 28

--The stick force gradient must not be excessively high or the aeroplane will be ........ Also, the stick force gradient must not be too low or the aeroplane may be ................. --When the aeroplane has high static stability, the manoeuvring stability will be ..... and a ......... stick force gradient will result. The forward CG limit could be set to prevent ................ As the CG moves aft, the stick force gradient ............. with ........... manoeuvring stability and the ........ limit of stick force gradient may be reached.

Answer 28

--The stick force gradient must not be excessively high or the aeroplane will be difficult and tiring to manoeuver. Also, the stick force gradient must not be too low or the aeroplane may be overstressed inadvertently when light control forces exist. --When the aeroplane has high static stability, the manoeuvring stability will be high and a high stick force gradient will result. The forward CG limit could be set to prevent an excessively high manoeuvring stick force gradient. As the CG moves aft, the stick force gradient decreases with decreasing manoeuvring stability and the lower limit of stick force gradient may be reached.

Question 29

--At high altitudes, the high TAS ............. the ................... for a given pitching velocity and ....... the pitch damping. Thus, a ........... in manoeuvring stick force stability can be expected with increased altitude. --A flying control system may employ ..............................

Answer 29

--At high altitudes, the high TAS reduces the change in tail angle of attack for a given pitching velocity and reduces the pitch damping. Thus, a decrease in manoeuvring stick force stability can be expected with increased altitude. --A flying control system may employ centring springs, down springs or bob weights to provide satisfactory control forces throughout the speed, CG and altitude range of an aircraft.

Question 30

--While static stability is concerned with the ................, dynamic stability is defined by .................. --An aircraft will demonstrate positive dynamic stability if the amplitude of motion ............. --When natural aerodynamic damping cannot be obtained, ..........................................

Answer 30

--While static stability is concerned with the initial tendency of an aircraft to return to equilibrium, dynamic stability is defined by the resulting motion with time. --An aircraft will demonstrate positive dynamic stability if the amplitude of motion decreases with time. --When natural aerodynamic damping cannot be obtained, artificial damping must be provided to give the necessary positive dynamic stability.

Question 31

--The longitudinal dynamic stability of an aeroplane generally consists of two basic modes of oscillation:- a) ........... b) ................. --The phugoid oscillation occurs with ................... angle of attack. --The period of oscillation is so great, the pilot is .....................

Answer 31

--The longitudinal dynamic stability of an aeroplane generally consists of two basic modes of oscillation:- a) long period (phugoid) b) short period --The phugoid oscillation occurs with nearly constant angle of attack. --The period of oscillation is so great, the pilot is easily able to counteract long period oscillation.

Question 32

--Short period oscillation involves ............ changes in angle of attack. --Short period oscillation is ............. controlled by the pilot. --The problems of dynamic stability can become acute at ....... altitude because of ......... ............

Answer 32

--Short period oscillation involves significant changes in angle of attack. --Short period oscillation is not easily controlled by the pilot. --The problems of dynamic stability can become acute at high altitude because of reduced aerodynamic damping.

Question 33

--To overcome the directional instability in the fuselage it is possible to .............. --The ........... is the major source of directional stability for the aeroplane. --T - tail makes the fin more effective by acting ............... --Because the dorsal fin stalls at a ............. .......... angle of attack, it takes over the stabilizing role of the fin at ................

Answer 33

--To overcome the directional instability in the fuselage it is possible to incorporate into the overall design, dorsal or ventral fins. --The fin is the major source of directional stability for the aeroplane. --T - tail makes the fin more effective by acting as an “end plate”. --Because the dorsal fin stalls at a very much higher angle of attack, it takes over the stabilizing role of the fin at large angles of sideslip.

Question 34

--Sweepback produces a directional ....... effect, which increases with increase in .......... --Ventral fins ......... directional stability at ........... / ................ requirements may limit their size, require them to be retractable, or require ........... ventral fins to be fitted instead of one large one. --Generally, good handling qualities are obtained with a relatively .................., lateral stability.

Answer 34

--Sweepback produces a directional stabilising effect, which increases with increase in CL. --Ventral fins increase directional stability at high angles of attack. Landing clearance requirements may limit their size, require them to be retractable, or require two smaller ventral fins to be fitted instead of one large one. --Generally, good handling qualities are obtained with a relatively 1ight, or weak positive, lateral stability.

Question 35

--The principal surface contributing to the lateral stability of an aeroplane is the .... The effect of .... ..... is a powerful contribution to lateral stability. --A low wing position gives an ........ contribution to static lateral stability. --A high wing location gives a ..... contribution to static lateral stability.

Answer 35

--The principal surface contributing to the lateral stability of an aeroplane is the wing. The effect of geometric dihedral is a powerful contribution to lateral stability. --A low wing position gives an unstable contribution to static lateral stability. --A high wing location gives a stable contribution to static lateral stability.

Question 36

--The magnitude of “dihedral effect” contributed by the vertical position of the wing is large and may require a noticeable ........ angle for the low wing configuration. A high wing position, on the other hand, usually requires ............... at all. --The swept back wing contributes a .......... “dihedral effect”. --An aircraft with a swept back wing requires .................... dihedral than a straight wing.

Answer 36

--The magnitude of “dihedral effect” contributed by the vertical position of the wing is large and may require a noticeable dihedral angle for the low wing configuration. A high wing position, on the other hand, usually requires no geometric dihedral at all. --The swept back wing contributes a positive “dihedral effect”. --An aircraft with a swept back wing requires less geometric dihedral than a straight wing.

Question 37

--The fin contribution to purely lateral static stability, is ........... --Excessive “dihedral effect” can lead to ...........,” difficult ........., or place extreme demands for lateral ....................... --Deploying partial span flaps gives a ......... dihedral effect.

Answer 37

--The fin contribution to purely lateral static stability, is usually very small. --Excessive “dihedral effect” can lead to “Dutch roll,” difficult rudder coordination in rolling manoeuvres, or place extreme demands for lateral control power during crosswind takeoff and landing. --Deploying partial span flaps gives a reduced dihedral effect.

Question 38

--A sweptback wing requires much ....... geometric dihedral than a straight wing. If a requirement also exists for the wing to be mounted on top of the fuselage, an ............ A high mounted and sweptback wing would give .............”, so .................. used to reduce “dihedral effect” to the required level. --When an aeroplane is placed in a sideslip, the ..................... will be coupled, i.e. sideslip will simultaneously produce a .......................

Answer 38

--A sweptback wing requires much less geometric dihedral than a straight wing. If a requirement also exists for the wing to be mounted on top of the fuselage, an additional “dihedral effect” is present. A high mounted and sweptback wing would give excessive “dihedral effect”, so anhedral is used to reduce “dihedral effect” to the required level. --When an aeroplane is placed in a sideslip, the lateral and directional response will be coupled, i.e. sideslip will simultaneously produce a rolling and a yawing moment.

Question 39

--Spiral divergence will exist when .......... stability is very large when compared to the ................ --The rate of divergence in the spiral motion is usually ........... that the pilot ......... control the tendency ........ --Dutch roll will occur when the ............. is large when compared to ............. stability.

Answer 39

--Spiral divergence will exist when static directional stability is very large when compared to the “dihedral effect”. --The rate of divergence in the spiral motion is usually so gradual that the pilot can control the tendency without difficulty. --Dutch roll will occur when the “dihedral effect” is large when compared to static directional stability.

Question 40

--Aircraft which Dutch Roll are fitted with a ........ This automatically displaces the rudder proportional to ................. --If the Yaw Damper fails in flight, it is recommended that the ........... be used by the pilot to damp-out Dutch Roll. --If the pilot uses the rudder, ............. will result and the Dutch Roll may very ....... become ........, leading to ...................

Answer 40

--Aircraft which Dutch Roll are fitted with a Yaw Damper. This automatically displaces the rudder proportional to the rate of yaw to damp-out the oscillations. --If the Yaw Damper fails in flight, it is recommended that the ailerons be used by the pilot to damp-out Dutch Roll. --If the pilot uses the rudder, pilot induced oscillation (PIO) will result and the Dutch Roll may very quickly become divergent, leading to loss of control.

Question 41

--When the swept wing aeroplane is at low CL the “dihedral effect” is ....... and the spiral tendency may be ........... When the swept wing aeroplane is at high CL the “dihedral effect” is .......... and the Dutch Roll oscillatory tendency is ............ --When pilot induced oscillation is encountered, the most effective solution is an .............. Any attempt to forcibly damp the oscillation simply .........................................

Answer 41

--When the swept wing aeroplane is at low CL the “dihedral effect” is small and the spiral tendency may be apparent. When the swept wing aeroplane is at high CL the “dihedral effect” is increased and the Dutch Roll oscillatory tendency is increased. --When pilot induced oscillation is encountered, the most effective solution is an immediate release of the controls. Any attempt to forcibly damp the oscillation simply continues the excitation and amplifies the oscillation.

Question 42

--Higher TAS common to high altitude flight ............ the angle of attack changes and .......... aerodynamic damping. --Mach Tuck is caused by ................................ and .......................................... --The Mach trim system will ............................ and operates only at ...............

Answer 42

--Higher TAS common to high altitude flight reduces the angle of attack changes and reduces aerodynamic damping. --Mach Tuck is caused by loss of lift in front of the CG and reduced downwash at the tail due to the formation of a shockwave on a swept back wing at high Mach numbers. --The Mach trim system will adjust longitudinal trim to maintain the required stick force gradient and operates only at high Mach numbers.

Question 44

An aeroplane which is inherently stable will: a. require less effort to control. b. be difficult to stall. c. not spin. d. have a built-in tendency to return to its original state following the removal of any disturbing force.

Answer 44

D

Question 45

After a disturbance in pitch an aircraft oscillates in pitch with increasing amplitude. It is: a. statically and dynamically unstable. b. statically stable but dynamically unstable. c. statically unstable but dynamically stable. d. statically and dynamically stable.

Answer 45

B

Question 46

Longitudinal stability is given by: a. the fin. b. the wing dihedral. c. the horizontal tailplane. d. the ailerons.

Answer 46

C

Question 47

An aircraft is constructed with dihedral to provide: a. lateral stability about the longitudinal axis. b. longitudinal stability about the lateral axis. c. lateral stability about the normal axis. d. directional stability about the normal axis.

Answer 47

A

Question 48

Lateral stability is reduced by increasing: a. Anhedral. b. Dihedral. c. Sweepback. d. Fuselage and fin area.

Answer 48

A

Question 49

If the wing AC is forward of the CG: a. changes in lift produce a wing pitching moment which acts to reduce the change of lift. b. changes in lift produce a wing pitching moment which acts to increase the change of lift. c. changes in lift give no change in wing pitching moment. d. when the aircraft sideslips the CG causes the nose to turn into the sideslip thus applying a restoring moment.

Answer 49

B

Question 50

The longitudinal static stability of an aircraft: a. is reduced by the effects of wing downwash. b. is increased by the effects of wing downwash. c. is not affected by wing downwash. d. is reduced for nose up displacements, but increased for nose down displacements by the effects of wing downwash.

Answer 50

A

Question 51

To ensure some degree of longitudinal stability in flight, the position of the CG: a. must always coincide with the AC. b. must be forward of the Neutral Point. c. must be aft of the Neutral Point. d. must not be forward of the aft CG limit.

Answer 51

B

Question 52

When the CG is close to the forward limit: a. very small forces are required on the control column to produce pitch. b. longitudinal stability is reduced. c. very high stick forces are required to pitch because the aircraft is very stable. d. stick forces are the same as for an aft CG.

Answer 52

C

Question 53

The static margin is equal to the distance between: a. the CG and the AC. b. the AC and the neutral point. c. the CG and the neutral point. d. the CG and the CG datum point.

Answer 53

C

Question 54

If a disturbing force causes the aircraft to roll: a. wing dihedral will cause a rolling moment which reduces the sideslip. b. the fin will cause a rolling moment which reduces the sideslip. c. dihedral will cause a yawing moment which reduces the sideslip. d. dihedral will cause a nose up pitching moment.

Answer 54

A

Question 55

With flaps lowered, lateral stability: a. will be increased because of the effective increase of dihedral. b. will be increased because of increased lift. c. will be reduced because the centre of lift of each semi-span is closer to the wing root. d. will not be affected.

Answer 55

C

Question 56

Dihedral gives a stabilising rolling moment by causing an increase in lift: a. on the up going wing when the aircraft rolls. b. on the down going wing when the aircraft rolls. c. on the lower wing if the aircraft is sideslipping. d. on the lower wing whenever the aircraft is in a banked attitude

Answer 56

C

Question 57

A high wing configuration with no dihedral, compared to a low wing configuration with no dihedral, will provide: a. greater longitudinal stability. b. the same degree of longitudinal stability as any other configuration because dihedral gives longitudinal stability. c. less lateral stability than a low wing configuration. d. greater lateral stability due to the airflow pattern around the fuselage when the aircraft is sideslipping increasing the effective angle of attack of the lower wing.

Answer 57

D

Question 58

At a constant IAS, what affect will increasing altitude have on damping in roll: a. remains the same. b. increases because the TAS increases. c. decreases because the ailerons are less effective. d. decreases because the density decreases.

Answer 58

D

Question 59

Sweepback of the wings will: a. not affect lateral stability. b. decrease lateral stability. c. increases lateral stability at high speeds only. d. increases lateral stability at all speeds.

Answer 59

D

Question 60

At low forward speed: a. increased downwash from the wing will cause the elevators to be more responsive. b. due to the increased angle of attack of the wing the air will flow faster over the wing giving improved aileron control. c. a large sideslip angle could cause the fin to stall. d. a swept back wing will give an increased degree of longitudinal stability.

Answer 60

C

Question 61

Following a lateral disturbance, an aircraft with Dutch roll instability will: a. go into a spiral dive. b. develop simultaneous oscillations in roll and yaw. c. develop oscillations in pitch. d. develop an unchecked roll.

Answer 61

B

Question 62

To correct dutch roll on an aircraft with no automatic protection system: a. use roll inputs b. use yaw inputs c. move the CG d. reduce speed below M MO

Answer 62

A

Question 63

A yaw damper: a. increases rudder effectiveness. b. must be disengaged before making a turn. c. augments stability. d. increases the rate of yaw.

Answer 63

C

Question 64

A wing which is inclined downwards from root to tip is said to have: a. wash out. b. taper. c. sweep. d. anhedral.

Answer 64

D

Question 65

The lateral axis of an aircraft is a line which: a. passes through the wing tips. b. passes through the centre of pressure, at right angles to the direction of the airflow. c. passes through the quarter chord point of the wing root, at right angles to the longitudinal axis. d. passes through the centre of gravity, parallel to a line through the wing tips.

Answer 65

D

Question 66

Loading an aircraft so that the CG exceeds the aft limits could result in: a. loss of longitudinal stability, and the nose to pitch up at slow speeds b. excessive upward force on the tail, and the nose to pitch down c. excessive load factor in turns d. high stick forces

Answer 66

A

Question 67

The tendency of an aircraft to suffer from dutch roll instability can be reduced: a. by sweeping the wings b. by giving the wings anhedral c. by reducing the size of the fin d. by longitudinal dihedral

Answer 67

B

Question 68

What determines the longitudinal static stability of an aeroplane? a. The relationship of thrust and lift to weight and drag b. The effectiveness of the horizontal stabilizer, rudder, and rudder trim tab c. The location of the CG with respect to the AC d. the size of the pitching moment which can be generated by the elevator

Answer 68

C

Question 69

Dihedral angle is: a. the angle between the main plane and the longitudinal axis b. the angle measured between the main plane and the normal axis c. the angle between the quarter chord line and the horizontal datum d. the upward and outward inclination of the main planes to the horizontal datum

Answer 69

D

Question 70

Stability around the normal axis: a. is increased if the keel surface behind the CG is increased b. is given by the lateral dihedral c. depends on the longitudinal dihedral d. is greater if the wing has no sweepback

Answer 70

A

Question 71

The Centre of Gravity of an aircraft is found to be within limits for take-off:: a. the C of G will be within limits for landing b. the C of G for landing must be checked, allowing for fuel consumed c. the C of G will not change during the flight d. the flight crew can adjust the CG during flight to keep it within acceptable limits for landing

Answer 71

B

Question 72

The ailerons are deployed and returned to neutral when the aircraft has attained a small angle of bank. If the aircraft then returns to a wings-level attitude without further control movement it is: a. neutrally stable b. statically and dynamically stable c. statically stable, dynamically neutral d. statically stable

Answer 72

B

Question 73

The property which tends to decreases rate of displacement about any axis, but only while displacement is taking place, is known as: a. stability b. controllability c. aerodynamic damping d. manoeuverability

Answer 73

C

Question 74

If an aircraft is loaded such that the stick force required to change the speed is zero a. the CG is on the neutral point b. the CG is behind the neutral point c. the CG is on the manouevre point d. the CG is on the forward CG limit

Answer 74

A

Question 75

Ordinarily, the static longitudinal stability of a conventional aeroplane configuration ........ ...... with lift coefficient. In other words, the slope of CM versus CL does not change with CL. However, if: -- the aeroplane has ..............., -- there is a large contribution of ................... on stability, or --there are significant changes in ................ at the horizontal tail ................ changes in static stability can occur at high lift coefficients (low speed). noticeable changes in static stability can occur at high lift coefficients (low speed).

Answer 75

Ordinarily, the static longitudinal stability of a conventional aeroplane configuration does not vary with lift coefficient. In other words, the slope of CM versus CL does not change with CL. However, if: -- the aeroplane has sweepback, -- there is a large contribution of “power effect” on stability, or --there are significant changes in downwash at the horizontal tail

Question 76

Aerodynamic centre is defined .....................

Answer 76

Aerodynamic centre is defined as the point on the wing Mean Aerodynamic Chord (MAC) where the wing pitching moment coefficient does not vary with lift coefficient. All changes in lift coefficient effectively take place at the wing aerodynamic centre.

Question 77

The degree of positive camber of the wing has .............. on longitudinal stability. The pitching moment about the AC is always ...................

Answer 77

The degree of positive camber of the wing has no effect on longitudinal stability. The pitching moment about the AC is always negative regardless of angle of attack.

Question 78

Because the ...................................., the wing contributes an .............. pitching moment to the aircraft.

Answer 78

As the wing AC is forward of the CG of the wing , the wing contributes an unstable pitching moment to the aircraft.

Question 79

The tail moment is determined by the CG position AND the effectiveness of the tailplane. For a given moment arm (CG position), the effectiveness of the tailplane is dependent upon:

Answer 79

-- Downwash from the wing -- Dynamic pressure at the tailplane -- Longitudinal dihedral

Question 80

LONGITUDINAL DIHEDRAL Page 262

Answer 80

--The difference between tailplane and wing incidence. For longitudinal static stability the tailplane incidence is smaller. This will generate a greater percentage increase in tailplane lift than wing lift for a given verticalgust. --This guarantees that the positive contribution of the tailplane to static longitudinal stability will be sufficient to overcome the sum of the de-stabilising moments from the other components of the aeroplane.

Question 81

Any factor which alters the rate of change of downwash at the horizontal tail (e.g. flaps or propeller slipstream) will directly affect the tail contribution and aeroplane stability. Downwash ...... static longitudinal stability.

Answer 81

Any factor which alters the rate of change of downwash at the horizontal tail (e.g. flaps or propeller slipstream) will directly affect the tail contribution and aeroplane stability. Downwash decreases static longitudinal stability.

Question 82

The CG position which produces ....................... is referred to as the “neutral point”. The neutral point may be imagined as the ..........................., i.e., with the CG at the neutral point, all changes in net lift ...................................... The neutral point defines the most aft CG position without ............

Answer 82

The CG position which produces zero slope and neutral static stability is referred to as the “neutral point”. The neutral point may be imagined as the effective aerodynamic centre of the entire aeroplane configuration, i.e., with the CG at the neutral point, all changes in net lift effectively occur at that point and no change in pitching moment results. The neutral point defines the most aft CG position without static instability.

Question 83

Power effects on stability will be most significant when .......................

Answer 83

Power effects on stability will be most significant when the aeroplane operates at high power and low airspeeds such as during approach and while taking-off.

Question 84

The effects of power are considered in two main categories.

Answer 84

The effects of power are considered in two main categories. --First, there are the direct effects resulting from the forces created by the propulsion unit. The vertical location of the thrust line defines one of the direct contributions to stability. If the thrust line is below the CG,as illustrated, a thrust increase will produce a positive or nose up moment and the effect is destabilising. A propeller located ahead of the CG contributes a destabilising effect.The magnitude of the unstable contribution depends on the distance from the CG to the propeller and is largest at high power and low dynamic pressure. --Next, there are the indirect effects ofthe slipstream and other associated flow which alter the forces and moments of the aerodynamic surfaces

Question 85

A propeller located ahead of the CG contributes a ........... effect.The magnitude of the unstable contribution depends on the ........................ and is largest at .................... and ...................

Answer 85

A propeller located ahead of the CG contributes a destabilising effect.The magnitude of the unstable contribution depends on the distance from the CG to the propeller and is largest at high power and low dynamic pressure.

Question 86

The indirect effects of power are of greatest concern in the .............................

Answer 86

The indirect effects of power are of greatest concern in the propeller powered aeroplane rather than the jet powered aeroplane.

Question 87

Ordinarily, the induced flow at the horizontal tail of a jet aeroplane is slight and is ......... when the jet passes ................

Answer 87

Ordinarily, the induced flow at the horizontal tail of a jet aeroplane is slight and is destabilising when the jet passes underneath the horizontal tail.

Question 88

--The combined direct and indirect power effects contribute to a ....... of static stability at ......, ...... CL and ....... dynamic pressure. -- It is generally true that any aeroplane will experience the ............ of static longitudinal stability under these conditions. Because of the greater magnitude of both direct and indirect power effects, the propeller powered aeroplane usually experiences a ......... effect than the jet powered aeroplane.

Answer 88

The combined direct and indirect power effects contribute to a general reduction of static stability at high power, high CL and low dynamic pressure. It is generally true that any aeroplane will experience the lowest level of static longitudinal stability under these conditions. Because of the greater magnitude of both direct and indirect power effects, the propeller powered aeroplane usually experiences a greater effect than the jet powered aeroplane.

Question 89

--The contribution of High lift devices on longitudinal stability ? --Hence, the aeroplane may experience the most critical forward neutral point during .......... ...... or ........... During this condition of flight the static stability is ........ ................. --The power - on neutral point may set the .............. limit of CG position.

Answer 89

--High lift devices tend to increase downwash at the tail and reduce the dynamic pressure at the tail, both of which are destabilising. --Hence, the aeroplane may experience the most critical forward neutral point during the power approach or overshoot/missed approach. During this condition of flight the static stability is usually the weakest and particular attention must be given to precise control of the aeroplane. --The power - on neutral point may set the most aft limit of CG position.

Question 90

Change in Elevator deflection and its contribution to stability

Answer 90

A CHANGE IN ELEVATOR POSITION DOES NOT ALTER THE TAIL CONTRIBUTION TO STABILITY

Question 91

What is stick position stability ?

Answer 91

--If the CG location is ahead of the neutral point and control position is directly related to surface deflection, the aeroplane will give evidence of stick position stability. In other words, the aeroplane will require the stick to be moved aft to increase the angle of attack and trim at a lower airspeed and to be moved forward to decrease the angle of attack and trim at a higher airspeed. --It is highly desirable to have an aeroplane demonstrate this feature. If the aeroplane were to have stick position instability, the aeroplane would require the stick to be moved aft to trim at a higher airspeed or to be moved forward to trim at a lower airspeed.

Question 92

MANOEUVRE STABILITY ...........Page 274

Answer 92

--When the pilot pitches the aircraft, it rotates about the CG and the tailplane is subject to a pitching velocity, in this example, downwards. Due to the pitching velocity in manoeuvring flight, the longitudinal stability of the aeroplane is slightly greater than in steady flight conditions. --The tailplane experiences an upwards component of airflow due to its downwards pitching velocity. The vector addition of this vertical component to the TAS provides an increase in effective angle of attack of the tail, which creates an increase in tail lift, opposing the nose up pitch displacement. --Since the negative pitching moment opposes the nose up pitch displacement but is due to the nose up pitching motion, the effect is a damping in pitch (aerodynamic damping).

Question 93

Effect of Altitude on Aerodynamic Damping

Answer 93

INCREASING ALTITUDE AT A CONSTANT IAS DECREASES AERODYNAMIC DAMPING --The pitch damping of the aeroplane is related to air density. At high altitudes, the high TAS reduces the change in tail angle of attack for a given pitching velocity and reduces the pitch damping. Thus, a decrease in manoeuvring stick force stability can be expected with increased altitude.

Question 94

Manoeuvre point

Answer 94

The CG position when the tail moment would be the same as the wing moment during manoeuvring is known as the manoeuvre point and this “neutral point” will be further aft than for 1g flight.

Question 95

--When the aeroplane has high static stability, the manoeuvring stability will be .... and a ....... stick force gradient will result. --A possibility exists that the forward CG limit could be set to prevent an ............ ....... As the CG moves aft, the stick force gradient .......... with ........ manoeuvring stability and the ....... limit of stick force gradient may be reached.

Answer 95

--When the aeroplane has high static stability, the manoeuvring stability will be high and a high stick force gradient will result. --A possibility exists that the forward CG limit could be set to prevent an excessively high manoeuvring stick force gradient. As the CG moves aft, the stick force gradient decreases with decreasing manoeuvring stability and the lower limit of stick force gradient may be reached.

Question 96

The aeroplane should have sufficient longitudinal control power to ........... or the .......... during manoeuvres. This particular control requirement can be most critical for an aeroplane in supersonic flight. Supersonic flight is usually accompanied by large ........... in static longitudinal stability (due to ...... CP movement) and a ...................... effectiveness of control surfaces. In order to cope with these trends, .................. must be used to attain .......... or .............. in supersonic flight

Answer 96

The aeroplane should have sufficient longitudinal control power to attain the maximum usable lift coefficient or the limit load factor during manoeuvres.This particular control requirement can be most critical for an aeroplane in supersonic flight. Supersonic flight is usually accompanied by large increases in static longitudinal stability (due to aft CP movement) and a reduction in the effectiveness of control surfaces. In order to cope with these trends, powerful all-moving surfaces must be used to attain limit load factor or maximum usable CL in supersonic flight

Question 97

--The CG ahead of the main gear contributes a .............. moment and this consideration could decide the most aft location of the .......................... --To balance these two nose down moments, the horizontal tail must be capable of producing a ................... moment big enough to attain the takeoff attitude at the specified speed.

Answer 97

--The CG ahead of the main gear contributes a nose down moment and this consideration could decide the most aft location of the main landing gear during design. --To balance these two nose down moments, the horizontal tail must be capable of producing a nose up moment big enough to attain the takeoff attitude at the specified speed.

Question 98

At landing, the aeroplane must have sufficient control power to ensure adequate control at specified landing speeds. The most critical requirement will exist when the CG is in ................., flaps are ....... ............., and power is ................ This configuration will provide the ......... condition which is most demanding of ................

Answer 98

At landing, the aeroplane must have sufficient control power to ensure adequate control at specified landing speeds. The most critical requirement will exist when the CG is in the most forward position, flaps are fully extended, and power is set at idle. This configuration will provide the most stable condition which is most demanding of controllability.

Question 99

As the aircraft enters ground effect, the .......... in downwash at the tail tends to ....... the static stability and produce a .......... moment from the reduction in .......... on the tail. Thus, the aeroplane just off the runway surface, will require ............... to trim at a given lift coefficient and the landing control requirement may be critical in the design of longitudinal control power.

Answer 99

As the aircraft enters ground effect, the decrease in downwash at the tail tends to increase the static stability and produce a nose down moment from the reduction in download on the tail. Thus, the aeroplane just off the runway surface, will require additional control deflection to trim at a given lift coefficient and the landing control requirement may be critical in the design of longitudinal control power.

Question 100

DYNAMIC STABILITY

Answer 100

While static stability is concerned with the initial tendency of an aircraft to return to equilibrium, dynamic stability is defined by the resulting motion with time. If an aircraft is disturbed from equilibrium, the time history of the resulting motion indicates its dynamic stability. In general, an aircraft will demonstrate positive dynamic stability if the amplitude of motion decreases with time. --The initial tendency to continue in the displacement direction is evidence of static instability and the increasing amplitude is proof of dynamic instability.

Question 101

What is the pre-requisite for positive dynamic stability.

Answer 101

The reduction of amplitude with time indicates there is resistance to motion and that energy is being dissipated. Dissipation of energy or damping is necessary to provide positive dynamic stability. --When natural aerodynamic damping cannot be obtained, artificial damping must be provided to give the necessary positive dynamic stability.

Question 102

What is the origin of PIO ?

Answer 102

--Divergent oscillation results when energy is supplied to the motion rather than dissipated by positive damping. An example of divergent oscillation occurs if a pilot unknowingly makes control inputs which are near the natural frequency of the aeroplane in pitch; energy is added to the system, negative damping exists, and Pilot Induced Oscillation ( P.I.O.) results. --IF AN AIRCRAFT IS STATICALLY UNSTABLE, IT CANNOT BE DYNAMICALLY STABLE

Question 103

--The aeroplane in free flight has ..... degrees of freedom: ............ ............ ................. --Since the aeroplane is usually ..........., there will be no need to consider coupling between longitudinal and lateral / directional motions.

Answer 103

--The aeroplane in free flight has six degrees of freedom: rotation in roll, pitch, and yaw and translation in the horizontal, vertical, and lateral directions. --Since the aeroplane is usually symmetrical from left to right, there will be no need to consider coupling between longitudinal and lateral / directional motions.

Question 104

LONG PERIOD OSCILLATION (PHUGOID)

Answer 104

--The phugoid is a gradual interchange of potential and kinetic energy about some equilibrium airspeed and altitude. --The phugoid or long period oscillation involves noticeable variations in:- >> pitch attitude, >> altitude and >> airspeed, but >> nearly constant angle of attack. (not much change in load factor) --The period of oscillation in the phugoid is between 1 and 2 minutes. Since the pitch rate is quite low and only negligible changes in angle of attack take place, damping of the phugoid is weak. However, such weak damping does not necessarily have any great consequence. Since the period of oscillation is so great, long period oscillation is easily controlled by the pilot. Due to the nature of the phugoid, it is not necessary to make any specific aerodynamic provisions to counteract it.

Question 105

SHORT PERIOD OSCILLATION

Answer 105

Short period oscillation involves significant changes in angle of attack (load factor), with approximately constant speed, height and pitch attitude; it consists of rapid pitch oscillations during which the aeroplane is constantly being restored towards equilibrium by its static stability and the amplitude of the short period oscillations being decreased by pitch damping. --Short period oscillation at high dynamic pressures with large changes in angle of attack could produce severe ‘g’ loads. (Large changes in load factor). --Short period Oscillations that correspond closely with the normal pilot response lag time, e.g., 1 or 2 seconds or less. There is the possibility that an attempt by the pilot to forcibly damp an oscillation may actually reinforce the oscillation (PIO) and produce instability. --Short period oscillation is not easily controlled by the pilot. --If short period oscillation occurs, release the controls; the aeroplane is designed to demonstrate the necessary damping. --Modern large high speed jet transport aircraft are fitted with pitch dampers, which automatically compensate for any dynamic longitudinal instability.

Question 106

Why is short period oscillation more dangerous ?

Answer 106

--Of the two modes of dynamic longitudinal stability, the short period oscillation is of greatest importance. The short period oscillation can generate damaging flight loads due to the rapid changes in ‘g’ loading, and it is adversely affected by pilot response lag (PIO).

Question 107

DYNAMIC STABILITY IS REDUCED AT HIGH ALTITUDE DUE TO .........................

Answer 107

DYNAMIC STABILITY IS REDUCED AT HIGH ALTITUDE DUE TO REDUCED AERODYNAMIC DAMPING

Question 108

The axis system of an aeroplane defines a positive yawing moment, N, as a moment .............

Answer 108

The axis system of an aeroplane defines a positive yawing moment, N, as a moment about the normal axis which tends to rotate the nose to the right.

Question 109

SIDESLIP ANGLE

Answer 109

--The sideslip angle relates the displacement of the aeroplane centerline from the relative airflow. Sideslip angle is provided the symbol β (beta) and is positive when the relative wind is displaced to the right of the aeroplane centerline. --The sideslip angle, β, is essentially the “directional angle of attack” of the aeroplane. --

Question 110

Static directional stability will exist when the curve of Cn versus β has a ....... slope and the degree of stability will be a function of ............

Answer 110

Static directional stability will exist when the curve of Cn versus β has a positive slope and the degree of stability will be a function of the slope of this curve.

Question 111

CONTRIBUTION OF THE AEROPLANE COMPONENTS to Directional Stability

Answer 111

--FUSELAGE: It is an aerodynamic body and a condition of sideslip can be likened to an “angle of attack”, so that an aerodynamic side force is created. This side force acts through the fuselage aerodynamic centre (AC), which is close to the quarter-length point. If this aerodynamic centre is ahead of aircraft centre of gravity, as is usually the case, the effect is de-stabilizing. --DORSAL AND VENTRAL FINS: To overcome the instability in the fuselage it is possible to incorporate into the overall design, dorsal or ventral fins. A dorsal fin is a small aerofoil, of very low aspect ratio, mounted on top of the fuselage near the rear. A ventral fin is mounted below.Dorsal and ventral fins become more effective at relatively high sideslip angles. Due to their low aspect ratio they do not tend to stall at any sideslip angle which an aircraft is likely to experience in service. --FIN: The magnitude of the fin contribution to static directional stability depends on both the change in fin lift and the fin moment arm. Clearly, the fin moment arm is a powerful factor.The contribution of the fin to directional stability depends on its ability to produce changes in lift, or side force, for a given change in sideslip angle. --WING and NACELLES:The contribution of the wing to static directional stability is usually small.Sweepback produces a stabilizing effect, which increases with increase in CL (i.e. at lower IAS). --

Question 112

--While dorsal and ventral fins contribute in exactly the same way to directional static stability, a dorsal fin contributes ............ to lateral static stability, while a ventral fin is ............... in this mode. --In relation to Directional Stability -Sweepback produces a ......... effect, which increases with ........ in CL (i.e. at lower IAS).

Answer 112

While dorsal and ventral fins contribute in exactly the same way to directional static stability, a dorsal fin contributes positively to lateral static stability, while a ventral fin is destabilising in this mode --In relation to Directional Stability -Sweepback produces a stabilizing effect, which increases with increase in CL (i.e. at lower IAS).

Question 113

How does Dorsal Fin contribute to the Directional Stability ?

Answer 113

The dorsal fin has a powerful effect on preserving the directional stability at large angles of sideslip which would produce stall of the fin. The addition of a dorsal fin to the aeroplane will reduce the decay of directional stability at high sideslip in two ways. -- The least obvious but most important effect is a large increase in the fuselage stability at large sideslip angles. -- In addition, the effective aspect ratio of the fin is reduced which increases the stall angle for the surface. --By this twofold effect, the addition of the dorsal fin is a very useful device. The decreased lift curve slope of a sweptback fin will also decrease the tendency for the fin to stall at high sideslip angles.