Longitudinal Stability

Question 1

When will an aircraft have static longitudinal stability?

Answer 1

If nose down (Negative) tail moment is greater than the nose up
(Positive) wing moment, the aircraft will have Static longitudinal stability

Question 2

What provides longitudinal stability?

Answer 2

Horizontal stabiliser, if you pitch up a restoring force will pitch down.

Question 3

What does static longitudinal stability depend upon?

Answer 3

Relationship between AOA and Pitching moment.

M=Cm Q S (MAC)

M - pitching moment about the CG
Q - dynamic pressure
S - wing area

Question 4

Positive static longitudinal stability

Answer 4

Positive to negative coefficient, increasing AOA.

Question 5

Neutral stability

Answer 5

Horizontal line.
Increasing AOA

Question 6

Negative stability

Answer 6

Negative to positive coefficient.
Increasing AOA.

Question 7

The tail plane

Answer 7

Most important contributor to longitudinal static stable.
Size of moment depends on: area of tail plane and distance from CG.
Collectively known as tail volume: too little, weak longitudinal static stability and too much, excessively strong longitudinal stability.

Tail area x arm = tail volume.

Question 8

The wing

Answer 8

Increased absolute AOA, increased lift, increased restoring moment.

Question 9

Longitudinal dihedral

Answer 9

Wing has bigger angle of incidence.
Main effect, to vary the trim point.

Question 10

Effect of CG

Answer 10

Aft CG limit, minimum acceptable restoring force.
Fwd CG limit, maximum acceptable restoring forces.

Question 11

Neutral point

Answer 11

Position if the CG where tail and wing moment are equal

Question 12

Neutral point and static margin

Answer 12

Static margin is between neutral point and aft CG limit.
Quoted as % of MAC
The longer the static margin:
-greater longitudinal static stability

Question 13

CG/MAC influence on longitudinal stability.

Answer 13

Lower % MAC = more stable

Question 14

Stick force per g

Answer 14

Amount of control column force to produce a given load factor.
Affected by: CG, aerodynamic damping, altitude, mass.
CAT has lower limit load.

Increases as CG moves forward.
Increase at lower altitude.

Question 15

Aerodynamic damping

Answer 15

Greater rate of pitch, greater reduction in tail plane downforce

Question 16

Altitude

Answer 16

Lower altitude, lower TAS, steeper flow, more damping.
And visa versa.

Question 17

Longitudinal dynamic stability

Answer 17

Cannot exist unless there is static stability.
Degree of dynamic stability is defined by time taken for oscillations to reduce to half amplitude.

Two types:
Short period oscillation
Long period oscillation

Question 18

Short period oscillation

Answer 18

Approximately 1 second.
Little variation in height or speed.
Large variations in AOA and load factor.

Question 19

Long period (phugoid) oscillations.

Answer 19

Approximately 1-2minutes.
Large variation in height and speed.
Little change AOA and load factor.
Easy to correct.
May be triggered by horizontal gust.

Question 20

Static and dynamic stability decrease with altitude due to reduced effect of aerodynamic damping.

Question 21

Greater difference between CG and AC is a greater pitching moment.