Chapter 23: Longitudinal Stability

Question 1

Pitch Coefficient Equation

Answer 1

CM = M / (qSc) where c = MAC chord

Question 2

Pitch Stability Stabilizing / Destabilizing Elements on AC (4)

Answer 2

Tailplane - stabilizing
Wings - stabilizing
Body - destabilizing
Engine - destabilizing (under wing)

Question 3

CM vs AoAa Graph

Stability, Neutral Stability, Instability, Sweptback Wing

Answer 3

AC trimmed at AoAa for which CM = 0
Stability -> downward slope
Neutral -> horizontal
Instability -> upward slope
Sweptback -> downward (stable) at low AoAa, neutral near stall and upward (unstable) after stall

Question 4

Neutral Point

What is? How does stick free/fixed affect it?

Answer 4

If CG aft of neutral point = unstable
Stick free -> neutral point more forward
Stick fix -> neutral point more aft

Question 5

Static Margin

Answer 5

Safety distance between CG aft limit and neutral point

Question 6

Flap Effect on Stability (2)

Answer 6

Stable - CP moves back

Unstable - wake turbulence on H stabilizer

Question 7

Stick-Position Stability

Answer 7

Same control column input for trim and manoeuvre. Stability if CG ahead of AC (normal flight)

Question 8

Stick-Force Stability

Airliner high or low stick-force?

Answer 8

Required Stick force and movement in same direction

Airliner = high for fine controls

Question 9

Aerodynamic Damping

Answer 9

While pitching up -> tail goes down -> higher AoA -> pitch down moment fighting pitch up

Question 10

Stick-Force and Aerodynamic Damping

How does stick-force differ

Answer 10

Low altitude -> more aerodynamic damping -> higher stick-force

Question 11

Longitudinal Oscillations (2)
Types, duration, what parameter changes, what can first one produce

Answer 11

Short Period, 1 - 2s, high n, high AoA variation
can produce PIO (pilot induced oscillation)

Phugoid, 1 - 2 min, high H variation

Question 12

Dynamic. Static Stability
Decreases with (2)

Answer 12

Altitude

Aft CG