81 04 Longitudinal Stability

Question 1

A positive Cm is what kind of motion?

Answer 1

A nose up motion

Question 2

What effect on AOA do the following have?

Answer 2

• Up gust: increases AOA

- Down gust: decreases AOA

Question 3

What is the Absolute AOA?

Answer 3

Absolute AOA is AOA corrected such that at 0 degrees AOA, L = 0

Given by Alpha a

Question 4

What factors increases the pitching moment coefficient?

Which has the most impact?

Answer 4

• Increased Dynamic pressure
• Increased surface area of the wings
• Increased MAC
• q, Dynamic pressure has the greatest impact. So Pitching moment will be greater at higher speeds.

Question 5

What gives the greatest contribution to the longitudinal stability of an AC?

Answer 5

The Tail Plane

Question 6

What factors effect the size of the moment produced by the Tail Plane?

What is this know as?

Answer 6

• Surface are of the Tail Plane
• The distance of the Tail plane from the CG
• Collectively this is known as the Tail Volume

Question 7

What impact do the following have on the Longitudinal Stability of an AC?

• Wings
• Fuselage
• Engine Nacelles (assuming pod mounted)

Answer 7

• Wings: Stabilising
• Fuselage: De-stabilising
• Engine Nacelles: De-stabilising
• Over all AC is Stable though due to the strong influence of the Tail Plane.

Question 8

What is the impact of CG on the Longitudinal stability of an AC?

Answer 8

• Further forward the CG the more stable the AC is

- AC is most stable when the CG is at the forward limit and least stable when it is at the aft limit

Question 9

What is the Neutral Point?

Answer 9

• The aft most point on an aerofoil the CG can move to before the AC becomes statically unstable.
• This is a fixed point decided on during the design of the AC

Question 10

What is the Static Margin?

Answer 10

This is the safety margin between the neutral point and the aft CG limit

Question 11

What is the effect of flaps on stability and why?

Answer 11

• Trailing edge flaps themselves are stabilising
• Overall high lift devises are De-stabilising. This is due to the increase in downwash which reduces the effectiveness of the Tail Plane

Question 12

What is the Stick Force Gradient?

Answer 12

The amount of extra stick force which must be applied per unit increase of LF

(LF = n = g)

Stick Force Gradient = Stick Force x Increase in LF

Question 13

What is an important factor to remember when calculating Stick Force Gradient (= Stick Force x LF increase)?

Answer 13

An AC starts at 1g so must minus 1 from the LF

Question 14

What is Aerodynamic dampening?

Answer 14

As an AC pitches up (+ve CM) the RAF on the Tail Plane changes. This changes the AOA on the Tail Plane which reduces the downforce the tail is producing.

Therefore, during the pitching manoeuvre, the AC is resistant to pitching up.

Question 15

What is the effect of Altitude on Aerodynamic dampening?

Answer 15

As altitude increases, TAS increases, so there is a shallower AOA to the tail plane.

Therefore higher altitude reduces Aerodynamic dampening.

Question 16

What are the two types of periodic Dynamic Stability?

Answer 16

• Short Period Oscillations

- Long Period Oscillations (Aka Phugoid)

Question 17

How long are short period oscillations and what effect do they have?

Answer 17

• Aprox. 1sec
• Little impact on height and airspeed
• Large impact on AOA and LF

Question 18

How long are long period (phugoid) oscillations and what do they effect?

Answer 18

• 1 to 2mins
• Little change on AOA and LF
• Large impact on height and airspeed

Question 19

What is the effect of Altitude on static and dynamic Longitudinal stability?

Answer 19

Static and dynamic stability reduce with altitude

Question 20

What is the effect of CG on static and dynamic Longitudinal stability?

Answer 20

A forward CG will increase Static and dynamic stability

Question 21

What simple system can be used to increase the stick force felt when increasing LF?

Answer 21

Bob Weight