Longitudinal Static Stability

Question 1

Define trim

Answer 1

control surface deflection required to get aircraft into the steady state

Question 2

Define stability

Answer 2

if aircraft will settle into steady state after a disturbance

Question 3

Define dynamic response

Answer 3

motion caused by atmospheric disturbances or changes to the control inputs

Question 4

Describe body fixed reference frame

Answer 4

origin centre of gravity
x axis - along body out of nose
y axis - along starboard side
x axis - down

Question 5

Why use CG as origin, what are the difficulties with it

Answer 5

why - makes equations of motion simpler

difficulties - CG moves with mass change

Question 6

Define quasi-static

Answer 6

unsteady aerodynamic and inertial effects are ignored

Question 7

Define statically stable

Answer 7

after a disturbance restoring moments generated to tilt aircraft back into equilibrium

Question 8

Define statically unstable

Answer 8

after a disturbance moments generated to tilt aircraft away from equilibrium

Question 9

Define neutrally stable

Answer 9

no restoring moments generated

Question 10

Define aerodynamic centre

Answer 10

where changes in angle of attack do not change the pitching moment
not typically 0, just a constant

Question 11

Why does stability increase as CG moves forward

Answer 11

• moment arm of horizontal stabiliser increases

- contribution of wings lift to pitching moment is also stabilising

Question 12

Why is it desirable to have trim alpha as positive

Answer 12

the wing will produce more lift

Question 13

List all contributions for stability of an aircraft

Answer 13

• wing
• tail
• fuselage
• propulsion system

Question 14

List method to derive expression for the contribution of the wing

Answer 14

• resolve lift and drag into normal and chord wise forces
• sum moments around CG
• substitute Lw and D
• divide by 0.5pV^2Sc to get moments
• assume alpha is small (cosx = 1, sinx = x)
• assume Clw&raquo_space; Cdw and Zgcw = 0
• apply conditions for statically stable

Question 15

Describe wing contribution

Answer 15

• indicates ac must lie aft of cg
• requires negative-cambered aerofoil section
• none of these fulfilled due to other considerations (Aerodynamics)
• wing contribution is always destabilising

Question 16

List method to derive expression for the contribution of the tail

Answer 16

• total lift L = Lw + Lt
• use lift equation
• i.e. 0.5pV^2SCl = 0.5pVw^2SClw + 0.5pVt^2SClt
• divide by QS, assume V = Vw
• substitute n = tail efficiency = 0.5pVt^2/0.5pV^2
• sum pitching moments around CG
• assume small angles and Lt&raquo_space; Dt
• nondimensionalise (divide by QSc)
• Substitute Vh
• substitute Clt = Cla x at
• substitute at = aw - iw - e - it
• split into Cmot + Cmat x a

Question 17

List advantages and disadvantages of canard

Answer 17

A:

• free from wing/propulsion system flow interference
• more attractive from trimming nose-down moment of high-lift devices

D
- destabilises contribution to static stability

Question 18

Define stick fixed neutral point

Answer 18

• the rear most position to which the CG can be moved aft before it becomes unstable. At neutral point, the aircraft is neutrally stable
• stick-fixed related to the fact the elevator angle is controlled by the pilot through the control circuit (stick deflection constant at this point hence ‘stick-fixed’

Question 19

define static margin

Answer 19

the distance through which CG can move rearwards before reaching the neutral point. It is a measure of longitudinal stability.

Question 20

How to find stick fixed neutral point

Answer 20

• substitute Xnp = Xcg
• make Cma = 0
• solve for Xnp

Question 21

List primary aerodynamic controls

Answer 21

roll - differential deflection of ailerons
Pitch - elevators
Yaw - rudder

high lift devices - flaps, slats and spoilers

Question 22

Factors effecting control surface design

Answer 22

• control effectiveness (flap size/tail volume)
• hinge moments (force required to overcome aerodynamic moments to rotate control surfaces)
• aerodynamic and mass balancing (ensure stick force within acceptable range)