Longitudinal Static Stability Flashcards Preview

AER324 - Aircraft Dynamics and Control > Longitudinal Static Stability > Flashcards

Flashcards in Longitudinal Static Stability Deck (22)
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1
Q

Define trim

A

control surface deflection required to get aircraft into the steady state

2
Q

Define stability

A

if aircraft will settle into steady state after a disturbance

3
Q

Define dynamic response

A

motion caused by atmospheric disturbances or changes to the control inputs

4
Q

Describe body fixed reference frame

A

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

5
Q

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

A

why - makes equations of motion simpler

difficulties - CG moves with mass change

6
Q

Define quasi-static

A

unsteady aerodynamic and inertial effects are ignored

7
Q

Define statically stable

A

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

8
Q

Define statically unstable

A

after a disturbance moments generated to tilt aircraft away from equilibrium

9
Q

Define neutrally stable

A

no restoring moments generated

10
Q

Define aerodynamic centre

A

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

11
Q

Why does stability increase as CG moves forward

A
  • moment arm of horizontal stabiliser increases

- contribution of wings lift to pitching moment is also stabilising

12
Q

Why is it desirable to have trim alpha as positive

A

the wing will produce more lift

13
Q

List all contributions for stability of an aircraft

A
  • wing
  • tail
  • fuselage
  • propulsion system
14
Q

List method to derive expression for the contribution of the wing

A
  • 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
15
Q

Describe wing contribution

A
  • 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
16
Q

List method to derive expression for the contribution of the tail

A
  • 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
17
Q

List advantages and disadvantages of canard

A

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

18
Q

Define stick fixed neutral point

A
  • 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’
19
Q

define static margin

A

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

20
Q

How to find stick fixed neutral point

A
  • substitute Xnp = Xcg
  • make Cma = 0
  • solve for Xnp
21
Q

List primary aerodynamic controls

A

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

high lift devices - flaps, slats and spoilers

22
Q

Factors effecting control surface design

A
  • 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)