Tutorials Flashcards
Notation for coefficient of lift of a) wing b) aerofoil
a) CL
b)Cl
Notation for lift and drag of a) wing and b) horizontal tail
a) Lw, Dw
b)Lt, Dt
Are pitching moment coefficient notation Cm and CM different?
No, they are used interchangeably
What does ε represent
the downwash angle behind the wing, i.e. the angle by which the airflow is deflected behind a wing due to the effects of vortices
What is the angle of incidence, i?
The angle between the chord line of the wing where the wing is mounted to the fuselage, and a reference axis along the fuselage
What is xac,w?
The wing aerodynamic centre location from the wing leading edge
What is xcg?
The location of the centre of gravity from the leading edge of the aircraft
How are wing reference area, wing span, and mean aerodynamic chord denoted?
S or Sw
b
Cbar
What symbols correspond to a) ailerons, b) elevators, and c) rudders
a) ξ (xi)
b) η (eta)
c) ζ (zeta)
What are the sign conventions for control surface movement?
1) positive aileron=> port wing down
2) Positive elevator=>nose down
3)positive rudder=>nose to port
Which side is port and which is starboard
port=left
starboard=right
What rotation does a) elevator, b) ailerons, c) rudder provide
a) pitch
b) roll
c) yaw
Where is the body fixed coordinate system centred?
At the centre of gravity of the aircraft
Is centre of gravity fixed?
No it moves as payload changes, fuel is consumed and as other factors changes
The simplest trim condition is straight and level flight, what does this mean?
The forces are balanced and the moments about the CoG sum to zero. Accelerations and angular accelerations are all zero.
Which surface most strongly influences longitudinal trim
The elevator
What is the best way to begin discussing airplane trim and stability?
With quasi-static analysis:
pitching moment about CoG is first considered, followed by the changes in pitching moment generated when the vehicle changes pitch
What does quasi-static mean
Unsteady aerodynamic and inertial effects are neglected
When is an aircraft said to be statically stable?
When the direction of the moment is restorative.
When is an aircraft said to be neutrally stable?
When no moment is produced
When is an aircraft said to be statically unstable?
When the moment produced further increases the angle of attack
Where is the aerodynamic centre usually positioned?
1/4 aft of the leading edge
What happens as CoG moves forward and why
the longitudinal static stability increases because:
a) the moment arm of the horizontal stabiliser increases
b) the contribution of the wing
Where is the most convenient place to locate lift, drag, and moment?
Aerodynamic centre
Define longitudinal static stability
It characterises the initial tendency of an aircraft to return to equilibrium after a pitch disturbance
Where is the trim point for a longitudinally marginally stable aircraft
Cm,cg=0
What 2 conditions have to be satisfied for an aircraft to be longitudinally statically stable? (numerical)
Cm,0 = Cm,α|α=0 > 0 and 2) Cmα < 0.
How can CLw be determined
CLαw*αw
WHere CLαw is the slope of the lift curve and αw is wing angle of attack
What are the two conditions for static stability? (wordy+numerical)
Cm,αw = dCm,cg,w/dαw < 0, meaning aerodynamic centre must lie aft of the centre of gravity, i.e.
xcg-xac,w < 0
Cm,0w > 0, this requires a negative-cambered airfoil section or a an airfoil section that has a reflexed
trailing edge.
If the eigen values are complex (have real and imaginary) and real is minus what do you say about free response
damped sinusoid
Comment on the impact of the short- and long-period longitudinal modes on aircraft handling
and pilot workload.
[Emily’s made up answer couldn’t find answers] If short period is unstable pilot will be unable to correct it.. tbc
Briefly explain, using diagrams if appropriate, the effect of varying the centre of gravity
position on the longitudinal dynamic stability.
5) Discuss, using examples as appropriate, the implications of flying qualities on the design
of aircraft
What type of flow does a) a canard and b) an aft tail experience?
a) upwash flow
b) downwash flow
Is it correct to assume elliptical lift distribution for an aft tail?
Yes
What is the efficiency equation for the horizontal tail?
Ratio of dynamic pressure at the tail to dynamic pressure at the wing
η=(ρVt^2)/(ρVw^2)
Where Vt=velocity at tail
Vw= velocity at wing
Cm0 is negative for an aerofoil with positive camber. How can an aft tail be used to create a positive Cm0 for the whole aircraft?
By ensuring it is properly designed to change the overall Cm0 to be positive, and Cmα is negative. This ensures static stability.
2 advantages and 1 disadvantage of a canard
A:
Relatively free from wing and engine flow interference
Better at trimming nose-down moment of high lift devices
D:
Destabilising contribution to static stability
How does the position of CoG impact stability at the stick fixed neutral point?
When CoG is forward of the SFNP the aircraft is neutrally stable.
If CoG moves after of SFNP the aircraft will be statically unstable- as the slope of the pitching moment curve becomes positive
What is the stick fixed neutral point?
The point at which Cmα=0
What is the equation for steady state roll response?
pss=-((Lδa)/Lp)Δδa
Where Lδa and Lp are defined according to the equation sheet
Subbing these in and cancelling terms
pssb/2u0=-(Clδa/Clp)Δδa
What is the characteristic equation for pure yawing?
λ^2+Nrλ+Nβ=0
where Nr=(∂N/∂r)/Iz
and Nβ=(∂N/∂β)/Iz
where Iz=moment of inertia
What is the value for ωn and ζ when comparing the yawing characteristic equation to the second-order characteristic equation?
ωn=√Nβ
ζ=-(Nr)/2*√Nβ
Note: frequency only depends on static stability, and damping ratio depends on yaw rate too so is a function of aerodynamic damping
What is the basic idea of inertial cross-coupling?
That rotation about one axis can cause unwanted rotation about the other axes
When can Ixz not be assumed as zero?
In fighter aircraft
What two static stability characteristics allow for high rates of roll and which type of coupling they lead to? Give an example of each
1: Small longitudinal and large directional static stabilities e.g slender supersonic aircraft at subsonic speeds: roll-pitch
2: Small directional and large longitudinal static stabilities e.g slender supersonic aircraft at supersonic speeds: roll-yaw
Why does Roll-Yaw inertial coupling happen
P=roll vector
q=pitch vector
w=new axis of rotation
w=p+q
The pitching moment is resisted by the large LSS. As the aircraft has small DSS it rotates to align with w. This causes a large yawing moment
