Descriptive questions Flashcards
Describe phugoid mode
- easily controlled by pilot
- slow, lightly damped, characterised by change in pitch angle, height and velocity at nearly constant CL/a
- takes a long time to grow/decay
- affects disturbances in u and θ
Describe short period mode
- more important over phugoid with regards to safety
- vertical movement caused by change in AoA
- high freq. well damped, short period changes in AoA
- characterised with 2 pairs of eigenvalues (comples)
- 10 x faster than phugoid, linked with high changes in load factor
Describe centre of gravity
the point where aircraft would balance if you suspend it at that point
theoretical point at which total weight is concentrated
CG is not a fixed point, position changes during flight as fuel used and passengers move.
Describe centre of pressure
point where resultant lift and drag forces act. point where moment of all forces on the wing is 0
Centre of pressure varies with angle of attack
aerodynamic centre
point about which the pitching moment doesn’t vary with angle of attack (providing velocity is constant)
used as a reference point for defining the pitching moment acting on the wing.
usually found at c/4 aft of leading edge
Describe flying qualities
how well a (long term) task can be fulfilled. Related to damping ratio and natural frequency if dynamic modes. Also important to a design how aircraft responds to the pilot.
- minimal acceptable are described by airworthiness requirements
Describe handling qualities
Deal with the response of the aircraft. How the aircraft responds (short term) to inputs
- properties which describe the ease and effectiveness with which the aircraft responds to pilot commands.
2 types of airworthiness requirements
civil - concerned with safety
military - specify stricter requirement for flying qualities
Describe how dutch roll arises
- aircraft is disturbed in the yawing sense
- the fin can provide the restoring moment
- there will be an overshoot, the aircraft will oscillate about its equilibrium point
- as the aircraft yaws, a rolling motion develops as one wing travels faster than the other
- sideslip will also occur during yaw
- yaw and roll oscillate at same freq. but out of phase
- period is a few seconds
List features desirable for military aircraft
- easy access to cargo bay in fuselage (T-tail allow for this)
- T tail provides greater longitudinal stability
- high wing enhances stability and keeps engines further from the ground
- anheadral wing to make it more manoueverable
- high wing means fuselage closer to the ground so loading and unloading easier
- high wing more stable as CG is beneath it
Drawbacks:
- increased drag near fuselage wing box junction
- land gear integration more complex (needs to be stiffer which adds weight)
Explain the principle of ILS - Instrument Landing System
- useful in difficult meteorological conditions
- provides lateral and vertical guidance of the aircraft
- composed of ground based signal transmitting a 90Hz and 150Hz signal
- when the approach is on the centre line, the strengths of both of these two are equal
- use similar thing for glide slope (3deg desired)
Explain the gimbal lock phenomenon and some solutions for it
- the loss of 1 DOF in a 3D, 3-gimbal mechanism that occurs when the axes of 2 of the 3 gimbals are driven into parallel configuration
solutions:
1) addition of a 4th gimbal driven by a motor so always 90 deg out of phase with inner most axis
2) ‘reset’ the device by rotating gimbals to an arbitrary position
3) use quaternions instead (sensors)
Advantages and disadvantages of T tail
A
1) moves tail away from exhaust
2) tail length of the horizontal can be increased
3) can have a larger/easier cargo bay entrance for military
D
1) higher vertical fin loads
2) potential flutter difficulties
3) problems with deep-stall due to wake of main wing
Advantages and disadvantages of V tail
A
1) combines rudder and elevator
2) out of wake of the wing
3) less interference drag and fewer tail surfaces
4) less tendency for rudder lock
D
1) increased load
2) possible interaction between rudder and elevator i.e. cross-wind landing control system needs a control system mixer
Explain difference between reversible and irreversible flight control
Reversible:
where there is a direct mechanical linkage connection between the control stick and the control surface
in a reversible system the pilot directly feels hinge moments applied to the surface
Irreversible:
the motion from the controller stick to the control surface cannot be returned by the elevator motion to the controller stick
(i.e. hydro-mechanical system in large aircraft)
