Tech Block 6 Flashcards
- Regulation of AC voltage
o Through use of a CSD (Constant Speed Drive)
- How do you control AC current?
o Through a Generator Control Unit (GCU)
- Purpose of bonding regarding static charges?
o To ensure that all sections of the airframe have the same potential and therefore prevent arcing
- For V2 over-speed take offs what are the obstacle considerations
o Close in obstacle clearance reduced
o Distant obstacles cleared better due to higher speed
- What is the advantage of an increased V2?
o An improved climb gradient
o An increase in Take off weight
- What are the advantages of a swept wing?
It allows a high mach number cruise speed due to it’s lower drag. This is because the swept wing is only sensitive to the component of airflow velocity across the
chord of the wing. The apparent airspeed across the chord is less than the real airspeed. This means that wing can be flown to a higher speed before the critical mach number is reached. Obviously a thin wing is also required so as reduce the camber and so reduce the acceleration of air over the upper surface.
- What are the disadvantages of a swept wing?
It can be subject to tip stalling which due to the wing tips being behind the center of gravity causes pitch up.
This has largely been fixed by washout (reducing the incidence at the wingtips so the wingroot stalls first).
It has a higher stall speed because the sweep reduces the lift in the same way it reduces the drag. This means that advanced and complicated flap and leading edge devices
are required to reduce the airspeeds for takeoff and landing.
It requires a higher angle of attack to produce the same amount of lift as an unswept wing. This produces high nose up attitudes for takeoff (possibility of tailstrikes) and landing. Also means that the profile drag is more – higher thrust required on approach and landing.
It has poor oscillatory stability. It has marked roll with yaw due to the reduced sweepback on the advancing wing producing more lift and also the increased projected
span. This leads to Dutch Roll.
On swept wing aircraft with podded engines far out on the wing, there is an increased possibility of scrapping them on the runway on takeoff or landing if the aircraft is rolled significantly. This is because the outer part of the wing is behind the main gear which is the pivot for the manoeuvre.
- How would you reduce Dutch Yaw if designing an aircraft?
For the same amount of sweepback you could enlarge the fin and rudder. This would make it more directionally stable butless stable laterally possibly giving spiral instability. You could reduce the sweepback. Yaw dampers are also used.
- How does a Yaw Damper work?
o It is a gyro system that is sensitive to changes in yaw which feeds a signal into the rudder controls so that rudder is applied to oppose the yaw.
- How do you stop Dutch Roll in flight?
o You apply opposite aileron to the up going wing.
- Why does the aircraft tend to pitch nose down when the Critical Mach number is reached? (Mach Tuck)
There are 3 reasons.
The shockwave on the upper surface upsets the lift distribution chordwise and causes the center of lift to move rearwards.
The swept wing tends to experience shock wave effect first at the wing root because this is thickest and has a higher angle of incidence. This causes a loss of lift inboard and thus forwards.
The shockwave can cause a reduction in downwash over the tailplane.
- What would you do to recover the aircraft from a MMO overspeed?
o Deploy the speed brake, roll aircraft level, hold back elevator pressure, use the elevator / stabiliser trim in small amounts.
o Power levers may be closed depending on the aircraft type. I.e. 747 – 400 low thrust line – closing power levers will reduce thrust causing more pitch down.
- Which part of the wing normally stalls first?
The wingroot.
The reason for this is so that you still have roll control and so that the nose pitches down on a swept wing aircraft. This is a stable movement.
Earlier aircraft without enough washout stalled at the tips first which pitched the aircraft up further increasing the angle of attack and drag. Aircraft with T tails suffered lack of tailplane and elevator effectiveness because the tail was in the path of the disturbed air coming off the wing. These aircraft became superstalled or deepstalled, and some could not be recovered. Thus stickshakers and stickpushers were developed for aircraft with unacceptable stall characteristics.
- What is a Mach Trimmer?
It is a device fitted to some Jet aircraft which trims the stabiliser up at mach numbers exceeding MCrit. It is used because some aircraft experience either lack of elevator effectiveness or very heavy elevator forces at high mach numbers above MCrit.
- How is stability affected by high speed, high altitude flight?
Aerodynamic damping is reduced at high altitude. There is less restoring force when a displacement happens. ½ Rho V2 is the reason why (less lift). Less air density at high altitude and the V2 forces are not high as V2 is based on IAS.
Directional control can be affected. If right rudder is applied it can accelerate the left wing to it’s critical mach number which thus loses lift and has increased drag so the aircraft yaws to the left and rolls left. This means that spiral stability is increased. The aircraft won’t enter a spiral dive.
Lateral control can be affected as normally the outboard ailerons are locked out due to wing twist that they cause leading to an opposite roll. Thus only the inboard
ailerons are available and possibly the differential spoilers. Oscillatory stability is reduced due to less roll control.
Longitudinal stability is reduced. (See Mach Tuck, Mach trimmers)
Note: spoilers are normally locked out as well due to the high drag penalties associated with their use at high speed.
As altitude increases, true air speed increases for the given equivalent air speed, resulting in decreased aerodynamic forces. Thus, at higher altitudes the pilot must apply greater opposite control movements to arrest rotation.
- How do designers increase the Critical Mach Number of an aircraft?
Number of ways:
Lift / Drag Formula: ½ Rho V2 S CL ½Rho V2 S CD
Low Wing Area: A larger wing will have increased drag but better lift.
Aspect Ratio: High aspect ratio causes less induced drag but can be a problem for the structural people. I.e. High bending forces involved and also a very thick wing root to support the long wing. A thick wing has a lower critical mach number so it is a tradeoff in many respects.
Sweep: To little sweep causes a high drag rise at a low mach number. Too much sweep causes poor oscillatory stability and a tendency for the tips to stall. Also as fuel burns off there is a large center of gravity change in a
highly swept wing.
Taper: This is the ratio of root chord to the wingtip chord. Optimum is about 2 ½ to 1. Each section of the wing will produce the correct proportion of lift. If it is too small then the wing will be heavier from a structural point of view. If too large then high local coefficients of lift are produced which tends to make the tips stall first or the wing to suffer bad stalling characteristics.
Thickness / Chord Ratio: A thin wing is required for high mach numbers. A thick wing is required for structural strength, accommodating fuel, landing gear, flaps, and also to lower the stalling speeds.
