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.
- Why is a 747 – 400 loaded or flown with an aft C of G?
o Because then the stabiliser is trimmed so as to produce lift. This means that there is more lift to drag so the aircraft will fly further or use less fuel for a given flight.
- How does an aft C of G affect stability?
o It reduces longitudinal and directional stability. Stabiliser and Fin, elevator and rudder effectiveness are reduced.
o The further aft the CP the more it gives a pitch up movement after a disturbance. The further aft the C of G, the shorter the restoring arm for the rudder and elevator.
- What are the advantages and disadvantages of engines mounted on the wings?
Advantages:
Engines provide bending relief thus reducing wing structure weight.
Intake efficiency is not compromised except perhaps in reverse.
The wing profile is not compromised.
At high angles of attack the engine pylons tend to act as fences, controlling spanwise flow.
Interference drag is low.
Thrust reverser design is not compromised.
Engine accessibility is good.
Less containment devices needed in the event of failure.
Disadvantages:
More yaw following engine failure. Bigger rudders and fins required.
Roll freedom can be limited on the ground.
A low thrust line can cause pitch up with power and pitch down with reducing power.
The reversed flow from the inners can affect the outboards on a 4 engined type on landing.
FOD damage is higher.
- How do you fly for best range?
You have to cover the most ground nautical miles per pound of fuel used. In terms of engine efficiency this is achieved at high RPM which can only be achieved at high altitudes with jet engines. In terms of airframe efficiency it is achieved at the lowest drag to airspeed / thrust / fuel consumption. At high altitudes the TAS is greater for a given IAS so more nautical miles will be covered than at low altitudes so flying high has
two advantages – better groundspeeds and better fuel efficiency.
- How do you fly for best endurance?
You have to stay airborne as long as possible on a given amount of fuel. The lowest fuel consumption rate is needed. Minimum drag and hence minimum thrust is required. Altitude improves endurance as turbine engines are more efficient at the higher RPMs necessary to maintain the minimum thrust at altitude.
Jets at high altitudes
Piston- sea level
Turbo-props below 10,000ft.
- Why are high bypass engines more efficient than low bypass engines?
Propulsive efficiency is made up of the Froude Efficiency (the thrust power produced divided by the kinetic energy added to the air).
U is the speed of the aircraft and V is the jet velocity relative to the aircraft.
High bypass engines moves a large mass of air and accelerates it to a reasonably high speed. This is most efficient up to quite high mach numbers.
Low bypass engines move a smaller amount of air but accelerate it to a lot high speed. This is only efficient at very high mach numbers.
The high bypass engine is thus more efficient and achieves a lower Specific Fuel Consumption (lb of fuel / lb of thrust / per hour)
Also high bypass engines are less noisy.
- What is bypass ratio?
It is the ratio of air that does not enter the turbine section of the engine to the ratio that does. I.e. If 10 parts of air goes does not enter the engine and 1 part does, the engine has a 10:1 bypass ratio.
- What is the advantage of the 3 Spool engine?
It has greater efficiency over a wider range of operating conditions.
Each spool can be rotated at it’s optimum design speed.
It is also less noisy.
Easier to start.
The front spool sections rotate slower and thus achieve better reliability.
A 3 spool engine is one that has three sets of compressors before the combustor and three sets of turbines behind it.A spool is made up of a compressor and a corresponding turbine used to extract the power from the exhaust gasses to turn the compressor.Each spool is given a name. N1, N2, and N3. N1 is the large fan section in front of the engine. N2 is the low pressure compressor section. And N3 is the high pressure compressor section. Some engines incorporate N2 And N3 into one rotating mass and call it N2. Hence the double spool engine.Each section of the compressor wants to rotate at it’s own speed, and if allowed to do so as in a triple spool engine, it is able to operate more efficiently. It can turn at it’s optimum speed, and not have to compromise between the optimum speed for the N2 and N3 sections when attached in the double spool engine
Disadvantages:
More sets of blades results in a greater engine weight, but the corresponding increase in thrust possible more than offsets the weight increase.Major advantage of triple spool engine design is it’s ability to minimise engine surgesThe drawbacks of a 3 spool engine are increased weight, complexity, and cost to purchase and overhaul, but they are the most efficient engine flying.
