AC-SYS
AC-SYSTEMS, Manfriani: AC-Structure, the Landing Gear, Propulsion System (Propellers, Piston Engines, Turbine Engines),....
AC-SYSTEMS, Manfriani: AC-Structure, the Landing Gear, Propulsion System (Propellers, Piston Engines, Turbine Engines),....
In a tension test, what does the constant E mean?
E is the normal modulus of elasticity or Young's modulus.
In an AC wing, the cross sectional area of the web carry all the
The web carries most of the
The torsion loading on a trapezoidal wing section with no sweepback is maximum at
The local aerodynamic heating at stagnation point is
The hottes part of an AC flying at supersonic speed is
At low speeds the load factor is limited by
At high speed, the limit is set by
- What are the load factor limits for the Pilatus-21 turboprop trainer?
- For the Lockheed Martin F-22 Raptor jet fighter?
- +8/-4
- +9.5/-3.5
- How is the load factor increment when encountering vertical gust defined?
- What does this equation tell us?
- \(delta n=delta L/(m*g)=Kg*(rho*U*V*CLalpha)/(2*m*g/S)\)
- The AC sensitivity to gust is a function of wing loading (2 m g/S), lift curve slope (cL alpha-->depends on wing aspect ration), flight altitude and airspeed.
How do AC actually react to vertical gust?
They accelerate upwards, so the net gust velocity is reduced.
How do Airworthiness specifications define the gust velocities to determine load factors due to turbulence?
They define the standard gust velocities (in equivalent airspeed).
The AC weight influences loads on the AC structure. What kind of loads are these?
- Fuselage bending
- Wing bending
- Shear loads
- Tailplane loads
How do airworthiness regulations limit the minimum MLW for the PC-12?
95% of the MTOW--> the AC must be able to land immediately in case of engine failure and cannot dump fuel in emergency.
For multi engine AC equipped with a fuel jettison system, a lower MLW can be specified.
- What is the MZFW critical for?
- What is the MRW critical for?
- The MZFW is critical for wing bending and shear at the limit load factor-> fuel weight alleviates these loads
- The MRW is critical for rolling, towing, braking loads on ground
- What weight does the MZFW limit?
- What does it mean to keep the MZFW balanced with the MTOW?
- The payload weight that can be carried. The MZFW should be as high as possible, while staying in balance with the MTOW.
- This means that the wing bending and shear at the limit load factor should be approximately the same at MTOW (with full tanks) and at MZFW.
What defines the zoning system in the ATA-100 specification?
The zoning system is specified to simplify the location of parts in modern large transport AC.
How many hierarchies of AC zones are defined?
3: Major zones, Intermediate zones, Minor zones
- What do odd numbers in the AC zoning system indicate?
- What do even numbers indicate?
- Odd numbers mark the zones of the left side
- Even numbers zones on the right side of the AC, as seen by the pilot.
What can the skin of the AC's fuselage be made of?
Fabric, plywood (Sperrholz), thin metal sheet or synthetic materials.
Layout of a tricycle undercarriage:
How much should the pitch angle Θ be at which the tail touches the ground?
10-15°
How much should the roll angle be for large transport AC and for light AC?
- at least 8° for large transport AC
- 15° for light AC
Which two limit angles define the required length of the main landing gear legs?
Ground pitch and roll angles
What loads should be on the nosewheel of the AC?
8-15% of the AC weight
For a general aviation or commuter AC, what maximum value of lift can be assumed for design and testing?
2/3 of the AC weight
On which value does the vertical speed at touchdown depend?
On the efficiency of the shock absorber.
What is the safety margin of a stroke?
2.5cm
What is a typical value for the internal gas pressure p0 of AC oleo shock absorbers under static load?
1800psi (12.5 MPa)
What is shimmy and why can it occur?
Shimmy is high speed vibration of the nose wheel and can occur because of the wheel imbalance, tire wear or free play in the wheel suspension mechanism. A hydraulic damper (shimmy damper) must be fitted.
What is the typical lifetime of a wheel?
300-400 landings
Propeller efficiency:
A short take off AC with a low cruise speed will certainly benefit from a...
...large propeller providing high efficiency and high thrusts at low speeds.
For a fast AC,....
....a smaller propeller diameter will be more suitable.
Which peak efficiencies reach modern variable pitch propellers?
0.85-0.9
What describes feathering?
In the event of an engine failure, propeller pitch must be set to a value that reduces drag to a minimum. (90°)
Fuel Systems:
Fuel tanks:
How much weighs the fuel mass in an AC (except for small general aviation AC)?
25-50% of the MTOM
Fuel pumps:
What are transfer pumps?
Transfer pumps move fuel between tanks to maintain mass balance and to supply fuel to the collector ranks from where fuel is drawn to feed the engines.
What are booster pumps?
Also called engine feed pumps. They are used to boost (antreiben) the fuel pressure in the engine feed lines.
Another reason is to prevent cavitation (vapour bubbles) at high levels.
Valves:
What is the function of shut-off valves?
Shutting off fuel when required, preventing fuel transfer from one rank to another.
What are the functions of refuel and defuel valves?
Filling the tanks to allow flow from the refueling gallery to the fuel tanks, or from the tanks to the refueling gallery during defueling (emptying the tanks for maintenance)
Cross-feed valves?
Shifting fuel from one side of the AC to the other side.
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If i.e. System B engine driven pump pressure drop below limits, how can pressure still be provided?
If the System B engine driven pump pressure drop below limits, a power transfer unit (System A drives a hydraulic motor which in turn drives a hydraulic pump pressurizing System B) provides a backup power source to operate the slats.
In a A320: When will the PTU automatically operate?
In flight, in case of loss of one engine, the PTU will automatically operate when the differential pressure between the systems is greater than 500psi. On ground, while operating the yellow system using the electric motor driven pump, the PTU will also allow the Green system to be pressurised
When does the RAT (Ram air turbine) extend?
The ram air turbine (RAT) extends automatically in flight in the event of failure of both engines and the APU.
How can wing bending and shear loads due to gust also be alleviated?
In the A320, the ailerons and the two outer spoilers are used to alleviate wing bending and shear loads due to gusts. When the load factor increase due to a vertical gust exceeds 0.3 g, the ailerons and the outer spoilers on both wings are automatically deflected upwards to reduce lift at the wing tips.
Which are the two main methods of mechanically connecting the pilot's controls to the rest of the flight control system?
- push-pull control rod systems: more precise and they have lower friction
- cable and pulley systems: easier to route through the airframe.
Which are the major components which are required to enable a flight control system to work while providing safe and pleasant handling characteristics to the pilot?
- trim actuators in tailplane (pitch), rudder (yaw) and aileron (roll) control systems;
- spring feel units in tailplane (pitch), rudder (yaw) and aileron (roll) control systems;
- ‘q’ feel unit in the rudder (yaw) control system:
- power control units (PCUs) for tailplane, rudder and aileron actuation.
What is the reversion mode of the actuators after failure?
The actuators themselves have a simple reversion mode following failure, that is to centre automatically under the influence of aerodynamic forces. This reversion mode is called aerodynamic centering and is generally preferred for obvious reasons over a control surface freezing at some intermediate point in its travel.
Explain the linear actuator with autopilot interface.
See scheme!
Why do systems generally use simplex electrical signalling?
For autostabilisation. In these systems the electrical demand is a stabilisation signal derived within a computer unit. The simplest form of autostabilisation is the yaw damper which damps out the cyclic cross-coupled oscillations which occur in roll and yaw, known as Dutch roll.
Which kind of systems do have linear actuators?
The linear actuators described so far are commonly used to power aileron, elevator and rudder control surfaces where a rapid response is required but the aerodynamic loads are reasonably light.
What kind of actuator does the Trimmable Horizontal stabiliser (THS) use?
The mechanical screwjack actuator. (A screwjack provides a slow response with a large mechanical advantage).
How does the EHA (Electro-Hydrostatic Actuator) accomplish to draw significant power when a control demand is sought?
The EHA accomplishes this by using the three-phase AC power to feed power drive electronics which in turn drive a variable speed pump together with a constant displacement hydraulic pump.
What is the difference between IAP (Integrated Actuator Package) and EHA (Electric-Hydrostatic Actuator)?
The difference being that when there is no demand the only power drawn is that to maintain the control electronics for the EHA.
Explain the Fly-by-wire system (FBW).
In a Fly-by-wire (FBW) system the conventional manual flight controls of an aircraft are replaced with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires (hence the fly-by-wire term), and flight control computers determine how to move the actuators at each control surface to provide the required response. The fly-by-wire system also allows automatic signals sent by the aircraft computers to perform functions without pilot input, as in systems that automatically help stabilize the aircraft.
What are the advantages of the FBW architecture?
- flight envelope protection (the computers will reject or modify pilot's demands that could lead to a violation of the aircraft limitations);
- improved stability and handling qualities across the full flight envelope, including the possibility of flying unstable vehicles;
- turbulence suppression and consequent decrease of fatigue loads and increase of passenger comfort;
- possibility of using thrust vectoring to augment or replace lift aerodynamic control,
thus extending the aircraft flight envelope; - drag reduction by automatic trim optimisation;
- higher stability during release of tanks and weapons;
- easier interfacing to auto-pilot and other automatic flight control systems;
- weight reduction (mechanical linkages are substituted by wirings);
- lower maintenance costs;
- reduction of pilot training costs (flight handling becomes very similar for all aircraft of a family).
How many types of computers are in an A320 for Flight Control?
- two ELACs (Elevator-Aileron Computers); normal pitch control function, aileron control function;
- three SECs (Spoiler Elevator Computers); spoiler control function, standby pitch control function;
- two FACs (Flight Augmentation Computers); rudder control.
Explain the Flight control, guidance and management loops.
- In the inner loop, the attitude of the aircraft is controlled by the fly-by-wire (FBW) system, that receives inputs from the pilot controls and from the autopilot.
- In the middle loop, the trajectory of the aircraft is controlled by the autopilot and flight director system (AFDS), that receives inputs from the flight control unit (FCU) and from the flight management system (FMS).
- In the outer loop, the entire flight mission is controlled by the FMS, programmed by the pilots through the multipurpose control and display unit (MCDS).
- The attitude, trajectory and mission data are presented on the electronic instrument system (primary and secondary flight display).
What is bleed air at high pressure and temperature used for?
- air conditioning and cabin pressurization;
- cargo compartment heating;
- ice protection;
- turbine engine starting;
- windscreen de-mist and rain dispersal;
- pressurization of hydraulic oil reservoirs, fuel tanks, water tanks;
- power generation by compressed air turbines;
- actuation
At which flight phases is the bleeding port typically open?
Typically this bleeding port is open during taxiing or descent, with the engine at or just above idle conditions.
On modern airliners, engine bleed systems are similar. What are the tasks of each system?
- select the air source compressor stage
- regulate bleed air pressure
- regulate bleed air temperature
Where is bleed air drawn in normal operation?
From the intermediate stage of the engine compressors to minimise engine efficiency loss.
When there is a high demand of bleed air where can it also be drawn?
If there is high demand, it can be supplemented by air from the HP stage through a restrictor valve (HPV) that reduces pressure to 36 psi (2.5 bar).
Why is the APU needed to supply compressed air for air conditioning during take-off?
During take-off it can supply compressed air for air conditioning, thus avoiding the reduction in engine thrust that would be caused by the use of engine bleed air when optimum aircraft performance is required.
What is the principal use of bleed air?
Environmental control
Where does the main pneumatic system of the A340 supply high pressure air?
- air conditioning and pressurisation;
- wing anti-icing;
- engine starting;
- pressurised water system;
- hydraulic reservoirs;
- pack bay ventilation turbofan supply;
- rain repellent purge system;
- thrust reverser system (GE engines only).
On ground, which pneumatic power to drive the engine starters can be used?
- by use of a ground air supply cart;
- by using air from the APU load compressor;
- by using bleed air from another engine which is already running
Where in the aircraft structure are overheat detection elements installed to warn the crew in case of hot gas leak?
An overheat detection system will have elements adjacent to the air conditioning packs, wing leading edge and engine nacelle areas.
What happens in the system in case of a wing leak signal?
- both bleed valve and HP valve on the related side are automatically closed;
- both associated FAULT light on the AIR panel are illuminated;
- the cross-bleed valve automatically closes ;
- if the APU bleed valve is open, and if the leak concerns the left wing, it automatically closes.
In case of a pylon leak signal, what happens with the valves, etc.?
- both bleed valve and HP valve on the related side are automatically closed;
- a FAULT light associated with the related engine is illuminated on the AIR panel;
- the cross-bleed valve automatically closes.
What happens in case of an APU leak signal?
- the APU bleed valve automatically closes;
- the FAULT light illuminates on the APU bleed pushbutton on the AIR panel;
- the cross-bleed valve automatically closes.
Environmental control systems: Under which conditions are humans comfortable and safe in an AC?
- the pressure is higher than 750 mb;
- the temperature lies between 18 and 23C;
- the relative humidity lies between 20 and 70%;
- the CO2 volume concentration is lower than 0.5%;
- the CO volume concentration is lower than 0.005%;
- the O3 volume concentration is lower than 0.25 parts per million
What does the pack controller regulate?
A pack controller regulates air flow by modulating the pack flow control valve and outlet air temperature by modulating the bypass valve, the ram air inlet flaps, and the ram air outlet flaps.
Which are the three independently controlled air conditioning system zones?
- CKPT (Cockpit)
- FWD of cabin
- AFT of cabin
Via which system is fresh air supplied?
- Two pack flow control valves
- Two air conditioning packs
- the mixer unit.
Which criterias determine the ratio between recycled and fresh flow rate?
- external air flow necessary to remove contaminants;
- conditioned air flow necessary to maintain a comfortable cabin temperature;
- total air flow to provide adequate circulation;
- external air flow needed to pressurise.
At which cabin altitude is the warning tone active?
If the cabin altitude exceeds 10,000ft (3048m).
- What is the pressure rate of change during climb?
- What is the pressure rate of change during descent?
- 500ft/min (2.5 m/s)
- 400ft/min (2 m/s)
To avoid pressure surge at rotation, what does the cabin pressurisation controller do?
The controller pre-pressurises the aircraft at a rate of 400 feet per minute, until the pressure differential reaches 0.1 psi; the valves are then closed. At lift-off, the controller initiates the climb phase.
How is the outflow valve slam shut initiated?
When the aircraft lifts off, the landing gear weight on wheels switch tells the cabin pressure regulator that the aircraft is now airborne, which then causes the outflow valves to slam shut, so the cabin pressure rises suddenly (a pressurisation bump is felt).
How is the fuselage structure sized for pressurisation loads?
With the maximum allowable differential pressure i.e. the difference between maximum cabin pressure during cruise and minimum atmospheric pressure at the service ceiling plus a safety margin.
