Block 2 Flashcards
(49 cards)
Identify the different types of engine.
Piston, Jet (turbojet and turbofan) and Turboprop
Describe piston engines
Similar to an internal combustion engine. Engine drives a propeller. Can be air or liquid cooled. Two kinds radial (odd number of pistons) and horizontally opposed (even number of pistons). Disadvantages: many complex parts and greater weight ratio (the engine’s weight as a ratio to the entire weight of the aircraft). Operates efficiently up to around 12,000’.
Describe jet engines (turbojets)
Two types: turbojet and turbofan. Turbojets take in air, compress it and force it into the combustion chamber where it is mixed with fuel injected at high pressure. The fuel-air mixture ignites and the expansion of high pressure gases through the turbine nozzles drives the turbine. At lower altitudes higher air density requires more fuel to acheive the correct fuel-to-air ratio. At higher altitudes less power is produced in the thinner air, however the reduced drag makes high altitude operations more economical. Max efficiency around 36,000’. Advantages include few moving and intricate parts, reduced drag (no prop, small frontal area) and good power to weight ratio. Disadvantages: consume excessive fuel at low altitude, slow to respond to application of power. Examples include the DC8, B707, and most military jet fighters.
Describe jet engines (turbofans)
Turbofan is an improved version of the turbojet. It uses a fan ahead of the engine to accelerate air. The extra power is available for better take-off, climb and cruise performance, reduced fuel consumption and better payload/range. Operates best between 33000’ and 37000’. Turbofans are more fuel efficient, quieter, have better low altitude performance and lower landing speeds than turbojet engines. Found on most commercial and executive jets eg Airbus 320, C550 (citation two) B767.
Describe turboprop engines
Combines the properties of the propeller and jet engines. Turbines constructed to drive propellers. Most of the energy of the heated gas is used by the turbine to drive the propeller with only a small amount creating thrust at the exhaust. About 90-95% of power from propeller, about 5-10% from thrust. Turboprops operate most efficiently between 13,000’ and 25,000’ ( trade off between the prop more efficient at lower altitudes, engine more efficient at higher altitudes. Lower engine-weight ratio than props however cannot operate at as high an altitude as a turbojet. Most common in commuter passenger ( eg DH8B, BE9L) or business aircraft (eg PAY2, MU2)
Describe the performance charateristics of the different types of engine: altitudes
Piston: 0’ - 12,000’
Turboprop: 13,000’ - 25,000’
Jet: 25,000’ +
Turbo charged piston craft may reach 25,000’ causing problems as slow moving piston mix with fater jet and turbo-props. Modern turboprop may reach as high as 29,000’, causing problems as they mix with faster jet engines.
Describe the performance charateristics of the different types of engine: speed
Piston: <250 KT (modern general aviation eg C172 maintain speeds between 100 and 120 KT)
Turboprop: 200 - 300 KT
Jet: 300 - 500 KT
Describe the performance charateristics of the different types of engine: climb rate
Piston: 500 - 1500 FPM
Turboprop: 1500 - 3000 FPM
Jet: 1500 - 6000 FPM
There is a large discrepancy in climb performance of jet engine aircraft. Commercial airliners cannot/will not climb at more than 2000 - 3000 FPM. eg a heavily loaded 747 on a hot day may only climb at 1000 FPM. On the other hand many excecutive jets can climb at 3000 - 6000 FPM. The rate of climb will also vary - the aircraft will normally reach the max rate of climb in the first 5000’ before decreasing to the max altitude.
Describe the performance charateristics of the different types of engine: rate of descent
Piston: 500 - 1500 FPM
Turboprop: 1500 - 4000 FPM
Jet: 2000 - 6000 FPM
Descent performance will vary somewhat based on high/lower performance aircraft and with altitude (jet aircraft will descend quicker at high altitudes, slowing their descent as they approach lower altitudes.
Describe the performance charateristics of the different types of engine: turns
Piston: Rate 1 (3 deg/s)
Turboprop: Rate 1 (3 deg/s)
Jet: Rate 1/2 (1.5 deg/s)
Turn radius depends on rate of turn and airspeed. ie jet engines have the largest turning radius.
Describe the performance charateristics of the different types of engine: run up
Piston: Long (the more engines on the aircraft the longer run-up time. Normally checks are done in the holding bay or taxiway)
Turboprop: Short (checks can usually be carried out with the aircraft is taxiing)
Jet: None (checks can quickly be performed during taxi)
Describe the performance charateristics of the different types of engine: acceleration
Piston: No delay
Turboprop: Slight delay
Jet: Long delay
Plan sufficient time for larger aircraft to comply with ATC instructions.
Describe the performance charateristics of the different types of engine: economy
Piston: Efficient at low altituded (<12,000’)
Turboprop: Efficient at normal operating altitudes, less efficient at low altitudes
Jet: ineffecient at low altitudes - minimize time spent at low altitudes.
Describe the performance charateristics of the different types of engine: FOD (Foreign Object Damage)
Piston: Does not ingest
Turboprop: Does not ingest
Jet: Ingests, warn pilots of potential hazards such as slush, loose stones, birds etc.
Describe how propeller pitch affects performance
The pitch is the distance the propeller travels forward in one revolution and is controlled by the angle of attack of the propeller blades. A coarse pitch (steep angle of attack) means the propeller travels a larger distance per revolution than a fine pitch (shallow angle of attack). In the real world this distance is reduced to the effective pitch with the difference being the propeller slip. A coarse pitch provides greater effective distance at a given RPM and is more efficient for cruising. A fine pitch has less drag and rotates faster giving more power for take-off and climb performance but is inefficient for cruising.
Describe different propeller types: fixed pitch
Fixed pitch: compromise between coarse and fine pitch. Found on most training aircraft eg Cessna 150
Describe different propeller types: variable pitch
Variable pitch: adjustable (blades may be adjusted on the ground) or controllable (may be adjusted in flight). Allows pilot to select best pitch for take-off and cruise performance.
Describe different propeller types: constant speed
Constant speed: variable pitch propeller fitted with a governor. The governor alters the blade angle to maintain a constant RPM for all conditions
Describe different propeller types: reversible pitch
Reversible pitch: achieved by turning the blades to the full reverse pitch so that a pushing rather than pulling is achieved. For safety this is only possible when the extended nose wheel is in contact with the ground.
Describe different propeller types: feathering
Feathering: When necessary to stop an engine it is desirable to feather the engine by turning it to an extreme coarse pitch. This stops the prop from rotating, reducing drag, windmilling and reducing vibration. Used to reduce drag in the even of a lost engine or to prevent wear caused by the propeller rotating in the wind when not in use.
Explain the use of thrust reversal by jet aircraft
Used to slow down jet aircraft once they have landed. Two types:
a) mechanical blockage system consists of two clamshells which when stowed are located near the rear of the jet engine. When deployed they deflect the exhaust forward, producing reverse thrust. Older B737 models use this method.
b) aerodynamic blockage system (cold stream reverser) causes the exhaust gases to redirect thrus outward and forward. Are controlled by thrust levers in the cockpit (as are mechanical blockage reversers)
Explain the consequences of engine failure on single engine aircraft
The aircraft can only glide. On take-off most will proceed straight ahead for an emergency landing, if terrain is good. Turning back toward the runway is risky as stall speed increases in the turn and the plane is alread losing speed and lift because of the failed engine. Most single engine aircraft have good gliding capabilities. The outcome depends on the altitude and pilot’s abilities.
Explain the consequences of engine failure on multi engine aircraft
Multi-engine aircraft can usually stay in the air, but at reduced airspeed.
During and immediately after take-off the loss of thrust reduces the ability to clear obstacles and increases the time to accelerate to climbing speed.
During take-off run the loss of engine will increase the time and distance required to take-off, requiring the pilot to decide whether there is enough runway to complete the take-off (and then clear obstacles) or enough runway remaining to stop safely.
If an engine fails while airborne the aircraft loses 50%, 33%, 25% power (2,3 and 4 engine respectively). 3 and 4 engine aircraft will usually maintain sufficient speed to successfully climb out, provided the aircraft has reached its critical speed (which depends on which engine failed). Twin engine aircraft can have serious problems (a 50% power loss is more like a 75% performance loss).
Rate of climb depends on excess horsepower. Often in a twin engine the 2nd engine provides all the excess power and it’s loss means the aircraft can barely keep the plane in the air. In cruise flight the engine loss will often force the plane to operate at a lower altitude.
Explain asymmetrical thrust
The aircraft tends to yaw after losing an engine due to the unbalanced thrust of the working engine and increased drag of the non-working. The asymmetric thrust is greater when the outboard engines fail (on 4 engine aircraft) and when propellor engines fail (than jet engines, because the propeller has greater drag).
Flying on asymmetric power is difficult.
Each engine turning to the right applies forces turning the aircraft to the left. One of these forces is the “P” factor which acts on the right engine side of the prop disk. It is farther from the vertical axis for the right engine than the left. The failure of the left engine produces greater unbalanced moment and is it’s failure is more critical than that of the right engine. (The opposite is when the props turn to the left - then the critical engine is the right engine).
The airspeed at which adequate directional control can be maintained with the critical engine failed but not feathered and with full power on the right is called the critical single engine speed. Adequate directional control is maintained by applying rudder opposite to the engine that has failed. The airplane must be at it’s critical speed for this to be effective. The critical speed varies depending on which engine was lost.
