Engines Flashcards
What is thermodynamics?
Thermodynamics is the study of heat / pressure and the behaviour of gasses with different heat / pressure variations.
What is Bernoulli’s theorem?
Bernoulli’s theorem is that in the steady flow of an ideal fluid, the sum of the pressure and kinetic energy remains constant.
Pressure + KE = Constant.
Explain a venturi?
A venturi (convergent / divergent duct) shows the practical application of Bernoulli’s theorem.
Converging:
Static Pressure - Decreasing
Dynamic Pressure - Increasing
Temperature - Decreasing
Velocity - Increasing
Diverging:
Static Pressure - Increasing
Dynamic Pressure - Decreasing
Temperature - Increasing
Velocity - Decreasing
What is the combustion cycle of an aeropiston engine?
WORKS ON CONSTANT VOLUME
Induction
Compression
Expansion
Exhaust
What is the compression ratio in a piston engine?
Compression Ratio = Total volume / Clearence volume
The ratio of the total cylinder volume with the piston at BDC and the volume remaining at the end of the compression stroke with the piston at TDC.
What produces the ignition in a piston engine?
Magnetos are used in a piston engine to supply the high tenstion voltage neccessary to create an electric spark at the spark plug.
What does blue / black / white exhaust smoke indicate from a piston engine?
Blue - engine is burning oil (indicative of wear on the piston ring).
Black - Engine is running too rich and carbon build up is caused in the cylinder where not all of the fuel can be burnt.
White - Often indicates a high water content in the combustion chamber which is exhausted as white steam.
What is engine torque?
Torque is a force that causes rotation (eg of the crankshaft) and is measured in pounds per inch.
Torque = Force x Distance
What is a “normally aspirated engine”?
A normally asperated engine is an engine which does not have a turbocharger or a supercharger and uses the normal atmospheric air density. Therefore the power output it is restricted by the cylinder capacity.
What are the disadvantages to a piston engine? (3)
Lack of power output, especially with increased altitude
Lower airspeed due to propellor RPM limitiations
Mechanical inneficiency
What is a supercharged piston engine?
A supercharger simply increases the air delivered to the engine cylinder above its normal aspirated capacity by compressing the intake air. This requires more fuel to be delivered at the carbouretta and produces a greater power output, allowing greater efficiency of the engine.
How is the piston engine output increased to compensate for low atmospheric pressure? (2)
Superchargers artificailly increase the manifold pressure to compensate for low atmospheric pressure either:
to increase the engine power output for take off / climb (ground boost).
OR
to increase the engines performance at higher altitudes by maintaining MSL pressure (altitude boost).
What regulates the supercharger to deliver a constant MAP?
Turbo - Absolute Pressure Controller (APC) uses an aneroid capsule that is sensitive to the compressor outlet pressure which changes the position of the wastegate with use of oil within the engine.
Supercharger - An Auto Boost Controller (ABC) is designed to maintain a given pressure at altitude. At low altitudes, in order to avoide detonation and engine stress due to excessively high combustion pressure, the throttle valve is only partially open. As the aircraft climbs, this valve is increasingly opened.
How is engine power monitored? (2)
MAP / Boost
On an ISA day, the MAP will read 29.92 inHg or Zero boost.
The gauge reading before ngine start is known as static boost.
What advantages does an aircraft gain from a propellor? (4)
Slipstream creates more lift over the wing
Increased fin / rudder effectiveness
Slipstream suppresses the stall speed of the aircraft
Quicker “low speed” recovery with propellow and piston engine aircraft
What is the main disadvantage of a propellor?
It is restricted by its RPM limitations which are due to compressability when the blade becomes sonic.
Therefore it is restricted by:
its lower TAS and shorter range.
What produces the thrust on a propellor driven aircraft? (2)
This is an example of Newtons Third Law: “The application of a force will cause an equal and opposite reaction”.
The power output of the engine which drives the prop shaft, combined with the blade angle of attack produces the thrust of a propellow driven aircraft.
Why is a propellor blade twisted?
The propellor blade is twisted in order to maintain a constant agle of attack.
How do you define propellor efficiency?
Prop Efficiency = Prop thrust (air mass x velocity) / Engine BHP
What are the disadvantages of a fix pitch propellor?
It can only be efficient for one predetermined RPM, at one stage of the flight. This is because as TAS increases the AoA on the propellor blade becomes smaller and causes a reduced angle of attack. Therefore the propellor blade needs to be coarsened with an increase of TAS.
What is a variable pitch propellor and why is it used?
A variable pitch propellor uses and oil / spring medium in order to change the ptich angle of the propello from fine to course pitch (known as the RPM). A constant speed unit govner (which uses flyweights and centripetal force) adjusts the angle of the blade in order to keep the RPM at a constant speed. This increases the speed range of the aircraft and maximises the efficiency of the aircraft.
Is there a critical engine on a propellor aircraft?
Unless engines are counter rotating (Seneca V). The critical engine is the one that determines the critical control speeds of the aircraft and has the largest yawing moment in the event of an engine failure.
Why is the number 1 engine the critcal engine on a multiengine propellor aircraft? (2)
Slipstream effect:
Because engine number 1 creates a slip stream force on the fin; this helps to counteract the yawing moment from number 2 engine should it fail. However, should engine number 1 fail the the slipstream from engine number 2 will produce a sideways force that will aggrevate the yawing moment, which will result in a more critcal situation. This would mean that a higher speed would be required in order to increase the effectiveness of the rudder, determining engine number 1 as the dritical engine.
Asymmetric blade effect:
As the downgoing blade produces more thrust that the ugoing blade, this increases or decreases the thrust moment arm. The engine that creases the largest moment arm will be the critical engine as it creates the largest amount of yaw.
How do propellers generate noise?
Noise from propellors is generated from the sheer that is different between the propellers faster displaced air and the air that is surrounding it.
What are thrust reversers and how do they work? (3)
Reverse thrust on jet engines reverse the airflow forward and create a braking action. They are all typically operated by hydraulic or pneumatic actuators and redirect the gas flow.
There are 3 different types of reverse thrusters:
Clam shell doors: The use of cascade vanes which are revealed when operated.
Bucket doors: The rear of the exhaust pipe is shaped like 2 halves of a bucket and they swing open and reverse the thrust.
Blockers doors (high bypass engines only): These block the cold bypass air and redirect them via cascade vanes.
