2 - Engines Flashcards
Applied Laws
What is thermodynamics?
Thermodynamics is the study of heat/pressure Energy or the behavior of gases (including air) and vapors under variations of temperature and pressure.
Applied Laws
Explain Bernoulli’s theorem.
Bernoulli’s theorem is that the total energy in a moving fluid or gas is made up of three forms of energy:
1. Potential energy - (the energy due to the position)
2. Pressure/temperature energy - (the energy due to the pressure)
3. Kinetic energy - (the energy due to the movement)
When considering the flow of air, the potential energy can be ignored;
therefore, for practical purposes, it can be said that the kinetic energy plus the pressure/temperature energy of a smooth flow of air is always constant.
Thus, if the kinetic energy is increased, the
pressure/temperature energy drops proportionally, and vice versa, so as to keep the total energy constant.
This is Bernoulli’s theorem.
Applied Laws
Explain a venturi.
A Venturi is a practical application of Bernoulli’s theorem, sometimes called a convergent / divergent duct.
A venturi tube has an inlet that narrows to a throat, forming a converging duct and resulting in
(1 ) velocity increasing,
(2) pressure (static) decreasing and...
(3) temperature decreasing.
The outlet section is relatively longer with an increasing diameter, forming a diverging
duct and resulting in…
(1 ) velocity decreasing
(2) pressure (static) increasing, and…
(3) temperature increasing.
For a flow of air to remain streamlined, the mass flow through a venturi must remain constant.
To do this and still pass through the reduced cross section of the venturi throat, the speed of flow through the throat must be increased.
In accordance with Bernoulli’s theorem,
this brings about an accompanying drop in pressure and temperature.
As the venturi becomes a divergent duct, the speed reduces, and thus the pressure and temperature increase.
Piston Engines
What is the combustion cycle of an aeropiston engine?
Induction, compression, combustion (expansion), and exhaust.
The combustion of a piston engine occurs at a constant volume.
Piston Engines
What is compression ratio in a piston Engine?
Compression ratio in a piston engine is the ratio of the total volume enclosed in a cylinder with the piston at bottom dead center (BDC) to the volume remaining at the end of the compression stroke with the piston at top dead center (TDC).
CR = BDC/TDC
Piston Engines
What produces the ignition in a piston engine?
Magnetos are used on internal combustion engines to supply the high-tension voltage necessary to cause an electric spark at the spark plug.
Piston Engines
What does blue, black, and white exhaust smoke indicate?
Blue exhaust smoke - indicates an oil burn in the cylinders, probably due to broken piston rings that allow oil seepage into the combustion chamber.
Black exhaust smoke - indicates carbon granules burning in the cylinders.
This occurs if the mixture is too rich, resulting in some of the fuel not being burnt and turning into carbon granules, which are then exhausted as black smoke.
White exhaust smoke indicates a high
water content in the combustion chamber, which is exhausted as white “steam” smoke.
Piston Engines
What is engine torque?
Torque is a force causing rotation.
In particular, torque is the force created within an engine, which causes rotation of the rotating parts, e.g., the crankshaft.
Torque is a measure of the load experience, expressed in pounds per
inch or feet.
A quantity of torque, or twisting moment, is involved in the measurement of the engine brake horsepower (bhp).
Torque = force X distance (at right angles to the force)
Piston Engines
What is a normally aspirated piston engine?
A normally aspirated engine works as a result of “the breathing of the cylinder is due to the pressure differential below the standard sea level 14.7 Ib/in2.”
In other words, it only uses the atmospheric air density that is available to produce a charge in its cylinders and is not boosted by a supercharger; therefore, the normally aspirated piston engine’s power output is restricted by its cylinder capacity.
Piston Engines
What are the disadvantages of a piston engine?
A piston engine suffers from three main disadvantages:
1. A lack of power output, especially with increased altitudes 2. A low produced airspeed due to propeller rpm limitations 3. Mechanical inefficiency
Piston Engines
What is a supercharged (piston) engine?
A supercharger simply increases the air delivered to the engine cylinder above its normally aspirated capacity by compressing the intake air, which in turn requires more fuel to be delivered to the carburetor to maintain the correct mixture ratio, which in turn produces a greater power output (horsepower).
Therefore, a supercharged engine is capable of producing a greater power output than a normally aspirated engine
of the same cylinder size.
Piston Engines
How is the (piston) engine power output increased to compensate for low atmospheric pressure?
Superchargers are used to artificially raise the engine manifold pressure to compensate for low atmospheric pressure either
(1 ) to increase engine power output for takeoff and the initial climb (ground-boosted engine) or
(2) to maintain mean sea level (MSL) engine power at high altitudes (altitude-boosted engine).
Piston Engines
What regulates the supercharger to deliver a constant boost/manifold absolute pressure (MAP)?
The auto boost control (ABC) keeps the boost pressure constant
during a climb or a descent.
Piston Engines
How is engine power monitored?
There are two main engine power monitoring/indication systems:
(1 ) - manifold absolute pressure (MAP) and…
(2) - boost pressure.
Piston Engines
What is carburetor icing?
Ice formation can occur in the engine induction system and in the carburetor
of piston engines, particularly in the venturi and around the throttle valve, where acceleration of the air can produce a temperature fall by as much as 25°C.
This, combined with the heat absorbed as the fuel evaporates, can cause serious icing, even when there is no visible moisture present.
Such a buildup of ice in a carburetor can disturb or even prevent the flow of air and fuel into the engine manifold, causing
it to lose power, run roughly, and even to stop the engine in extreme circumstances.
This effect is called carburetor icing.
Throttle icing (i.e., around the throttle valve) is more likely to occur at
low power settings (e.g., descents), when the partially closed butterfly
creates its own venturi cooling effect.
Piston Engines
When would you expect carburetor icing in a piston engine?
Carburetor icing should be expected in a piston engine when the outside air temperature (OAT) is between -1O°C to +30°C with a high humidity and/or visible moisture present in the air.
However, carburetor / throttle icing is most likely to occur between + 10 and + 15°C with a relative humidity greater than 40 percent.
Note: Carburetor icing can be found on a warm day in moist air, especially with descent power settings.
Piston Engines
What actions should you take to prevent or remove carburetor/throttle icing?
You should use the carburetor heat system (hot air) at regular intervals
to treat icing in the carburetor when in carburetor icing conditions.
The carb heat system delivers hot air from the engine compartment into the carburetor that melts the buildup of ice.
Note: It is also advisable to apply carb heat at the start of a descent to protect against throttle icing occurring and thereby ensuring that full power is available in the event of a go-around engine application.
However, carb heat always should be switched off before the throttle is advance to ensure maximum thrust delivery, and therefore, it should
be off in the final landing approach in anticipation of an emergency go around
near to the ground.
However, the use of carburetor heat should be avoided when the OAT is colder than -1OC because by applying carburetor heat you raise the air temperature in the carburetor into the icing temperature range, i.e., -1OC to +30C.
Propellers
What advantages does an aircraft gain from a propeller?
The propeller provides the following advantages:
1. The propeller creates a high-energy slipstream, which has three
main effects on the aircraft:
a. The slipstream creates extra lift over the wing.
b. The slipstream suppresses the stall speed of the aircraft.
c. The slipstream makes the fin/rudder more effective.
2. The propeller/piston engine has a quick response rate to a throttle input, which gives an early application of the slipstream effects on the aircraft.
Therefore, a propeller-driven aircraft has good slow-speed recovery.
Propellers
What is the main disadvantage of a propeller?
A propeller-driven aircraft suffers from a lack of airspeed due to propeller rpm limitations as a result of propeller compressibility losses.
This is so because the propeller suffers the effects of compressibility when the speed of the propeller blade tips becomes sonic.
Therefore, propellers have a revolution speed limited to just below sonic speed,
which limits the thrust force produced by propellers and therefore the aircraft’s airspeed.
This results in…
(1 ) a lower true airspeed (TAS) and
(2) a shorter range.
Propellers
What produces thrust on a propeller-driven aircraft?
The forward thrust (horizontal lift) of a propeller-driven aircraft is a result of Newton’s third law. “The application of a force on a body will cause an equal and opposite reaction.”
The propeller drives an accelerated
mass of air rearward, and the thrust force developed by the propeller is equal to the mass of the air and the rate of change in
momentum given to the air, which has the reaction of driving the aircraft forward.
The amount of the force created by the propeller is governed by two factors:
1. The output of the engine, which drives the prop shaft.
2. The blade angle, angle of attack, and pitch of the propeller.
Propellers
What restrictions does the propeller design have?
The degree to which the propeller design can be increased to absorb engine power is restricted in the following areas:
1. Blade length is restricted because of the following:
a. The need for adequate ground and fuselage clearance.
b. The need to maintain subsonic blade-tip speeds (the longer the propeller blade, the greater are the tip speeds, and supersonic propeller speeds are inefficient).
2. Blade chord size is restricted because of the following:
a. An increase in the chord size will reduce the aspect ratio (blade diameter/chord), and as with the wing, this gives a lower efficiency output than the optimal ratio.
b. Larger chord lengths also increase the centrifugal twisting moment, which tends to twist the blade to a finer pitch, causing...
(1 ) Higher loads on the root fittings.
(2) Higher torque values, which give a lower resultant force / lower thrust output (forward thrust force minus torque force equals resultant force).
3. The number of blades can be increased to a certain value, but with more blades, the hub diameter and weight become excessive, and blade interference begins to reduce efficiency.
Propellers
How does the propeller convert engine horsepower to produce thrust?
The propeller is connected to the engine via a prop shaft that rotates the propeller, which generates an accelerated mass of air rearward, thereby converting the shaft horsepower of the engine into a thrust force.
Thrust force = air mass x velocity
Air mass is determined by
( 1) blade angle,
(2) blade angle of attack and
(3) pitch.
Air velocity is determined by propeller rpm, which is set by
( 1) engine power output and
(2) blade angle of attack.
Propellers
Why is the propeller blade twisted?
The propeller blade is twisted along its length to maintain a constant blade angle of attack.
Propellers
How do you define propeller efficiency?
The propeller efficiency in producing thrust to propel an aircraft forward is determined as the ratio of the useful work done by the propeller (propeller thrust) in moving the aircraft to the work supplied by the engine (engine bhp).
Prop Efficency = Prop Thrust (Air-Mass x Velocity) / Engine BHP
Propellers
What are the disadvantages of a fixed-pitch propeller?ngle of attack,
The failing of a fixed-pitch propeller is that it only produces its maximum
efficiency (i.e., best blade angle/pitch for rpm speed) at one predetermined
engine rpm, altitude, and forward airspeed condition.
This is so because the forward airspeed affects the blade angle of attack (an increase in forward airspeed causes a decrease in blade angle of attack,which reduces the rearward air displacement and thereby decreases the thrust produced), and therefore, propeller efficiency is reduced away from its single predetermined condition.
(See Q: How do you define propeller efficiency? page 56.)
Therefore, for a fixed-pitch propeller, a single setting must be made between the blade angle (and therefore the angle of attack and pitch) desirable for takeoff and that required for cruise conditions.
And if a blade angle for high-speed cruise is chosen (coarse), then the aircraft
has the disadvantage of poor propeller performance for takeoff, and vice versa.
(See Q: What is a variable-pitch propeller, and why is it used? page 57.)
Propellers
What is a variable-pitch propeller, and why is it used?
Variable-pitch propellers are a development of the fixed-pitch propeller
that have a variable and controllable blade angle between the coarse and fine positions.
This is used to adjust the blade angle of
attack to its optimal setting in order to maintain propeller efficiency and aircraft thrust over a wide range of aircraft speeds, phases of flight, and differing operating conditions.
A variable-pitch propeller maximizes the propeller’s efficiency through a large speed range by maintaining a constant blade angle of attack that thereby produces a constant thrust value.
Propellers
What controls the propeller blade angle/speed?
A constant-speed unit (CSU) (sometimes called a governor unit) controls the
propeller’s blade angle/pitch to maintain an rpm speed.
Propellers
Why is a turboprop aircraft better suited for short regional operations?
Turboprop aircraft in general are better suited for short regional operations
because short sector routes normally have a restricted cruise altitude, which the turboprop aircraft is normally better suited to for the following reasons:
1. A jet engine is suited to high altitudes (i.e., 30,000 ft and above),
which an aircraft is not capable of reaching during a short sector route.
2. The turboprop engine is designed to operate at its most efficient at a medium altitude, which is associated with short regional operations.
3. In addition, many short regional operations operate out of restrictive airfields, and the turboprop aircraft's high-lift straight wing is capable of meeting the field lengths and climb and descent gradients of these restrictive airfields.
4. In addition, short-range sectors usually are flown more frequently and with a smaller passenger demand per trip. Therefore, a turboprop aircraft, which typically has a smaller passenger capacity than a jet aircraft, is a better economical design for this type of short sector demand, i.e., 20- to 60-passenger loads.
However, it should be noted that this trend is changing, with 30 to 70 seat jet airliners now being built that are efficient at medium altitudes over short regional routes and as such are replacing the turboprop fleets operated by regional airlines.
Propellers
Is there a critical engine on a propeller aircraft?
Yes, especially on aircraft with propellers rotating in the same direction.
A critical engine is the engine that determines the critical control speed
of the aircraft.
Propellers
Why is the number 1 engine the critical engine on a multi-engine propeller
aircraft?
There are two main reasons why the number 1 engine is the critical
engine on an aircraft with propellers rotating in the same direction:
( 1) slipstream effect and
(2) asymmetric blade effect.
