Propulsion Exam Questions

The constant speed unit (CSU) in an aircraft propeller system is responsible for maintaining a selected engine speed within the power available. This means that regardless of changes in load or other factors, the CSU adjusts the propeller pitch to ensure that the engine maintains a consistent speed. This is important for efficient operation and to prevent engine damage. The CSU does not feather the propeller, retard ignition timing, or increase the mixture strength.

Using liquid to cool an aircraft piston engine instead of air provides the benefit of more easily maintaining a steady operating temperature. Liquids have higher specific heat capacities compared to air, meaning they can absorb and dissipate heat more effectively. This allows for better heat regulation and prevents overheating or temperature fluctuations in the engine. By maintaining a consistent temperature, the engine can operate more efficiently and reliably.

A magneto is a device used in an aircraft's piston engine to generate the spark needed for the spark plugs. It functions as a small generator, producing high voltage electrical pulses that ignite the fuel-air mixture in the engine cylinders. This eliminates the need for an external power source, such as a battery or an alternator, to provide the spark. The magneto is a reliable and self-contained ignition system commonly used in aircraft engines.

Windmilling refers to the situation where the airflow over the propeller blades of a failed engine keeps the propeller turning. This can occur when the aircraft is descending or when there is sufficient airflow from the forward motion of the aircraft. The term "windmilling" is used because the propeller blades rotate due to the force of the airflow, similar to how a windmill operates. It is important for pilots to be aware of windmilling as it can affect the handling characteristics of the aircraft and may require specific procedures to be followed.

Gudgeon pins, also known as wrist pins, are used in a piston engine to attach the pistons to their connecting rods. These pins go through the small end of the connecting rod and the piston, allowing the piston to move up and down while maintaining a secure connection with the connecting rod. This attachment is crucial for the proper functioning of the engine, as it allows the transfer of power from the pistons to the crankshaft, which ultimately drives the vehicle.

The function of the turbine in a turbojet engine is to drive the compressor. The compressor is responsible for compressing the incoming air, which is then mixed with fuel and ignited in the combustion chamber to produce high-pressure gases. These gases then expand through the turbine, causing it to rotate. The rotation of the turbine is used to drive the compressor, ensuring a continuous flow of compressed air into the engine. This process allows the engine to maintain its power output and efficiency.

The Olympus 593 is a turbojet engine.

The turboshaft engine is most suited for helicopters because it is designed to provide a high amount of power at low speeds, which is ideal for vertical takeoff and landing, hovering, and maneuvering in tight spaces. Unlike fixed-wing aircraft that require forward motion to generate lift, helicopters rely on the power of the turboshaft engine to rotate the rotor blades and create lift. This engine type also offers good fuel efficiency and the ability to quickly respond to changes in power demand, making it well-suited for the unique requirements of helicopter operations.

After-burning in a jet engine refers to the process of burning additional fuel in the jet pipe. The jet pipe is the section of the engine where the exhaust gases pass through before being expelled from the engine. By injecting extra fuel into the jet pipe and igniting it, the temperature and velocity of the exhaust gases are increased, resulting in a significant boost in thrust. This allows the jet engine to generate more power and achieve higher speeds, making it a crucial component for increasing the performance of the aircraft.

Turbo-chargers are devices that increase the efficiency and power output of an engine by compressing the incoming air. They are driven by exhaust gases, which means that they use the energy from the engine's exhaust to spin a turbine, which in turn drives a compressor. This compressor then forces more air into the engine, resulting in increased power. Therefore, the statement "Driven by exhaust gases" correctly refers to turbo-chargers.

The purpose of the "fins" arranged about the cylinder and cylinder head of an air-cooled engine is to allow heat to dissipate rapidly. The fins increase the surface area of the engine components, which enhances the heat transfer process. As air passes over the fins, it carries away the heat generated by the engine, helping to prevent overheating and ensuring efficient operation.

The arrow pointing to the main jet in the diagram of the carburettor is X.

When a propeller is "feathered," it means that the blades have been rotated parallel to the airflow, with the leading edges facing forwards in the direction of flight. This position minimizes drag and allows the aircraft to maintain a higher speed during engine failure or shutdown. It is not related to the propeller's rotational speed or power production.

The distributor in a piston aero engine is responsible for distributing high voltage electrical impulses to the cylinders. This is essential for igniting the air-fuel mixture in each cylinder at the correct time, ensuring efficient combustion and power generation. Without the distributor, the spark plugs in the cylinders would not receive the necessary electrical impulses, leading to misfires and engine performance issues.

In the modified 4-stroke cycle, ignition takes place just before TDC on the compression stroke. This is because the fuel-air mixture needs to be ignited before the piston reaches the top dead center (TDC) in order to achieve maximum combustion efficiency. Igniting the mixture just before TDC ensures that the expanding gases from the combustion can push the piston down with maximum force during the power stroke, resulting in better engine performance.

Blade twist in a propeller helps to even out the thrust along the length of the blade. This is because the angle of attack of the blade changes along its length, with a higher angle of attack at the root and a lower angle of attack at the tip. This variation in angle of attack helps to distribute the thrust more evenly, resulting in smoother operation and better performance of the propeller.

Coil springs are used to return the valves onto their seats after they have been opened in a piston engine. When the valves are opened, the coil springs are compressed. Once the camshaft or cam pressing on the valve stems releases the pressure, the coil springs expand and push the valves back onto their seats, ensuring a proper seal. This allows for efficient combustion and prevents any leakage of gases during the engine's operation.

Air cooling has an advantage over liquid cooling because it is much cheaper to make. Liquid cooling systems require additional components such as radiators, coolant pumps, and hoses, which increase the cost of manufacturing. On the other hand, air cooling systems do not require these additional components, making them more cost-effective.

The direct fuel injection system does not require a choke on the intake. This means that unlike in a carburetor system, there is no need for a device that restricts the flow of air to increase the fuel mixture when starting a cold engine. In a direct fuel injection system, the fuel is injected directly into the combustion chamber, bypassing the need for vaporization. The system also does not require a throttle butterfly, which is a device used in carburetors to regulate the flow of air and fuel mixture. However, the statement does not provide any information about the system's ability to operate inverted.

The correct answer is: To transfer oil from the crankcase to the oil tank.

In a dry sump lubrication system, the scavenge pump plays a crucial role in ensuring proper lubrication of the engine by:

Removing used oil: It collects the oil that drains from the engine components and bearings after lubrication. This oil might contain contaminants like metal shavings or carbon deposits.

Transferring oil: It pumps the collected oil from the crankcase (where it accumulates) to the external oil tank.

Maintaining oil circulation: This continuous loop of collecting and transferring oil helps maintain a constant flow and prevents stagnation in the crankcase, ensuring all engine components receive lubrication.

While the scavenge pump might contribute to contaminant removal to some extent by collecting used oil, its primary function is not specifically focused on filtration. Other components, like oil filters, handle that task further downstream.

The pressure chamber receives oil from the oil tank, not directly from the scavenge pump. The pressure pump then pressurizes this oil and delivers it to various engine components for lubrication.

Finally, while both scavenge and pressure pumps contribute to overall lubrication, the scavenge pump solely focuses on oil removal and circulation, not ensuring direct lubrication of all engine components. That's the combined job of both scavenge and pressure pumps working together.

Therefore, the main purpose of the scavenge pump in a dry sump lubrication system is to efficiently transfer oil from the crankcase to the oil tank.

The largest obtainable pitch angle on a variable pitch propeller is known as the coarse pitch. This refers to the propeller blades being set at a steeper angle, which allows for greater thrust and power during takeoff and climbing. The coarse pitch is typically used in situations where maximum performance is required, such as during takeoff or when operating at high altitudes.

The correct answer is the cam shaft. In the diagram, the cam shaft is shown both inside the engine and outside the engine, enlarged for clarity. This suggests that the cam shaft is an important component of the engine and is being highlighted for better understanding. The other options, such as the connecting rod assembly, tappet assembly, and crankshaft, are not mentioned or emphasized in the explanation, indicating that they are not the correct answer.

The Dart engine is a turboprop engine. A turboprop engine is a type of aircraft engine that uses a gas turbine to drive a propeller. The Dart engine, developed by Rolls-Royce, is a well-known turboprop engine that has been used in various aircraft, including the British Aerospace 748 and the Fokker F27 Friendship. It is known for its reliability and efficiency in powering regional and commuter aircraft.

Valve lead refers to the timing of the valve opening or closing relative to the position of the piston. In the modified 4-stroke cycle, the exhaust valve opening before the piston reaches BDC is considered as valve lead. This means that the exhaust valve starts to open before the piston has completed its downward stroke, allowing the exhaust gases to begin exiting the cylinder before the piston starts its upward stroke. This helps in improving engine efficiency and performance by ensuring better scavenging of exhaust gases and reducing back pressure.