Cathay Pacific I Can Fly Programme General Aviation Knowledge. Aerodynamics

Transcription

1 Aerodynamics 1. Definition: Aerodynamics is the science of air flow and the motion of aircraft through the air. 2. In a level flight, the 'weight' and 'lift' of the aircraft respectively pulls and holds it in the air. Hence, the aircraft maintains the same height when the two forces - weight and lift balance each other. If, - Thrust > Drag: the plane will speed up; Drag > Thrust: the plane will slow down - Lift > Weight: the plane will climb; Weight > Lift: the plane will descend Lift 升力 Thrust 推力 Drag 阻力 Weight 重力 3. The top surface of a wing is more curved than the bottom. If two air particles split up at the tip of the wing and come back at the trailing edge at the same time, the particle on top must be travelling faster, given the top has a larger surface than the bottom.

2 4. The fundamental of fluid dynamics stated that as the speed of a fluid flow increases, its pressure decreases. While the faster moving air develops a lower pressure on the top surface, the slower moving air maintains a higher pressure on the bottom surface. This pressure difference essentially 'sucks' the wing upward. 升力 Lift 移動較快的空氣粒子會引致氣壓較低 Lower pressure due to faster-moving air 氣流 Air stream 機翼橫切面 WING Higher pressure exerted by slower-moving air 移動較慢的空氣粒子會造成氣壓較高 5. If the 'standard shape' of the wing is deformed, for example, if there is ice on the surface of the wings, the 'lift-producing capability' will be greatly reduced or may totally disappear. The aircraft may not be able to take-off at all. 6. Wing is a very important aspect of the aviation field. All Cathay Pacific pilots wear a pin that bears the shape of a pair of wings. 7. Apart from holding the aircraft in the air, airflow can also be used to control the direction of the aircraft when the airflow hits the control surfaces. 8. There are three types of control surfaces that can steer an aircraft's movement, namely, the elevator, the rudder and the aileron.

3 9. The elevator is controlled by the forward/backward movement of the control stick. Pulling the controls will cause the elevator to deflect upwards, causing the aircraft to pitch upwards about the lateral axis. 10. The ailerons are controlled by the sideway movement of the control stick. Turning the controls to the right will cause the right aileron to deflect upwards and the left one to deflect downwards. This causes the plane to roll right about the longitudinal axis.

4 11. The rudder is controlled by the movement of the rudder pedals. Pushing the left pedal will cause the rudder to deflect to the left. The plane will yaw left about the normal axis.

5 Engines 1. How do piston engines work? Almost all cars and light aircraft currently use what is called a four-stroke combustion cycle to convert gasoline into motion. The four-stroke approach was invented in 1867 and is comprised of: - Intake stroke: The stroke commences with the piston moving downward. The inlet valve open carrying the petrol mixture with it, the exhaust valve closed. - Compression stroke: The stroke commences with the piston moving back up to compress the gases into the space above the piston. The compression of the fuel produces an immense increase in the efficiency of an engine contributing to power output. The stroke ends with the passing of the spark. - Combustion stroke: Both valves remained closed. The gas in the combustion chamber being ignited by the spark burns rapidly. The heat generated causes the gases to exert a high pressure on the piston that forces it to descend to its original position. - Exhaust stroke: The exhaust stroke begins with the opening of the exhaust valve. The ascending piston forces the burnt or exhaust gases out of the cylinder through the exhaust port. This stroke, which ends at TDC, completes the cycle. (Four-stroke cycle)

6 2. How do turbine engines work? Gas turbine engines have three parts: - Compressor: Compresses the incoming air to high pressure - Combustion area: Burns the fuel and produces high-pressure, high-velocity gas - Turbine: Extracts the energy from the high-pressure, high-velocity gas flowing from the combustion chamber (The Working Cycle) 3. Comparisons between turbine/piston engines: - Gas turbine engines have a great power-to-weight ratio compared to piston engines. That is, the amount of power you get out of the engine compared to the weight of the engine itself is very good. - Gas turbine engines are smaller than their counterparts of the same power. - Gas turbines operates at high temperatures, making designing and manufacturing gas turbines a tough problem from both the engineering and materials standpoint. - Gas turbine engines are also more expensive compared to a reciprocating engine of the same size and tend to use more fuel when they are idle. 4. Differences between turbine/piston engines: - Turbine engines have a higher compressibility of air and hence better performance. This is especially true at high altitude. - Turbine engines have different variations to produce the thrust, for example turbo-jet, turbo-fan and turbo-prop. Piston engines, however, can only rely on the propellers to produce the thrust.

7 5. Variations of turbine engines: - Turbo-jet engines: Consist of a compressor, combustion and turbine section. They function by accelerating a relatively small amount of air to very high speeds to produce the reactive force required to move the aircraft. - Turbo- fan engines: They are turbo-jet engines with a large fan in front. The final turbine stage therefore drives a shaft that makes its way back to the front of the engine to power the fan. - Turbo-prop engines: Turbo-prop engines are similar to turbo-fan but instead have a conventional propeller at the front of the engine.