7 WWW.GOFLY.ONLINE Y ONLI NE FLI GHT SCHOOL PRINCIPLES OF FLIGHT Types of Drag The types of drag that affect an aircraft in flight, are discussed below. Parasite Parasite drag is caused by the aircraft and its components being exposed to the relative airflow. Parasite drag is all drag that is not associated with the production of lift, and includes form drag, skin friction, and interference drag. Form Form drag is drag due to the shape of the aircraft. A more streamlined aircraft will suffer from less form drag. Interference Interference drag is created when air flowing over points of the airframe where surfaces meet or join, such as where the wing meets the aircraft fuselage, is disrupted. Shaping, or fairing, of these joints may reduce interference drag. Skin friction Skin friction drag is produced as air flows over the surfaces of the aircraft, and is largely due to the smoothness or roughness of the aircraft’s skin. Induced (lift dependent) Induced drag is drag caused as a byproduct of lift being produced by the wing. Total drag relative to airspeed As an aircraft’s speed increases in level flight, the angle of attack is reduced, and induced drag decreases. Induced drag decreases as airspeed increases. When an aircraft is stationary, with no airflow moving over it, there is no parasite drag. As the airspeed increases, parasite drag starts to increase. • Parasite drag increases as airspeed increases. • Total drag is a mixture of both induced and parasite drag. In relation to airspeed, total drag initially decreases as airspeed increases due to induced drag decreasing, before reaching an airspeed known as minimum drag speed, where total drag starts to increase as a result of increasing parasite drag. Attitude, Angle of Attack and Airspeed The relationship between attitude, angle of attack and airspeed in level flight In level flight, there is a relationship between attitude, airspeed and angle of attack. Most small training aircraft do not have an angle of attack indicator, but in level flight at normal operating speeds, the attitude is closely related to the angle of attack. The attitude is also closely related to the aircraft airspeed. Let’s look at what happens when an aircraft slows down in level flight. We know that lift is a factor of velocity, so as the aircraft slows down, the lift starts to reduce. If nothing changes the aircraft will start to descend. The only way for the aircraft to stay level is to create more lift by increasing the angle of attack. The pilot controls the angle of attack through the use of pitch by observing the aircraft’s attitude. Angle of attack changes pressure changes above and below the wing At low angles of attack, the airflow over the wing is relatively smooth, resulting in lower pressure on the upper surface of the wing compared to the lower surface. As the angle of attack increases, the airflow over the upper surface of the wing becomes more curved, leading to a decrease in pressure and an increase in airflow velocity according to Bernoulli’s principle. On top of the wing we have the centre of pressure. The centre of pressure is the average point where all the different pressure forces are applied on the wing. As the wing’s angle of attack starts to increase, so does the lift and drag, and the centre of pressure starts to move forward.
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