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Trailing edge flaps once extended:
  • A
    degrade the minimum glide angle.
  • B
    significantly increase the angle of attack for maximum lift.
  • C
    increase the zero lift angle of attack.
  • D
    significantly lower the drag.

Refer to figure.
The total drag is the resulting factor if you add induced drag and parasite drag together. It acts in the direction of the relative wind. Lift is perpendicular to the relative wind.

Parasite drag is caused if an airfoil is moving through the air. It is the result of form drag and skin friction drag. Form drag arises from the shape of the object. The more streamlined the object, the less form drag is present. Shape and size are most important in the amount of form drag resulting. Form drag increases with increasing velocity, making it very important for high speed aircraft. Skin friction drag is caused by the friction of the fluid (in case of aircraft this fluid is the atmosphere) against the skin of the object moving through it. It is directly related to the surface area in contact with the fluid. Skin friction increases with the square of the velocity.

Induced drag is caused when an object redirects the airflow. A wing produces lift by creating a pressure difference. This pressure difference is caused by the shape of the airfoil, causing the airflow on top to accelerate, creating a low pressure on top of the wing and high pressure below the wing. This pressure difference naturally wants to restore towards a uniform pressure, which creates vortices around the wingtip. These vortices are deflected downwards, this phenomenon is also called downwash. The induced drag decreases with airspeed (with a rate of 1/V²) and increases with angle of attack and increasing weight.

In the image we can see that the two vectors (induced and parasite drag) create a resulting vector, the total drag. Where the induced and parasite drag vectors collide, we have the minimum drag speed (VMD); the speed where the total drag is the lowest. This leads us to understand that during an acceleration from a total stand still, the total drag first decreases up until the minimum drag speed and then starts increasing again.If we add flaps to the clean configuration, CL will increase, but so will the drag. This degrades glide performance. The rate of descent will increase and glide range will decrease.

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