Drag Flashcards
Explain skin-friction drag and state factors affecting it.
Skin- friction drag is the result of shear stress between successive levels of air with the boundary layer. Shear stress is the force required to separate the air particles.
factors affecting skin friction drag are:
- Speed
- Shape
- Surface condition
- Size
- Angle of attack
State the drag formula, and the three basic functions contained within it.
Parasite( Skin-friction Drag + Form Drag) + Induced = TOTAL DRAG
Identify and explain the components of total drag.
Components are: Induced drag and Parasite drag. Parasite drag is broken into separate sections:
profile drag, form drag, skin friction and interference drag.
Induced drag is created by lift. The relative air flow is changed creating a steeper AOA.
Profile drag is split into skin friction drag and form drag.
Skin friction drag is the air sticking to the aircraft surface as it passes over it. At first it is laminar flow, only 2 mm thick. Then it turns to turbulent flow which is 2 cm thick.
Form drag is caused by the shape of the wing. The front area will have high pressure while the rear of the wing will have low pressure.
Describe interference drag and the measures for reducing it.
Interference drag is caused by different air flows from different surfaces (such as wings and fuselage) coming into contact. To decrease this drag fairings are used to smooth out the connecting area.
Describe:
a) laminar boundary layer flow;
The initial part of flow over most smooth surfaces is laminar in nature.
Laminar boundary layers are very thin, in the order of 2mm in depth. Each successive sheet of air slides smoothly over the one nearer the surface.
Describe:
b) turbulent boundary layer flow;
After progressing over a distance over the surface, the flow in the boundary layer normally becomes turbulent and the layer becomes much more thicker. Turbulent flow boundary layers are about 10 times thicker than laminar flows, about 2cm thick.
Describe:
c) transition point;
The point at which the flow changes from laminar to turbulent.
State the meaning of the term coefficient of drag (CD); and describe the main features of a typical curve of CD versus (angle of attack)
The total drag on an aircraft is a combination of parasite drag and induced drag. The coefficient of total drag is therefore CD= parasite+induced. Minimum CD will generally occur around 0 degrees angle of attack. Beyond the stalling angle, the increase in CD becomes more rapid due mainly the effect of separation and increased form drag.
Explain the term boundary layer.
When air moves past a solid body, its viscosity causes the particles next to the surface to adhere to the surface of the body, and the airflow in the immediate vicinity to be slowed down. This layer of retarded flow, which is sandwiched between the local freestream flow and the surface of the body, is known as the boundary layer.
Flow within the boundary layer exists in 2 forms, laminar flow and turbulent flow.
Laminar flow is where initially the flow is streamlined in nature. The laminar flow boundary layer is very thin, about 2mm thick.
After progressing as laminar flow for a distance the flow becomes turbulent and thus the layer becomes much thicker. This happens at a point called the transition point. It is now called turbulent flow and is usually about 2cm thick.
Describe the effect of streamlining in reducing form drag.
Streamlining reduces the effect of adverse pressure gradients by making curvature of surfaces more gradual. This delays separation which reduces the size of turbulent wake and reduces pressure difference between forward and rearward surfaces. This reduces form drag dramatically.
From the information provided or a diagram identify the speed for minimum drag and maximum lift/drag ratio.
Fig 5-17.
Explain form drag and state the factors affecting it.
Part of the drag on an aerofoil which arises from its shape. It varies according to the angle of attack and can be decreased by streamlining.
Describe typical curves of induced drag, all other drag, and total drag versus IAS in straight and level flight.
Fig 5-17.
Distinguish between graphs for CD and total drag.
See graph 5-16 and 5-17. Remember CD = Parasite + Induced (Total Drag)
Explain from a typical graph the most efficient angle of attack, the zero lift position and the stalling angle.
Most efficient angle of attack is 4 degrees. Zero lift position is below zero (-4 degrees on a general purpose aerofoil). The stalling angle is any angle past 15 degrees.
