Types of Drag
What are the different types of Drag ?
Drag as we have seen previously is a force, generated (usually) by air moving over an object.
There are various different kinds of Drag and they are generated by different parts of aircraft for different reasons, the terminology surrounding them seems to cause much confusion with different organisations having slightly different definitions of very similar things (as is usual in aviation).
What is important to remember is that drag is merely that part of the force generated by air passing by an object, that acts in line with the air flow towards the object, some of which touches the object, some of which doesn't.
It is also important to understand that Drag is not Power, power is consumed to overcome Drag but they are not the same thing.
Defining Types of Drag
These defintions are from aerodynamics texts, not helicopter books, we will deal with helicopter specifics later. Some of the drag types are due to viscous effects of fluids, the others are due to pressure effects of the fluids concerned.
That part of the drag which is caused by shear stress at all points on the body concerned where it is touched by the fluid in which it is immersed, it acts tangentially to the surface. It is due entirely to the
viscosity of the fluid in which the body is immersed.
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Is pressure drag due to flow seperation.
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Profile Drag is the sum of Form drag and Skin Friction drag. It is the drag to which the Drag Coefficient C
d refers, remember that the coefficients in text books are for two dimensional airfoil sections and that wings and rotor blades are three dimensional.
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Induced drag is dependant upon lift production and is not usually associated with viscous effects. In fixed wing terms the induced drag is caused by part of the lift vector being inclined rearward due to the downwash created by the formation of vortices at the trailing edge which causes the relative wind to be inclined downwards, reducing the angle of attack of the aerofoil. Remember
lift is defined as that part of the force which acts at 90 degrees to the relative airflow.
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A blunt body is one where most of the drag is pressure drag.
A streamlined body is one where most of the drag is skin friction drag.
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Bibliography
Aerodynamics for engineering students, Houghton and Carpenter.
Fundamentals of Aerodynamics, John D. Anderson.