2. Drag

2.1 What is friction ?

Friction is the force that holds objects in motion back — or so it may seem.

Friction is the rubbing between solid objects. For instance, when you rub your hands together you will feel friction, the force that resists motion and creates heat. Friction can be used for positive purposes: athletes wear sneakers on a gym floor so that they don't fall down while running.

Actually, friction is essential for everyday living. Imagine a world without friction:

NO WAY to drive a car on the road,
NO WAY to walk on pavement,
NO WAY to pick up the ham sandwich or your phone.

Friction may seem like an enemy to the hearty physics follower, but it’s also your friend.

Friction comes from the interaction of surface irregularities. If you put together and press two surfaces that have plenty of microscopic pits and projections, you produce friction. And the harder you press thos two surfaces, the more friction you create as the irregularities interlock more and more.

2.2 What is drag ?

Forces on airplane

Drag is the friction between a solid object and fluid (liquid or gas). For instance, when you sail a boat across a lake, the force that resists the movement of the boat through the water is drag. Heat is generated by drag just as heat is generated by friction.

You can experience drag at home in your bath tub or at school in a tank filled with water. Try to move your hand through the water, first with your fingers put together and facing the water and then with your palm facing the water. In which way the movement is easier?

When you ride a bicycle at high speed, drag makes the bike harder to pedal and increase speed. Racing cyclists crouch over their handlebars to decrease drag and increase speed.

Which are the factors influencing drag?

When a solid body is moved through a fluid (gas or liquid), the fluid resist the motion. The object is subjected to an aerodynamic force in a direction opposed to the motion which we call drag.

This is the force that opposes movement of a boat through the water.

This is the force that opposes movement of a plane through the air.

As with aircraft lift, there are many factors that affect drag. We can group these factors into those associated with the object, those associated with the motion of the object through the air, and those associated with the air itself.

The Object: Shape and Size
The Motion: Velocity and Inclination to Flow
The Air: Mass, Viscosity, Compressibility

2.3 The Object

Geometry has a large effect on the amount of drag generated by an object. As with lift, the drag depends linearly on the size of the object moving through the air. The cross-sectional shape of an object determines the form drag created by the pressure variation around the object. The three dimensional plan form shape affects the induced drag of a lifting wing. If we think of drag as aerodynamic friction, the amount of drag depends on the surface roughness of the object; a smooth, waxed surface produces less drag than a roughened surface. This effect is called skin friction and is usually included in the measured drag coefficient of the object.

Rough surface: produces a big amount of drag. Smooth surface: the surface of a plane is very smooth and polished, producing a small amount of drag.

Between all the shapes in the next picture, the smallest amount of drag is produced by the aerodynamic profile (airfoil).

2.4 Motion of the Air

Drag is associated with the movement of the aircraft through the air, so drag depends on the velocity of the air. Like lift, drag actually varies with the square of the relative velocity between the object and the air. The inclination of the object to the flow also affects the amount of drag generated by a given shaped object. If the object moves through the air at speeds near the speed of sound, shock waves are formed on the object which create an additional drag component called wave drag. The motion of the object through the air also causes boundary layers to form on the object. A boundary layer is a region of very low speed flow near the surface which contributes to the skin friction.

2.5 Properties of the Air

Drag depends directly on the mass of the flow going past the aircraft. The drag also depends in a complex way on two other properties of the air: its viscosity and its compressibility.

These factors affect the wave drag and skin friction, which are described above. We can gather all of this information on the factors that affect drag into a single mathematical equation called the Drag Equation. With the drag equation we can predict how much drag force is generated by a given body moving at a given speed through a given fluid.

2.6 The Drag Equation

The drag coefficient Cd contains all the complex dependencies, like the dependence on body shapes, inclination, air viscosity, and compressibility and is usually determined experimentally.

Drag depends on the density of the air, the square of the velocity, the air's viscosity and compressibility, the size and shape of the body, and the body's inclination to the flow. In general, the dependence on body shape, inclination, air viscosity, and compressibility is very complex. One way to deal with complex dependencies is to characterize the dependence by a single variable. For drag, this variable is called the drag coefficient, designated "Cd."

This allows us to collect all the effects, simple and complex, into a single equation.

Reducing drag

The drag equation states that drag D is equal to the drag coefficient, cd, times the density,?, times half of the velocity, V, squared, times the reference area A. D=Cd.(p.V²/2).A

To decrease friction, low-friction materials such as Teflon and other plastics are used. Contact surfaces are designed to be smooth as possible. Lubricants are introduced to reduce friction. To decrease drag on an airplane, smooth surfaces and a streamlined design are used. In addition, high aspect ratio wings are used as well as an optimum camber.

How can drag be reduced?

Using smooth surfaces: the surface of a plane is very smooth and polished, producing a small amount of drag.
Streamlined design hat is a flow? A flow is the continuous movement of a fluid, either a liquid or a gas, from one