Question

Explain the reason why friction is necessary for the creation of lift.

Answer 1

Every physical body that is propelled through the air will experience resistance to the air flow. This resistance is called drag. Drag is the result of a number of physical phenonmena. Pressure drag is that which you feel when running on a windy day. The pressure of the wind in front of you is greater than the pressure of the wake behind you. Skin friction, or viscous drag, is that which swimmers may experience. The flow of water along a swimmer's body creates a frictional force that slows the swimmer down. A rough surface will induce more frictional drag than a smooth surface. To reduce viscous drag, swimmers attempt to make contact surfaces as smooth as possible by wearing swim caps and shaving their legs. Likewise, an aircraft's wing is designed to be smooth to reduce drag ,  putting your hand out of a moving car window and feeling it pull back.

Like lift, drag is proportional to dynamic pressure and the area on which it acts. The drag coefficient, analgous to the lift coefficent, is a measure of the amount of dynamic pressure gets converted into drag. Unlike the lift coefficient however, engineers usually design the drag coefficient to be as low as possible. Low drag coefficients are desirable because an aircraft's efficiency increases as drag decreases.