How Everything Works
How Everything Works How Everything Works

Question 519

How do helicopters fly with such small wings without them breaking off?
As you suggest, the blades of a helicopter are really rotating wings. But unlike the wings of a normal airplane, the helicopter blades are always moving through the air, even when the helicopter's body is not. That's why a helicopter can obtain an upward "lift" force from the air while it's hovering motionless—the wings keep moving and obtaining that lift force. A second difference between a helicopter's rotating blades and the wings of a normal aircraft is that a helicopter's blades are under enormous tension. Were it not for this tension, the end of each blade would naturally travel in a straight line at constant speed, a behavior that we associate with inertia—objects that are free of outside forces travel at constant velocity (they follow straightline paths at constant speeds). To make the end of its blade travel in a circle (which is certainly not a straight line), the helicopter must pull the end of the blade toward the pivot about which the blade is turning. Thus as the blades turn, each blade experiences an enormous tension pulls the parts of the blade toward the pivot. This tension is what stiffens the blade, just as tension stiffens the strings of a guitar or a violin. Just as it's hard to break a guitar string by bending it, it's hard to break a helicopter blade by bending it. However, both guitar strings and helicopter blades will snap if they're exposed to more tension than they can tolerate. The manufacturers of the blades work hard to make each blade strong enough to withstand the enormous tension it experiences in use. As long as the blades can tolerate this tension, they won't break and will have no trouble supporting the body of the helicopter.

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