Aircraft

A helicopter is an aircraft that has the capability of maneuvering in both horizontal and vertical directions. It accomplishes these maneuvers by means of a single elaborate and rather remarkable propeller-like device mounted to its top. Although the device looks like an ordinary propeller, it is much more complicated. In fact, the device is more properly thought of as a pair of wings (rotor blades) that spin around a common center. By varying the pitch, position, and angle of attack of the rotor blades, the helicopter pilot can direct the thrust upward, downward, forward, backward, or at an angle. For example, if the pilot wants the aircraft to go upward, he or she increases the pitch in each blade of the rotor, increasing the upward lift that is generated. If the pilot wants to move the aircraft in a forward direction, the whole rotor shift can be tipped forward to change the lifting action to a forward thrust.

Helicopters present some difficult design problems for aeronautical engineers. One of the most serious problems is that the spinning of the rotor causes—as Newton's second law would predict—a reaction in the helicopter body itself. As the rotors spin in one direction, the aircraft has a tendency to spin at an equal speed in the opposite direction. A number of inventions have been developed to deal with this problem. Some helicopters have two sets of rotors, one turning in one direction, and the other in the opposite direction. A more common approach is to add a second propeller on the rear tail of the helicopter, mounted either horizontally or vertically. Either design helps to stabilize the helicopter and to prevent it from spinning out of control.