Chapter 8, Problem 24Q

To determine

The reason for a helicopter to have more than one rotor to keep it stable and the ways in which

the second propeller can operate to keep the helicopter stable

Answer to Problem 24Q

Solution:

For the helicopter, the single rotor provides the lift. In order to keep the rotor spinning a torque needs to be applied. An equal and opposite torque will be produced. The opposite torque will be applied to the body of the helicopter. If we do not apply the opposite torque, the helicopter will collapse. In order to avoid this situation, a second rotor or a tail-rotor is needed. It helps to provide the opposite torque in another axis of rotation.

Explanation of Solution

When the propeller of a helicopter changes it angular velocity while flying, to conserve the angular momentum, a balancing torque has to be provided. In case of single propeller helicopters, this torque is provided by the body of the helicopter itself, which can cause loss of control. To prevent this, many helicopters have a propeller at the tail, which provides an opposing torque and keeps the flight smooth.

Instead of having a propeller at the tail, a second propeller can be added in the same plane as the first propeller, and this second propeller’s velocity can be controlled as required to keep the helicopter balanced.