# Questions about helicopter rotors - conservation of angular momentum

I have two basic questions about helicopters. Let's say the z-axis is the vertical axis from ground to sky. I know the tail rotor is there to counteract the angular momentum effects of the main rotor, and keep the total angular momentum about the z-axis at 0.

1. Say the tail rotor spins about the x-axis. What keeps the body of the helicopter spinning about the x-axis in the opposite direction? I assume it's a slight tilt of the main rotor forward to balance this effect. Am I right?

2. How are the speeds of the two rotors are kept coordinated? I assume there must be a sensor/feedback mechanism by which rotor speeds change to counteract spinning.

• I think your first question is like; Why wheels rotate instead of cars?. For the second one, I think you can follow a link like this; Helicopter Gearbox Animation ps: Currently I have no time for wider explanation, just wanted to give you a clue. Dec 30 '16 at 8:35

A conventional helicopter has three main flight controls.

• Torque pedals : these function is a similar way to the rudder on a fixed wing aircraft. They work by adjusting the pitch of the blades of the tail rotor to produce more or less thrust, allowing the pilot to balance the torque from the rotor blades and also control the yaw of the aircraft.

• The cyclic lever : this controls the pitch of individual rotor blades at a specific point in their rotation, controlling pitch and yaw. This is analogous to tilting the rotor disk at about its centre.

• Collective pitch : this controls the pitch of all of the rotor blades simultaneously controlling overall lift. As the pitch is changed the throttle must also be used to adjust engine power to maintain a more or less constant rotational speed as the load on the blades changes.

It is worth reiterating that it is the pitch of the blades which provides the direct control over thrust and lift and engine power is adjusted to keep their rotational speed withing an optimum range.

In a conventional helicopter the tail rotor and main rotor are directly coupled together via the gearbox and only their blade pitch rather than speed can be adjusted independently ie the tail rotor speed is proportional to the main rotor speed.

There are designs which deviate from the conventional layout though for example there are tail designs which use ducted airflow rather then an external rotor and also various configurations of twin rotor which dispense with tail thrust altogether eg the Chinook which has fore and aft rotors and various Russian military helicopters which have coaxial rotors.

You're right on Q1, but typically the blades are designed in such a way as to take advantage of Bernoullis principle. Notice the pitch of the blades: this animation is pretty cool: https://m.youtube.com/watch?v=C7N3ME5NTOs

Control systems engineering. Feedback loops, pid control, lots of computational control of dynamic systems keeps them in order.

Considering that the helicopter's forward direction is the y-axis, the rear rotor should deliver torque, but the thrust created by the main rotor is connected to a swashplate so the thrust is not directly orthogonal. The main rotor can tilt into the y-axis in order to accelerate forward and vice versa.