Understanding Gyroscopic Precession

Airplanes use gyroscopes in many of their instruments. There are also some aerodynamic effects from the rotating movement of the propeller that require an understanding of gyroscopes. This post will focus only on precession and not on the main attributes of gyroscopes.

Gyroscopic precession affects rotating objects like a propeller or the classic example, a bicycle wheel. As an object rotates it will translate forces applied to it by 90 degrees.

If you push sideways on the top of a moving bicycle wheel, the bicycle will not fall over. Instead, it will turn because the front of the wheel will move in response to the force.

Now let’s think about the rotor of a helicopter, which is also subject to gyroscopic precession. Imagine that the blades are spinning around flat and just one of them is tilted for an instant to gather more lift. During this instant the tilted blade will have an upward force applied to it. It also already has an enormous amount of momentum and will continue spinning at a high speed.

The lift will cause it to begin accelerating up, but the momentum will very quickly move it farther around the rotor disk. Combining these two actions results in the blade rising until it reaches its peak 90 degrees from the point where it was pushed up.

Source: FAA Helicopter Flying Handbook

For example, if the blade (rotating counter-clockwise if you look at it from above) tilted momentarily while passing over the back of the helicopter then the whole rotor disk would tilt left because the right side would go up.

Confused yet? Gyroscopic precession is weird but it makes sense. If you can understand this 90 degree translation of force it will help you to understand:

  • Why airplanes have left turning tendencies
  • Why helicopter controls are mounted 90 degrees from the direction intended
  • How some airplane instruments work
  • Why your bike or motorcycle doesn’t tip over

Here are some additional resources I recommend: