Q: Why Do Helicopters Always Have Two Blades, and How Does That Help Cats?

A: To answer this question, let's begin with an activity. Get a chair or stool that swivels. Sit on the chair, and push something (such as another chair or table) or someone (a friend) with your right hand and then with your lefthand. Make sure your feet aren't on the ground to prevent you from moving, and make sure you do this with your arms at your sides instead of in front of you (see Figure 1).

So what happened? If you did this correctly, you rotated in a direction opposite to the direction in which you pushed on the object or person. This is an example of Newton's third law. When you push on something, that something pushes back on you (see Figure 2).

When that something pushed back on you, you rotated because the push on you was off-center, meaning the push wasn't directed at the center of rotation. This is also shown in Figure 2. For those who like vocabulary words, a push that is not directed at the center of rotation of something is called a torque. Torques tend to cause objects to change their state of rotation (in this case, the torque causes you to go from rest to a state of rotating), just as linear forces tend to cause objects to change their state of linear motion.

Now let's look at helicopters. Imagine a helicopter that has a single blade (by single blade, I mean a single-blade assembly, which can have two, three, or four blades in total). In order to get that blade rotating, the mechanism attached to the helicopter must exert a torque (by exerting an off-center force) on the blade assembly. Because Newton's third law is always in effect, the blade assembly exerts a force (and thus a torque) back on the helicopter. With only one blade, the blade will rotate in one direction, but the helicopter will rotate in the opposite direction. If you try to fly a helicopter with just one blade, or blade assembly, you will be spinning the entire flight (see Figure 3).

We can fix this problem by adding another blade to the helicopter, either on top and spinning horizontally with the first blade or on the back at the tail...