Text 2. Rifling

Let us take a look at the other end of the size scale – a rifle bullet. A gun barrel is rifled to give the bullet a spin in flight. In many types of projectiles the center of air resistance is forward of the center of gravity. This means that if nothing were done, a torque would be set up which could turn the bullet over in flight and make the tip face backward. With the spin resulting from rifling, the bullet acts like a small gyro and the angular momentum tends to resist the air resistance torque. Like any other gyro, though, the torque does produce a precession. It makes the bullet drift in azimuth. This drift is allowed for in sighting the gun. The larger the projectile, or the faster the spin, the less is the precession. There is nothing to sustain the spin throughout the entire flight against the friction drag of the air and the bullet spin rate slows down with increased range. If the air resistance torque does not reduce as quickly, the azimuth drift rate increases with range.

Text 3. Bikes, Hoops and Tops

One of the things that keeps you up on a bike is gyroscopic action. You unconsciously use the law of gyroscopic precession to right yourself, if the bike begins to tip. Suppose, for example, you were riding down the street with your hands off the handlebars, and your bike begins to tip over to the right. You might grab for the handlebar to turn the front wheel to the left. But that does not work. If you turn the wheel to the right, into the tipping direction, this action rights the bicycle. By turning the wheel to the right, it begins to precess about a horizontal axis (at right angles to the direction of spin). This is the correct way to restore the bike to an upright position. Centrifugal forces help too – but we are speaking about gyroscopes.

Remember the hoop and the spinning top? Both are kinds of gyroscopes. To turn the hoop, it did not do any good to try to push the back or front. This just caused the hoop to tip over. Instead, you tried to push the hoop over at the top of the rim. Spin vector into the torque vector, and the hoop turns by precessing about the vertical axis. When a top is slowed down, it does not just drop over. The spinning axis described a zone which slowly got flatter as the center of gravity descended. Precession came from a torque produced by the reaction forces at the tip, and the weight times the horizontal distance between the center of gravity and the tip.

Text 4. Ship Stabilizers

Let us look at some other ways that gyroscopes have been used. To reduce a ship's rolling one should use a gyroscope as a brute force ship stabilizer. This is basically a single-degree-of-freedom gyro with the spin axis vertical and gimbal axis along the athwartship axis of the boat. Suppose the boat starts to roll. This means that a torque is being applied about the fore-aft axis of the ship, in quadrature to both the spin axis and the gimbal axis. At the same time, gyroscopic torque is developed about the fore-aft axis which opposes the force producing the roll. The gyroscopic torque will continue to oppose the rolling torque until the gyro precesses so that the spin axis aligns with the ship's roll axis. At this point, the gyro ceases to act as a roll stabilizer. This system would work fine if all stray torques were eliminated so that the gyro would not drift; and if all rolling torques were of such magnitude and duration, and also symmetrical, so that the spin axis would never precess to be in alignment with the ship's fore-aft axis. These necessary conditions make it impossible to use an uncontrolled gyro.

There is another way. The rolling motion of the ship can be detected by a smaller single-degree-of-freedom control gyro mounted so that the spin axis is along the ship's athwartship axis and the gimbal axis is along the vertical axis. The stabilizing gyro is mounted as before, except that a small torque motor has been added which is geared to exert torque about the gimbal axis of the big gyro. This torquing arrangement is really very much the same as used in the centering system of a stable platform. When the ship rolls, the control gyro precesses about the vertical axis and contacts are closed which start the precession motor. The motor torques the big gyro around the gimbal axis which is the same as the ship's athwartship axis. Law of gyroscopics, and the big gyro begins to precess about the roll axis of the ship in such a way as to counteract the roll motion which the control gyro detected. With this control gyro method it is possible to overcome the difficulties encountered by using a brute force gyro for stabilization. These stabilizing gyros come big-some more than twice as tall as a man and weighing about 100 tons. Units this size are for an ocean liner, generally.