Aircraft, warships and merchant vessels have derived the most striking benefits from the application of the gyroscope to various instruments, but the gyroscopic principle is involved in many fields of engineering
MOST people have at one time owned a gyroscope, generally in their early youth. Many will be able to recall this interesting toy top in its circular frame that gives a “curious twist” when moved, spinning, in the hand. That curious twist is one of the manifestations of many peculiar properties of a rapidly spinning wheel, or “gyro”, to use its scientific name. The working principles of this mysterious device permit a large railway car to run balanced on a single rail. The gyro is the instrument that keeps the deadly torpedo running straight for its target, that controls the broadside salvos from the guns of a mighty warship. The gyroscope makes possible the pilotless aeroplane, the automatic helmsman on board ship, the True North gyro, the directional gyro and gyro-horizons used by the air pilot.
THE GYROCOMPASS is used in ships of every description. The Sperry merchant marine gyrocompass contains a single gyrowheel with a diameter of 10in. It weighs 55 lb. and is spun at 6,000 revolutions a minute by a self-contained induction motor.
Repeatedly in engineering practice the principles of gyroscopic action must be studied and applied, in the super-elevation of a railway track on curves, in the design and firing of artillery projectiles, in camera control for aerial photography and for many other purposes. Possibly the most spectacular application of the gyroscope is found in the gyroscopic stabilizer that resists the rolling of a great liner in a heavy sea. In this instance the instrument develops into a huge machine with a gyrowheel that weighs many tons.
A gyroscope comprises a gyrowheel spinning within a number of pivoted frames arranged in a manner similar to the gimbals of a ship’s compass. The number of frames may be varied within certain limits to give different degrees of “freedom of rotation”. When a gyrowheel is spinning it has one degree of freedom of rotation. If the wheel shaft be placed in a frame that allows it to be swung, weathercock fashion, to any point on the horizon, the gyroscope has two degrees of freedom of rotation. Three degrees of freedom are attained by providing another pivoted frame that enables the shaft to be up-ended in any direction.
A revolving gyrowheel that is thus free to move about in any direction will continue to spin with its axle “fixed in space”, no matter how the supporting frames are shifted. It was with a rapidly spinning wheel so mounted that the scientist Leon Foucault, in 1852, demonstrated the rotation of the earth. Because the instrument exhibited this property he named it the gyroscope.
This remarkable property of the gyroscope that resists any attempt to alter the direction of its spin-axis in space is governed by what is known in science as “the first law of gyrodynamics”. There is also a second law — that when pressure is exerted against
the inner frame of a gyroscope, the gyro axle will tilt, or precess, in a direction at right angles to that of the force applied.
Any revolving body exhibiting the above laws may be a gyroscope — a heavy wheel from a garden barrow, for example. Suppose a boy trundles the wheel away from him along a path; then by pushing down on the left-hand axle he can make the wheel turn to the left. A push down on the right-hand axle produces a right turn — the spin-axis (the axle) of the wheel has precessed.
Again, the driving wheels of a locomotive act as gyroscopes. On rounding a curve the spin-axes (the axles) precess and the driving wheels on the inside of the curve tend to lift from their rail. Here then is one reason for raising the level of the outer rail, or super-elevating the track.
Precession also plays an important part in the behaviour of various types of craft at sea and in the air. The wheels of a paddle steamer constitute a gyroscope and affect the behaviour of the vessel at sea. When the ship rolls to port (to the left) the wheel on that side dips deeper into the water than the starboard wheel and the ship’s bow is accordingly turned to starboard (to the right). At the same time the precession of the paddle shaft tends to turn the bow to port so that the first movement is counteracted. It is for this reason that paddle steamers roll less but pitch fore and aft more than screw steamers of corresponding size. A paddle steamer when turning, say, to port under the control of the rudder will, because of precession, heel to starboard. The action here is similar to that of a locomotive on a curve.
In multiple-screw steamers the use of two or four screws revolving in opposite directions cancels out the gyroscopic effect of the propellers and their shafts. The gyroscopic effect of a single propeller is, however, considerable. When the propeller shaft is horizontal the gyroscopic effect on rolling is nil, although rolling is influenced by the torque or “twist” of the screw. If seas are lifting the bow of the ship, it is turned to port or starboard, according to the direction of propeller rotation, by precession of the propeller shaft. The precession then tends to decrease the amount of the pitching.
It is in single-engined aircraft, however, that the greatest gyroscopic effect is obtained from a propeller, and due allowance has to be made to counteract the movements so produced. The phenomenon is particularly noticeable, for instance, in powerful single-engined fighting machines during aerobatics or “stunting”.
When, for example, the pilot pulls back the joystick and the elevator lifts the nose of the machine, precession by the propeller shaft will cause a turn to right or left according to the direction of engine rotation. Similarly a turn to left or right will tend to be accompanied by a corresponding rise or fall of the machine’s nose.
Yet another form of gyroscope is the shell or bullet from a rifled gun. Grooves in the gun barrel impart a spin to the projectile, and because the spin-axis is “rigid in space” a greater range is obtained than would be possible with a smooth bore.
Gyroscopic action has another important effect, in combination with air resistance, and that is “drift” to the right or left of the target. The drift may vary from a yard at a range of 1,000 yards to perhaps 2,000 feet in a distance of twenty miles, and allowance must be made for this drift in the setting of gun sights. Due allowance must be made for all gyroscopic tendencies existing in machines or appliances, but when used in the service of the engineer the gyroscope is of enormous value. A gyrowheel in its frames, applied by the engineer as a controlling device for machinery, is termed a gyrostat.
STABILIZING EQUIPMENT for the Italian liner Conte di Savoia weighs 660 tons. The photograph shows one of the three huge gyrowheels mounted inside a casing. The wheel has a diameter of 13 feet and spins at 800 revolutions a minute. A small electric gyrowheel, when precessed (or tilted) by a slight roll of the ship, starts up an electric motor that precesses the big gyro, which exerts an enormous pressure on the casing trunnions attached to the ship’s hull, thus correcting the roll.
An interesting application of the gyroscope is the steering of naval torpedoes. Briefly, the modern torpedo comprises a long cigar-shaped missile carrying a charge of explosive. It is shot from a tube on board a warship and is driven by propellers operated by compressed-air engines. Through the combined actions of a pendulum and a hydrostatic valve on horizontal rudders the torpedo can be set to run on an even keel at any predetermined depth below the surface of the water.
The function of the gyroscopic control is to operate vertical rudders and to keep the torpedo running dead on its target, perhaps 11,000 yards distant. To this end advantage is taken of the fact that once set in motion a gyrowheel, free to move in all directions, will continue to spin with its axis pointing in the same direction in space. There are many types of gyroscopic control mechanisms for torpedoes, and details of most of them are secret, but the following illustrates the application of the principles involved.
In the engine compartment of the torpedo is a gyrowheel mounted with its axle pointed fore-and-aft in frames that permit the necessary movement. The rim of the gyrowheel is provided with small blades or buckets and on these are directed jets of compressed air, causing the wheel to revolve at high speed.
On top of the outer frame of the gyrostat a disk and cam are mounted, with a stop. Bridging the stop are two tappets, mounted on a pivot, that make contact with the disk and cam respectively. The tappets are attached to an arm that operates a series of levers connected with the valve of a compressed-air steering motor. The motor is in turn linked to the vertical steering rudder of the torpedo.
The gyrostat, while the torpedo is still in the firing tube, is locked so that the wheel can revolve but cannot move about in the frames. With the torpedo firing tube pointing in the required direction the gyrowheel is then started up by an auxiliary geared turbine motor and runs at a speed of about 10,000 revolutions a minute — with its axis pointing in the required direction in space.
The torpedo is fired by a charge of cordite, and when it reaches the water the gyrostat, is unlocked, the auxiliary motor is automatically withdrawn from the gyrowheel and is replaced by the compressed-air jets. Thus, no matter how the torpedo is buffeted and turned aside by the waves, the gyrostat with its cam-disk and tappets must always follow the dictates of the wheel within and so bring the missile back to the course on which it was originally set.
An interesting feature of this control by a gyrowheel with its spin-axis “fixed in space” is that during the run of a torpedo taking say ten minutes, the earth has moved two and a half degrees. This movement of the earth must be allowed for and adjustments made to the gyrostat according to the latitude and direction of the torpedo attack.
There are also devices for controlling torpedoes so that they can be fired at an angle away from their target (as from a bow tube) and after running on a curved course, through a predetermined angle, will “straighten out” and run dead true to their target. In this type of control gear indirect firing is achieved by turning the cam-disk through the required angle. Then, when the torpedo has steered its predetermined curved course with the rudder hard over, the torpedo axis coincides with the ahead position of the cam-disk, the stop comes into place between the tappets and the torpedo runs on a straight course. There are also devices in use that enable a torpedo to run straight ahead for a predetermined distance and then set off on a curved course.
THE LAW OF PRECESSION is illustrated by this diagram. When a wheel is rotated-in the direction indicated at A and a downward pressure is exerted on the axle or spin-axis at B, the axle will turn, or precess, in a direction at right angles to that of the force applied. In this instance, the wheel and axle will turn in the direction indicated by C.
The control of a warship’s guns that are capable of dropping shells on a target many miles away is a matter of the utmost complexity. Not only must the gyroscopic “drift” of a shell be allowed for in training a gun, but also the speed and course of the ship, the target and the wind must all be computed. The rolling of the ship presents one of the greatest of the naval gunners’ problems, because this affects the elevation and thus the range of guns that are trained outboard for the firing of a broadside. The correct gun elevation is attained only at one point during a roll of the ship.
Suppose that a warship in a heavy sea has her guns loaded and trained ready for the firing of a broadside. The fire control officer has pressed the trigger of the firing mechanism. The great gun muzzles are slowly lifting as the ship rolls and the movement of the vessel brings the gun mountings slowly to their correct angle. Suddenly the angle is attained — the roll has also brought the guns to their correct elevation. Then, and only then, at the bidding of the gyroscopic control gear, is the electrical circuit completed and the salvo crashes out over miles of sea.
The modern marine gyrocompass is a wonderfully reliable instrument, but its action is subject to a number of natural errors due to variations in latitude, the velocity of the ship, rolling and other causes. These errors, are, however, all eliminated by mechanism so that only True North indications are shown by the repeater compass. The principle on which the ingenious correcting
gyrocompass works is, however, comparatively simple. A free gyrowheel spins with its axis “fixed in space”, but this is not the same, thing as a wheel whose axle will point along a meridian of longitude and so indicate the direction of the north and south poles of the earth.
A spinning gyroscope can be mounted in such a way that it will aline its axle with the axis of any angular motion to which it may be subjected. In the gyrocompass the earth’s rotation provides the angular motion which causes the axle of the gyrowheel to take up a position in line with the earth’s poles, thus indicating True North. The earth has been rotating for many million years, but the gyrocompass is the only man-made device which makes use of this rotation.
Modern marine gyrocompasses are operated electrically and any automatic steering apparatus that may be used for control is also electrical. Once set to a given course the “automatic helmsman” will continue to hold the ship to that course without any human interference whatever. Aircraft can be controlled in a similar way except that two gyroscopes are required, one horizontal, the other vertical, for the control of the respective rudder systems.
One of the most spectacular instances of gyroscopic control is the stabilizing, or anti-rolling, gear of the Italian liner Conte di Savoia. The equipment comprises three main gyrowheels driven electrically and mounted with their axles vertical inside casings. Each casing is provided with trunnions arranged athwartships so that the gyrowheel can be swung in a fore-and-aft direction. Each wheel has a diameter of 13 feet and spins at a speed of 800 revolutions a minute. Round the casing, close to one trunnion, is a toothed ring, with which is geared the armature of an electric motor that is provided with braking gear.
In addition to the large gyros, there are small electric gyrowheels, free to move, also arranged with their axles athwartships. When the ship rolls, if only to the extent of one degree, the control gyro precesses and closes an electrical circuit. The main gyro motor immediately starts up and precesses the great wheel, which exerts an enormous pressure on the casing trunnions and so corrects the roll.
THE AUTOMATIC PILOT will keep a plane on its predetermined course at any desired flight level. Two small gyroscopes, spinning at 13,000 revolutions a minute, control the ailerons and rudder. The unit, which weighs 60 lb., is housed behind the rectangular control panel in the centre of the instrument board. Relative movement between the gyros and their supporting members is converted into corrective movement of the aeroplane.