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An ingenious device in which pilots may practise or learn instrument flying on the ground

PRACTICE WITH THE COCKPIT OPEN is required by every pilot until he becomes acquainted with the sensitiveness of the controls. The Link trainer may be fitted with either a stick or wheel type of control column. Although the instruments appear normal and give normal indications, special schemes for their operation had to be devised before the desired effects were obtained.

FOR many years attempts have been made to produce a device to simulate on the ground the impressions of flying. With the development of the Link trainer success has been achieved. It is possible for a synthetic spin in the Link trainer to be so realistic that the pilot becomes giddy and airsick. But the Link trainer has a much more serious task to perform than the mere reproduction of sensations. Its object is to teach instrument flying of an elementary or advanced nature.

In appearance the Link trainer has the general lines of an aeroplane. There are short wings and a tail unit on a single-seater fuselage, the cockpit of which may be completely closed by a hood. The wings and the aeroplane shape of the trainer are not necessary to its operation, but they make it more clear to an onlooker what the pilot is doing. The wingspan is about 13 feet and the length of the fuselage approximately 8 feet. The overall height is a few inches over 7 feet when the hood is closed down.

With the trainer there is a control desk about 6 feet by 3 feet in area. This desk carries duplicates of some of the instruments in the cockpit, the necessary electrical and radio apparatus to imitate radio transmissions and a device known as the Crab.

The Crab is one of the most important items of the trainer. It is a three-wheeled device which can move in any direction and traces by means of an ink line the route followed by the pilot in the cockpit.

The inventor of the idea on which the Link trainer is based was Edwin C. Link, a flying instructor and an authority on the manufacture of automatic pianos. Originally the device, in a simple form, was produced for use in amusement parks. The bellows used were designed on similar lines to those found in automatic pianos. An interesting point concerning the amusement model was that pilots obtained more interest from them than members of the general public. Most people complained that there was insufficient thrill, although pilots considered that flying conditions were well simulated. When, however, instrument flying began to attain great importance in commercial flying, the Link trainer was developed by the inventor in conjunction with Casey Jones, an American racing pilot, and with other pilots and engineers.

The cockpit part of the trainer is mounted on a turntable which may be revolved continuously. Supporting the cockpit on the turntable are four bellows, which keep the device balanced on a universal joint. The front and back bellows are linked by valves, as also are the side bellows. The valves are controlled by movements of the joystick in the cockpit.

When the joystick is put forward it allows air to pass from the front bellows to the back bellows, and the nose tips downwards. Similarly the movement of the stick sideways causes the “aeroplane” to bank. This method of controlling movement gives a sensitivity and “feel” to the joystick which are similar to those experienced in an aeroplane in flight.

There is no inherent stability in the device, which means that the pilot must keep the “aeroplane” continually balanced. This makes the control more difficult than it would be in normal conditions. It is desirable that the pilot should learn to fly by instruments in the most difficult conditions because then he will be more at ease when carrying out manoeuvres in normal conditions. There is a tendency for all pilots when learning to fly by instruments to over-control — that is, to make the movements of the joystick and rudder too large. The great sensitivity of the Link trainer to movements of the controls helps toward the rapid eradication of this fault. Another tendency which has to be overcome is the placing of reliance on the human senses instead of being guided entirely by the instruments. Practice alone removes such a tendency, and the Link trainer enables the practice to be obtained in conditions in which the fault can have no harm if persisted in. The trainer can be made to bank, climb or dive at angles up to 50 degrees. The connexions between

the Crab on the control desk and cockpit are entirely electrical. Two of the wheels of the Crab are driven by electric motors; the third is used to draw an ink line. The speed of the Crab is such that it passes over the map or paper on the desk at a speed proportionate to that at which the “aeroplane” is assumed to be flying. It also follows with complete accuracy every turn made by the “pilot”.

The cockpit contains ten instruments. Stick or wheel control may be fitted and the pressure required on the controls can be altered to simulate that in any particular type of aircraft. The instruments are a magnetic compass, air speed indicator, turn and bank indicator, rate of climb indicator, directional gyro, artificial horizon, altimeter, revolution counter, clock and indicator for Lorenz blind-approach radio system. The altimeter, rate of climb meter, air speed indicator and Lorenz indicator readings are duplicated on the instruments on the control desk.

Of all the instruments in the cockpit, the directional gyro is the only one which is entirely normal. The others are interconnected in many ingenious ways to make them respond in the same way as the instruments on an aeroplane in flight. The way in which the instruments give the correct indications is almost uncanny.

If bank by itself is applied to an aeroplane it will cause the aircraft to begin to turn, because of the aileron drag.

IPILOT IN THE LINK TRAINERn the Link trainer the corresponding turn will be indicated as well as the bank if the control stick is held over to one side. If the throttle is closed below the necessary point to maintain level flight, the sink which occurs before the stall will be indicated on the rate of climb meter.

THE PILOT IN THE LINK TRAINER is in communication with the instructor at the desk by means of a microphone and headphones similar to those he would use when flying an air liner. Special electrical apparatus enables the instructor to transmit signals which sound identical with those heard by the pilot when flying.

If the stick is not put forward to regain speed the trainer will automatically go into a spin. So accurately do the instruments imitate flying conditions that experienced commercial pilots used to instrument flying completely forget that they are not in the air. One pilot in a spin is said to have broken a rudder bar in trying to bring the trainer out of a spin before his altitude became too low. The engine revolutions rise or fall, irrespective of the throttle setting, according to the fore and aft positions of the joystick. The altimeter is made to change by an automatic pressure control. When the trainer is kept in a climb for a period the pressure is reduced and a higher reading is shown on the altimeter.

Air bumps may be introduced by the instructor at the control desk, who may also simulate the effects of ice formation on the wings. About the only thing the Link trainer is unable to represent is the drop which occurs when an aeroplane flies into a descending column of air, which causes it to drop momentarily and to produce a sensation similar to a descending lift. The value of the trainer is largely dependent on the skill of the instructor, who has to act as the radio operator at ground stations and to control the beam at radio-approach-equipped aerodromes.

The value of the Link trainer has been recognized by aircraft operators and Governments in many parts of the world. Different forms of radio-approach systems may be practised. On the trainers used in Great Britain Lorenz indicators are fitted because this is the system mostly used in Europe. Practice is essential with all radio approach systems, but facilities for unlimited practice at airports are not available because of other traffic which has to be accommodated. There must also be considered the high cost of such practice and the danger of accident when an inexperienced pilot is learning to use the system.

All these drawbacks are overcome by the Link trainer. But the value of the trainer for practice in ordinary instrument flying must not be overlooked.

The “Experienced” Novice

One man who had never handled an aeroplane learnt to fly by instruments on a Link trainer. When he was taken up in an aeroplane he flew so well by the instruments that the pilot accompanying him found it difficult to believe that he was making his first flight in control of an aeroplane. The Link trainer is a great time and money saver in the periodic checking of the flying of pilots employed on air lines. The checks may be conducted at any time without the operating costs of an aircraft for the purpose.

With the aid of suitable maps and a knowledge of the radio stations in the vicinity it is possible for a pilot to practise over a course on which he has never flown. Thus a pilot going onto a fresh route may obtain a working knowledge of it before he leaves the ground.

Because of the sensitivity of the controls on the Link trainer and of the fact that there is no inherent stability in it, even experienced pilots find it necessary to do a little practice with the hood up before attempting to fly “blind” in the trainer. Many instances have occurred of recovery from spins after ground level has been reached. The Crab has been known to pass right off the map and to be in danger of leaving the desk altogether because of inaccurate flying. The Crab is fitted with high-speed or low-speed motors which drive it forward at a constant rate of speed, which on 1938 models is 160 miles an hour. Provision is to be made, however, to provide a range of speeds between 120 and 180 miles an hour. The high-speed motors are used for practising blind approaches and the low-speed motors for cross-country flying. All three wheels of the Crab are geared to a single automatic synchronous motor which turns in accordance with the trainer on its turntable, and so turns all three wheels of the Crab. The automatic synchronous motor is connected by wires to a similar motor fitted to the spindle of the trainer.

THE INSTRUCTOR’S DESK for the Link trainer

ON THE INSTRUCTOR’S DESK the course taken by the pilot in the trainer is plotted on a chart or map by a device which runs on three wheels. Two of the wheels are driven by electric motors, while the third inks out the course. The connexions to the device, which is known as the Crab, are entirely electrical, and the smallest movement made by the pilot to his “aeroplane” controls is accurately recorded on the instructor’s desk by the Crab.

When a pupil is practising blind approaches to an aerodrome, the instructor has on his desk a plan of the particular arrangement of the radio beacons with the aerodrome in the centre. The plan is prepared to scale in all ways, and a series of concentric circles are drawn around the aerodrome. Each circle may, for example, represent a distance of two miles — which is the distance travelled in one minute at a speed of 120 miles an hour. Compass degrees are marked on the circles surrounding the aerodrome, and the directions in relation to these of the approach paths are marked. The positions of marker beacons along the approach paths are also shown.

The instructor places the Crab on his chart at a position unknown to the pupil, but pointing in the same compass direction as the trainer. By noting the position of the Crab marker wheel in relation to the aerodrome and to the direction of the correct approach path, the instructor knows what radio signal to transmit and how strong to make it. It is now the pilot’s task to interpret properly the signal he is receiving, and to fly so as to arrive on the proper approach path. He has then to follow it down to the aerodrome, decreasing height at the right speed in relation to his position as indicated by the marker beacons. The method in which the Lorenz radio approach system works is described in the chapter “Blind Flying”.

During the pupil’s performance of his task the instructor has to concentrate on the position of the Crab on the chart so that he may vary the strength of the signals and their nature as the position of the aeroplane changes in relation to the radio transmitters.

For instance, the distinguishing signal of a marker beacon must be sent out exactly at the moment that the inking wheel passes over the position at which the marker beacon is indicated on the chart. The controls on the radio equipment at the side of the desk enable the instructor to make the necessary changes to simulate the conditions experienced by pilots when in the air.

By studying the course marked on the chart after the completion of his test, the pilot is enabled to obtain a clearer grasp of the problems of blind approach than is possible when practice is conducted in the air, and there is no record of the course followed by the pilot.

Artificial Weather Changes

The chart also prevents misunderstandings between pupil and instructor as to the course followed. By carefully watching the movements of the Crab and making suitable adjustments of the radio controls, the instructor can imitate unusual effects such as distorted approach beams.

Before a practice “flight” the pilot and instructor agree on the route, say a triangular one, to be “flown”. Then they decide the nature of the weather to be expected over the route and the force and direction of the wind. These are all details that would be known to any pilot setting off for a flight in a real aeroplane. The instructor is at liberty to introduce storms, rough weather or wind changes as the pilot in the trainer “flies” his course. Such conditions and changes should be of a nature to accord with the agreed weather conditions. This is one of the ways in which a good instructor will show to advantage.

The pilot climbs into the trainer, lowers the hood and “takes off”. The hum of the motor which drives the mechanism is such that the impression of flying “blind” becomes strong as soon as the pilot is concentrating on his task.

When the pilot has gained sufficient height he will inform his instructor that he is setting off on the course. He will keep to the course by the use of his directional gyro, which he will check from time to time against his magnetic compass. A magnet is arranged to cause the magnetic compass to swing in a realistic manner whenever the course is changed.

During the first leg of the course the instructor may let the pilot have an easy time. After the first turn the pilot may call up a radio station in the vicinity of his course to check his position. If the instructor likes he need not answer, thus indicating that the pilot’s call was not “heard”. At the same time he may introduce drift unknown to the pilot. This will throw the Crab off the correct course. Having received no reply to his first call, the pilot tries another station. This time he probably receives a reply from the instructor. By the time he has obtained his position he will have drifted some way off his course, because of the supposed change of wind. For a while he may now be busy returning to the route and working out a new course to allow for the change of wind. If, while he is doing this the instructor introduces bumpy conditions, the pilot will have to watch his flying carefully and be particularly busy.

After the pilot has made the second turn he is called up by the instructor taking the part of the radio operator at the pilot’s destination, to inform the pilot that there is a bad storm area right in his path which must be avoided. This requires a carefully worked out course, and another change of wind by the instructor puts the pilot right off his course when he is under the impression that he is nearing the aerodrome at the end of his journey. At the same time he is informed he will have to come in on the blind-approach system.

As he is off his course he will now have a complicated task to find the beam of the approach system. Once truly on the beam the pilot will have a sense of relief. If the pilot is an old hand at instrument flying in air liners, he will expect his co-pilot to take over any moment as the ground becomes visible through the haze. Probably a feeling of fatigue will come over the pilot as he realizes he is in the Link trainer and has been concentrating over a synthetic trip in bad conditions.

Simulating Night Flying

The latest development of the Link trainer is a night room which is being produced for training in night flying. The trainer will be arranged in a dark room and will have windows so that the pilot may look out. Round the walls and ceiling of the room will be stars, arranged in their proper order. Varying weather conditions such as fog, snow, squalls and electrical storms will be available to be introduced at will.

That the Link trainer is an extremely valuable invention is proved by the fact that it is generally the experienced pilots who are most enthusiastic about its value for practice purposes.

As an example of the saving which the trainer provides in time spent in tuition in the air, the average times for a course of instrument flying may be given. Five hours are first spent in the

trainer, followed by two hours practising in the air what has been learnt. By ordinary methods the whole seven hours would have to be spent in the air to reach an equivalent standard. To this would have to be added the time taken in taxying the aircraft and in climbing to a suitable height before each lesson. The Link trainer is ready for use as soon as its driving motor is started.

The first hour of instruction is spent by the pupil in familiarizing himself with the controls and instruments. During this hour the hood is open. All instruction thereafter is conducted with the hood closed. The first half-hour lesson with the hood closed is occupied by practice turns to gain smoothness. Then follows an hour practising turns to any given compass bearing. After this, five half-hour periods are spent in the following ways: a check flight with the instructor correcting all faults, fast and slow turns, turns with and without compass under bumpy weather conditions, spins and recovery from spins and, finally, a check flight covering all previous instruction. At this stage the pupil is ready to fly by instruments in the air, and the two hours of instruction in an aeroplane follow. Further practice can always be carried out in the trainer.

THERE IS NO INHERENT STABILITY in the Link trainer. Once the cockpit cover is lowered, the pilot has to be guided entirely by the instruments in keeping the “ aeroplane ” level. Should the pilot allow the nose to point upwards too steeply, the instruments will indicate a stall and, unless the nose is lowered, the trainer will begin to revolve in a manner that feels to the occupant just like a spin.

[From Part 24, published 16 August 1938]

You can read more on “Blind Flying”, “How Aircraft Instruments Work” and “Learning to Fly” on this website.

The Link Trainer