Proving new types of aircraft and new models of existing types
THE FAIREY BATTLE ON TEST. The test pilot, after exhaustive trials on the ground, takes the aircraft into the air and performs various manoeuvres before reporting on its characteristics and behaviour. The Fairey Battle is a low-wing cantilever monoplane day bomber with a span of 54 feet and a length of 52 ft 2-in. Its speed at 15,000 feet is 257 miles an hour. The engine is a Roll-Royce Merlin.
THE most important part of the test pilot’s job is the testing of prototypes, or first machines of new types of aircraft. He has not only to assure himself that the aeroplane is right in every particular, but he has to be completely satisfied in his own judgment. In other words, he must know that the new type is not defective in any way.
In the initial stages of testing a prototype aircraft, and also in some of the tests which are called for by the Director of Technical Development at the Air Ministry, the test pilot’s opinions have to be relied upon. Test pilots, in the same way as everybody else, vary in their opinions, and sometimes there are conflicting views on certain subjects. Yet the test pilots of leading firms, who have had a considerable amount of experience, would have no hesitation in unanimously rejecting an aeroplane that was bad or appeared vicious in its habits.
The test pilot’s job begins long before the aircraft is flown. It begins in the initial stages of the design, as it is his duty to give advice about many things, such as the pilot’s view, gunner’s view, bomb aimer’s view, cockpit layout and all the small details which go to make an aeroplane first-class from the pilot’s standpoint.
This has to be done first of all in the drawing office. Then a full-size “mock-up” of the aeroplane is built in wood and covered with brown paper. This mock-up is virtually a true reproduction of what the aeroplane will be when it is built. All equipment is placed in it so that the various people concerned can check clearances and the suitability of the equipment, and make sure that the operating crew have enough room in which to carry out their various duties.
After the mock-up has been finally approved by the manufacturers, it must further be approved by the officials appointed by the Director of Technical Development. Then the building of the aeroplane may proceed.
Before the aeroplane leaves the experimental shop in the factory it is fully assembled, and it has to be dismantled for transport to the aerodrome, where it is assembled again. Reassembly at the aerodrome gives the test pilot who is going to put the type through its trials an opportunity of seeing that it is reassembled to his satisfaction.
This work of recent years has become considerably more complicated, and therefore takes more time, with such devices as retractable undercarriages, retractable bomb carriages and flaps, all of which have to undergo severe tests before flight. This complication enables the pilot to operate these units himself under test, so that he becomes familiar with the aircraft from the first.
The usual way of testing these components when finally reassembled at the aerodrome is to trestle the aircraft up in flying position, or in any other position that may be required in the schedule of tests, and then by means of an electric motor to drive the hydraulic pump which operates the components. Certain types of aeroplane, such as the Fairey Battle, have a hydraulic pump to operate the components. The undercarriages, flaps and bomb carriages of the Battle are all operated by the Lockheed hydraulic unit.
The electric motor is arranged to drive the hydraulic pump at a reasonable speed. The electric motor is kept running, during which time the chassis, flaps and bomb carriages have to do a certain number of retractions. Chassis retractions must be done not only at the ordinary static load, but also at increased loads, which represent such factors as wind pressure and centrifugal force. The method of reproducing these load conditions is generally to hang a predetermined weight on the wheels.
The aeroplane having passed these tests is ready to be taken outside for an engine run. This is carried out generally by a responsible representative of the engine makers. The aeroplane is wheeled out of its hangar. The machine is generally surrounded by fire extinguishers, as no chances are taken, because the aeroplane has cost a considerable sum of money, and on it may depend the earnings of the manufacturers for some years to come.
For the first time the engine is run up without any of the cowling on. This is to observe anything undue that may be happening to the engine bearers, or in the engine installation, such as leaking oil pipes or coolant pipes. Generally it is found that the engine has come from the makers in perfect condition, and perhaps only trifling adjustments may be necessary. Any such difficulty is soon remedied, and the machine is then cowled up ready for its first taxying and flight trials.
The accuracy of all instruments is of paramount importance, as one faulty instrument alone can ruin several hours of work. Accuracy is so important that it is the general practice for test pilots to have their own special instruments. They have a cupboard full of the various instruments which they use during test flying. The instruments must be frequently calibrated to keep a constant check on them. Otherwise it may often be found that, say, an air speed indicator is working normally one day and abnormally the next - an irregularity that would not be noticeable except on a calibrator.
The chief instruments required are an airspeed indicator with a reasonably open calibrated scale, a revolution counter, also clearly marked, a boost gauge, an altimeter, a stopwatch and an outside air temperature thermometer. Another necessary item of equipment, although it is not an instrument at all, is a knee pad for all the notes which must be taken during flight. Another small but important point is the necessity for carrying at least two pencils in a readily accessible position; for if the test pilot is in the middle of a timed climb, and he drops his only pencil, the job has to be begun again.
There are numerous other instruments which, although they affect the performance of the aircraft, do not bear upon it directly. These include such fittings as oil pressure gauges, oil scavenge pressure gauges, oil temperature gauges for the oil going into the engine, for the oil coming out of the engine and for the oil after it has passed the cooler.
In yet another category there are such instruments as the gauge for the hydraulic pressure which operates the undercarriage, and so forth. There are also a flap indicator gauge, various chassis warning indicators, fore and aft and directional trim indicators and - of great importance - the mechanism for operating the variable pitch airscrew.
Although the first flight is of great importance, there is a danger in presuming too much from it. The first impressions of an aeroplane may not be borne out by further flights. Moreover, if many things are criticized and altered before the next flight, it will often be found that many of the alterations have been useless.
On no account should more than one alteration be carried out between each flight, where such alteration would have any bearing on another. For example, if two control surfaces appear to need slight attention, it is better to correct one at a time; for often it will be found that when one control surface has been corrected the other will automatically come into line with it and need not be modified.
It is of great importance that the weather and any local conditions, such as the aerodrome surface, should be good. It is galling to the designer and builders of the aircraft to have to wait two or three days for fair weather, but this cannot be helped, and on no account should the first flight be undertaken except in favourable weather conditions.
Before the test pilot starts the engine for the first handling trial, it is advisable for him to check over the flying controls and all other mechanisms which he will operate, even though this may have been done by him on previous occasions.
This having been done, the engine is warmed up carefully and run up even more carefully. During this time all the relevant engine figures are noted down. These include oil pressures and temperatures, cylinder temperatures if the engine is air-cooled, and liquid temperatures if it is liquid-cooled. These particulars are all taken at regular intervals during the warming up period; and during the run up to maximum permissible boost, the number of revolutions a minute is noted, as are the boost pressure and all pressures and temperatures.
While the engine is being warmed up the flap mechanism may be tested, but not the undercarriage. After the run up the engine should be kept running fast enough to test the variable pitch airscrew.
Having satisfied himself that everything is working correctly, the pilot may start taxying trials.
First of all the machine is taxied slowly about the aerodrome. This is for several reasons. One is to accustom the test pilot to the attitude and general characteristics of the aircraft on the ground; another is to find out if the undercarriage and tail wheel are doing their jobs properly; and yet another is to bed in the wheel brakes. This last is done by running the aeroplane with the wheel brakes slightly on over short periods, care being taken not to overheat the brakes.
STRAPPED TO THE KNEE of a test pilot about to carry out test flights is a knee pad, on which he can make the necessary notes during the flight and record the performance of the machine. On his other knee is strapped a stop-watch.
The pilot then taxies the aeroplane to the lee side of the aerodrome, taking all the available room possible. He sets the tail trim and any other trim, and also sets the flaps to the position which he thinks will be most suitable for taking off. The pilot then proceeds to carry out what are called “straights”. These straights are short runs into the wind over the aerodrome, and each one is generally slightly faster than the last. The object of these straights is gradually to work the aeroplane up to its taking-off speed. When this has been reached the throttle is closed and the aeroplane is brought to rest.
During each of these straights particular attention is paid to the working of the chassis and the wheel brakes. Further, the general stability of the aircraft on the ground is investigated and, should there be any tendency to nose heaviness, this is noted down and due care is taken when using the brakes. With modern types of aircraft the chassis is generally so far in front of the centre of gravity that trouble of this nature is scarcely ever experienced.
It is now time to find out if the aeroplane will fly. This again is set about carefully, and, at first, if space permits, more straights are carried out. On these occasions, however, the aeroplane is allowed to leave the ground before the throttle is closed, and the machine is brought to rest within the limits of the aerodrome.
During these short hops all the controls are tested to make sure that there is adequate control in all planes. At such slow speeds it is hard to determine this, but with practice and experience it can be done. Another point which must be noted is the maximum air-speed recorded.
Once it has been shown that the aeroplane is stable on the ground and that all the flying controls work, it can then be taxied to the edge of the aerodrome and taken off. It is advisable not to take it off at full throttle, but at reasonable power only. This will probably be at half to three-quarters throttle, as these first flights are always carried out at a much reduced all-up load.
After it has left the ground the aeroplane should be climbed at a speed not more than that attained during the short hops, provided that this was a reasonable speed. As it has been discovered that the aeroplane will fly satisfactorily at this speed, it should not be exceeded until a safe height has been reached, say, 4,000 or 5,000 feet.
Once at this height the speed of the aircraft can be increased gradually, and when a height of 8,000 feet or so has been reached the business of finding out the characteristics and faults of the aeroplane can start in earnest.
Generally such defects as overbalanced controls are apt to be features of first flights, and this is one of the reasons why the aeroplane should not be climbed from the ground at a speed faster than that reached on the short hops. Should the controls have shown themselves to be overbalanced on the short hops, the first flight would have been postponed until the necessary modifications had been carried out. On the other hand, if the speed is increased slowly, as suggested, an overbalanced control will be discovered gradually; in other words, control will not be snatched straight out of the pilot’s hand.
Other points to be carefully noted are any structure movements or any vibration periods, with reference to the attitude of the aeroplane and the range of revolutions at which these vibrations occur. Satisfactory working of flaps, undercarriage, variable pitch airscrew, instruments and so forth should be tested and, if necessary criticized, as also should be the pilot’s view and the general layout of the aircraft’s cockpit.
Instrument Board of the Fairey Battle
1. Automatic Pilot Controls; 2. Automatic Pilot Pressure Gauge; 3. Brake Pressure Gauge; 4. Air Speed Indicator; 5. Chassis Position Indicator; 6. Revolution Counter; 7. Boost Pressure Gauge;
11. Main Oil Pressure; 12. Oil Inlet Temperature; 13. Oil Outlet Temperature; 14. Fuel Pressure Gauge; 15. Outside Air Temperature Gauge; 16. Flap Position Indicator; 17. Hydraulic Pressure Gauge; 18. Oil Temperature at Oil Cooler Outlet.
If possible, it is always a good thing to have an approximate idea of the speed of the aeroplane. This, however, can be only a bare indication, as before proper speeds are measured, or before the machine is put up to its full load, it will probably have undergone several modifications. If an approximate speed is obtained it gives an indication of the suitability of the airscrew setting.
After the first flight all the various points are discussed with the chief engineer and chief designer, and a programme of work is laid out for carrying out modifications where necessary. Modifications may be required to rectify overbalanced, heavy, ineffective or unstable controls. Alterations to the pilot’s or the gunner’s windscreen may be needed. The sliding hood, as fitted to some machines, may not be satisfactory and may be found to be moving dangerously in flight. All these points, and possibly many others, have to be considered carefully and must be put right.
It is always desirable to get the machine as nearly perfect as possible before performance trials are carried out. Flying controls must be well harmonized, the aeroplane must be positively, or at any rate neutrally, stable in all planes, and all the other controls must work freely and correctly.
The machine goes through handling trials at all its service loads. These include its full load; its load at the forward limit of centre of gravity, which is arrived at generally through the expenditure of fuel and ammunition and when the crew is at its most forward position; and its load at the after limit of the centre of gravity, which is the furthermost point aft to which the centre of gravity will move in service conditions.
At these varying loads the trim of the aircraft has to be adjusted. On modern aircraft this is done by the adjustment of strips attached to the elevators and rudder.
Handling at the Stall
It is most essential for the elevator strips to be correctly set. They must be set to allow the aircraft to be trimmed to fly hands off with the centre of gravity at any position and at any speed from about 10 miles an hour above the stalling speed up to maximum speed, with the engine either on or off. Too big a range to these strips must not be allowed, and stops should be fitted, as it must not be possible to make the aeroplane unduly tail heavy. If a pilot takes it off in this state, before he has had time to readjust the trim, the machine may have climbed up to the stall and dropped to the ground.
Another test during these handling trials must be a full-throttle climb at full load to check the temperatures of the liquid (or, in an air-cooled engine, of the cylinders) and the oil temperatures. The limiting temperatures are laid down by the engine makers, and must not be exceeded. If it is found during a full-throttle climb, or at full-throttle speed, that these temperatures are becoming excessive, then modifications must be carried out to the various cooling systems.
Not only do all these flight trials have to be carried out, but the electrical installations, bomb installations and the like all have to be tested. Details are generally submitted to the staff of the Director of Technical Development.
In addition to these trials, the aircraft has to be tested for handling at the stall. This is possibly one of the most important tests, for if the aeroplane has bad characteristics at the stall it will be rejected immediately. These tests are rather lengthy and take anything up to as much as four hours’ flying time to complete. The aeroplane has to be flown at slow speeds and its characteristics in these conditions have to be discovered. Further, the aeroplane must be stalled in various attitudes and with the controls set in various positions, so that again the characteristics can be noted.
During these trials the aircraft must not show any vicious tendencies such as rapidly dropping a wing, or, even worse still, flicking straight into a spin. If the machine does this it must be put right before it leaves the makers.
UP TO ITS FULL LOAD, this Fairey light bomber is waiting for the test pilot to carry out a climb at full throttle. He will observe also the machine's behaviour at various speeds in level flight under full load. Tests are made with various loads, the trim being adjusted accordingly.
Included in these tests are spinning trials. These trials apply in different ways to different aircraft. Single-seaters, two-seater fighters and trainers, for instance, have to complete eight turns of a spin in either direction, with varying tail settings, and at the various loads and positions of centre of gravity. Other aircraft may have to complete only two turns, according to their size and the purpose for which they are required. The bigger aircraft, such as light bombers, flying boats and the like, do not have to carry out spinning trials.
Spinning trials have to be approached cautiously, as there have been frequent instances of prototypes remaining in spins. Generally the worst type is the flat spin, in which the nose goes up high and, as the term expresses, the aeroplane spins flat, more or less round its vertical axis. Often, once an aeroplane has got into this attitude, it cannot be brought out by use of the controls. Such things as ballast can often be jettisoned to move the centre of gravity farther forward, a measure which sometimes helps to bring the aircraft out of the spin.
Other anti-spin devices include the tail parachute, which is a small parachute that can be released by the pilot should the aeroplane stick in a stable spin. This parachute merely jerks the aeroplane and alters either the speed of the spin or the attitude of the aeroplane, thereby allowing it to come out of the spin.
Other tests called for are diving tests, whose nature depends on the type of aeroplane. Such aircraft as single-seaters, two-seater fighters and trainers are dived to their terminal velocity speed or until such time as maximum permissible engine speed is reached with the throttle one-third open. Other types will have limiting speeds, such as the maximum engine speed, or it may be that the diving speed is allowed only up to one-third in some instances, or in others one-half over the maximum level speed of the aircraft concerned. Here, again, these dives have to be carried out at all the varying loads and positions of centre of gravity.
Instability and Flutter
During these dives the main structure has to be watched carefully for any undue movement, and it is generally advisable to approach the diving tests cautiously, gradually working up to the maximum permissible speed.
All controls, too, have to be tested in the dives for stability. If instability is present it is generally shown by the controls snatching, and can be most unpleasant. Also flutter may develop, which must be stopped at once before it builds up and breaks the aeroplane. There are certain types of flutter which, once started, cannot be stopped, and may cause the loss of the aircraft.
A good many of these tests have to be carried out several times, not only for the various loads and positions of centre of gravity, but also with the flaps up and chassis up, and flaps down and chassis down, at these different loads and positions of centre of gravity. This multiplies the work considerably. Further flights have to be carried out to ascertain the final rate of climb of the aeroplane, to discover speeds at height and to determine the general suitability of the various cooling systems.
The testing of an aeroplane takes a long time, for the modifications necessary may, and frequently do, involve many weeks of work.
The testing of the first production type of an aircraft is similar to the testing of the prototype, but modifications have probably been carried out between the time the prototype finished flying and the time when the first production type was built. Although such modifications may cause trouble, it should not take long to finish testing this aircraft. The controls and other fittings may need some attention, for although they are supposed to be made the same as in the prototype, there are often slight variations.
The first production machine goes through the same schedule of Air Ministry tests as the prototype: in other words, diving trials, handling at the stall, spinning trials, full-load climbs and speeds, measured take-off, landing-tests and so forth.
Finally, the test pilot’s work includes ordinary production testing. For this the usual allowance is half an hour for each aircraft. During this time the trim of the aeroplane is checked, as are all the controls, instruments, seat ratchet gear, petrol cock and the like. For each machine a schedule of tests is carefully worked out.
A FAIREY SWORDFISH taking off for its half-hour acceptance trials. The Fairey Swordfish is available either as a seaplane (as shown above) or as a landplane. It is a torpedo spotter reconnaissance biplane, with a 655-690 horse-power Bristol Pegasus III M radial air-cooled engine. The span is 45 ft 6-in, the width folded 17 ft 3-in, and the length (as seaplane) 40 ft 11-in.