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A system whereby aeroplanes have remained in the air for weeks without landing


AN AEROPLANE RECEIVING PETROL while in flight. The machines are those of the American Hunter brothers, who set up an endurance record of 553 hours 40 minutes over Chicago in 1930. Their record was beaten by nearly 100 hours in 1935 by two other brothers, A. and F. Key, who remained in the air for more than twenty seven days. This photograph was taken over Los Angeles during experimental tests.

OF all the problems connected with air transport on a large scale, there are few as fascinating as that of refuelling a machine in the air. The idea is not new; refuelling was tried experimentally in Great Britain during the later stages of the war of 1914-18; but no progress was made for some years, chiefly because the attendant disadvantages greatly outweighed the possible advantages to the aircraft of the period. Now, however, with modern aeroplanes and apparatus, the possibilities of refuelling during flight are becoming increasingly more interesting.

In recent years aviation has entered a new phase of its history. In the same way as the large, long-distance motor coach has been evolved from the smaller vehicle, so has the large passenger aeroplane been developed from the light machine. Already, with twenty- or thirty-seater aeroplanes flying to regular schedule, there are plans for flying boats which will carry 100 or more passengers. Such a great advance has uncovered an entirely new set of problems; the greatest of these is the problem of undesirable weight at the take-off. Once in the air, ail aeroplane is capable of carrying a heavy load, but it is another matter to raise that load from the ground.

Airframes can be built to almost any reasonable size, and engines can be designed to produce enormous power; but much power is wasted in the engines, for designers have to allow for a big margin of power necessary to give the initial lift. Thus it is easy to see where refuelling can help. If an aeroplane can take off with a full payload and then, while in the air, can take on board the extra fuel load, all the un-necessary engine power will be saved.

About 1928 experiments were resumed in Great Britain by the Air Ministry, and certain Royal Air Force machines were equipped with comparatively simple apparatus for the purpose of testing the possibilities of refuelling during flight. Two machines were used, one as a “tanker” and the other fitted with a specially designed auxiliary tank for receiving supplies of petrol. The experiments were of a hazardous nature in their early form, and they depended upon the ability of one man, standing up in an open cockpit, to catch the end of a hosepipe lowered from the machine above. The two greatest dangers were that the man might be struck by the hose, or that the hose might come in contact with the propeller. Yet, in spite of these risks, contacts were successfully established between the two machines and fuel was passed down the pipe.

Important data were collected in these trials and an immediate improvement was effected by lowering a weighted line. This was allowed to dangle within reach of the man standing in the cockpit; when he had caught the line he was able to haul in the heavier hosepipe from above. With a light line, however, and the force of the wind created by speed, it was still necessary to use a fairly heavy weight, and this was dangerous. Experiments were then tried with something less deadly - a soft container filled with water, which would burst without damage if a gust, or bump, swung it at the waiting man below.

This method was fairly successful and a decided advance on previous methods; but it soon became obsolete when it was found that the line could be caught against the leading edge of the wing, and “steered” round to the fuselage. Once preliminary contact had been established, it was a simple matter for the man in charge on board the tanker to pay out the hose and to turn on the fuel tap; but there were still serious difficulties to be overcome. For the sake of safety the hose had to be long; this meant great wind pressure.

A sudden gust could easily tear the pipe away unless it was securely fixed; if it was fixed too securely a breakage might occur, causing damage to one or both aircraft. Thus it became obvious that certain important points had to be considered before any real advances could be made. First, the method of contact had to be simplified and made as automatic as possible; secondly, the most useful length of fuel hose had to be determined; and finally, the method of attaching the hose to the second aircraft had to be quick, secure and yet safe in emergencies. Another factor which had to be considered, once all these points had been settled, was the control of fuel at either end. In the event of a sudden parting of the hose from the aircraft being refuelled, it was undesirable to have a gush of petrol, particularly as the “snatch” which caused the parting might fling the hose end forward towards the engine. It was found in practice that a hose about 150 feet long was best for general purposes; it followed that a large quantity of petrol would always be suspended between the two aircraft.

Contacts at 90 Miles an Hour

Because of the great wind drag, the hose assumed the form of a loop, and the head of petrol was not 150 feet; yet, even a head of 50 feet created plenty of pressure at the nozzle.

As the experiments progressed, it was found that a valve could be built into the nozzle of the hose, which would close immediately a parting occurred. A second valve at the other end of the hose was designed so that it could be closed almost instantaneously. Next came the development of an ingenious clip; the clip enabled the nozzle of the hose to be secured at the instant of contact, but it would part immediately under a sudden strain. Meanwhile, the problem of refuelling an aeroplane during flight had captured the interest of several people in the United States, and in 1923 was made the first of a series of spectacular refuelling flights. Two American-built De Havilland machines were used for the first test. One machine, a D.H.4, was remodelled at the Rockwell air station, San Diego, California, and fitted with an extra petrol tank behind the rear cockpit; this tank had a large opening capable of taking the nozzle of a hose. The other machine, D.H.4B1, was equipped with a 50-feet length of hose, embodying a quick-acting shut-off valve and a special tank system from which petrol supplies could be drawn.

On the morning of June 27, 1923, Lieutenants Lowell Smith and Paul Richter took off in the D.H.4 and flew round Rockwell Field while the tanker, piloted by Lieutenants Virgil Hine and F. W. Seifert, manoeuvred into position above them. Two experimental contacts were made, and fuel was passed between the two machines on both occasions - 25 gallons at the first and 50 gallons at the second attempt. The D.H.4 remained in the air for 6 hours 38 minutes.

A LIGHT GUIDE LINE is first passed between the machines which are to carry out refuelling in the air

A LIGHT GUIDE LINE is first passed between the machines which are to carry out refuelling in the air. When the operator in the lower aircraft has caught hold of the guide line, the nozzle of the petrol feed-pipe may be lowered at the other end of the line. This illustration shows one of Sir Alan Cobham’s experiments being carried out near Ford Aerodrome, Sussex.

This, however, was but a preliminary, for the pilots wished to demonstrate that a machine could be kept in the air for a long period by means of refuelling. They wished, in addition, to show that this could be done in all weather conditions. They made minor adjustments to the equipment on both machines, and at 4.44 a.m. on August 27 they took off again, flying round and round on a triangular course of 31 miles.

During the first twenty laps two contacts were made, at an air speed of some 90 miles an hour. The tanker machine landed and took in more fuel, while the D.H.4 continued on its course. In all, some 294 gallons of petrol were transferred during the day, and 15 gallons of oil were lowered in containers.

After a final refuelling at nightfall the tanker was refilled and prepared for the morning. At 3.0 a.m. on August 28 a message was dropped from the D.H.4 saying that more fuel was required, and within an hour several attempts were made to establish contact. Thick fog rolled over the field, however, and the D.H.4 was eventually forced to land, the reserve tank having failed to work. The flight had lasted 37 hours minutes; during that time 309 gallons of petrol and 15 gallons of oil had been transferred.

This flight was of special interest, not merely because it created a world’s record for endurance, but because it marked the beginning of refuelling experiments in America. No more important refuelling tests were made in that country for nearly six years. In 1929, however, there occurred a number of refuelling endurance flights, some of which were remarkable. Though most of them were made for the purpose of establishing spectacular records, they formed an important part of the whole story. The main reason for the sudden revival of interest in refuelling in America was the establishment of a surprising endurance record in Europe. Between July 5 and July 7, 1928, Johann Risticz and Wilhelm Zimmermann had stayed aloft over Dessau, Germany, for 65 hours 25 minutes without refuelling.

In the early part of 1929 announcements were made by a number of pilots who were determined to beat the endurance record by a wide margin. By the end of the year about forty attempts had been made. The first notable flight began on January 1, 1929, when the Question Mark, a Fokker monoplane fitted with three Wright Whirlwind engines, took off and began to cruise up and down the coast of south California. The flight was elaborately planned by the U.S. Army Air Corps, and the Question Mark was manned by a crew of five. These were Major Carl Spatz (commander of the flight), Captain Ira Eaker, Lieutenant Elwood R. Quesada, Lieutenant H. A. Halverson and Staff-Sergeant Roy Hooe, the mechanic.

Airship Record Beaten

The Question Mark had 100 gallons of petrol on board at the take-off, and it was intended to raise the record for time in the air to at least 100 hours. A Douglas transport aeroplane was chosen for the refuelling tanker, and when the first signal had been received for fuel, Lieutenant Moon took off and flew overhead. The hose was lowered, according to prearranged signals and, when Major Splatz had secured it, another signal was given. Petrol flowed down the pipe, and in about one and a half minutes 100 gallons had been taken on board. The manoeuvre was repeated again and again with complete success as the hours passed. Oil, hot food and newspapers were passed down in containers on the end of lines.

The most interesting feature of this refuelling flight was the fact that refuelling was done by day and by night; night refuelling was extremely difficult. The European aeroplane record for endurance was soon surpassed, but the big Fokker flew on. Another endurance record remained to be beaten; this was the record set up by the German airship L 72, surrendered to France and renamed Dixmude. In 1923 the Dixmude had made a sustained flight of 118 hours 41 minutes in a cruise over France and north Africa.

SIR ALAN COBHAM inspecting the nozzle of the petrol pipe before taking off for a test of refuelling

SIR ALAN COBHAM (left) inspecting the nozzle of the petrol pipe before taking off for a test of refuelling during flight. In early experiments, before the guide line was introduced, the petrol tubing itself with the nozzle on the end was lowered. The operator in the aeroplane to be refuelled had to catch the nozzle as it swayed in the air. This was found to be a hazardous procedure, because the weight of the nozzle involved a risk of damage to the aeroplane or of injury to the operator.

It was not until the seventh day of the flight that the engines began to give trouble. Then one engine failed; so the machine was headed for the Los Angeles Municipal Airport and landed. No fewer than forty-six refuelling contacts were made during this flight, at all altitudes, in all types of weather, and at a comprehensive range of speeds. Over 5,660 gallons of petrol and 245 gallons of oil had been transferred from the tanker to the Question Mark. The total flying time was 150 hours 14 minutes - over six and a quarter days - and the mileage covered was approximately 11,000.

On May 19, 1929, two Texas pilots, J. Kelly and R. L. Robbins, took off from Fort Worth, Texas, in an attempt to beat the record set up by the Question Mark. Their machine was a single-engined Ryan Brougham monoplane, with a Wright Whirlwind engine; the petrol capacity was 250 gallons - enough to keep them in the air for about twenty-four hours. They refuelled three times a day, with the exception of one day, when their reserve was big enough to demand only one contact, and each manoeuvre was performed without a hitch. In all, seventeen contacts were made, some in violent rain and wind, and the machine eventually landed after having been in the air for 172 hours 32 minutes - a flight of about 12,900 miles.

By this time there was considerable interest in the possibilities of refuelling, and during the next month there were several flights of considerable duration. Two pilots, Bryon K. Newcomb and R. L. Mitchell, took off on June 28. They flew a Stinson Detroiter, with a Wright Whirlwind engine, and remained in the air for 174 hours.

While Newcomb and Mitchell were still flying, a second machine took off, piloted by L. W. Mendell and R. B. Reinheart. The second machine remained aloft more than ten days. For this attempt a Curtiss Carrier Pigeon, with a Liberty engine, was fitted up as a tanker, and 4,085 gallons of petrol were transferred. This record flight covered about 19,760 miles in 246 hours 43 minutes.

In the Air for 17½ Days

Though these flights, and many others, aimed at the spectacular, they supplied the engine manufacturers with valuable data; they also proved beyond any doubt that refuelling in the air could be developed into something really important. In most instances the refuelling equipment consisted of little more than a hose and a large-mouthed tank; but the fact that so much could be done, so regularly, and with such simple apparatus, was significant in itself.

Two other endurance flights deserve mention, one because of the large number of successful refuelling contacts involved, and the other because of its possible bearing upon refuelling as applied to everyday civil aviation.

On July 6, 1929, Dale Jackson and Forest O’Brine, two pilots of the Curtiss-Robertson Manufacturing Co., took off at St. Louis, Missouri, to make an endurance test flight. Their machine, the St. Louis Robin, was equipped with special tanks for receiving extra fuel supplies from above. The refuelling tanker was fitted with a rubber hose 35 feet long and 2 inches in diameter.

The pilots signalled that they intended to surpass the recently raised record; this they did by flying for 300 hours. Not satisfied, however, they flew on for another 100 hours, and then decided to try for a 500-hours record. The president of the company sponsoring the flight signalled for them to come down, and they landed after having been in the air for 420 hours 21 minutes. During this flight of about seventeen and a half days’ duration, the pilots had flown some 25,500 miles. They had taken on board 3,500 gallons of petrol from the tanker, and forty-eight contacts had been made without a hitch.

PETROL-PIPE DRUM fitted in the underside of the fuselage of an Armstrong Whitworth 23

PETROL-PIPE DRUM fitted in the underside of the fuselage of an Armstrong Whitworth 23 which was used as the tanker in refuelling experiments made over Southampton Water during the early part of 1938. The aircraft which was refuelled was the Empire flying boat Cambria, equipped with the large petrol tanks used for the experimental transatlantic flights. The tests were carried out under the supervision of the Air Ministry and were reported to be most satisfactory.

It remained for Lieutenants N. B. Mamaer and A. Walker to apply the results of all this experience to practical air line operation. Hitherto all refuelling flights had been aimed at endurance records, no matter how carefully they had been disguised; all the flying had been done above aerodromes, generally in a small circuit. It was argued, with reason, that totally different conditions would prevail on a long distance, non-stop flight. The Texas Government decided to sponsor such a flight to see exactly what refuelling might effect if properly organized and developed to meet the needs of civil aviation.

Flying a Buhl Sesquiplane, with a Wright Whirlwind engine, the two pilots took off from Felts Field, Spokane, Washington, on August 15, 1929, climbed above the mountain peaks, and headed down the Pacific coast to San Francisco. Refuelling aircraft had been stationed at various points along the route; the first of these tankers rose into the air as the machine was sighted shortly after dawn. Mamaer and Walker took on board sufficient petrol for the next stage of the flight, and then headed east. They passed over Elko (Nevada), Rock Springs (Wyoming), North Platte (Nebraska), Cleveland (Ohio), and flew on for New York, refuelling at intervals. On every occasion the tanker took off as they approached, gained height, manoeuvred above them and flew with them as the petrol was transferred by hose. They reached New York, turned, and flew back to Spokane, landing after a non-stop flight of 115 hours 45 minutes - a cross-country trip of about 7,200 miles.

This flight was of great interest and value, for it proved that refuelling could be carried out on long-distance passenger routes. The American authorities, civil and military, went into the subject thoroughly, and it is probable that aviation will eventually benefit to no small degree from the many endurance flights that were made in 1929.

Particular interest was aroused when the matter of refuelling during flight was mentioned in connexion with bombers. At that time the average range of a fully laden bomber was between 1,000 and 1,500 miles, and it was thought that this figure could be increased by at least 25 per cent by refuelling. A bomber’s load has to be carefully adjusted, the loading depending upon the weight of bombs to be carried and on the distance to be flown. By taking off with a full bomb load and a light petrol load, and adding petrol when in the air, a great advance in performance could be expected. From the viewpoint of safety the refuelling of bombers seemed attractive. The most critical stage of a flight with bombs is the take-off; with a cargo of high explosives, every additional ounce of reserve power is of great value.

R.A.F. Experiments

The result of the discussions in America is not available for publication, but so far as the civil aspect of refuelling is concerned, it is probable that big developments may be seen in the future. At the time of the first cross-country refuelling flight, it was said that six hours could be saved on a single transcontinental flight. In Great Britain experiments have been of a more conservative nature, and have received considerably less publicity. Much, however, has been done behind the scenes; unfortunately, much is not yet available for publication. R.A.F. machines and personnel have been responsible for refuelling experiments extending over more than ten years, and these experiments are still progressing. On the civil side, many thousands of people have seen demonstrations of refuelling by aircraft operated by Sir Alan Cobham. At the Royal Air Force pageant at Hendon, in 1937, two military machines gave an exhibition of one of the more up-to-date methods of establishing contact in mid-air.

THE METHOD OF REFUELLING used in the tests with the Cambria

THE METHOD OF REFUELLING used in the tests with the Cambria was devised by Sir Alan Cobham. In this picture a clear view is obtained of the nozzle on the end of the petrol pipe. The petrol pipe runs out from near the nose of the tanker aircraft and is fed into the tail of the Empire flying boat. Light lines were used to make the first link between the two aircraft.

By this method a light, weighted line is dropped from the tanker machine, and a drogue - or small wind stocking - is paid out on a line from the machine to be refuelled. When both lines have been paid out - the one hanging more or less vertically, and the other streaming out horizontally - the machines manoeuvre so as to link the lines together. After this, the drogue line is wound in to the lower machine on a winch until the vertical line from the tanker has been secured. Then, at a given signal, the operator in charge on board the tanker begins to unwind the hose, which is pulled in to the other machine. Contact is made automatically to a pipe on the side of the fuselage and a signal is given for the release of fuel from the tanker. A valve is opened and petrol rushes down the pipe, opening the valve at the hose nozzle.

The hoses at present in use are about 150 feet long, and have an internal diameter of about 1½ inches. The flow thus obtained by gravity is approximately 100 gallons a minute. Should an emergency arise, or a sudden parting take place, the flow ceases immediately and automatically, without damage to the system or the machines. By the use of a hose of larger diameter it would be possible to increase the rate of flow and, therefore, to decrease the time of contact This is desirable, but it means an increase of weight, and a loss of quick manoeuvring.

It is this side of the problem which is engaging the attention of British engineers at the moment, rather than a series of spectacular flights. Refuelling in the air is recognized as practicable; and though flights of long duration, with a series of contacts in all weathers, and at different speeds, over an unbroken period, undoubtedly prove much, the more practical side of the business seems to be of greater importance.

Automatic Coupling Device

It must not be inferred that America has been satisfied with a series of endurance records, leaving the development of refuelling to others. On the contrary, there are a number of experiments in progress in that country. When the system is adopted by air lines, American endeavour will have done a great deal to bring it into being.

The fact that refuelling experiments are still creating interest is borne out by a fairly recent flight made at Meridian, Mississippi. Two brothers, A. and F. Key, took off in a Curtiss Robin monoplane on June 4, 1935, and remained in the air for 653 hours 34 minutes. This beat by nearly 100 hours the record of 553 hours 40 minutes made by the Hunter brothers over Chicago in 1930.

In the early part of 1938 some interesting experiments were carried out above Southampton Water, with the Empire flying boat Cambria. This machine, equipped with tanks capable of holding 2,320 gallons of petrol, was used on the experimental transatlantic flights, and offered a unique advantage for refuelling tests. An automatic coupling device was fitted, and a number of contacts were made by the latest method, light lines being used. The experiments were carried out under the supervision of the Air Ministry, and it is reported that they ware most satisfactory. An Armstrong Whitworth machine, one of the latest types supplied to the R.A.F., was converted. As yet, neither tanker aircraft nor transport machine has been specially designed for refuelling, and all experiments have been made with converted aeroplanes.

The value of refuelling during flight can best be judged by considering the recent experiments carried out with the Short-Mayo composite aircraft. Here, to increase the initial lifting power, it was considered advantageous to devote a parent aeroplane to the task of raising a second machine into the air. The gain in payload is so great when the take-off difficulty is overcome by outside means, and the ultimate performance so superior, that it pays to add on four extra engines for the first effort. Refuelling in flight means tackling that same problem from a different angle. Instead of making a detachable engine unit, capable of lifting a full payload and fuel load, it enables a normal aeroplane to take off under normal power and to receive the extra load when the extra power is available.

AN ENDURANCE FLIGHT of over 420 hours was made in 1929 by the St. Louis Robin, shown here being refuelled during flight. The pilots of the St. Louis Robin were Dale Jackson and Forest O’Brine. During their flight they took on board 3,500 gallons of petrol and made forty-eight contacts with the tanker aircraft without a hitch occurring.

[From Part 19, published 12 July 1938]

You can read more on “Cobham’s Pioneer Empire Flights”, “Short-Mayo Aircraft” and

“Short Empire Flying Boat” on this website.

You can read more on “Empire Flying Boats” in Wonders of World Engineering

Refuelling During Flight