Page images
PDF
EPUB

and the other at room temperature. Periodical weighings showed that in 26 hours the weight had increased by 1.9 per cent, due to the condensation of the moisture.

The result, thus obtained in a few hours might require years if the faces were securely boarded in, but in practice it was impossible to seal off the material perfectly. There was always some breathing due either to alterations of the barometric pressure, or to the practice of reducing the temperature of a cold store during the day, so as to avoid the necessity for keeping machinery running at night. It was important, therefore, that the surface of an insulator should be coated with a substance of low permeability. It was, accordingly, decided to measure at the National Physical Laboratory the rate of diffusion of gases through various materials.

The apparatus is represented in Fig. 5, where B denotes the material under examination, which was fixed in a metal case A. The cover C was used for experiments with other gases than air, and for the purpose of testing the tightness of the wax seal. The manometers E and D served for measuring the pressure difference between the two faces of the slab. The ballonets H served to steady the gas supply.

The author said that not many reliable experiments on the component parts of refrigerators had been made in this country, but that a good deal was being done in America. The York Manufacturing Company had equipped a laboratory for this purpose, at a cost of £15,000 for plant alone, and the weekly pay roll averaged £130. Here different types of compressor had been tested, and much information had also been obtained as to the effect of air leakage on the efficiency of the ammonia condensers. In these it was often found that the temperature in the condenser was sometimes as much as 10 degrees F. lower than that corresponding to the pressure.

Dr. Griffiths next dealt with investigations carried out by his brother on behalf of the Union of South Africa, in connection with the transport of fruit both by rail and by ocean. Some of the fruit had to be carried by rail for four or five days before reaching the point of shipment. The investigations made showed that, even when iced en route, the refrigerated fruit trucks sometimes reached their destination with portions of their consignment at a temperature of 100 degrees F. Other observations made on these trucks showed that the temperature might differ by 12 degrees F. at different points. Smoke observations showed that with any practical type of truck having end bunkers for the ice there were regions inside where the air was stagnant. As the result of these observations an experimental truck had been designed, in which the ice tank covered almost the whole of the roof. With this truck the coolest part of the interior was at the top, and the temperature did not differ by more than 3 degrees F. at different points, whilst the average temperature was 4 degrees to 5 degrees lower than with end-bunker trucks. Comparative records taken with the different types of truck are reproduced in Fig. 6. South African experience showed the advisability of pre-cooling the fruit before it was placed on ship board.

The author said that he had also taken part in the investigations made into the conditions of the refrigerated chambers on the service between Australia and the United Kingdom. Four ships, representing different systems of refrigeration, were equipped with temperature-measuring and gas-analysis instruments, and a fifth ship with an automatic carbon dioxide and oxygen recorder only. The electrical resistance thermometer used was placed inside the actual fruit boxes. The general arrangement of the plant used is represented diagrammatically in Fig. 7. The large box, constituting an airtight enclosure for four boxes of apples, was taken down into the hold, the apple boxes selected at random from the cargo, being loaded and the lid sealed down. The enclosure was then built into the bulk of the cargo so as to be subjected to precisely the same conditions as regards temperature. Pipes led away from the enclosure to a convenient position outside the hold and

the observer was able to make gas analyses of the atmosphere in the box, for if the experiment were started with ordinary air in the box the respiration of the apples resulted in the absorption of oxygen and the formation of carbon dioxide. The carbon dioxide content could, with the arrangement shown, be kept within any desired limits by pumping in fresh air.

In the discussion which followed the reading of the paper, it was stated that, owing to the fact that Tasmanian apples reached the ports largely by river transport, the fruit was not pre-cooled before shipment. As a consequence, therefore, much of it arrived in bad condition and the price obtained per box was only half that paid for Californian apples. The latter were pre-cooled before shipment, as also were most of the apples shipped from Australia. Several speakers also referred to the very frequent destruction by dry rot of the wood used in constructing refrigerating chambers. In order to reduce heat losses, this wood was carefully protected from air currents, and in these circumstances dry rot was very likely to develop."Engineering," Nov. 28, 1924.

PROBLEMS OF THE ENGINEER IN AVIATION.

GROVER LOENING.*

PRESIDENT, THE LOENING AERONAUTICAL ENGINEERING CORPORATION. We often hear that the public is intensely interested in aeronautics, but there is clear evidence that engineers have failed to spread their technical knowledge of aviation widely enough.

Human interest has been gripped by the great flying feats that follow, one after another, but the practical engineering developments that make greater and greater feats possible, year after year, are unfamiliar to the public. New engineering developments of worth for the automobile are instantly appreciated, and the engineers are stimulated to new efforts by a demand from the public that is most competent. The layman, let us say for example, knows that he wants motor cars for steering in a much shorter radius to facilitate parking, or that he wants less trappy bodies, or hopes to do away with the gear shift, and so on. No engineer successfully developing features of this sort need sell them. The public is away ahead, impatiently waiting.

Much the same thing is true in radio.

But the case of aviation is quite different. The wide interest in the great feats that are performed is largely in the spectacular side. If the Roundthe-World Flight had been an easy jaunt there would have been much less interest, but the perils faced by the flyers, the great weather hazards of storm and fog, and the unknown nature of the lands through which they flew so successfully, have a compelling and dramatic interest. The hardships endured and the heroism of the pilots are justifiably emphasized; but am I entirely wrong in saying that this is to the detriment of a healthy public demand for air transport, because flying is still left too much in the realm of the spectacular and remains a feat-not for the layman and his wife and family-but for the hero and the daredevil.

As a matter of fact, the World Flight from an engineering standpoint was a revelation in showing us that airplanes of the present day have a service endurance and weather resisting qualities far above what was commonly believed possible, and the machines used stood up to mooring and weathering conditions that many boats and automobiles would have trouble equalling. But the very feature of the World Flight that makes it a convincing demonstration of the future possibilities of air travel is, that one

From an address delivered recently at the New York University, under the auspices of Aeronautic Division of College of Engineering.

half to two-thirds of it was not at all spectacular, and represented a series of easy flights well within the power of the machines and motors, and easily within the scope of competent pilots and navigators.

We rarely read of the millions of miles flown by planes all over the country without a serious accident of any kind, and yet, as you know, accidents themselves hardly ever miss the front page. This is entirely understandable, of course, because the crash of an airplane to the ground is eminently spectacular and generally seen by a great many people.

As an engineer in this business, however, I submit that the public gets two aspects of aviation, which we engineers know are not true. In the tremendous featuring of the great flights that are made, the public is not made to realize that these feats are well within the possibilities of the machines that the engineers devised. Secondly, the attention to accidents gives aviation an unfair reputation for danger, which is not justified by the actual figures.

Flying at present is admittedly dangerous; so is polo, steeplechasing, mining, and for that matter the Twentieth Century Limited; and need I add that more people die in bed than anywhere else. In short, the technical people intimately acquainted with aviation are admittedly somewhat hard boiled, but they do know without fooling themselves that flying is not nearly as bad as is generally believed. The question has often been asked, why there is not a more general use of flying by the public, and where the blame may be placed. I do not hesitate to blame the engineers first for being slow in making those perfections which we know will make flying a "go." The engineers, in turn, blame first the lack of available capital for development, and secondly, some of them blame the war. Whether aviation was helped or hindered by the war is, in the opinion of many engineers, a fifty-fifty proposition. The technical stimulus given by a forced development of the theory of planes and motors when added to the enormous amount of flying that was done, has undeniably given us a vast technical knowledge. But what good is this, if it must remain locked up in the engineer's drafting room, because of the lack of outside understanding and enthusiastic public interest and encouragement, which the great war distinctly hindered.

Engineers today recognize all over the world that the airplane as used in the war is not a practical vehicle for travel by the public. We now have air transport ideas, in addition to exclusively military ideas, and the two are quite different. We are, therefore, at the stage, and all engineers in this business as well as the public, must realize it, where we are entering upon a new start in aviation, for a civilian customer is being added to our military and naval customer. The military and naval side of aviation is fixed beyond any question of argument, and is absolutely indispensable, if not commanding. The civilian side of aviation, in the opinion of the engineering mind, has really just barely begun.

There is only one limit to the lightness of an airplane, and that is the lightest humanly possible. Never must it be forgotten in aeronautical engineering that the only reason the airplane flies is because it is light enough to do so. The refinements that this requirement of lightness have led to in the engineering structures that we build are most remarkable, and far ahead of any other branch of structural engineering. For example: Is it realized that we consider it nothing extraordinary in aviation to build a wing structure, really a bridge, fifty feet span weighing only six hundred pounds which will support, without failure, a load of over twenty tons?

We have ribs, for example, that will support a distributed load of eight hundred pounds, without failure, and that weigh only twelve ounces; and, of course, it is the lightness requirement of airplanes which has given the great impetus to the development of the new alloy, duralumin, which we find has absolutely got the strength of mild steel, and weighs only one-third as much. Aeronautical engineers now accept duralumin as a fact incapable

of any further argument, and many others go even further, and wonder why so many other branches of engineering are failing to re-design their structure for the use of this material. Railroad cars, automobiles, and ships can all be lightened enormously; many of us believe we are about to pass out of the steel age into a duralumin age.

The use of other light structural materials has been developed to a high degree, also. For example: The use of balsa wood which we find on test is, per pound of weight, the strongest structural material known to man. It is a light wood, largely pithy, weighing only eight pounds a cubic foot, and one-third the weight of the lightest pine wood ordinarily seen, but it is so soft that we have had to learn a great deal about how to take hold of it, and curiously, where duralumin fittings are fine for taking hold of the harder woods, we find that hard wood is the best thing with which to take hold of balsa.

The engineer chooses an engine out of several available, or perhaps installs one of new design, and what applies to the plane on lightness and reliability applies, of course, to its engine. But here we are treating with the branch of our work, which, in the opinion of trained aeronautical engineers, has suffered somewhat from its similarity to automobile engine work and to get into this phase of how the gasoline automobile engine has been marvelously lightened and perfected to high horsepower for the airplane is another long story.

I have recently had the good fortune to participate in a development that carries strong proof of this, in the inverting of the Liberty motor. This experience is typical of the unfortunate way that engineers have of sticking to things that really have no reason at all. The automobile requires road clearance, and at the same time requires the lowest position for the driving shaft so as to drive straight back to the axle. Therefore, automobile engines have developed so that their cylinders and their parts are largely on top of the crank shaft. Then we come along to the development of the Liberty motor, and nobody gives sufficient attention to this fundamental, so that the airplane motor proceeds along the same lines. But in the airplane we have a totally different condition, because we have to swing a big propeller ten feet more or less in diameter. That right away puts the crank shaft way up in the air, and using the automobile type of motor, the cylinders and all the works are added on top of that, so that the whole thing sets up on stilts, and the space easily available and much more convenient for the engine below the crank shaft is, from a design standpoint, sheer waste.

Landing conditions have been operating today as a great deterrent to flying. Fields that are big enough are not provided and the airplane has largely had to remain a vehicle with no roads to ride on. New developments, however, are improving this condition daily. New designs of planes are being brought out that require less and less space to start from and alight on, and right here we should say that the popular conception that slow landing speed solves this problem is quite wrong, since it is even more important above all things to have excellent control at landing speed in order to make safe and easy landings.

The development of the amphibian airplane which can alight on land or water alike, has been most encouraging and I consider myself fortunate to be identified with this development. We are confident that we have arrived at the point where the airplane has become a vehicle that can land anywhere, and the old distinction between the seaplane and the land plane is going to disappear. Common sense alone would indicate that the airplane must be able, even when on trips in the interior of the country, to land on lakes and rivers as well as fields, and this fact alone opens up a great deal more flying, since the lakes and the rivers and the harbors already exist as fine landing fields and generally are in the heart of cities. The cross-country flyer with an amphibian type has much more than doubled his available landing places

in case of difficulty, and the development of a machine of this type that is the absolute equal of the land airplane has long been the dream of aeronautical engineers as an enormous stride in greater safety and more practical usage.

Another fundamental we must realize is that an airplane, or for that matter any other air vehicle, be it a helicopter that rises straight up, or some other contraption, is mechanically suspended in the air, and if anything goes wrong it cannot be stopped by the roadside to be fixed, as can a motor car. There is always, therefore, in air travel the possibility of a fall, and competent engineers in time to come will not only realize this but will actually design the airplane so as to take a fall from several thousand feet entirely out of control with no injury to the occupants. We may, very likely, come to the development of crash proof cockpits, seating the passengers in a metal shell surrounded by shock absorbing material, so that the most complete breakdown and accident resulting in the collapse and fall of a plane will absolutely ruin the plane but will hurt no one. This is merely recognizing the principal that you can buy a new plane, but you cannot buy a new foot, and I venture to say, that if the railroads had not had the construction of the steel cars with their relative safety they would have suffered greatly in volume of traffic. Given the opportunity, the encouragement, the capital, and the time, aeronautical engineers are certain to work out the obvious limitations which aircraft of today have got, and that means that the airplane of tomorrow will become a very practical and safe vehicle quite soon-that the thirty or forty mile-an-hour world-wide transportation of mail, passengers, and goods by railroad, motor car, and ship will be succeeded quite suddenly by a one hundred mile-an-hour transportation through the limitless fields of air.—“U. S. Air Services," Jan., 1925.

RECENT PROGRESS IN THE SCIENCE OF AERONAUTICS.* By JOSEPH S. AMES, PH.D., LL.D.

THE JOHNS HOPKINS UNIVERSITY.

MEMBER OF THE INSTITUTE.

The aeronautical engineer whose function is to design aircraft has, in the past, followed several methods, or combinations of methods.

One of these, the most obvious, is to rely largely upon past experience. One successful type leads to another, refined in some particulars, it is true, but essentially the same. This method was the one followed until very recently in the design of airships. As a matter of fact, there could be no other method until direct physical experiments were performed upon the distribution of pressures over the envelope, fins and elevators and rudder, and the stresses and strains in the members of the framework were measured. The defects of the method are sufficiently obvious, the main one being that ignorance of detailed knowledge would surely lead to making the structure too heavy or too weak.

The method most commonly used today is to obtain preliminary knowledge of the aerodynamical properties of a model of the aircraft or of models of its various parts. For this purpose, models, of possibly one foot or two feet in size, are made and tested in wind-tunnels. These last are circular tubes of large diameter, up to ten feet or more, through which a current of air may be drawn by fans at a velocity of 100 miles an hour or more. The model is suspended in the air-stream from a balance, to which the necessary weights may be added in order to keep the model stationary; and in this

Address delivered in the hall of The Franklin Institute, Wednesday, September 17, 1924, on the occasion of the centenary of the founding of The Franklin Institute.

« PreviousContinue »