The Limits of Aviation

I

OUT of all the mass of conflicting testimony that has appeared in the acres of newsprint that have been devoted to discussion of the significance of recent transoceanic flights, one fact stands clear and beyond dispute: the airplane has become an important element in the transportation systems of the world and in the armed forces of nations. It is undeniably destined to take a place of even greater importance in the future.

But how? And to what extent? Are we on the verge of regular transoceanic passenger and mail service by airplane? May we expect in a ‘next war’ to be subjected to bomb raids by enemy planes flying across the ocean?

Notwithstanding the exploits of Lindbergh and those who have followed him across the sea, consideration of the inescapable physical laws governing mechanical flight leads inevitably to a negative answer to the latter questions. The future of the airplane, both in peace and in war, lies in pursuits other than transoceanic transportation or other independent long-range operations.

Here are the reasons: —

The possible performance of an airplane is written almost entirely in the three words: pounds per horsepower. Everyone knows that an airplane depends for its support entirely upon the continuing output of power from the engine; when the engine stops the airplane must descend. It is clearly evident to everyone that a big airplane requires more power to keep it going than does a little one; that there must be some limit to the total weight which can be supported and propelled by the power of a given engine. Certainly we cannot hope to carry an infinite weight with zero power — stones don’t float in air.

The airplane, to justify its existence, must have high speed. The airplane builder and operator has learned as the result of a quarter century of experience that in practical service one horsepower will support and propel at reasonably high speed a maximum gross weight of about twenty-five pounds. One horsepower, twenty-five pounds; one hundred horsepower, twenty-five hundred pounds; and so on. This total weight must include the weight of the airplane structure, the engine and propeller which drive it, the fuel, freight, passengers — everything. Greater loads can be and have been carried, but always at prohibitive cost in respect of speed, controllability, and safety. A practical airplane, to use a common technical term, will not fly safely if the ‘power loading’ exceeds twenty-five pounds.

With this limitation in mind, it will be apparent that an attempt to build an airplane around an engine whose weight is twenty-five pounds for each horsepower that it develops is foredoomed to failure, because the whole weight allowance must be allotted to the engine, and nothing would remain for wings or for fuel to run the engine. No wings, no flight. The only possible way to provide wings and fuel, and still remain within the limiting total weight, is to reduce the weight of the engine. Every pound that is taken from the engine is a pound that may be allotted first to the construction of wings and to enough fuel for a short flight, and later to useful carrying capacity as development progresses.

Someone has truly said, ‘An airplane is merely a light engine with wings on it; the lighter the engine, the better the airplane.’ Mechanical flight as an idea is as old as man; stories of attempts to accomplish it date back into ancient mythology. The basic principles involved have been well understood for centuries; Leonardo da Vinci, who died in 1519, left behind him not only the famous Mona Lisa but also a number of crude mechanical sketches which showed a remarkably clear understanding of the principles of mechanical flight. Many other investigators studied the problem for centuries before the first flight was actually made, but all came to the same inevitable conclusion — that realization of mechanical flight must wait until engineering development had produced a light power plant.

The development of light internal combustion engines is the foundation on which has been built the entire structure of the modern airplane. The first internal combustion engine ever built — some seventy-five years ago — weighed more than five hundred pounds per horsepower. It took fifty years, and the tremendous impetus given by the appearance of the automobile, to reduce this weight, until in 1900 the lightest engines weighed about twenty-five pounds per horsepower. This weight was still too great to permit of successful mechanical flight.

In 1903 the Wright brothers were able to construct an engine of their own design which weighed about thirteen pounds per horsepower. At this point the age-old bar to mechanical flight was let down; the Wright brothers were able to build an airplane which would just barely fly. It is no disparagement of the splendid work they did to say that, had they conducted their gliding experiments five years earlier than they did, they would at that time have been scarcely more successful in their attempts at mechanical flight than were the many earlier investigators who preceded them. Mechanical flight required a light power plant. It was destined inevitably to come when — it could not come before — light power plants appeared.

Engine development has been by no means the only factor in airplane development, — wing and body construction has kept pace, — but the engine has been undeniably the controlling factor. Every outstanding advance in airplane performance, since and including the first flight, has followed upon advance in light engine construction.

Starting with an airplane which will just barely fly, — with such an airplane as the Wright brothers used in their first flights in 1903, — if we reduce the engine weight we can add an equal weight to the ‘useful’ load—passengers, freight, additional fuel for the engine, whatever we will. If we would add weight in one place we must first take out an equal weight somewhere else. The more freight or passengers we carry, the less fuel; the more fuel, the less freight. If we are looking for extreme distance flights (New York to Paris, for example) we must provide the maximum amount of fuel possible, and therefore eliminate passengers, freight, and nonessential equipment. We may skimp structural weight and thereby add to the useful carrying capacity, but if we go too far in that direction there is danger that the structure will fail. We may elect, as we save weight in engine and structure, to keep the power loading low and to use the reserve power to provide greater speed, quicker take-off, better manœuvrability, faster climb.

It is all a compromise. If we stress one characteristic we must subordinate others. We can have extreme high speed in one airplane, long range in another, great freight-carrying capacity in another, slow landing speed and quick take-off in another. But we cannot have all these characteristics in one airplane. Power loading — pounds per horsepower — is the criterion in all.

The size of the airplane makes no difference in respect of possible power loading. Big planes, little planes, middle-sized planes — all are subject to the same limitation; all must keep within the same limiting figure of about twenty-five pounds per horsepower. Within that limitation we may proportion the weights as we see fit; if we exceed it, the probabilities are that the airplane cannot lift itself from the ground; or, if it does, its flying characteristics are so poor that there is great difficulty in retaining control, and grave danger to the passengers. Lindbergh’s plane weighed 23.6 pounds per horsepower; Chamberlin’s 24.5; Byrd’s (which was three times as large and had three times the power) 22.5. They were all dangerously close to the limit; all had great difficulty in taking off. The tragic end of Lieutenant Commander Davis’s preparations for transatlantic flight came in a plane which was loaded to just under twentyfive pounds per horsepower. Safety demands lighter power loading. Our modern commercial weight carriers and military bombers all have a power loading of well under twenty pounds. Higher speed demands still lighter power loading: our fast military combat planes weigh about seven pounds per horsepower; world-record racers, five pounds or less.

No clearer or more concise explanation of the developments of the past, or of the probabilities of the future, can be given than by an engineering comparison between the power-plant weights of the first airplane to fly, in 1903, and the airplane in which Lindbergh crossed the Atlantic in 1927. Each machine carried one man. Each was loaded to its maximum total capacity. Neither carried any disposable load other than fuel and oil for the engine. The Wrights’ machine had a range of a few hundred yards; Lindbergh’s, a range of nearly four thousand miles. In the Wrights’ the fuel carried was necessarily, due to the engine weight, a negligible part of the total weight of the airplane; in Lindbergh’s the fuel comprised more than half the total weight.

Now compare the distribution of the total power-plant weights (including engine, fuel, and oil) in the two machines.

WEIGHT BROTHERS' 1903 LIND-BERGH'S 1927
Total weight of power plant, including fuel and oil 175 3400
Total horsepower 12 225
Pounds per horsepower of total power plant 14.6 15
Pounds per horsepower for bare engine 13 2.5
Pounds per horsepower for fuel and oil — ‘use-ful load’ 1.6 12.5

The pounds per horsepower of the total power plant, including fuel and oil, are practically the same for both machines, but the distribution of that weight, as between bare engine and fuel, is changed enormously. It is that difference in distribution — the conversion of eleven pounds dead engine weight into eleven pounds of useful carrying capacity — that is primarily responsible for Lindbergh’s success.

For all the advance in knowledge of aeronautical engineering, there has been no increase in total weight carried per unit of power. The Wrights’ first machine had a power loading in the neighborhood of fifty pounds, but it was not in the modern sense a practicable airplane; its maximum speed was less than thirty miles per hour; it required special launching devices to put it in flight; its possible climb was less than the height of a modern office building, its structure far from rugged, its manœuvrability near the vanishing point. The trend has been definitely downward in respect of power loading, rather than upward. This is natural, because improvement in speed, rate of climb, manœuvrability, safety, — in everything but extreme carrying capacity, — comes with decrease in power loading.

The explanation of the development in airplane performance and utility is not difficult to understand. Engineers have been taking dead weights out of engines and making available thereby an equal weight for ‘useful’ carrying capacity — available for fuel if one would break endurance records, for freight or mail or passengers if one is willing to dispense with a part of the fuel and accept a shorter cruising range. The saving of about eleven pounds per horsepower in engine weights, coupled with corresponding improvement in structures to take advantage of it, has been translated into increase in cruising range, increase in speed, increase in climbing ability, increase in utility, increase in safety.

Reduction in engine weights has been the predominating influence in improvement in airplane performance. We may expect no startling advance in performance in the future because we are now approaching the ultimate limit in respect of weight reduction. Engines now operating in service types of planes, both military and commercial, weigh in some instances less than one and one half pounds per horsepower. Certainly they can never be reduced to zero weight. But even if they could be reduced to zero, the gain in possible performance would not be startling. Lindbergh carried twenty-seven hundred pounds of fuel; his engine weighed five hundred pounds. A weightless engine would have permitted him to add five hundred pounds—about eighty-three gallons — to his fuel weight, and would have increased his cruising range by something less than twenty per cent.

We may expect some further reduction in engine weights, some improvement in fuel economy, refinements in wing and body construction which will enable us to use the engine power more efficiently. But these advances will come so slowly as to be scarcely perceptible to the casual observer, except perhaps as some new long-distance record is established. We shall never reach the ultimate limit, of course, but we are approaching it; and as we approach it the rate of progress becomes ever less rapid.

The Pacific Ocean will be spanned, as the Atlantic has been. The airline distance between Honolulu and Tokyo is almost exactly equal to the distance from New York to Berlin. Chamberlin has flown within a few miles of that distance; a handbag full of additional gasoline would have taken him all the way. Lindbergh had enough fuel left on arrival at Paris to take him on to Berlin without landing.

There are no halfway measures in transoceanic flights. They are either completely successful or complete failures. The difference between glorious success and tragic failure is very small indeed; measured in terms of airplane performance it may be almost negligible. When the barrier is finally broken down success comes with such startling suddenness that one is very likely to conclude that some new invention or some revolutionary advance in the art of airplane building has suddenly come to light. We are prone to forget, if indeed we ever knew, that Lindbergh’s plane was an adaptation of a similar type which has been in continuous operation for a long time in regular passenger-carrying service in California; to forget that Chamberlin’s plane was actually built some three years ago, and has been in intermittent flight service since that time; to forget that the Wright Whirlwind engine used in all the transoceanic flights was designed seven years ago, and has for at least four years, in substantially its present form, been in continuous operation in naval planes which have operated from Iceland to the South Sea Islands; to forget that Kelly and Macready made a nonstop flight crosscontinent in 1924, covering a distance greater than from San Francisco to Honolulu; to forget that a German airplane in 1914 remained in the air without refueling for twenty-four hours.

There have been great strides in reliability and safety, and in adaptation to utilitarian purposes. But there has been no startling increase in possible performance during the past few years; there is no likelihood that there will be in the years to come.

II

But what of the ‘giant’ airplane? We have read much in the daily newspapers of huge air liners of the future. Big steamers can handle ocean freight and passengers far more efficiently than can little ones; the experience of centuries has taught us to accept as a matter of course, in practically all forms of transportation, that efficiency goes with large size. What more natural than that we should look forward to the development of larger and still larger airplanes, with ever-increasing efficiency as a result?

Unfortunately the old rule fails us when it comes to airplanes, because the method of support is entirely different from that of any other known vehicle. All other forms of vehicles, including the dirigible, are supported by static forces. They are supported equally well whether they are at rest or in motion. Their propelling plants are entirely independent of the support, and the fuel carried is all available for driving them forward. Ships ranged the seas before ever an engine was heard of; but the airplane immediately ceases to be an airplane when its engine stops.

Our grammar-school physics tells us that every floating body is buoyed up by a force equal to the weight of the liquid which it displaces. The volume of any body increases as the cube of the dimensions. If we double the dimensions of a cubical box the volume becomes eight times as great, and therefore it will float eight times as much weight; but the weight of the box does not increase anywhere near so fast as does its contained volume. It is the story of the dry-goods box; less lumber is needed to enclose material in one big box than to enclose the same material in a lot of little boxes. In floating vessels, therefore, we can carry proportionately far more useful weight in large vessels than in small ones. And so with the surface ship, the submarine, the balloon, the dirigible airship, or the dry-goods box: when we want to increase the proportion of the total weight that can be allotted to useful pay load, or, if ships are self-propelled, to increase the distance they can go without refueling, all we have to do is to build the vessels larger. That is why we have enormous passenger ships like the Leviathan. That is why we can carry sixteen-inch armor on big battleships when we must be content with thin side plating in destroyers. That is why rigid airships increase in effectiveness as the size is increased. ‘Bigger and better’ is a fine slogan for displacement ships.

We have an entirely different story with the airplane. It does n’t float in the air, any more than the skipping stone which you shy along the surface of the water floats on the water. The airplane is an airplane; the stone is (so long as it keeps moving) a hydroplane. Both depend entirely for their support upon their continued movement — with relation to the air in the one case; with relation to the water in the other case. In both cases it is the area of the surface in contact that counts. The volume has nothing to do with the supporting force or with the total carrying capacity. If we double the area we double the total supporting force and the total carrying capacity; but at the same time we double the weight of the structure, double the resistance to forward motion, and hence double the effort required to keep it moving; and when, as in the airplane, the driving effort is supplied by an engine, there is required twice as powerful an engine, and twice as much fuel to go a given distance. The weight of structure, carrying capacity, and power and fuel required, all increase in exactly the same proportion as the size increases.

Generally speaking, then, and contrary to the rule which applies to floating vessels, small airplanes are just as efficient as are large ones. There is no advantage in great size. A small airplane can fly as far as a large one; two small airplanes can carry as much useful load as can one large one of double the size. It is not just an accident that the world’s records to-day for the longest sustained flight, the greatest nonstop distance, and the highest sustained speed for all distances from one mile to nearly four thousand miles, stand to the credit of airplanes of moderate or small size.

Byrd had three engines of exactly the same model as Chamberlin’s one, exactly three times the total power, and approximately three times the wing area. His plane weighed just a little less than three times as much as Chamberlin’s. He had no longer cruising range, he carried proportionately no greater useful load. Chamberlin’s one engine carried two men across the ocean; Byrd’s three engines carried four. Both were loaded to the very limit of safety.

There is one advantage which larger machines have over smaller ones — that within reasonable limits no larger flying crew is necessary for large machines than for small ones. For example, Lindbergh flew the Spirit of St. Louis alone. Conceivably Byrd might have flown alone, and have translated his ‘crew’ into ‘passengers.’ It is rather doubtful, however, if any pilot would care to make a regular practice of flying under such conditions.

But there is a limit even to this advantage. It is a demonstrated fact that, as size is increased, a point is reached, somewhere in the neighborhood of eighteen thousand pounds (Byrd’s plane weighed a little more than fourteen thousand pounds), beyond which it becomes necessary to allot a proportionately greater part of the gross weight to wing and body structure, and a correspondingly smaller part to useful load, than is necessary in the smaller types. The explanation of this involves a somewhat complicated technical discussion of the principles of structural design somewhat analogous to those involved in cantilever bridge construction — a discussion which has no place here. Suffice it to say that attempts to build airplanes considerably in excess of about eighteen thousand pounds necessarily result in decrease in efficiency.

There is no advantage in great size. This is the dictum of the engineer, this the evidence of actual performance, this invariably the lesson which has been learned in the many attempts that have been made in all countries in the world to construct giant airplanes. Byrd’s plane is often spoken of as a giant plane. In reality it is not a giant at all. The NC-4 which made the transatlantic flight via the Azores eight years ago was more than twice the size of Byrd’s plane. The army’s Barling bomber was three times the size of Byrd’s plane. None of these huge types has persisted.

The trend of development in recent years has been downward in size rather than upward. There is no reason to believe that the airplane of the future will be of great size. The ‘bigger and better’ complex is so strong that efforts to build giant airplanes will doubtless persist for years to come, but we have probably seen about as large airplanes as we shall ever see in commonplace operation.

The trend of airplane development, then, is toward greater reliability, dependability, safety; toward more general application to useful commonplace service, greater production, and marked decrease in cost of construction, operation, and maintenance. There will be slow and steady improvement in possible range, in maximum speed, in possible time of sustained flight, to be sure; but we need look forward to no very startling gain in any of these. Because of the great proportion of the total carrying capacity required for fuel in long flights, the airplane, as applied to practical carrying purposes, is to-day, and must inevitably remain, essentially a short-range vehicle. Its forte is high speed and unprecedented freedom of movement; its weakness, relatively limited range of independent operation.

Transatlantic flight by airplane is undoubtedly practicable in favorable conditions, just as nonstop transcontinental passage by motor bus is practicable. But that does not indicate that either is likely to become a commonplace commercial venture or military operation. A motor bus in a transcontinental passage will consume nearly two tons — or six hundred gallons — of fuel. No one can deny that a motor bus can successfully make a nonstop passage across the continent without refueling along the way; but can anyone picture as a paying commercial venture a bus line which must sacrifice enough passenger space to accommodate twelve fifty-gallon gasoline drums, or one hundred and twenty five-gallon gasoline tins, or eliminate the weight of some twenty passengers and their baggage? Would you invest your money in such a project?

The airplane must have frequent refueling and service stations along its route, just as automobiles must have them. These do not exist at sea; they cannot be provided or maintained except at prohibitive cost, if at all.

One does not say that regular transoceanic airplane service will never come, — it is a rash man who says that any thing will never come, — but it cannot and will not come until there is far more advance in the field of aeronautics than now exists or is remotely in view to-day.

III

What, then, have the recent transoceanic flights proved? They have probably done more to advance the cause of aeronautics, both commercial and military, in this country, than any other one series of events could possibly have done. Aeronautics has been developing rapidly under the surface. It has long since reached a point where it is ready to take the people of the country on its back with safety and assurance from one end of the country to the other at tremendous speed.

Our lack of commercial passenger lines in this country is not due to lack of excellence in flying equipment. Our airplanes are just as efficient, just as safe, as any in the world. Our engineers and pilots are just as good. No foreign airplane or pilot has surpassed the feats of Lindbergh, Byrd, Chamberlin, or Maitland. A very fair share of worldrecord performances stands to the credit of American airplanes. We have excellent airplanes, peerless flyers, lighted airways, frequent landing fields, well-equipped service stations throughout the country. All the groundwork in its necessary elements is laid for air transportation from anywhere to anywhere in the land. All that we have needed is the passengers to ride; the income from their fares to pay running expenses, to perfect existing facilities, to provide new facilities where they do not now exist. And all that is necessary to get passengers is to convince them that air transport is safe and dependable. It needed men with daring and unbounded confidence to perform some startling exploit which would arouse popular enthusiasm, focus the attention of the whole country on the things which have been under our very noses for years if we would but use them. These men who have faced the perils of the Atlantic and Pacific are the kind of men who are flying your mails across the country every day and every night of the year; the kind of men who are ready at your service to carry you wherever and whenever you wish to go. They will take you in planes which are conservatively loaded, which need no special runways to take off, over welllighted and regularly established airways, with accurate weather information for the path ahead always at hand, with marked and lighted landing fields every few miles, and competent service men distributed all along the route. They are propelled by engines which have plugged on unfaltering for dozens of hours over the Atlantic and Pacific, through fog, rain, and sleet, day and night, good weather and bad. Need you have any fear to fly with such pilots and such equipment? Many are ready to serve you now. Many more will spring to your service when you indicate your desire to ride, your willingness to pay a reasonable fare.

When you have come, as you doubtless will — and soon — to taking passage by air with as little thought of the consequences as you now have when taking the regular express train, you may look back and thank these men for bringing this service to your attention. Theirs has been the ‘punch’ which has been necessary to bring to the public consciousness the fact that the airplane in the field of public utilities has come to stay.