Aircraft in Warfare (1916)/Chapter 2
CHAPTER II.
(September 4th, 1914).
AEROPLANE versus AIRSHIP OR DIRIGIBLE.
§ 3. Aeroplane and Dirigible: Speed Limitations. Two questions are involved in the consideration of the relative merits of the aeroplane and dirigible. We are firstly concerned with their respective advantages and disadvantages in relation to their primary function—namely, as instruments of reconnaissance, attack, and defence; secondly, we have to take into account their secondary function—i.e. their relative power of mutual destruction; the question whether, for example, either can drive the other from the field, or whether each may have its own rôle to play in securing and holding the command of the air.
Before going into either of these questions in detail it is convenient to review a few of the facts by which limitations are imposed on the ultimate performance of either type of aircraft. We must avoid falling into error by judging each too closely by its performance of to-day.
The all-important question of speed is a matter depending primarily on the lightness {i.e. horse-power per given weight) of the prime mover, and the law of resistance. The horse-power per unit weight of motor is roughly the same whichever type of aircraft is in question, and any future advance in the art of motor construction tending to diminish weight will, we may presume, be equally available for either type. The laws of resistance of the aeroplane and dirigible are well understood; in the case of the former the resistance is approximated by a curve a a. Fig. 1, representing the sum of a resistance following the V-square law and a constant; the latter (the dirigible) may be taken as following the V-square law implicitly. Fig. 1, b b.
_-_Fig._1.png)
Fig. 1
Fig. 1 represents approximately actual values of the resistance coefficients, in tractive effort per cent., in machines of average size as they exist to day, for the speeds given in miles per hour.[1]
One salient fact is at once evident; the greater the horse-power available for a given engine weight the greater the advantage in the matter of speed in favour of the aeroplane; the highest speed of flight of an aeroplane attained to-day (through, i.e., relatively to, the air) is already more than twice that of which the fastest dirigible is capable. There is every prospect that its advantage in this respect will increase rather than diminish with the march of progress.
Beyond the above, it is well understood that an increase in size is conducive to a reduction in the resistance coefficient; this applies to both aeroplane and dirigible. This fact has been one of the controlling considerations in dirigible design; no dirigible, other than of comparatively large size, has been found to be of real service. It is, moreover, evident that, in the case of some of the large Zeppelins, it will not be found practicable to go very much further in the direction of increase. Here again the aeroplane is at an advantage; we can in nowise regard the aeroplane of to-day as defining the limit.
It is abundantly manifest therefore that the dirigible is at a permanent disadvantage of not less than two to one in the matter of speed.
§ 4. Aeroplane and Dirigible: other points of comparison. The question of range and duration of flight is largely determined by petrol-carrying capacity. In the aeroplane both range and duration depend definitely upon the petrol supply holding out; in the case of the dirgible the same applies to a limited extent; but here the duration and, to a less extent, the distance can be greatly prolonged by reducing the speed to the minimum possible without jeopardising the control. In the dirigible the gradual loss of buoyancy, due to the leakage and escape of hydrogen, is an independent determining factor. Taking everything into account there is not much to choose between the two types of aircraft in the matter of range or radius of action; on the other hand, under favourable conditions, the dirigible has undoubtedly the advantage on the score of duration of flight. The maximum is about 24 hours in the case of the aeroplane, against 48 hours in the case of the dirigible. This may be taken as a fair indication of their relative capacity, though of no quantitative value as a guide to what is to be expected under service conditions. The possibilities of the future are here rather in favour of the airship; there is an absolute limit both of range and duration where the aeroplane is concerned.
On the question of storage or housing the advantage of the aeroplane is overwhelming; the aeroplane, especially if furnished with folding wings, can be stowed away in any ordinary shed or barn, or may be anchored in the open without serious risk, whereas the "balloon hall" necessary for the safety of an airship is not only costly, but is an unmistakable landmark for hostile aircraft at 20 miles distance. Again, bad weather affects the storage of an aeroplane but little, whereas the housing or getting out of an airship in a strong wind is a difficult and risky business, even under the best of conditions. A large Zeppelin may sometimes call for the services of 300 men.
The foregoing by no means exhausts the grounds of comparison, but is sufficient for the present purpose. It is scarcely necessary to point out the very great disparity of weight, and, incidental thereto, carrying capacity, between the two classes of machine; the large German Zeppelins have a gross weight, taken from their displacement, of 22 tons (military) up to 35 tons (naval); of the aeroplanes in service, practically all the military machines are less than 1 ton "tare," and most types do not exceed 1 ton gross—i.e., with full complement, petrol, oil, etc.
If we were concerned with the primary function of the aeronautical arm alone, there appears to be no reason to doubt that both kinds of aircraft would have their place; the large air-ship has unquestionable advantages under suitable conditions: cruising at high altitudes over the battlefield, or over or in the rear of the enemy's lines, and reporting to headquarters by wireless every movement of strategic or tactical importance, it might render the most vital service. It is able to carry a complement of officers trained to observation, capable of giving an accurate interpretation of what they observe, and acting under most favourable conditions, such as are not possible in any existing aeroplane; it can move at some fifty miles per hour, if required, or remain to all intents and purposes stationary; it can follow continuously the course of events from sunrise to sunset, and remain the whole time in touch with headquarters, either for sending or receiving. On the other hand, for bearing despatches, for flying at low altitude within range of shot and shell, as may be necessary for detail reconnaissance or in cloudy or misty weather, for bringing machine-gun fire to bear at some important point or at a critical moment, etc., all these are duties for which the aeroplane is pre-eminently suited, as also for rapidly locating and signalling gun positions, directing fire, and duties of such-like character.
It is more than questionable whether actual fighting is any part of the primary function of a dirigible at all; it is at least becoming apparent that bomb-dropping is an entire misuse of the large airship; the results are incomparably small in view of the means employed, and can never affect decisively the course of any battle or campaign.
It is important to note that though it is possible effectively to armour an aeroplane, at least to be proof against small-arms fire, and that in any case the vulnerable target is small, the dirigible, presenting a mark larger than the proverbial haystack, cannot be effectively protected. In spite of the fact that injury to the envelope is not necessarily dangerous, it has been reported that such injury has already necessitated a hurried descent into a hostile country, with the effective loss of both vessel and crew. These are the considerations which place the dirigible at a formidable disadvantage when within reach of the enemy's guns.
§ 5. Aeroplane and Dirigible, analogy between Air and Naval Forces not tenable. We may now pass to the discussion of the secondary function of the aeronautical Arm in its present relation—that is to say, we shall consider the question of aeroplane versus dirigible in armed conflict.
At the outset it is desirable to dispose of the much-worried analogy that crops up again and again when the present subject is discussed. Some of the most strenuous supporters of the airship as an auxiliary to the aeronautical service are fond of drawing a parallel between the air service and the Navy, the airship being put forward as analogous or comparable to the battleship or battle-cruiser, and the aeroplane to the torpedo boat or destroyer. In the author's opinion any such analogy is totally fallacious. The effective area of the target presented by an aeroplane is but a few square feet. The effective target area of a torpedo boat or destroyer is more than one hundred times as great. The time during which an aeroplane is visible and under fire, owing to its small size and high speed, is short compared to that of torpedo craft at sea.[2] The armament which a Zeppelin can bring to bear on an attacking aeroplane is confined to that which she can carry on a platform arranged on top of the structure, since the hostile aeroplane making its attack from above _-_Plate_II.png)
Field tent for aeroplane: back view. Supported by two poles only;
Roof ridge formed by stretched wire rope. Designed R.A.F. 1913.
can manoeuvre to remain in billiard phraseology, "snookered" so far as the gondolas and their armament are concerned. Beyond the above, the speed of the aeroplane is approximately double that of the airship, whereas the speed of a fast destroyer is not more than 25 or 30 per cent, superior to that of a fast and heavily-armoured cruiser or battleship of modern type, and even this advantage is lost in heavy weather.
It will be realised in considering the above facts that the whole analogy breaks down—the continued existence of the battleship or cruiser in the face of torpedo-craft does not in the least degree imply or involve the continuance of the airship as a logical probability.
September 11th, 1914.
§ 6. Aeroplane and Dirigible in Armed Conflict. Having in the preceding sections devoted some attention to contrasting the respective merits and limitations of the aeroplane and airship or dirigible, and to disposing of the false analogy so frequently drawn between the air forces and the Fleet, we pass to the consideration in greater detail of their mutual relationship in matters of attack and defence. Firstly, it is evident that the attack will essentially be on the side of the aeroplane; the dirigible can do no more than act on the defensive. The great disparity of speed alone, whatever armament the airship may carry, settles this definitely; it is within the power of the aeroplane to choose precisely when, how, and where it will engage in conflict. The dirigible, like the submarine, is too slow to run the enemy to earth or to bring him to bay, and, to its disadvantage, cannot, like the submarine, make itself invisible and attack by stealth. Beyond this, its quarry (the aeroplane) is of small size, often scarcely visible at a mile or two distance, and when not actually in the air can be either concealed or efficiently protected. Any attempt at aggressive action on the part of the dirigible is totally and completely out of the question; it is, in fact, beyond the conceivable range of possibility.
On the other hand, if the airship is to continue as a factor in warfare at all, it must be able to defend itself against hostile aircraft, and in particular be capable of repelling the attack of the enemy's aeroplanes. Now the only power of defence possessed by a dirigible when attacked by an aeroplane is counter-attack by gun-fire; hence the extent, character, and distribution of its gun armament is one of the most important factors in its design.
In the earlier days of the development of the aeroplane when its horse-power was but little in excess of the minimum required for the bare necessities of flight, its rate of ascent was so extremely slow (if it could be said to have any real rate of ascent at all) that it was commonly assumed that a dirigible, or airship, could seek safety in altitude. To-day, however, many aeroplanes will make altitude at a speed of 700 ft, or 800 ft, per minute, thus being more than able to hold their own with the lighter-than-air machine, and can ascend to over 10,000 ft, altitude (even twice this height has been reached); again having the dirigible at a disadvantage.
§ 7. Aeroplane versus Dirigible, means of Attack and Defence. The method by which an aeroplane may most effectively attack a dirigible is a matter that remains for future experience to settle. If the aeroplane pilot is prepared to sacrifice himself, and has at his disposal a powerful machine of modern design, no dirigible can stand against him. Thus, if, as a matter of experience in actual service, men are found of sufficient grit and grim determination to adopt ramming tactics, and to hurl themselves and their craft bodily at the gas-bag of the dirigible, its destruction is immediate and complete. There is no defence possible against this mode of attack. The crew of the dirigible may not have even the most slender chance of stopping the aeroplane by machine-gun fire; 'the attack can be made from above by a steep vol plane or a vertical dive. In the case of a large airship of the Zeppelin type, even with machine guns mounted "on the roof," the chances of defeating such an attack are remote; the speed of a machine descending vertically, or steeply, is approximately that of its limiting velocity—commonly about 150 miles per hour—leaving a very brief period in which to score a hit. Beyond this, no ordinarily fatal hit is effective under the conditions in question; no injury to the motive-power installation is of the least effect as a stopper, and the pilot is in almost perfect security in his position behind the engine. If by an exceptional chance he should be wounded, he is still able to effect his purpose, unless totally disabled.
The steep or vertical descent is admittedly a dangerous feat of airmanship, but it is not intrinsically dangerous; the risk involved is due to the structural stresses to which the machine is subjected when "flattening out." These, it is well known, may become excessive; any objection on the score of danger has obviously no weight whatever under the conditions contemplated. It is an open question whether airmen will be found ready to step forward at the critical moment to go to certain death, and so the general feasibility of ramming tactics must for the time being remain in doubt. However, there are many other modes of attack open to the aeroplane pilot, all more or less untried at present; unquestionably also there are still other methods that will in due course be devised. In the case of the non-rigid dirigible, as in the ordinary spherical balloon, it is almost certain that a hundred or so yards of barbed wire trailed, beneath an aeroplane would be a quite sufficient weapon; equally effective would be an incendiary shell, or a rocket, presuming any part of the envelope to be hit. Ordinary small-arm or machine-gun fire is comparatively ineffective, since the bullet holes are, in any case, small, and in some, of the modern machines repairs can be effected without coming to earth. However, even rifle fire has proved sufficient to bring a balloon down. It is evident that the weak point of any dirigible or airship is its liability to attack from above; in the non-rigid type, without going to the length of any elaborate apparatus, and without endangering the attacking aeroplane, almost any angular and weighty object dropped from a height cannot fail to be of conclusive effect if it fairly, hits the envelope, and likewise in the case of the rigid type—such as the Zeppelin—the structure would not stand up under a blow from, say, a steel bar of any ordinary stock section of 70 lb, or 80 lb, weight dropped from a height of 200 ft, or 300 ft. Without saying that the above are suitable methods of attack, it may be claimed that they fairly indicate the inherent weakness of the dirigible in face of attack by an aeroplane of sufficient power to master it in the matter of altitude. There are methods not mentioned here which are actually in use or in contemplation, but which, for obvious reasons, require to be treated as confidential. It is, however, in the author's opinion, quite unnecessary to carry the matter further; the weaknesses of the dirigible on the defensive are so great and of such a character as to render it quite unfit to remain an active participant in aerial warfare. It may escape for a time, and may render a certain amount of useful service, but only thanks to the circumstance the number of high-powered, fast-climbing aeroplanes is comparatively limited, and to the fact that scientific methods of attack have not yet been fully worked out or put into practice. However, even to-day, the finest of Germany's fleet of Zeppelins would be absolutely at the mercy of a modern aeroplane in the hands of a man prepared to make his one and last sacrifice. So fragile and combustible a contrivance as a dirigible, whether rigid, or non-rigid, can never, in the author's opinion, survive in the face of the rapid development of the aeroplane and the engines of offence with which before long it will be furnished.
Before proceeding to the broader considerations, it has been thought desirable to dispose of the airship as a factor in the aeronautical service—its dismissal being an initial simplification. It is not altogether important whether or not this conclusion turns out to be literally true. It may be that, in spite of all that has been put forward, the large airship may retain some degree of utility; even if this be so, the main conclusions will be unaffected. It is the aeroplane, and the aeroplane only, either as a reconnaissance or a fighting machine, acting independently or in flights or squadrons, which will in effect constitute the aeronautical Arm; and whether the considerations we discuss are strategic or tactical, it is: the potential capabilities and limitations of the aeroplane that we require to keep constantly in mind.
- ↑ The maximum speed attained by an airship is approximately 50 miles per hour; the maximum in the case of an aeroplane is considerably over 100 miles per hour; thus (Fig. 1) the tractive coefficient in the case of the aeroplane is actually greater than in the case of the airship. The reason for this is that the dead-load in the airship—represented by the envelope and its appurtenances—is disproportionately great, and the proportion of the weight that can be devoted to the motive-power installation is relatively smaller than in the aeroplane. Were it not for this fact the airship would have held the advantage until speeds about 60 miles per hour had been reached and the aeroplane after.
- ↑ Added to this, in order to detect the approach of a hostile aeroplane, the sky has to be scanned in the three dimensions of space.