MONOPLANE OR BIPLAX1-4: ?
The Pros and Cons of Each Technically Reviewed.
By Clifford W. Tinson, FR.Ae.S.
THE question of monoplane or biplane has always been one of interest, and one upon which a general agreement has never been reached. Contending parties rarely see from the same angle in making comparisons, and the difficulty is to assess fairly the value of a variety of features in discussing the merits of the two types, the trouble being that it is not always possible to visualise a common datum to which to refer the values of features so widely differing in character as are the purely technical or aerodynamic ones and those of a practical or commercial nature.
In some respects, the difficulties referred to are comparable with those one meets in discussing the value of an automobile to an individual. In this case, we have a common ground in that all are agreed that the value is proportional to the performance, cost, comfort and depreciation, but in comparing the cost of running two different cars there is the difficulty that one cannot reduce everything to a common point of reference. Depreciation, for instance, is intimately connected with the time factor, whereas fuel and oil costs depend on distance, whilst tyres and repainting are governed to a great extent by both time and distance.
In much the same way, arguments for and against the monoplane or the biplane cannot be settled definitely to everyone’s satisfaction, for in place of the time and distance factors there are, as principal headings, the technical and commercial—or perhaps it is better to say the aerodynamic and practical sides to the question. Divested of all technicalities, there does not appear to be any marked superiority in the observed performance of either type, other than that one imagines some aircraft are travelling faster because of their small dimensions, or because they present a cleaner outline. That the cleaner appearance of a monoplane should be followed by a definite gain in performance is reasonable, yet it must be admitted that machines having the same
horse-power and designed for similar duties, whether they be monoplanes or biplanes, have approximately the same speed and rate of climb. Any superiority of one type must then be connected with carrying capacity or duration of flight without re-fuelling. If this advantage is claimed for the monoplane, it is definitely superior from the aerodynamic point of view, and should give the best values in ton-miles-per-gallon, but can such a claim be established ?
It is much easier to concentrate the argument round the comparative qualities from the point of view of general utility and suitability for operational requirements. The query has often been raised as to why it is that so many long distance flights, particularly the Transatlantic attempts, were made on monoplanes, the inference being that there must have been some subtle quality in the monoplane which was lacking in the biplane to influence those who were responsible for these projects in their selection of a suitable machine.
Perhaps, the reply is that, speaking generally, it was undoubtedly necessary to select a machine actually in existence and one which could be purchased at a reasonable price, since the design and construction of a special machine is a costly matter. Moreover, a special machine is not necessarily more reliable than one of a type which has been in production for some time ; usually the reverse is the case.
Apart from the advisability of selecting a machine of known merits and reasonable price, there was the need for a considerable space in which to mount additional petrol tanks for the Transatlantic attempts and possibly this, as much as anything, was the factor controlling the choice of the machines used.
A high or low wing cabin monoplane is readily adapted to carry additional petrol tanks, probably more readily adapted than any other class of aircraft if one excepts flying boats —which one excepts from this discussion as being a special type of aircraft. One can easily imagine that a fuselage suspended below a continuous wing structure, or placed on top of it, will require a minimum of internal bracing for torsional rigidity, and if this is indeed so, a considerable space inthe cabin portion is available for the disposal of load, whether it be additional petrol tankage, passengers,
or goods. No transverse bracing is necessary in the portion of the body which lies between the front and rear wing spars, and as the monoplane has a greater wing chord—and therefore a greater distance between the spars — than a biplane of similar wing area, there is at once a good reason for making a machine a monoplane when the greatest extent of unobstructed space inside the cabin part oi the fuselage is a desirable feature. The high wing monoplane, moreover, is usually found to be a more convenient type when one considers the facility for entering and leaving the cabin, and has the definite advantage of allowing for the installation of fuel tanks in a high enough position to give gravity feed to the engines. This advantage it shares with the biplane of course, in or on the top wing of which can be mounted fuel tanks, in con
trast to the low wing monoplane which may have equally good or better characteristics when considering cabin space and ease of entry, but must have pump feed for the fuel.
Comparing the monoplanes, one would expect the high wing machines to be at a disadvantage on a windy day whilst on the aerodrome because a cross wind would presumably create a stronger tendency to lift the windward wing and turn the machine over. This, however is not the case in practice, for the high wing design lends itself to an undercarriage of considerable track whilst a good broad track is what is required for stability in these circumstances.
Monoplanes, generally, fall into one of two distinct categories ; either they are what is called” full cantilever “or ” semicantilever.” Typical examples of the former type are the Fokker and Junkers machines, whilst nearly all the American cabin monaplanes fall in the semi-cantelever class.
With regard to the semi-cantilever class, it is clear that some saving in weight can be effected by running struts out to a point approximately half way between the fuselage and the wing tips at the expense of the resistance of the struts and the interference effect at their joints with fuselage and wing, and bracing in this way has a considerable advantage in making the wing structure more rigid, that is to say the bracing reduces the torsional flexibility of the wing. For the benefit of those unfamiliar with aeronautical work it may be as well to explain here that the air pressure distribution on a wing in flight is more concentrated, or rather of greater intensity, toward the leading edge when. the wing
is at a large angle of incidence (corresponding to low flight speed), and this concentration is referred to a point called the “centre of pressure” at which a single force which would produce similar results is supposed to act. At small angles of incidence (corresponding to high flight speed), the centre of pressure moves back along the chord.
A wing is a comparatively flexible structure ; it is susceptible to a certain amount of deformation in flight according to the position of the centre of pressure, and if the unbraced portion of the wing is of considerable length as in full cantilever type, the torsional deformation in certain conditions of flight would prove serious if steps were not taken to guard against it. Fig. 1 illustrates di agrammtically the deformation of the wing tip of a cantilever wing with respect to the root.
In order to prevent excessive deformation, it is usually necessary to cover cantilever wings with plywood or aluminium sheeting instead of fabric, thus ensuring that the amount of aileron control under all conditions is sufficient and not affected to an appreciable extent by the wing twisting.
At this point, it may be necessary to explain the relation between twisting of the wings and the efficiency of the aileron control. The movement of the aileron does two things ; it alters the camber of the wing and it alters the angle of incidence of the wing tip relative to the root. To obtain the desired rolling moment it is necessary for the downward moving aileron to produce an increase both in camber and angle of incidence, and the reverse applies, of course, to the upward moving
aileron. If the movement of the aileron is accompanied by a twist of the wing tip away from the desired change of incidence, then the effect of the aileron is seriously reduced and, in extreme cases may even be reversed. Fig. 2 illustrates the change of camber and incidence by downward aileron movement when there is no relative twist between the wing tip and wing root, and. the state of affairs which would result from a very flexible rear spar combined with a fairly rigid front spar. In the latter case, it will be seen that the change o: incidence it, in the opposite direction to that required. As the speed of the machine is increased by throttle opening or by diving, the centre of pressure retreats, and if the wing is flexible the power of the aileron is decreased. What is still more serious, there is the possibility of wing flutter setting in as well as loss of aileron power. A curious effect of flexi
bility is the reversal of the aileron control beyond a certain speed, without wing flutter occurring at the critical speed of the change over. In this case, the aileron power was normal at slow speeds and gave the usual response, but on increasing the speed the aileron power gradually failed until, beyond a fairly definite speed, the effect of the aileron was reversed, and this condition may be likened to riding a bicycle with the hands crossed over to the opposite handle bars. Needless to say, this peculiarity was discovered during the experimental stage and was rectified.
Such a state of affairs as this must not be allowed to take place at any speed, and consequently the majority of cantilever wings are covered with stiff material such as ply-wood to prevent twist in flight.
This requirement results in the wing of a full cantilever machine being rather heavier than might appear to be necessary on casual consideration, and the semicantilever machine is thus at an advantage from the point of view of wing weight even when the weight of bracing struts are taken into account and allowing for the power absorbed by and the resistance due to them. To eliminate the disadvantage of the flexibility of the cantilever wing, a system of internal bracing has been tried recently, for which the inventors claim exceptional rigidity for light weight and no increase of air resistance. This is illustrated in Fig. 3 and the explanation of the theory
of it would take more space than is available here, but the rigidity is obtained by a special system of internal struts and ties, shown in heavy lines in the diagram, which prevent the single spar from twisting, without assistance from the wing covering.
Should this system come into vogue, the objection to the wing weight of a cantilever machine may disappear. From the point of view of lightness and rigidity of wing structure there is no type as yet which can compete with the nor
of entering and leaving the cabin is equal to a monoplane, the biplane would be the better type. In the smaller sizes, however, such as four seaters, it is questionable whether the slightly greater structure weight of the monoplane is not compensated for by the advantages
gained in other, and purely practical directions.
While it is true that all machines used in the last Schneider Trophy race were monoplanes, it is not without interest to note that the Gloster Co., deserted the biplane type with some reluctance and purely by reason of the exceptional conditions imposed by the requirements. Their experience, backed by extensive wind-tunnel investigation and research, convinces them that, for pure speed, the biplane is the equal of the monoplane.
For weight carrying with a maximum cruising speed the biplane would again appear as good or better than the monoplane. At a recent meeting of the Royal Aeronautical Society, Mr. C. R. Fairey (in speaking of the Fairey long distance machine on which Squadron-Leader Jones-Williams and Flight-Lt. Jenkins unfortunately lost their lives in the Cape Flight), stated that he could have produced a biplane for a structure weight of 20% whereas his monoplane had a structure weight of 26%. The reason for sacrificing this economy was the purely practical one of the ability to utilise the wings as petrol tanks, thus permitting the fuselage to be made of small cross section and low resistance, and as it was necessary to
carry some 1,200 gallons of fuel and feed it to the .engine with the maximum of reliability, the accommodation of this fuel in the wings of a high wing monoplane was a very strong point in favour of selecting this type.
mally braced biplane, whilst the facility of arranging for folding wings in contrast to a monoplane may be a strong point in its favour in cases where housing accommodation is restricted. The fact that the biplane is lighter is an excellent reason for constructing civil aircraft in this form, for they will naturally carry more paying load for a given power and, in the larger sizes such as the twenty-seaters, where the convenience
In conclusion, one. may say fairly that practical considerations are ultimately the governing factor. Aerodynamic superiority would seem to be slightly favourable to the monoplane, structure weight rather strongly favourable to the biplane, the combination of the two giving the biplane a small advantage, but so small as frequently to be masked by operational requirements.