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conditions. It is certain that the tow-rope forces can be very considerable when flying in gusty air. The Use of Gliders in Civil This means that parts of the structure of both tug and glider have to be strengthened in order to withstand these extra loads, thus increasing the structural weight and lessening the pay-load Air Transport* capacity. Furthermore, the tow rope should be designed to break at a specified force in order to By Ir. L. L. Th. Huls, v.i . avoid over-stressing of the aeroplanes; this in creases the possibility of the transport glider T the start of a new era of civil air transport square power of the linear enlargement having to make an emergency landing under the possibilities offered by the numerous factor, but fuselage volume increases with the adverse weather conditions. new developments in the operation as cube power. Although a larger aeroplane well as in the design of aircraft should be analysed needs rather more cabin space per ton gross Flying an aerial train under bad weather con in order to exploit them to the full extent in weight than a small one, owing to the re ditions will make a heavy demand on the skill building up an efficient transport system. One of latively increased payload and higher stand and the endurance of the pilots. Concerning the the new appearances in the field o f air transporta ard of comfort, this value need not be in control problem it should be remarked that really tion is the towed glider, which reaped many creased in the indicated ratio. This effect is the pilot of a transport glider has one important laurels in the last war. Names like Crete, Sicily, strengthened by the fact that in a larger control lever less than his colleague in a powered Arnheim recall heroic battles in which glider fuselage an increased portion of the total aeroplane, namely the throttle, which is as im troops played a prominent part. These military available space can be used for the accommo portant a control lever as the elevator control. successes have given rise to many articles in dation of the pay load. Furthermore, the He will most appreciate this lack when landing; which the transport glider is announced as an roomier cabin and increased comfort of a because then the glider is released from the tug important discovery for economical air transport. large aeroplane will certainly promote the and is left to its own resources. If a glider pilot Aerial trains consisting of one tug and a number attraction of air transport for most classes undershoots while landing he cannot correct of gliders have been proclaimed to be a new and of passengers. this by opening up the throttles, a glider cannot valuable contribution to the future of civil perform a baulked landing. This drawback makes As the fuselage dimensions need not be in aviation. On economic as well as safety considera it impossible for the pilot of a transport glider to creased in the same ratio as the wing area, tions the author advises the advocates of the adhere to the normal air traffic regulations. the parasitic drag coefficient of the fuselage transport glider to restrain their enthusiasm and In particular, when coming in to land on a busy decreases with growing size. not to overestimate the future possibilities of airport under Q.B.I. conditions, this may lead to (2) The reduced drag results in a lower cruising this type of air transport. disastrous results. A glider cannot remain at a power per ton of gross weight; so the drag predetermined height, awaiting his turn to land. of the engine installation also decreases. The much-advertised advantages of the glider Into the present blind landing systems, which have train—high load-carrying capacity and low fuel Furthermore, in large aeroplanes the engines made such an important contribution towards costs per ton-mile—can only be obtained by can be accommodated in the wing, thus giving improving the safety as well as the regularity of sacrificing both range and cruising speed. If the a further reduction of the parasitic drag. air transportation, the transport glider cannot economic performances of a modern high-speed (3) The skin friction decreases with increasing be fitted. air liner are compared with those of a glider train Reynolds number; i.e. with increasing size. composed of the same air liner and one or more As the ratio of parasitic to total drag increases The fact that transport gliders are not suited transport gliders, a distorted view is given. All with the cruising speed the effect of the above- for scheduled air transport operations should be comparisons of pay-load capacity and fuel costs mentioned reduction of the parasitic drag, and clear from the foregoing considerations, but it of transport aeroplanes should be based on equal with it the advantage of the large aeroplane, still remains to be discussed whether a glider can ranges and cruising speeds. If the question is put becomes more marked at higher cruising speed. not successfully be used to cope with temporary thus: "Which is better suited to transport a The efficiency of a transport aeroplane de large increases of freight supply when weather certain quantity of goods over a given distance pends not only on the lift-drag ratio; the ratio conditions permit their operation. This proposi and with a prescribed cruising speed; a glider of pay load to gross weight is of equal importance. tion has a particular appeal for companies which train or one freight-carrying aeroplane?" the The structural weight of the aeroplane increases operate a small fleet of transport aeroplanes and answer is definitely in favour of the latter. This more rapidly than the gross weight; pessimists which are eager to be able to carry all freight answer could be based on a comparison of the have prophesied that this would limit the size offered. But one should not forget that a fully design characteristics of both glider train and of aeroplanes because the structural weight would loaded transport aeroplane is not capable of freighter, but it can also be based on the general increase with the cube of the linear enlargement towing a loaded transport glider without violat statement that with increasing size the efficiency factor, while the gross weight, which for a given ing the safety regulations. Perhaps someone of transport aeroplanes also increases. So if wing loading is proportional to the wing area, will remark: "We have seen Dakotas tow such large quantities of freight have to be transported only with the square of this factor. Happily, the large gliders as the Horsa." But the Dakotas were by air it is more economical to use one big pessimists are wrong, they forgot that in larger not fully loaded and, furthermore, in military aeroplane than, a number of smaller ones—as is planes the structural material can be better operations far greater risks were taken than can be the case with the glider train. The special ad exploited. But, nevertheless, there remains an accepted in civil air transport. Take-off and climb vantage of distributing the load-carrying capacity increase of the ratio of structural to gross weight of such a combination with one engine inoperative over a number of smaller aeroplanes—namely, with growing aeroplane size. This increase is will certainly not comply with civil require the increased frequency of service which can be compensated, however, by the reduction of the ments to ensure adequate safety for normal offered in this way—is lost when these smaller portion of the gross weight demanded by the transport operation. If, on the other hand, the units are combined into one train and operate engine installation and the fuel supply because air liners are designed to meet all requirements simultaneously. of the reduced drag. So it may be expected that while towing a transport glider under full load, the proportion of the gross weight which is In this connexion it is worth going into a more the air liner would be overpowered for normal available for pay loads remains substantially un detailed examination of the advantages of in operation without a glider and therefore not be changed when the size of an aeroplane increases; creasing the size of transport aeroplanes. efficient. remembering that we are still comparing a large The increase in the efficiency of a transport and a small aeroplane having identical design The use of auxiliary gliders has another dis aeroplane, determined by the ratio of pay load advantage ; that, in order to be able to cope with characteristics and cruising speed. to fuel consumed for a given range and cruising sudden increases of cargo supply, each station speed, should be attributed to the relative de Returning to the subject of transport gliders, it along the route has to be equipped with some crease of the parasitic drag of a large air liner has been shown that the splitting up of a given transport gliders, the annual utilization of which in comparison with a similar smaller aeroplane, if payload capacity by using several smaller units, will be very low and the operation therefore costly. the major design characteristics (e.g. the wing operating simultaneously, is unprofitable as far Furthermore, such demands for extra transport loading,† aspect ratios, etc.) of both aeroplanes are as the efficiency is concerned. But of even greater capacity often occur only in one direction, so in identical. For similar aeroplanes the lift coefficient importance than the efficiency is the safety of air order to avoid accumulation of all gliders on a few and the coefficient of the induced drag are iden transportation and from this standpoint the stations, the gliders have to be hauled back with tical; hence, a reduction in the parasite drag transport glider is certainly inferior to the normal out pay-load, thereby greatly reducing the coefficient results in a higher lift-drag ratio; transport aeroplane. efficiency of their operation. i.e. an increased aerodynamic efficiency of the Little is known as yet about the behaviour of an aeroplane. The reduction of the parasitic drag I am aware that I have been painting a very dis aerial train under adverse weather conditions. coefficient with increased size can be explained by couraging picture of the future of the transport Large though the experience accumulated by the the following considerations: use of transport gliders during the war may be, glider. But primarily with a view to the safety (1) Wing area and weight increase with the this experience cannot give definite proof of the of air transportation, which has to be kept at as possibilitity of operating aerial trains under all high a standard as possible, and besides, on con weather conditions. Large-scale airborne opera siderations relating to the economical side of the *Based on a lecture delivered to the Royal Netherlands Aero nautical Society (K.N.V.v.L.) tions were mainly performed when the weather problem, I am convinced that the transport glider, conditions were favourable, while scheduled air however valuable an acquisition it may be for †The optimum wins loading, as indicated by Warner in his Wright Memorial Lecture, 1943, only depends on the aerodynamic charac military purposes, will not play a leading role in transportation should be executed with nearly 100 teristics of the wing and the operating height of the aeroplane and civil air transport. per cent regularity regardless of the weather is not influenced by the parasitic drag. June 1946 199
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jun 1, 1946
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