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Aviation History
1945
1945 - 1301.PDF
F LIG HT JULY 5TH.-IO.45 TH.E MERCHANT FLYING SHIP development of.the large flying boat, the •"merchant flying ship." The four main contentions are' as follows: — The flying boat is inferior in all respects to the corresponding landplane built to the same operational specification, and with equal design merit. Briefly, this means the load-carrying capacity, or the pay-load, - , will be less, and the cruising speed lower, hence less economical as a commercial proposition. This is true of the small flying boat because it has to have a hull of greater depth than necessary for the required accommodation in order to obtain adequate seaworthiness, with -sufficient water clearance for the propellers and tail unit. Further, the best hull form to meet the required water performance has an air drag about ten per cent, greater than the best practical streamline landplane fuselage- However, this does not necessarily mean that the efficiency is relatively so much lower that it cannot be an economic proposition under certain operational requirements. No Upper.Limit On the other hand, when the flying ship is considered, the picture changes rapidly with increase in size. Both in this country and America, estimates based on the best design data available indicate convincingly that the struc- ture weight decreases—and hence the disposable load and the aerodynamic efficiency increases—progressively with size without predicting an \ipper limit in comparison with the corresponding landplane. It will be realised in this article it is possible to state only the reasons for this con- viction ; positive proof to satisfy the incredulous could be obtained only by building and operating the two types. The primary design feature, not unforeseen, responsible for the above reversal in this state of affairs is found when the landing and, of course, the take-off arrangements of the two types are compared. Secondary effects, dun: to wmg loading and hull dimensions, afford a iurther valuable contribution. No new problems associated with the take-off and land- ing requirements arise with increase in size of the flying boat. Hydrodynamic efficiency, dynamic and static water stability, clean running, etc., usually denoted by the term seaworthiness, are now well denned as the result of exten- sive tank tests of models and full scale operational experi- iii .. Two forms of wing-tip float retraction. (Left) The consolidated Cata- Hna. (Right) The Martin PBM. 1. Semi cantilever and cantilever wing-tip floats. The H.A. 138 and the Sea Ranger. ence. Increase in size, therefore, is primarily a changt in geometrical scale. It is otherwise, however, in the case of the landplane With each successive major step in size the landing geai presents a new and difficult problem, by no means yet solved. Four, probably more, separate landing units will be necessary, all capable of withstanding direct landing loads during all, or part, of the landing run, and obviously must be retracted into either the wings or fuselage. Provision of stowage space in the wings for the main undercart is by no means juasy, as there is an aerodynamic limit to the wing thickness ratio, and engine nacelle dimen- sions do not increase in proportion to scuie, a difficulty which would be increased further if the caterpillar track type had to be, substituted for wheels. It is not suggested the problem is incapable of solution, but irom the pro- posals so far examined it would seem likely that the additional weight and drag involved would not be found acceptable. On the other hand, the flying ship need have only two retractable units, namely, the wing tip floats, which take no share in the main landing 'loads, under normal con- ditions, but must be capable of standing up to a rough sea. Their function is to provide, when on the water, a -righting moment to counteract the inherent instability of the flying boat hull and, if present, the upsetting cross- wind rolling movement on the wing when at rest or manoeuvring, during take-off and alighting, at speeds up to about 20 knots. Above that speed hydro-planing begins and, until airborne, the hydrodynamic forc.es acting on the hull planing bottom endow dynamic stability. There- fore, at high speeds, they are arranged to be clear of the water, and so not subjected to heavy loads, except during a forced landing in a rough sea. Retraction into the wings is comparatively simple and has little detrimental effect on wing structure weight, as the stowage position is well outboard as against well . inboard for the usual • ' - undercarriage. Further, unless the requirement is either very high speed or extreme range, retraction (apart from the complication and cost involved) may not be worth while be- cause their low drag form will not be any greater handicap than the extra weight of re- traction gear. Furthermore, with the introduction of the new type engine pre- viously mentioned, and contra - rotating air- screws, adequate sea- worthiness is obtain- able without a propor- tional increase in hull depth. In both types,
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