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Aviation History
1947
1947 - 0185.PDF
FEBRUARY 6TH, 1947 FLIGHT 131- FLYING - BOATS Their Case Ably Argued by Two Distinguished Lecturers Especially Favourable in Large Sizes ^ DEFERENCE was made in our Editorial Comment last week to the two lectures given recently on •* the place of the flying-boat in commercial aviation of the future : " Recent Developments in Flying- boats " by Mr. Henry Knowler, Saunders-Roe chief designer, and " The Flying-boat and its Commercial Future," by Mr. Peter Masefield, Director-General of Long-Term Planning and Projects at the Ministry of Civil Aviation. Mr. Knowler was giving the nineteenth Thomas Lowe Gray Lecture to the Institution of Mechanical Engineers, and Mr. Masefield was addressing the Air Transport Session of the U.S. Institute of the Aeronautical Sciences in New York BY way of background to his lecture Mr. Knowlershowed pictures of a number of flying-boats, fromthe Supermarine Southampton of 1926 to the ShortShetland and Martin Mars of recent times. He explained that since the hull of a flying-boat was required to plane,it must be of chine form and have a step, and the wing must be well clear of the water to keep engines and air-screws away from spray. Consequently, flying-boats were always of the high-wing type. The tailmust also be set high for the same reason. Contrary to general belief, Mr. Knowlersaid, the structure weight of flying-boats was similar to that of contemporary land-plane counterparts. Wings, tail and power installations were about the same; the hullwas slightly heavier than the fuselage of a landplane, but the outboard stabilizingfloats were lighter than the landplane under- carriage. The table gives a weight per-centage comparison between recent types of landplane and flying-boats of comparablesize, and shows that the advantage is with the flying-boat. A curve was shown to indi-cate how percentage structure weight of flying-boats had decreased with size in the course of theyears. The same reduction applied to landplanes except for the undercarriage, which might be expected to increase2-4 per cent in 200,000 lb sizes. Increase in structure weight, Mr. Knowler said, was not likely to be the limitingfactor on the ultimate size of flying-boats. With increasing size there had been a tendency towardsincreased wing loadings and decreased power loadings, accompanied by a decrease in parasite drag by cleaning-up excrescences. Fig. 1 shows the profile drag of flying- boats of modern design plotted against all-up weight. Tomake the results comparable, the ordinate used was the drag at zero lift divided by the all-up weight. The reduc-tion in drag with increasing size followed from a reduction iii relative body drag due to smaller relative surface area ;reduction in relative wing drag ; slight reduction in skin • ". Phato£raphs_ friction and drag coefficients due to increase in ReynoldsNumber ; reduced specific power-plant drag with increasing engine power; and finally it was easier, the larger the air-craft, to suppress the parasite drag of excrescences, air leaks, gaps, etc. There was an interesting relationshipbetween weight and drag in this comparison. If the struc- ture weight of a flying-boat was 3 per cent less than thatof a similar landplane, then the hull drag could be about The metal hulled supermarine Southampton of 1927. Earliermodels had wocden hulls. The Short/Saro Shetland on the River Medway. This flying boat wasaccidently burnt at its moorings. 20 per cent greater than that of the fuselage and, withequal performance, the all-up weight of the flying-boat could be 5 per cent less. Shape affects Drag In the past, hull drag had been larger than the dragof a fuselage mainly because the hull size had been dictated by buoyancy considerations rather than by theload to be carried. With increase in size this difference was disappearing. The greater drag of a hull due to shapewas caused by the sweeping-up of the aft portion, the steps, and the chines. The worst offender was the main step,and Fig. 2 shows attempts to improve matters. It illus- trates models tested in the compressed-air tunnel at theN.P.L., the drag figures representing aircraft weighing 150,000 lb at 100 ft/sec. Mr. Knowler's comments on thedifferent shapes were as follows; " The conventional step E, in general use until recently,is about 9 per cent of the beam in depth, and is V-shaped, in plan view, at an angle of 20-30 deg. This shows a 50per cent increase in drag over the faired fuselage shape A. " The faired step D is an improvement on step E and isnow generally used, but it has sometimes a tendency to cause instability on the water. "Step C, which is streamlined in plan view, and is alsofaired in profile, has had considerable tank testing, and is likely to come into general use shortly, as it has good high-speed stability characteristics on the water as well as a fairly low drag. " The retracting step B, although it has the lowest drag,has not found much favour so far, due to its complication, vulnerability, and weight." Mr. Knowler then referred to a method of reducing stepdrag which is now beins tested on a small Saunders-Roe
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