FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1931
1931 - 0512.PDF
SUPPLEMENT TO FLIGHT 36 MAY 29, 1931 THE AIRCRAFT ENGINEER enclosed by the curve is the displacement to the loadwater line. The C.B. or centre of buoyancy is deter- mined by cutting out the shape of Fig. 5 from fairlystrong cardboard and suspending it freely from each of the points A, B, C in turn. In each case a plumbline is dropped from the point of suspension, and the point where all three plumb lines intersect is the centreof buoyancy. This may appear at first sight to be a rough and ready method, but is, in fact, quite reliable.It will be found in practice that for all working pur- poses the same result is arrived at by this means as bycalculating the displacement to load water line and position of centre of buoyancy by Simpson's rules. Tank Tests.—In tank tests there are important aspectswhich have to be neglected, such as the effects of alight- ing and the behaviour of the machine just at the periodof take-off. On the other hand, small differences in the shape of the planing bottom have unexpectedly largeresults, and tank tests show clearly the best forms to use to minimise resistance and make for clean running.Model float testing without the employment of a wave- making apparatus simulates the worst conditions asregards dynamic stability. Planing stability is improved on water with a moderately roughenedsurface. Wing Setting to Hull.—One of the important resultsof tank tests is to determine the most satisfactory wing setting and trim for any given design. The sum ofthe angle of incidence, the angle between steps and two degrees allowed for " squatting " by the stern shouldbe two or three degrees below the stalling angle of the wing section chosen. For commercial machines the take-off is generallyconsidered most important, and the wing setting is arranged to suit this condition. Take-off.—For the calculations on time to take-off,and length of run to take-off, the factors considered are: — 1. The effective thrust. 2. The water resistance of the hull. S. The air resistance. 4. The wing area. Maximum water resistance occurs at approximately30 per cent, of the take-off speed. The water drag and air drag are added to give totaldrag, at all speeds up to take-off speed. On the same graph are plotted the propeller thrust curves. Thedifference between thrust available and total drag at any speed gives the force available for acceleration,from which the time and take-off can be estimated. The determination of the lines of the hull, and veri-fication or modification of the same by means of tank tests represents the first stage in the design of the hull.When these lines are considered satisfactory, it is usual to draw them out full scale to fair them, and then thefollowing are investigated, in conjunction with the general arrangement of the machine: — Hull construction. Strength of hull. Stability of hull. ON SOLID RIVETS. BY M. LANOLBT, A.M.I.N.A., A.M.I.Ae.E. Rivets provide one of the best methods of fasteningtwo parts together permanently. They will, however, only carry their load in shear. Any tension on theheads tends to burst them apart, loading them in a way they are ill-fitted to stand. Fig. 1 shows a use of rivets which is bad. Understrain, the shackle will only be prevented from opening by the stiffness of the material and by pulling againstthe rivet heads. The result may be as illustrated in ITig. J. Had the shackle been designed as in Fig. 3, though not perfect, it would have been much better, and its ten- dency to straighten out would have been less. This simple and perhaps painfully obvious example serves as an introduction. The more complicated cases of rivet head tension, caused, perhaps, by drumming or vibra- tion of a structure, may not become so obvious until failure occurs, but the possibility should always be present in the designer's mind. A further example is in the popular single lap (see Fig. 4), which tends to alter to the form shown in Fig. 5. Here the number of rivets, as in a flying-boat hull seam, may be sufficient to ensure that the tension on each is minute. The double strap form of joint com- pletely removes the possibility (Fig. 6). Manv types and shapes of rivets are known to engi-neering, from the square-headed tap rivet to the big taper-shank pan head rivet. The only two in commonuse on aircraft structures are the snap head (Fig. 7i and the countersunk (Figs. 8 and 9). The snap headis made in all sizes from 1/16 in. diameter upwards,, but the countersunk rivet cannot be used in plates ofless than 20 s.w.g., and consequently it is not found in general practice below £ in. diameter. The standard sizes of heads and the length of shank which must be allowed for forming them are here tabulated. TABLE 1. DU. In. A 1 1 Snap Head. D. In.Oil 0-16: 0-22 0-270-82 0-440-56 0-86 T. In.004 006008 0'06011 015010 022 S. in.0-06 OOS012 0-15017 0-280-20 0-86 Lengthto form head. in.0-08 012016 0-160-28 0-81080 0-47 Countenunk Head. D. in.0-10 015020 0-260-80 0-400-50 0-60 T. in.0-03 0-040-05 0 06008 010018 01S Length to form head. In. OOS 0-06 0-oe 0-08 009 0 IS 018 019 The " Length to form head " is, of course, the length of shank standing proud of the plate surface when the rivet is inserted in its hole. The total length of met required is this dimension plus the thickness of materift through which it passes. The forming and holding-up tools require a certain space bigger than the head diameter, and, when tne rivet is used close to the flange of an angle, all™™"£ must be made. Table 2 gives average allowance, but w» conditions vary according to the type of tool useu. TABLE 2. Rivet Shank Dia. Dimension 'A." In. TV iA i In.0-13 0-170-21 0-240-28 0-360-44 0-53 Rivet Strengths. The sheaf strength of a rivet is given by lte sectional area of shank multiplied by the m
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
