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Aviation History
1915
1915 - 0061.PDF
JANUARY 22, 1915. (/JJGHT TABLE II. Model. A-i. C-i. D-i. N-i. N-2. 1. Number 19. Total resistance in water, Table I .. 20. Total resistance in water, Table I ... 21. Total resistance in water, Table I ... 22. Resistance in air, full size, at V m.p.h. 23. Resistance in air, full size, at V m.p.h. 24. Resistance in air, full size, at V m.p.h. 25. Resistance in air, full size Kpyn'HVr 26. Resistance in air of normal plane of equal maximum section C033AV3 = H5# 27. Ratio in per cent H,/H;! = 28. Fineness coefficient, assuming II., as best value H4/H5= F 29. Excess resistance at 60 m p.b., over 1602-2 = E 30. (Kpe0'14) from model tests, "form factor"... = F' ... Rr + R, = Rt ... R'r + Rt + R', K:irt = R", Rt/8i3 = II1# ... R't/8i3 = H2# R",Si3 = H3# ^^(Rayleigh) = IL,# 1350-15 1,967 1,968 2,640 2-42 2-42 3*24 3-48 io-10 107-2 o-34 * 0-0000258 1592-2 15,220 16,030 18,500 18-72 19-72 22-75 20-56 114-5 90-3 0-179 9"5# 0-0000364 1593-2 12,075 '8.515 15,45° '4-85 22-77 19-00 17-16 77'7 90-3 0-22I 9-5# 0-00002984 i59i-4 9,900 9,840 13,600 12-18 I2- 15 16-73 15-09 86-5 90-00 0-156 4'04# 0-0000239 1602-2 6,390 8,363 9,950 7-86 10-29 12-24 11-05 ni-6 90-3 0-0991 — 0-0000183 N-3. 1617-2 7,498 6,936 11,518 9-22 8-53 14-17 12-78 141-0 90-25 o • 0906 i-74# O'0000201 Model of radically different form from others. except the residual resistance is determined on the basis of relative areas and relative speeds, being proportioned to the latter in accord ance with the exponent determined for the variation of residual resistance of the model, instead of using the law of the square. Line 24 is an approximation, assuming the resistance to be directly proportional to the cube of the linear ratio at "corresponding speeds." Line 25 is computed on the basis of the Lord Rayleigh method, which has been found reasonably satisfactory for the comparison of dirigible models in England. In England ebonite models I in. in diameter were used in water, and in air gold-beater's skin models 3 ft. in diameter. The National Physical Laboratory formula, which is based on the Lord Rayleigh method, is H=icpi'0'14L1'88V1"86, in which K is a con stant of form to be derived by experiment, p is the density of the medium in which the experiment is carried on, v is the kinetic viscosity, L is the length in feet, and V is the velocity in feet per second. This method has been introduced at the suggestion of Naval Con structor J. C. Hunsaker, in commenting on the proofs of this paper as originally written. He further suggested the comparison, with line 24, which is of peculiar interest as shown by the percentage relation of these values. Investigation of the relation of the two methods as per line 27, shows that if we confine the use of the "approximate" method to the " corresponding speed," as per the Law of Comparison, the values as determined by Lord Rayleigh's method should be 90-25 per cent, of the values attained by the approximate method, for models one-ninth the full size, and 107 per cent, for models one-quarter the full size, so that the approximate method, which is much simpler, can be used with a fair degree of accuracy if we put it in the form Hfi = o-ooi76K2'i'rt This assumes p' Line 26 givea the head resistance of a plane ot the maximum section according to Eiffel's coefficient for fiat plates normal to the wind. Line 28 gives a fineness coefficient based on the comparison of lines 25 and 26. Line 30 gives the value of KpiO'i^ for each ot the boat models. These "form factors " are of interest when compared with the values of the same coefficients for models of dirigibles in which the form is unrestricted by requirements such as enter into the flying boat pro blem. Thus to make the comparison more ready, Table III is compiled : TABLE III. o 00123 and '3- Model. N.P.L. Beta ... Gamma B.F. 36 Lebaudy B.F. 32 Type. Dirigible . ,, ,, ,, • ,» ,, Kpv'u (air). •0000152 . -0000164 .. -0000165 .. -0000142 Model. A-i .. C-i ... D-i ... N-2 ... 0000124 N-3 ... .. -0000140 Type. Kpv'u (air). Pontoon ... -0000258 Flying boat -0000364 ,, -0000298 ,, OOOO183 Owl ... -0000261 It thus appears that the N-2 form, while superior in the air to the other flying boat forms, may still be improved, and if the efficiency of the Lebaudy form could be approached its head resist ance might be reduced to 68 per cent, of the present value. However, when we come to consider that the total head resistance of the N-2 model is only about 11 # in air at 60 m.p.h., and consider the difficulty of construction involved, particularly if the surface running qualities are to be retained, we see that the present forms are reasonably satisfactory. While the possible saving of 3-5 Ife. head resistance is worth considering, it must only be considered if its attainment does not involve increased weight, cost or difficulty of construction to such a degree as to outweigh the small gain possible. Such savings increase in importance in proportion to the square of the speed desired. It thus appears that increased efficiency must be aimed at in those members of the structure which offend to a greater degree than the hull, namely, the multiplicity of the truss members ; and the exposed power plant, especially the water-cooled power plant. The peculiar form of Model 1617-1 is due to an attempt to utilise the advantage of the flying boat arrangement of bottom and step, together with a good shape stream-line hood in place of the ordinary pontoon with the hydro-aeroplane type of machine. It is interesting to note that the coefficient of fineness of this model is less than that for Model 1602-2, which indicates that per unit of area of maximum section the resistance of this form is slightly less than that of the 1602 model. An inspection of line 29 will show that the probable reason for this is due to the very low value of the frictional component of the resistance of this model. How ever, when the comparison is based on /cpi'"'14, the form factor used by Lord Rayleigh, this form is much coarser than the 1602 model. An independent experiment is worthy of note at this time. An experiment was made to determine the existence and amount of " nosing" torque on model 1350, at various angles of incidence. Unfortunately the apparatus carried away before the experiments were completed, but it was found that there is a " nosing " torque of about 90 ft. lbs. when the deck of the pontoon is parallel with the line of flight. To this torque should be added that due to the head resistance, which is approximately 5-42 lbs. x 5 ft. = 26-10 ft. lbs. or a total torque due to the pontoon tending to make the machine head down of about 116 ft. lbs. This with the c.p. of the diving rudder 15 ft. abaft the e.g. would require the diving rudder to carry a negative load of about 7-75 lbs. if the machine were " balanced " for all other effects, at 60 m.p.h. Additional experiments on submerged models are contemplated with a view to determining the stream-line flow about the models as a means of arriving at improvement of form, and other experiments to determine the effects of the cockpit openings, sponsons, &c, and a more complete series for determining torque at different angles. ® ® ® ® Not a Zeppelin Raid. AN interesting report just issued by Professor Jenkins, of the Godlee Observatory, Manchester, recalls the rumours of a Zeppelin raid on the Cottonopolis district on October 13th, when "a strange luminous body was seen in the sky emitting what seemed to be explosions. A large meteorite was found next morning at Appley Bridge, near Wigan, and Professor Jenkins reports that it is the usual sort of meteorite, its total weight—it is in two pieces—being 28 lbs. 13 oz. It is composed chiefly of silica and magnesia, with traces of other elements, and its specific gravity is 3-3 times that of water. The size is 96 ins. by 9-1 by 6-6, and the stone ranks as the second largest known to Great Britain, the larger one being a meteorite which fell in Yorkshire in 1795. 6l
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