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Aviation History
1923
1923 - 0332.PDF
JUNE 21, 1923 " The distribution of material in the spars could be improved upon, with resulting reduction of weight towards the tip and an increase of strength and resilience, since it is safer to have a spar that is strongest at the root and whose strength slightly diminishes towards the tip than vice versa. " With regard to the wing construction, it is pointed out that the type of spar consisting of thick spruce flanges and deep, thin three-ply webs has proved to be unsatisfactory on account of the great difference in stiffness between the two. resulting in great stress being thrown on the webs, with consequent failure of these members. It is pointed out that the ribs would require diagonal bracing in the bays between the spars. "No arrangement or details of the controls have been shown ; it must be mentioned that these are among the most important features of any design. " The fin and rudder design appears correct, but the area of these, members would probably have to be increased." Space does not permit of publishing the complete calcula- tions, etc., carried out by the designer of " Norbet," but the following synopsis should be of interest. The wing section used is that known as airscrew No 4, particulars of which are contained in Advisory Committee for Aeronaiitics Reports and Memoranda No. 322, which can be obtained from H.M. Stationery Office, Kingsway, W.C. 2. The designer makes an estimate of the minimum sinking rate, based upon the maximum value of ( — )2 X KL> and arrives at the result that, with a wing loading of 2 lbs./sq. ft., the minimum sinking rate will occur at an angle of 4°, and will be 2-75 ft./sec, at a speed of 46-9 ft./sec. (31 -9 m.p.h.). The following approximate weight estimates are then made : Fuselage, 26 lbs. ; seating, 6 lbs. ; controls, 14 lbs. ; instruments, 7 lbs. ; wings, 96 lbs. ; tail unit, 16 lbs. ; under- carriage and tail skid, 15 lbs. , total empty weight, 180 lbs. The area assumed in these estimates is 160 sq. ft., and, assum- ing a pilot weight of 160 lbs., the total loaded weight becomes 340 lbs., giving a wing loading of 2-12 lbs./sq. ft., which is slightly above the original estimate, and would probably give a slightly higher sinking speed. As the maximum KI; of airscrew 4 is 0-774, the landing speed should be in the neigh- bourhood of 34 ft./sec, or 23 m.p.h. The plan form chosen for the wing is slightly tapered, with straight tips and a span of 32 ft. ; the maximum chord is 5 ft. 7 ins., and the chord at the tip 4 ft. 3 ins., giving an area of 160 sq. ft. Taking moments about the nose of the fuselage, using the item weights previously estimated, the e.g. is found to lie 4 ft. 6| ins. aft from the nose of the fuselage. The designer thinks that this position would probably result in a slight up-load having to be carried on the tail plane. [It is not possible to estimate this accurately, as in the tests described in R. and M. No 322 the position of the c.p. was not determined.—ED.] The following item drags are estimated at 4° incidence and a speed of 47 ft./sec. : wings, 20 lbs. ; body, 6 lbs. (drag coefficient of 0-15 assumed); wheels, 0-415 lbs.; axle, 0-025 lbs. ; tail unit, 1-26 lbs ; total drag, 21-7 lbs. By taking moments about the ground line, i.e., a horizontal line tangent to the wheel, the vertical position of the CO. is found to be 3 ft. 6£ ins. above ground line. As pointed .out in the Judge's criticism, no estimate has been made of the vertical position of the centre of drag in relation to the e.g. In getting out the spar stress calculations, the designer has . assumed the c.p. to be atv0-3 chord from the leading edge for the forward position, and at 0-5 chord for c.p. back position. The maximum fibre stress in the front spar for c.p. forward position is found to be 3,980 lbs./sq. in., which is satisfactory, as the strength of spruce and three-ply is assumed to lie 4,000 -lbs./sq. in. Still, with c.p. forward, the stresses in front spar at 6 ft. and 10 ft. out from centre line are 2,191 lbs./ sq. in. and 929 lbs./sq. in. respectively. As pointed out by the Judge, the distribution of material in the spars could be improved upon. In the case of the rear spar, with c.p. back, the maximum spar stress is found to be 3,903 lbs./sq. in., while at 6 ft. from centre line it is 1,904 Ibs./sq. in. These figures are based on a load factor of 4. Estimates of spar strengths laterally indicate that the crippling loads are above the stresses met with on a load factor of 4, so that the spars may be assumed to be up to strength. Very thorough stress calculations are also carried out for ribs, tail plane, elevator and rudder, and for the fuselage, both in bending and torsion, as well as for undercarriage and ailerons. Altogether the designer has taken very great care in his stress calculations, and so far as it is humanly possible to foresee, the structure should be up to strength if sound materials are used and good workmanship insisted upon. Nevertheless, neither we, the judges, nor the designer can accept any responsibility for possible breakages. The aerodynamics of the " Norbet " are shown in the accompanying curve of gliding angles on base of gliding speeds in ft./sec. It will be seen that the best gliding angle occurs at 47 ft./sec. (32 m.p.h.), and is 4 degs. 20 mins., equal to 1 in 14 approximately. Construction of " Norbet " The fuselage is a rectangular section, wire-braced girder, built of spruce. The maximum load in any longeron is found to be 551 lbs. and the necessary strength provided by using main longerons of spruce J ins. square. Generally speaking, the struts in the rear portion of the fuselage are h in. square, getting thicker as the pilot's cockpit is approached. For actual sizes see drawings. The maximum load in a wire is found to be 172 lbs., so that, assuming 80 ton wires, the area necessary will be 0-00096 sq. in., or a diameter of 0 0012 in. The nearest diameter 80-ton wire can therefore be used throughout. The designer does not provide any drawings, nor any indication whatever of the metal fittings to be used in the fuselage. A variety of types could be chosen, but personally we would rather have seen the fuselage built up without metal fittings, which are always a nuisance where amateur construction is concerned. The writer of these notes has used bracing wire without fittings of any sort, simply passing it through holes drilled in the longerons, one on each side of the strut end, and the wire bent to a flat U before being inserted. This method, however, tends to split the wood of the longeron, owing to the angle of the wire, unless a lashing is put around the wires and strut end near the longeron (To be continued.) ALEXANDRA DAY IN THE AIR : The operations of the rose-sellers are gradually extending. In ourphotographs are seen the bevy of beauties who went over to Paris on Wednesday, June 13, in the special Rolls-Royce Handley Page aeroplane. ; : 332* ^
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