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Aviation History
1946
1946 - 1532.PDF
140 FLIGHT UGUST 8TH, 1946 Portable Weighbridge Strain-gauge Method Used in Compact Equipment THE advantages obtained from the ability to takethe weighbridge to the aircraft rather than thereverse, are tolerably obvious. Not only is it some- times inconvenient to move aircraft, but it is also very often desirable to obtain the structure weight of various sections of a prototype during the process of construction. Again, there are many occasions when, following modifica- tions or the necessary work involved in renewing a C. of A., an aircraft must be weighed and a fully portable type of equipment will be of considerable advantage. In the American Cox and Stevens electronic weighing kit, the agency for which has been acquired over here by British Aviation Services, a now well-proved system of strain-gauge recording is used. Small cylindrical "cells" are placed between the jack heads and the three aircraft jacking points, and the readings from these are separately recorded on the main instrument. These "cells" are, essentially, spring balances of a rather unusual type in which the springs are. in fact, short steel columns. When these columns are compressed, resistance wire strain-gauges determine the very small deformations with varying loads. Naturally enough, the method of recording the resist- ance variation must be comparatively involved, since the changes are extremely small (10,oooth of an ohm) and cannot be measured by normal means, while allowance must be made for the additional and varying resistances of the plugs and contacts connecting the ceils with the recording instrument. For this reason, a four-arm bridge One of the three strain-gauge cells shown in position at the jack-head. , network is used. This is unbalanced under load, and any changes in the incidental resistances affect the result only in proportion to the total bridge resistance. The small voltage output is then measured by electronic means. Before weighing starts, each cell is adjusted to give a neutral reading on the electronic indicator. The aircraft is then raised on the jacks with the three cells resting be- tween the jack heads.and the necessary adapters at the jacking points. The needle is then brought back to zero for each cell in turn and the weight read off on the weight dial. After the figures have been taken, the jacks are lowered and the zero weight reading is again checked to The complete weighing kit is carried in a normal-size suitcase,with the cells, necessary adapters and measuring gear. M/5 make sure that nothing has changed during the process. Given reasonable conditions, and an aircraft with con- venient jacking points, the entire process can be completed in little more than half an hour, and most of this time is, of course, taken up in the business of ensuring that the loads are accurately distributed. During a demonstration with British Aviation Services' own Airspeed Consul, the work was completed quite quickly, and the figures obtained were 3,200, 3,012 and 163 lb—making a total of 6,375 lt>. The variation between the port and starboard figures was caused by the fact that the Consul was ready for flight, and the fuel loading was unbalanced. The complete set, which is carried in a normal-size suit- case, includes accessories such as levelling bars, measur- ing tape and plumb lines, to enable the operator to deter- mine the centre of gravity, and the power required is taken from the aircraft's normal 24-volt battery, or from a bat- tery cart. Suitable jacks are normally available at the scene of the weighing, and the set also includes the neces- sary jacking point adapters for all types of American air- craft. Unfortunately, our own aircraft types are by no means so conveniently standardized in this matter, and British Aviation Services have made a large number of additional adapters as well as a simple attachment to en- able the same type of cell to be used when an aircraft part, or its tail, is sling-hoisted. In order that there should be no recording errors, the cells, cables and instrument controls are each colour-coded. The weight of the entire kit is in the region of 50 lb, and the maximum test errors are less than 5 lb at reason- ably low maximum figures. Each cell is capable of record- ing loads up to 50,000 lb. The set we saw was designed for recording a maximum three-point weight of 75,000 lb. At present there are' only two of these kits in the country, but B.A.S. are obtaining a limited number of further sets. In the meantime they are instituting a system whereby they can "proceed with any necessary weighing at an" factory or airfield in the country. FLETCHER AIRSCREWS HIS many friends in the industry will be interested to learnthat Mr. A. A. Fletcher has resumed airscrew design: It may be remembered that during the war Fletcher-designed air-screws were used on Tiger Moths, Magisters arid Messengers, Auster IVs and Vis, and Oxfords. Under the title "FletcherAirscrews," a division of Philidas, Ltd., the firm will design and manufacture wooden airscrews, specializing particularly onlight aircraft types. The offices and factory are at the Aero- drome, Reading.
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