FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1932
1932 - 0277.PDF
March 25, 1932 Supplement to FLIGHT Edited by C. M. POULSEN March 25, 1932 CONTENTS "Wires Cut" Cases in Wing [Structures. By A. E. Russell, B.Sc. A.F.R.Ae.S Page 17 The Mutual Iufluence of Engine and 'Airscrew Characteristics. By Lt. Col. J. D. Blyth, O.B.E., A.F.R.Ae.S., M.I.Ae.E 21 Technical Literature— Papers at Institute of Metals 23 Summaries of Aeronautical Research Committee Reports .. .. 23 "WIRES CUT" CASES IN WING STRUCTURES. By A. E. RUSSELL, B.Sc., A.F.R.Ae.S. A broken lift wire sounds rather terrifying, and by calculations based on the usual assumption of ignoring redundant incidence bracing, the loads found in the centre section are certainly very severe. Very few aeroplanes in existence could have a factor of safety of even 1 under these loads. The fact that many hundreds of wires have broken in the air without disaster shows that normal assumptions do not give results approaching the true state of affairs. Mr. A. E. Bussell, who is head of the Stress Depart ment of the Bristol Aeroplane Co., Ltd., and who has jireviously contributed valuable articles to THE AIRCRAFT ENGINEER, points out that at the R.A.E. the method is to assume a similar wire broken on the opposite side of the wing structure, thus getting symmetrical loading. This assumption is based on investigations carried out at the B.A.E., but no results have been published, and no mention of the difficulty is made in Air Publication No. 970. Mr. Bussell has made certain general calcula tions, and the results are given in the following article. AN engineering framework is an arrangement of mem bers (struts, ties) suitably connected together by a series of joints, such that no geometrical distortion can take place under any condition of applied loads. It is possible to fulfil this condition with a certain mini mum number of members, depending on the number of joints. For a frame lying in one plane, the most simple form is an arrangement of three members, connected by three joints to form a triangle. For every extra joint added, two extra members are required. For such a " plane frame," the minimum number of members necessary is given by 2j—3, where j is the number of joints. Likewise, the most simple form of " space frame " is the tetrahedron, and for each joint added three more members are required. Thus for a frame work in three dimensions, the minimum number of members necessary is given by 3^—6. 260 A structure consisting of the minimum number of members required to give geometrical rigidity is said to be " just stiff." In this case, the failure of any one member results in complete collapse when under a system of forces. An apparently correctly designed member might fail due to unforeseen causes, such as chemical action, vibration or damage from outside sources. In order that local damage may not be disastrous, struc tures are usually made redundant, i.e., extra members are added, so that in the event of damage to certain of the principal members, the loads are balanced by a redistribution of stress. In normal aeroplane wing structures, redundancy is provided by bracing at the interplane struts, either by a pair of wires or by a single strut (i.e., incidence bracing). This arrangement provides an alternative path for loads normally taken in the lift wires or in the drag bracing. For example, if the front lift wire fails, the lift is taken through the incidence bracing to the rear lift wire. This obviously means a complete redistribution of loads. The compression in the front spar is reduced, and increased in the rear spar; also the loads in the drag bracing are altered, due to the new fore-and-aft component at the interplane strut panel. The failure of a lift wire may not be expected to occur simultaneously on both sides (port and starboard), so that the wing centre-sections are subjected to unsym- metrica] forces. If the loads on the undamaged side remain unaltered, the loads in the centre-section bracing are very severe. There is reason to suppose that the conditions on the undamaged side are considerably modi fied, giving great relief to the apparently highly-stressed members. This relief will be investigated in this article. The stresses in a redundant structure cannot be found by the methods of simple statics. A very close approxi mation, however, is given by the now well-known method of " Strain Energy." Briefly the theory involved in these calculations is as follows: — Calculate the loads in the structure, assuming that the redundant members are slack (i.e., can take no load). Next give each redundant member, in turn, ten sions, T,, T2, T3. These tensions will induce loads in all other members, so that the load in any one member may be expressed as P + aTr + 6T, + cT,. .. The work done in stretching this member will be U=i(P + oT1 + 6T1 + cTs..)e where e = extension
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
