From: firstname.lastname@example.org (Robert Dorsett) Organization: Netcom Online Communications Services (408-241-9760 login: guest) Date: 05 Aug 96 04:21:21 References: 1
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In article <airliners.1996.1565@ohare.Chicago.COM> "Mark A. Brown" <email@example.com> writes: >Anyway, the published design limit is the limit to which you can load >up the airframe and it will bend of course, but when you unload it, it >is guaranteed to bend back to its original shape. In order to >guarantee this behaviour there has to be a margin, of course. > >Loading over the design load but under what I recall as the ultimate >load (I may be wrong), the aircraft will bend but when you unload it >is not guaranteed to bend back to its original shape. However, it >shouldn't fall apart. The ultimate load is normally defined as the point at which it'll break. The ultimate load is calculated by applying a fudge factor to the limit load. This fudge factor is normally 1.5, and is normally tested. >Over the ultimate load and not only is it not guaranteed to bend back Here's more than anyone ever wanted to know on it. Sec. 25.305 Strength and deformation. (a) The structure must be able to support limit loads without detrimental permanent deformation. At any load up to limit loads, the deformation may not interfere with safe operation. (b) The structure must be able to support ultimate loads without failure for at least 3 seconds. However, when proof of strength is shown by dynamic tests simulating actual load conditions, the 3-second limit does not apply. Static tests conducted to ultimate load must include the ultimate deflections and ultimate deformation induced by the loading. When analytical methods are used to show compliance with the ultimate load strength requirements, it must be shown that-- (1) The effects of deformation are not significant; (2) The deformations involved are fully accounted for in the analysis; or (3) The methods and assumptions used are sufficient to cover the effects of these deformations. (c) Where structural flexibility is such that any rate of load application likely to occur in the operating conditions might produce transient stresses appreciably higher than those corresponding to static loads, the effects of this rate of application must be considered. (d) The dynamic response of the airplane to vertical and lateral continuous turbulence must be taken into account. The continuous gust design criteria of Appendix G of this part must be used to establish the dynamic response unless more rational criteria are shown. (e) The airplane must be designed to withstand any vibration and buffeting that might occur in any likely operating condition up to VD/MD, including stall and probable inadvertent excursions beyond the boundaries of the buffet onset envelope. This must be shown by analysis, flight tests, or other tests found necessary by the Administrator. (f) Unless shown to be extremely improbable, the airplane must be designed to withstand any forced structural vibration resulting from any failure, malfunction or adverse condition in the flight control system. These must be considered limit loads and must be investigated at airspeeds up to VC/MC.