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STRUCTURAL ANALYSIS AS CRITERIA OF STRENGTH Structural analyses are considered criteria of complete proof of strength only in the case of structural arrangements for which experience has shown such analyses to be reliable. References should be shown for all methods of analyses, formulas, theories and material properties which are not generally accepted as standard. In some cases the acceptability of a structural analysis may depend on the indicated excess strength incorporated in the structure.
Mechanical properties of materials.—The structural analysis should be based on the guaranteed minimum mechanical properties of the materials specified on the drawings, except in cases where exact mechanical properties of the materials used are determined. The effects of welding, brazing, form factors, stress concentrations, discontinuities, cutouts, instability, redundancies, secondary bending, joint slippage of wood beams, rigging, end fixity of columns, eccentricities, air loads on struts, and vibration shall be properly accounted for when such factors are present to such an extent as to influence the strength of the structure.
COMPUTING OF LOADS AND STRESSES Acceptable methods for computing the allowable loads and stresses corresponding to the minimum mechanical properties of various materials are given in Bulletin ANC-5 "Strength of Metal Aircraft Elements,” obtainable from the Superintendent of Documents, Washington, D.C.
PROOF OF WINGS
For proof of wings by structural analysis only, see "Structural Analysis as Criteria of Strength,” p. 46.
Compliance Suggestion The following points should be considered in the wing analysis: • Joint slippage.—When a joint in a wood beam is designed to
transmit bending from one section of the beam to another or to the fuselage, the stresses in each part of the structure should be calculated on the assumption that the joint is 100 percent efficient and also under the assumption that the bending moment transmitted by the joint is 75 percent of that obtained under the assumption of perfect continuity. Each part of the structure should be designed to carry the most severe loads
determined from the above assumptions. • Bolt holes.-In computing the area moment of inertia, et cetera,
of wood beams pierced by bolts, the diameter of the bolt hole should be assumed to be 1/16-inch greater than the diameter of the bolt.
• Box beams.-In computing the ability of box beams to resist
bending loads only that portion of the web with its grain parallel to the beam axis and one-half of that portion of the web with its grain at an angle of 45°, to the beam, should be considered. The more conservative method of neglecting the web entirely
maybe employed. • Drag trusses.-Drag struts should be assumed to have an end
fixity, with a coefficient of 1.0, except in cases of unusually rigid restraint, in which a coefficient of 1.5 may be used.
PROOF OF CONTROL SURFACES Suitable structural analyses of control surfaces will be accepted as complete proof of compliance with the ultimate load requirements provided that the surfaces are of conventional construction. However, proof tests are required to prove compliance with yield requirements. Inasmuch as many control surfaces do not lend themselves to rigorous analysis, it is recommended that strength tests be considered for proving compliance with the ultimate load requirements. The analysis and tests should include the horns, and should demonstrate compliance with multiplying factors of safety requirements contained in table 1-III.
DEFLECTION OF HINGE POINTS In analyzing movable control surfaces supported at several hinge points, care should be taken in the use of the "three moment” equations. In general, the assumption that the points of support lie in a straight line will give misleading results. When possible, the effects of the deflection of the points of support should be approximated in the analysis.
RIGGING LOADS The effects of initial rigging loads on the final internal loads are difficult to predict, but in certain cases may be serious enough to warrant some investigation. Methods based on least work or deflection theory offer the only “exact” solution. Approximate methods, however, are satisfactory if based on rational or conservative overlapping assumptions.
PROOF OF CONTROL SYSTEMS Structural analyses of control systems will be accepted as complete proof of compliance with ultimate load requirements when the structure conforms with conventional types for which reliable analytical methods are available. Proof tests, however, are required to prove compliance with yield load requirements.
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Analysis or individual load tests should be conducted to demonstrate compliance with the multiplying factor of safety requirements specified in Chap. 1 pertaining to control system joints subjected to angular motion.
In addition to the proof tests and analyses, operation tests are also required.
CRITICAL LOADINGS In some cases involving special leverage or gear arrangements, the critical loading on the control system may not occur when the surface is fully deflected. For example, in the case of wing flaps, the most critical load on the control system may be that corresponding to a relatively small flap displacement even after proper allowance is made for the change in hinge moment. This condition will occur when the mechanical advantage of the system becomes small at small flap deflections. The proof of control systems should include the most severe loading conditions for all parts of the system.
An investigation of the strength of a control system includes that of the various fittings and brackets used for support. In particular, the rigidity of the supporting structure is important especially in ailerons, wing flap, and tab control systems.
PROOF OF LANDING GEARS Structural analyses of landing gears will be accepted as complete proof of compliance with the load requirements when the structure conforms with conventional types for which reliable analytical methods are available. For gliders, equipped with an auxiliary powerplant and conventional airplane type arrangement of main and tail or main and nose wheels, compliance should be shown with the landing gear requirements of Civil Air Regulations Part 3.
WHEELS AND TIRES
For wheel type landing gears, the approved wheel rating shall equal or exceed the gross weight, if one main landing wheel is used, or shall be half the gross weight if two main landing wheels are used. When unrated wheels are employed, their ultimate strengths should not be less than the ultimate loads to which they are subjected. Any standard tire adaptable to the wheel will be considered acceptable.
SHOCK ABSORPTION There are no definite recommendations regarding the energy absorption characteristics of glider landing gears. On heavy gliders either air wheels or shock absorbing skids should be used.
PROOF OF FUSELAGES Proof of fuselages by structural analysis only should be in accordance with procedures described above.
PROOF OF FITTINGS AND PARTS
Compliance Suggestion In the analysis of a fitting it is advisable to tabulate all the forces which act on it for the various design conditions. This procedure will reduce the chances of overlooking some combination of loads which may be critical.
The additional ultimate factor of safety of 1.15 for fittings (table 1-III) is to account for various factors such as stress concentrations, eccentricity and uneven load distribution, which tend to decrease the strength of the fitting.
COMBINED STRUCTURAL ANALYSIS AND TESTS In certain cases it may be advisable to combine structural analysis procedure with the results of load tests of portions of the structure not subjected to accurate analysis.
Compliance Suggestion Generally, structural analyses are considered satisfactory for showing compliance of conventional type structures with the ultimate load requirements. However, certain 'limit load tests and operation tests should be conducted in accordance with the procedures outlined below. For unconventional type structures for which reliable stress analysis methods have not yet been developed, it is usually necessary to resort to combined analysis and limit load tests, or ultimate load tests alone, to show compliance with the ultimate load requirements.
D-nose wing spars, fuselages, and wings of sheet-stringer construction and structures which may be adversely affected by such factors as cutouts and discontinuities, covering behavior including wrinkling, shifting of neutral plane, changes in stress distribution, et cetera, are considered as unconventional structures because they cannot be substantiated by analysis methods alone.
Prior to deciding whether combined analysis and limit load tests or ultimate tests alone are most practicable, it is recommended that an investigation of the effort and cost involved be conducted. The following points should be considered: 1. Components which have been tested to their ultimate strengths
should not be in certificated gliders unless it can be shown that no damage including detrimental permanent set has occurred in any part of the structure or that such damage has been properly repaired.
2. Test results should be reduced to correspond to the minimum
mechanical properties of the materials used (which requires
additional tests). 3. In certain cases, the cost of preparing structural analyses may be
appreciably less than the cost of strength tests. This is especially true when extra aircraft components, test jigs, et cetera, must be constructed for test purposes and there are only a
relatively small number of gliders involved. 4. In certain cases, components of the gliders may be built suffi
ciently overstrength without excessive weight penalty, so that tests of the particular component may be conducted without causing failure or permanent set. When ultimate load tests are conducted, the stress analysis need include only the determination of the external loads onto the structure or component. Detailed stress analyses need not be prepared except for fittings. However, if the strengths of the fittings are demonstrated by static tests, the test loads should be equal in magnitude to the fitting design load. Material variation factors need not be used in fitting tests.
LOAD TESTS Demonstration of compliance with structural loading recommendations by means of load tests only is permissible provided that ultimate and limit tests are conducted to demonstrate compliance.
STATIC TESTING In static testing of structural components, no material correction factor is required. However, care should be taken to see that the strength of the component tested conservatively represents the strength of subsequent similar components to be used on gliders to be presented for certification. Included in the test report should be a statement certifying to this fact.
SPECIAL TESTS If load tests do not show compliance with the particular multiplying factor of safety recommendations, the tests should be supplemented by special tests or analyses to prove compliance with such recommendations.
SYSTEM AND COMPONENT TESTS Wing ribs.—The strength of ribs should be demonstrated by static tests to at least 125 percent of the ultimate loads for the critical loading conditions.
Control surfaces.Static tests of the control surfaces to limit loads should be conducted.