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Aircraft Structures

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David J. Perry's "Aircraft Structures" is indisputably a classic reference for the students and practicing engineers as well, and most likely a best text book ever written on the subject of the static strength and stability of metallic aircraft structures and its components.

However, both editions, the original from 1950, and 2nd from 1982, contain a minor error in "Joints and Fittings" Chapter (page 304 of 1950 edition, page 397 of 2nd edition):

The last paragraph contains a statement: " ... in Fig. 12.15.(c), which yields maximum bearing stresses, 4P/tb at the inside corner and 2P/tb at the outside corners." Conversely, these bearing stress expressions don't agree with those (i.e., 4P/td, 2P/td) shown in Fig. 12.15.(c). Because the text doesn't corroborate how it was arrived to the maximum pin bending moment 4Pt/27 [InLb] shown on page 305, this discrepancy may confuse the issue.

That this indeed may lead to the confusion is evident in Michael Niu's "Airframe Stress Analysis and Sizing", 2nd edition, page 275, Eq. 9.1.1. Here in all likelihood Niu copied Peery's equation (12.8), but apparently unable to reconcile mentioned discrepancies, in the equation simply left out plate width "b", and not addressing the maximum bearing stress in question, boldly claims that this tensile stress is for "1.0 inch - wide strap".

In the equation (12.8) Peery logically concludes, that if the axial stresses (as derived from the equation P/A ± M/W at both plate faces) are four times (resp. two times) of the average stress value, then same should be valid for the pin bearing stresses too. In other words, at a point, the load intensity (unit loading) producing maximum axial stress in the plate has to be equal to the load intensity causing the maximum bearing stress on the pin. This means, that the extreme load intensities on the pin will be obtained by dividing extreme plate stresses by plate width "b". Then by dividing these extreme load intensities by the pin diameter, one will obtain pin bearing stresses at both plate faces, and these expressions are in agreement with those shown in Fig. 12.15.(c).

And thus the text in last paragraph on this page should read: "...,which yields maximum bearing stresses, 4P/td at the inside corner and 2P/td at the outside corners ...".

If one constructs shear force diagram, then at the outside of the plate the pin shear

force equals to 0 [Lb], at 1/3t (measured from outside of the plate) the shear force equals to P/3 [Lb] , at 2/3t (measured from plate outside) the shear force equals to 0 [Lb] (i.e., the location of maximum bending moment), and on the inside of the plate the shear force has to balance with the applied load P [Lb].

For the moment diagram, the pin bending moments equal to zero at both plate faces, and the maximum moment, Mmax = 2 x (P/3 x 2/3 x t/3) = 4Pt/27 [InLb] exists in the location where the internal shear force in the pin changes its sign, i.e. at t/3 measured from the inside of the plate.

Perry's "Aircraft Structures", 1950 Edition, as well as Flabel's "Practical Stress Analysis for Design Engineers" are my favorite stress references, and both are highly recommended to the students and practicing engineers as well.

Jiri Cerny,

Stress Guesser (jobshopper)

[ASIN:0070491968 Aircraft Structures]