Modern research in experimental aerodynamics is largely concerned with the effects of the boundary layer. Interpretation of experimental results would be greatly assisted by the knowledge of the position and extent of the transition region and of the separation points. Frequently this has to be obtained, and it can only be done by the use of an elaborated technique involving hot-wires or total head creepers with complicated traversing gear, so that the work takes on the character of a major research.

In cases of tapered struts with hinged or built-in ends where the exact determination of the buckling load is complicated it may be useful to apply a method of successive approximations.

Let us first consider a bar of the length l with hinged ends under the actionof the compressive force P. The differential equation of the bending line becomes

where v is the deflection at the section u, v with the moment of inertia I (u) and E is Young's modulus. At the ends of the bar the deflection v is equal to zero (Fig. I).