Comment on "Effect of Stabilizer Dihedral and Static Lift on T-Tail Flutter"

Abstract

I is reassuring in an engineering sense to see in Ref. 1 that the archaic methods of strip theory and lifting-line theory can still be made to arrive at reasonable solutions to modern problems. However, the strip theory employed in Ref. 1 lacks many of the refinements of the modified strip theory of Yates and, more significantly, it does not possess the growth potential to treat the general problem of the T-tail with partial span elevators and rudders. Hence, the experimental correlation achieved (a posteriori?) in Ref. 1 is a necessary but insufficient condition for validation of the authors' method. It would have been more informative if the authors had utilized their experimental data to evaluate modern methods for calculating the aerodynamic interference loads on the T-tail. However, before surveying these modern methods we might add four historical references on early Ttail investigations to the authors' list. The earliest applications of lifting-surface theory to the Ttail configuration were made by Stark and Davies. General discussions of interference problems have been given by Ferrari, Ashley and Landahl, Landahl and Stark, Laschka, and Ashley and Rodden. A symposium on Unsteady Aerodynamics for Aeroelastic Analyses of Interfering Surfaces was sponsored by AGARD in 1970 and eight of the fifteen papers contained in the conference proceedings were related to the T-tail problem. A comparison of results computed by different methods for wingtail-fin combinations, taken two a time, is the subject of a recent AGARD report by this commentator. Interfering lifting-surface methods contain none of the limitations on aspect ratio, taper ratio, sweep, and compressibility effects that are implicit in strip theory and lifting-line theory.! The importance of static deformation of the stabilizer should not have come as such a surprise to the authors since the change in dihedral of a wing under load factor has been recognized for many years (although not in the stability and control textbooks) as an important aspect of lateraldirectional motion at limit load factor. This was first considered by Lovell in 1948, and an analysis of the problem by lifting-surface methods was outlined by this commentator in 1965; Ref. 20 also contains some experimental correlation but only with an earlier simplified method and not with a lifting-surface method. The fact that the dihedral effect of a flexible wing typically doubles at limit load factor suggests that the stabilizer trim load is an essential parameter in T-tail flutter analysis, as the authors of Ref. 1 eventually observed. A final criticism of Ref. 1 is the authors' inconsistency in treating the rotary effects of the stabilizer. The rolling moment due to yawing was included, but the yawing moment