An investigation was conducted in the NASA Langley 20-Foot Vertical Spin Tunnel to determine the developed spin and spin-recovery characteristics of a 1/28-scale, free-spinning model of the NASA F-18 HARV (High Alpha Research Vehicle) airplane that can be configured with and without the vertical tails installed. The purpose of the test was to determine what effects, if any, the absence of vertical tails (and rudders) had on the spin and spin-recovery capabilities of the HARV. The model was ballasted to dynamically represent the full-scale airplane at an altitude of 25 000 feet. Erect and inverted spin tests with symmetric mass loadings were conducted with the free-spinning model. The model results indicate that the basic airplane with vertical tails installed (with unaugmented control system) will exhibit fast, flat erect and inverted spins from which acceptable recoveries can be made. Removing the vertical tails had little effect on the erect spin mode, but did degrade recoveries from erect spins. In contrast, inverted spins without the vertical tails were significantly more severe than those with the tails installed. Introduction Currently, there is an interest in exploring the feasibility of flight without the use of vertical stabilizers. The maturation of thrust vectoring has allowed designers to begin considering tailless designs as a means of reducing drag, for example. But the primary driver behind this interest is the pursuit of low radar cross section (RCS), or OstealthO characteristics that are superior to those available on contemporary designs such as the F-117A and F-22. On these configurations, radarreflecting vertical tails must be compensated for using various stealth techniques. However, it is unlikely that any current technology could be expected to provide the low-RCS characteristics that could be realized by removing a source of radar reflections altogether. An investigation was conducted in the NASA Langley 20-Foot Vertical Spin Tunnel to determine what effects, if any, the lack of vertical tails had on the spin and spin-recovery characteristics of a 1/28-scale free-spinning model of the NASA F-18 HARV airplane. The HARV was chosen as the subject for this study because i t represents a current fighter configuration and is equipped with thrust vectoring which could be used to compensate for the lack of vertical tails. This investigation consisted of developed (i.e., equilibrium) erect and inverted spins and recoveries, with and without the vertical tails installed. Both erect and inverted tests were conducted. Data, in the form of motion time histories, were obtained via an optical data acquisition system installed in the Spin Tunnel (ref. 1). Note that the present test was not an exhaustive free spin test (e.g., the F/A-18 test described in reference 2). In a typical free spin test program, a model is launched into the Spin Tunnel upwards of one thousand times. With such a large number of tests, all of the equilibrium spin modes that will be possible for the airplane in question are identified. In contrast, the current test program was meant only to identify major trends in the results that resulted from varying geometric parameters on a model whose basic spin modes were already well documented. Therefore, all of the spin modes possible with the modified model may not have been identified in the present test. Symbols b wing span, ft c wing mean aerodynamic chord, ft Cn body axis yawing moment coefficient Ix, Iy, Iz moment of inertia about the x, y, or z body axis, respectively, slug-ft l linear dimension, ft m mass of model or airplane, slugs N model-to-airplane scale ratio S wing area, ft 2 Re Reynolds number, Vl n