NACA-RM-A52L04 REV A

Abstract

INTRODUCTION <br>In the classical study of the longitudinal motion of an aircraft, it is usually found that the motion resulting from a small equilibrium destroying disturbance consist of two modes: one, a lightly damped, low-frequency motio at essentially constant attitude, called the phugoid oscillation; the other, a rotary-pitching and plunging oscillation of high frequency (relative to the phugoid frequency) called the shortperiod oscillation. The phugoid oscillation has generally been described as resulting from a slow interchange of potential and kinetic energy as the aircraft edperiences periodic variations in airspeed and altitude. The character of the phugoid motion as influenced by airspeed, altitude, and aircraft geometry has been well understood for some time (see, e.g., ref.2) The short period motion, on the other hand having in the past been found to be highly damped and of short duration short duration, has been the cause of no concern. Its characteristics therefore have not been as fully investigated as those of the phugoid oscillation. With the advent of the loss of rotary damping occurring in practically all aircraft at speeds near the sonic speed has caused renewed interest in the short period pitching mode. Unlike the easily controlled phugoid, oscillation, the deterioration of damping in the short period mode is of serious concern to the pilot, since the period of the oscillation can be of the same order of magnitude as the [o;p's reaction time. The oscillation may therefore be difficult or even impossible for the pilot to control manually. Furthermore, the additional load imposed upon the airframe due to a rapid growth of the amplitude of a negatively damped oscillation makes possible the occurrence of structural failure. It is therefore of considerable interest to obtain an understanding of the nature of the short period mode, parallel to that which has been gained of the phugoid mode. <br>One means of viewing the aerodynamic phenomena occurring during the short period oscillation from a fundamental standpoint is through application of the concept of indicial functions. In this approach, the variations with time of the aircraft angle of attack and angular velocity during the oscillation are replaced by a large number of small instant-neous or step changes. The transient aerodynamic reactions to these step changes are termed "indicial functions," and have been calculated theoretically for several classes of wings (refers. 2 to 16). By suitable superposition of these results (refs. 7 to 9) the aerodynamic forces and moments caused by the given maneuver can be studied. It will be the purpose of this report to make such a study for the simplified case of an aircraft performing single degree of freedom rotary oscillations. For this maneuver, which corresponds to the short period oscillation when the plunging velocity of the aircraft is zero the use of simple physical relationships associated with the indicial function concept enables qualitative studies to be made of the separate effects on the aerodynamic forces and moments of changes in mach number, aspect ratio, plan form, frequency, and thickness. Results of this investigation are then compared with the results of experiments with a groupo f low aspect ratio wing-body combinations. The tests were conducted in the Ames 6- by 6-foot supersonic wind tunnel and were similar in technique to those reported in reference 10. <br>INTRODUCTION