P
rior to the use of free-flight models, designers relied on qualitative guidelines for aircraft configurations and design details such as the areas required for vertical and horizontal tail surfaces. In some cases, simple theories with broad assumptions were used to predict dynamic motions in response to pilot inputs.The NACA approached flying model experiments with an awareness of potential scale effects that might be caused by the relatively low values of Reynolds number associated with small model tests. After establish-ing the relative accuracy of results from model tests by correlation with full-scale vehicle experiences, a concentrated effort was undertaken to investigate and demonstrate the effects of configuration variables such as wing planform shape, tail configurations, and other geometric characteristics on the dynamic stabil-ity and control characteristics of conventional aircraft designs. By combining free-flight testing with theory, the researchers were able to quantify desirable design features, such as the amount of wing-dihedral angle and the relative size of vertical tail. With these data in hand, methods were developed to theoretically solve equations of motion of aircraft and determine the dynamic stability characteristics such as the frequency of inherent rigid-body oscillations and the damping of those motions.
When Langley began operations of its 12-Foot Free-Flight Tunnel in 1939, high priority was placed on establishing correlation with full-scale flight results. Immediately, requests came from the Army and Navy for correlation of model tests with flight results for the North American BT-9, Brewster XF2A-1, Vought-Sikorsky V-173, Naval Aircraft factory SBN-1, and Vought-Vought-Sikorsky XF4U-1. Meanwhile, the NACA used a powered model of the Curtiss P-36 fighter for an in-house detailed calibration of the free-flight process.1
The results of the P-36 study were, in general, in fair agreement with airplane flight results, but the dynamic longitudinal stability of the model was found to be greater (more damped) than that of the airplane, and the effectiveness of the model’s ailerons was less than that for the airplane. Both discrepancies were attributed to aerodynamic deficiencies of the model caused by the low Reynolds number of the tunnel test and led to one of the first significant modifications to the free-flight technique. The critical lesson learned in
CHAPTER 4: DYNAMIC STABILITY AND CONTROL
48 MODELING FLIGHT
Dynamic stability and control evaluation of the Navy Vought-Sikorsky XF4U-1 Corsair fighter in the Langley 12-Foot Free-Flight Tunnel in September 1940. Early testing in the Free-Flight Tunnel concentrated on correlation of model predictions and full-scale results for dynamic stability and control characteristics.
this early study was that using the specific full-scale P-36 airfoil shape (NACA 2210 airfoil) for the model resulted in poor wing aerodynamic performance at the low Reynolds number of the model flight tests. In particular, a major degradation in lift characteristics was experienced for the 2210 airfoil shape. After this experience, researchers conducted an exhaustive investigation of other airfoils that would have satisfactory performance at low Reynolds numbers. In planning for subsequent tests, the researchers were trained to anticipate the potential existence of scale effects for certain airfoils, even at relatively low angles of attack.
As a result of this experience, the wing airfoils of free-flight tunnel models were frequently modified to airfoil shapes that provided better results at low Reynolds number. One specific airfoil substituted for some full-scale wing airfoils in free-flight model testing was the Rhode St. Genese 35 section, which provided a high maximum lift coefficient at low Reynolds number.2 Even though the modified airfoil affected the fidelity of the predicted characteristics at lower angles of attack, researchers used this approach in attempts to match stall phenomena expected at high angles of attack and full-scale values of Reynolds number.
49 As early as the 1920s and 1930s, researchers in several wind tunnel and full-scale aircraft flight groups at Langley conducted analytical and experimental investigations to develop design guidelines ensuring satisfactory stability and control behavior.3 The objective of such studies was to reliably predict the inherent flight characteristics of aircraft as affected by design variables such as the wing-dihedral angle, size and locations of the vertical and horizontal tails, wing planform shape, engine power, mass distribution, and control surface geometry. The staff of the Free-Flight Tunnel joined in these efforts with several studies that correlated the qualitative behavior of free-flight models with analytical predictions of dynamic stability and control characteristics. For example, flight tests of models with excessive wing-dihedral angles dramatically confirmed analytical predictions of undesirable large-amplitude Dutch roll yawing and rolling motions, which were difficult or impossible to control. In other studies, the vertical tail was increased in size, and a marked degradation in the dynamic directional stability was noted, as predicted by the analytical studies. In particu-lar, as the tail size became excessive, the free-flight model exhibited spiral instability, which demanded constant attention and corrective control from the pilot to prevent crashes. Coupled with the results from other facilities and analytical groups, the free-flight results accelerated the maturity of design tools for future aircraft from a qualitative basis to a quantitative methodology, and many of the methods and design data derived from these studies became classic textbook material.4
NACA free-flight study of the effect of negative wing dihedral. Note the large nega-tive dihedral angle (15 degrees) of the wing. Simple generic models were used in flight studies of the effects of variations in geometric design variables such as dihedral angle, tail size, and center-of-gravity location. Based on the results, design guidelines were formulated.
Other factors influencing the dynamic stability of aircraft also received atten-tion during studies in the Free-Flight Tunnel. Fundamental investigations of the effect of mass distribution were con-ducted as designers began to increase the spanwise distribution of weight through multiengine configurations and flying wing or tailless designs.5 Again, the research staff very successfully cou-pled free-flying model tests with analyti-cal predictions of model behavior. In another example of investigations of the effects of physical phenomena on dynamic stability and control, the effects of fuel sloshing in unbaffled fuel cells was explored in the Free-Flight Tunnel.6 Design trends after World War II had led to enlarged fuselage fuel tanks on high-performance aircraft, and considerable concern had developed over the
poten-tial adverse coupling of fuel motions with inherent aircraft dynamic modes of motion to cause degradation of flying qualities. Flight tests of a simple model equipped with liquid-filled spherical fuselage tanks were used to investigate the motion coupling phenomena. The erratic, jerky flight motions of the free-flight model used in the study clearly demonstrated the potential for highly unacceptable flight characteristics that could result if a designer did not appreciate control of internal fuel movements by baffling.
CHAPTER 4: DYNAMIC STABILITY AND CONTROL
50 MODELING FLIGHT
As high-performance aircraft configura-tions evolved in the early 1950s, the rela-tive length of the fuselage forebody became much longer compared with air-craft of World War II, and in some cases, the cross-sectional shape of the forebody changed from a traditional circular or slab-sided shape to a shape having a relatively flat oval cross section with the major cross-sectional axis horizontal.
These design trends resulted in large impacts on aircraft dynamic stability and control. The staff of the Free-Flight Tunnel had encountered an unconventional aero-dynamic characteristic related to these features when testing a canard-type con-figuration in the late 1940s. During those tests, the canard-fuselage combination was directionally unstable at low angles of attack but became stable at high angles of
attack, at which sidewash from the canard caused a flow reversal on the fuselage so that the combination became directionally stable.7 As researchers anticipated similar effects for radical oval fuselage cross-sectional shapes, programs were carried out with models in the Free-Flight Tunnel, leading to a compilation of data and design information. As high-performance U.S. military aircraft have evolved, flattened fuselage forebodies have become commonplace for fighters (such as the Northrop F-5 series), and the fundamental understanding derived from the free-flight model research of the 1950s has proven to be valuable guidance for aircraft development programs to this day.
Side and front sketches of the free-flight model used in the fuel sloshing inves-tigation. Spherical fuselage tanks containing various levels of water were used to simulate various arrangements of fuel loading and sloshing characteristics.
During the final model flight projects in the Free-Flight Tunnel in the mid-1950s, various Langley organiza-tions teamed to quantify the effects of critical aerodynamic dynamic stability parameters on flying character-istics. These efforts included correlation of experimentally determined aerodynamic stability derivatives with theoretical predictions and comparisons of the results of qualitative free-flight tests with theoretical predic-tions of dynamic stability characteristics. In some cases, rate gyroscopes and servos were used to artificially vary the magnitudes of dynamic aerodynamic stability parameters, such as yawing moment caused by roll-ing.8 In these studies, the free-flight model result served as a critical test of the validity of theory.