William G. Bousman

Evaluation of Airfoil Dynamic Stall Characteristics for Maneuverability

Abstract : The loading of an airfoil during dynamic stall is examined in terms of the augmented lift and the associated penalties in pitching moment and drag. It is shown that once stall occurs and a leading-edge vortex is shed from the airfoil there is a unique relationship between the augmented lift, the negative pitching moment, and the increase in drag. This relationship, referred to here as the dynamic stall function, shows limited sensitivity to many parameters that influence rotors in flight. For single-element airfoils it appears that there is little that can be done to improve rotorcraft maneuverability except to provide good static clmax characteristics and the chord or blade number that is required to provide the necessary rotor thrust. The loading on a helicopter blade during a severe maneuver is examined and it is shown that the blades dynamic stall function is similar to that obtained in two-dimensional wind tunnel testing. An evaluation of three-dimensional effects for flight and an oscillating wing in a wind tunnel suggests that the two problems are not proper analogues. The utility of the dynamic stall function is demonstrated by evaluating sample theoretical predictions based on semi-empirical stall models and CFD computations. The approach is also shown to be useful in evaluating multi-element airfoil data obtained from dynamic stall tests.

The response of helicopter rotors to vibratory airload

Abstract : Structural response data from flight or wind tunnel tests of eight full-scale rotors were examined and compared for high-speed flight conditions and in the absence of blade stall or maneuvers. Both similarities and differences in the behavior of the rotors were observed, and these findings are useful in determining appropriate tests for development of theoretical methods. Limited use is made of airload measurements and theoretical calculation in examining these data. Major similarities observed in the rotor behavior include: 1) 3/rev vibratory flap bending moments are remarkably similar among all the rotors at high speed; 2) the root oscillatory chord bending induced by lag dampers is similar for three of the articulated rotors despite differences in the damper type; and 3) torsion moment and pitch-link loads show same positive- negative loading over the advancing side of the disk caused by the unsteady pitching moments at the blade tip. Differences that were observed include: 1) the vibratory chord bending-moment behavior appears to be dependent on rotor stiffness in part, but differences seen are not easily explained; 2) the CH-53A root oscillatory chord bending-moment data do not show the damper-induced loads that are seen on the other articulated rotors with hydraulic lag dampers; and 3) the AH-1G torsion response is very different from that of the articulated rotors. Rotor blades; Rotary wings. (edc)

Assessment of Predictive Capability of Aeromechanics Methods

A technique is described, based on simple statistics, that can be used to assess the accuracy of analytical predictions compared to measurements. It is demonstrated that the approach can be applied to a broad range of problems in the area of helicopter aeromechanics, including hover and forward flight performance, blade aerodynamic and structural loads, vibratory forces, stability, and rotorcraft icing. The method described has both strengths and weaknesses and these are shown with examples and discussed. It is also shown that there is currently a hierarchy of accuracy in aeromechanics problems, ranging from the necessarily accurate methods for performance prediction to the inaccurate and untrustworthy calculations of fixed-system vibration.

Rotorcraft Airloads Measurements: Extraordinary Costs, Extraordinary Benefits The 31st Alexander Nikolsky Honorary Lecture

The first airloads measurements were made in the 1950s at NACA Langley on a 15.3-ft-diameter model rotor, stimulated by the invention of miniaturized pressure transducers. The inability to predict higher harmonic loads in those early years led the U.S. Army to fund airloads measurements on the CH-34 and the UH-1A aircraft. Nine additional comprehensive airloads tests have been done since that early work, including the recent test of an instrumented UH-60A rotor in the 40× 80-ft Wind Tunnel at NASA Ames. This historical narrative discusses the 12 airloads tests and how the results were integrated with analytical efforts. The recent history of the UH-60A Airloads Workshops is presented, and it is shown that new developments in analytical methods have transformed our capability to predict airloads that are critical for design.

Experimentally determined flutter from two- and three-bladed model bearingless rotors in hover

A series of experiments was performed on a 1.8-m-diam model rotor in hover for the principal purpose of investigating the lead-lag stability of isolated bearingless rotors. Incidental to those tests, at least three types of pitch-flap flutter were encountered. Type 1 flutter occurred approximately at the second flap-mode frequency on both two-and three-bladed rotors for both small and large pitch angles and appeared to be a classic pitch-flap flutter. Type 2 flutter showed mostly torsional motion and was seen on both two- and three-bladed rotors. Type 3 flutter was a regressing flap flutter that occurred for only the three-bladed rotor configurations and appears to be a wake excited flutter. Although flutter occurred on a number of different configurations, no rotor parameters were identified that were clearly stabilizing or destabilizing.