Flow field characteristics of a complex blade tip at high angles of attack

Abstract

I N T R O D U C T I O N Rotating blades exhibit aerodynamic characteristics which cannot be predicted from fixed-wing results. The differences become more pronounced at high angles of attack. The most famous example of this is the experiment of Himmelskampl who reported steady lift coefficients exceeding 3.2 near the root of a propeller blade. Recently, interest in this subject has increased for several reasons. If lift coefficients on the retreating blade can be increased, the forward flight speed of conventional helicopters can be increased. If the inboard portions of a blade can be operated at very high lift coefficients, rotor blade designs may be optimized at lower diameters and tip speeds, with attendant advantages i n power, weight, and noise reduction. Proprotor climb performance can be enhanced. This paper examines the flow field near the tip of a rotor blade with a complex tip shape, at very high pitch angles. Flow visualization and velocity measurements are presented from experiments where the blade is operated as a rotor and then as a fixed wing. These are used to examine flow separation and vortex formation effects, using digitized image data and contours of velocity data. The flow field is dominated by the strong vortices formed at high incidence. The tip shape is a generic representation of features found on modern helicopter blade tips. The blade planform and the single-bladed rotor configuration are shown in Fig. 1. The untwisted constant chord NACA0012 inboard shape changes to interpolated NACAOOxx profiles for the tip. The tip planform has some similarities to the BERP tip used by Westland Helicopters, but no conclusions are drawn about the BERP tip since the rotor geometry and airfoil sections are quite different. The experiments reported here extend past work on this blade tip. The rotating tests were performed in the Georgia Tech 2.74m (9') hover facility, and the blade was then used as a fixed wing in the John J. Harper 2.13m x 2.74m (7' x 9') wind tunnel. Komerath and Liou2 presented measurements around this blade at 15 deg. pitch in hover. Tsung et a13 presented Navier-Stokes computations of the flow around the same configuration, under both rotating and nonrotating conditions, and made qualitative comparisons with the rotor experiments. Further experiments reported in Ref. 4 compared results from the rotating and fixed-wing case. The separation was weaker on the rotating blade compared to the fixed wing. However, the experiments were performed at identical geometric incidence settings i n both cases, so that the wake-induced inflow correction was not incorporated. Also, the rotor case showed weaker inwarddirected flow along the span than in the fixed-wing case, but since the measurements could not go close to the blade surface, i t was not possible to determine the extent of centrifugal pumping as a factor i n controlling flow separation. These shortcomings were corrected here. *: Post Doctoral Fellow. Member, AIAA ISSUES AND 0B. IECTIVES **: Associate Professor. Senior Member, AIAA #: Formerly, Prcsidential Fellow. Copyright