Airfoils for a Gyroplane Rotor
by Dan M. Somers, President
State College, Pennsylvania
A family of natural-laminar-flow airfoils for the rotor of the Groen Brothers Aviation (GBA) H2X gyroplane has been designed and analyzed theoretically. The family consists of a primary and a tip airfoil intended for the 0.75 and 1.00 blade radial stations, respectively.
The design specifications for the airfoils were determined from rotor-performance calculations performed by Mark D. Maughmer (Associate Professor, Department of Aerospace Engineering, Pennsylvania State University), under contract to GBA, using a computer code developed especially for gyroplane rotors by Barnes W. McCormick (Boeing Professor Emeritus, Department of Aerospace Engineering, Pennsylvania State University), also under contract to GBA. The specifications were refined through discussions with GBA personnel and Peter G. Dixon of Advanced Technologies Incorporated (Newport News, Virginia), the manufacturer of the prototype blades.
Two primary objectives are evident from the design specifications. The first objective is to achieve a high maximum lift coefficient. This objective corresponds to the retreating blade at high airspeeds. A requirement related to this objective is that the maximum lift coefficient not decrease significantly with transition fixed near the leading edge on both surfaces. The second objective is to obtain low profile-drag coefficients over a wide range of lift coefficients. The lower limit of the low-drag range corresponds to the advancing blade at high airspeeds; the upper limit, to the blade when pointing aft.
Two major constraints were placed on the designs. First, the absolute value of the zero-lift pitching-moment coefficient must be no greater than a certain limit. Second, the airfoils must have certain thicknesses to satisfy structural requirements and to provide sufficient volume for tip weights.
The design of the airfoils is complicated by the myriad operating conditions. Specifically, the local flow should remain subsonic for all operating conditions.
The philosophy employed in the design of the airfoils was to satisfy the constraints while meeting the objectives to the greatest extent possible. Of course, compromises between the various objectives were inevitable. In particular, the relatively thin airfoil thicknesses necessary to maintain locally subsonic flow resulted in low-drag ranges narrower than specified.
The Eppler Airfoil Design and Analysis Code (refs. 1 and 2, below) was used because of its unique capability for multipoint design and because of confidence gained during the design, analysis and experimental verification of many other airfoils. (See ref 3, below, for example.)
The primary airfoil is designated the S401. The tip airfoil, the S402, was derived from the S401 airfoil to increase the aerodynamic and geometric compatibilities of the airfoils.
The theoretical results show that the two primary objectives of a high maximum lift coefficient, insensitive to leading-edge roughness, and low profile-drag coefficients have been achieved; the constraints on the zero-lift pitching-moment coefficient and the airfoil thicknesses have been satisfied. Comparisons with other airfoils typically used on rotor blades illustrate the higher maximum lift coefficient and the lower profile-drag coefficients, thus confirming the achievement of the design objectives.
1. Eppler, Richard: Airfoil Design and Data. Springer-Vorlag (Berlin), 1990.
2. Eppler, Richard: Airfoil Program System. User’s Guide. R. Eppler, c. 1994.
3. Somers, Dan M.: Subsonic Natural-Laminar-Flow Airfoils. Natural Laminar Flow and Laminar Flow Controlas, R. W. Barnwell and M. Y. Hussaini, eds., Springer-Verlag New York, Inc., 1992, pp. 143-176.