. Agency Use Only(leave Blank) 2. Report Date 3. Report Type and Dates Covered Experimental Evaluation of a Flat Wake Theory for Predicting Rotor Innow-wake Velocities

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, Unclassiied{Unlimited Subject Category 02 13. ABSTRACT (Maximum 200 words) The theory for predicting helicopter innow-wake velocities called at wake theory was correlated with several sets of experimental data. The theory was developed by V. E. Baskin of the USSR, and a computer code known as DOWN was developed at Princeton University to implement the theory. The theory treats the wake geometry as rigid without interaction between induced velocities and wake structure. The wake structure is assumed to be a at sheet of vorticity composed of trailing elements whose strength depends on the azimuthal and radial distributions of circulation on a rotor blade. The code predicts the three orthogonal components of ow velocity in the eld surrounding the rotor. The predictions can be utilized in rotor performance and helicopter real-time ight-path simulation. The predictive capability of the coded version of the theory was correlated with ow velocity data from several sources. In general, the coded version of at wake theory provides vertical innow patterns similar to experimental patterns. Summary The theory for predicting helicopter innow-wake velocities called at wake theory was correlated with several sets of experimental data. The theory was developed by V. E. Baskin of the USSR in 1976, and a computer code known as DOWN was developed later at Princeton University to implement the theory. The theory treats the wake geometry as rigid without interaction between induced velocities and wake structure. The wake structure is assumed to be a at sheet of vorticity composed of trailing elements whose strength depends on the azimuthal and radial distribution of circulation on a rotor blade. The code predicts the three orthogonal components of ow velocity in the eld surrounding the rotor. The predictions can be utilized in rotor performance and helicopter real-time ight-path simulation. The predictive capability of the coded version of the theory was correlated with ow velocity data from several sources. In general, the coded version of at wake theory provides vertical innow patterns similar to experimental patterns. Several descriptions of rotor blade circulation were used in the …