PDI Flight Probes
Droplet size and velocity measurements

The PDI Flight Probe measures all the drops passing through a measurement volume to form statistical distributions of droplet size and velocity. Since every droplet is measured, the time history of the droplet size and velocity are provided. This allows the observation of temporal changes in the icing cloud and enables the user to sample over discrete periods of time to form the statistical information and perform higher-level analyses on the character of the simulated icing simulation clouds. A long record can be re-processed into suitable time intervals, if desired. The instrument provides real-time calculations of all the mean values including MVD, number density and cumulative volume distributions. Since the droplet size and velocity or measured for each droplet, the droplet dynamics information is available using the size velocity correlation plot. These results can be used to determine whether there is an aerodynamic bias to the sampling due to the smaller drops responding more readily to the airflow in the vicinity of the probe. Results have also demonstrated that in wind tunnels, the larger drops may not reach the test section flow velocity which will bias the sampling results obtained with such devices as the J-W probes.

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Accurate measurements of cloud of water content requires that all droplets passing the sample volume be detected and measured accurately in terms of their size and velocity. Accurate characterization of the instrument measurement volume is also required. A key advantage of the PDI method is that the signals have a sinusoidal character which can be easily detected in the presence of noise as compared to, for example, methods that simply use the threshold level detection system. Digital signal detection invented by our team has been perfected and incorporated into our latest signal processing system. This method continuously samples the incoming voltage from the detectors and analyzes overlapping segments to detect coherent signals (sinusoidal voltage fluctuations). If the signal to noise ratio exceeds a specified level that indicates the presence of a sinusoidal Doppler signal which sets the gate signal alerting the signal processor to send that packet to the computer for high level analysis. These detection means are reliable even under low signal-to-noise ratio measurement conditions. For aircraft icing applications, the signals are generally well-defined due to the fact that the droplet number densities are not high as compared to other applications.

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The PDI-FP and the PDI-FPDR have been tested under typical aircraft icing conditions in various aircraft icing facilities including the U.S. Air Force McKinley Climatic Laboratory, CIRA Italy, and the Boeing Research Aerodynamics Icing Tunnel (BRAIT). It has been flown on various fixed wing aircraft including a U.S. Navy Twin Otter, a C-130, and the UH 60 helicopter. These results have been compared to the PMS FSSP and OAP.