A Three-Aircraft Intercomparison of Two Types of Air Motion Measurement Systems

We present procedures to evaluate air motion measurements on two or more aircraft by flying them in formation at a known lateral displacement. The analysis is applied to two formation flights involving three aircraft—the NCAR Electra, Sabreliner and King Air—in a clear convective boundary layer to compare two types of air motion sensing probes mounted on different aircraft. The lateral separation between the Electra in the center, and the other two aircraft was ≈30 m. One sensing system utilized constrained vanes and the other differential pressure measurements across ports on a nose radome to obtain the two airflow angles that are used to calculate the transverse air velocity components. Both systems used a Pitot-static pressure difference for obtaining the longitudinal velocity component. We compare differences in means and variances, spectra and cospectra, and spatial coherences between the same velocity components measured on the different aircraft. The differences are, in most cases, comparable to what is predicted on the basis of making identical measurements of the same variable laterally displaced by 30 m in a turbulent velocity field. Measurements from a constrained vane gust probe and a differential pressure gust probe mounted less than 0.2 m apart on the Electra noseboom also compared well with each other. Thus, we have some assurance that both systems are measuring the true air velocity components.