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An aircraft in flight generates multiple wake vortices, the largest of which are a result of the lift on the wings. These vortices rapidly roll up into a counter-rotating vortex pair behind the aircraft. The initial separation between the centroids of vorticity behind the aircraft, or the cores of the resulting vortex pair, is commonly denoted as bₒ. This separation distance is used to estimate the initial vortex strength (circulation) and to nondimensionalize both time and circulation behind the aircraft. Thus, bₒ is an important parameter in nondimensionalizing aircraft wake measurements. For an elliptically loaded wing, bₒ = (π/4)s, where s is the wingspan of the generating aircraft. The initial descent velocity of the vortex system behind the aircraft, Vₒ, is proportional to bₒ-². Thus, from estimates of Vₒ from lidar measurements, bₒ can be estimated. Estimates of Vₒ from lidar measurements of vortex altitude vs. time are presented in the Out of Ground Effect (OGE) region for aircraft arriving at Denver International Airport and San Francisco International Airport. Estimates of bₒ for these aircraft are then obtained. These measurements suggest that, while some estimates of bₒ from commercial aircraft are close to the value based on elliptical wing loading, some aircraft have distinctly nonelliptical values of bₒ.

Estimates of the Initial Vortex Separation Distance, b<sub>o</sub>, of Commercial Aircraft from Pulsed Lidar Data