In the work the electrohydrodynamics (EHD) at unipolar corona discharge, when intensive EHD phenomena, e.g. electric wind, take place owing to free-space charges, is analyzed.  Density scale (characteristic value) for the free-space charge is defined as: ,  (1) where ;  and U are the corona on-set and applied voltage; l is the characteristic dimension of corona discharge electrode system. Using (1), the voltage-current characteristic (VCC) of Townsend type was obtained: (2) where А and  are parameters which depend on the electrode system geometry and gas properties. These parameters are calculated on the basis of experimental data using the method of the least squares. By re-writing of equation (2) in nondimensional form, the general canonical form of the VCC is obtained - the equation of bisector segment: ,     (3)       ; ; ; ; ,  (4) and (*) denotes non-dimensional values while ( ) denotes dimensional, scale ones.  The equation (3) is successfully used for analysis and generalization of experimental data of corona discharge in gases at various conditions. The non-dimensional parameters in (3) and (4) can be considered as criteria of similarity. From (3) and (4) it follows that two corona discharges are similar if their non-dimensional voltages, and hence non-dimensional currents, are the same. The similarity of VCC can be interpreted as “electrodynamic” oppositely to “electrohydrodynamic”, which is defined by the criteria of similarity obtained from Navier-Stokes equations. It is shown that in this equation appears the main criterion of similarity which characterizes the ratio of Coulomb force to viscous force: .   (5) Two EHD flows are similar if their numbers K are the same. It is possible to emphasize that the results of laboratory tests are valid for full-scale ones if they are similar. It is shown that the mean volumetric speed, up to a constant factor, is equal to    .   (6) The exponents depends on K (1>m(K)>0,5) and decreases from 1 to 0,5 with flow enhancement from laminar to turbulent (increase of К). Omitting the const in the equation (6) as insignificant, from this formula an important parameter can be defined     which plays the role of so called electric Reynolds number, the fundamental EHD characteristic of corona discharge that can be used to characterize the heat and mass transfer phenomena.