The theoretical model of Tu (1988b) for the turbulent evolution of solar wind fluctuations assumed the constancy of the ratio &agr;1 between the energy in inward and outward propagating Alfv¿n waves. Hence we extend this model to reflect the observed evolution in the propagation directions of the interplanetary fluctuations. The radial dependence of &agr;1 is determined by direct time domain correlations of the normalized cross helicity. The theoretical results match the observations at least as well as those of the previous, constant &agr;1, model. Measured values of 1.25 for this constant is very close to the value observed in fluid turbulence if we assume the correspondence between fluid and magnetofluid theories should be made for uncorrelated (zero cross helicity) MHD turbulence. ¿ American Geophysical Union 1989