Chiffon cakes rely primarily on egg white foam (bubbles) for the characteristic cloud like texture. Egg white bubbles are more stable (won’t pop as easy) when beaten to stiff peaks. there is too much verbage to write to explain when this happens. Instead, please use the recommended amount of cream tartar and beat away! it is impossible to overbeat whites when cream tartar is used. I beat till the foam reseembles rough dollops and mountains of snow rather than looking like whipped cream or frosting. Often, a big dollop gathers on the middle of the mixer’s whisk.
Also, be sure your whites are free from any yolks and your mixer bowl and whisk attachment are free from grease. I always wash these in very hot water and dry with paper towels, just prior whipping.