In this paper we offer a general theory of rotorcraft trim. The theory
is set in the context of control theory. It allows for completely arb
itrary trim controls and trim settings for multi-rotor aircraft with t
ests to ensure that a system is trimmable. In addition, the theory all
ows for ''optimal trim'' in which some variable is minimized or maximi
zed rather than set to a specified value. The theory shows that sequen
tial trim cannot work for free flight. The theory is not tied to any p
articular trim algorithm; but, in this paper, it is exercised with per
iodic shooting to show how free-flying rotorcraft can be trimmed in a
variety of ways (zero yaw, zero pitch, zero roll, minimum power, etc.)
by use of the general theory. The paper also discusses applications t
o harmonic balance and auto-pilot trim techniques.