This is the first of a pair of papers on the theory and implementation of a direction-cosine-matrix (DCM) based inertial measurement unit for application in model planes and helicopters. Actually, at this point, it is still a draft, there is still a lot more work to be done. Several reviewers, especially Louis LeGrand and UFO-man, have made good suggestions on additions and revisions that we should make and prepared some figures that we have not included yet.