Although altitude control of multirotor flying vehicles has been studied for years, it is still required to design and analyze the system by a practical-oriented procedure. The most challenge is due to the nonlinearities and uncertainties of thrust force characteristics. To overcome the aforementioned issues, this paper proposes a systematic approach for altitude control using disturbance observer. In particularly, this paper shows that the overall system can be equivalently modelled as a multi-agent system with rank-1 physical interaction and sector bounded nonlinearities. Thanks to this modelling, a condition for absolute stability is derived. The condition is independent of the number of propeller actuators, and hence, considerably reducing the complexity of system design. The effectiveness of the proposed approach is demonstrated by both numerical simulations and experiments using a quadrotor vehicle system.