Flight Envelope Determination Using Physically Motivated Margin Indicators for Unmanned Helicopters

Determining the flight envelope is a crucial step in the development process for Unmanned Aerial Vehicle (UAV). Research in the recent years focused on extending the useable flight envelope safely. As the number of available unmanned helicopters increases, maintaining low development costs is a key aspect to enable many civil business cases. Therefore, this paper presents a method to detect flight envelope limits tailored to commercially available unmanned helicopters. The proposed method consists of the following two steps: First, a set of dominating limiting effects of the flight envelope is identified and the concrete thresholds are determined. For the helicopter example used in this paper, these effects are engine power, actuator authority, rotor hub moments, load factor limitations, and the vortex ring state. Second, we propose to use indicators to measure the margin to these limits for each flight condition. A comprehensive rotorcraft model is used to calculate the indicators for the flight conditions. This model determines steady state responses or trim points. Thus, the margin for each trim point to the limit of the flight envelope can be estimated. In this paper, we apply this method to a helicopter in intermeshing rotor configuration and present a verification of the method. Furthermore, we compare the flight envelope of the proposed method to the known of this specific rotorcraft to assess the potential in respect of flight envelope expansion.