Comprehensive System Identification of Ducted Fan UAVs - Cal Poly

3.2 Bare-Airframe IDThe bare-airframe dynamics are perhaps the most unique aspect of these vehiclesand the way they fly. A small inertia with a large concentration of mass near the center ofthe duct is inherent in the design. Combined with this, there is heavy coupling betweenpitch and roll due to the gyroscopic effects of the fast spinning propeller. All of thevehicles looked at utilize fixed pitch propellers. Figure 1.11 showed that the pitchingmoment characteristics together with the whole of the bare-airframe rigid body dynamicscharacterize the vehicle in uncontrolled flight.3.2.1 Aerovironment/Honeywell OAVThe goal of the CIFER ® system identification was to achieve an accurate Multi-Input Multi-Output (MIMO) state-space model to support flight control development andvehicle sizing for the DARPA Phase I test vehicle. The frequency range of interest was0.1 –10 rad/sec. Frequency response analyses show that the important dynamiccharacteristics in this frequency range are the rigid body dynamics.Examination of the eigenvalues of the identified model reveals low frequencyunstable periodic modes in both the pitch and roll degrees of freedom. Excellent matchesbetween the model and flight data for the on-axis time responses confirm the accuracy ofthe of the identified state-space dynamic model.- 17 -