Comprehensive System Identification of Ducted Fan UAVs - Cal Poly

3.2.5 Vehicle Scaling Laws and ComparisonsIt becomes apparent that the ducted fans looked at all share some basiccharacteristics in one way or another. One of the main advantages of the RUAV designsmentioned in Chapter 1 is that these vehicles can hover. Hovering flight leaves thesevehicles highly susceptible to wind in station-keeping applications. Of particular interestis the derivative M u . This derivative characterizes the vehicle very well in hovering flight(as seen with OAV flight test: Equation 3.15) in the hovering cubic. To understand thenature of the vehicles and fully characterize and identify their flight, some time is neededto understand the pitching moment characteristics.In order to compare the pitching moment characteristics of the four vehicles, M umust be nondimensionalized to take into account the size of the vehicles, the propellereffects, and the ducts themselves. To do this, the nondimensional pitching momentdefinition for rotorcraft is applied:CM=M( Ω ) 2ρ A R RM ~ pitching momentρ ~densityΩ ~ blade rotation speed (rad/sec)R ~ duct radiusA ~ duct areaThis method primarily accounts for duct size with the radius terms, and fan speed Ω.Because the condition we are most interested in is low speed around hover, welook at the derivative about zero to 20 knots airspeed for the vehicles. In other words, theslope of a line fit to the pitching moment vs. airspeed data is calculated for only the lowspeed condition. This value is then nondimensionalized with the above method. It is- 52 -