Real-time modeling of agile fixed-wing UAV aerodynamics

Agile UAVs is a special class of fixed-wing aircraft characterized by high thrust-to-weight ratios (around 2 to 3) and big control surfaces (around 40 to 50% chord) with deflections as large as 50 degrees; and hence capable of extreme maneuvers and aerobatics. The intent of this paper is to model the aerodynamics of agile UAVs for real-time applications such as pilot-in-loop aircraft simulation, accounting for the unique geometry of the aerodynamic/control surfaces, high angles of attack encountered during maneuvers and aerobatics, and also unforeseen changes in the aerodynamics in the event of a crash/accident. Conventional modeling techniques, such as the stability derivatives approach, are not suitable due to the highly nonlinear nature of the aerodynamics and in particular because of the strong coupling of the aircraft states. Hence in the present work, a component breakdown approach is utilized to model agile UAV aerodynamics for the complete angle of attack range. Simplifications have made in the model to retain real-time functionality without losing much accuracy. For the purpose of validation, wind-tunnel testing is done for different angle of attack conditions including completely reversed flow. A good agreement between the simulation and experiments establishes the validity of the proposed approach and the aerodynamics model, with max. rms errors of ~0.15 N and 0.05 N.m in aerodynamic forces and moments.

I. Introduction

The wide variety of UAVs available today can be broadly classified into either fixed-wing (conventional) UAVs or rotary-wing UAVs (such as helicopters and quadcopters). The former type has traditionally been associated with conventional cruise flight, and only the later type has been assumed to have maneuvering capabilities. This is no longer true with the recent emergence of a special class of fixed-wing UAVs called agile UAVs that, with their low weight, control surfaces as big as 50% chord, large deflections of over 50 degrees, and powerful thrusters producing high thrust-to-weight ratio of ∼2 and a strong slipstream, attain maneuverability paralleling that of rotary-wing UAVs, while retaining their capability of long distance cruise flight, see Fig. 1. As such, agile UAVs are suitable for a wide range of tasks such as conventional flight, V/STOL, hovering, perching, rapid evasive maneuvering etc.