1 Introduction

In this tutorial we will see how to model VTOL to horizontal flight transition. We consider a 5 meters span hybrid UAV, having 4 electric VTOL propellers, and a push propeller driven by a 2 strokes DA100L engine. We will make a RANS simulation of the hovering phase and another simulation in the middle of the transition with horizontal velocity, and check the forces equilibrium in these cases.
 

 

The geometry was simplified differently compared to the last tutorial. We will model the full span this time but the undercowl details were removed. All propellers will be modeled rotating. For that, we need to remove the blades from the CAD and replace them by disc shaped volumes. The flow in the cells contained in these volumes will be treated with a special model recreating the acceleration of the air normally generated by the blades.
 

Please find the stl files for all these parts in the ACE of Aircraft directory, in the compressed folder named "STLfiles.zip", or use this link : STLfiles.zip.
 

We will study the hovering to horizontal flight transition. At the start of the transition the horizontal velocity is 0m/s and the VTOL propellers are near their maximum velocity to generate lift. The push propeller is also running to create a forward force and start the transition. In order to maintain altitude the total lift must be kept above a given threshold. As the UAV accelerates to the cruise speed, the lift from the wings and canards increase and the VTOL propeller lift must be decreased, as shown in the following image. 

 

At the start of the transition the aerodynamic forces from the airframe are weak, too small to use elevators or ailerons to maneuver. The VTOL propellers are what drives the equilibrium. 

At the end of the transition, when we are near the cruise speed, it is the opposite: the maneuvering surfaces are strong, but the VTOL propellers were not designed to work in a very strong lateral wind, and we are unsure of their performances in such conditions. 

The weight of this UAV is about 450N, and the stall speed is around 24 m/s. The UAV was designed to be able to fly and have good maneuverability above that threshold without the help of the VTOL propellers. Even if the VTOL thrust is dropped sharply past this point, the lift can be recovered quickly with a slight increase of angle-of-attack. The airframe can lift the MTOW with only 3° of angle of attack at 32m/s. 

 
In this particular tutorial we will focus on two points of this transition. We will make one run for the hovering state, at 0m/s. Then a second run at the stall speed (24m/s) in the middle of the transition.