Validation study of a nominal drone rotor behavior

Simulation in aerospace industry is a key tool, which enables the prediction of products behavior. This is the case in the drone industry, and in this study we will use CFD (Computer fluid Dynamics) and motion analysis to compare the nominal behavior of the drone rotors coming from the manufacturer specifications against the real loading coming from a CFD simulation.

Motion analysis allows the accurate replication of any kind of drone movement. In this study, to feed the simulation we will start from the CFD loading obtained in a previous work. This is the so-called co-simulation study that enables a more realistic behavior analysis of the drone flight.

Figure 1 Drone Flight Simulation

The first step in the motion study, is to introduce the data related to the revolutions per minute (rpm) of the drone rotors. However, introducing the rpm of the rotors will not cause the flight of the drone by itself, as doing so will result in a motion of the propellers without any lifting. So we need to couple this motion with the lift forces. These forces come from the CFD analysis with a polynomic interpolation to turn into equations. The result of this methodology provides the equation of force versus angular velocity of the rotors F(w) and this transformed into thrust, knowing the static thrust produced by a propeller is:

So, when the table of nominal rotations coming from datasheet is introduced for each propeller, thrust is created by the rotation of the propellers. In this way, any motion and animation can be created with these tools. The combination of CFD simulation and Motion Study gives you unlimited possibilities to simulate any flight conditions and allows the extraction of different output data, such as: Center of mass as position, speed, angular velocity, etc.

In this study we make the comparison between the linear behavior of the drone given by manufacturers datasheets, and the real flight coming from the CFD polynomic behavior. The results of the comparison of both behaviors are illustrated in the below animation coming from the simulation. We can see a green drone (real – polynomic behavior) and a blue drone (theoretical– lineal behavior).

https://www.ibticae.com/wp-content/uploads/2021/03/quadcopter_motion_animation_x2.mp4

Why both drone behaviors are different?

In principle the angular velocity of both scenarios is the same (figure 2), so what is causing such difference?

 

One of the possible answers comes from the fact that, the forces for each rpm level are different in the two cases. For example:

We can easily check this, by plotting the center of mass position (CM) with respect to time (Figure-3 ), which shows that the different force levels produce between data available from the manufacturers and the CFD calculated polynomic equation.

From the plot we can conclude that the instantaneous height of the drone flight caused by polynomic force equation is lower than the one coming from the theorical linear equation. This is also seen in the polynomic equation obtained from our CFD analysis, represented in the next picture (Figure 4):

The difference between drone flights behaviors shown are easily seen in this study. The accuracy of the CFD study plus the use of 3D animation and Motion analysis is surely a very useful methodology, that could be of great interest for the drone manufacturers in particular and for the drone industry in general. Also, the use of 3D animations is a very useful tool to inspect the visualization and detection of unrealistic behaviors.

I would like to thank the support of the team of IBTICAE SL which is a firm focused on high end engineering solutions in different areas of the product Design cycle, with specific expertise in drones. The present study has been developed in the frame of a collaboration with a Research institution. The company experience in studying complex FE Analysis and CFD studies with drones has enabled the good cooperation to perform this work.

If you find this post interesting and useful and want to know more about the details. You can contact me on my Linkedin.

If you find this post interesting and useful for you or your company please contact me on my Linkedin to know more.

Autor: Pablo Hidalgo Gómez