Drone design #2: 3D Flow Analysis

For part 1, check this link:    • Drone Design #1 - Selecting an Airfoil  

In this video, we’ll be looking at what happens when we move to three-dimensional shapes.

For the full report of our Generic Drone Simulation, click here:
https://airshaper.com/assets/reports/...

To get started, let’s have a look at the aerodynamics of a fixed-wing drone. I have a simple 3D model that I’m uploading to AirShaper. I’ll just call it “GENERIC FIXED WING DRONE” and drag the STL file into the box. It’s a flying drone, so I’ll set it to “above the ground” and “moving” and give it a 15 m/s velocity. Now let’s rotate the nose forward and then pull it upwards, giving it an angle of attack of 15°. That’s not a very efficient flight regime, but it will allow us to see some more interesting flow results. The model was exported in meters and that’s it, now let’s order the simulation.

Ok, we’re back, 4 hours later and the simulation has finished. We’re interested in what happens at an angle of attack of 15°, which should cause a stall. But first, let’s relate to the drag & lift theory of the previous video. This drone has a lift of 21.61N versus a drag of 4.94N. That’s a lift-over-drag ratio of 4.37. Not very impressive, as we saw values of more than 100 in the previous video! Let’s have a look at some techniques to find out what is going on.
One way of finding sources of drag is to look at the total pressure coefficient. Plotting a 3D cloud of where this equals 0 is a good indication of where you have a wake or a draft zone, and thus a cause of drag. As we can see, the outer parts of the wings seem to cause quite a big wake.

To get more insight, surface streamlines & surface friction can help. Remember when we talked about flow separation and adverse pressure gradients? You can detect the edge of a separated zone by looking at low values of surface friction. Looking at the top of the drone, we can see that the outer parts of the wing feature such a separation zone. In that area, you can see the surface streamlines move in multiple directions, even opposite to the wind direction.

You can also see this reflected in the surface pressure map: for a wing to function properly, we need high pressure at the bottom surface and low pressure at the top surface. In this case, the air stays nicely attached to the centre part of the drone. High velocity, low pressure. But at the outer parts of the wing, the air separates right after the leading edge, rapidly increasing pressure and thus reducing lift.

Let’s build a few theories on why this happens. First of all, there is the airfoil that is used. At the centre, the airfoil has a high relative thickness. At the outer parts of the wing, the relative thickness is much lower. Airfoils with a high relative thickness tend to have a higher stall angle of attack. We can see this when comparing lift curves of the NACA006, NACA0012 and NACA0018, which are basically the same airfoil with a different thickness.

Another theory is the shape of this fixed-wing drone. It’s a blended wing body, with a triangular platform, the shape when viewed from above or below. The air first hits the bottom of the nose and is pushed sideways in the process. As it bends around the nose towards the top side, it is pushed back towards the centre by the surrounding air. As the left & right join at the top centre, they fill the wake that is there and prevent separation. This inward & upward pull of air towards the centre could increase the local angle of attack at the outer parts of the wings, triggering stall.

So then how do we prevent this? Well, the question could also be should we prevent this? We’ve deliberately triggered stall by setting the angle of attack to 15°, which is not a standard flight regime. We’ve also seen that stall only happened on parts of the drone, which can be a good thing: a gradual stall in function of the angle of attack will make the drone easier to fly at the limit for example, while it could still be very efficient at lower angles of attack.

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The AirShaper videos cover the basics of aerodynamics (aerodynamic drag, drag & lift coefficients, boundary layer theory, flow separation, reynolds number...), simulation aspects (computational fluid dynamics, CFD meshing, ...) and aerodynamic testing (wind tunnel testing, flow visualization, ...).

We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!

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