Part 1/3: Side Force Generators (SFGs) in FPV Racing

Summary: FPV racing multirotors with side force generators (SFGs), will drift less during turns, making them potentially faster on race tracks (watch the video comparison here). But the SFGs need the be carefully placed and dimensioned, so that aerodynamic centre and centre of mass are at the same position. Otherwise it will not work. 

Introduction

Recently, I was thinking about how to improve the handling of racing copters. I noticed since quite a long time, that different copters behave quite differently in fast turns: Some drift quite a lot (especially when they carry a larger battery or an action cam), others drift less. I was always preferring copters that drift as little as  possible. That is also why I am trying to reduce the weight of my copters as much as feasible.
The amount of drift depends on the weight of the copter and the lift and drag of the fuselage for sideways air streams (side-slip). The larger the lift and drag, the more side force the fuselage produces at a given side slip angle. More weight = more drift, more side force = less drift.
If there is only very little side force available from the copter, then you have to point the nose of the copter towards the centre of the turn ('drift') to keep the desired trajectory. This is ok and fun most of the time. But on some tracks with a lot of fast and quick turns, you will drift so much that you will be really busy with pointing the nose towards the centre of the turn.

Drift vs. grip. Drift: the nose of the copter always has to point towards the centre of the turn to keep the desired trajectory. Grip: The copter mostly follows the direction of the nose, it doesn't slip sideways.

Side note: The name 'side force generators' (SFGs) is taken from RC aircraft: SFGs are winglet-like structures at the wing tips of aerobatic RC aircraft. They are used to enhance the performance in 3D flight by enhancing the effect of the rudder. Manoeuvers that require an effective rudder (e.g. knife edge flight) highly benefit from these devices.

Side force generators (red) in aerobatic RC aircraft. (image from Hacker Motor GmbH)

To reduce the amount of drift during turns, you can increase the side force of your copter by enlarging the area in sideview. The people from RedBull Dr.one increased the sideview area quite dramatically (thanks, Heiko Schenk, for the image) but pilots experienced huge problems with it. The large area creates huge amounts of torque around all axes when it is not perfectly aligned with the flow (and perfect alignment will never ever happen). Then, the motors will be extremely busy to fight this torque. In the end, the RedBull fin had to be removed, because it was unflyable (well, not really a big surprise).

RedBull Dr.one fin causing problems with aerodynamic torque

The problem with the above fin is that the aerodynamic centre of the fin is far away from the centre of mass. Flying objects become unstable when the aerodynamic centre is in front of the centre of mass (see picture below). However, they become stable, when the aerodynamic centre is behind the centre of mass. This is the case for fixed wing airplanes. For multirotors, it is best when aerodynamic centre and centre of mass are in the same position, otherwise you will lose maneuverability.

Aerodynamic stability of a flying object

Enlarging the area at the correct position

There are probably many solutions to enlarge the area in sideview of a copter and keep the aerodynamic centre at the same position as the centre of mass. Here is my solution: I measured the position of the centre of mass on my copter, an then designed a simple shape with the aerodynamic centre at the correct position.

SFGs on one of my copter designs

I was calculationg the aerodynamic centre with the copter inclined at 45 degrees (because that is the inclination it might have on average on a racetrack).

The aerodynamic centre was calculated for 45 degrees inclination.

Ideally, you would calculate the aerodynamic centre of the whole copter (including the area of the frame, lipo battery, motors etc.). But I think it is ok for now to ignore the other areas (assuming that the quad was flying ok without SFGs before). I am also ignoring the effect of the propeller jet and many more aerodynamic interactions for now.

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SFGs will only work if the aerodynamic centre coincides with the centre of mass.

The location of the aerodynamic centre of SFGs can easily determined by taking 25% of the average chord length of the 'wings':

Aerodynamic centre on an airfoil

For my initial tests, I was using 3D printed airfoils. Later, I was using 1 mm thick carbon plates as wings.

The first SFG prototypes were 3D printed wings

When the copter flies straight, the SFGs do not add much area and will not increase drag too much.

Frontal area does not increase much

Flight test results

After waiting a long time for some minutes without rain, I could finally do some test flight today. I like the results. The copter drifts much less, that makes fast flying easier. As long as some throttle is applied, you will not notice any negative side effects of the SFGs. But, when you fly at full speed, then completely cut the throttle, and then apply 90° yaw, then you will hear that some motors are spinning up a little bit to compensate the aerodynamic moments. But the copter does not do any unwanted movements (if PIDs and especially the 'I' are tuned correctly).

Although I have a serious lack of flying practice, I did some side-by-side comparsion on a small racetrack. With SFGs, I am almost a second faster. And also the standard deviation (how much does the lap time vary) is much lower - I think because I do less pilot errors, because it is easier to fly. Fastest lap is lmost the same. BUT: On the setup without SFGs, I was skipping one gate (because of bad lighting conditions that my old CMOS cam couldn't handle). Without skipping the gate, the lap times would be worse for the setup without SFGs.

Here is the video link.

Lap times on a small track

How about trying it yourself? You can experiment with the shape of the blades easily by 3D printing different types of 2D blades.

I uploaded some of my blade holders on thingiverse, feel free to adapt them to your quad:
https://www.thingiverse.com/thing:2610750