Badminton Shuttlecock Spin & Aerodynamics

As well as being a Mac and iOS developer, I’m a keen Badminton player and have started playing again after a 10 year absence.

As a former academic researcher, one thing that has always intrigued me about badminton is just how the badminton shuttlecock actually behaves when in flight and especially precisely how it reacts to spin.

As in many other racquet sports, players regularly slice shots, but because of the unique shape of the shuttlecock and its sharp deceleration it behaves very differently to a ball.

The aerodynamics of Tennis or Table Tennis balls are well understood and have been studied extensively, but the same is not true of the Badminton shuttlecock, which is surprising given that it is the most popular racquet sport in the wold by far (a fact that is not easily understood in the Western World where Badminton is still fairly niche).

In 2006, I asked this question on a badminton forum and was shocked to find that even people who were directly involved in designing shuttlecocks did not seem to have a really good understanding of what is actually happening when you slice a shuttle cock. 10 years on there have been some aerodynamic wind tunnel studies and I’ll summarize what I’ve found:

Firstly, the aerodynamics of actual feather shuttle cocks and synthetic ones are very different and advanced players will use only feature shuttle cocks, so we won’t go into the details of the synthetic ones.

All feather shuttle cocks are constructed so that they have a natural counterclockwise spin as seen by the hitter when the shuttle cock is moving away from him/ her. This is due to the overlapping of the feathers, which creates an asymmetrical shape. This “natural” spin stabilizes the shuttle cock while it flies and is caused by the air passing over the feathers. This spin across the central axis of the shuttle cock gets faster as shuttle cock travels faster. When the shuttle cock slows down, so does the spinning and it becomes less stable.

So far, this “natural” spin is present simply through the shuttle cock construction and is not due to player intervention. You can observe this spin by dropping the shuttle cock from a raised platform, e.g. a balcony.

As far as understand it, until a certain speed is reached the spin of the shuttle cock has little effect on its drag coefficient but simply stabilizes the shuttle cock much like a spinning top. Once the spinning goes over a threshold, however, the centrifugal force that it exerts on the shuttle cock pushes the “skirt” outwards thus increasing drag and leading to a significantly faster deceleration of the shuttle cock. I’m not sure from the studies I’ve seen whether this is due to the feathers themselves bending or only the strings that keep them together “giving” a little.

When a right handed player slices the shuttle cock in the “normal” left to right direction (clockwise), this will add to the “natural” counterclockwise rotation of the shuttle as it inverses its path. This rotation will thus be faster than it would be at the same speed without the slicing action.

Under some circumstances, the slicing action will thus cause the shuttle to decelerate more sharply due to the skirt deformation increasing its drag and the shuttle will then fall shorter. If I understand correctly, this will only be the case if the shuttle rotates quickly enough to cause this skirt deformation. If the counterclockwise spin is increased but still remains under the skirt deformation threshold, the shuttle should simply travel in a more stable trajectory. Whether this stability increase is significant or not, I don’t know and can’t find any research on.

It is clear though that applying spin to the shuttle cock through the racquet slicing action will have a significant influence  on the trajectory of the shuttle cock when shuttle is hit at great speed. The skirt will then deform and increase drag, resulting in a shorter distance travelled.

When applied to a flat drive or to an attacking clear, the slicing action will allow the player to hit the shuttle much harder while still being able to keep it inside of the court where a straight shot leaving the racquet at the same speed would go long.

In this scenario, the shuttle will travel faster on average and thus overall until it comes to a stop and drops straight down towards the floor. The slowing effect will be the strongest initially and cut off altogether at some stage during its flight path when the natural spin rate will reimpose itself due to the construction of the shuttle cock. So the later part of the shuttle’s flight path will be identical between the sliced and straight shot. The increased deceleration effect will cut off when the rotational forces become too small to result in skirt deformation.

The difference in speed thus stems entirely from the higher initial speed of the shuttle cock.

Some people believe that the rotation of the shuttle cock itself could provide a propeller-like speed increase, but this is not true. The rotation only influences its drag coefficient but does not provide forward or backwards momentum.

In ball sports, top spin and slice work by creating pressure differentials around the ball. It looks like the “gap” between the base (cork) of the shuttle cock and the skirt (feathers) produces a pressure differential that is crucial to generating the strong deceleration of the shuttle cock, but I haven’t seen any evidence that pressure differentials are influenced by the axial rotation of the shuttle cock.

So “normally” sliced shuttles decelerate quicker than when hit “straight” and they might move somewhat more stably, but what happens when a “reverse slice” is applied through the racquet head moving right-to-left over the shuttle?

Well, I haven’t been able to find any research on this at all. In forum discussions some people claim that there is no difference, but this is obviously false because shuttle cocks are constructed with a “natural” anti-clockwise rotation and the “reverse slice” will apply a clockwise rotation.

There also certainly seems to be a difference when you actually reverse slice a shuttle cock in normal play, but everything happens so fast that it is impossible to observe exactly what is happening. I use reverse slice almost exclusively for left rear court cross court drop shots, particularly because of the deceptive element of the racquet moving in the opposite direction to the actual shot. I also feel that reverse slicing the shuttle on deep net pushes (such as when taking serves in doubles) makes it less likely to go out.

Interestingly, left handed players slice the shuttle in the opposite direction, meaning their “straight slices” are in fact “reverse slices” (imparting clockwise rotation) and their “reverse slices” are “straight slices” (imparting counterclockwise rotation). When you watch Lin Dan play for instance, his shuttles certainly seem to be taking a different trajectory from that of most other players and perhaps this is one explanation for this.

Unfortunately, there seems to be no firm evidence on this at all, so the remainder is mostly speculation, some of it inspired by forum posts.

It would seem logical that making the shuttle spin in the opposite direction to its natural spin would cause it to move less stably. At high speeds, the effect would likely be insignificant, but at lower speeds there should be more tumbling.

It would also be logical that the “natural” spin imposed by shuttle construction and air resistance would counteract the “reverse” spin and might (and probably would) cause the rotation of the shuttle to move from clockwise to counterclockwise at some stage along its trajectory.

We would thus be left with a high degree of drag as the shuttle leaves the racket, followed by a drop in drag as the centrifugal forces become too small to cause the skirt to deform, followed by a stop of the clockwise rotation and finally a re-establishment of the natural counterclockwise rotation of the shuttle. Only at the beginning of the shot could the centrifugal forces be great enough to decelerate the shuttle quicker than for a straight shots.

The big unknown in all of this is whether the amount of skirt deformation is the same for clockwise or counterclockwise rotation. If it is the same, then a straight sliced shot will decelerate for longer and thus always fall shorter. If it is greater, it depends on how much greater it is. If it is a lot greater, this would compensate for the shorter amount of time that it is effective and the shuttle would fall shorter.

As far as I can see, there have been no studies on this, but just looking at the shuttle cock construction, it certainly seems possible that the clockwise rotation against the “grain” of the features would significantly impact the airflow around them and create turbulence. This will definitely cause it to stop spinning clockwise rapidly, but whether it increases or reduces drag and by how much I wouldn’t want to hazard a guess at.

Of course, whether you want to play a shot straight, sliced or reverse sliced depends on more than just the flight characteristics that it imparts. Body mechanics make it much easier to “straight slice” than to “reverse slice” for almost all shots. “Reverse slicing” may still be justified because it can be deceptive both in terms of racquet swing and flight path.

For maximum power, slicing smashes is probably not a good idea as it will make the shot slower. For check smashes or half-smashes using “straight slicing” is probably most effective, but “reverse slicing” may have a different flight path and deceleration characteristics which might inconvenience opponents.

Attacking clears can be heavily sliced so that they can get to the back faster because more initial speed can be imparted. Reverse slicing a clear is probably not a good idea as it reduces the amount of power that can be put into the shuttle as it presents inferior body mechanics.

The situation is less clear for drop shots, where both approaches seem to make sense. The reverse slicing action is more deceptive than the straight slicing action and deception is very important for drop shots. Slicing the drop shot will allow it to be played faster than if played straight, so drop shots should generally be sliced.

The body mechanics of the forehand cross court drop shot would make it hard to use reverse slice and just hitting the shuttle with a straight swing but angled racquet head provides a great way of playing “straight” sliced cross-court drop shots and thus seems the only way to go.

Similarly, playing a left-of-the-head cross-court drop shot with a straight slice would be very hard to do and the reverse slice is much easier to execute and more deceptive and thus the obvious choice.

When it comes to straight drops, things are rather more finely balanced. As we suspect that the straight slice is more effective at slowing down the shuttle, you can probably produce a more effective shot using this technique and its advantage will increase with the speed. So the closer we are getting a half-smash the more we should prefer the straight slice. At lower velocities, however, it is not clear whether the any slice actually decelerates the shuttle at all; it might only make it more stable.

The reverse slice at lower speeds probably makes the shuttle less stable in the middle of its flight path as the “natural” spin reimposes itself and the shuttle briefly tumbles. This might be an advantage as the shuttle will travel under perfect control while it still rotates clockwise, letting you place it precisely. Then if the timing is right, it will start becoming unstable after it crosses the net and thus inconvenience your opponent.

Clearly, the reverse slice motion, while harder to perform, is also much more deceptive. So slow drop shots should probably be executed using the reverse slice.

On the forehand side, fast mid-court drives have a high risk of going long, but body mechanics make it practical to hit them with both forms of slice. On the backhand side, it is hard to see how one would be able to play a hard reverse sliced drive and few players will have enough strength to have to worry about sending the shuttle out anyway. So we only really have a choice on the forehand side, but there seems to be no advantage to trying to execute the harder reverse slice.

In summary then, sliced shots definitely decelerate faster in the beginning of their trajectory and thus fall shorter than straight shots. Reverse sliced shots definitely decelerate faster than straight shots, but probably decelerate both differently and probably less so than straight sliced shots. Even straight shots cause the shuttle to spin counterclockwise at high speeds.

Any insights or corrections would be very much welcome, as I’m keen to understand this whole area better. Any pointers to relevant research or articles would also be much appreciated.