The Physics of Disc Flight
September 19, 2023 – Disc Golfing
A flying disc is a little like a cross between a gyroscope and a wing.
A wing works by having the top and bottom sides of different lengths. The airflow moves over the longer surface faster, creating a low-pressure region on top of the wing. The higher-pressure region underneath the wing tries to move toward the low-pressure region and, as a result, lifts the wing.
A disc is like a gyroscope in that it uses gyroscopic inertia, the tendency of a gyroscope to keep spinning in the same plane without twisting and turning.
With no spin, a disc is inherently unstable, and with no speed, no lift is generated, so a combination of these factors is needed for a disc to fly. The exact combination varies significantly from disc to disc since different discs have different “wing” shapes. Combining the wing and the gyroscope also introduces other factors, as the two are not mutually independent.
The most obvious one is a torque on the disc. If the disc is spinning clockwise and moving forward when viewed from the top, then the left side is moving faster than the right. There is a corresponding pressure drop over the left side, and that side will produce greater lift, and the disc will try to twist clockwise when viewed from behind. As mentioned above, the gyroscopic inertia acts to counter this, and the greater the spin, the higher the inertia.
Spin
There is almost never any reason for not trying to put as much spin as possible on a disc, all other factors being equal. Spin provides stability so that the disc will continue to fly in the direction it is already flying. A disc that is lacking in spin will tend to “turn over”, that is, twist about the axis of flight, and will generally not go as far as one which has more spin. Lack of spin is probably one of the two major problems encountered when trying to throw accurately over any reasonable distance.
One time when too much spin is possible is when throwing some golf discs. Their flight characteristics are such that they have a very narrow window of stability. Too much spin and they can twist one way, and too little spin and they will twist the other. In general, however, too much spin is better countered by other factors in the throw, such as speed and angle of release, rather than by simply spinning the disc less.
Particular discs are termed “overstable”, “stable,” and “understable,” depending on how they to spin and speed. A disc that needs a lot of spins to be stable at a given speed is called “understable”, while a disc that needs little spin to be stable at a given speed is called “overstable”. “Stable” generally refers to discs that are stable at a wide range of speeds for a given spin.
Most discs can be thrown “understably” or “overstably” by decreasing or increasing the amount of spin put on the disc.
Speed
The speed of a disc is a major (but not the only) factor in determining how far a disc goes and how quickly it gets there. Unlike spin, it is possible to put too much speed on a disc. The stability of a disc changes as it gets faster, and the stable range of the disc is determined both by the shape of the disc and its spin rate. Most discs used for throwing to other people are stable, that is, they fly flat at a range of speeds. Golf discs on the other hand are usually not very stable – they tend to fly flat only for brief periods during their flight. The rest of the time they are banked either one way or the other.
A generally stable disc thrown with too much speed in relation to the spin will act understandably. That is, it will turn over. For this reason, hucking the disc requires plenty of spin and speed. Not enough speed results in the throw landing short, while not enough spin results in the throw diving into the ground.
Most inexperienced players find it easier to get more spin on their backhand, and as a result, most inexperienced players will huck on their backhand side.
Rotation Angle
The title does a poor job of explaining what this section is about. Apart from the amount of spin, the other major factor in the lack of accuracy and distance is the difference between the angle of spin of the disc and the angle of the plane of the disc. The disc often wobbles at the start of its flight, which is the problem. Ideally, the disc should be spinning flat and wobble-free. If the thrower puts a spin on the disc at an angle to the flat plane of the disc, it will wobble and lack control.
This is usually most obvious when teaching beginners the air bounce. For an air bounce, the disc is thrown downwards, and at release, the thumb is dragged across the trailing edge of the disc. This results in a slow upward flight. The thrower’s wrist is at a sharp angle to their arm, and beginners often find that getting the arm to move one way while spinning the disc with their wrist in another direction entirely is very difficult.
The result is a wobbly throw that lacks spin in the right direction and lacks stability and the ability to get much distance. Similar problems usually result when teaching the high-release backhand.
There is often no easy solution. The cure is generally practiced in the case of air bounces and high-release backhand. The wrist needs time to learn that it can impart spin at a different angle than the arm. In the case of normal throws, the thrower may be swinging the disc in a loop instead of straight back and straight through. The trick may be to get them moving the disc straight back on the backswing and straight through on the follow-through. This is often harder to do than it sounds. Also, it may not be the problem. Plenty of good players use a slightly loopy backswing, but they are able to get their wrist and arm in the right line just before release, usually just through practice.
Attitude (Pitch)
Here, we are borrowing a term from aviation to describe one of the angles of release. Pitch refers to how steeply an aircraft is pointed, rotating about an axis through the wings. For a disc, it refers to the angle of release where the front edge of the disc is pointed up or down. The pitch helps determine how far the disc travels and, more importantly, which path. Keeping the front edge well up, the disc will travel well into the air but will probably stall and float down at the end of the flight. Keeping the front edge down results in a low trajectory and a relatively fast flight.
Roll (Bank)
Roll, or bank, refers to the angle of the disc rotated about the direction of travel. This means whether the outside edge of the disc is held up or down. The amount of bank on the disc mainly determines the flight path and can be used to good effect to throw around players who are between thrower and target. The disc will curve in the direction of the lowest edge of the disc.
Wind
Wind can have a marked effect on all of the five factors mentioned above, depending mostly on which direction it is coming from. An important point to remember is that regardless of how much wind there appears to be, there is no wind at ground level, and there is less wind the closest to the ground you are. This means that to avoid the effects of the wind as much as possible, it is a good idea to release and keep throws as close to the ground as practicable.
A headwind increases the apparent air speed of the disc. This means the amount of spin necessary to keep it stable becomes greater. A headwind also reduces the distance possible and lifts under discs that are pitched with the leading edge up, sending them into the air.
A tailwind conversely decreases the air speed of the disc but can increase ground speed. While distance may increase, there is a drop in lift, which means that the disc needs to be thrown at a higher angle of attack to counteract the wind that tends to push the disc into the ground.
Crosswinds have the most effect on banked discs, either by pushing the raised edge up and lifting the disc or by pushing down on a lowered edge and possibly making the disc dive into the ground. Crosswinds also make it difficult to keep the disc flat on release, and even experienced players may find it difficult to keep the angle of spin in the plane of the disc.
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