Parachutes have holes to keep them stable during descent. Without a hole at the top, air gets trapped beneath the canopy and escapes unevenly around the edges, causing the parachute to tilt, swing, and oscillate like a pendulum. That small opening, called the apex vent, lets a controlled stream of air pass straight through, which smooths out the airflow and keeps the parachute falling in a steady, predictable path.
What Happens Without a Hole
Picture a solid, fully sealed parachute canopy descending through the air. As it falls, high-pressure air builds up underneath and has nowhere to go but sideways, spilling out from under the rim in an uneven, chaotic pattern. This creates rotating pockets of turbulence in the wake behind the canopy. Those asymmetric forces push the parachute off to one side, then the other, producing a swinging motion that can grow more extreme over time.
NASA engineers studying the Orion spacecraft’s parachute system documented exactly this problem. They found that parachutes inherently produce unstable side forces that cause oscillatory motion with respect to the vertical axis. In testing, when even one of the spacecraft’s three main parachutes failed to deploy, the remaining two created a weak axis of rotation that allowed pendulum-like swinging to grow to large amplitudes. The apex vent is one of the primary design features that keeps this swinging in check under normal conditions.
How the Vent Stabilizes Airflow
The hole at the top of a round parachute works by letting a small amount of air bleed straight through the center of the canopy. This does two important things at once. First, it prevents pressure from building up asymmetrically underneath the canopy, which is what triggers the side-to-side swinging. Second, it stabilizes the turbulent wake region directly below the parachute by preventing large, rotating masses of air (vortices) from forming on the downstream surface.
Research in aerospace engineering has confirmed that even an extremely small center opening in a dome-shaped object reduces drag slightly while suppressing the asymmetric wake oscillations that cause instability. The vent either prevents the airflow asymmetry entirely or pushes it far enough downstream that it no longer affects the canopy itself. The result is a parachute that descends smoothly rather than lurching from side to side.
The vent also helps during the critical moment of inflation. When a parachute first opens, air rushes into the canopy from below. Without a vent, that air has no escape path, which can cause the canopy to over-inflate on one side, collapse partially, then snap open again in a violent cycle. The vent gives the incoming air a release valve, allowing the canopy to fill evenly and open in a controlled sequence.
Holes That Help You Steer
The apex vent isn’t the only hole that matters. Early in the development of round parachutes, designers began adding holes around the sides of the canopy to give jumpers some ability to steer. By selectively closing off certain vents, a parachutist could direct airflow out one side, generating enough thrust to change direction. High-performance round parachutes took this further: designs known as the Para Commander class pulled the apex of the canopy downward to create higher-pressure airflow, then directed that air through vent holes in the rear section of the parachute. This produced forward drive, turning a simple drag device into something with limited glide capability.
How Modern Rectangular Parachutes Use Openings
If you’ve watched skydivers land with precision, they were likely using a ram-air parachute, the rectangular, wing-shaped canopy that has largely replaced round parachutes for sport jumping. These work on a completely different principle, but they rely on openings just as much.
A ram-air canopy is made up of a series of fabric cells, open at the front edge and sealed at the back. As the parachute moves forward through the air, wind rushes into those open cells and inflates them, creating a rigid, wing-shaped airfoil. The internal air pressure even pushes a small cushion of stagnant air ahead of the opening, forming an artificial leading edge that improves aerodynamic performance. Inside the canopy, small holes cut into the fabric walls between cells (called crossports) allow air pressure to equalize across the full width of the wing, ensuring that every cell stays evenly inflated.
So while round parachutes use a top vent to let air escape and prevent instability, ram-air parachutes use front-facing openings to capture air and maintain the rigid wing shape that gives them their speed and steerability. In both cases, the holes are doing the same fundamental job: controlling where air goes so the canopy behaves predictably.
Size of the Vent Matters
The apex vent on a round parachute is carefully sized relative to the overall canopy. Too small, and it won’t bleed enough air to prevent oscillation. Too large, and the parachute loses too much drag, meaning the descent speed increases and the parachute becomes less effective at slowing you down. Most round parachutes strike a balance where the vent is large enough to keep the canopy stable but small enough that the slight reduction in drag is negligible compared to the gain in reliability.
Canopies designed for very low porosity (tightly woven fabric with minimal airflow through the material itself) are especially prone to a specific type of instability called coning oscillation, where the vent traces a circular path around the vertical axis instead of staying centered above the payload. This is why the vent size and overall fabric porosity are tuned together as part of the same design problem. The goal is always the same: a smooth, straight descent with minimal swinging.