I Rented A Helicopter To Settle A Physics Debate

A helicopter’s rotor wash doesn’t meaningfully reach the suspended cable, so a uniform, flexible cable flying at constant speed settles into a straight diagonal line—making the correct multiple-choice answer B. The result settles a long-running physics-debate tied to a 2014 qualifying exam for the US Physics Team, where students argued over whether the cable should hang straight down, tilt, form a hook, invert into a different hook, or develop an S-bend.

To test the competing predictions, the experiment team rented a helicopter and suspended a long, heavy battle rope beneath it while flying nearly 100 km/h. The rope initially “whipped” as rotor wash interacted with it during deployment, but once the rotor wash stopped affecting the rope and the motion stabilized, the cable hung diagonally to the left—matching option B. The team also noted a practical constraint: the pilot kept the rope on the helicopter’s right side to monitor it and avoid any risk of the rope drifting into the rotors or tail rotor.

The physics behind the diagonal shape comes from balancing forces along the cable. Gravity pulls each segment downward, while air resistance pushes sideways. At constant speed, tension in the cable must counter the combined weight of the segments below plus the air drag acting on them. Because each small segment has the same weight and experiences the same drag per unit length (same cross-sectional area and same speed through air), the tension grows from near zero at the bottom to a maximum at the top, but its direction stays consistent. That constant ratio of air resistance to weight along the cable is what keeps the cable from curving into a hook or S-shape; it remains a straight diagonal line.

The team then varied what hung from the rope’s end to show why other answer choices can appear. Adding a heavy kettlebell at the bottom changes the boundary condition: the bottom tension must be nearly vertical to support the large weight, and as air resistance becomes more important higher up, the cable shifts toward a more horizontal orientation near the bottom. During a high-speed run with the kettlebell, the rope formed an inverted J shape consistent with option D.

Finally, the experiment replaced the end weight with a Veritasium flag to introduce drag without much mass, and then added a small parachute. With extra air resistance at the end but relatively little weight, the rope became a J shape—matching option C. The takeaway is that the “correct” diagram depends on the end condition: a uniform cable with no special end load gives B, while heavy loads or draggy end attachments can produce C or D.

Overall, the helicopter test ties the exam controversy to a clear rule: for a uniform flexible cable at constant speed, the diagonal straight-line shape follows from local force balance and a constant drag-to-weight ratio along the length, not from rotor wash reaching far below the aircraft.