Optimal ski jump

Gale force winds, a 60-kilo test dummy, and one of Norway’s largest wind tunnels are helping researchers at the Norwegian University of Science and Technology perfect the optimal ski jump. Next year Tommy Ingebrigtsen and the other members of Norway’s national ski jumping team could have new ski boots as a result.

Photo: Rune Petter Ness

At the heart of researchers’ quest is a 60-kilo test dummy dressed in a ski jumping suit and boots, ensconced in a wind tunnel at NTNU. The wind tunnel is sized to allow a human to stand upright inside and is the only such in Norway. Even though humans have tested techniques in the stand-up tunnel, researchers like the test dummy best. It squats day in and out with its knees bent – with nary a complaint about the lactic acid build-up in its legs.

Researchers from NTNU and Olympiatoppen (the organization responsible for top-level sports at the Norwegian Olympic Committee and Confederation of Sports (NIF)) can put the dummy through its paces, in winds that vary from a gentle breeze to a roaring gale. New tests will check the validity of a technique that has been used in the past to simulate ski jumps in the tunnel. In preparation, the dummy and all its equipment have been specially reinstalled in the tunnel.

In pursuit of gold
Researcher Steinar Bråten pushes and twists the dummy to fix it in a steady position. Bråten is a former ski jump champion and trainer for Norway’s national ski jumping team who now works for Olympiatoppen. He has worked with the Sports Sciences Programme at NTNU; these recent experiments have been in cooperation with Professor Gertjan Ettema from the university’s Human Movement Sciences.

The first part of a ski jump, when the jumper hurtles down the steep tracked slope, takes just a few seconds. But this part of the jump, called the in-run, decides just how far the jumper will fly before he lands. By studying the jumpers’ aerodynamics and balance during the in-run, researchers hope they will help Tommy Ingebrigtsen, Lars Bystøl and the rest of the Norwegian national ski jumping team come home draped with medals.

The lifeless ski jumper is about to start a new session. The winds in the tunnel increase, and the researchers stare fixedly at the motionless dummy. Suddenly, something shifts – a movement that shouldn’t happen.
“I bet it will fall to the ground”, Bråten exclaims. Seconds later, the dummy tumbles over, head-first.

Blown away
Current sports science textbooks focus almost exclusively on a ski jumper’s centre of gravity. Researchers now suspect they may have overlooked an important interaction between the jumper’s centre of gravity and drag in the inrun. The centre of air resistance is the place on the jumper’s body where the drag is strongest.

Back at the wind tunnel, the researchers place the dummy on a special panel that is connected to a computer that registers every little movement of the dummy. The researchers now can monitor how the dummy’s centre of gravity shifts with the strength of the wind.
In order to measure the interaction of gravity and drag the researchers have put the dummy and its special panel on a board that is moveable. They tie a rope to the shoulders of the dummy near its centre of gravity. The two ends of the rope meet at a single rod in front of the dummy. Together the rope and the rolling board are meant to mimic the gravity and drag that a ski jumper experiences on his way down the hill.

“Earlier experiments have shown that both the drag centre and centre of gravity are of great importance for the jumper. If these two forces are not in balance, the jumper will either tip forwards or backwards”, Bråten reasons. – “The dummy’s crash proves this. We estimated the drag centre to be too high up on the dummy”, Bråten says as he prepares for a new trial.

As the new trial proceeds, Bråten eagerly monitors the dummy and the images on the computer screen. He soon gets the answer he anticipated: the new and more realistic test method yields results that are completely opposite the old test methods. The dummy actually increases its lean slightly forwards as the wind force becomes greater, rather than leaning backwards, in what would be a dramatic shift in the centre of gravity.
“This confirms that the very first measures we did were correct”, Bråten notes with satisfaction.

These boots are made for jumping
The wind tunnel experiments may mean that textbooks ought to be revised. But the results also suggest that ski jumpers’ boots need modification.

“Earlier it was assumed that it was important to build up the boots with blocks to create a more aggressive posture down the jump. The idea was that the drag forced the jumper to lean backwards, and the blocks were meant to help stabilize the jumper. But the new test trials show the jumper’s centre of gravity doesn’t shift backwards – but forwards”, Bråten explains. “This suggests that equipment should be adjusted to individual jumpers to achieve the optimal balance”.

The results are too new to affect athletes’ training for the coming ski jump season. But Bråten thinks the design of new equipment, especially boots, will start early in 2004. Athletes have benefited in the past from earlier experiments in the wind tunnel. But Bråten is discreetly silent when he’s asked if the new findings will mean more medals for Norway’s national ski jumping team.

“But why”, you might ask, “haven’t researchers tested the interaction of centre of gravity and drag centre before?”
“A very good question that is difficult, if not impossible, to answer. You might as well ask why the car wasn’t invented earlier – or for that matter, the V-style of jumping”.

Text: ELIN FUGELSNES

Contact: Steinar Bråten, Program for idrettsvitenskap, NTNU
Tel: +47 73 55 06 39. Email: steinarb@svt.ntnu.no