skiing!

a minipost

With Beijing approaching I started into a piece on skiing, but the number of drawing needed to illustrate carved turns, sidecut radius, reverse camber, self-turning, plowing and so on was daunting.  Looking around I found a good, not very technical, discussion of downhill skiing. The discussion falls apart on freestyle and ski jumping. I'll save freestyle for later as it's a rich area.  But I know a little about ski jumping.

 

The point of ski jumping is to jump as far down a hill as possible while showing style. Contestants are judged on how far their landing is from a critical line know as the K line (kritisch is critical in German).   The main hill at the PyeongChang Olympics was K98 - the K line was 98 meters from the end of the jump.  The long hill was K125.

 

The first phase is the in-run.  A ski jumper releases from a metal bar at the top of the ramp and begins to turn  gravitational potential energy from the height they're at for kinetic energy.  They crouch to lower the area their body presents to the oncoming air.  Aerodynamic helmets and tight fitting suits are used and their arms are out behind them.   Ski jumps have refrigerated ceramic tracks that form a very slippery layer of ice.  New snow or warm sunlight can dramatically change frictional drag and ski waxes are picked for specific conditions.

 

The ski jumper has a good deal of speed and momentum as they reach the takeoff table at the end of the ramp.  It may look angled up, but that's an illusion. It's still slightly down and the jumper needs to change from the crouch they've been in and physically jump into the air.  You don't just fly off.  The timing and power of the jump are critical elements.

 

 The ski jumper/ski combination is a glider at this point subject to three forces:  gravity, aerodynamic drag, and lift.  They've changed from a minimal aerodynamic profile to something more like an airfoil with greater surface area and the best possible shape. The skis move to form a V shape and  with the ski jumper in control of the angle of each ski (a total of six angles) and their hip and chest angles. Their arms are held slightly apart from the body to add some additional area while keeping aerodynamic drag at a minimum.   The best lift to drag in competition is about 1.6 - for every meter a ski jumper drops, they travel 1.6 meters forward. A common sea gull has a L/D of about 10 and modern airliners are in the 15 to 18 range.  

 

The V ski position revolutionized the sport in the mid 80s. Sorting out all of the angles and learning how to  change them for differing conditions is necessary for winning.  The aerodynamic problem is still very computationally difficult, so large low speed wind tunnels as well as racks on top of vans are used for experimentation and training.

 

You might think that a heavy jumper, coming off the ramp with greater momentum, would have an advantage, but the force of gravity is more important and lighter jumpers fly farther.   There were body mass index requirements, now lighter skiers are penalized with shorter skis that reduce lift.  Ski jumpers still tend to be very thin.

 

The flight is rarely more than four meters above the ground, the flight plath roughly following the hill's downward slope. Landings are highly technical and judged for style.  The transition from the V flying configuration to parallel skis one foot ahead of the other requires timing, strength and balance. 

The longest ski jumping events are on K185 and longer hills - "ski flying"  It's not an Olympic event, but shows off the flying portion:

 

But one of the most memorable performances was that of Eddie the Eagle in the Calgary 88 games.  (it won't embed .. watch at the link) You can place last and be the most celebrated athlete Calgary.  If you've stood at the top of a ramp, you get an appreciation of what it must take to even try it.