<BLOCKQUOTE>quote:</font><HR>Originally posted by matt_davis:PM, I was told by a speed skier once that when he hit a certain speed he was on a cushion of air, not the melted snow suface us mortals ski on. is this true? what is that speed? Thank you, Matt :
We all ride around on a ultra thin cushion of liquid water when we are skiing. However, on equipment that is normally used for SL, GS or DH, I don't think anyone is ever going to get completely airborne at DH speeds or below, even if they are on wide jumping skis (or any other ski with wide tips).
Clearly, someone already airborne on wide jumping skis is going to get a substantial degree of lift from the air hitting the bottom of their skis, but this is very different from the situation you are interested in where the ski is flat on the snow, and the airstream is only hitting the turned up tip.
When skiers report that they feel like they are "floating on a cushion of air", I think that what they are most often feeling is a reduction in tip pressure caused by the ram effect of the air on and under the tip. The whole ski doesn't need to lift off the snow for the ski to feel "light" or "quiet" or unusually easy to pivot.
The conditions under which this would happen require:
1) An *extremely* smooth but fairly hard surface;
2) Skis with wide tips;
3) Skis with a soft forebody;
4) Moderately high speeds (50 mph might do);
5) A perfectly flat ski.
To get a feeling for the plausibility of this effect, stick your hand out a car window at 50 mph. Angle it up 45 degrees, and you will feel several pounds of lift on your hand. Your hand is roughly the same surface area as a ski tip, thus, there could be several pounds of force lifting up on your skis tip. Now, while someone is standing on the snow, lift up on their ski tips just enough get them off the snow. Somewhat surprisingly, for most skis, but particularly, soft powder skis, it only takes a few pounds to begin to lift the tips. Thus, there might be enough aerodynamic lift on the tip to lift it up a bit, but it would require VASTLY more force to lift the whole ski off the ground. There might be enough lift to do this at speed skiing speeds, but there are too many variables for me to predict this accurately.
Other phenomena occur at high speeds on flat skis that could generate a similar sensation. For example, enough heat can be generated when a fully loaded skis is traveling at high speed not only for the snow to turn to liquid, but for the wax to melt. When this happens, there will be a sudden change in friction that might also be mistaken for "going airborne".
The reason I am familiar with this general phenomena is that one day last season, I was caught at the end of the day on a wide, long, totally empty low blue cruiser on my powder skis (Volkl Explosivs with 120 mm wide soft tips). The snow was fairly hard and groomed / melted / refrozen to mirror smoothness. I was in a big hurry, so I straightlined the run on flat skis. I have no idea what speed I was going (certainly no more than 50 mph, and probably more like 40), but above a certain speed, my skis suddenly get very quiet, suddenly become frighteningly easy to pivot, etc. Subsequently, I wondered what was going on, chatted with a bunch of people about it, did a bunch of calculations (eg, stagnation pressure, lift, etc.), and came to the conclusions I stated above.
If anybody is interested in the details of the feasibility calculations, I can describe them in subsequent posts, but I think that the only absolute proof of this tip lightening phenomena would be to instrument up some skis and run some RW tests.
Hope this helped,
Tom / PM
<FONT COLOR="#800080" SIZE="1">[ November 24, 2001 01:57 AM: Message edited 1 time, by PhysicsMan ]</font>