Airborne skiers

[Captain Spod]

Yet again I heard this morning that speeding skiers need to be in contact with the snow, and lessen time in the air during jumps to increase their speed. Surely friction is (even slightly) greater when in contact with the surface than when "flying" so would slow them down? Perplexed.

[kempie]

Some points to ponder:

Contact with the snow keeps the skier stable and able to maintain the correct line for the fastest route - it is difficult to correct your direction while airborne.

When the skis make contact with the snow after being airborne the force can be substantial. Increased friction and snow displacement at the point of impact steal energy and therefore speed.

[Clanad]

The skier experiences a forward component of force (part of the downhill ramp force) only while in contact with the ground.
In the air, the skier experiences only gravity and a backward drag force. With no forward forces, the skier slows down. On the ground, the skier experiences gravity, drag, a little sliding friction, and a support force. The support force is tilted forward because it's at right angles to the slope. This support force exerts a forward force on the skier, keeping that skier moving forward quickly. (Thanks to Physics 105).

[Tim123]

Imagine the extreme situation where a bump is so steep that it converts all of the skier's horizontal momentum into vertical momentum (ie it throws them vertically upwards). Not only do they make no useful progress while in the air, they land (heavily) at a standstill.
A less extreme bump will have a similar but smaller effect, so downhill skiers 'pre jump' the worst of the bumps to minimise the loss of horizontal momentum.
I'm not sure what the 'downhill ramp force' that Clanad cites is all about. The forces at work are simply gravity and various flavours of drag.

[Clanad]

Further, for Tim123...
"... The only forward force the skier experiences is the downhill ramp force that comes from his contact with the ground. This force is the result of his downward weight and the forward-tilted support force that the mountain slope exerts on his (it's at right angles to the slope, so it's upward and forward). When he is in the air, the only force he experiences along the slope is the backward force of drag. When when he is on the ground, he also experiences the small backward force of frictional force and this large forward ramp force..."
I suspect it's another way of describing the gravitational forces referenced...

[Space]

During a jump, some of the forward momentum is temporarily converted into an upwards momentum.
But the only thing that matters to win the race is going forwards... towards the finish line
So by jumping, a skier is losing ground compared to a competitor that doesn't.