Wind powered speed record

Web site for a British range of wind powered vehicles attempting to break the speed records on land water and ice.

One technical point I noticed is that the wing twists off like and windsurfers sail. This is done because the direction of the apparent wind changes with height because the real wind closer to the ground is travelling more slowly due to ground friction.

I always thought that windsurfer sails twisted off to release power in gusts. I did not know that this could also help gaining power because of the direction of the apparent wind.

Michael Chare

Yep, all windsurf sails should have some amount of twist due to wind gradient (ie surface friction causing the wind to get lighter towards the surface).

The wind speed at 1m (3ft) off the water can be (depending on other factors) quite a bit lighter than the wind at 5m (15ft) off the water.

This means than the wind at the top of the sail is coming from a more side-on direction than the wind at the bottom thats coming more front-on.

If you look at the prop on a light aircraft, you'll see the same twist due to increasing speed towards the tip of the blades.

Cheers

A windsurf rig has much more in common with an aircraft propeller blade than an aircraft wing. If you look at a GE plane's prop the twist is almost identical to most sails.

The weird thing is that the windsurf Industry has a lingo that talks about the sail's leech "dumping power" or "flattening off" in gusts, implying that the sail becomes less powerful in a gust. Not totally true, the whole upper section of the sail is automatically changing it's angle of attack relative to the area around the boom, which keeps the air flow from separating from the foil. Those upper sections should still have some shape, esp in cambered and free ride sails. Less power actually is transmitted to the rider from, say, the top panel vs. the panel near the center of effort even though the wind speed is a bit faster up there. If you think of the sail geometrically as a three point triangle (foot, clew and mast top) then proportionally the greatest force will be near the center of that triangle, which is where the boom should be raised if you want the most power.

The only sails that seem to truly "flatten off" are dedicated wave sails, not much shaping up top and less power for their size - wave boards have straps being near the board's centerline for carving and this helps avoid getting overpowered.

Another twistless sail today is on Sailrocket, which just did some unintentional aerial freestyle. Might not be a coincidence.

Old twistless windsurfing sails just stalled from separated flow at the top when there was a severe gradient wind due to gusty conditions -terrible. It's a good thing the sport lasted long enough to get some decent aerodynamics engineers involved. Otherwise, i'd be kiting by now.