How long we windsurfers need to wait to have to proper airfoil sails instead of simple sheets?
When the properly build sail will be invented and what difference it could make to the ride?
As you look at airplanes and wind turbine blades nobody longer use plain sheets for that sort of aerodynamics (beside maybe poor para gliders and ancient dutch wind mills.
Soon to be invented proper windsurfing sail consist of two sheets of standard sails , sealed at at edges and pumped with air inside like the kiter's do.
Two sheets are separated at strategically important distance, hold by bridges together to create proper 3Dimensional airfoil sail not twisted and banded 2D surface.
Sort or sailing with air-materace attached to your mast.
There some advantages:
-improved aerodynamics
-extra buoyancy
but also drawbacks since such area could be very easy to punch a hole.
That could be mitigated with permanent structure similar to the board but lighter -ie filled with airgel. Or two sheets separated and resting on the skeleton -rigid 3D sails ?
Can't wait to see my sails on the water :)

At low airspeeds such as the speeds windsurfers go, the single sided aerofoil is nearly as efficient as a double sided one, and very much lighter, so the benefit of the double side is outweighed by the disadvantage of the extra weight and expense.
If we start sailing at 50 knots and above then it will be worth it, but that won't be me.

redsurfbus said...

Sound great if you want to go in one direction

ok, rigid then that will flip like cams and battens.
Specifically designed cams makes this battens longer on one side and shorter on another with each flip.
May looks a bit like two sails on one mast..

Macroscien said...

pweedas said...

At low airspeeds such as the speeds windsurfers go, the single sided aerofoil is nearly as efficient as a double sided one, and very much lighter, so the benefit of the double side is outweighed by the disadvantage of the extra weight and expense.
If we start sailing at 50 knots and above then it will be worth it, but that won't be me.

Wind turbines are designed and rated at 9-12 m/s for maximum efficiency which similar to our requirement.
Even glider use that proper airfoil to hang in the air for hours, since kiters and paragliders feels that gravity force due to inefficiency of their propulsion systems.

That's true, but they are referring to the wind speed, not the speed of the turbine blade.
Believe it or not, the tips of the turbine blades are not that far below supersonic.
When you stand underneath them on a windy day you get an appreciation for this. They really whistle past. I worked it out a while back at about 400 miles per hour for the RPM stated on a windy day.
I can't see me sailing anywhere near that fast even with a rocket up my butt.

dinsdale said...

These things (see link below), and many others before them (eg Little America's Cup) have very successfully used solid aerofoils. The aerofoils tack quite successfully, with a canterlever system to "reshape" the aerofoil for the port ans starboard tacks. As previously noted there'd be an awful weight disadvantage, especially with a rig which must be lifted and held aloft by hand. The expense would be horrendous too I'd reckon.

Definitely Americas Cup getting more excitement with this new multi-hull designs and now airfoil. I wonder if this new catamaran is planing/skimming over water or still relay on buoyancy. If I were boat designer I could go further and try cat on the hydrofoils - which works in opposite direction. On one hull standard one -for planing/ skimming over water surface but on another pulling down to re balance the boat to horizontal position. This way I could use max power that sail could provide.
To be fair I don't think that human reflex will be quick enough to re balance using pulling down underwater wings , so the best will be some electronics and "sailing by wire" or smart mechanical depth regulator is above is not allowed on Americas Cup competition.

Macroscien said...
This is possibly the biggest drawback in efficiency of the blade or any other propeller design. Subsonic speed in one place and slow motion in another closer to the axis. I imagine that is only one "sweet spot or narrow range " balance that airfoil is most efficient in given wind speed - so theoretically vertical axis wind turbines could prove more efficient. If so this same idea could not be used to propel planes? vertical axis propellers ? never heard about but we never known if not impossible...funny that at specific speed one side could travel at subsonic and another supersonic speed. Take two propellers like that and aircraft could be propelled /powered by the drag on sonic boom - in my opinion - first supersonic vertical axis propeller craft

This is in fact the big speed limiting factor for choppers. The forward moving rotor blade approaches mach while the rearwards moving rotor stalls. The rearwards moving blade is the biggest problem, as supersonic travel (or in this case, transonic) is quite passe now-a-days. There have been piston/propeller aircraft which have operated with the prop tips traveling at >mach 1. The last Schneider Trophy winning aircraft, a Supermarine S.6B, in 1931, was known to have its prop tips traveling at >mach 1.

Modern turboprop aircraft have truly constant speed props, hence their props can be designed optimally for the one speed, though the pitch does vary all the way from feathered to 0 degrees, or in some cases (eg C-130) reverse.