Downwind Faster Than The Wind - update

[Bille]

...

No, but if you have a couple of hours to waste on a really boring video, you can watch me try to explain
...

Boring ??

I had way too much coffee this morning, and i had
to pee , when i started your video ; forgot all about the bladder, till
the video ended !!!


Seriously ----
That was Fun , and educational to watch !!!!! ---you get (3)

Bille

[Regis-de-giens]

Hi,

I would like to relaunch the topic and discuss my point of view on it ; we had a side discussion on another topic about what is the impact of apparent wind on kitesurfing hydrofoil speed ; and we derived on the DDWFTW with one disagreement with Herman. last post is :

Régis-de-glens

The point is, no relevant closed sailing system works because of apparent wind. The DTW generates apparent wind because it has a rotor. The apparent wind on the foil is created by the sum of the rotation and vehicle movement. The concept that is not easily explained, imho, is that the energy input can only be from the existing wind but the forces involved at any given instant of time have to be analysed by including the accumulation effects of the craft's acceleration-velocity. It becomes easy to make the intellectual mistake,imho, that this is bringing something or driving the system.

Not that it matters from a kiting or sailing point of view. When riding or sailing it is the art of continuously exploiting the actual flow over the aerofoil at each instant that maximizes the potential of the wind. Of course, the actual flow is the summation of the real and apparent wind.

Regards Herman

I have a "personal view on it" which is : the wing act neither like a rotor (where commonly it should accelerate particules coming from upstream the vehicle) nor a wind turbine (where rotation is energized by the wind) ... it acts like a ... rotating spinacker ... (with massive and pure drag) : rotation is here to always present a surface with less speed vs ground than the vehicle its self = compensate for related speed of the vehicle (so do not "suffer" from the related wind) and constantly offer a surface to be pushed by particules coming from its back !

Does that sounds ok to you ?

[vela99]

Really interesting. I read many but not all posts in this thread, so apologies if this is a repeat.

Did I undertand correctly that there are two states?

1) wind pushes the vehicle down and the wheels spin the propeller.

2) when the vehicle reaches a certain speed, close to the windspeed, the propeller starts to act as a propeller and moves the wheels.

If the above is correct, I think that when kitesurfing downwind and downlooping intensively I am in state 2. Please confirm.

[Herman]

Hi Regis, I also added that conservation of energy and momentum are important to analysing the problem, and so, if you wanted you could regard apparent wind as the conduit for this conservation.

To be honest I doubt that a descriptive discussion will reach a satisfactory conclusion. I suspect an indisputable conclusion could only be achieved mathematically. Perhaps this would not be that easy either!

The analogy of the rotor and a diving kite has merit. If you want a fuller understanding it might be worth examining the effect of LD ratio on an aerofoils ability to go DWFTTW. This is touched on in the excellent presentation.

Regards Herman.

[kiteykitekite]

Yes you can go down wind faster than the wind. I have done it many times. You use a wave to drive you there. Wind window switches 180degrees and kite can fly upwind.

As to the mechanical device used on land to do this, maybe it could be recreated on water. It would be much harder though I think.

[Herman]

DWFTTW can be achieved with just wind power. If memory serves the 2013 America's Cup was expecting to achieve a ratio of 1.6 faster than wind speed. Of course it is relatively easy on ice.

[spork]

>> the energy input can only be from the existing wind but the forces involved at any given instant of time have to be analysed by including the accumulation effects of the craft's acceleration-velocity.

This is not the case. We can look at the vehicle moving at any steady state speed without considering any momentum or acceleration effects. The rotor makes things seem more complicated than they really are. In some sense it's easiest to consider only the tips of the rotor and analyse what's happening there. At steady state, the tips of the rotor will be directly analogous to a traditional sailing craft operating at a specific steady state reach at some angle off the wind.

>> Did I undertand correctly that there are two states?

Not in the way that you stated. Nothing fundamentally changes as we approach wind speed. The closest we come to having two states is that the entire vehicle and rotor initially act as a bluff body being pushed downwind. From a stand-still the wheels turn the rotor. As the vehicle speed increases, the tips of the rotor begin to develop attached flow. As the speed increases, the rotor speed of course also increases, and the region of attached flow grows toward the root of the blades. Still the wheels are turning the rotor. This continues as we reach and pass the speed of the wind. But at all times, the wheels are turning the rotor when we're operating in the downwind mode.

>> the wing act neither like a rotor (where commonly it should accelerate particules coming from upstream the vehicle) nor a wind turbine (where rotation is energized by the wind) ... it acts like a ... rotating spinacker ...

The analogous situation for a traditional sailing craft will always put the craft on a broad reach. In this case the kite or sail would be analogous to the tip of the rotor blade. It's not acting as a spinnaker, but as a propeller blade at that point. Interestingly, it is not possible for a kitesurfer to go directly downwind faster than the wind at steady state by looping the kite. This is because the interaction of the foil with the water can't contribute to the effort to extract energy from the *relative* motion of air and water when it's going directly downwind.

>> Yes you can go down wind faster than the wind. I have done it many times. You use a wave to drive you there. Wind window switches 180degrees and kite can fly upwind.

A kiter can go downwind faster than the wind, as long as they maintain a broad reach. But they can't go *directly* downwind faster than the wind, steady-state, even if brought to that condition through external means.

>> DWFTTW can be achieved with just wind power. If memory serves the 2013 America's Cup was expecting to achieve a ratio of 1.6 faster than wind speed. Of course it is relatively easy on ice.

Yes - DWFTTW is possible with a traditional sailing craft as I describe above (a broad reach). But DDWFTTW (Directly DWFTTW) is not possible with a traditional sailing craft on water or ice.

If there's a specific case you guys are trying to analyse perhaps I can take a crack at it with a simple vector analysis.

[Herman]

>> DWFTTW can be achieved with just wind power. If memory serves the 2013 America's Cup was expecting to achieve a ratio of 1.6 faster than wind speed. Of course it is relatively easy on ice.

Yes - DWFTTW is possible with a traditional sailing craft as I describe above (a broad reach). But DDWFTTW (Directly DWFTTW) is not possible with a traditional sailing craft on water or ice.

If there's a specific case you guys are trying to analyse perhaps I can take a crack at it with a simple vector analysis.

Could you please do the vmg for the 34th America's Cup Catamaran for a typical down wind leg in 18 knts of breeze. If you can't find the relevant polar diag the Olympic Tornado would do. That polar diagram is definitely available on line.

To make it clear I was referring to vmg when getting down wind quicker than the wind - I thought it was obvious that sailing craft cannot run directly down wind faster than the wind!!!

[Herman]

Sporks comment about looking at vehicles in steady state without considering momentum is of course true but depends on your definition of looking and is imho an example why, without doing it mathematically, this sort of discussion would go............

Looking would have to be happy to ignore how it got to that steady state!!!!

Imho anybody who wants a simple analogy for the DDWFTTW MACHINE thinking of the rotor tips as sails/aerofoils tacking downwind faster than the speed of the wind is as simple as it can get.

[spork]

>> Looking would have to be happy to ignore how it got to that steady state!!!!

Exactly right. For purposes of analysis we can push the thing up to speed with a car. The question then is whether it can maintain that speed indefinitely.

>> Could you please do the vmg for the 34th America's Cup Catamaran

Sure, if you'll provide the L/D for both the hull/keel/foil and the sails.

>> To make it clear I was referring to vmg when getting down wind quicker than the wind - I thought it was obvious that sailing craft cannot run directly down wind faster than the wind!!!

I'm not sure everyone understands that. The idea of looping your kite so your board can go DDWFTTW has come up before.