Down wind faster than the wind

[Michael C]

See here: http://www.nextenergynews.com/news08/next-energy-news8.29.08b.html

Here's some more on the Ventomobil:

Running along a road:


Contruction (with braided carbon fibers, very neat):


The Ventomobil managed to go upwind at 64% of wind speed. That means that its speed along the ground was 64% of wind speed. Therefore its speed through the air was 164% of wind speed. Pretty impressive.

It looks as if it could become a downwind vehicle if it is possible to change the gearing appropriately. In any case, the makers of the Ventomobil are certainly capable of making a DDWFTTW vehicle if they wish to. Spork, maybe you should get in touch with them?

 

[dv82matt]

You say that like you think the Ventomobile is 36% short of actually traveling upwind.

If you have a 10mph wind coming directly from the north, and you build a cart that can use that wind to make your cart move north, you are going directly up wind. If you can attain 0.1mph, you are going directly upwind at 1% of windspeed. The Ventomobile can do this, but >60x faster! 60 times faster!

No, I realise that the Ventomobile travels upwind. It is short of travelling upwind faster than the wind. This is not a debatable point. As impressive as the Ventomobile may be it is not travelling directly upwind faster than the wind measuring both speeds relative to the ground.

 

[ThinAirDesigns]

dv82matt:
>I agree that sailboats that are optimised for tacking upwind
>may be better at tacking upwind than downwind.

My point would be this:

It takes *very little* optimization to create a sailboat that will tack upwind -- with hardware store tools and materials I could build one in a weekend. It's a rather simple task.

Building a sailboat that can achieve a downwind VMG greater than 1.0 is a far for difficult task requiring far more skill, knowledge, specialized tools and materials.

When it comes to sailboats, DWFTTW is not even in the same league difficulty wise with upwind progress.

Heck, most sailors will argue to this day that DWFTTW isn't even possible in *any* sailboat.

JB

 

[screensnot]

No, I realise that the Ventomobile travels upwind. It is short of travelling upwind faster than the wind. This is not a debatable point. As impressive as the Ventomobile may be it is not travelling directly upwind faster than the wind measuring both speeds relative to the ground.

So, then you also agree that a vehicle can also travel DDWFTTW, but not twice the windspeed?

A car about as efficient as the Ventmobile (but set up for downwind travel) can go downwind in a 10mph wind, but it will only get to ~16mph, right?

 

[sol invictus]

I don't see any point of contention then. I agree that sailboats that are optimised for tacking upwind may be better at tacking upwind than downwind. What I don't see is how it's relevant.

It's relevant because it demonstrates that it's generally easier to go upwind than to go downwind faster than the wind.

I think you are mistaken. The upwind scenario will have greater losses due to drag, but other than that the two scenarios DDWFTTW and DUWFTTW are equivalent as far as I can tell.

No, that's not correct, and that's the source of your confusion. When you're at rest with respect to the ground/water, the air is moving at the speed of the wind. When you're at rest with respect to the air, the ground/water is moving at the speed of the wind. Those two situations are the most similar - they differ only because air differs from ground/water (and one can imagine a zeppelin floating at an interface between two atmospheric layers in relative motion to eliminate even that difference).

So again - moving upwind at speed v is (roughly) equivalent to moving downwind at at wind speed plus v. Moving upwind at wind speed is (roughly) equivalent to moving downwind at twice windspeed.

 

[sol invictus]

When it comes to sailboats, DWFTTW is not even in the same league difficulty wise with upwind progress.

Heck, most sailors will argue to this day that DWFTTW isn't even possible in *any* sailboat.

Actually, I'm not entirely sure it is possible (in water rather than ice, that is). Not for any physics reason, just because there's a lot of friction.

Just to be clear, I'm talking about a situation where the downwind component of velocity is larger than the total wind speed on a steady reach. It might be pretty hard to achieve that - on a windsurfer (which are faster than any conventional sailboat) the max speed is on a beam reach (perpendicular to the wind), or maybe a little downwind from that. But as you turn further downwind you lose speed pretty rapidly with the angle. And my guess is that max speeds are maybe 1.5x as fast as the wind, or perhaps as much as 2x on a kite surfer.

 

[ThinAirDesigns]

Sol:

Actually, I'm not entirely sure it is possible (in water rather than ice, that is).

http://www.boatdesign.net/forums/sailboats/tacking-downwind-faster-than-wind-24761.html


There is actually quite a list of boats these days that can do it. My buddy Stan Honey as navigated quite a number of them.

JB

 

[Michael C]

So, then you also agree that a vehicle can also travel DDWFTTW, but not twice the windspeed?

A car about as efficient as the Ventmobile (but set up for downwind travel) can go downwind in a 10mph wind, but it will only get to ~16mph, right?

I'm not sure, but that sounds like a reasonable assumption. The actual values will, I imagine, depend on the proportions of drag and friction losses. When the Ventomobil goes at 6 mph upwind in a 10 mph wind, it is fighting a 16 mph headwind. If a similar cart were to go at 16 mph downwind in a 10 mph wind, it would only be fighting a 6 mph headwind. The losses due to unwanted air drag would be less important in the downwind case. There would, however, be more loss due to friction in the wheels in the downwind case, since the speed on the ground is higher than in the upwind case.

Maybe someone could help us here with a more rigourous mathematical analysis?

 

[spork]

The upwind scenario will have greater losses due to drag, but other than that the two scenarios DDWFTTW and DUWFTTW are equivalent as far as I can tell.

This makes sense when you think of a sailboat that has one wing extending into the water (the keel) and one wing extending into the air (the sail). But what if we imagine a vehicle that floats between two air masses (a shear). In this case going upwind for one wing IS going downwind for the other wing (and vice versa). This demonstrates that there is really no inherrent difference between upwind vs. downwind faster than the wind. I hope it also makes clear that you don't have to go upwind faster than the wind in order to demonstrate the same principle as going downwind faster than the wind.

EDIT: It looks like Sol saw what I was planning to write about a vehicle floating between two airmasses and wrote it before me - in fact before I was awake : ) Perhaps I'd better start reading to the end of the thread before responding.

 

[sol invictus]

http://www.boatdesign.net/forums/sailboats/tacking-downwind-faster-than-wind-24761.html

You mean that comment by stumble? It wasn't totally clear to me whether he was saying it's actually been done (by sailboats in water) or just that it's possible - but you're saying it has? I'm not doubting you if you say so; you've proven to be pretty reliable .

Actually maybe thinking about windsurfers is a little misleading - even though they're faster than conventional boats on beam/broad reaches, they're generally not as good going upwind.

 

[ThinAirDesigns]

Sol:
>...but you're saying it has?

I'll make sure I'm clear as the questions and comments have gotten fuzzy.

In real life, sailboats exist which can achieve downwind VMGs of greater than 1.0 steady state. This include PlayStation/Cheyenne, Phillips and others in that league.

I am told that there is more a common racing class (18s?) whose boats have proven this since the early 1980s, though I don't have the personal knowledge to vouch for those as I do in the first case.

JB

 

[screensnot]

I'm not sure, but that sounds like a reasonable assumption. The actual values will, I imagine, depend on the proportions of drag and friction losses. When the Ventomobil goes at 6 mph upwind in a 10 mph wind, it is fighting a 16 mph headwind. If a similar cart were to go at 16 mph downwind in a 10 mph wind, it would only be fighting a 6 mph headwind. The losses due to unwanted air drag would be less important in the downwind case. There would, however, be more loss due to friction in the wheels in the downwind case, since the speed on the ground is higher than in the upwind case.

Maybe someone could help us here with a more rigourous mathematical analysis?

I think if the losses (drag and friction) and efficiencies (of the gears and propeller) were equal for both circumstances, then yes, one car would go 6mph upwind, if the other car can go 16mph downwind.

But, all those losses are not likely to be equal in any car you build. You'd end up having to 'handicap' one type of loss (purposely design it with extra loss) so that it matched its counterpart loss in the other vehicle. For example, you may not be able to use good wheel bearings because its counterpart loss, air drag, can't be lowered as far. Maybe you have to use bushings instead, so that the friction is equal to the friction against the air.

 

[Modified]

Informally, when the streams differ (air/surface, air/water) I would say it's an easier task to build a vehicle that will go downstream faster than the one with the better "grip" on the vehicle.

 

[dv82matt]

dv82matt:
>I agree that sailboats that are optimised for tacking upwind
>may be better at tacking upwind than downwind.

My point would be this:

It takes *very little* optimization to create a sailboat that will tack upwind -- with hardware store tools and materials I could build one in a weekend. It's a rather simple task.

Building a sailboat that can achieve a downwind VMG greater than 1.0 is a far for difficult task requiring far more skill, knowledge, specialized tools and materials.

When it comes to sailboats, DWFTTW is not even in the same league difficulty wise with upwind progress.

Heck, most sailors will argue to this day that DWFTTW isn't even possible in *any* sailboat.

JB

I don't disagree but comparing downwind faster than the wind to upwind slower than the wind is irrelevant in my view. The drastic differences between the two that you point out are an indication that you are comparing apples and oranges I think.

So, then you also agree that a vehicle can also travel DDWFTTW, but not twice the windspeed?

I'm not sure what the basis for this is but right now I do not agree or disagree with this. At the moment I am unclear what the maximum speed of the cart relative to the wind is.

A car about as efficient as the Ventmobile (but set up for downwind travel) can go downwind in a 10mph wind, but it will only get to ~16mph, right?

I would suspect that it would go somewhat slower than that but I couldn't really hazard a guess.

It's relevant because it demonstrates that it's generally easier to go upwind than to go downwind faster than the wind.

Sure and that point is not in contention.

No, that's not correct, and that's the source of your confusion. When you're at rest with respect to the ground/water, the air is moving at the speed of the wind. When you're at rest with respect to the air, the ground/water is moving at the speed of the wind. Those two situations are the most similar - they differ only because air differs from ground/water (and one can imagine a zeppelin floating at an interface between two atmospheric layers in relative motion to eliminate even that difference).

So again - moving upwind at speed v is (roughly) equivalent to moving downwind at at wind speed plus v. Moving upwind at wind speed is (roughly) equivalent to moving downwind at twice windspeed.

I think I understand your point now but I disagree here's why. We are speaking of what is fundamentally a ground vehicle. It is only loosely coupled to the air and is firmly coupled to the ground. The wheels have virtually no slip compared to the propeller. Since this is essentially a wind powered ground vehicle it makes more sense to measure its speed relative to the ground rather than its speed relative to the wind. I am sure you would agree that it would be absolutly trivial to achieve %100 of ground speed relative to the wind simply by applying the brake but achieving %100 wind speed relative to the ground while still maintaining contact with the ground is nontrivial as the cart has to make up for losses due to friction.

This makes sense when you think of a sailboat that has one wing extending into the water (the keel) and one wing extending into the air (the sail). But what if we imagine a vehicle that floats between two air masses (a shear). In this case going upwind for one wing IS going downwind for the other wing (and vice versa). This demonstrates that there is really no inherrent difference between upwind vs. downwind faster than the wind. I hope it also makes clear that you don't have to go upwind faster than the wind in order to demonstrate the same principle as going downwind faster than the wind.

I get the hypothetical but the interface we are talking about is a ground air interface. Granting that we must take the increased drag into account doesn't the greatly stronger coupling of the cart to the ground dictate that a different approach is appropriate.

For example, instead of two air masses let's imagine two solid surfaces say a treadmill and the floor moving with respect to each other. Couldn't a correctly designed vehicle move almost equally fast either upground or downground with respect to the floor due to the reduced slippage?

 

[screensnot]

We are speaking of what is fundamentally a ground vehicle. It is only loosely coupled to the air and is firmly coupled to the ground. The wheels have virtually no slip compared to the propeller. Since this is essentially a wind powered ground vehicle it makes more sense to measure its speed relative to the ground rather than its speed relative to the wind. I am sure you would agree that it would be absolutly trivial to achieve %100 of ground speed relative to the wind simply by applying the brake but achieving %100 wind speed relative to the ground while still maintaining contact with the ground is nontrivial as the cart has to make up for losses due to friction.

What if the vehicle you are starting with is a blimp?

It moves along at 100% windspeed because it is 'anchored' to the wind.

 

[Modified]

For example, instead of two air masses let's imagine two solid surfaces say a treadmill and the floor moving with respect to each other. Couldn't a correctly designed vehicle move almost equally fast either upground or downground with respect to the floor due to the reduced slippage?

It depends what you mean by "correctly designed". For a vehicle that is symmetrical with respect to both surfaces, down belt slightly faster than the belt and up-belt slightly faster than zero (down floor slightly faster than the floor) would be equally easy, with equivalent surfaces and assuming equal wind (wind at half belt speed).

Think of it this way: maintaining speed with respect to the floor would be the mirror image of maintaining speed with respect to the belt, so the "neutral point" for a symmetrical vehicle would be half belt speed.

 

[tsig]

If I say "from the motion of the treadmill", will you tell us what you think wind is?

Treadmills are not fans.

 

[dv82matt]

What if the vehicle you are starting with is a blimp?

It moves along at 100% windspeed because it is 'anchored' to the wind.

I'm not sure in what sense we can consider a blimp "fundamentally a ground vehicle".

Are you perhaps making a Socratic point that I managed to miss? I honestly don't know what I was supposed to glean from this.

It depends what you mean by "correctly designed". For a vehicle that is symmetrical with respect to both surfaces, down belt slightly faster than the belt and up-belt slightly faster than zero (down floor slightly faster than the floor) would be equally easy, with equivalent surfaces and assuming equal wind (wind at half belt speed).

Think of it this way: maintaining speed with respect to the floor would be the mirror image of maintaining speed with respect to the belt, so the "neutral point" for a symmetrical vehicle would be half belt speed.

Ok yes you are correct, thanks for the clear explanation. But I still think my conclusions in the case of the DDWFTTW cart may hold some water although admittedly my reasoning was incorrect.

Consider that the "surfaces", that is the air and the ground, are not symetrical with regard to the cart. Shouldn't this raise the "neutral point" with respect to the ground and lower it with respect to the wind.

 

[tsig]

Actually, I'm not entirely sure it is possible (in water rather than ice, that is). Not for any physics reason, just because there's a lot of friction.

Just to be clear, I'm talking about a situation where the downwind component of velocity is larger than the total wind speed on a steady reach. It might be pretty hard to achieve that - on a windsurfer (which are faster than any conventional sailboat) the max speed is on a beam reach (perpendicular to the wind), or maybe a little downwind from that. But as you turn further downwind you lose speed pretty rapidly with the angle. And my guess is that max speeds are maybe 1.5x as fast as the wind, or perhaps as much as 2x on a kite surfer.

Ice boats and treadmills are red herrings.

 

[Modified]

Consider that the "surfaces", that is the air and the ground, are not symetrical with regard to the cart. Shouldn't this raise the "neutral point" with respect to the ground and lower it with respect to the wind.

Sure, but it's still somewhere in between, so in answer to your statement that started this, "But I'm curious as to whether the cart could (with some minor tweaking such as adjusting gear ratios and/or optimizing propeller/turbine efficiency) go directly upwind faster than the wind.", the answer is "Possibly, but more efficiency would be required so minor tweaking might not be enough."