Downwind Faster than the Wind - Successful Run by manned cart

[Keen_Ed]

I confess I see a cart that appears to go faster than the windspeed whilst going dead down wind. But I'm afraid I don't believe everything I see on the internet.

Well, I understand that they're going to have NALSA - the US Land Sailing body - do some timing. So when NALSA say that "the cart went downwind at 2.5x windspeed (or whatever the final number will be)", will you believe that? Bearing in mind NALSA are the people who ratified Greenbird's outright land sailing world record.

 

[Keen_Ed]

Of course. It's really very simple to understand. What is really very disturbing is that so many who claim numerous qualifications do not seem to understand it.

Well, it's fine if people with numerous qualifications don't believe it - I can understand that, particularly if all their qualifications are in basket weaving or something similar. What's worrying is the number of people with numerical qualifications who don't understand what's going on.

 

[Ubergeekian]

ie the wind blows the cart along - get it? The fact that the pitch of the prop is reversed explains the difficulty of getting the thing to move initially and, partially, its slow acceleration - so the wind is blowing the cart which is driving the prop counter to the wind. ie if the prop was free spinning it would turn in the opposite direction (viewed from behind) Right- geddit?

So far so good.

As the cart approaches wind speed the braking effect of the prop reduces and the cart's inertia is transferred via the drive train to the prop which is now turning in the correct direction relative to the wind.

But that's where you go wrong. By the time the cart gets to wind speed the propeller is whizzing round like fury: it does not suddenly start acting like an aerodynamic brake.

If it did, the propeller of a Spitfire would start slowing it down as soon as the Spitfire reached windspeed, which means that on a calm day it could never move.

 

[RAI]

There is no comparison here between BMWO, and this contraption. BMWO broad reaches, so at no point does the the true wind drop to nothing before the apparent wind takes over.

When does the apparent wind drop to zero on the propeller blades of this cart? Certainly not when the cart is at wind speed.

BMWO is always sailing in apparent wind. Only when it's stationary in harbour does the true wind equal the apparent wind for very long.

 

[Ubergeekian]

Now unless this cart is not going dead down wind, I would like to know where it is getting it's energy from once it exceeds the true wind speed. Gears on the wheels, and a big prop are all very well, but once the true wind speed ceases to push the cart, then friction would surely start to slow the thing down again. To say that the propeller drives the cart, and the wheels on the cart drive the propeller is not something that can work for any length of time once the wind that initially pushes the cart ceases to be present.

Sigh. Once the cart is at windspeed the propeller still works. How many times, oh lord, how many times?

 

[colingr]

Sigh. Once the cart is at windspeed the propeller still works. How many times, oh lord, how many times?

I'll confess I was very much in the No camp, but then started to waver.

Ignoring the mathe, which is beyond me, those videos clearly show it working. The "how it works" bit is still a bit confusing but then so is nuclear physics.

I don't need to know how something works to accept that it does.

 

[Ubergeekian]

Then may I ask, and this is a genuine question: what do you see in this video?

A truly beautiful engineering feat. I take back everything I said about Heath Robinson.

 

[Woodlouse]

Sigh. Once the cart is at windspeed the propeller still works. How many times, oh lord, how many times?

Yes, the propeller still works, but it has nothing to turn it except the wheels. Once the cart exceeds the true wind speed then it's the propeller that is then turning the wheels, which means it has no driving force of it's own, which then means it has to start slowing down.

And stop comparing this to BMWO, there are no comparisons.

 

[Ubergeekian]

Yes, the propeller still works, but it has nothing to turn it except the wheels. Once the cart exceeds the true wind speed then it's the propeller that is then turning the wheels, which means it has no driving force of it's own, which then means it has to start slowing down.

Don't be silly. A propeller works perfectly well in still air and if the propeller can push back against the air then - thank you, Mr Newton - the air must be pushing forwards against the propeller.

As I have pointed out many times before, the work done by the air on a propeller can easily exceed the shaft work needed to turn the propeller.

And stop comparing this to BMWO, there are no comparisons.

Except that they have both been demonstrated to work. Tee hee.

 

[snowleopard]

BMWO broad reaches, so at no point does the the true wind drop to nothing before the apparent wind takes over.

OK, let's try to show you how the aerofoil surfaces on BMWO and the cart behave in exactly the same way.

Here is a diagram of the air flow over the wing of BMWO.

When it starts from rest it has just the true wind but as its motion builds up the apparent wind moves forward

Now here is a diagram of the air flow over the prop blade of the cart

When it first starts the only wind is the true wind but as it begins to move the apparent wind moves forward.

The motion of BMWO is its forward motion through the water. The motion of the blade of the cart is a combination of the forward motion of the cart and the rotation of the blade thus:

 

[RAI]

Yes, the propeller still works, but it has nothing to turn it except the wheels. Once the cart exceeds the true wind speed then it's the propeller that is then turning the wheels, which means it has no driving force of it's own, which then means it has to start slowing down.

The wheels are always pushing the propeller round. On this cart there is no point at which the wheels are not pushing the propeller round. The faster it goes the harder they have to push. When the torque on the propeller gets as high as the wheels can push the maximum speed is reached. That point is at a higher cart speed than the wind speed because the propeller has good aerodynamics.

 

[Keen_Ed]

So you're saying that it can accelerate through wind speed, the AWA can go from 180 to 0, so the cart can proceed downwind at greater than TWS for a minute and a half, and then some extra drag is magically produced from somewhere so it slows down?

Newton died 283 years ago, you know.

 

[colingr]

The wheels are always pushing the propeller round. On this cart there is no point at which the wheels are not pushing the propeller round. The faster it goes the harder they have to push. When the torque on the propeller gets as high as the wheels can push the maximum speed is reached. That point is at a higher cart speed than the wind speed because the propeller has good aerodynamics.

That pretty much sums it up. I must admit at the beginning of all this I thought it was the prop driving the cart then the cart driving the prop, which clearly cannot work.

I'm still amazed that it does work though.

 

[RAI]

I'm still amazed that it does work though.

Yes, cute isn't it. I wonder if they will start a new sport category for it and have a DDWFTTW race.

 

[Woodlouse]

What I'm saying is that once you exceed the true wind speed, whilst running dead down wind, the force that has initially pushed the cart has stopped acting upon it, which means that the energy is coming from somewhere else.

Snowleopards drawings are wrong, as the whole point of this discussion is to show motion in the same direction as the true wind, where as the diagram shows a broad reach in relation to true wind and motion.

So I'll try again, the wheels are turning the propeller, which then pushes the cart that turns the wheels, if this concept works, why after a little push to get the wheels, and propeller turning, can it not work in no true wind at all, and just run off the apparent that it creates for its self?

 

[colingr]

What I'm saying is that once you exceed the true wind speed, whilst running dead down wind, the force that has initially pushed the cart has stopped acting upon it, which means that the energy is coming from somewhere else.

Snowleopards drawings are wrong, as the whole point of this discussion is to show motion in the same direction as the true wind, where as the diagram shows a broad reach in relation to true wind and motion.

So I'll try again, the wheels are turning the propeller, which then pushes the cart that turns the wheels, if this concept works, why after a little push to get the wheels, and propeller turning, can it not work in no true wind at all, and just run off the apparent that it creates for its self?

I think it should work with a bit of a shove to get it started.

 

[RAI]

What I'm saying is that once you exceed the true wind speed, whilst running dead down wind, the force that has initially pushed the cart has stopped acting upon it, which means that the energy is coming from somewhere else.

No, sorry, the energy is coming from the apparent wind on the propeller blades.

Snowleopards drawings are wrong, as the whole point of this discussion is to show motion in the same direction as the true wind, where as the diagram shows a broad reach in relation to true wind and motion.

No, sorry, they are correct. aerodynamics works with apparent air speed.

So I'll try again, the wheels are turning the propeller, which then pushes the cart that turns the wheels, if this concept works, why after a little push to get the wheels, and propeller turning, can it not work in no true wind at all, and just run off the apparent that it creates for its self?

Because no wind implies no wind to ground differential. If the wind dropped after starting, the cart and its propeller would slow down and stop.

 

[Keen_Ed]

I think it should work with a bit of a shove to get it started.

Doesn't need a shove. Starts perfectly well on it's own from stationary. Quicker with a shove, though.

 

[colingr]

Doesn't need a shove. Starts perfectly well on it's own from stationary. Quicker with a shove, though.

Yes if there is a wind blowing. I was responding to Woodlouse's comment about it working in zero wind. I think in that situation it should work with a shove.

 

[Keen_Ed]

What I'm saying is that once you exceed the true wind speed, whilst running dead down wind, the force that has initially pushed the cart has stopped acting upon it, which means that the energy is coming from somewhere else.

You're thinking about the apparent wind felt by the cart, which is irrelevant. It's the apparent felt by the blades which is important. 2 different things.

Snowleopards drawings are wrong, as the whole point of this discussion is to show motion in the same direction as the true wind, where as the diagram shows a broad reach in relation to true wind and motion.

No, the drawings are right. It's exactly the ability of the prop to see a different apparent wind from that seen by the cart that makes it possible.

One way of thinking about it: "You can also think of it as the wheels constraining the prop to follow a spiral downwind angle, so the wheels act as a keel/runners and the prop blades act as sails.
The gearing system means that the angle that the motion of any portion of the prop makes with a DDW course is fixed so the motion of each section of the blade has a constant angle to the true wind direction. The cart goes DDW but the blades of the prop do not."

So I'll try again, the wheels are turning the propeller, which then pushes the cart that turns the wheels, if this concept works, why after a little push to get the wheels, and propeller turning, can it not work in no true wind at all, and just run off the apparent that it creates for its self?

Another view from Spork. (one of the team, AIUI)

The prop produces the thrust that pushes the cart, which turns the wheels, which provide the torque to turn the prop, which produces the thrust... Of course this sounds like perpetual motion, but it's really a simple feedback loop being powered by the wind (motion of the air relative to the ground).

No wind = no feedback loop = it stops. In the same way you can't sail without wind.