Downwind faster than the wind. Poll

[Pierrome]

Another question for Bosun Higgs

Dear Bosun

You seem to be saying that it is impossible for a moving object to obtain energy from a stationary medium, because a stationary medium "has no energy".

So, mount a wind turbine on the back of a pickup truck.
On a windy day the wind-turbine can get energy from the wind when the truck is standing still. (You must agree this).

However, today there is no wind, so you drive off to about 30mph.

Are you saying the the wind-turbine wont turn and give energy in the relative breeze going past the vehicle "because the air is not moving"?.......

 

[Salty John]

I find it extraordinary that 29% of respondents believe this April Fool's joke!

 

[Bosun Higgs]

Dear Bosun

You seem to be saying that it is impossible for a moving object to obtain energy from a stationary medium, because a stationary medium "has no energy".

No I'm not saying that - it would be possible to make up a scenario in which this is happening. But there is no sensible mechanism by which a road can provide energy to a moving vehicle.

So, mount a wind turbine on the back of a pickup truck.
On a windy day the wind-turbine can get energy from the wind when the truck is standing still. (You must agree this).

However, today there is no wind, so you drive off to about 30mph.

Are you saying the the wind-turbine wont turn and give energy in the relative breeze going past the vehicle "because the air is not moving"?.......

Once you are doing your 30mph, the wind is moving relative to the turbine and will turn the turbine blades. The drag that this causes will mean the truck has to use more power and hence more fuel. What you effectively would be doing is liberating the chemical energy locked up in the diesel fuel to power the truck and to turn the blades. If you then took electrical power from the turbine to do something like run a light bulb you would further increase the drag caused by the blades and use up even more diesel - and that diesel would be being turned into light energy.

I did my degree in Physics. I'd be the first to admit that was a long time ago and my brain isnt what it was when I was a young man. But with those caveats I can see no way that the vehicle could go dead down wind faster than the speed of the wind without breaking the laws of physics. Problem is trying to explain this in words rather than equations.

 

[RAI]

Yes the energy to drive the cart would come from the wind passing the cart but as soon as the cart gets up to wind speed, there is no wind passing it.

The blades of the propeller are still moving when the cart is at wind speed moving down wind. They don't stop producing lift or thrust until their incidence angle gets too small.

If you look at the all the carts so far, they have precious little area for using drag to accelerate them. It is the propeller that does nearly all the work on extracting energy from the wind. The point at which the propeller stops extracting energy is not necessarily the point where the cart's speed reaches wind speed. Were it a spinnaker, you would be right, but its not a spinnaker or simple drag device. It's a propeller with the aerodynamic properties of a propeller.

 

[RAI]

Problem is trying to explain this in words rather than equations.

Read Mark Drela's analysis and put some numbers in, see if you get a zero net force at wind speed.

 

[Pierrome]

No I'm not saying that - it would be possible to make up a scenario in which this is happening. But there is no sensible mechanism by which a road can provide energy to a moving vehicle.
I did my degree in Physics. I'd be the first to admit that was a long time ago and my brain isnt what it was when I was a young man. But with those caveats I can see no way that the vehicle could go dead down wind faster than the speed of the wind without breaking the laws of physics. Problem is trying to explain this in words rather than equations.

Bosun thanks for your reply.

I think the road could provide energy by use of a wheel running along it. Would it not be similar to a rotating wind turbine? And this is what the cart uses anyway.

Yes, you're quite right to say that there would be drag caused to slow the vehicle down. But, this is not just "drag" it is energy being fed into the prop to counteract the "slowing down" effect.

Someone here might say "but a propeller cannot provide more thrust than the force required to rotate it" - actually it can. Because it has aerofoils like a plane's wing that provide greater lift than they do drag.

There is no way the cart can go faster than the wind without using energy from the road, because the wind has stopped blowing in the right direction relative to the cart.

Well, if you don't believe it you are not alone, non-believers are in the majority I think.

 

[DownWest]

What you're saying is nonsense. Of course it is possible to transfer the energy of its movement into the cart.

I will give you an example and I ask you to explain it.
Some jet airliners have an emergency system which is called RAT (ram air turbine) it is a wind turbine that can be deployed into the airstream in the event of electrical failure, and it provides enough power for essential services in the aircraft. It works because the aircraft is travelling through the air and there is a relative difference in velocity between the air and the aircraft.
You therefore are saying that it cannot work because the air is static although the aircraft is moving through.

I don't know why they would choose to fit such things on planes when it is impossible for them to get energy from (static) air going past!

Please explain why the (static) air going past the moving aircraft is different from a (static) road rolling beneath the moving cart.

Another irrelevent analogy. Aircraft have hight (energy) and in the case of loss of hydraulic power, trade some of the energy for useful power by sticking a fan out in the breeze to keep the controls working. What this has to do with the current discussion escapes me.
A

 

[Bosun Higgs]

You therefore are saying that it cannot work because the air is static although the aircraft is moving through.

No I'm not. What happens with the turbine you describe is that the drag of it slows the plane down a bit, converting some of the planes' kinetic energy into power from the turbine. Once is has slowed down a bit it is (with throttles untouched) then using up more power from the engines than it would normally do at that speed and the surplus power is what you are getting out of the turbine. Just as you would do if you were towing the turbine through the water behind your boat.

You are not taking energy out of the air. In fact you are giving it some energy because the turbulence your turbine will create. Thats where the drag comes in.

Just out of curiosity, what is your technical background?

 

[Bosun Higgs]

Someone here might say "but a propeller cannot provide more thrust than the force required to rotate it" - actually it can. Because it has aerofoils like a plane's wing that provide greater lift than they do drag.

If a plane were able to do this it could fly without an engine. Not in the way that gliders do by making use of upward air currents, but in still air. Indeed you would have a perpetual motion machine.

 

[Bosun Higgs]

There is no way the cart can go faster than the wind without using energy from the road, because the wind has stopped blowing in the right direction relative to the cart.

Thats absolutely right. So ask yourself, what energy is the road losing and how is the road different after the vehicle has passed on? Cos just like a cup of tea gets colder as it loses energy, so there must be a change in the energy state of the road if its given something up.

 

[Pierrome]

Another irrelevent analogy. Aircraft have hight (energy) and in the case of loss of hydraulic power, trade some of the energy for useful power by sticking a fan out in the breeze to keep the controls working. What this has to do with the current discussion escapes me.
A

Anyone can see the relevance. Bosun said it was "impossible" to extract energy from a static road. This example shows that you can extract energy from a static medium.

Incidentally, they don't have to have height. It works just as well if they run along the runway driven by their engines. No one is saying you can get energy from nothing.
You can get energy from two adjacent mediums which are travelling at different velocities. It doesn't matter which one is static and which one is moving. It's as simple as that. That's how it is possible to sail normally, and even go downwind faster than the wind. Either by sailing (as in the case of BMW Oracle) or using rotary motion (as in the case of the downwind cart).

 

[halfway]

RAI: At the end of the video there is a boat with a red flag. It shows the last leg to be a direct down wind leg so it is not relevant. Try again, and find a gybing down wind faster than the wind where there is no observable difference to tacking.


boomerangben: "Trying to compare this cart to sailing down wind (or vice versa) "

But I'm not. I am saying the comparison is invalid. The treadmill is not a proof of faster than the wind.



fireball: "Your paragraph about apparent wind travelling across the bow during a gybe is complete ****** "

Really, so I am going faster than the wind, down wind, and so the apparent wind is in front of the beam somewhere. If at the end of the manoeuvre I am on the opposite tack doing the same thing. Therefore the apparent wind is still in front of the beam (but the other one).

At the point that the stern moves through the REAL wind I will still be going faster than the wind. Therefore the apparent wind is directly ahead of the bows.

So it starts ahead of the beam. It moves through a position ahead of the bows and ends up in front of the other beam. That does not seem to be related to spherical items in any way.


Ubergeekian: The situation you want to consider is using a 20' wind tunnel.
1. Assume a 2' prop driven at constant speed in the tunnel. So that the generated "blow" is towards the wind tunnel "blowing" fans.

Start the fans but divert all the air outside the tunnel.

Measure the power going to the motor

Now suddenly divert the air into the tunnel.

What will be the final power to the motor after it is settled back to the steady state.

What will be the effects just after the pressure wave has passed the prop and the majority of the tunnel is seeing 10kts of wind.

2. A plane is flying along at 100kts on a calm day. With a speed of the prop governed to a fixed value.

Suddenly a wind arrives of 10kts from the aft.

What happens to the power during the time the plane accelerates to the new 110kt ground speed. (I am assuming you agree that tail winds result in faster travel across the Atlantic)

 

[Pierrome]

If a plane were able to do this it could fly without an engine. Not in the way that gliders do by making use of upward air currents, but in still air. Indeed you would have a perpetual motion machine.

So you are now saying that an aerofoil cannot have more lift than it has drag.

You have an example of this in the upwind windmill boat, in this case the air turbine (windmill) is providing more power than the drag it creates, because it moves towards the wind that is driving it. I fear you may say this is impossible too!

 

[halfway]

A road can not provide energy to a vehicle. That is completely true.

However, a moving vehicle can extract some of ITS energy by rotating a wheel against a road.

If however the vehicle was able to obtain the same amount of energy from another source it would not slow down.

This is the basis of the force part of the proof. All you need to do is to assume that the energy extracted is used to rotate a prop.


Now if that prop was so arranged that it acted as a sail on the wind, using the rotation to create an apparent wind, it could extract energy out of the wind.


The apparent wind part of the proof then is used to show the lift angle has a larger forward than rotational component. Therefore the net force is to accelerate the vehicle.


But the limitation is that when the vehicle speed approaches approx 3x the wind speed, the best aerofoil will have a lift vector at 45 degrees so can not produce any more acceleration. Past 45 degrees the rotational force is greater the forward thrust.

 

[RAI]

RAI: At the end of the video there is a boat with a red flag. It shows the last leg to be a direct down wind leg so it is not relevant. Try again, and find a gybing down wind faster than the wind where there is no observable difference to tacking.

BMW Oracle Racing's web site, videos, America's cup, race 1, about at 1 hr 58 minutes, both "platforms" gybe. Watch how the foresails behave and the speed through the water. When you there, note the time at the windward mark 1:39) and the time at the finishing line (2:42) and work out the average speed for the 20 nm down wind leg. Explain how a device driven by the wind can have a downwind VMG well above the prevailing wind speed.

 

[DownWest]

Pierrome,
What I am saying is that there are many people coming up with better arguments than you are. " Harvesting energy from the road" No. It comes from the movement of the object passing over it.
The aircraft you quote is trading energy produced by its engines, height + velocity, into powering its hydraulics with a fan. The air may be fixed relative to the plane, but it is using its kinetic energy to drive the fan, and slows down as a result.
I will wait until the full size proofs prevail until I decide who is right. But I doubt if downwind sailing IN BOATS is happening soon.
A
Oh dear, I meant downwind faster than the wind. Head down....

 

[halfway]

Pierrome: "This example shows that you can extract energy from a static medium."

No it does not. I am beginning to think you enjoy this world you live in. A fan pushed out of a moving plane on a runway or while flying is extracting energy from the movement of the plane using the wind as a reference. It will slow the plane.

 

[halfway]

RAI: I do not accuse people of being trolls lightly, but we started by discussing gybing and tacking differences, now you are trying to prove Down wind faster than the wind can be done, to someone that openly supports it!

Original words: "Would you know if this was a tack or a gybe "
Now you say: "Explain how a device driven by the wind can have a downwind VMG well above the prevailing wind speed."


If you want to troll, try and keep to the plot. It makes it more fun.

 

[Pierrome]

Pierrome: "This example shows that you can extract energy from a static medium."

.

I will re-phrase that, because I didn't think it would be so easily misunderstood.

"This example shows that you can extract energy from the relative movement between a medium that is static with respect to the earth, if you yourself are moving with respect to the earth."
"Static" or "Moving" have no relevance unless you state with respect to what. If you are unable to imagine a different frame of reference, then you are unlikely to understand how DDWFTTW is possible.

 

[RAI]

RAI: I do not accuse people of being trolls lightly,

On the contrary, you do it very rapidly.