Downwind Faster than the Wind - Successful Run by manned cart

Saltwater Gypsy,

Empty your mind of pre conceptions, fill it with nothingness and then start again. I have to say it worked for me. At the moment, you think this mechanism creates energy from nowhere, where as I think your logic destroys energy.

Even when the cart is at the same speed as the wind, the air is still moving and therefore still has energy. That energy is up for grabs. If you stood next to the cart as it went past you can see the cart has energy (kinetic energy due to its speed) and you can feel the air has energy (wind in your face). Just by jumping on the cart as it went past doesn't mean that the energy in the air disappears.

Newton's laws are working very well in this cart. He would be very proud.
 
Last edited:
saltwater_gypsy said:
As ever, these discussions are getting bogged down in detail.
Forget about propellors , gearing variable pitch etc and enclose whatever clever mechanism you can think of in an open ended wheeled box which allows free flow of the wind from front to back.
No matter what mechanism you put in the box,it will accelerate downwind until, at best, it achieves the same velocity as the wind..... end of story...unless you can find some extra force from some additional energy source
You are stuck with Newtons Law that a mass will continue at uniform velocity unless some external force is applied.
You have used up all your external forces, the wind..... and your argument!
Click to expand...
Here's my mechanism, in a very big box that lets the wind through. The box is on wheels, which allows it to be towed down wind. I put in two land yachts, each of which can tack/gybe downwind. I tie a rope between the box and each of the two yachts. If they sail down wind, the box will be pulled down wind too. They yachts set off on opposite tacks. Each can achieve a VMG down wind of 3 x the wind speed and tacks before reaching the edge of the box or each other in the middle. As the yachts achieve 3 x downwind VMG the towed box does too. Now we have a box travelling down wind faster than the wind.
 
shaunksb said:
No - aeroplanes work - really. Its all to do with the opposing forces of thrust, drag, lift..........................AAAhhhh! Your giving me back some banter aren't you?

I like that - jump on in...........................



just make sure you can swim!
Click to expand...


I've never heard a drowning man quite so happy, roll on NALSA verification... :)
 
RAI said:
The force on the propeller blade can be resolved into lift and drag. However it can also be resolved into along track and cross track components. The along track component is the thrust driving the cart forward. The cross track component is opposing the propeller's rotation. This later component is countered by the torque provide from the wheels.

As long as the torque from the wheels is greater than the cross track component (summed over both blades x radius) the propeller can spin faster.

As long as the down track component is greater than the sum of all forces opposing forward motion, the cart can accelerate. The biggest of these forces is the resistance of the drive wheels, which stems from the cross track force on the blades.

All rather circular, but it means if the blade's aerofoil produces a lot more lift than drag, this approximates to a lot more along track force than cross track force. So we have a lot more force pushing forwards than force opposing forward motion. The cart can accelerate.

However, higher forward speed changes the angles in SL's vector diagram, which means the propeller reaches a point where it is producing less lift for greater drag. (Were the propeller adjustable in pitch, this need not happen.)

BUT - the vectors in the along track force and cross track force force change with the apparent wind vector too, lift is contributing more to cross track and drag is contributing more to slowing the vehicle.

Consequently, a limit is reached where the along track force is just balancing the cross track force expressed in terms of torque through the wheels and gearbox.
Click to expand...

Carefull RAI,

Your crosstrack force (to me at least) infers that there is a tendency to move the cart sideways as it moves down the track. I think you are meaning that the crosstrack force is the planar (in the same plane the in which the propeller rotates) component of the net drag/lift force, which is of course opposed by the same force on the other blade(s). These result in no net force perpendicular to the wind, but causes a torque that tends to roll the cart over, which of course is reacted by the fact the vertical component of reactions at the wheels are different on each side.
 
snowleopard said:
As it has a variable pitch prop I think setting the blades to fully fine would do the job. If they are set square to the wind they would act like a square sail then, once they are turning at a reasonable speed, they could be pitched up to give some drive.
Click to expand...

Look at the second video, showing the start. The prop blades appear to have a lot of pitch at the start, whereas I would expect them to be arranged for near zero pitch at start off, to propel the thing with the most push from the wind and least resistance from prop rotation.
 
Norman_E said:
Look at the second video, showing the start. The prop blades appear to have a lot of pitch at the start, whereas I would expect them to be arranged for near zero pitch at start off, to propel the thing with the most push from the wind and least resistance from prop rotation.
Click to expand...

If the driver can't adjust the pitch I guess they have to set the blades for optimum top speed which is going to make it tricky to get started as the wind is trying to turn the blades one way and the mechanism is trying to turn them the other way. It's noticeable in the videos that it sometimes needs a nudge to get it going. (Of course the deniers are going to say that it uses the energy from the nudge to exceed wind speed ;))
 
snowleopard said:
It's noticeable in the videos that it sometimes needs a nudge to get it going. (Of course the deniers are going to say that it uses the energy from the nudge to exceed wind speed ;))
Click to expand...

NALSA rules explicitly allow a push start for record attempts.

So, while the vehicle is already being tested, recorded on video, it's design and build is well documented online and arrangements are being made by the official body to speed test it, there are still people who deny it can possibly work. :confused:

Confused and amused.... :D
 
RAI said:
Here's my mechanism, in a very big box that lets the wind through. The box is on wheels, which allows it to be towed down wind. I put in two land yachts, each of which can tack/gybe downwind. I tie a rope between the box and each of the two yachts. If they sail down wind, the box will be pulled down wind too. They yachts set off on opposite tacks. Each can achieve a VMG down wind of 3 x the wind speed and tacks before reaching the edge of the box or each other in the middle. As the yachts achieve 3 x downwind VMG the towed box does too. Now we have a box travelling down wind faster than the wind.
Click to expand...

I get the idea
BUT​

...the problem here is that if landyachts set off on opposite tacks how could you continuously maintain a connection to the box?Its back to the water skier sortof thing but in reverse.
BUT​

.... is the idea that the blades of the propellor are pitched so that they are contiuously on a broad reach so that, even when the vehicle is going dead downwind, the angle of attack of the prop blades to the wind is such that is still generating power. Mmmmmm... will have to think some more.... and what is this wind I'm referring to..............
 
Last edited:
DDWFTTW convert

Ubergeek and SL, an aplogy.

I now understand how it can work, even though my intuition screams otherwise. It's nothing to do with conservation of energy or conservation of momentum. It simply a matter of leverage.

Good, though lofty explanation here:

http://projects.m-qp-m.us/donkeypuss/wp-content/uploads/2009/05/drela_efficiency.pdf

More down to earth explanation here.

http://docrampage.blogspot.com/2009/11/ddwfttw-directly-downwind-faster-than.html

Uber and SL. Neither of you are teachers are you?
 
RAI said:
Here's my mechanism, in a very big box that lets the wind through. The box is on wheels, which allows it to be towed down wind. I put in two land yachts, each of which can tack/gybe downwind. I tie a rope between the box and each of the two yachts. If they sail down wind, the box will be pulled down wind too. They yachts set off on opposite tacks. Each can achieve a VMG down wind of 3 x the wind speed and tacks before reaching the edge of the box or each other in the middle. As the yachts achieve 3 x downwind VMG the towed box does too. Now we have a box travelling down wind faster than the wind.
Click to expand...

Thats brilliant!!!

For land yachts reaching downwind substitute propellor blades (which are doing exactly the same job) and you have this machine.

The sceptics forget thet the blades are not travelling directly downwind - they travel in a downwind spiral analagous to tacking
 
boomerangben said:
But the propeller acts on some of the air in the box blowing it backwards (relative to the cart). So a portion of air in the box is not traveling away from the starting post at the same velocity as the cart, it is traveling away from the starting post at a lower velocity. Therefore that air has less kinetic energy relative to the starting post. It is that kinetic energy which goes to overcome friction - the energy balance is heat energy gained because of friction = loss of kinetic energy in the air that is slowed by the propeller.
Click to expand...

Spot on.
 
ubergeekian: "I'm sorry that Halfway hasn't felt able to answer the three simple questions I asked. Nevertheless, here's a simple one for you: relative to a reference frame fixed to the cart (or boat) how much kinetic energy does it gain as its ground (or water) speed increases?"

What an arrogant unforgiving, and completely wrong person you are. How can you assume I was even reading the forum. Now I am back, temporarily, it is clear to see some major developments.

As for your question. a fixed reference point on a cart means the cart CANNOT have any energy due to its momentum. Since it can not have any momentum. That is what you are missing all along. Your arguments are dogmatic and wrong. It can not have any linear or rotational momentum as it is fixed to the reference. So no energy can be gained or lost by the cart.

ubergeekian then goes on to confirm that he has not altered from the energy relative to the ground theory by saying "Air resistance increasing with the square of the apparent headwind, to a large extent. " So confirming his belief in friction being the fundamental limit.

No change there, but others seems to have noticed the problem!




RAI seems to have developed a bit. From a complete belief in the ridiculed professor's energy proof, he is now proposing my explanation of the speed and apparent winds which SL kindly created as a diagram.

Example of my words: "9. The wheel is trying to slow down the vehicle to counter the West force."

RAI: " This later component is countered by the torque provide from the wheels."

But then his newly found belief system falls flat on its face when he says:
"However, higher forward speed changes the angles in SL's vector diagram, which means the propeller reaches a point where it is producing less lift for greater drag. (Were the propeller adjustable in pitch, this need not happen.)"

Which is completely and utterly wrong. The prop pitch can not be adjusted so that the cross track component is always smaller than the forward component. There is a limit for a given aerofoil that can not be exceeded.

Never mind RAI, a good try.


The Opposition: Good arguments against the bad proofs. But with your arguments I assume none of you actually sail to windward. As conceptionally it is harder to go into the wind, than sail down wind faster than the wind. You are asking someone to believe that atoms with momentum, going the WRONG way, make it possible to oppose them. But you can not see if you run faster than the wind you do not bump into more atoms that have energy going your intended way. Kind of strange.


Anyway well done to the team for demonstrating it.
Suggestions
1. When doing relative wind indicators try 2 suspended weights on thin rods. On one fit a disk to cause wind resistance. From the angles of both you can deduce if it is acceleration or wind that is doing the shift. (Good having the forward indicator.)
2. Redesign the prop. It is very inefficient for that forward helix.
3. I am glad that you did not kill the driver when the prop shaft broke. Obviously you did not understand the huge forces involved due to the forward force being the difference of 2 vectors otherwise you would not have broken the drive chain. This was brought up earlier. Take care. Next time put the prop on the back of the stand to give a some protection.
4. Borrow a better camera! Camera operator that could hold it steady might help.


Back to real world, so Ubergeekian can insult me again in my absence. (I notice he never answered the better specified situations either, I did actually answer his as being impossible. But then why let actual facts get in the way of a good insult)
 
shaunksb: "Link 2 is poppycock


Link 1 is just confusing"


I suggest both are poppycock as proofs. I understand your complete opposition when confronted by these explanations, but the real proof is very simple. The proof is no more complex than sailing to windward and has all the same stages of resolving speed vectors and force vectors.

It has nothing to do with friction, all you need is one real life aerofoil characterization graph. No models and no approximations.
 
shaunksb said:
Link 2 is poppycock


Link 1 is just confusing
Click to expand...

Link 1 isn't a proof, it simply shows how the energy conversions happen. The inclusion of a water turbine is an unnecessary complication for our case. The equations of course are no help to those who don't have the background to understand them.

Link 2 isn't a proof, it is an aid to understanding that it is possible. I don't feel it adds a lot to what has already been said. The logic of it however is valid.
 
You must log in or register to reply here.

Share this page

Top