Wind Power Vehicle Traveling Down Wind Faster Than The Wind

Click For Summary
The discussion centers on the feasibility of a wind-powered vehicle traveling faster than the wind itself, a concept known as Directly Downwind Faster Than The Wind (DDWFTTW). Participants share insights on past experiments, including a full-scale model that achieved notable speeds, and debate the mechanics involved in achieving such speeds, particularly when moving at angles to the wind. The conversation highlights the theoretical underpinnings of the vehicle's propulsion system, which utilizes wheels to drive a propeller for downwind travel and a turbine for upwind movement. Additionally, there is a focus on the efficiency of these systems and the potential for future developments in wind-powered vehicle design. Overall, the thread emphasizes ongoing interest and research in this area of physics and engineering.
yn3
Messages
8
Reaction score
0
There was and still is a hot debate on the matter of a wind power car that travels down the wind faster than the wind. As a part of larger interest in the subject I put the theory for this case, it can be found in my website
https://sites.google.com/site/yedidianeumeier/"
I welcome your comments
 
Last edited by a moderator:
Physics news on Phys.org
"There are several past threads on the subject containing thousands of posts" Indeed so, and all the previous threads are closed, at list one of them because of improper language. My experience with published theory on this subject suggest that I can contribute to the matter. I hope that this thread will remain civil and to the point. I am welcoming your feedback.
 
The most recent thread includes links to videos of a working full scale (human pilot) model that accomplished about 28 mph speed with a 10 mph tail wind:

https://www.physicsforums.com/showthread.php?t=390801

Videos of this model before the outer skin was added can be seen here:

http://www.youtube.com/user/TraderTurok

Some sailcraft, such as a "Skeeter" ice boat, when tacking downwind (at an angle, not directly downwind), can also achieve a net downwind speed greater than the wind.
 
Thanks, you are right I am very familiar with Rick blackbird. NALSA actually ratified a 2.8X speed record on his behalf, you can see it in http://www.nalsa.org/index.htm
Note that unlike sail car, prop-turbine wind car does not tack the wind and thus a race of these cars could take place on a regular closed loop race track. I intend to develop the theory for all direction car and hopefully build one myself.
 
Anyone with a half-decent brain can ponder this question for a few minutes and realize that YES, it is possible to make a vehicle go faster than the wind, directly down the wind, solely on wind power.

It is also possible to go directly against the wind, but this one is even easier to figure out.
 
Well, the "half-decent brain can ponder this question for a few minutes and realize that YES" seems at that point like http://en.wikipedia.org/wiki/Egg_of_Columbus" . I personally communicated with two distinguished professors, one of which share the Nobel Prize with Al Gore and is currently advising the president on energy matters, the other one run a blog about physics where he explained why it cannot be done. Both admitted to me that now they are confused, they don't know how it can be explained. Now, leave this aside and proceed to ask the following. To move with (down) the wind the car uses a propeller driven by the wheels, to move upwind one needs a wind turbine the shaft of which drives the wheels. How fast can this car go? Now, what happens if you go in an angle to the wind? Is there an angle where the propeller is inefficient and the turbine likewise so that it is so to say a blind spot to the car, meaning an angle it cannot drive or is it a point of overlapping manning both propeller and turbine are effective, if so what is the maximum velocity at this angle? All this can be readily answered, however it will take me a bit more than few minutes to put the equations run the simulation type it neatly and post it.
 
Last edited by a moderator:
yn3 said:
... two distinguished professors, one of which share the Nobel Prize with Al Gore
That of course is the the highly political Peace prize, and not for physics or the other sciences.
 
Well, I guess I don't have half-decent brain power because I thought about for a few minutes and couldn't come up with it. Damn my less than half decent brain...
 
  • #10
"That of course is the the highly political Peace prize, and not for physics or the other sciences. " Indeed so but the guy, according to his bio is
Professor of Nuclear Engineering and Ph.D (1998) & MA (1986) Harvard University (Physics),
AB (1984) Cornell University (Physics)
The AB is in his bio it may be a typo for BA but you see my point.

"Damn my less than half decent brain" Can I offer you half of mine? My wife says my brain is full of useless physics and otherwise useless historical knowledge.

To the point, humor and gossip is always great but what about discussing the all direction car?
 
  • #11
> Angle to wind ...

Once at a significant angle, a standard sail setup would be better. In a DDWFTTW vehicle, the ground force opposes the forward motion of the vehicle, because that force is used by the wheels to drive the propeller. In a sailcraft, except for drag or rolling resistance, the ground force is perpendicular to the forward motion of the sailcraft, and consumes no energy, and this would be a better setup when moving at an angle to the wind.
 
Last edited:
  • #12
"Once at an angle, a standard sail setup would be better" This is a qualitative statement. What angle do you have in mind 1deg? 10 degrees? However, let's wait and see what other thinks.
 
  • #13
yn3 said:
To move with (down) the wind the car uses a propeller driven by the wheels, to move upwind one needs a wind turbine the shaft of which drives the wheels. How fast can this car go?
In terms of wind-speed-multiple there is no particular limit in either direction. It just depends on efficiency.

in terms speed relative to the air, there are practical limits related to the gas properties. I doubt the sound barrier could be broken.

in terms speed relative to the ground there the speed of light limit

yn3 said:
Now, what happens if you go in an angle to the wind? Is there an angle where the propeller is inefficient and the turbine likewise so that it is so to say a blind spot to the car, meaning an angle it cannot drive or is it a point of overlapping manning both propeller and turbine are effective,
The blind spot for faster than wind would be at 90°. But with cyclic pitch control propeller/turbine ranges would overlap.
 
Last edited:
  • #14
The top class ice boats, called Skeeters, would out perform the DDWFTTW Blackbird, given sufficient room to tach at an angle to the wind (30 to 40 degrees). Based on information about the Skeeter in this pdf file:

http://www.nalsa.org/Articles/Cetus/Iceboat Sailing Performance-Cetus.pdf

I calculated that a Skeeter's downwind component of speed was 3.36x wind speed in an 18 mph wind (over 60.6 mph downwind component while at taching at 30 degrees offset from wind). The ratio would be higher in a 10 mph wind due to less aerodynamic drag from the apparent headwind. See post # 28 in this thread:

https://www.physicsforums.com/showthread.php?t=283813
 
  • #15
There were runs as high as 3.48x WS but were not used for the record because they were borderline. The team wanted an absolutely indisputable run for the initial record.

The purpose of establishing the record was more about proof of DDW travel by a recognized independent authority than the highest number achievable - I'm sure that others will go for the big numbers in years to come.
 
  • #16
Surprise surprise: That is if my calculations are true, and rest assure that I am and will double checking them for a while.
The propeller wind car can do extremely well in side winds. Assuming that the propeller shaft can be rotated sideways, (which is not possible in the current ThinAirDesigns car), a car that can do 3XW down the wind can do more than 4XW with 90 degrees wind, the propeller will be swayed about 70 degrees sideways to 20 degrees off the wind. Moreover, the calculations show that even with the fix front thrust propeller, the aforementioned car capable of 3XW down the wind will do still better than 2XW at 45 degrees to the wind. This part of my calculation should be able to be confirmed or rejected by the experience of ThiAirDesigns, I am sure that they took some runs with angle to the wind. mender what say you?
 
  • #17
yn3 said:
Moreover, the calculations show that even with the fix front thrust propeller, the aforementioned car capable of 3XW down the wind will do still better than 2XW at 45 degrees to the wind. This part of my calculation should be able to be confirmed or rejected by the experience of ThiAirDesigns...

We never ran the vehicle in anger in full 45 cross, but we have data showing ~2.5x in sustained 30-40 cross.

JB
 
  • #18
"We never ran the vehicle in anger in full 45 cross, but we have data showing ~2.5x in sustained 30-40 cross."
That's seems to support my claim, if I know your transmission efficiency and the maximum down the wind speed ratio, I know it is around 3.5 but would like to know your simulation prediction, I then could run my calculations to provide a so called polara, the maximum ratio as function of angle.
 
  • #19
Sorry to see this topic dropped. I was looking to seeing Yn3's maximum angle. Rumor has it that with it's fixed prop Blackbird functions best directly downwind. Any thoughts on that?
 
  • #20
RCP, it's not the 'fixed pitch' nature of the Blackbird (which has gone DDWFTTW in both fixed pitch and variable pitch configurations) that makes it function best DDW, it's the circular nature of it's sail path.

Fixed or standard 'variable pitch' makes no difference in it's off axis performance. Even if it were equipped with a 'cyclic pitch' hub, it would still perform best DDW -- but the off axis performance would degrade slower off DDW.

But the you knew that already -- I'm just responding to the thread to answer your question on the record.

JB
 
  • #21
Here we go again :-)

As others said, you do not deserve your marks on mechanics 101 if you can't figure out that this is trivially possible to go faster than the wind. The elementary theory of it (of the order of an exercise of medium difficulty in said mechanics 101 course) has been posted several times and it is quite disturbing if people with a degree which should have mastered such a course cannot understand it almost immediately, worse, dispute it. You could just as well dispute that things heavier than air can fly.

The long, locked threads cited earlier must contain about everything that can possibly be discussed about it.
 
  • #22
I have read a dozen of pages on this topic but I cannot understand it. Could someone please explain it or give a link to an explanation? Isn't it true that once you are up to wind speed and use the wheels start to power the propeller, the energy will come from the kinetic energy of the cart and since the propeller cannot propel air at 100% efficiency some energy will be lost and shouldn't the cart slow down? I simply don't understand how it works.
 
  • #23
chingel said:
I have read a dozen of pages on this topic but I cannot understand it.
I read them all.
Could someone please explain it or give a link to an explanation? Isn't it true that once you are up to wind speed and use the wheels start to power the propeller, the energy will come from the kinetic energy of the cart and since the propeller cannot propel air at 100% efficiency some energy will be lost and shouldn't the cart slow down? I simply don't understand how it works.

It's meant to be an exercise in aerodynamics. Break the problem down into a free body diagram and figure out if it is possible or not.
 
  • #24
chingel said:
I have read a dozen of pages on this topic but I cannot understand it. Could someone please explain it or give a link to an explanation? Isn't it true that once you are up to wind speed and use the wheels start to power the propeller, the energy will come from the kinetic energy of the cart and since the propeller cannot propel air at 100% efficiency some energy will be lost and shouldn't the cart slow down? I simply don't understand how it works.
If there was no wind, the cart would have to be 100% efficient just to maintain speed, which of course in the real world isn't possible. Since the air is moving wrt the ground, the thrust at the propeller only has to be a portion of what it would be if there was no wind. Given reasonable losses to friction and such, a properly designed cart is quite capable of doing so.

Did that help?
 
  • #25
mender said:
If there was no wind, the cart would have to be 100% efficient just to maintain speed, which of course in the real world isn't possible. Since the air is moving wrt the ground, the thrust at the propeller only has to be a portion of what it would be if there was no wind. Given reasonable losses to friction and such, a properly designed cart is quite capable of doing so.

Did that help?

I can understand that the thrust has to be a portion of what it would have to be when there is no wind, but nevertheless, no matter how little thrust, it has to come from the kinetic energy of the moving cart, I would think. Then the propeller uses some of that energy to accelerate air particles in one direction which accelerates the cart in the other direction. Isn't it true that the propeller cannot accelerate air particles with more force than it is getting from the wheels? What am I missing?
 
  • #27
chingel said:
I can understand that the thrust has to be a portion of what it would have to be when there is no wind, but nevertheless, no matter how little thrust, it has to come from the kinetic energy of the moving cart, I would think. Then the propeller uses some of that energy to accelerate air particles in one direction which accelerates the cart in the other direction. Isn't it true that the propeller cannot accelerate air particles with more force than it is getting from the wheels? What am I missing?
You're forgetting that the thrust at the prop can be higher than the drag at the wheels but only because the ground is moving past the cart faster than the air. If there is no wind, the air is moving past the cart at the same speed.

The cart essentially gears down the force at the wheels and turns the prop slower than the ground speed but with more force against the air.

The drag at the wheels is x, the speed across the ground is v; since the prop is acting on the air, with no losses, the thrust from the prop can be 2x when it is turning at a rate to move the air back at half the ground speed (v/2). Same principle as a lever; the force at one end can be much greater than at the other but the distance is less.

Better?
 
Last edited:
  • #28
Uglybb said:
A simple wikipedia search would have given you all the answers

http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind

The formula and maths etc ... everything its all there!
But even with all the answers, sometimes it has to be explained in the right order and the right way for the light bulb to go on.
 
  • #29
mender said:
You're forgetting that the thrust at the prop can be higher than the drag at the wheels but only because the ground is moving past the cart faster than the air. If there is no wind, the air is moving past the cart at the same speed.

The cart essentially gears down the force at the wheels and turns the prop slower than the ground speed but with more force against the air.

The drag at the wheels is x, the speed across the ground is v; since the prop is acting on the air, with no losses, the thrust from the prop can be 2x when it is turning at a rate to move the air back at half the ground speed (v/2). Same principle as a lever; the force at one end can be much greater than at the other but the distance is less.

Better?

I'm sorry I do not understand how. How can the thrust be higher than the drag at the wheel? Aren't they are mechanically connected? Move air back at v/2 relative to what, the cart or the ground? If you use gears to make the propeller move faster than the wheels, it will displace more air but also drag the wheels more, because it does more work and needs more energy, if you make it spin slower it provides less acceleration but also causes less drag, at least the way I understand it.
 
  • #30
chingel said:
How can the thrust be higher than the drag at the wheel? Aren't they are mechanically connected?
Thrust is a force, and drag is a force. You do know that by using a lever or gears, you can put x amount of force into one end and get 2x, 3x, or 10x force out of the other end right?

Gears are mechanically connected yet change the amount of force put in at one end to a different force at the other; no magic, just leverage or mechanical advantage.

chingel said:
Move air back at v/2 relative to what, the cart or the ground?
Relative to the cart.

chingel said:
If you use gears to make the propeller move faster than the wheels, it will displace more air but also drag the wheels more, because it does more work and needs more energy, if you make it spin slower it provides less acceleration but also causes less drag, at least the way I understand it.
But we use gears to slow down the propeller, displacing less air but supplying more force for the same amount of work taken from the wheels.

For the sake of clarity, let's take losses out of the picture for a moment.

The wind is blowing at 10 mph. The cart, without the propeller engaged, gets pushed up to 10 mph. The pitch of the prop and the gearing of the cart is set up to push air back relative to the cart at half the speed of the wheels. The prop is engaged; the air around the cart gets pushed back at 5 mph, half the speed of the wheels. Since the force at the wheels is geared down, the force at the prop is twice as much as the drag at the wheels and the cart accelerates. F=MA so if the net force acting on the cart is positive, as it is so far, the cart will accelerate.

The cart gets to 15 mph; the prop is now turning fast enough to push air back at 7.5 mph and the force at the prop is still higher than at the wheels, so the cart will still keep accelerating. About now you're thinking that if that is true,the cart will keep accelerating forever; it won't, because the gear ratio determines the top speed in accordance to the speed of the wind.When the cart gets to 20 mph, the prop is pushing air back at 10 mph and the wind is still blowing at 10 mph, meaning that the prop is no longer exerting any force on the air, it's just freewheeling. With this gear ratio, the absolute best the cart can travel across the ground is twice the speed of the wind.

Now the wind drops to 7.5 mph; what happens to the cart? At 20 mph, it is still pushing air back at 10 mph but with the wind at only 7.5 mph, that means that instead pushing air back, it is actually trying to drag air forward at 2.5 mph. The net force acting on the cart is now negative, so the only thing that can happen is that the cart will slow down. What speed does it stabilize at? Again, twice the speed of the wind, or 15 mph.

Gearing the wheel speed down by half (0.5) to power the propeller gives a theoretical top speed of twice the wind speed. A different ratio will give a different theoretical multiple of the wind speed. Gearing the cart down less means the prop is spinning faster but with less force; it has the potential to faster for the same wind speed but it needs to be more efficient because the prop is supplying less force than before to overcome the real world losses.
 

Similar threads

Undergrad Power available to a wind powered vehicle traveling directly downwind
  • · Replies 60 ·
3
Replies
60
Views
4K
High School How Does Blackbird Leverage Wind Power to Move Faster Than the Wind?
  • · Replies 69 ·
3
Replies
69
Views
15K
Undergrad Focussing on the Treadmill of Veritasium's Blackbird faster than wind
  • · Replies 15 ·
Replies
15
Views
3K
How can an vehicle move faster than the wind that is powering it?
  • · Replies 175 ·
6
Replies
175
Views
34K
Undergrad Sail craft and apparent wind theory
  • · Replies 5 ·
Replies
5
Views
2K
Propeller thrust with backwards air flow
  • · Replies 3 ·
Replies
3
Views
2K
Undergrad Why is it claimed that Betz's limit is a law?
  • · Replies 30 ·
2
Replies
30
Views
3K
Can a sailing boat go faster than the wind when going in the downwind direction?
  • · Replies 14 ·
Replies
14
Views
4K
Wind powered vehicle (Derivation)
  • · Replies 15 ·
Replies
15
Views
5K
Transformations of Energy in Wind Power
  • · Replies 3 ·
Replies
3
Views
2K