# DDWFTTW - Directly Downwind Faster Than The Wind

Discussion in 'Propulsion' started by Guest625101138, Jan 4, 2009.

1. Joined: Jun 2021
Posts: 3
Likes: 0, Points: 1

### rushiNew Member

two problems in the calculation.

first minor problem, wheel or cart is limited reservoir of kinetic energy. as 600w is taken from the wheel and given to prop, wheels energy will decrease that will reduce the speed lets say by 0.5 m/s

second and main issue.
yes, 100N drag will produce 600N forward push for prop but it is not the prop only that has to go forward, it has to push the wheel as well which is going at faster speed so that 600N push from prop will be 100N for wheel that will increase wheel speed by 0.5 m/s, same as speed penalty in first case.

2. Joined: Mar 2008
Posts: 820
Likes: 246, Points: 43, Legacy Rep: 743
Location: Huddersfield, UK

### Tiny TurnipSenior Member

Here is the clearest video, showing acceleration from a standing start, and, from about 2 minutes 50 seconds in, the change in direction of the red streamers, which also show that the vehicle continues to accelerate until the end of the clip, and the vehicle speed does not drop back below wind speed:

3. Joined: Jul 2006
Posts: 1,262
Likes: 187, Points: 63, Legacy Rep: 358
Location: finland

### keroseneSenior Member

Rushi, take a momentary snap shot of 600N pushing the whole cart forward and 100N resistance trying to slow it down.
What will happen?
Instead of justifying why it don’t you accept the proven success and try to understand how it works (instead of “if it works” or “why it doesn’t work”).

4. Joined: Jun 2021
Posts: 3
Likes: 0, Points: 1

### rushiNew Member

take a momentary snapshot when cart is traveling at 12 m/s in 10 m/s wind.

"cart is going in 2 m/s head wind and speed is not decreasing "

btw more the moment of inertia of propes, more time period and amplitude of high speed low speed oscillation. use heavier props in higher wind speed to have more vehicle speed to wind speed ratio.

5. Joined: Jul 2006
Posts: 1,262
Likes: 187, Points: 63, Legacy Rep: 358
Location: finland

### keroseneSenior Member

Exactly. It is going 2 m/s headwind, wheels traveling 12 m/s
Plenty if excess with ratio of 1:6

As speed increases the ratio decreases. For example 15m/s (10 m/s wind) we are down to 1:3
At 20 m/s is down to 1:2

naturally overall drag of vehicle grows with the square of headwind too so it is actually quite impressive that Blackbird did reach such high speeds. Propeller and aerodynamics were well tuned. Also it operated in higher wind I believe.

I don’t get your oscillation point but it is not how it works.
The propeller blade is very analogous to a sail traveling at an angle to the wind.

6. Joined: Jun 2021
Posts: 1
Likes: 0, Points: 1

### chen.gNew Member

I registered specifically to talk about this, I think I have a good explanation of what's going on. Simply put, this vehicle operates differently at different speeds relative to the wind. It starts off the same as a normal sail, but once it exceeds wind speed it transitions into a headwind windmill, and becomes wheel-driven rather than sail-driven.

Ok so I'll start with why the sail boat around the cylindrical Earth idea doesn't work, simply because the sail boat cannot go faster than the wind, in the SAME DIRECTION as the wind. If wind is blowing in Y direction and your sail boat is angled to make it go faster than the wind, no matter what you do, the y component of that speed still won't be faster than the wind. Also if this is how it works then there would be no need to connect the propeller to the wheels, you should be able to just put up a free propeller on a free cart and we all know that wouldn't work. The idea that a sail boat can go faster than the wind in a different direction, ONLY applies to the fact that a windmill can spin faster than the wind, or more precisely, the tip speed can be faster than the wind.

Second I'll point out the undeniable fact that once you have a vehicle like this going faster than the wind in a steady state, it is a headwind windmill vehicle. You have all kinds of resistance slowing the vehicle down, and the ONLY source of thrust you have is the torque in your propeller and therefore wheels. It should be obvious that at this stage the vehicle is powered by windmill effect, via the wheels, and this is the secret of it being able to sustain a faster than tail wind speed.

Thirdly, it's obvious but yea, there's no way you can sail down wind faster than the wind, as soon as you reach the wind speed, you're no longer sailing. Clearly you have some other mechanism at play here.

I think most people are so confused by this vehicle is the fact that it's a hybrid of both drive modes, and in fact it's a hybrid of three drive modes. It's not really sailing down wind faster than the wind, it's a car that can sail down wind decently quick, and a windmill car that can go into the wind, and it's a fan-thrust vehicle that can accelerate from below to above wind speed.

This third drive mode is exactly what's demonstrated by the treadmill experiment, what it shows is if this vehicle is somehow accelerated to a certain speed, the propeller will be driven fast enough to accelerate the vehicle. This may appear to violate some kind of energy conservation, like what if you have a giant ring with this car on the surface, you spin the ring and the car would just run around it faster and faster. That won't happen because the car imparts a drag force on the ring, which will just slow it down.
Also the car cannot accelerate infinitely against the wind because the effectiveness of the propeller reduces with airspeed. So if you have an infinite treadmill, what would happen is it would get (either up or down) to a certain speed, and just stay still while the treadmill has to overcome all the friction in the system, no perpetual motion.

The way this works, is the vehicle starts off as a simple sail, the wind just pushes it forward. It works because the propeller isn't free spinning and so some forward force is transferred.
Then as you get up speed, the wheels will drive the propeller to generate thrust, where the wheels are driven simply by the inertia of the vehicle. As you approach the speed of the wind, it's hard to tell the comparative magnitude of all these forces so it's not clear if the car can just straight accelerate into a faster than tailwind situation. I suspect it can't, maybe it requires some change in either the wind speed of configuration of the vehicle to actually enter the final stage. For example maybe it needs the wind to temporarily slow down so that it enters the third stage. Or maybe it requires you to manually change the pitch of or gearing of the propeller so that it steals some of the momentum of the vehicle to turn it into thrust which pushes you into an effective headwind. But I guess it is entirely possible that these things just work out to perfectly overlap for a smooth transition between the drive modes.
Once you're going past the speed of the tailwind, you are now in the headwind, windmill operation where there again should be no controversies that this works.

Last edited: Jun 12, 2021
7. Joined: Jul 2006
Posts: 1,262
Likes: 187, Points: 63, Legacy Rep: 358
Location: finland

### keroseneSenior Member

this is incorrect. land sailing carts reach downwind goals (distance made good) much faster than the wind.

People assume that sailboats downwind speed component cannot beat wind speed but it totally can.

and no the cart is never propelled by the wheels.

8. Joined: Jul 2006
Posts: 1,262
Likes: 187, Points: 63, Legacy Rep: 358
Location: finland

### keroseneSenior Member

you state several "facts" that are not facts.
It does not at any point operate as a turbine ("windmill"). If it did It couldn't beat the wind speed.

the operation is best understood from the relative speed differences of the two interfaces:
1) ground to vehicle
2) air to vehicle

1) is larger than 2) when traveling downwind.
P=FV (Power equals Force * Velocity) and
F=P/V

Let's assume 10m/s wind and 15m/s downwind land speed.
Let's call force1 (F1) the braking force at the wheel. we can pick arbitrary 100N as the force.

100N * 15m/s = 1500 Watts

the vehicle is facing a 5m/s (apparent) headwind at this speed.

how much force can we get at 5m/s from 1500W?
F=P/V
so
F= 1500W / 5 m/s = 300N
lets call this F2. this is the force we have to work with (minus losses) to push the cart forward.
As F2 is 3x F1 we have plenty of room for propeller efficiency losses, other aerodynic drag etc.

it might help think it as having an electronic generator pulling energy from wheels and using that to power a propeller that pushes the vehicle forward.

because of the relative speed differences there is excess.

as speed goes up the _relative_ speed differences diminish and aerodynamic drag goes up so eventually vehicle will reach equilibrium and max speed (for that wind).

The creators of blackbird agree with this principle and made a vehicle that broke the wind speed with great margin.

it makes sense to try to understand the principle that has been proven than to insist it doesn't work that way.

9. Joined: Aug 2017
Posts: 810
Likes: 333, Points: 63
Location: Littleton, nh

### Will GilmoreSenior Member

Notice that if the the vector arrow for the boat's DDW speed (VMG) has been included in this diagram, it would have been significantly larger that the true winds speed arrow.

-Will

10. Joined: Dec 2008
Posts: 127
Likes: 6, Points: 18, Legacy Rep: 40
Location: USA

### ThinAirDesignsSenior Member

kerosene likes this.
11. Joined: Jul 2012
Posts: 347
Likes: 110, Points: 53, Legacy Rep: 14
Location: s.e. england

### alan craigSenior Member

In the video posted by Tiny Turnip above, the propeller IS a propeller, not a turbine; this confirmed by the designer/poster of original video in YT comments. Watch its direction of rotation relative to the pitch angle; it turns the "wrong" way for a turbine powering the wheels. If you jacked up the wheels while the vehicle was static, the propeller would turn backwards (trailing edge first) and if you changed the gearing so that the wheels turned much more slowly, the torque applied by the propeller turning backwards would drive the vehicle upwind - I've done this decades ago, it's very easy.
I've put these comments here because they might help people to understand what is happening; I'm still trying to fully understand it myself.

12. Joined: Jul 2006
Posts: 1,262
Likes: 187, Points: 63, Legacy Rep: 358
Location: finland

### keroseneSenior Member

The lates.
UCLA Physics prof Alex Kushenko claimed that Blackbird had to use gusts and that principle was not sound. Sadly Bill Nye sided with Alex.

1st my response to some if Alex’s criticism and another way of visualizing the concept.

and here is the lates update by Derek, Alex did pay his \$10k.

rxcomposite likes this.
13. Joined: Jan 2005
Posts: 2,625
Likes: 502, Points: 113, Legacy Rep: 1110
Location: Philippines

### rxcompositeSenior Member

Want to take sides and place a bet. Say \$10,000.

14. Joined: Oct 2009
Posts: 5,199
Likes: 598, Points: 113, Legacy Rep: 1485
Location: Midcoast Maine

### DCockeySenior Member

rxcomposite likes this.

15. Joined: Jun 2013
Posts: 266
Likes: 97, Points: 28, Legacy Rep: 10
Location: Cambridge, UK

### tlouth7Senior Member

This is wrong, boats can absolutely have a VMG (component of velocity) dead downwind that is greater than windspeed.
This is also wrong. A free spinning propeller could act as a gyro-rotor, but that would be the same as a normal sail in that the vessel as a whole would have to travel at an angle to the wind.

This is not how these vehicles work. The propeller is being driven by the wheels, not the other way round.

It can
It doesn't

kerosene likes this.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.