DDWFTTW Circular Wind Tunnel Test

[A.T.]

You could simply have a sail attached to a wheel driven conveyor belt, that moves the sail backwards at some fraction of the carts forward speed.

Yes, that is what happens on the "http://www.youtube.com/watch?v=Ufk6HVWdSzE"". The air interface is moving backwards in the carts frame.

Propellers are used because they are more efficient than paddle wheel type devices, not because of the wedge effect.

Yes, the wedge effect and sidewards movement of the air interface isn't needed. But the propeller cart does use it just like tacking ice boats do. Once you understand it for sail vehicles, you just roll the sail's path together to get the helical path of a propeller blade. Both air interfaces are constrained in their movment which results in the wedge effect.

 

[rcgldr]

Yes, the wedge effect and sidewards movement of the air interface isn't needed. But the propeller cart does use it just like tacking ice boats do. Once you understand it for sail vehicles, you just roll the sail's path together to get the helical path of a propeller blade. Both air interfaces are constrained in their movment which results in the wedge effect.

There's a key difference. In the case of a tacking ice boat, you have an apparent crosswind component that is only dependent of the direction of the ice boat with respect to the wind equal to : wind_speed x sin(angle_between_wind_and_ice_boat_path), and which is independent of the ice boat's speed. In the case of the cart, the helical path with respect to the air changes depending on wind speed, cart speed and prop speed. It's also not needed. You can just consider the prop as a nearly 2 dimensional disc and still be able to do the math involved for a DDWFTTW cart. The key factors are prop thrust and speed output, the apparent headwind (or tailwind) and the corresponding prop torque and rpm.

 

[A.T.]

There's a key difference. In the case of a tacking ice boat, you have an apparent crosswind component that is only dependent of the direction of the ice boat with respect to the wind equal to : wind_speed x sin(angle_between_wind_and_ice_boat_path), and which is independent of the ice boat's speed.

By "apparent crosswind component" do you mean the velocity of the air measured relative to the boat...
- perpendicular to air velocity relative to ground (wind direction)?
- perpendicular to boat velocity relative to ground (boat direction)?

In the case of the cart, the helical path with respect to the air changes depending on wind speed, cart speed and prop speed.

The helical path is static in the ground's frame just like the path of the ice boat. In the air's frame both paths are moving in the opposite way the air moves in the ground's frame. So I don't see any difference besides the shape of the paths.

 

[rcgldr]

In the case of a tacking ice boat, you have an apparent crosswind component that is only dependent of the direction of the ice boat with respect to the wind equal to : wind_speed x sin(angle_between_wind_and_ice_boat_path), and which is independent of the ice boat's speed.

By "apparent crosswind component" do you mean the velocity of the air measured relative to the boat...

Apparent wind is relative to the ice boat. Apparent crosswind is the component of apparent wind perpendicular to the direction of travel (wrt ground/ice) of the ice boat.

The helical path is static in the ground's frame just like the path of the ice boat.

OK, back to my main point then. An ice boat can't achieve DDWFTTW, it has to tack at some angle with respect to the wind in order for it's downwind component to exceed wind speed. It can do this because the apparent crosswind is indpendent of the ice boats speed. A DDWFTTW cart doesn't need a rotating prop in order to achieve DDWFTTW. It could use a sail on long conveyor belt driven by the wheels. The "wedge" effect of the prop is a side issue, and not a requirement for a DDWFTTW cart, but the "wedge" effect is required for the ice boat to achieve a downwind component faster than the wind.

Another difference is that the thrust from the sail on an ice boat comes from diversion of the apparent crosswind (a sail can't generate thrust from an apparent headwind), and the ground force opposing the apparent crosswind consumes no power because the opposing ground force is perpendicular to the direction of travel of the ice boat. In a DDWFTTW cart, the thrust come from a multiplcation of an opposing ground force and a corresponding division of the ground speed via some form of gearing effect; and since the opposing ground force is in the direction of travel, it consumes power.

 

[mender]

I see I didn't comment on the SJSU project, although I remember seeing it. What an awesome engineering project. Those kids are going to learn so much.

btw, your "Continous" video is exactly what was going through my head one day in an effort to comprehend what was going on with this D&D stuff. Thank you for putting that together for me.

OmCheeto, did I miss your epiphany on DDWFTTW? If so, what was your turning point?

 

[rcgldr]

"continous" video

OmCheeto, did I miss your epiphany on DDWFTTW? If so, what was your turning point?

It's my guess is that the "continuous" video demonstrates that when traveling DDWFTTW, that since the ground speed is faster than the air speed relative to a DDWFTTW device, it allows DDWFTTW devices to to effectively gear down the faster ground speed to the slower air speed, with the leverage effect of the gearing increasing the force at the air interface (but at a slower speed).

 

[OmCheeto]

OmCheeto, did I miss your epiphany on DDWFTTW?

No. But that doesn't mean I don't like watching people build these devices. They are always great fun to watch and make my brain hurt trying to understand their operation. I look forward to the test of the SJSU machine. It is a beautiful piece of engineering.

 

[mender]

They're doing an impressive job, aren't they!

So you're saying you're not quite there yet? I lean heavily on physical evidence then take a while to fill in the blanks. Maybe I covered some of the ground you're on and can help?

 

[OmCheeto]

They're doing an impressive job, aren't they!

So you're saying you're not quite there yet? I lean heavily on physical evidence then take a while to fill in the blanks. Maybe I covered some of the ground you're on and can help?

Great!

Describe the effect with only equations.

ie. no words, graphs, pictures, nor pseudo-vector diagrams are allowed.

 

[rcgldr]

Describe the effect with only equations.

wind_speed_wrt_ground = +10 mph
ddwfttw_cart_speed_wrt_ground = +15 mph

ground_speed_to_effective_thrust_speed_gear_ratio = .5
wind_speed_wrt_ddwfttw_cart = -5 mph
ground_speed_wrt_ddwfttw_cart = -15 mph
effective_prop_thrust_speed_wrt_ddwfttw_cart = -15 mph x .5 = -7.5 mph
ground_force_required_to_drive_prop = -1 lb
effective_prop_thrust = +1.5 lb
drag_plus_rolling_resistance = -.5 lb

net_force = -1 lb + 1.5 lb - .5 lb = 0 lb
net_acceleration = 0 mph / second

power_input = 15 mph x 1 lb = .04 horsepower
power_output = 7.5 mph x 1.5 lb = .03 horsepower
losses_in_power_conversion = .01 horsepower

 

[mender]

Great!

Describe the effect with only equations.

ie. no words, graphs, pictures, nor pseudo-vector diagrams are allowed.

How about in Norske?

Thanks, Jeff! So OmCheeto, all is good now?

 

[mender]

And was that you over on the other forum?

 

[OmCheeto]

How about in Norske?

So OmCheeto, all is good now?

Only one thing comes to mind:

Gentlemen, congratulations. You're everything we've come to expect from years of public education.

shamelessly stolen from http://www.angelfire.com/oh/quotations/movies/m/meninblack.html" .

 

[mender]

Likewise, I'm sure!

"Then I saw little Tiffany. I'm thinking, y'know, eight-year-old white girl, middle of the ghetto, bunch of monsters, this time of night with quantum physics books? She about to start some ****, Zed. She's about eight years old, those books are WAY too advanced for her. If you ask me, I'd say she's up to something. And to be honest, I'd appreciate it if you eased up off my back about it."

 

[A.T.]

Spork & JB finally tested their cart in natural wind:



http://www.youtube.com/watch?v=aDzWh9J1dk4#t=1m23s

Their blog:

http://www.fasterthanthewind.org/

 

[rcgldr]

Very impressive results. In the meantime I found a youtube video of some sort of "upwind" cart competition.



Spork and JB should be able to acomplish the same thing by increasing the pitch of the prop enough that prop pitch speed (prop_pitch x (revolutions / second)) > ground speed, perhaps starting off with prop pitch speed at 3x to 4x ground speed. In this case the prop would be acting at a turbine driving the wheels (as opposed to DDWFTTW mode, where the wheels drive the prop).

 

[A.T.]

In the meantime I found a youtube video of some sort of "upwind" cart competition.



Spork and JB should be able to acomplish the same thing by increasing the pitch of the prop enough that prop pitch speed (prop_pitch x (revolutions / second)) > ground speed, perhaps starting off with prop pitch speed at 3x to 4x ground speed. In this case the prop would be acting at a turbine driving the wheels (as opposed to DDWFTTW mode, where the wheels drive the prop).

I think they have some limits on turbine size in the upwind races. Here is a lecture by one of the participants (or even organizers) of those races:
http://www.youtube.com/view_play_list?p=6C046C08399626CA

Here is an animation that shows the symmetry between directly upwind & directly downwind faster than wind:

 

[rcgldr]

I think they have some limits on turbine size in the upwind races. Here is a lecture by one of the participants (or even organizers) of those races:
http://www.youtube.com/view_play_list?p=6C046C08399626CA

It also appears that the upwind carts are allowed to use clutches as the turbines are moving when the carts are not. This allows for faster launches.

For DDTFTTW carts, a variable pitch prop would help with launches, since ground to prop pitch (thrust) speed can be any value below 1 (including negative ratios, < 0 is just slower than wind speed) but there would be some additional losses due to whatever mechanism was used to control prop pitch, unless it was something similar to a torsion spring controlling the pitch based on the amount of thrust being generated, and it's an uneeded complication as you could just give the carts an initial push if the goal was to achieve a top speed.