5
   

I don't understand how this car works.

 
 
rosborne979
 
  1  
Reply Tue 8 Jun, 2010 08:23 pm
@Cycloptichorn,
Cycloptichorn wrote:

Quote:
With wind directly behind your sails there is zero force from that wind at the wind vel/speed.


And due to the design of the props here, the wind is never directly behind the 'sails' - even when it is directly behind the vehicle. Do you not understand this?

Cycloptichorn

No, he doesn't understand it.
0 Replies
 
rosborne979
 
  1  
Reply Tue 8 Jun, 2010 08:26 pm
@ThinAirDesigns,
JB, is there any practical use for such a vehicle design, or is this just an academic exercise?
ThinAirDesigns
 
  5  
Reply Tue 8 Jun, 2010 08:58 pm
@rosborne979,
rosborne979 wrote:
JB, is there any practical use for such a vehicle design, or is this just an academic exercise?


We can see no use whatsoever for a wind powered vehicle of this sort -- totally and completely impractical. It took on the whole project for a couple reasons:

A: mostly to prove a point.

B: to provide an attention getting platform to showcase a technology that few understand -- dynamic wind energy extraction (extraction while in motion)

Our primary sponsor (Joby Energy) is spending millions on wind energy research and one of the interesting tidbits about this device is that it isn't constrained by the Betz' limit the way a turbine is. What this means is it can extract energy from the entire 'tube' of wind that is the diameter of the rotor rather than just ~59% of it as a turbine can. Also, while a turbine can only *slow* this remaining portion of the tube, this device can essentially bring the entire tube of air to a stop (not absolutely perfectly due to swirl, etc. but close), harvesting much more from the same tube. Additionally, using this method allows more air to be processed -- a stationary turbine in a 15mph wind can only process a 'tube' of air 15 miles long in an hour. A device like this moving downwind at 2x ws can of course double that.

All told we're pulling more than 5x the HP out of our rotor at 15mph than can a stationary turbine of the same size in the same wind. Does that mean the vehicle is practical? -- nope.

JB
0 Replies
 
Cycloptichorn
 
  2  
Reply Tue 8 Jun, 2010 09:19 pm
@BillRM,
Quote:
So what is the point of this hoax?


What, indeed? Laughing

You might want to consider the idea that it isn't a hoax.

Cycloptichorn
0 Replies
 
Eorl
 
  2  
Reply Tue 8 Jun, 2010 11:01 pm
Sorry I can't be bothered reading the whole 14 pages thus far but I think I can see what's going on. It's difficult to imagine that the power to travel faster than the wind can be drawn from said wind.

If you imagine a massive funnel with wind blowing into it with pea-sized hole at the narrow end, then you can easily imagine a pea being spat out of it that would travel much faster than the wind providing the energy. This car is like that, it clearly has a large catchment abillity and a small friction profile.

The mistake is to imagine it works like a sail, which has a friction barrier (the sail) that is stronger than it's abilty to harness the energry (that sail)
ThinAirDesigns
 
  1  
Reply Tue 8 Jun, 2010 11:06 pm
@Eorl,
Erol wrote:
The mistake is to imagine it works like a sail, ...


Except not only does it work *like* a sail, it actually has *two* very nice ones.



ossobuco
 
  1  
Reply Tue 8 Jun, 2010 11:08 pm
@Eorl,
Read back, Eorl, it's worth it.
Eorl
 
  1  
Reply Tue 8 Jun, 2010 11:24 pm
@ossobuco,
You're right ossobuco (as usual)

You'd think I'd know better by now!

Cool topic... be back after reading, hopefully less ignorant.
ossobuco
 
  1  
Reply Tue 8 Jun, 2010 11:46 pm
@Eorl,
Well, it can be annoying.
0 Replies
 
DrewDad
 
  1  
Reply Wed 9 Jun, 2010 07:59 am
@ThinAirDesigns,
Still working on getting my head around the iceboat performance. Sad
DrewDad
 
  1  
Reply Wed 9 Jun, 2010 08:19 am
@DrewDad,
Apparently, I need to start a new thread titled "I don't understand how this iceboat works"

http://sports.groups.yahoo.com/group/2nalsa/message/168

Quote:
> Isn't the basic issue that it is lift that is driving a landsailer
> so the apparent wind speed across the sail increases as the roll
> speed increases and the "boat" keeps accelerating until one kind of
> drag or another exceeds the power from lift?

Yes. The basic sailing performance relationship is:

Vb = Vt * sin(gamma - beta) /sin(beta)
Vb: yacht speed
Vt: true wind speed
gamma: course sailed relative to true wind (gamma = 0 is straight
into the wind)
beta: apparent wind angle, measured between course over the ground
and the apparent wind vector

The closer the yacht can sail to the apparent wind (small beta), the
faster it goes. A typical beta for an efficient landyacht is on the
order of 14 degrees, which yields a maximum boat-speed ratio of 4
times the true wind, and a maximum Vmg downwind of 2.5 times the true
wind.

Beta, in turn, depends on the lift/drag ratio (actually the inverse -
the drag/lift ratio) of everything exposed to the air, and the lift/
drag ratio of the tires.

beta = arctan(aero_drag/aero_lift) + arctan(tire_drag/tire_lift)

Landyacht performance is all about reducing the aerodynamic drag in
all its forms.

Cheers,

Tom Speer


lulz
ThinAirDesigns
 
  1  
Reply Wed 9 Jun, 2010 08:40 am
@DrewDad,
@DrewDad

DD, thanks for at least responding on that subject ... I was beginning you feel as though you simply ignore anything that doesn't fit your view of how it should be.

I can give you many links supporting the FACT that traditional sailing rigs will stomp the hell out of the balloon all day long - there are reputable sailing books supporting the same.

Why would the organization that is resposible for verifying and ratifying the records for wind powered land vehicles lie about their performance?

And hell with what the BMW Oracle crew says in their press conference (where they discuss their ability to double the speed of the wind VMG), just go grab the data from the race and use your own head: The yacht covered its downwind leg at VMG of 19 knots, in winds of 5-10 knots. That's a VMG of well over 2 times wind speed. Find a full video on the internet and place your own stop watch on the downwind leg -- wind conditions for the race are published all over the place ... hundreds of thousands of people were watching all over the world. It's not a trick, it's a fact.

JB
0 Replies
 
ThinAirDesigns
 
  1  
Reply Wed 9 Jun, 2010 08:52 am
@DrewDad,
DrewDad wrote:
Apparently, I need to start a new thread titled "I don't understand how this iceboat works"


Yes you do -- and until you come to grips that that confounding 'it does exactly what I say can't be done' ice-boat, you've got no chance at the ddwfttw vehicle because they both utilize the same exact aerodynamics to propel themselves faster than the wind.

JB
DrewDad
 
  1  
Reply Wed 9 Jun, 2010 09:04 am
@ThinAirDesigns,
So, the wheels replace a boat's keel, or an iceboat's runners.

The wheels spin the propeller not to provide thrust out the back of the machine, but to keep the blades of the propeller angled correctly against the "apparent wind" at the leading edge of the propeller? The thrust is provided because the apparent wind creates lift against the propeller blades?

The key component isn't apparent wind across the hood of the vehicle, but the apparent wind at the leading edge of the propeller.

gah. Still don't understand how it extracts the energy. I can repeat talking points all day long....
ebrown p
 
  1  
Reply Wed 9 Jun, 2010 09:34 am
@DrewDad,
I really don't think the boat sail is the most helpful way to describe this (other to show on principle that extracting energy with a higher speed is possible). Of course, in Physics there are usually more then one way to look at a problem that will get the same, correct, answer... I just don't think this is the most helpful way to answer DrewDad's questions.

The answer (in my way of looking at the problem) is that the propeller pushing back against the tailwind interacts with the tailwind. In this way the tailwind can do work (through action/reaction) on the propeller even when the propeller is going faster then the wind (this is because the propeller is forcefully pushing air backwards against the tailwind). This is how the cart can "extract" energy from the tailwind (remember a key part of the system is the ground doing work on the wheels).

ThinAirDesigns
 
  1  
Reply Wed 9 Jun, 2010 09:35 am
@DrewDad,
DD, I apologize that I must be short in the following answer -- I have to leave to go give a presentation to the SF Saint Francis Yacht Club on this very topic. I'll be brief (which may must mean confusing).

Quote:
So, the wheels replace a boat's keel, or an iceboat's runners.


Yes, the gearing between the wheels and the prop perform the exact same kinematic constraint as a keel or skates/wheels on a traditional rig.

Quote:
The wheels spin the propeller not to provide thrust out the back of the machine, but to keep the blades of the propeller angled correctly against the "apparent wind" at the leading edge of the propeller? The thrust is provided because the apparent wind creates lift against the propeller blades?


Here's where I don't have time to carefully answer -- I'll get to you later, but currently you'll just say I'm full of crap when I answer this: The wheels DO spin the prop to provide thrust just like a regular old propeller AND they operate just like a sail on a broad reach. There's no conflict in that statement ... it's that most folks (even most sailors) don't realize that a sail isn't alway acting as a turbine but is often acting as a prop.

Quote:
The key component isn't apparent wind across the hood of the vehicle, but the apparent wind at the leading edge of the propeller.


Yes, yes, yes. !!!

JB

DrewDad
 
  1  
Reply Wed 9 Jun, 2010 09:37 am
@ebrown p,
ebrown p wrote:
The answer (in my way of looking at the problem) is that the propeller pushing back against the tailwind interacts with the tailwind. In this way the tailwind can do work (through action/reaction) on the propeller and the cart can "extract" energy from the tailwind (remember a key part of the system is the ground doing work on the wheels).

I'm glad that explanation works for you, but it doesn't work for me. I've told you why it doesn't work for me, and I don't care to rehash my explanation yet again.
0 Replies
 
DrewDad
 
  1  
Reply Wed 9 Jun, 2010 09:49 am
@ThinAirDesigns,
ThinAirDesigns wrote:
Quote:
The wheels spin the propeller not to provide thrust out the back of the machine, but to keep the blades of the propeller angled correctly against the "apparent wind" at the leading edge of the propeller? The thrust is provided because the apparent wind creates lift against the propeller blades?


Here's where I don't have time to carefully answer -- I'll get to you later, but currently you'll just say I'm full of crap when I answer this: The wheels DO spin the prop to provide thrust just like a regular old propeller AND they operate just like a sail on a broad reach. There's no conflict in that statement ... it's that most folks (even most sailors) don't realize that a sail isn't alway acting as a turbine but is often acting as a prop.

Well, I get that there will be some thrust developed just because the thing is turning, but I don't see how the thrust from it acting as a propeller can be any greater than the drag created by the wheels; these two forces should cancel out if you ignore friction losses.

The actual gain in energy (from the vehicle's perspective) should be from it acting as a sail.
ThinAirDesigns
 
  2  
Reply Wed 9 Jun, 2010 09:56 am
@DrewDad,
DrewDad wrote:
I don't see how the thrust from it acting as a propeller can be any greater than the drag created by the wheels


I know you don't -- and that's because you are as wrong on the following point as you are on the ice-boats: You claim that the wheels and the prop are calculated over the same distance (Work = Force * Distance). You're just plain wrong.

If you were calculating the power required of the propeller to thrust an airplane downwind you would never use distance over the ground -- you would use distance through the air (what the hell does the ground have to do with a propeller 5 thousand feet in the air?)

Likewise if you calculate the power required of the ddwfttw vehicle's propeller to thrust it downwind you don't use distance over the ground -- you use distance through the air. (what the hell does the ground have to do with a propeller 15ft in the air?)

If you'll give up your hard headed wrongness on the point of the ice-boats and the point of the distance, we'll be done here and you can go on your merry way on the correct side of this brainteaser.

JB
DrewDad
 
  1  
Reply Wed 9 Jun, 2010 04:18 pm
@ThinAirDesigns,
ThinAirDesigns wrote:
and that's because you are as wrong on the following point as you are on the ice-boats:

Now, I was wrong on the iceboats. Admitting that I don't understand where they're getting their energy from isn't continuing to be wrong; it's just admitting a gap in my knowledge.

ThinAirDesigns wrote:
If you'll give up your hard headed wrongness on the point of the ice-boats and the point of the distance, we'll be done here and you can go on your merry way on the correct side of this brainteaser.

Well, there's a difference between taking something on faith, which is religion, and actually understanding something, which is science.
 

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