5

# I don't understand how this car works.

Cycloptichorn

1
Tue 8 Jun, 2010 10:05 am

Cycloptichorn wrote:
No - drag on the wheels should match thrust from the propeller + thrust from the sail in a perfect world...

If drag on the wheels = propeller thrust + wind thrust, then the thing would never accelerate. Propeller thrust + wind thrust would never overcome the drag.

I really don't believe that this is true. Take the example of an iceboat. It clearly overcomes the drag in order to accelerate. It does so by using the stiff keel of the runners to cancel out some of the force vectors while harvesting others, and the sail-suck effect does the rest.

This is the exact same thing! Except the nature of the propeller means that you can face any direction you want, and still have your sail tacking the wind.

Watch the video that I linked above - even if you don't believe it's going faster then the wind straight downwind, it's clearly accelerating.

Cycloptichorn
Cycloptichorn

1
Tue 8 Jun, 2010 10:14 am
@Cycloptichorn,
And here's a similar car that goes directly INTO the wind.

http://www.popsci.com/node/32069

Cycloptichorn

1
Tue 8 Jun, 2010 10:26 am
@Cycloptichorn,
Into the wind is trivial. Wind blows on wind vanes, wind vanes turn, transfers power to the axle.

You could do this with direct power transfer, or you could convert to electricity first, then run an electric motor.

Your speed is limited by the size of the wind mill you have attached to your vehicle. Bigger windmill = more power.

But if you turn away, and run with the wind, the wind relative to the windmill will reduce, and you will run out of power to accelerate further.

1
Tue 8 Jun, 2010 10:31 am
@Cycloptichorn,
Cycloptichorn wrote:
I really don't believe that this is true. Take the example of an iceboat. It clearly overcomes the drag in order to accelerate. It does so by using the stiff keel of the runners to cancel out some of the force vectors while harvesting others, and the sail-suck effect does the rest.

That's not drag = thrust, though. That's thrust > drag.

You're the one that claimed drag = thrust (propeller) + thrust (wind).

Drag = thrust (propeller) + thrust (wind) is only true when the car is at equilibrium. (i.e., no acceleration)

Cycloptichorn wrote:
This is the exact same thing! Except the nature of the propeller means that you can face any direction you want, and still have your sail tacking the wind.

True. But your downwind component will never average greater than the wind speed.

Cycloptichorn wrote:
Watch the video that I linked above - even if you don't believe it's going faster then the wind straight downwind, it's clearly accelerating.

I'm not claiming that the thing will not move. I'm saying that I don't believe it when they say it will run steadily at 2.85 times the average wind speed.

Show me the wind speed measured independently from the vehicle, and show me ground speed measured against marks on the ground. Then I'll believe it.

1
Tue 8 Jun, 2010 10:35 am

Into the wind is trivial. Wind blows on wind vanes, wind vanes turn, transfers power to the axle.

You could do this with direct power transfer, or you could convert to electricity first, then run an electric motor.

Your speed is limited by the size of the wind mill you have attached to your vehicle. Bigger windmill = more power.

Whoops! This is in error. Bigger windmill = more power, but it also equals more drag. I can visualize mechanical advantage that will pull the car into the wind, but the speed will be limited.
0 Replies

Cycloptichorn

1
Tue 8 Jun, 2010 10:37 am
Quote:
I'm saying that I don't believe it when they say it will run steadily at 2.85 times the average wind speed.

This is a complex argument, so it's important to be accurate. The creators did not claim that it would run steadily at 2.85x the wind speed; they claimed that this was the TOP velocity it hit, the absolute peak.

Cycloptichorn

1
Tue 8 Jun, 2010 10:40 am
@Cycloptichorn,
Cycloptichorn wrote:
So, the car is now moving at equal speed to the wind (from directly behind) and the wheels are turning. Your contention seems to be that there's no way that the force of the wheels turning could possibly provide more thrust out of the propeller then it slows down thrust due to increased drag on the wheels.

That is exactly my contention.

Cycloptichorn wrote:
But it does in fact do this - because the spinning propeller tips actually provide an angled surface for the wind to push against, as well as the additional thrust due to air which is pushed backwards by them - at least, if I understand this thing right.

And I ask you... what wind? The car is moving at a speed equal to the wind.

Cycloptichorn wrote:
Like I said, peep the videos - the thing clearly accelerates.

And like I said: I don't doubt that it accelerates. I grant that it accelerates. It accelerates just as an iceboat would. (Not at exactly the same rate.)

Edit: You removed the post to which I am replying, but I felt like posting my response anyway.
0 Replies

1
Tue 8 Jun, 2010 10:42 am
@Cycloptichorn,
Cycloptichorn wrote:

Quote:
I'm saying that I don't believe it when they say it will run steadily at 2.85 times the average wind speed.

This is a complex argument, so it's important to be accurate. The creators did not claim that it would run steadily at 2.85x the wind speed; they claimed that this was the TOP velocity it hit, the absolute peak.

Cycloptichorn

They seem to claim on other forums that it will continue to run steady state at that speed.
0 Replies

ThinAirDesigns

3
Tue 8 Jun, 2010 10:44 am
Hi folks. I'm one of the two primary designer/builders of the craft in question.

I'll try to go back through the posts on the thread and come up with some relevent comments over the next bit, but in the mean time feel free to fire away with any questions you might have.

One thing to remember is that the press has done a very poor job of explaining this thing and as such I'm completely understanding of those who say "it's being explained wrong". We find we are credited with many, many statements that we never made at all.

Basics:
-- the spinning rotor is a propeller, not a turbine
-- the wheels provide the torque to turn the rotor (always).
-- the rotor does not provide the torque to turn the wheels (ever).
-- it will take off from a standing start on it's own.
-- we sometimes push it up to speed to save time during testing
-- there's no "null point' at windspeed.
-- It will maintain it's speed well above wind speed indefinitely
-- By design, this particular one works best when aimed directly downwind.
-- One can be built to go faster than the wind in any direction
-- A simple gearing change will cause it to go upwind rather than down

We have lots of videos if this site allows me (a new member) to post links - perhaps someone can tell me if this is OK ... I don't want to 'un-welcome' myself by doing something uncool.

JB

1
Tue 8 Jun, 2010 10:48 am
@ThinAirDesigns,
You can embed a Youtube link by using the custom youtube tags:

ebrown p

1
Tue 8 Jun, 2010 10:51 am
@ThinAirDesigns,
Welcome ThinAir,

I am sure glad you are here.... I have been doing my best!

Could you clarify one thing... the rotor does provide torque to turn the wheels when it is going upwind, right?

1
Tue 8 Jun, 2010 10:54 am
@ThinAirDesigns,
ThinAirDesigns wrote:
Basics:
-- the spinning rotor is a propeller, not a turbine
-- the wheels provide the torque to turn the rotor (always).
-- the rotor does not provide the torque to turn the wheels (ever).
-- it will take off from a standing start on it's own.
-- we sometimes push it up to speed to save time during testing

I don't have any trouble with these claims.

ThinAirDesigns wrote:
-- there's no "null point' at windspeed.
-- It will maintain it's speed well above wind speed indefinitely

These are the two claims I have trouble believing.

ThinAirDesigns wrote:
-- By design, this particular one works best when aimed directly downwind.
-- One can be built to go faster than the wind in any direction
-- A simple gearing change will cause it to go upwind rather than down

I don't have trouble with these, either.
ThinAirDesigns

1
Tue 8 Jun, 2010 10:55 am
Thanks DrewDad -- that will be a bit help.

JB
0 Replies

ThinAirDesigns

1
Tue 8 Jun, 2010 10:58 am
@ebrown p,
ebrown p, our vehicle was designed for and is currently only equipped to go directly downwind so there is no real way to answer your question directly.

I'll say this. If we designed a craft to go directly upwind (or modified this one to do it) the rotor would have to function as a turbine (rather than a prop as it does now) and the torque from said turbine would provide the force to turn the wheels.

JB
0 Replies

ThinAirDesigns

1
Tue 8 Jun, 2010 11:08 am
These are the two claims I have trouble believing.

Fair enough. Have you seen our demonstrations done at windspeed on the treadmill which show the vehicle passing repeatedly from below windspeed to above windspeed?

JB

1
Tue 8 Jun, 2010 11:11 am
@ThinAirDesigns,
I haven't watched them all in detail, but I've viewed several.

The most compelling one (#4 on the list of videos on Youtube), shows the vehicle accelerating up an inclined plane.

Unfortunately, much of the room is out of the frame. It would be trivial to place a fan at the base of the treadmill during the initial exposition, and have it blowing up the treadmill for the rest of the demonstration.

As I've said before, I would be persuaded by independent confirmation, or by video of a controlled experiment which includes both an visible measurement of the wind independent of the vehicle, and markings showing how fast the vehicle is moving.
ThinAirDesigns

1
Tue 8 Jun, 2010 11:17 am
Unfortunately, much of the room is out of the frame. It would be trivial to place a fan at the base of the treadmill during the initial exposition, and have it blowing up the treadmill for the rest of the demonstration.

We have quite a number of videos where we walk around the room showing the lack of a fan. Also, we have the treadmill near the curtains on purpose -- if there was a fan blowing the curtains would be moving. Try this one:

JB
0 Replies

1
Tue 8 Jun, 2010 11:19 am
An explanation of why the car continues to accelerate past wind speed would be interesting as well.

If the car cannot accelerate from rest when there is no relative wind, then I don't see how it can accelerate from a point when there is slack wind, even if the device is moving..

Thrust from the propeller has to be less than drag from the wheels.
ThinAirDesigns

1
Tue 8 Jun, 2010 11:21 am
This one also is a good one showing that it's the treadmill alone with no fan or other:

0 Replies

rosborne979

1
Tue 8 Jun, 2010 11:30 am
@ThinAirDesigns,
ThinAirDesigns wrote:
Hi folks. I'm one of the two primary designer/builders of the craft in question.

Ah ha, so you're one of the evil hoaxers (just kidding).

Welcome to A2K.

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