Ubergeekian
Well-Known Member
The skeptics - like myself - have been convinced. It's the fingers-in-the-ears, la-la-la-I-can't-hear you brigade who have run away.
Downwind Faster than the Wind - Successful Run by manned cart
- Thread starter Pierrome
- Start date
The skeptics - like myself - have been convinced. It's the fingers-in-the-ears, la-la-la-I-can't-hear you brigade who have run away.
Kelpie
Well-Known Member
Implying that you remain unconvinced? Yes?
By the wya you may want to read Phil's follow-up post where he calls the freewheeling run a 'red herring' due to changing windspeed conditons.
mikemonty
Well-Known Member
Excellent Uber! Your trolling is coming along nicely.
And as in all good trolling a slippery grasp of logic and language is the key.
Your point - well made in trolling terms - is that there are/were only two possible camps other than "Believers" namely "Sceptics" and "Closed-minded Cowards".
We're not sure which camp you include yourself in because we don't know whether "like myself" meant "I included myself in the sceptics" or "I have been convinced".
In two sentences you have claimed, without any basis in fact, that the argument is won - that there are no more sceptics. You claim they have all been convinced (I'd suggest because you have run out of people to argue with. All it means is that there are no more people HERE who can be bothered.)
AND those whe remain unconvinced are, by default, in the other camp -"Closed-minded Cowards".
Presumably engineered to provoke some form of reaction because I cannot think of any other reason for the labelling.
Again, another gallant attempt at trolling - but the game, as they say, is not worth the candle.
You can however , if you feel like it, sit and mumble in the dark to see if anyone else will take the bait.
Bosun Higgs
N/A
Well on the basis of the latest videos and bearing in mind their provenance, I'm prepared to think I might have been mistaken in believing it to be impossible. But what I do not understand is where the vehicle is getting the energy from once it reaches true wind speed.
Something else has to have less energy. What?
Something else has to have less energy. What?
Elemental
Well-Known Member
Only the cart is experiencing zero apparent wind. The blades are rotating very quickly and are experiencing a lot of wind...
If you powered up a turbine in a still room, would it provide a propulsive force (because that's what you have here...).
The air.
Pierrome
Well-Known Member
How about this:
Consider when the cart reaches exactly windspeed or faster.
It requires some energy to keep it there. The propeller is turning to provide thrust, and relative to the cart it is pushing air backwards, but relative to the wind it is slowing part of the wind down. Therefore, locally, the wind has less energy.
Isn't this a similar process to a wind-turbine which extracts energy from the wind, and after the wind passes through it the wind is slowed locally and and some of the wind's energy has been extracted.
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Ubergeekian
Well-Known Member
Oh goody. A long personal rant without addressing any of the issues.
I was a skeptic, of course. I'm a professional (and highly qualified) scientist - I'm skeptical about everything. It's what I do. It's what we do. In this case it didn't take much work to convince myself of the truth, since both the maths and the physics, although initially counter-intuitive, are fairly straightforward.
We skeptics base our views on evidence. Since there is no irrefutable evidence, any real skeptic will be convinced.
Do you still claim that conservation of energy and the second law of thermodynamics make that faster than the wind cart (which many people have seen) impossible?
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Ubergeekian
Well-Known Member
The wind. Some of it is being pushed backwards by the fan, and is therefore slowing down.
glashen
Well-Known Member
Sorry I've read most of this thread, and not commented yet, but the statement above encapsulates the problem as I see it. I can just about accept the possibility of accelerating above wind speed, (taking the energy from the windage on the cart) but then there is no energy source, the cart is now going against the apparent wind, and don't say its the propeller blades that are moving they are only spinning at right angles to the wind. It has also been stated on here they are not driving the wheels, it is the wheels that are driving the prop. So the once the cart exceeds wind speed it must start to run out of energy and slow down.
RAI
Well-Known Member
But it doesn't until it reaches 2 to 3 times the wind speed downwind. So there must be a reason.
shaunksb
Well-Known Member
Its complete and utter poppycock
shaunksb
Well-Known Member
Absolutely and irrefutably correct.
Tommyrot
Well-Known Member
snowleopard
Well-Known Member
Actually no, they aren't spinning at right angles to the wind. The motion of the blades is a combination of the rotation of the prop and the forward motion of the cart so it is travelling at an angle to the wind thus:
That means that the apparent wind over the blades is as shown, producing a forward component of thrust which drives the cart. The vectors of wind and force are exactly the same as a fast boat or land yacht tacking downwind and we all know that works.
Just set aside the unhelpful idea of apparent wind relative to the cart, think about the air flow over the airfoils.
shaunksb
Well-Known Member
Sorry I thought this thread had died a death.
I gave an explanation of how the treadmill demo worked on post 157 - nobody seems to have challanged that yet.
I'ts the same explanation on our fellow formites model.
Should you stick it outside in a gust the extra energy it holds onto will keep it moving forward for a while when the wind drops. Sadly this will always come to an end as it is moving into a headwind (drag) as well as suffering frictional loses - there can be no other outcome.
You have to think how a vehicle with only one drivetrain can move forwards with the wind behind and also when it is in front.
I gave an explanation of how the treadmill demo worked on post 157 - nobody seems to have challanged that yet.
I'ts the same explanation on our fellow formites model.
Should you stick it outside in a gust the extra energy it holds onto will keep it moving forward for a while when the wind drops. Sadly this will always come to an end as it is moving into a headwind (drag) as well as suffering frictional loses - there can be no other outcome.
You have to think how a vehicle with only one drivetrain can move forwards with the wind behind and also when it is in front.
shaunksb
Well-Known Member
I understand and respect your view on the apperent wind but I just don't think it's relevent to how the model works.
![[32511]](https://ybw-data.community.forum/avatars/m/32/32511.jpg?1576058889){kind=avatar}
[32511]
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Oh dear, oh dear, said he, banging his head against the wall!
We all, or nearly all, know a sand yacht, running on a broad reach at 45 degrees to the downwind direction, can get an apparent wind over its sail to travel faster than the wind speed.
Now, my little cart has wheels 100mm diameter, so one revolution is 314mm. The prop has a diameter of 400mm so halfway along the blades is 200mm diameter, giving 628mm distance travelled for one rev. I realised this was wrong after the first runs; the prop is now geared at 1/2 axle revs, so a half revolution is travelling through 314mm. So the prop hub and cart move directly downwind through 314mm; the prop blade rotates through 314mm; nett result, the point on the prop blade is travelling at 45 degrees to the direction of motion, albeit in a spiral. Now the important bit; the wheels are only driving the prop to produce the rotational speed, NOT fast enough to produce any thrust . The prop blade airfoil is now in exactly the same situation as the sand yacht's sail airfoil, travelling at 45 degrees to the downwind direction, and will be developing a forward thrust component FROM THE TAILWIND via the apparent wind passing over it.
If you don't believe me, build a model and try for yourself.
No laws of physics are broken, there is no perpetual motion, and this is my last post on the subject (cue solemn trumpet in background!)
We all, or nearly all, know a sand yacht, running on a broad reach at 45 degrees to the downwind direction, can get an apparent wind over its sail to travel faster than the wind speed.
Now, my little cart has wheels 100mm diameter, so one revolution is 314mm. The prop has a diameter of 400mm so halfway along the blades is 200mm diameter, giving 628mm distance travelled for one rev. I realised this was wrong after the first runs; the prop is now geared at 1/2 axle revs, so a half revolution is travelling through 314mm. So the prop hub and cart move directly downwind through 314mm; the prop blade rotates through 314mm; nett result, the point on the prop blade is travelling at 45 degrees to the direction of motion, albeit in a spiral. Now the important bit; the wheels are only driving the prop to produce the rotational speed, NOT fast enough to produce any thrust . The prop blade airfoil is now in exactly the same situation as the sand yacht's sail airfoil, travelling at 45 degrees to the downwind direction, and will be developing a forward thrust component FROM THE TAILWIND via the apparent wind passing over it.
If you don't believe me, build a model and try for yourself.
No laws of physics are broken, there is no perpetual motion, and this is my last post on the subject (cue solemn trumpet in background!)
shaunksb
Well-Known Member
shaunksb
Well-Known Member
By the way SL I think you just put the wrong slide up.
Aren't the rules implying that the "motion" has to be in the same direction as true wind?
"Dead downwind" faster than the wind
Aren't the rules implying that the "motion" has to be in the same direction as true wind?
"Dead downwind" faster than the wind
Keen_Ed
Well-Known Member
This film completely refutes your explanation. Or are you suggesting that string is good in compression? Steady state DDWFTTW for 1 minute 15 seconds