Downwind faster than the wind. Poll

[Ubergeekian]

Like I said, conseration of energy. The road has no energy in and of itself, only in relation to the cart. Honestly, if you step off the cart the road stops moving.

You're making the mistake of changing your reference frame. Think of it relative relative to the ground. When you step off the cart you slow down and the momentum you lose is transferred to the ground, which then speeds up. Bingo - energy transfer.

In practice the mass of the earth is so large that its velocity can be assumed to be unchanging, whichever reference frame you use. Note that in the boat case there will be significant energy transfers to and from the sea.

So using the road to power the cart is exactly the same as using the cart to power itself.

No.

 

[Ubergeekian]

I can see one fly in the ointment.

The air around the belt is indeed instantaneously moving at x mph in the opposite direction to a point on the belt. A man standing still on the belt would feel a headwind due to his motion through the air If, however he was running fast enough to retain his position relative to the room, he would experience no air movement at all, no wind.

Since it is the movement of the belt relative to the room's airmass which is creating this relative wind, the device sees no wind since the device is not moving in relation to the rooms airmass . This means that whatever is causing the aircraft to NOT just follow along the belt, it is not air movement. Repeat: there is no air movement relative to the aircraft.

Since you're new here, I'll explain again. The propeller is pushing back against the air. No bulk air movement necessary.

 

[Ubergeekian]

Sadly I'm forced to agree; I have an innately optimistic view of human nature and thought I saw people with a genuine misunderstanding I could help correct.

Oddly enough, I thought that too.

 

[Ubergeekian]

,It is simply that at its maximum pitch angle, and maximum throttle, if you take it into a dive at high speed. the apparent wind on the prop stalls on the blade and the prop is useless.

Nope. It's not stalling - quite the opposite. Propellers stop working when the angle of attack gets too small, not too big.

The forward velocity makes a wind component . The rotational prop speed makes a wind component. Add them together and they hit the prop blade at an angle.

On the ground the rotational speed makes the apparent wind angle close to the plane of the prop. As the air speed increases the apparent angle bends forward of the craft.

Think about this. If the angle of attack was as you say, the prop would be pushing backwards.

When it gets bigger than the minimum angle of the attack the prop it is useless. That determines the speed limit.

Most aerofoils stall with an angle of attack of around 18 degrees. That's why many props have variable pitch blades: they use a low angle of attack at low speeds and a high angle of attack at high speeds. The limit of effectiveness comes in three ways: either (1) the angle of attack can't physically be increased any more or (b) the prop can't produce enough thrust to overcome airframe drag or (c) compressibility effects at high tip speeds. Or a combination.

 

[Ubergeekian]

T You have wronged science, and that saddens me and is something I cannot allow to pass unchallenged. Call me a sentimental fool but public understanding of science is at an all time low in our society, and anything which harms it further in the minds of the easily impressionable must be resisted.

That is ridiculous. If anyone is harming science here - and frankly I doubt if science is over concerned with the perceptions of a few people ho can't quite remember GCSE Combined Science - it is those who glibly cite things they don't understand to bolster their irrational prejudices.

Newtons Laws. The Law of Thermodynamics. Conservation of Energy. All good principles and every one easily applied to the cases under discussion.

 

[lw395]

Urinal Moment

If you use a reference frame moving with the trolley it does! How? By exerting a force on the rim of the wheels while moving.

Well I suppose you have to close the loop and consider that who/whatever is pushing the ruler is exerting a force between the ruler and the road.

However, was thinking about this in very dull meeting.

If you change the gear ratio of the cart, so that the relative speed of the ruler to the cart is less than the road to the cart, it works, because the gearing causes the 'go faster' force to be greater than the 'go slower' force.
e.g if the cart speed is 1 and the ruler speed is a half, (road = still) then the force on the ruler can be twice the force from the wheels.
From the cart's view point: power in at the wheels =FV, power out at the ruler = 2F*(V/2)

You can change the ratio but where I have 2, you need >1 or the net force is the wrong way. If you use 1.00000...1 then the cart goes 1.999999.... rulerspeed. (and needs 99.999..% efficiency). Call that 2:1.

Next physicist question, is this limit fundamental,or just for simple geared carts?
What VMG downwind do ice yachts do?

I tried looking at the vid last night, but VoodooBone mobile broadband was only giving me 1kbaud and I gave up.

 

[Wunja]

OK, only refering to the video and not discussing Oracle's downwind performance.

The point that is being missed is that the cart has zero velocity. It isn't going anywhere. So KE=1/2 M V2, v=0 and therefore KE will also equal zero. Being pedantic you could say the wheels are going round, but that is less relevant.

I don't have a treadmill to hand, but I'm willing to bet that if I put a marble or ball bearing on it, the ball rould revolve quite happily but remain stationary.
It is only when the inertia of the object is overcome that it willstart to move. The cleverness of the cart is that there is so little friction in the mechanism, that the wheels can revolve without causing a breaking effect and overcoming the inertia.

I have practical experience of the opposite effect too. Riding a bicycle on rollers, where the wheel rotation indicated a 'speed' of 20 mph, I accidently rode off the side, landing on the floor. Rather than shooting off through the garage wall at 20mph, I just stopped. Because my velocity was zero.

 

[Keen_Ed]

The point that is being missed is that the cart has zero velocity. It isn't going anywhere. So KE=1/2 M V2, v=0 and therefore KE will also equal zero. Being pedantic you could say the wheels are going round, but that is less relevant.

No, it's very relevant.

The treadmill isn't going anywhere either. But the belt is moving.

I have practical experience of the opposite effect too. Riding a bicycle on rollers, where the wheel rotation indicated a 'speed' of 20 mph, I accidently rode off the side, landing on the floor. Rather than shooting off through the garage wall at 20mph, I just stopped. Because my velocity was zero.

But what happens if you ride off your rollers and the first thing to touch the floor are your bike wheels?

 

[cliffordpope]

Try driving a mini up a sloping ramp on the back of a moving furniture van. Do you crash through the driver's cab at 60 mph? I doubt it.

 

[Wunja]

Being pedantic you could say the wheels are going round, but that is less relevant.

No, it's very relevant.

The wheels also have zero velocity. The rim may have the same speed as the belt, but that is countered by the speed of the rim at the opposite side - the wheel only goes around, it doesn't move.

Why do they demonstrate it on a treadmill and not using a fan? Because it won't work!

 

[Keen_Ed]

Why a treadmill rather than a fan? Because it's easier to see what's going on, and the calibration is much simpler.

Question: You're in the middle of the Atlantic. There is nobody in sight. You're sailing in a 2kt westerly. Is this southerly the result of the air moving east at 2 kts, or the water moving west? You have switched off your GPS, so you can't just look at that.

 

[lw395]

Try driving a mini up a sloping ramp on the back of a moving furniture van. Do you crash through the driver's cab at 60 mph? I doubt it.

Love it.

Bit like the bicycle coming off the rollers.
The bikes wheels have very little momentum so stop very easily, whereas the cyclist is much heavier, so conserving momentum, the wheels virtually stop and the cyclist moves forward a little bit.

The mini is a little different, the engine and wheels have a lot of angular momentum, so the torque to stop them turning will probably, a) cause the wheels to spin on the ramp or b) cause the CV joints to explode. The momentum of the engine etc will cause the mini to accelerate up the ramp, so a long ramp and van may be called for. The rise of the ramp will use up the kinetic energy of the transmission as potential energy I suppose.
Maybe if you try this it is best if the clutch is sliding the whole time to remove the shock load of the engine? Perhaps an automatic has its uses after all.

I don't know how they did it in the Italian Job!

 

[RAI]

Nobody understand the maths in Mark Drela's analysis, done for both force and energy?

Nobody bothered to look on the boat design forum?

http://www.boatdesign.net/forums/propulsion/ddwfttw-directly-downwind-faster-than-wind-25527-14.html

His conclusion: "This confirms that the DDWFTTW condition V/W>1 is achievable with a wheeled vehicle without too much difficulty.

The many nay-sayers here might like to check out his maths before feeling too foolish.

 

[xxyyzz]

Every prop aircraft has a speed limit. It has nothing to do with its strength. It has nothing to do with friction. Nothing to do with air friction. Nothing to do with any energy loss system.
(...)
the apparent wind on the prop stalls on the blade and the prop is useless.

In your bonkers world the prop feels no effects from apparent wind because there is no friction.
I agree that as blade speed hits the speed of sound there's a problem, but I'm not sure that would still be an issue if there were no friction.
Fundamentally, trying to analyse the energy transfer of a mechanical system ignoring friction isn't going to work.
In fact, with no friction this device wouldn't work at all because the wheels wouldn't grip the ground - that's how bad an assumption it is.

Now, the ruler video again, we all agree now that the geared device moves around 3 times faster than the ruler.
Now imagine that instead of a hand, the ruler had a sail driving it.
How fast can the geared thingummybob travel?

 

[Keen_Ed]

The many nay-sayers here might like to check out his maths before feeling too foolish.

Sadly, those who can understand the proof don't need it, and those who need it can't understand it.

 

[halfway]

ubergeekian: I said "It is simply that at its maximum pitch angle, and maximum throttle, if you take it into a dive at high speed. the apparent wind on the prop stalls on the blade"

A prop is designed to push air backwards and as such has a asymmetric aerofoil. So as the apparent wind moves to the axis, as the wind speed increases in a "dive at high speed", the prop first goes through a region where it creates zero pull. Then as it continues to speed up in the gravity assisted "dive", due to the aerofoil now being the wrong way up, the air over the blade stalls very easily. At this point the drag from the prop is a bit severe. It is also on the front of the craft and will lead to "odd" deadly outcomes. The last time I saw the yellow plane I refer to later it was spread down a hill!

"Think about this. If the angle of attack was as you say, the prop would be pushing backwards."

That is correct, the prop does push the air backwards when above the wind speed. But the lift is forwards. The apparent wind comes from forward. The vector direction points backwards. As long as the effective component of the aerofoil lift, in line with the axis, is bigger than the rotational component the vehicle speeds up.


Please stay in the realm of low multiples of wind speed. I only used the speed of sound to disprove the issue of the energy of the wind being referenced to the ground. It was in my view a ridiculously high number in this context. Now everyone seems to reference it is a relevant limiting factor. The issue is number (1) the plane can not fly faster than its pitch and revs will allow due to the lack of attack angle. This is the key to faster than wind. It is not a limitless system and it is NOT friction limited. It is hampered by friction, but the limit is the aerofoil design and apparent angle. I would say that with normal technology you are not going to see past 3 times wind speed.

I agree a nice jet plane will be limited by friction only, but I was thinking about ones with big props and covered with yellow canvas that flew very slowly and with old slow engines. The type that had trouble getting home into a headwind. Something more akin to the subject we are discussing.


xxyyzz: Looking at energy transfer does not solve the system anyway. Forces do. Just like you would NOT analyse the ruler and gears with energy. My point is that friction is NOT relevant. As long as you assume an aerofoil works the rest of the energy losses due to friction in the system are meaningless. There is good friction and bad. You can just ignore all bad loss friction and still explain this system. Like you can when you explain the sailing to windward. Who uses energy transfer to do that?

I can safely say that Mark Drela's proof is a total waste of time, and does not prove anything except his ability to analyse performance of known systems. It does not in any way convince me, even now I know it works. "Proof" like these are the reason certain fundamental concepts do not gain acceptance.

xxyyzz: "Now imagine that instead of a hand, the ruler had a sail driving it." Now that is missing the point entirely, the sail is part of the gear system and the wind is the ruler. If the ruler had a sail on, it would not be on the vehicle and falling behind rapidly. Unless you want to get around the problem by having a vehicle that gets longer and longer.


I will stick to the explanation that it works using forces and apparent wind. Just like when you analyse the ruler and gears, or a sailboat operation. It was all the talk of energy and referencing it to the ground that had me doubt it to start with.

(I did like the video of the magic ball bearing. Kind of put it all in perspective. Also the idea that everything has to be super light weight to work is not right. Maybe if they designed them properly they would not have the marginal performance. Maybe if they did not read the supplied "proof" they would fair better. To start with they should be more like fans than props. The prop centres do not work at these low speeds and should be coned either side. Or better still not obstructed at all. In fact a short ducting might help (but not compressive). The fan angle should be right angles to the ground. The wheels should be big. The fan should be high off the ground. The other wheels should be up wind, so lifting, reducing friction. The fan blades should be closer to sail construction, as they do not need the strength of high speed props. Basically nothing like the demonstrated designs.)

 

[Ubergeekian]

Sadly, those who can understand the proof don't need it, and those who need it can't understand it.

Amen to dat, brudda. Ain't it de troof.

I'll confess that when I first saw descriptions of these things I thought "That's impossible, surely." Unlike the fanatical opponents, though, I took the time to work through the physics properly and soon worked out that it isn't impossible at all. It's an interesting problem, and also quite a difficult one to get your head round since it involves three different motions and a choice of axes systems. A lovely exercise for final year engineering students, perhaps less so for those whose knowledge of physics is low level and rusty.

 

[BrendanS]

I foresee some serious thread drift
http://www.youtube.com/watch?v=J9iWaCMbw60

 

[Pierrome]

Fuel from Water vs Downwind faster than the wind

I foresee some serious thread drift
http://www.youtube.com/watch?v=J9iWaCMbw60

Further to the thread drift.
Presumably this link has been posted because the poster thinks there is some similarity between the idea of getting fuel from water, which is a deception, and using the wind to power directly downwind faster than the wind. However, one being a deception does not necessarily make the other a deception, or maybe the poster is saying that if something is on Youtube it must therefore be a deception, which is also not necessarily true.

News media have found an exciting headline in "Using Water as a fuel", but in fact these inventors are not actually using it as a fuel anymore than a steam engine uses water as a "fuel". It's just an intermediary.

Electrolysis, or splitting water into hydrogen and oxygen by passing an electric current through it, has been know for probably a hundred years. Yes, you can use the resultant gas - known as "Brown's gas" as a fuel, but the truth is that the energy you can get from Brown's gas is always less than the electrical energy you put into it to make the Brown's gas in the first place. Some devices have been sold to allow the electrical system of a car to split water and feed it into combustion, and these have claimed to give enhanced mileage, but they never seem to be adopted so we must presume that they are ultimately ineffective.

By contrast, "downwind faster than the wind" trolleys and test carts, to be used on land, are the most simple mechanisms possible. There are only two parts, a wheel and a propeller which are connected together! Therefore, they are so simple to construct and test, that anyone who doubts that they work can easily put one together and see for themselves.

The doubters are really on a losing wicket and swimming against the tide when you consider the following:
1. DDWFTTW carts have been made, demonstrated in two different forms by several different unconnected people.
2. Instructions for making them are available on the web.
3. Mathematical proofs that they work are available.
4. In 2 weeks time the 2nd man-carrying version will be independently tested at Ivanpah dry lake California.

I hope all those that all those not been able to accept ddwfttw is possible will have the good grace to admit they were mistaken. But, I fear all we will get from them is a frosty silence!

 

[lw395]

Sadly, those who can understand the proof don't need it, and those who need it can't understand it.

It does not help when some of the 'proofs' are false!
It shows that some of us (inc me) could be better at explaining things too.

Now here's a thought:
Instead of exploiting the speed difference between the wind and the ground, could you exploit the speed difference between different levels of wind close to the ground? A wind powered aircraft perhaps?