Down wind faster than the wind

[spork]

I've already tried that. Such questions quickly get left behind by the flood of lessons in incorrect physics, bickering, justifications and other nonsense. If anybody can suggest a place where I can actually discuss DDWFTTW, I'm going there.

Michael, please don't leave this thread because of the humber spam (or even our responses to it). If your questions are left behind, ask them again. PM them to us, etc.

Sadly, every forum I've seen has at least one complete jackass trying to derail it.

 

[Dan O.]

Hello Dan,

yes, that would be a good idea. And then, preferably make the split about 40 or so pages ago ....

Greetings,

Chris

There was only one supermod with the ability to unravel a thread this entangled and she is no longer here. I think we'll have to settle for making a clean split and moving forward.

 

[Michael C]

Michael, please don't leave this thread because of the humber spam (or even our responses to it). If your questions are left behind, ask them again. PM them to us, etc.

OK, I'll try. One question at a time:

For a ride-on DDWFTTW cart, what would be the best way to control acceleration and deceleration?

 

[spork]

For a ride-on DDWFTTW cart, what would be the best way to control acceleration and deceleration?

That's a good question. But I have to admit, I may be no better suited to answer it than anyone else.

We are contemplating a ride-on cart with either fixed pitch prop, or one with slightly variable pitch. We would alter the pitch only to find the best position for fastest downwind speed, not for control. So acceleration would simply max out at the value given by the wind at our backs.

For braking we've discussed braking at the propeller itself, or at the wheels. In either case you'd want brakes that provided zero drag when not engaged. We've talked about many transmission options, and JB seems to be leaning toward a "spool and take-up reel" design. In this case you'd want to be very careful to make sure you don't let inertia send the filament flying off the spool. That could be done with proper braking of the prop, spool, axle, or through a dedicated friction brake that operates on the spool only when the filament goes slack.

 

[John Freestone]

I've already tried that. Such questions quickly get left behind by the flood of lessons in incorrect physics, bickering, justifications and other nonsense. If anybody can suggest a place where I can actually discuss DDWFTTW, I'm going there.

Ok, Michael. Sorry, I was a bit miffed by once again being told I'm doing it all wrong. Anyway, in the time it took to write my last, the quality of the discussion deteriorated from 'virtually pointless' to 'absolutely banal', and you're probably right. Actually, what the world of DDWFTTW needs now, probably, is (love sweet love?, no...) a secret forum, so that it can't be found by any of those Nordic mythical personalities who live under bridges.

 

[Michael C]

We are contemplating a ride-on cart with either fixed pitch prop, or one with slightly variable pitch. We would alter the pitch only to find the best position for fastest downwind speed, not for control. So acceleration would simply max out at the value given by the wind at our backs.

I'm wondering about the start. If the prop is optimised for greater-than-wind speeds, are there risks of skidding wheels at start, or too much stress on the frame or transmission? Is it worth making a variable-pitch prop that can also be adjusted to negative pitch, in order to better control less-than-wind speeds?

For braking we've discussed braking at the propeller itself, or at the wheels. In either case you'd want brakes that provided zero drag when not engaged. We've talked about many transmission options, and JB seems to be leaning toward a "spool and take-up reel" design. In this case you'd want to be very careful to make sure you don't let inertia send the filament flying off the spool. That could be done with proper braking of the prop, spool, axle, or through a dedicated friction brake that operates on the spool only when the filament goes slack.

Sorry, I'm not sure what a "spool and take-up reel" design is. Could you provide a drawing/explanation or link to one? When the brakes are applied, would the trasmission between prop and wheels be disengaged?

 

[spork]

what the world of DDWFTTW needs now, probably, is (love sweet love?, no...) a secret forum, so that it can't be found by any of those Nordic mythical personalities who live under bridges.

Actually, not a secret forum, but a forum controlled by someone with a direct interest in DDWFTTW. I see that a number of people have blogs that are followed by what amounts to a forum. Given the tools and resources I'd be happy to run such a forum - or nominate someone like you, Sol, Michael, or JB to do so. Obviously we would want to address the sceptics, but would draw the line at people simply spamming the board with pure nonsense (in our own sole discretion).

I'm wondering about the start. If the prop is optimised for greater-than-wind speeds, are there risks of skidding wheels at start, or too much stress on the frame or transmission? Is it worth making a variable-pitch prop that can also be adjusted to negative pitch, in order to better control less-than-wind speeds?

Clearly variable pitch prop would give you more flexibility (at the cost of complexity). But I think it would be quite doable without. I doubt that skidding wheels would be an issue with high pressure bike tires, but that's just a guess or a feeling I suppose.

Sorry, I'm not sure what a "spool and take-up reel" design is. Could you provide a drawing/explanation or link to one? When the brakes are applied, would the trasmission between prop and wheels be disengaged?

Just imagine two of your cotton reels. One is empty and the other has 1000 meters of filament. The prop-shaft has the full spool, and the axle has the empty one. As the axle turns it pulls the filament onto its spool. This can give you a nearly 100% efficient transmission - even with the 90 degree turn that's required. But it has an obvious range limitation, and the possibility of spewing filament everywhere if not designed well.

 

[humber]

Clearly variable pitch prop would give you more flexibility (at the cost of complexity). But I think it would be quite doable without. I doubt that skidding wheels would be an issue with high pressure bike tires, but that's just a guess or a feeling I suppose.

The cart had trouble stating, because it not efficient under those conditions.
It is true that that when rotating CCW, that does the same for the wheels and that blocks travel, but that is too simplistic a view.
To drive the prop, there need only be a pressure differential one way or the other. The wind changes, producing a varying condition. If the average one way is greater than the other, the cart will move.
If the wind drives the prop CCW, that stresses the shaft and components, including tires. That stores energy. When the force driving the prop CCW falls, the energy will be returned, and the cart will move forward.
Two energies and forces at work. Normal CW and CW as a result of stored energy plus direct drive from the elasticity of the wheels or tires.
Try pushing a car, it rocks on the tires elasticity.
You can see the cart do at least part of this in the street video. All that is required is an average force over time to produce the required acceleration, until forward motion is sustained by other means.

 

[jjcote]

Happy St. Stephen and a half day to all o' y'all, or whatever it is. Jeez, even during the holidays and the database servicing you people created such a blizzard of posts that I'm not sure I have time to wade through them all. When I was last here, I believe I was waiting for humber to answer the question of whether he would accept a flat rubber surface in the foggy dark, with a 10 m/s steady wind, as a test bed, or if he needed to see more of the surroundings. Did he ever provide more of an answer to than than "Not cart, not treadmill"?

While I was visiting the extended family, I posed the DDWFTTW puzzle to my brother-in-law (college freshman at a respected engineering school) and his father (graduate of same school). I didn't have enough time to take the conversation as far as I would have liked (and other family members were annoyed that we were having it), but they were pretty skeptical and were insisting that I was doing things wrong. They almost understood the "stretch tumbleweed" design, but were trying to insist that it would either move the wrong way or else stand still, and when I did the yo-yo demonstration with a thread bobbin, they were trying to figure out what subtle way I had devised to cheat. Fun times.

 

[humber]

Did he ever provide more of an answer to than than "Not cart, not treadmill"?

The belt is OK. That remark means that I don't want to discuss the car itself, but the claim of its operation on the treadmill.

I did the yo-yo demonstration with a thread bobbin, they were trying to figure out what subtle way I had devised to cheat. Fun times.

There are many such puzzles. The fact that they contradict your notions, is why they are fun.

 

[humber]

I love humb He is even humberer than humber. Maybe if humber sees himself enough in a mirror he will learn something.

Tragic. Perhaps you would like to tell me how the stored energy of the elastic members won't work?
In the wind blow consistently so the props turns CCW, then it won't move at all. However, if it blows consistently CW, then it should. So why the fuss about starting?
Perhaps it doesn't generally start well, but sometimes if conditions are right, it does. Do you think then that anything that makes that process more efficient, may broaden that range of conditions?

 

[Dan O.]

The easiest way to build a variable pitch prop (though not necessarily the lightest) is to have a fixed axel that goes through the hub and mount the blades on a pair of hollow tubes that fit over the axel. A linkage with either a swash plate like on your helicopters or a control rod through the center axle like Bauer's cart can then rotate the blades. There are other linkage possibilities that involve differential gears that could also be used.

If you don't need to transmit a lot of power, a continuously variable transmission could give you 0 to infinite and even negative gearing. At infinity, it even acts like a break.

 

[humber]

The easiest way to build a variable pitch prop (though not necessarily the lightest) is to have a fixed axel that goes through the hub and mount the blades on a pair of hollow tubes that fit over the axel. A linkage with either a swash plate like on your helicopters or a control rod through the center axle like Bauer's cart can then rotate the blades. There are other linkage possibilities that involve differential gears that could also be used.

If you don't need to transmit a lot of power, a continuously variable transmission could give you 0 to infinite and even negative gearing. At infinity, it even acts like a break.

I wonder if it would be useful if such a cart also exploited momentum?

 

[John Freestone]

OK, I'll try. One question at a time:

For a ride-on DDWFTTW cart, what would be the best way to control acceleration and deceleration?

I'm not sure why you'd want much control, other than a simple but good, mechanical brake and simple steering. Acceleration would be dealt with by disengaging the brake, and deceleration by applying it. I'm thinking of the DD cart as a phenomenon and brainteaser, rather than the latest transport option or sport, and since in good tradition form follows function, it would be best if extra gizmos were for the purposes of trying to ascertain lateral wind movement (so as to minimize it), windspeed, ground speed, etc., if necessary. I would have thought that something under a force 10 gale would do the trick, but if you want to 'extreme-DDFTTW', variable pitch (or just a disengage blades mode) would be useful to get it on the tarmac/salt without killing someone.

I suppose it could lead to sporting versions that might use a prop to go in all directions relative to the wind, but that's a different question.

For racing DDW, i.e. for max acceleration and best top speed, I haven't got a clue.

 

[Gaspode]

As requested, all further discussion of alternate inertial reference frames and other claims regarding the validity of classical physics split to new thread.

Some off-topic posts have been moved to AAH.

Replying to this modbox in thread will be off topic  Posted By: Gaspode

 

[John Freestone]

I wonder if it would be useful if such a cart also exploited momentum?

No.

 

[John Freestone]

I also keep wondering about a mechanism for monitoring lateral air movement that automatically adjusts the steering, so that if the wind isn't veering all over the place, but just keeps changing a little, the cart automatically directs itself. A simple rudder-affair attached to the steering mechanism would be nice except for the problem that the wind reverses w.r.t. the cart. Maybe some bright spark can come up with something, or maybe it's just another unnecessary complication. It does seem that the directness of travel wrt the wind is one of the big problems though.

 

[spork]

I'm not sure why you'd want much control, other than a simple but good, mechanical brake and simple steering. Acceleration would be dealt with by disengaging the brake, and deceleration by applying it. I'm thinking of the DD cart as a phenomenon and brainteaser, rather than the latest transport option or sport...

That happens to be in line with my own interests as well. I understand that some might want to do any number of other performance tests with them. My only goal would be to demonstrate the cart clearly outpacing the wind by a large margin.

I also keep wondering about a mechanism for monitoring lateral air movement that automatically adjusts the steering, so that if the wind isn't veering all over the place, but just keeps changing a little...

I like that idea. For the ride-on version we could take the same approach we use on sailplanes - just a simple piece of yarn.

 

[Dan O.]

Here is a quick concept sketch of a continuously variable transmission for a larger cart. The lower wheel rolls on the ground and the upper wheel connects to the propeller shaft. The vertical transmission shaft connects the two smaller wheels that roll on the sides of the larger wheels.

 

[Christian Klippel]

I also keep wondering about a mechanism for monitoring lateral air movement that automatically adjusts the steering, so that if the wind isn't veering all over the place, but just keeps changing a little, the cart automatically directs itself. A simple rudder-affair attached to the steering mechanism would be nice except for the problem that the wind reverses w.r.t. the cart. Maybe some bright spark can come up with something, or maybe it's just another unnecessary complication. It does seem that the directness of travel wrt the wind is one of the big problems though.

Hello John,

well, if one wants to have such a cart that can travel DWFTW at some greater angle instead of directly, then yes, it would be a problem. But then, i think that a certain amount of "non-directness" should be possible already, if it isn't too big.

I'm wondering how big the influence of the prop size for this. Maybe some combination of an "bigger than needed" prop together with variable pitch blades would do the trick? Also maybe together with (or only) a prop-mount that can rotate, similar to a weather-vane on a roof? In any case, since the main purpose is to go _directly_ downwind, i wouldn't bother too much. There are these cars that are driven by rockets, just for pure speed. They cant really steer as well, only very minimal, since they have only one purpose: being fast. So, personally i would be happy with a "rideable" cart that can only go directly downwind.

As for Michaels question of braking the cart, what about a magnetic brake? Just a bunch of strong magnets in the wheels, and for braking one moves or "folds" a steel or aluminum plate close to them, so they brake due to eddy currents? This way there wouldn't be extra parts mounted on the axles, only some extra weight in the wheels, and extra weight is already added because of the potential driver so the magnets don't add too much in relation to the overall weight addition. Or the straight and simple way that bicycles brake, with a , working directly on the wheels themself. There
would be some kind of bicycle wheels anyways on such a cart, i guess.

But then, these are just some ideas that popped in my mind immediately after reading the questions...

Greetings,

Chris

Edit: Of course the magnets-in-wheels can be replaced by using wheels with an aluminum body, and a electromagnet on the frame next to it, plus the needed battery. But that would be heavier in the end, i guess. In any case, we have street-trains here that use exactly this principle for braking by having a "loosely" mounted electromagnet that hovers over the tracks. And boy do they brake quickly.