Down wind faster than the wind

[ThinAirDesigns]

Huh:

I strongly oppose to the "spool and take-up reel" design. It can be argued that it violates the steady state requirement. Even if agreed it doesn't it would again be unstylish to be able to do only short runs without "reloading".

You guys act like this will be a commuter vehicle. "Short runs" is all the one I build will ever see anyway. If designed efficient enough to do what I ask of it initially, it will likely only see a half dozen short runs in it's entire life.

My design parameters call for the ability to do a mile run without reload. This is longer than any course I plan to set up for speed trap purposes. I will run out of measured course before I run out of spool.

>If there truly is a problem with efficient gearing, just
>go for a bigger propeller or wait for force 10 wind

When it's your money and life on the line you are welcome to use a bigger prop or wait for that wind.


Michael C:

I agree. There will certainly be peole who complain that you are "winding the cart up".

And this is offset by the very, very large group of people currently who will not believe that the propeller is not driving the wheels -- something the spool design proves beyond a shadow of a doubt. Can't push a string after all.

The design should permit the cart to run for an indefinite time, at least theoretically.

Our claim is "steady state", not "indefinite". Run out of lake bed, run out of wind, run out of string -- Wright Brothers proved steady state flight, but even they ran out of gas pretty quickly.

What sort of gearing have human-powered aircraft used

Interestingly enough, to my knowledge (jjcotes would be more qualified than I to state for sure) the only human powered helicopter to sustain hover is powered by spool.

For what it's worth, I haven't decided on the spool transmission yet, but am leaning that way for a variety of reasons. The "style" arguments however haven't swayed me. Technical ones very well could.

I'm listening.

JB

 

[Dan O.]

I was in that force 10 wind last night, measured at over 30 m/s for gusts, over 20 m/s sustained. That would get almost any cart moving but keeping up to verify that it is going faster than the wind would be tricky.

 

[CyCrow]

One good reason to avoid a spool is braking. If you put a brake on the wheels, prop inertia could release string tension and unravel it. If you put the brake on the prop, the string has to be strong enough to stop the cart. The required string strength would depend on the string gear ratio, which again determines the required length. The gear ratio would also change during the run if the spool is multi-layered.

A one-way bearing would also show that power goes from the wheels to the prop, but I'm not sure how much extra drag that would entail.

I like the CVT idea, as it is probable different windspeeds will have different optimal gear ratios.

The other critical design aspect is the prop. It would work at much lower rpm than typical props in normal use. Clearly a candidate for some trillions of CPU cycles and some carbon fibre.

// CyCrow

 

[Subduction Zone]

So JB you are making plans for a mansized cart. I would be glad to hear of any progress reports. Also braking is an issue, you might want to use some sort of fairly simple caliper brakes on both the wheels and the prop. You would have to keep an eye on your "string" to keep either the wheels or the prop from stopping to quickly.

 

[mender]

Or set up the two brakes to work in series with the prop brake applied first. Have the prop brake caliper movable to actuate the wheel brake in proportion to the prop braking torque. Adjust the linkage to keep the right amount of tension on the string while allowing considerably more braking on both the prop and the wheels. One brake pedal or handle, no worries.

 

[ThinAirDesigns]

One good reason to avoid a spool is braking. If you put a brake on the wheels, prop inertia could release string tension and unravel it. If you put the brake on the prop, the string has to be strong enough to stop the cart.

Unconcerned. This device will only be used on a dry lake bed or large abandoned airstrip. The course layed out for trap purposes will be no longer than a half a mile and to get smooth laminar winds there will need to be miles of level, unobstructed terrain surrounding. Braking needs are very gentle.

One bicycle style disk brake on the prop shaft. Carbon tow (carbon thread) will be sized to allow reasonable braking. One emergency brake on the drive wheels in case of some unseen failure. Integrate the two or not ... braking solved.

The gear ratio would also change during the run if the spool is multi-layered.

Yes, got that covered: First, size the spools and perhaps levelwind so that there is not deep spooling, then size the spools so that you are at optimum ratio at the 2/3 course point -- or whatever point you expect to reach top speed.

A one-way bearing would also show that power goes from the wheels to the prop, but I'm not sure how much extra drag that would entail.

That's been mentioned by others before and I don't think drag would be a problem ... it would engage 100% during load and freewheel otherwise. Problem is it isn't visual enough -- you have to explain that there is a one way clutch in the system and then get people to believe you. The limitations of the thread is immediately visible.

JB

 

[ThinAirDesigns]

So JB you are making plans for a mansized cart. I would be glad to hear of any progress reports. Also braking is an issue, you might want to use some sort of fairly simple caliper brakes on both the wheels and the prop. You would have to keep an eye on your "string" to keep either the wheels or the prop from stopping to quickly.

I'll keep everyone posted. I'm also sure I'll be throwing out crazy ideas for everyone to toss about and will be tossing about those that everyone else comes up with.

JB

 

[huh34]

When it's your money and life on the line you are welcome to use a bigger prop or wait for that wind.

Njah, don't be such a bore

And this is offset by the very, very large group of people currently who will not believe that the propeller is not driving the wheels -- something the spool design proves beyond a shadow of a doubt. Can't push a string after all.

The wheels driving the prop is proved beyond doubt already when you look at the gearing and pitch. Although you can't push a strig, you can wind it up the spool the other way round. So I think the argument is not offset. I think you just fail to "think" as the unbelievers do. Anyway I think this is irrelevant.

Our claim is "steady state", not "indefinite". Run out of lake bed, run out of wind, run out of string -- Wright Brothers proved steady state flight, but even they ran out of gas pretty quickly.

Again, you use that sort of logical thinking that doesn't nesessarily apply to nonbelievers. I said it _can be argued_ that it violates steady state, did not say it does. It would be of advantage to be able to theoretically run to infinity. But this is a minor issue, it would be more important to include self start to your parameters, please do!

ETA: There is a big difference between running out of thread and running out of wind/gas (or lake bed). The latter are external issues, running out of thread is a fault of the device. If you run out of fuel or whatever your power source, no problem, but if you run out of thread when the power source (wind) is still available, that is a problem with the steady state claim. Remember you saying "it keeps going as long as we keep paying our power bills" in the treadmill video.

Unconcerned. This device will only be used on a dry lake bed or large abandoned airstrip. The course layed out for trap purposes will be no longer than a half a mile and to get smooth laminar winds there will need to be miles of level, unobstructed terrain surrounding. Braking needs are very gentle.

But keep in mind, that the more braking power you have, the shorter airstrip (or whatever test area) you need. Proper braking may make the task of finding a suitable test area a lot easier.

One crazy idea for you to toss about. If you do use the spool and takeup-reel solution, perhaps the slack could be dealt with a few blocks and a counterweight (or rubber string or a spring). Between the spool and reel, run the thread through 2 ball-bearing blocks. These blocks are attached to counterweights that pull them apart when the thread goes slack. When tight, the thread looks like "I" and when slack the blocks pull it to a "Z" thus taking up the slack. There will be some added friction and I wonder if the about 1-2 m of slack that this system could handle is enough, but perhaps an idea worth considering.

Do you have some rough figures of the spool-reel system details. How long carbon thread do you need (ain't that pretty expensive stuff)? Diameter and breaking load?

 

[John Freestone]

My design parameters call for the ability to do a mile run without reload. This is longer than any course I plan to set up for speed trap purposes. I will run out of measured course before I run out of spool.

And this is offset by the very, very large group of people currently who will not believe that the propeller is not driving the wheels -- something the spool design proves beyond a shadow of a doubt. Can't push a string after all.

For what it's worth, I haven't decided on the spool transmission yet, but am leaning that way for a variety of reasons. The "style" arguments however haven't swayed me. Technical ones very well could.

I'm listening.

JB

JB, I don't get the technical advantages of the spool transmission, myself, and wouldn't do it that way for the following reasons, some of which may have been covered:

1. You're nervous about big props and fast winds for safety/financial reasons, but I hate to think of the dangers of a cord with sufficient tensile strength accidentally getting unwound for a moment or two and then the torque of the machine tightening any slack up again after it's been flapping about, perhaps getting caught on other moving parts, while you try to remember which brake to apply first! I've only chatted with you online, but I like you enough not to want to hear of your unfortunate garrotting during a freak wind-carting accident!

2. Demonstrating that a string can't push is quite good, but it could be open to someone saying that you changed it round (as has been said); also, it's not particularly difficult to demonstrate a freewheel mechanism, if I'm thinking of the same thing as has been mentioned, people can see that it can't easily be altered and the things are common enough on bicycles. Besides, I'm not so sure that the direction of the prop and people understanding it is particularly important in the scepticism stakes - if the machine does what it's supposed to do and they don't know how, I don't see there's such a problem.

3. The likelihood of somewhat variable wind conditions means that it would be an advantage to be able to start a run with miniumum setting up time, and I don't know how you're going to do that with spools, with a mile without reload as the parameter (how much cordage will that involve, BTW?). In fact, the only ways I can think of doing the winding sensibly involve some other machinery and/or the ability to uncouple the spools from their axles - again allowing a window for sceptics to swear something dodgy is going on.

3. Then you're talking about being fairly precise in the way you wind it for the best gearing at a particular point of travel, and you may find that conditions don't fit with that. Again, by the time you've worked this out and adjusted (if it's worth it) the conditions might have changed again.

4. Pretty much all the parts for a conventional gear system, freewheel and brakes can be bought at your local bike shop; you've probably got most of them in the garage right now (ok, maybe you've got spools and a very long, strong piece of cord!).

5. If the land cart you build is as near as possible a scaled-up version of the treadmill cart, it's easier for people to make the mental switch and also see that the treadmill is an equivalent situation.

6. I can't think of one positive advantage a spool transmission offers.

 

[CyCrow]

If I was writing a mythbusters episode...

0. Intro the myth that you can't travel DDWFTTW, illustrate with sailboats. Show that tacking gets you there faster, show that iceboats easily get VMG downwind much faster than the wind. Maybe show direct upwind vehicles.

1. Build the spork cart and test it on a treadmill. Explain the equivalence.

2. Find a long conveyor belt/walkway and run it up that. Maybe RC steering would be needed.

3. With plausibility demonstrated, do a buildoff and race.

One team could go as simple and cheap as possible, by just scaling up the spork cart to carry a human. Bicycle wheels, windsurfer mast for propshaft, aluminum tubing frame, a seat and simple rear steering would do. I would also add a brake for safety. The prop would have to be manufactured, perhaps by scanning the small prop and scaling it up, milling it out of foam. Then create a mold and make the blades using epoxy, fibreglass and carbon fibre.

The other team could go all scientific and hi-tech. Get some aerodynamicists to analyse and design an optimal prop, variable pitch, variable gearing, complete instrumentation showing wheel speed, prop speed, advance ratio, prop torque, gps track and speed, relative windspeed etc. Use lots of carbon fibre and expensive materials.

Then race...

// CyCrow

 

[spork]

I've only chatted with you online, but I like you enough not to want to hear of your unfortunate garrotting during a freak wind-carting accident!

That could change. You've got to get to know him like I do.

6. I can't think of one positive advantage a spool transmission offers.

I think there are some advantages to the spool and take-up reel. You could get ludicrously high efficiency with incredibly simple design (at least until we start thinking of braking and devices to keep from turning our whole contraption into a hair-ball). Even so, I tend to agree with most folks on this list. I'm still hoping to go with a belt or gearing solution. But I think JB is open to having both options in the field.

 

[John Freestone]

That could change. You've got to get to know him like I do.

With friends like you...

I think there are some advantages to the spool and take-up reel. You could get ludicrously high efficiency with incredibly simple design (at least until we start thinking of braking and devices to keep from turning our whole contraption into a hair-ball). Even so, I tend to agree with most folks on this list. I'm still hoping to go with a belt or gearing solution. But I think JB is open to having both options in the field.

Any idea how much loss would be caused by traditional gears? I'm surprised if the efficiency of the gears is of much concern (says me, setting myself up for another lesson). People were saying that props can be quite inefficient, so wouldn't those gear-train losses be relatively insignificant? The gearbox of my car gets a little warm, but mostly from being in contact with the engine, surely? I guess those 90d gears (forgot the name) are a bit less efficient than, say, the derailleurs on my bike, but even so, a bit of grease and I can't see it's going to take that much off the efficiency.

 

[mender]

Well, if you guys get to use a couple of spools and some string, I get to use a finite system too!

My design is going to be using the same length string as the proposed run with one end anchored to the ground, run forward to a pulley cart which is attached to a big parachute and back to my ultra-mini lay down mancart. I figure I should be able to average at least 1.5x windspeed with my non-prop cart.

There won't be any question about directly downwind or how my cart is powered!

ETA: John, those are bevel or spur gears.

 

[ThinAirDesigns]

I think you just fail to "think" as the unbelievers do.

Having been involved now in literally *scores* of forums containing again literally tens of thousands of posts I can say that I have a pretty good idea how the unbelievers think. I'm not interested in thinking like individual unbelievers, but as a whole. How does one best focus on answering the questions of the *majority* since one cannot answer the questions of all.

it would be more important to include self start to your parameters, please do!

Here we agree -- it's not part of our claim but becomes entangled by the vast majority of unbelievers. Additionally, it's trivial to demonstrate and so will most certainly be included.

If you run out of fuel or whatever your power source, no problem, but if you run out of thread when the power source (wind) is still available, that is a problem with the steady state claim.

If my thread is longer than my course (which it will be) the half empty thread spool issue no more violates "steady state" than the brakes which I will apply at the end of the course.

But keep in mind, that the more braking power you have, the shorter airstrip (or whatever test area) you need. Proper braking may make the task of finding a suitable test area a lot easier.

I've spent a fair amount of time here in CA on dry lake beds while towing soaring craft. Ideal test area and plenty of room. Non-issue.

One crazy idea for you to toss about. If you do use the spool and takeup-reel solution, perhaps the slack could be dealt with a few blocks and a counterweight (or rubber string or a spring). Between the spool and reel, run the thread through 2 ball-bearing blocks. These blocks are attached to counterweights that pull them apart when the thread goes slack. When tight, the thread looks like "I" and when slack the blocks pull it to a "Z" thus taking up the slack. There will be some added friction and I wonder if the about 1-2 m of slack that this system could handle is enough, but perhaps an idea worth considering.

I think spork came up with a simple and clever way to deal with any potential slack, but let us work out the details before we present.

How long carbon thread do you need (ain't that pretty expensive stuff)?

Unbelievably cheap it turns out. You can buy reels of it on ebay for under a hundred bucks that would do our job many times over. (or at least we currently believe so). Testing required.

JB

 

[ThinAirDesigns]

JB, I don't get the technical advantages of the spool transmission, myself,

John, there is one advantage and one only that I know of - and it happens to be meaningful to me ... speed.

I'm going to build this once. I'm going to put it though it's paces as few as times as needed to wring the last bit of speed out of it. Then I'm done.

I want the meanest, leanest, most efficient device that I can build and I will demonstrate, document and put it way. Like Goodman who has the prop from his cart on the wall and says: "I've gone as far as I need to"", what I get this next round will be the end of the road for me.

A top fuel dragster is a very impractical device that takes a full crew just to get it to the line -- the result of which blows people away. That's what I'm looking for here ... the best results available this side of a corporate funded project.

1. You're nervous about big props and fast winds for safety/financial reasons, but I hate to think of the dangers of a cord with sufficient tensile strength accidentally getting unwound for a moment or two and then the torque of the machine tightening any slack up again after it's been flapping about, perhaps getting caught on other moving parts, while you try to remember which brake to apply first! I've only chatted with you online, but I like you enough not to want to hear of your unfortunate garrotting during a freak wind-carting accident!

First, easy to deal with -- keep the spool further from the prop than a thread break can reach. Second, size the thread to handle working loads, but to snap in case of tangle with the frame. Third, coast to a stop.

2. Demonstrating that a string can't push is quite good, but it could be open to someone saying that you changed it round (as has been said); also, it's not particularly difficult to demonstrate a freewheel mechanism, if I'm thinking of the same thing as has been mentioned, people can see that it can't easily be altered and the things are common enough on bicycles. Besides, I'm not so sure that the direction of the prop and people understanding it is particularly important in the scepticism stakes - if the machine does what it's supposed to do and they don't know how, I don't see there's such a problem.

Every system has it's explanations -- only one is the most efficient.

3. The likelihood of somewhat variable wind conditions means that it would be an advantage to be able to start a run with miniumum setting up time, and I don't know how you're going to do that with spools, with a mile without reload as the parameter (how much cordage will that involve, BTW?).

A simple cordless drill with a socket on it rewinds the top spool from the bottom in less that 30 seconds.

In fact, the only ways I can think of doing the winding sensibly involve some other machinery and/or the ability to uncouple the spools from their axles - again allowing a window for sceptics to swear something dodgy is going on.

No matter what system I use I will be able to disconnect the prop from the wheels. This is needed to safely get the device from the finish line back to the start in short order. Disconnect and tie the prop down and then tow back to do it again.

3. Then you're talking about being fairly precise in the way you wind it for the best gearing at a particular point of travel, and you may find that conditions don't fit with that. Again, by the time you've worked this out and adjusted (if it's worth it) the conditions might have changed again.

Changing gearing with a spool system is WAY quicker than with a belt or chain or gear system. WAAAAAY quicker. A belt system required either changing both pulleys or altering the distance between prop shaft and wheel shaft or an energy robbing takeup system. The prop shaft can be open ended, but the wheel shaft is not, so getting the belt clear off is very time consuming. Changing the gearing on a spool system only requires changing one spool and it can be the easy one (prop). With a second spool loaded and ready I can change gearing in less than one minute.

4. Pretty much all the parts for a conventional gear system, freewheel and brakes can be bought at your local bike shop;

What they sell at bike shops will not make the 90 degree twist we need from wheel to prop. Bike stuff useless.

5. If the land cart you build is as near as possible a scaled-up version of the treadmill cart, it's easier for people to make the mental switch and also see that the treadmill is an equivalent situation.

My first choice in form would be exactly as you say. In search of speed however I simply can't afford the ~85% efficiency of a 90d gearbox. Find me a 95%+ right angle drive and my decision is easily made.

6. I can't think of one positive advantage a spool transmission offers

Now I've given you several.

JB

 

[ThinAirDesigns]

Well, if you guys get to use a couple of spools and some string, I get to use a finite system too!

The age old challenge from one racer to another.

Let's line'm up and run what'cha brung.



PS: Actually I would think it would be trivial for the system you describe to wallop the one I describe. Put 4x1 pullies, a light cart and a big chute -- done. I think the DDWFTTWRA may have an issue with the stake in the ground though.

 

[mender]

Stake in the ground = kinematic restraint. Mount a reel to the road wheel axle that has an o.d. close to the road wheel diameter to wind up the string and it'll do essentially the same thing as a stake. Also acts as a brake when all the rope is wound up and the cart gets to the parachute! Instant lock-up!

But I also think the pulley system would be way too easy. Of course, it's likely that the sanctioning body will impose a "wind interface" area limitation and a minimum total weight to even things up. Gotta get that rule book written!

 

[Dan O.]

My first choice in form would be exactly as you say. In search of speed however I simply can't afford the ~85% efficiency of a 90d gearbox. Find me a 95%+ right angle drive and my decision is easily made.

Spiral Bevel Gearbox

 

[ThinAirDesigns]

Spiral Bevel Gearbox

Ooooohhh. Very nice Dano.

Now to figure out if I can find on that's light enough for our purposes.

Cool -- thanks again.

JB

 

[ThinAirDesigns]

But I also think the pulley system would be way too easy. Of course, it's likely that the sanctioning body will impose a "wind interface" area limitation and a minimum total weight to even things up. Gotta get that rule book written!

Personally I would insist on the CG of the vehicle to be going DDWFTTW steady state in any rule book I contributed to. But that's just me.

JB