Wind powered cart traveling directly down wind faster than the wind

[Jim Logajan]

When the cart is sitting still and the wind is blowing across it then the cart has zero kinetic energy. After the cart gets moving it has kinetic energy that came from the wind.

The maximum kinetic energy is 1/2mv^2 where v is the wind speed if the cart is 100% efficient at capturing the wind energy. But it isn't 100% efficient. It's less than that and if it's more then it's magic.

What value do you think should be entered for "m" into that kinetic energy equation, and why?

(Hint: You're starting off on the wrong path in the analysis, which is why you've immediately run into a road block by starting where you did.)

 

[ebacon]

What value do you think should be entered for "m" into that kinetic energy equation, and why?

(Hint: You're starting off on the wrong path in the analysis, which is why you've immediately run into a road block by starting where you did.)

m is the mass of the cart.

Where is the error?

 

[Jim Logajan]

m is the mass of the cart.

Where is the error?

The error is that the cart isn't the source of energy. After all, when the cart is stationary in the earth's reference frame, its kinetic energy in that frame is zero.

If you are going to claim a physical limit due to conservation of energy, then at the beginning (before the cart starts moving) the only thing moving in the earth's reference frame is the wind. What is the mass of the wind? Answer that and you have an upper limit on how much energy is available in the system.

 

[ebacon]

The error is that the cart isn't the source of energy. After all, when the cart is stationary in the earth's reference frame, its kinetic energy in that frame is zero.

If you are going to claim a physical limit due to conservation of energy, then at the beginning (before the cart starts moving) the only thing moving in the earth's reference frame is the wind. What is the mass of the wind? Answer that and you have an upper limit on how much energy is available in the system.

The mass flow rate of working wind comes into the equation as an expression of force. For example assume two different carts having equal masses, but each presents a different area to the wind. One more than the other.

The cart that presents more wind area could generate more force (F) because it interacts with a greater mass flow rate of the wind. If the masses of the two carts are equal then the bigger cart will accelerate faster (A=F/m) but their limit velocities and final kinetic energy (E.sub.k) will be the same (E.sub.k=1/2mv^2).

Another way of looking at the problem would be to race two identical carts, but one in a helium atmosphere and the other in a nitrogen atmosphere. The helium cart will accelerate slower than the nitrogen cart. That's because the force generated by the less dense helium will be less.

No?

 

[Jim Logajan]

The mass flow rate of working wind comes into the equation as an expression of force. For example assume two different carts, but each presents a different area to the wind. One more than the other.

The cart that presents more wind area could generate more force (F) because it interacts with a greater mass flow rate of the wind. If the masses of the two carts are equal then the bigger cart will accelerate faster (A=F/m) but their limit velocities and final kinetic energy (E.sub.k) will be the same (E.sub.k=1/2mv^2).

Another way of looking at the problem would be to race two identical carts, but one in a helium atmosphere and the other in a nitrogen atmosphere. The helium cart will accelerate slower than the nitrogen cart. That's because the force generated by the less dense helium will be less.

No?

No. The V and m in the kinetic energy equation must both describe the same object. You can't use m = mass of cart and V = velocity of wind and plug them into the kinetic energy equation and expect to get anything meaningful.

 

[ebacon]

m = mass of the cart
v = velocity of the cart. It starts at 0 and accelerates (A), at best, up to the wind speed.

If I understand what you are saying then m is the mass of the wind. How do you arrive at that m?

 

[PilotAlan]

Let me try to get an explanation in here.

Hypothetical cart with geared wheel and a prop. Hypothetical 100% efficiency of the prop and gears, and zero rolling resistance from the wheels.

You start the cart rolling. With no rolling resistance and perfect efficiency, the cart will continue to roll forever, with the thrust from the prop and the drag from the energy production at the wheels equaling each other out.

In the real world, you have losses from the gears and the prop. But, the wind is blowing from behind and you have essentially zero drag.

Plus, the wind is introducing energy into the system, with a forward vector. So, if you can get the efficiency of the gears and prop high enough, you can use the energy being introduced by the wind to accelerate the cart.

 

[COFlyBoy]

m = mass of the cart
v = velocity of the cart. It starts at 0 and accelerates (A), at best, up to the wind speed.

If I understand what you are saying then m is the mass of the wind. How do you arrive at that m?

At t=0 the cart is moving at v=0. Therefore the cart has 0 kinetic energy.

Soooooo, the kinetic energy must come from the wind. To calculate the available kinetic energy you need to know the mass of the wind. So the question is what do you think the mass of the wind is?

 

[Jim Logajan]

m = mass of the cart
v = velocity of the cart. It starts at 0 and accelerates (A), at best, up to the wind speed.

If I understand what you are saying then m is the mass of the wind. How do you arrive at that m?

Earlier you said V was the wind speed. To use wind speed meaningfully in the kinetic energy equation one has to plug in the mass of the wind for m, not the mass of the cart.

If V and m are now consistently describing the state of the cart, we can plug them into kinetic energy equation. But all that equation tells us is that the cart's kinetic energy increases with V. So too does its momentum. But they do not per se indicate any limit in the cart's speed.

I suspect your reference to the kinetic energy equation was to show that, once the relative wind on the blades drops to zero the available kinetic energy from it drops to zero? If so, then no disagreement. (There is just the, ahem, small matter now of what the wheels of the cart are doing....)

 

[Cap'n Jack]

What's the name of the outfit that designed the car? Thin Air Designs. They design brain teasers. Hoaxes. That's what they do and they are damn good at it.

That's my story and I'm sticking to it.

EdFred!!

Do you have a model cart? Someone suggested you might. I fly out of KPTK and could be there in a jiffy.

Spork has a model now and he invited you over to see it. Why don't you fly out and talk to the guy that made both a model and a man-carrying one, and inspect his model?

I'd have to build one, but Spork has one ready to go.

 

[spork]

You say it's going directly downwind when it's actually on a broad reach. That's a verbal misdirection.

You're simply wrong on both accounts. This is a direct downwind cart. It goes directly downwind. Can you drive you car directly downwind? I think everyone but you would say "yes". But what about the pistons? They're not going directly downwind. What about the tires. The front is going down and the back is going up. I think it's intuitively obvious to even the most casual observer that this cart has moving parts. That's not supposed to be the tricky bit. If there's "verbal misdirection", it's not intentional. If you simply have trouble understanding our simple definition, in which a cart, its driver, and it's C.G. go directly downwind, I can accept that - but don't accuse me of scamming, trolling, or misdirection.

Steady state means "unchanging in time" ... your cart changes constantly. You are not doing what you claim to do.

Steady state means that it's not in any way gaining or losing energy. It's steady state. You think a spinning top can't be steady-state?

When we allow that it is not steady state but average, then ANY sailboat can do exactly what you do, maintain a broad reach over a long distance and average the changes so that the vehicle maintains an average course directly downwind. You're bizarre definition of steady state would suggest no motion of any kind. Somehow we will have to find the one true absolute reference frame and make sure the cart is stationary with respect to it in order to meet your definition.

You are just on a run with these false accusations. You do not go directly downwind

Very nice - you accuse me of making false accusations, and follow that up immediately with a false accusation!

I'm not saying your cart doesn't move. I'm just pointing out that you've chosen a deliberately obscure way of describing a simple device in order to foster argument.

There is nothing deliberately obscure about what I've described. In fact I don't think there's anything remotely obscure about my definition for most anyone else on this thread.

Maybe scammer should be replaced with "troll"....

Should I take that as a compliment too? I'm sorry you have trouble understanding a perfectly simple, straight-forward definition.

Explain how the power provided by the prop exceeds the power consumed by the wheels.

IT DOESN'T. When are you going to sit still and listen for 10 seconds!? It's NOT PERPETUAL MOTION.

Recall that there is no external source of power here.

You just couldn't be more wrong. You will not come to understand how this works by making random invalid assertions. It might require reading what we write.

No treadmill motor.

You're still believing a plane can't take off from a treadmill - huh? Most people don't find treadmills all that confusing.

Just the wind, and that was used up getting the car up to the speed of the wind.

Really? We used up all the energy of the wind getting the vehicle up to wind speed? Amazing. We'd better not operate this around any wind farms. They would hate it if we used up all the energy of the wind.

Where is the additional power coming from?

There is no EXTRA energy.

A winky smiley doesn't help me undertand.

Making crazy assertions when you should be listening and thinking won't help you understand.

But if the efficiency of the wheel/prop system is <=1 then the cart can't punch through the zero wind barrier.

There is no "wind barrier" and no need to "punch through it". This cart can pull a trailer all day long, going at exactly wind speed.

Are saying that if 1 HP is applied to the wheels then > 1 HP will come from the prop?

Only you are saying that - repeatedly - no matter how many times we tell you it's not the case.

 

[Cap'n Jack]

This thread is turning into an example for this one:
http://www.pilotsofamerica.com/forum/showthread.php?t=41503&highlight=science

 

[olasek]

There is no "wind barrier" and no need to "punch through it". This cart can pull a trailer all day long, going at exactly wind speed

Correct. Unfortunately some here with extremely poor understanding of physics think that the wind provide only energy up to its own speed and everything over it is a "magic" extra that needs special explanation where it came from. Complete BS, pardon my language.
There is nothing in the law of conservation of energy that limits the cart's speed to the speed of the wind.

 

[spork]

Correct. Unfortunately some here with extremely poor understanding of physics think that the wind provide only energy up to its own speed and everything over it is a "magic" extra that needs special explanation where it came from. Complete BS, pardon my language.

That is in fact the crux of the brain-teaser. It might be intuitive to most that you can't extract wind energy when going downwind at wind speed. I don't fault anyone for that.

I do fault people for just insisting how things "must be" when how they "must be" conflicts with the real world evidence and the analysis that I'd be happy to explain.

I also get a little torqued about being told I'm intentionally misdirecting people just because they fail to understand the correct answer to a brain-teaser instantaneously.

 

[COFlyBoy]

This thread is turning into an example for this one:
http://www.pilotsofamerica.com/forum/showthread.php?t=41503&highlight=science

Mmmmm. Good thread. I also liked the one where we denied the conservation of energy...which would prevent any theoretical proof of the concept for this thread.

 

[Skylane81E]

SNIP
Now, let's remove the drive chain and replace it with a generator on the wheels, and an electric motor driving the prop. Wind speed is 15 mph, and we tow the cart up to 30 mph. At this point the cart is going over the ground at 30 mph, and there is a retarding force on the wheels because they're turning the electric generator. If we want to know how much power we're generating it's simply going to be that retarding force multiplied by 30 mph (remember, power is force x speed).

Now let's see how much power we need our prop to put out in order to keep us going at that speed - or accelerate. If we produce a thrust equal to the retarding force, we will maintain our speed (ignoring losses for the moment). So our prop is only moving through the air at 15 mph (30 mph - 15 mph tailwind). The work it does is thrust x 15 mph.

So the work the prop needs to do in this case is only one half of the work available from the generator. But in real life we will have aerodynamic drag, rolling resistance, and transmission losses. The secret is to keep the overall efficiency above 50%. If we can do that, we can accelerate after cutting the tow rope.
SNIP

numbers

 

[spork]

numbers

O.K...

Ground speed = 30 mph
wind speed = 15 mph
retarding force = thrust = 15 lbs

This will show you that we're harvesting twice as much energy as is needed with no losses. From there we can add in rolling resistance, aero drag, transmission inefficiency and prop inefficiency. If we keep the overall number better than 50%, it shows that we can maintain 2X wind speed.


Of course these numbers are chosen somewhat randomly to prove the concept works. If you want the numbers for the small cart on a treadmill, that's a bit tougher since it was never instrumented in any significant way.

I can give you the numbers from the Blackbird that I have, but keep in mind, they are the result of estimates, a little research, some simulation, etc. The speeds are carefully measured. The forces and torques are not.

 

[ebacon]

Earlier you said V was the wind speed. To use wind speed meaningfully in the kinetic energy equation one has to plug in the mass of the wind for m, not the mass of the cart.

If V and m are now consistently describing the state of the cart, we can plug them into kinetic energy equation. But all that equation tells us is that the cart's kinetic energy increases with V. So too does its momentum. But they do not per se indicate any limit in the cart's speed.

I suspect your reference to the kinetic energy equation was to show that, once the relative wind on the blades drops to zero the available kinetic energy from it drops to zero? If so, then no disagreement. (There is just the, ahem, small matter now of what the wheels of the cart are doing....)

Jim,

I'm still not understanding what you mean by "mass of the wind". Wind has a mass flow rate, but not a static mass that's uniformly defined. The static mass would depend on the volume of air in the wind. If that mass described the total wind kinetic energy available then the acceleration of the cart would be dependent on how much air is in the volume, not how fast the wind is blowing, e.g. the mass flow rate. That's clearly erroneous.

Spork,

What limits the cart's speed? My recollection is that the torque gets to be too much for the chain drive. Is that true? If so what is the torque at a point, any point? Axle, prop shaft, whatever. I'll accept whatever data or estimates you have. I'm not asking you to do any more experiments or calculations.

 

[EdFred]

If I roll a heavy ball into a lighter ball where does the lighter ball get the energy from?

 

[ebacon]

If I roll a heavy ball into a lighter ball where does the lighter ball get the energy from?

The heavy ball.

How fast does the lighter ball travel after teh impact? Faster than the heavy ball, slower than the heavy ball, or at the same speed as the heavy ball. Assume the coefficient of restitution is unity.

 

[spork]

Jim,

I'm still not understanding what you mean by "mass of the wind". Wind has a mass flow rate, but not a static mass that's uniformly defined. The static mass would depend on the volume of air in the wind. If that mass described the total wind kinetic energy available then the acceleration of the cart would be dependent on how much air is in the volume, not how fast the wind is blowing, e.g. the mass flow rate. That's clearly erroneous.

The cart is affected by a "tube" or "cylinder" of air. That would be the air that flows through the disk formed by the propeller (with some complications we can get into later). It's common to treat the propeller as a disk to perform such calculations (known as "actuator disk theory). Whether you talk about mass flow rate or mass doesn't really matter so much. It's just depends on how you want to setup your analysis. If you want to think in terms of mass, you can ask how much wind the cart sees in a 1 second period.

You can then see how much that entire mas of air was slowed and conclude that it did work on the cart in one way or another. Some of that will be useful work and some not. But ultimately you can look at the work done on the cart by the wind, and the total energy dissipated by the cart through aero drag, rolling resistance, etc. and determine whether the cart would have sped up or slowed.

Spork,

What limits the cart's speed? My recollection is that the torque gets to be too much for the chain drive. Is that true?

There are several ways to answer that question. For the Blackbird in particular, the strength of the chain is a limiting factor if we have enough wind to get it above 54 mph. If we're operating in say 10 mph winds, it seems the overall efficiency is our limitation. Our record run put us at 28 mph in a 10 mph wind, but we've had unofficial runs in the area of 3.5 times wind speed. Ultimately we'll be limited by either efficiency or mechanical failure - depending on the wind strength.

If so what is the torque at a point, any point?

At the upper chain wheel on the prop shaft, at 54 mph I believe we computed a torque of about 700 ft-lbs. JB tells me this is the same as a turbo charged V8 Corvette engine. This is NOT to be confused with power. Our prop is turning MUCH slower than a V8 at peak torque. My recollection is that we were somewhere in the ballpark of 30 h.p.

 

[EdFred]

The heavy ball.

How fast does the lighter ball travel after teh impact? Faster than the heavy ball, slower than the heavy ball, or at the same speed as the heavy ball. Assume the coefficient of restitution is unity.

So what's the mass of the atmosphere (wind), and the mass of the cart?

m1v1 = m2v2

 

[COFlyBoy]

So what's the mass of the atmosphere (wind), and the mass of the cart?

m1v1 = m2v2

So if the entire atmosphere were moving at 10 mph and all of the KE was transferred to a cart with 150kg mass, then the cart would travel 41000000 times the speed of light. AWESOME. (I really did work the numbers)

 

[Jim Logajan]

Jim,

I'm still not understanding what you mean by "mass of the wind". Wind has a mass flow rate, but not a static mass that's uniformly defined. The static mass would depend on the volume of air in the wind. If that mass described the total wind kinetic energy available then the acceleration of the cart would be dependent on how much air is in the volume, not how fast the wind is blowing, e.g. the mass flow rate. That's clearly erroneous.

Air has mass, therefore wind has mass, though you are correct that most fluid problems like this are best solved by using mass flow rates. I did not choose to use mass - you first brought it into the discussion with the equation for kinetic energy - I was merely trying to draw out the fact that your usage was ambiguous at best.

I only have a lowly BSc in physics - no experience in education - so I do not know what the best approach would be here. I think I'll let spork and others take it from here. I think it odd that you can consider finding the time and money to fly to see a demonstration rather than build and try a model device yourself, but I guess some pilots will invent any excuse to fly! That's understandable! :wink2:

 

[Jim Logajan]

So if the entire atmosphere were moving at 10 mph and all of the KE was transferred to a cart with 150kg mass, then the cart would travel 41000000 times the speed of light. AWESOME. (I really did work the numbers)

"Scotty - I need more wind power!"

"Captain, the propeller's at warp 9 - she canna take no more!"

 

[ebacon]

Here are my numbers for Blackbird. I used the m1v1=m2v2 formula, spork's explanation of the wind cylinder as I understood it, and the Blackbird data as published on jalopnik.com. I arrived at 1,398 MPH over a ten second run at max steady-state speed. That ain't right. Where did I muff it?

 

[spork]

...then the cart would travel 41000000 times the speed of light. AWESOME.

We chose to exploit less than the entire atmosphere because we suffer compressibility losses when the prop tips exceed about 0.3 mach. Otherwise I'd have gone for your number, but I think I'd outfit the Blackbird with a seat-belt.

Air has mass, therefore wind has mass, though you are correct that most fluid problems like this are best solved by using mass flow rates.

I agree that it's more common to use mass flow rates, but I sometime do look at it over a fixed period of time because it's sometimes a bit more intuitive for me (e.g. rate of change of momentum = force is less intuitive to me than momentum change = force * time). In any event, you can either talk mass and energy or mass flow rate and power - to-may-to / to-mah-to.

I only have a lowly BSc in physics - no experience in education - so I do not know what the best approach would be here.

A BSc in physics, and a critical but open mind, beats a Physics PhD university professor that already knows all the answers. Let me know if you need names.

I think I'll let spork and others take it from here.

I'd be more than happy to explain it - all nice and calm. I'll go at it from any angle needed (there are several), if just given a chance. Simply telling me that I'm wrong, lying, scamming, etc. isn't the best way to foster that conversation. A sentence of two ending with question marks will do wonders.

I think it odd that you can consider finding the time and money to fly to see a demonstration rather than build and try a model device yourself...

I have to agree. Does he even know where I live (S.F. bay area)? Isn't he in the midwest? That's a long way to go in a Quicksilver MX II.

Also odd that he won't be convinced by the experiment in carefully controlled conditions (i.e. model on the treadmill), and that he will be convinced by the cart simply self-starting.

 

[spork]

Here are my numbers for Blackbird. I used the m1v1=m2v2 formula, spork's explanation of the wind cylinder as I understood it, and the Blackbird data as published on jalopnik.com. I arrived at 1,398 MPH over a ten second run at max steady-state speed. That ain't right. Where did I muff it?

I can confirm that we never reached 1398 mph. I'll take a closer look at your analysis.

 

[spork]

O.K. I took a brief look at your analysis. There are some problems.

You say: M1*V1 = M2*V2

This would suggest that the cart is initially at 0 velocity and that the wind will be slowed from its nominal velocity to 0 velocity. Of course we don't slow the wind to a dead stop.

It looks to me like you're trying to set the analysis up to see how fast the cart will be going from a dead stop after 10 seconds. Is that correct?

Personally, I'm not too keen on such an analysis because it get's pretty tricky. Initially the cart is pushed as a bluff body by the tailwind. The prop isn't producing any thrust at this point. But it is producing a torque that counters the cart's forward movement. To make matters more complicated, this torque is produced due to reversed flow. Finding an analysis that handles reverse flow on a prop is not easy. Presumably you could model the propeller and do a computational fluid dynamics simulation - but I didn't see the need for several reasons. All the prop analyses I could find only showed the "off design" performance numbers down to 0 mph free-stream (i.e. static thrust) - and those numbers tend not to be very accurate.

As the prop begins to turn faster, the tips of the prop eventually become unstalled. At this point the prop begins to produce thrust. As it continues to speed up, the unstalled portion of the blades grows toward the root.

In any event, we found that whole thing to be too complicated and pointless to address in any particularly accurate way. Although our carts have all been demonstrated to self-start in a tailwind, that's not part of our claim. Our only claim is that they can go directly downwind, faster than the wind, powered only by the wind, steady-state. We could tow them up to speed as far as the claim is concerned. We only claim they'll maintain that speed as long as the wind blows.

Therefore, the analysis I perform assumes the cart is at some speed above wind speed. From there I do the power or force analysis to determine whether the cart (towed up to that speed) would maintain, accelerate, or slow down. That's a reasonably simple calculation.

By the way, the Blackird is 450 lbs and I'm another 200.

 

[spork]

I have a proposal ebacon - how about if I guide you through a relatively simple analysis? I'll give you some realistic numbers for the Blackbird, and we won't proceed with a step until you agree we're on course and using reasonable numbers. I predict you'll convince yourself this thing works. You can still fly out here for a beer though.

 

[ebacon]

I have a proposal ebacon - how about if I guide you through a relatively simple analysis? I'll give you some realistic numbers for the Blackbird, and we won't proceed with a step until you agree we're on course and using reasonable numbers. I predict you'll convince yourself this thing works. You can still fly out here for a beer though.

That's reasonable.

I'll admit up front that prop calculations are beyond me. One thing that does fascinate me about props is what I *think* are wakes that they push in front of them, much like bow waves of ships.

In my head the idea of going upwind faster than the wind should be easy in the first approximation. Where I get stuck and shut up fast is the idea of air getting plowed in front of the prop. That's why I am excited about witnessing Blackbird's second mission.

If in the meantime you get some funny video of me blowing a cart and getting proved wrong about self-start then it's all good. If the cart goes forward then others have more data to debate. If it goes backwards and cuts off my lips then that's funny too.

 

[spork]

That's reasonable.

Great.

I'll admit up front that prop calculations are beyond me.

No worries. We'll keep it simple.

One thing that does fascinate me about props is what I *think* are wakes that they push in front of them, much like bow waves of ships.

A propeller in normal operation has low pressure in front of it (meaning in front of the prop disk - not the prop blade). A free-wheeling propeller on the other hand acts as a turbine - and can create approximately the same drag as a parachute of the same diameter. It's doing the same thing as a helicopter in autorotation - which is really no different than two gliders attached wing-tip to wing-tip, circling each other as they glide to the ground.

A turbine, or a freewheeling prop, does have a high pressure area in front of it. For that reason, not all the air that's headed for it actually goes through the disk. That's one of the reasons a turbine is subject to the Betz limit while a prop is not (i.e. no turbine can capture more than 59.3 percent of the kinetic energy in the wind - depending on how you define that).

In my head the idea of going upwind faster than the wind should be easy in the first approximation.

Most people think it's more counter-intuitive to go directly downwind faster than the wind. Some think it's trickier to go upwind. So far, no one has gone directly upwind *faster* than the wind. We hope to do so soon.

So - to start off this analysis...

I want to prove (or have you prove) that the cart is capable of going directly downwind, faster than the wind, powered only by the wind, steady-state. In other words, I want to prove that if I tow the car up to 2X wind speed, it can stay at that speed or even accelerate. I'm less interested at this point in the analysis that proves it can self-start. Fair enough?

 

[EdFred]

Question: I know you probably want to be as light as possible, and keep it as simple as possible, but is there a way to use a derailleur on the drive chain, to get better efficiency or are you already doing that?

 

[spork]

Question: I know you probably want to be as light as possible, and keep it as simple as possible, but is there a way to use a derailleur on the drive chain, to get better efficiency or are you already doing that?

We cannot change gearing while underway, and it would be very difficult to do so since our gear cluster is on the driving end (the axle) rather than the driven end (the prop shaft). This would put the derailleur on the tight side of the chain (which would never work with a traditional derailleur). Also, wouldn't improve our efficiency, but could possibly be used to improve our initial acceleration.

We can achieve roughly the same result by using a variable pitch prop (which we didn't do in our first runs, but did use in the record runs).

 

[EdFred]

Bikes have derailleurs on both ends, why wouldn't it work?

 

[spork]

Bikes have derailleurs on both ends, why wouldn't it work?

That's a good point. I was only thinking of the rear one. On the rear one they have that chain-tensioning mechanism - which wouldn't work on our already extremely tight chain (on that side). But the front type would probably work.

Nevertheless, we didn't see a need for one. We could effectively get that lower gearing by reducing our prop pitch.

 

[jhausch]

Ever think about a sail to later retract for faster initial acceleration?

 

[spork]

Ever think about a sail to later retract for faster initial acceleration?

Yes. We talked about using a spinnaker or drogue 'chute that could be stowed as we approached wind speed. We also considered making big hinged panels that swing out from our pylons, and then automatically fold back as we pass wind speed. Ultimately we decided to keep it simple and use more runway. If runway length or time of trial were important we'd do several things to get off the line more quickly. Perhaps even pitch the prop negatively and redesign the transmission to allow the prop to drive the wheels.

On another matter - I thought ebacon and I were going to run through a simple analysis of the cart. Anyone seen him around?

 

[ebacon]

. . . On another matter - I thought ebacon and I were going to run through a simple analysis of the cart. Anyone seen him around?

Sorry, my brain has been preoccupied with a project at work.

My understanding is that I still owe you an analysis.

 

[spork]

Sorry, my brain has been preoccupied with a project at work.

No worries.

My understanding is that I still owe you an analysis.

Nope - I think you agreed that we can walk through the analysis together - step-by-step; making sure that I don't make any jumps you don't agree with. I'm just waiting for an answer to the first step in the analysis. This is it...

For now, I don't wish to address the issue of the cart self-starting or getting up to speed. Instead I want to tow the cart up to speed behind my car. When we get to 2X wind speed we'll do an analysis to see how much tension there is in the two rope. If there is tension in the tow rope, that means our cart is trying to slow down - and would if not for the V8 pulling it. If our analysis shows there is no tension in the tow rope, it means we could cut the rope and the cart would continue on at that speed. If there is "negative" tension in the tow rope, it means the cart wants to accelerate from that point. Do you agree with this approach?