Google-Backed Wind-Powered Car Goes Faster Than the Wind

sterlingda writes "A wind-powered car has been clocked in the US traveling downwind 2.85 times faster than the 13.5 mph wind. The definitive research by Rick Cavallaro of FasterThanTheWind.org is being funded by Google and Joby Energy. The run should now settle the DWFTTW (downwind faster than the wind) debate that has been raging for some time on the Internet about whether or not such a feat was possible."

Comments

Debate?

[Mikkeles]

Sailing vessels can go faster than the wind, why shouldn't a car be able to?

Re:Debate?

[wrook]

IANAP (physicist) - The wind hits the sail. Since it's hitting the sail on one side and not the other, the pressure is higher on the side where the wind is hitting. This means that the sail is sucked towards the low pressure side. The speed of the craft is dependent upon the pressure difference that you are able to make and the amount of inertia and friction of the vehicle. It's not really related to the speed of the wind (other than higher speeds can create higher pressure differentials).

Re:Debate?

[wrook]

I started to think about this more. And the more I thought about it, the more confused I became.

Finally, I came to the conclusion: Thank god I'm not a physicist!

Re:Debate?

[prionic6]

http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind

Re:Debate?

[Anonymous Coward]

Sailing vessels only go faster than the wind when they travel with the wind coming from the side. No matter how fast the vessel goes, the wind keeps blowing from the side and delivering energy to the vehicle. When you try to go faster than the wind in the direction of the wind, the relative motion to the air goes down to zero and then you start going against a head wind. Obviously the wind can not be the propelling force beyond the point where you go as fast as the wind in the conventional sailing sense, because at that speed there is no wind (motion is relative). The described device uses the sailing force to accelerate and then produces its own faster wind, so to speak, by driving a propeller via a transmission from the wheels.

Re:Debate?

[Anonymous Coward]

Sailing vessels can go faster than the wind ACROSS the wind. They use a keel to do this. But the keel does nothing (except drag) when they travel directly downwind. They can't go faster than the wind in the direction of the wind. If you think it through what these guys are doing is pretty clever, making the car travel faster than the wind providing it's power in the direction the wind is blowing is cool.

Re:Debate?

[ls671]

Read TFA, parent is correct concerning the wheels :

Cavallaro explained the car is able to move faster than the wind because the propeller is not turned by the wind. The wind pushes the vehicle forward, and once moving the wheels turn the propeller. The propeller spins in the opposite direction to that expected, pushing the wind backwards, which in turn pushes the car forwards, turning the wheels, and thus turning the propeller faster still.

Re:Debate?

[Rigrig]

But sailing vessels can't go faster than the wind directly downwind, this car does.

FTFA:

the propeller is not turned by the wind. The wind pushes the vehicle forward, and once moving the wheels turn the propeller. The propeller spins in the opposite direction to that expected, pushing the wind backwards, which in turn pushes the car forwards, turning the wheels, and thus turning the propeller faster still.

Re:Debate?

[supercrisp]

And the propellor is a "sail" whose surfaces are not perpendicular to the wind.

Re:Debate?

[dtmos]

Ah, no. I can do no better than to quote ThinAirDesigns:

The key thing to remember is that due to the tailwind, the wheels are traveling over the ground much further than the propeller is traveling through the air[1] -- thus using the force x distance calculations for work and power it's easy to see that when we are traveling the speed of the wind, we can gain more power from the wheels (faster moving ground) than we have to expend in the air (slower moving air).

________
[1] As he is about to mention, this is best considered at the moment when the car is moving at the speed of the wind. In this case, there is no wind over the propeller, since the car and the wind are traveling at the same rate in the same direction; however, the wheels are moving relative to the ground at the speed of the wind, and therefore turn the propeller and supply additional thrust to the car, accelerating it.

Re:Debate?

[Pharmboy]

I hate spelling nazis, but since you are suggesting a search, the correct spelling is Ascher H. Shapiro. I only found this out as I was trying to take your advice and search for him.

Re:Debate?

[SerpentMage]

Look at the experiment I would say sure its possible because there is no resistance. As they say they can't do this on a gym floor, but a tread mill.

As a mechanical engineer who studied dynamics I would say the reason is because the resistance that is normally hit due to acceleration or keeping the thing moving is not present. Thus this thing could accelerate faster than the wind.

What I would find interesting is what are they exploiting in specific? As they say, is there some neato resistance, aero-dynamic trick that nobody has yet thought of?

It reminds me of the ram jet that below a certain speed is useless. YET at higher speeds it becomes more effective than a regular jet.

Re:Debate?

[dtmos]

The rotating sails (i.e., propeller blades) are inclined to the wind; it's how all propellers work -- when rotated, they provide the force to the car that accelerates it to speeds higher than the wind. However, the power flow is from the wheels to rotate the propeller, not the other way around.

He states quite clearly, several times, that they don't use the "sail" word, because it confuses people into thinking that the power flow is in the opposite direction. Like all propeller-driven craft, the power flow is from the craft to the propeller.

The energy comes, ultimately, from the wind. One might as well ask, "Where would the energy to move a sailboat faster than the prevailing wind come from?"

Re:Debate?

[Anonymous Coward]

however, the wheels are moving relative to the ground at the speed of the wind, and therefore turn the propeller and supply additional thrust to the car, accelerating it.

Am I stupid, or is there another reality where the laws of thermodynamics don't apply?

What you described can't increase the vehicle's momentum, because it's adding resistance to the car wheels (there's no actual gain in energy being described in your statement, just gibberish about potentials).

The only way to go faster than the wind is to resist gaining speed (store energy), and when wind speed is eventually reached, expend the stored energy to go a bit faster than the wind. After that stored energy is used up, the vehicle slows back down to wind speed. Not magic.

Re:Debate?

[anegg]

Because the comparing the velocity of the wind and the velocity of the car is a bogus comparison. In order to not be a perpetual motion machine, it is only necessary for the energy of the wind (available to the car) to be more than the energy required for the car to accelerate to and maintain a particular velocity. The tricky part is in extracting the energy from the wind when the velocity of the car exceeds the velocity of the wind. But that is an engineering problem, not a metaphysical problem.

Re:Debate?

[v1]

The more I look at that description and think about it, the more skeptical I become. It reminds me of someone trying to sell a perpetual motion machine. You have this battery to start it, and it shines this light. And all around it are solar panels, that absorb the light and keep the battery charged. Of course this doesn't work, there's never any net gain, and since there's losses in the system, it fails.

Here, the wind speeds the vehicle up to say 15mph, same as the wind. The wheels rob the vehicle of some speed in exchange to spin the propeller backwards. (which I must admit is a very interesting, novel approach!) which provides a force on the wind blowing the vehicle, which by itself would appear to accelerate the vehicle faster than 15mph.

But I see no reason why the drag from the wheels isn't exactly canceling out the benefit of rotating the propeller. And then the losses of friction etc step in, and you end up with a vehicle traveling slightly slower than the speed of the wind.

Basically, you can't turn the propeller without investing energy, because you're turning it against a resistance, namely the wind blowing on it. The more you want to resist the wind, (the faster you want to go than the wind) the more energy is required on the prop. And so as this theoretical vehicle accelerates and more energy is available from the wheels, (and is being robbed from the vehicle speed) the more energy you have to invest in the propeller. It's the same as a perpetual motion machine. You can't get more out without putting more in, and you can't put more in until you've gotten more out.

But then of course everyone asks "but he proved it with his prototype. I would ask if this was a sustained speed. Here's a scenario where it could work for a short time only:

First it looks like the blades on the prop can be pitched. That makes sense for control anyway. Lets say the car is blown up to speed while the blades are pitched at 0 degrees. Power is drawn from the wheels to spin up the prop. This slows the vehicle initially but the wind is constant and eventually the prop is up to full speed (perhaps very fast!) and the wind continues to blow and brings the vehicle back up to about it's speed. (probably only close, due to various friction elements)

Then suddenly the blades on the prop are pitched heavily, and now there's a good wind blowing out the back.

The vehicle would surely lurch forward. This is spending the energy of the inertia of the prop to accelerate the vehicle. This will only last a short time. Yes, the vehicle is traveling faster now and the wheels are turning faster, but you can't rob power off the wheels to keep the prop up to speed because that would slow the vehicle down. Remember the perpetual motion machine above. Any energy you take from the wheels to spin the prop to keep it up to speed must provide equal or less energy in the end to the prop than you are taking from the wheels. And the vehicle's acceleration crests.

At this point the prop will be spinning slower but still backward, enough to reach equilibrium, such that the energy of pushing on the prop to accelerate the vehicle is equal to the energy being taken from the wheels.

And then it starts to slow down, slowly, due to drag. And during which the prop slows down, STOPS, and reverses direction.

How can it stop and even reverse direction while the wheels still turn? Good question! The prop is currently working against the wind. Energy must be invested in the motor, not merely to spin it in the direction you want, but to even resist being spup the other way. It's easier to understand if you look at a prop being spun freely by the wind. If you put a dead battery on the terminals, the battery will start to charge, but the blades will slow down. You are providing a load on the prop, and are withdrawing energy from the prop's speed and transferring it to the battery. Energy is always moved around, never created or destroyed. In the

Re:Debate?

[Puls4r]

Thanks for rehashing basically the entire debate.

However, there's one hitch. They've already done this out on the salt flats with an actual person in the vehicle, and done it dozens of times in front of hundreds of people. No tricks, a very simple mechanism. It isn't perpetual motion any more than a DN Iceboat that can go insane speeds of 70+ mph in a breeze of 20 - downwind.

In addition, you'll find hundreds of pages of discussion on sailing websites. Specifically here:
http://forums.sailinganarchy.com/index.php?showtopic=82175

The engineers who put that machine together are there. I've talked with them dozens of times and they've explained literally every step of their process. You'll find no end of skeptics - right up until the point where they start posting videos of them actually doing it. There's nothing secret here, they'll explain it right down to the nuts and bolts so you can build your own scale model to prove it to yourself, on a treadmill or in the real world.

This isn't perpetual motion. It's elegant engineering. There have been boats in the water developing this same concept (though they can't go dead down wind yet) for a long long time.

http://www.navagear.com/wp-content/uploads/2007/02/windowslivewriterwindturbinepoweredsailboat-1270awindmill-boat-thumb51.jpg

http://www.boatdesign.net/forums/attachments/projects-proposals/13565d1180209267-windmill-wind-turbine-powered-boats-how-many-out-there-they-viable-vertical_windmill_catamaran.jpg

Their main issue with going DDW is the losses incurred because of the friction with the water. The ground (or even ice) is far less lossy in transferring the power from the wind-driven motion back to the vehicle.

Re:Debate?

[sznupi]

All the energy there comes from the wind. The wheels act as keel, providing resistance against the surface. The propeller (rotating sail, really) is what tacks to the side (by rotating...but it doesn't make any difference from "to the side" as far as wind is concerned), so the whole vehicle doesn't have to and can move "forward", with the wind.

Re:What debate ?

[Jedi Alec]

If you'd read some of the provided links, you'd have seen that the requirement was for the vehicle to be powered solely by wind, so no gravity involved except in its usual role of keeping the wheels on the ground ;-)

In other news, scientists actually getting their hands dirty turn out to know more about their chosen field than a bunch of people on the interwebz.

Very old news.

[StikyPad]

It shouldn't really be a debate -- sailors have done this for decades. Essentially you turn your vessel/vehicle at an angle to the wind such that you utilize both the positive pressure from the wind and the negative pressure created by the curved sails which create an air foil. Positive pressure pushes you forward while negative pressure pulls you forward == faster than the wind. The same effect is at play with the "propeller" on the car. It's also the same principle that keeps planes in the air -- higher pressure on the bottom of the wing relative to the top.

Re:Very old news.

[AnonymousClown]

So all this time, Wile E Coyote could have stood to the side, blown just as hard on his sail and gotten away from his bombs that the Road Runner turned on him?

Some genius!

Re:Very old news.

[waimate]

You are correct, but only for boats sailing across the wind or to windward. Modern yachts cannot sail faster than the wind *downwind*. Indeed, downwind is their slowest point of sailing, which is why many yachts tack downwind rather than sail dead downwind. Sailing boats cannot do what is claimed here. What is claimed here is substantially cool.

Re:Very old news.

[natehoy]

For sailors: By using a propeller rather than a sail, the "sail" this boat is using is simulating a continuous optimal downwind tack (the propeller blades are at a tack angle to the wind)

For cyclists: The wind is being turned into rotational force like the cranks on a bicycle. Since they now have rotational force, they can use gearing to take maximum advantage of that force.

Does that make it clearer?

Re:What debate ?

[Bazman]

True. I bought some water skis and spent ages looking for a lake with a slope.

A million monkeys at a million keyboards...

[jtownatpunk.net]

"The run should now settle the DWFTTW (downwind faster than the wind) debate that has been raging for some time on the Internet about whether or not such a feat was possible."

You're new to the internet, aren't you, son? No amount of reality can end an internet debate.

Another way to look at this.

[queazocotal]

Firstly, ignore that it's moving.
You have 0m/s ground, and a 10m/s wind.

You put up a wind turbine - it can extract power from this 10m/s difference.

The funky part of this idea is that this still works when you're moving faster than 10m/s.

For the moment - imagine that the turbine is a pure 'airscrew'.

It describes a helix in space - like the DNA molecule.
For every meter the air moves "forward" relative to it, it turns 1m clockwise.
Considering the air as completely rigid for the moment, the airscrew goes forward in a rigid helix, unchanged by load.

So - 10m/s wind - airscrew turns at 10m/s. Simple.
You can extract - say - 100N * 10m/s = 1kW of power.

Funky part coming up.

Now. You're moving at 20m/s. Twice as fast as the wind.
Of course this will slow you down - you can't use this to make power!

Well - not quite.

If you are moving at 20m/s in the direction of the wind - for a total speed with regards to the wind of
30m/s then the blades need to be spinning at 30m/s in order to keep up.

But, you can use gearing from the wheels so that the 'base' speed of this spin is 20m/s.

That is - when you push the car along on a windless day - the airscrew creates no drag - because it is spun at exactly the right speed by gearing from the wheels. It has effectively - by rotating at the right speed - cancelled out the movement of the car.

This cancellation then allows you to ignore the speed of the car, and instead work off the speed difference between the wind and ground!

In reality - it's very far from an airscrew, and turbines have a lot of drag. It's the same basic concept though.

Another beautiful and 'obvious' when you think of it bit of physics.

Re:What debate ?

[History's Coming To]

The things you're looking for are called "waves" ;) http://www.youtube.com/watch?v=y7mmO0CsyZA

In related news, kitebuggies will generally travel at three times the wind speed, depending on the aspect ration of the kite.

Not impressed

[thoughtsatthemoment]

Let me know when you have a solar powered car traveling faster than light.

The Oracle Trimaran..250% faster than the wind...

[droopus]

The Oracle trimaran that recently won the America's Cup had no problem exceeding wind speed due to aerodynamics, and the insanely cool carbon fiber wing that added to sail volume and power, and allowed them to use a fixed-shape sail - a huge advantage. They had no problem sailing between 16 and 24 knots upwind in 5 to 10 knots of wind—that’s 2.5 times wind speed.

They went even quicker periodically, and had a five knot downwind advantage. The first race report shows that the Oracle trimaran was able to almost constantly fly both outer and center hulls (amazing on a boat this big.,.I sail Hobies and this shit is HARD) and execute some slick pre-race maneuvers (which is how you really win sailing races).

So yes, sailboats have been exceeding wind speed for a while, but not by 250%..until now. When a car does that, I'll be impressed.

Re:Stupid exercise

[Goaway]

Boats can not go downwind faster than the wind. Rather than jump out and try to announce to the world how much smarter you are than the people who actually did stuff, maybe you should first go read and comprehend what they actually did.

DWFTTW--Except where the car couples to the wind!

[karlandtanya]

Read carefully the excerpt in the parent's post.
This is a demonstration of some basic physics and geometry, but it is not "DWFTTW" at the point where the car actually couples to the wind.

A science project where the participants and the public learn some interesting physics and engineering principles--or are entertained by watching--this is a very good thing. It gets the public (if you can call /. the public) talking about science.
We need more demonstrations like this--no, what we really need is another Sputnik!

Be careful.
People have been known to use the counterintuitive nature of the physical world to argue they have discovered a new way to get rich quick--and you can get in on it if you want! We like to think were too hip for perpetual motion, but a lot of folks will still hand over real green (dollars) for bogus green (environmental scams). Don't you care about the environment?

So, what is the "magic" here, and what's the physics?

The fundamental error in the statement "DWFTTW" is the fallacy of dual definitions.
This is kind of cheating--a really good science demonstrator doesn't actually lie to you; they just show you something that exposes your misconceptions. Either way, the point is to get you to say "I see it, but it's impossible!". Then you are more ready to learn some science. (or maybe to invest in a free-energy scam).
DWFTTW is simply the koan. It actually means nothing--just gets us ready to study and learn something new.

When the experimenters say "faster than the wind", they are referring to motion of the bulk (center of mass) of the car.
BUT--the wind couples to a very specific portion of the car, which has a completely different (and somewhat more complex) velocity than the center of mass of the car.
The propeller--or more specifically, the surface of the propeller that pushes against the wind.
And the part of the car that connects to the wind NOT traveling "DWFTTW".

Read the article and look at the pictures--this is why they took such care to "streamline" the car. The rest of the car (except the propeller) is built so that it presents the very minimum cross-section (drag coefficient), and is effectively transparent to the wind. So, it is the part of the propeller that pushes against the wind that matters when we try to analyze the downwind motion.

So--what is the the portion of the propeller's motion that is "downwind"?
You could say "parallel to the direction of the wind" if you like, but for this case, "downwind" works fine.

A little math (just two equations, I promise--and only to describe the geometry!):
The propeller surface has a pitch angle, theta, from zero (parallel to the plane in which the propeller rotates) to 90 degrees (parallel to the propeller shaft), and it spins at some angular velocity w (omega).
At any instant, the linear velocity, v, of a point a distance r from the shaft of the propeller is simply v=Rw
And the perpendicular (downwind) component is just v(p)=v*sin(theta).

By controlling the diameter of the propeller, the pitch angle, and the rotational speed, the experimenters cause the relevant part of the car--that is, that portion of the car that connects to the wind!--to travel downwind much slower than the wind.

But, I hear you say "We keep talking about "slower" than the wind, and cars move fast.".
This seems strange because we started with the reference frame of the road, and we compare the velocity of the car and the air. The comparative term "Faster" describes the downwind velocity of the car, which, for consistency, we continue to reference.

In Newtonian physics, there are no preferred reference frames. This is true in other cases as well, but they are not significant at the speeds this car is traveling. This means we are permitted to say "the car is traveling slower than the wind" or "the wind is traveling faster than the car" and they mean EXACTLY the same thing.

Recall again that the pertinent part of the car is that part

Re:What debate ?

[Dracker]

I'm not trolling, honestly. How is this not perpetual motion?

Re:Oh come on man think!

[takev]

But maybe I should actually read the article instead of just the pictures.

Re:What debate ?

[pthisis]

Because there is an energy input into the system (wind). The car's momentum will tend to keep it moving at its current speed, so the wind power only has to be enough to overcome friction/drag for the car to accelerate. The wheels and prop are directly linked (they can't rotate independently--if you held the prop still and turned the wheels or vice-versa, you'd break the car).

Basically, the blades of the prop act like tacking sails; once you get your head around that it becomes easy to see that it works.

Re:Why cannot boats do that?

[Goaway]

YOur intuition tells you it was not possible for a car, and now they prove you wrong.

I am not sure who you are talking to. I know how the car works and that it works. I am saying that regular sailboats can not do the same that the car does, and that all the people yelling about how this is "nothing new" do not understand what is being done.

Re:What debate ?

[Arthur Grumbine]

In related news, kitebuggies will generally travel at three times the wind speed, depending on the aspect ration of the kite.

Many sail-based vehicles travel much faster than wind-speed. What they don't do, though, is travel directly downwind faster than wind-speed. This vehicle does.

Re:The Oracle Trimaran..250% faster than the wind.

[Quantumstate]

Sailing boats don't go faster than the wind when moving directly downwind. They tack at an angle which allows them to go faster than the wind speed in the downwind direction while never actually travelling directly downwind.

Energy != Velocity.

[spaceturtle]

But I see no reason why the drag from the wheels isn't exactly canceling out the benefit of rotating the propeller.

The Energy generated from the wheels has to match the Energy lost by the propeller. Thanks to gearing, the force is not the same.

Energy isn't the problem, a decent sized windmill can generate a megawatt of power. And it can generate the energy perpetually (assuming perpetual wind).

Consider if the vehicle was stationary, then we could easily generate the power from the wind: the force against the wheels wouldn't lose us any energy because E=mv^2 and so dE/dv=0 when v=0. Now imagine we are travelling at exactly the speed of the wind. Then our velocity relative to the wind is 0 so dE/dv=0. Thus we can push against wind without losing any energy, the same way a stationary windmill can push against the ground without losing energy. And so we can generate energy from the ground speed without losing kinetic energy (ignoring for the moment that the propeller doesn't have perfect grip on the air)

So we are currently travelling at wind speed, and generating energy from the ground. We now use that energy to push against the the wind to make us go even faster. Note that even a 50KW engine feels powerful when we are going slow and in first gear, and even a 200KWH engine can't burn rubber when we are going at 100KM/h. This comes back to E=mv^2, because Energy is proportional to the square of the Velocity, it takes more energy to speed up the faster we are going.

Note that we are still travelling faster relative to the ground than the air. Thus we can use the same trick as gears in an engine, we use a high gear relative the ground so have only a small force. We use a low gear relative to the air so we generate more force (for the same energy). We continue to speed up until the energy we gain from the different gearing ceases to make up for friction and other inefficiencies in the system (such as the propeller not having perfect grip on the air).

Re:How it could possibly work

[jbengt]

Sailboats have long been able to go faster than the wind when tacking at an angle to the wind.
What you are ignoring is that once the prop gets rotating, it has a velocity component perpendicular to the wind. Therefore, the prop blade is not going downwind, and can generate a forward force even when the vehicle is going the same velocity as the wind, or faster.

Re:What debate ?

[pthisis]

Yeah, but the big problem is that it doesn't correlate to that analogy- you can't go faster than the wind you're tacking against with a sail

Huh? That's news to all the sailors out there who do routinely sail faster than the wind.

Physics explained here in the "How can boats sail faster than the wind?" section:
http://www.animations.physics.unsw.edu.au/jw/sailing.html

You don't need input energy to maintain your momentum--you only need enough input energy to overcome friction/drag, anything beyond that can accelerate you.

Re:How it could possibly work

[pthisis]

Physicsforum explained it well at http://www.physicsforums.com/showthread.php?t=274996

The kinetic energy of the net movement of earth and of the atmosphere (in the centre of mass frame) is thermodynamically available and (neglecting friction and relativity) can accerate a sufficiently light mass arbitrarilly fast. Thus, DDWFTTW is obviously not physically impossible (and numerous conceptually-trivial schemes have been suggested).

It is empirically known that yachts or sailing boats can continuously travel diagonally with (as well as against) the wind at speeds (such that even the component of their velocity in the direction of the wind is) significantly faster than the wind.
        In the boat's frame of reference (assuming steady nonaccelerating state, inviscid flow, etc) the sail can be oriented such that any (sufficiently) diagonal headwind (or tailwind) is redirected more into the sternward direction (with the same speed, by conservation of energy), such that the reaction force on the boat (while mostly perpendicular to the keel) has a positive component in the direction of the bow; (neglecting friction) a sailboat can accelerate forward as long as the wind relative to the boat is not arriving directly from the front.
        In the water's frame of reference (assuming the boat is already moving forward) this redirected breeze is always slower than the incoming wind (draw the trig'), losing energy and momentum to the boat (later frictioned to the water), independent of how fast this lets the boat accelerate compared to the windspeed. (Learn More)

The simple idea behind the fan and wheeled trolly contraption is that the belt (which couples their respective axels) performs exactly the role of the sailboat's keel (imagine sailing on a ringworld).
        The belt/gear ratio constrains the propellor-tip to move through space on a fixed helical trajectory of constant diagonalness (the ratio of forward to transverse motion, or pitch to circumference), ensuring that (as long as the atmosphere is moving forward relative to the ground) the propellor tips are never moving directly into the wind and thus (identifying the propellor blade with a yacht's sail) a forward thrust component can be obtained regardless of whether the velocity of the cart itself is less, equal or more than the wind velocity.
        The limiting factors are the aforementioned ratio, the fixed-angle pitch of the propellor-tip's blade-sail, the windspeed (relative to the ground), drag and friction. For any constant windspeed, the ratio and the propellor sail-pitch together determine a maximum cart velocity (downwind relative to the ground) at which forward thrust can be produced (this can be larger than the wind velocity but not infinite) but it is a tradeoff because the ratio simultaneously increases drag, and (with the ratio also fixed) tuning the propellor sail-pitch for higher cart-velocity decreases its efficiency at lower cart velocities.

Re:What debate ?

[White Yeti]

You make a good point, but do you mean "dirigible" rather than "derringer"?