Yesterday I was fortunate to be invited to attend a talk at the St. Francis Yacht Club in San Francisco given by Rick Cavallaro, driver and principal of the curious cart shown here. It does something that even experienced aerodynamicists say is impossible: powered only by the wind, it beats the wind going directly downwind.
This year, the Guinness Book of World Records created a category to accommodate Cavallaro’s current record of 2.8× wind speed. On hand to certify the speed was sailing journalist Kimball Livingston, who hosted the event I attended. Livingston said that when he first heard of the concept, he would himself have dismissed it as impossible, if he’d heard it from anyone other than who he did: Stan Honey, founder of Sportvision that puts all those NFL and NASCAR graphics on your TV screen. Honey is also a sailing obsessive, and is currently putting his company’s talents to work on the most difficult project of all, the America’s Cup. Unlike football and car racing, neither the boats nor the helicopter cameras have any fixed position. The wind is shifting around. Most people find the sport boring, and Honey’s job is to make what’s happening crystal clear and exciting.
Honey also happened to be Cavallaro’s friend and employer.
Cavallaro gave a number of simple examples that show the same principle using more familiar mechanisms, and here is the one I found most exquisite: Lay a wine glass on its side. Put your finger under the stem. Push the wine glass away from you. The wine glass will roll away about twice as fast as you push it, because of the “gearing up” effect from the small-curcumference stem and the large-circumference rim. The DDWFTTW cart (Directly Down Wind Faster Than The Wind) uses the same basic principle of leverage to push away from the wind faster than the wind is going.
The important point, which is often misunderstood, is that the wheels drive the propeller; the propeller is not driving the wheels. Here’s how to drive the cart:
- The propeller is variable pitch. Point the cart directly downwind. At this point, the propeller is facing the “wrong” way; if the wind began to turn it, the cart would creep upwind. Flatten the prop’s pitch so this doesn’t happen.
- You’re now basically a sail, just an inert surface that the wind begins to push.
- As the wind rolls the cart, the moving wheels turn the prop. The faster the wind pushes it, the faster the prop turns. Its pitch is still flat, so its rotation neither aids nor hinders the cart’s speed.
- Note that the gear ratio between the wheels and the prop is such that at some point, the pitch of the prop blade can be gradually increased, and it will begin driving the vehicle against the wind.
- The system is now rocking and rolling. The prop is pushing back against the tailwind with enough force that the cart is driven forward faster than the wind, just as the wine glass would push away from your finger faster than your finger is moving.
As I said, lots of people who should know better still assert that this is impossible. Most of the discussion I’ve seen that asserts this is based on a misunderstanding — that it’s more about apparent wind angles and complex sailing technology, when it’s really about simple leverage.
(Note that I blogged about this cart about two years ago, but this update is a much better-informed explanation.)
UPDATE. Rick Cavallaro emailed me the following comments on this post:
“Honey is also a sailing obsessive, an…”
I think you need to lose “a”.
Incidentally, Stan is probably best known in the sailing community not because of his technology, but because he is an extraordinary sailor with many of his own world records. I’m pretty certain he’s generally regarded as the best sailing navigator on the planet these days.
“At this point, the propeller is facing the “wrong” way; if the wind began to turn it, the cart would creep upwind. Flatten the prop’s pitch so this doesn’t happen.”
True – mostly. The propeller is in a battle between the wind and the gearing to the wheels. As the wind tries to push the cart downwind, the wheels try to turn the prop CW (viewed from the rear). Absent that torque, the wind is trying to turn the prop CCW as a turbine – and with the trailing edge leading. That makes for a very inefficient turbine, but you saw in the video that it will spin up pretty good when the chain is removed.
“Flatten the prop’s pitch so this doesn’t happen.”
Again – sort of. In our first tests in Ivanpah we had not yet implemented the variable pitch mechanism. Even with the prop pitch fixed, it can self-start from a stand-still in a tailwind and achieve better than 2X wind speed. However, if the prop pitch were too great, it would indeed go upwind instead of downwind. Interestingly, just changing the size of the wheels can also make that change.
“You’re now basically a sail, just an inert surface that the wind begins to push.”
True. But as you know a typical sail is an airfoil – just like our prop blade. At the start the prop blades are simply a bluff body to the wind – more like a spinnaker (a particular type of sail that you’re probably familiar with).
“Note that the gear ratio between the wheels and the prop is such that at some point, the pitch of the prop blade can be gradually increased, and it will begin driving the vehicle against the wind.”
I’d be careful to say that it’s driving the cart against the *relative* wind. The cart is going directly downwind at all times. What we’re basically doing is clawing our way forward through that mass of air that’s already passed over us. In fact, we sometimes wait until after a gust has passed and then we let off the brakes so we can catch up with that gust at the peak of our run to get the best speed. How twisted is that!?
“The prop is pushing back against the tailwind with enough force that the cart is driven forward faster than the wind, just as the wine glass would push away from your finger faster than your finger is moving.”
Again, sort of correct. But this one gets dangerously close to the very common misconception that the wind is able to push the cart even when the cart is outpacing the wind because the prop is creating some type of air-buffer – like there’s a convergence of flow from the tailwind and the prop. The reality is that the prop is operating in the air-mass exactly as the prop on a small plane does in flight. No tufts or instrumentation would demonstrate any difference between the two. The “advantage” that allows the cart to go downwind faster than the wind is due to the fact that the propeller is operating on an air-mass that it is moving through more slowly than its wheels are moving over the ground.
“As I said, lots of people who should know better still assert that this is impossible.”
Right – but unlike cold-fusion or the moon landing, people can prove this to themselves for about $40.