Faster Than The Wind And Flames

Not seen at Burning Man. Way too mindblowing.

On July 16th, 2012, Rick Cavallaro was at the controls as the Blackbird land yacht, powered only by the wind, accelerated from a stop to 20 mph. However, the 10 mph breeze was blowing directly in his face for the entire run, giving the Thin Air team the first North American Land Sailing Association (NALSA) record for traveling directly upwind at a speed faster than the wind itself.

This new record has attracted far less attention and controversy than Blackbird’s spectacular downwind speed record, set in 2010. But some people still believe both runs were faked. While the ideas that the wind could push against itself or push something faster than itself seem obviously impossible, I’m confident that this is not a hoax, and I arrogantly believe I can explain upwind and downwind carts to my fellow Toasters. [1]

Simplifications and Assumptions:

– The vehicle always moves to the right in the illustrations.
– The blade profile always moves down in the illustrations.
– The efficiency of the blade decreases linearly with speed. The efficiency of a real blade will rise to and then fall away from a peak efficiency at some non-zero speed.
– Even the weird arrowheads indicate direction.

When sailing directly upwind from a stop, the wind blows on the front of the Blackbird, and tries to push it backwards. Ratchets in the wheels prevent this. The turbine [2] is free to turn, just like blowing on a pinwheel. The turbine blades force the wind to change direction, which creates both a torque force and a drag force. The turbine is geared to the wheels, and as long as the torque reaction at the wheels is greater than the total drag, the craft will accelerate forward.

Directly Upwind – From A Stop

When the Blackbird is moving upwind, the vehicle speed and the tangential speed of a turbine blade are added to the ambient wind to create an apparent wind, which is the Galilean equivalent that would create the same forces if the blade was perfectly still [3]. The apparent wind is still forced to change direction by the blade. Compared to the stopped cart, this makes less torque relative to drag, and the land yacht will not accelerate as hard. Once the torque reaction at the wheels is equal to the total drag, the craft will maintain speed. If the wind slows, and the total drag is greater than the torque reaction at the wheels, the craft will decelerate.

Directly Upwind – At Speed

To get more power out of the wind, the Blackbird’s blades are are formed into airfoils. They still cause the wind to change direction. But now, the torque and drag generated by low pressure air following the convex side are added to the forces from the high pressure stream flowing against the concave side.

Upwind – Airfoils

Running the Blackbird directly downwind is only slightly different. The key to the whole concept is that the wheels now drive a propeller, instead of a turbine driving the wheels. Furthermore, the illustrations show that the orientation of the blades is reversed. [4][5]

Starting out, the wind pushes on the back of the Blackbird and makes it roll forward. The wheels make the propeller turn, and each blade sees an apparent wind. The blade changes the direction of the apparent wind. Guiding the air backward creates thrust and a resistant torque. As long as the thrust is greater than the torque reaction needed by the wheels to turn the propeller, the vehicle will accelerate forward.

Directly Downwind – Below Windspeed

As vehicle speed increases, the apparent wind at each blade is increased and rotated. This makes less thrust relative to the torque needed to turn the propeller, so acceleration will decrease until the Blackbird eventually reaches its top speed.

Directly Downwind – Above Windspeed

All of this obeys the laws of conservation of energy. In the upwind case, the craft gets energy by using the wind to turn the turbine. For the downwind case, the next illustration shows how a tailwind pushes on the back of the propeller, even when the vehicle is moving forward faster than the wind.

Force From Apparent Wind

Moving directly forward, an object going faster than the wind experiences a headwind. No one is disputing this. However, when an object moves across the wind, even with a parallel component greater than wind speed, the apparent wind comes from the side, and pushes on the object along the dotted line. Even though the Blackbird moves faster than the wind, the wind can push on the propeller blades [6] because they move across.

This also shows why the Blackbird can’t push off still air. With no wind, the apparent wind comes only from motion, and will always be a perfectly parallel to the motion. A propeller or a turbine cannot exert enough force on a cart to keep itself turning or moving forward. A propeller needs the extra push, and a turbine needs that extra turn to break even or accelerate. [7]

We’ve done the easy part: looking at something that works and trying to understand why. I wonder what I would have thought in the early discussions. When thought experiments are this hard to understand, much less explain, too often we forget the obvious solution of building a model and trying it out. It was a proud moment when the results of both record attempts were handed down. But I imagine it feels even better to outrun the wind, have it blowing back at you, and use that to go even faster.

[1] This may be asking for trouble. Directly Downwind Faster Than The Wind (DDWFTTW) was one of the most protracted battles between very serious, skeptical, and intelligent people yet seen by the internet. Here is a 60 page thread, and here is an 80 page thread. While I’m usually all for deep-diving with research, I have not read any forum discussions since I first tried to understand the Blackbird back in 2010. For the early history of the debate, I recommend Mark Frauenfelder’s piece for the MAKE Magazine blog.

[2] Pedantically, a turbine converts fluid power into mechanical power, while a propeller converts mechanical power into thrust.

[3] This is an example of using different reference frames for relative motion.

[4] The Blackbird used different optimized blades for each attempt, but reversed the direction of rotation instead of the orientation of the blades.

[5] None of this is Photoshopped. I’m an engineer; I use MSPaint.

[6] Moving across is also required to create the low pressure forces (lift).

[7] That question had me doing the most pacing. I think the gears in my head sheared of some teeth trying to write this paragraph without lapsing into tautology. I am eternally grateful that I wandered across Doc Rampage’s take on it.

Sources And Further Reading:

Main Image: Stephen Morris, via Wikipedia

http://www.nalsa.org/DownWind.html

http://www.fasterthanthewind.org/

What I’ve Learned About Wind Carts – Mark Frauenfelder

Analysis – Doc Rampage

One Man’s Quest to Outrace Wind – Adam Fisher There is a good explanation of how Rick Cavallaro originally figured out how to go faster than the wind. Followed by the statement that an airplane on a treadmill will take off normally. One step forward, two steps back.