Terrific.
About 6'30"

Works sort of how a sail boat tacks when the wind is behind it.
The sail boat moves MUCH faster than the wind.

Sails work by vacuum. The boat is sucked along, it is not blown by the wind.

Sprintcyclist wrote on May 9^th, 2022 at 8:11pm:


1/2 right. If you travel exactly downwind you can only go as fast as the wind.
At beam or broad reach, you get push from the wind and lift from the apparent wind. The lift from the way the air travels around your sail adds the extra speed that allows your to travel faster than the wind.

Can you make sense of this?

https://www.quora.com/Can-a-sailboat-travel-faster-than-wind-to-the-direction-of-wind-using-beating-For-example-can-a-sail-boat-win-a-balloon-floating-downwind

Can a sailboat travel faster than wind to the direction of wind using beating? For example, can a sail boat win a balloon floating downwind?

Yes the sailboat will sail downwind faster than a balloon provided it use a spinnaker and jibe downwind. If you do it right the speed margin can be a significant amount.
If that was not thru no sailors would spend big money on a good spinnaker. We all use them and develop a skill set that work well downwind using one. After 50 years of racing sailboats and studying the aerodynamics related to sails, I pretty much got both the downwind and upwind working reasonably well.

The wheels turn the propeller.

freediver wrote on May 9^th, 2022 at 9:33pm:


When they're talking about beating it means sailing upwind and yes some sailboats can sail faster than the wind while going upwind. The 18ft Skiffs i sailed could do 14 knots upwind in 10 knots of wind tacking through 90 deg which means they're sailing 45 deg away from true wind.

There are boats that don't have spinnakers that sail faster than the wind while tacking downwind.

The Moth is a small boat that can sail faster than the wind upwind, across the wind and downwind if there is enough wind for them to get up on hydrofoils and not too much wind so they can control it.



I saw a huge debate on the video you posted about that concept around 10 years ago. The one doubt i have with your video is with that streamer on the bow flowing back was that caused by apparent wind coming from forward direction or suction from the prop?

Blackbird is powered by the difference in speed between the ground and the air. If it were able to work in no wind, then it would be a perpetual motion machine, that is, violate conservation of energy.

The challenge of travelling downwind faster than the wind is essentially one of gearbox design (in which the whole vehicle is a gearbox), and achieving a effective coupling between the vehicle and the air.

Blackbird's propeller is coupled to the wheels. It is not a conventional vehicle powered by a wind turbine. It is an airplane powered by the ground. The ground drives the wheels, the wheels drive the propeller, and the propeller pushes the vehicle forward. If the propeller thrust force is greater than the braking force at the wheels, blackbird will accelerate and achieve greater than wind speed. 'Thrust' because this force acts to speed the vehicle up. 'Braking' force because this force acts to slow the vehicle down.

Consider the blackbird travelling at wind speed. This is equivalent to the treadmill prototype (shown in the video in my opening post) being held stationary on a treadmill, just with a different reference frame. Power is transferred to the vehicle through the wheels. This power is the braking force into the wheels multiplied by the relative speed between the vehicle and the ground. Thrust power is the thrust force from the propeller multiplied by the relative speed between the vehicle and the air. Because the air and the ground are travelling at different speeds, it is theoretically possible for the thrust force to be higher than the braking force, but the thrust power to be less than the braking power. Energy is conserved, and the difference in power is accounted for by all the inefficiencies. The trick here is to use the bulk air speed. The local air speed variation generated around the propeller is part of the 'coupling problem', one of the inefficiencies that limited blackbird to 2.8 times air speed.

I think the treadmill prototype is a better experiment, because it simplifies the analysis conceptually. The vehicle could be clamped to the treadmill frame so it does not move, and the question then becomes whether the thrust force can exceed the braking force. You are effectively turning the treadmill into a fan, and the interaction between the wheels and the tread is a cog in an elaborate gearbox. You can then adjust the gear ratio so that the force on the final gear (the propeller) is higher than on one of the intermediate gears (the wheels), with energy conserved by gearing them to turn at different speeds.

An even simpler analogy would be if you used a prototype fixed to a treadmill, and replaced the propeller with a screw (or threaded rod) mounted into a fixed plate. You could generate extremely high thrust forces because you do not have the coupling problem that a fan does.

Blackbird is a more complicated concept because all three parts are moving relative to each other: the ground, the wind, and the vehicle. A good analogy for this is a planetary gearbox in which all 3 shafts can turn, allowing the ring gear to turn. Using concentric shafts (one inside the other), such a gearbox can have two input shafts (analogous to the ground and air) turning at different speeds, and an output shaft (analogous to the vehicle frame) turning at a third speed. You can achieve any output speed you want with the right gear ratio.

Another good analogy is a 'car' mounted between two moving walls, with wheels coupled to both walls and a gearbox that makes the wheels turn at different speeds (and possibly opposite directions). Depending on the gear ratio and the relative speed of the two walls, the car could move in either direction, and at speeds far higher than either wall. In this case there is no limit to the car speed, because it does not have the coupling problem that blackbird does. You can bring this analogy one step closer to blackbird by thinking of one of the walls as a plate mounted under a blimp, allowing blackbird to get an almost 'rigid' coupling to the air.

In the vehicle's internal drive train, power is always going from the wheels to the propeller. If it is moving with the wind.