Land Yachts -- Why don't cars and trucks also use sails?

[cjl]

There is a transmission system. The turbine drives the wheels, the wheels drive the vehicle across the ground.

Worth noting that the prop on Blackbird only acts as a turbine when the vehicle is in its upwind configuration. When set up to go downwind, it is acting as a propeller, and the driving force is coming from the wheels.

When going upwind, the turbine harvests energy from the air and uses it to drive the wheels. When going downwind, the wheels harvest energy from the ground, and use it to drive the propeller. In both cases, the reason this works is because the vehicle has a higher relative speed compared to where it is extracting energy than where it is expending it for propulsion.

 

[cjl]

So, why don't we have aircraft with self-propelling propellers? Instead of drive to the ground they can go through a gearbox to a second pushing propeller at the back that runs quicker than the front one?

It's like someone saying you can go read up on cold fusion if you don't believe it. All very convincing. Whole conventions run by established scientists on the subject.

Extraordinary claims .. etc ...

The fundamental principle that makes Blackbird work is that the vehicle's speed relative to the ground is different than its speed relative to the air. As a result, it can extract power from one and use it to propel itself against the other. This obviously doesn't work for an aircraft. This is also why the Blackbird only works when there's wind.

 

[cmb]

I see this as a 'large body' problem

If there is a cross-wind, it can be quite small, a body can be constructed to cause the air to scatter from the side and redirected to behind. If the efficiency of that process is high then momentum from the air in the opposite direction of body travel will provide a source of power for the body to continue along its path.

I remain hesitant on a conclusion. One might consider a vehicle braked and stationary, putting wind power into some batteries. That could be done. One can then imagine (wind not changing) the vehicle then sets off under battery power.

In principle, the energy can be gained when stationary, I see no big difference if it in motion providing the traction force is higher than the wind loading, and the mechanical power used directly rather than via a storage medium.

... I will have to consider further ... I still would regard this as a case of momentum via redirection of air, as I can see the cross wind in the photo. I'd like to see real data with calibrated vane anemometers to show direction of travel versus speed, etc..

 

[ardnog]

Drela's maths paper is quite accessible, and shows the equations for propulsion without a crosswind.

If you mean the photo in wiki, I don't see how you can see crosswind in that. Trace the perspective lines and look at the shadows. It's inexact enough that you wouldn't be able to tell if there was a slight crosswind or not - nor do I understand why this is a productive activity.

Examining the mathematics would be a better use of time.

 

[cjl]

I see this as a 'large body' problem

If there is a cross-wind, it can be quite small, a body can be constructed to cause the air to scatter from the side and redirected to behind. If the efficiency of that process is high then momentum from the air in the opposite direction of body travel will provide a source of power for the body to continue along its path.

Yes, but it's very obvious that's not what the Blackbird is designed to do. We have methods of very efficiently curving a slight crosswind component to the rearward, and that is a large sail or airfoil. What you're describing is exactly what a high performance sailcraft (be it an ice boat, catamaran, or land sailer) does. However, the blackbird has no large sail, and has no method by which a large sail could be controlled anyways (since to achieve optimal performance you would need to be able to rotate one around a vertical axis).

It seems odd to say that, somehow, purely by accident, the builders of the blackbird accidentally made its superstructure such an efficient sail as to make a VMG of over double windspeed directly downwind based only on a small unintentional crosswind component, rather than that the designed mechanism works as intended.