General > General Technical Chat

Mess with your minds: A wind powered craft going faster than a tail wind speed.

<< < (259/285) > >>

electrodacus:

--- Quote from: fourfathom on December 28, 2021, 09:20:30 pm ---OK.  I will shut up and wait until friction is re-introduced.  I want to see the situation when the vehicle speed = windspeed, and force has to be found to overcome friction.  I know this works, but how the math applies to the actual real-world blades, gears, and wheels still isn't clear to me.

--- End quote ---


A real vehicle that has friction will never be able to get to wind speed it will stay at some lower than wind speed forever. The only way to even get to wind speed or exceed that is to use an external energy source or an energy storage device to store wind energy when it is available as it is the case for direct downwind blackbird.

electrodacus:

--- Quote from: IanB on December 28, 2021, 09:24:02 pm ---
--- Quote from: gnuarm on December 28, 2021, 08:19:33 pm ---The equation 0.5 * air density * area * (wind speed - vehicle speed)3 may be the right equation, but for the wrong purpose.

--- End quote ---

There is an equation, 0.5 x (air density) x (area) x (wind speed)^3

This equation gives the flow of wind kinetic energy through the swept area of a turbine.

It breaks down into three parts:

1) kinetic energy of wind per unit mass of air
2) mass flow of air per unit area
3) swept area of turbine

Hence:

(flow of kinetic energy) = (energy per unit mass of air) x (mass flow per unit area) x (area)

If any one of those terms is zero, the available energy will be zero.

If you try to apply this to a sail, you find that the mass flow of air though the sail is zero (because the sail is a wind barrier). So you cannot use this equation in this form for sails.

When I brought up this point with ED and asked him to explain it, he quickly changed the subject and avoided giving an answer.

--- End quote ---

Actually a sail on ideal wheels friction less will be 100% efficient in using that available wind power.

Air is a fluid so trillion of small particles that have mass and an average speed in a particular direction.
So each time a particle hits that area of the sail it will lose half of the kinetic energy since the other half will be transferred to sail.
Stop involving brakes or an anchor. It is like a wind turbine had the electrical brakes enabled of course there will be no output power.   

IanB:

--- Quote from: fourfathom on December 28, 2021, 09:20:30 pm ---OK.  I will shut up and wait until friction is re-introduced.  I want to see the situation when the vehicle speed = windspeed, and force has to be found to overcome friction.  I know this works, but how the math applies to the actual real-world blades, gears, and wheels still isn't clear to me.

--- End quote ---

The same general theory still applies. The golden equation, which is always true, is this one:

(power applied to vehicle) = (force applied to vehicle) x (speed of vehicle)

You can apply this equation twice. You can apply it first to the wind pushing the vehicle along (power supplied), and you can apply it second to the rolling resistance and drag on the vehicle as it moves (power consumed).

When (power supplied) = (power consumed) then everything is balanced and the vehicle reaches a steady speed. For a simple sail, this steady speed will be lower than the wind speed.

For Blackbird, the spinning prop adds some extra thrust, allowing the (force applied to vehicle) to be positive even when exceeding the wind speed. Therefore, Blackbird can reach a steady speed faster than the wind where it is still true that (power supplied) = (power consumed).

For Blackbird to do this requires careful design of gear ratios on the propeller shaft and pitch angle on the propeller blades.

bdunham7:

--- Quote from: electrodacus on December 28, 2021, 09:31:51 pm ---So each time a particle hits that area of the sail it will lose half of the kinetic energy since the other half will be transferred to sail.

--- End quote ---

Say what?  Is this another gem that 'university professors' don't understand?

IanB:

--- Quote from: electrodacus on December 28, 2021, 09:31:51 pm ---Actually a sail on ideal wheels friction less will be 100% efficient in using that available wind power.
--- End quote ---

Newton's first law of motion says that an object will not change its motion unless a force acts on it. If the sail boat is moving at the same speed as the wind there is no wind force: (w-v) = 0. If there is no friction, there is no friction force. Therefore no force at all. If there is no friction, the boat requires no power at all to continue moving at the same speed, so it neither needs nor gets any power from the wind. The wind power in this situation is zero, so the efficiency must be 0%.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod