General > General Technical Chat

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

<< < (131/285) > >>

IanB:

--- Quote from: electrodacus on December 12, 2021, 06:56:26 pm ---you do not have an extension cord to a stationary location
--- End quote ---

You do have an extension cord to a stationary location. It is the wheels of the vehicle, which are sitting on the ground, which is stationary.

electrodacus:

--- Quote from: IanB on December 12, 2021, 07:03:40 pm ---
--- Quote from: electrodacus on December 12, 2021, 06:56:26 pm ---you do not have an extension cord to a stationary location
--- End quote ---

You do have an extension cord to a stationary location. It is the wheels of the vehicle, which are sitting on the ground, which is stationary.

--- End quote ---

Wind power is applied to vehicle body (including propeller or anything that has a surface facing the wind) that will be the area in the formula and then part of that applied power (you can not get all) can be taken from the wheel and transferred to the propeller.

So say at some point in time 100W are available to the vehicle as wind power then you can chose to use all that to accelerate the vehicle (increase kinetic energy) or you can decide to only use 50% for vehicle acceleration (it will take longer to get to a certain speed) and use the other 50% to power the propeller by taking 50W from the wheel basically this 50W from the wheels are braking.
Now that propeller may be 50% efficient so you will end up with 25W of thrust. This will be fairly useless and wasteful in more cases but air is a compressible fluid so most of this energy will get stored by increasing the pressure differential on each side of the propeller.
This stored pressure differential is what allows blackbird to exceed wind speed even by significant margins but this will last for at most a few minutes depending how low friction the vehicle is then vehicle will start to slow down as there will no longer be enough stored energy to cover the losses and to accelerate.

fourfathom:
electrodacus, perhaps you should ponder the demonstrations with the small vehicles on the treadmill.  I recall that you dismissed these demonstrations, but do reconsider.  In these there is no appreciable energy storage, and the vehicle is propelled faster than the treadmill speed for extended periods of time.  In these, the frame of reference has shifted, with still air and moving ground.  Perhaps this will help.

If the vehicles work, but your models and equations don't, you should suspect that your models and equations are wrong.

electrodacus:

--- Quote from: fourfathom on December 12, 2021, 07:22:18 pm ---electrodacus, perhaps you should ponder the demonstrations with the small vehicles on the treadmill.  I recall that you dismissed these demonstrations, but do reconsider.  In these there is no appreciable energy storage, and the vehicle is propelled faster than the treadmill speed for extended periods of time.  In these, the frame of reference has shifted, with still air and moving ground.  Perhaps this will help.

If the vehicles work, but your models and equations don't, you should suspect that your models and equations are wrong.

--- End quote ---

Not sure why you think I did not agree with any experiment.  The treadmill model showed in Derek's video works exactly according to my explanation.
While you set the vehicle on the treadmill you need to keep it there until all energy storage sources are fully charged up then you can release.
During that period you charge all flywheels (vehicle wheels and propeller so all spinning mass) and the important part you are also charging the pressure differential as tis is what will power the vehicle against the treadmill travel direction.  The thing is that this stored energy is large enough that the treadmill is way to short to demonstrate how vehicle will start to slow down.
If you try to interact with the vehicle by keeping it moving forward you are just charging the pressure differential back to the same initial level.
So keeping the vehicle forced in place will result in you having an inefficient treadmill powered fan.

fourfathom:

--- Quote from: electrodacus on December 12, 2021, 07:31:57 pm ---The treadmill model showed in Derek's video works exactly according to my explanation.
While you set the vehicle on the treadmill you need to keep it there until all energy storage sources are fully charged up then you can release.
During that period you charge all flywheels (vehicle wheels and propeller so all spinning mass) and the important part you are also charging the pressure differential as tis is what will power the vehicle against the treadmill travel direction.  The thing is that this stored energy is large enough that the treadmill is way to short to demonstrate how vehicle will start to slow down.
If you try to interact with the vehicle by keeping it moving forward you are just charging the pressure differential back to the same initial level.
So keeping the vehicle forced in place will result in you having an inefficient treadmill powered fan.

--- End quote ---

When you are holding the vehicle in place on the moving treadmill (relative airspeed zero) the vehicle is pulling on the string.  Using your equation there should be zero force, but there obviously is.  Remember, if you swapped the reference plane, this is identical to the vehicle rolling across the ground at windspeed.  In your equation, where is this stored energy coming from when (vehicle speed = windspeed)?

Hint:  There is no appreciable stored energy.  There is potential energy that can be extracted.

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