General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
<< < (253/285) > >>
gnuarm:

--- Quote from: electrodacus on December 28, 2021, 06:06:55 am ---
--- Quote from: IanB on December 28, 2021, 05:56:39 am ---
But where does the stored energy in the pressure differential come from? There is no external power source to charge it up. The vehicle starts from a standstill and gets blown along by the wind like a sail, until it speeds up and goes faster than the wind. Until this point, all the wind energy is being used to make Blackbird go faster. Where does the extra energy come from to charge up a pressure bubble?

--- End quote ---

It comes from the wind while vehicle is well below wind speed. As soon as the vehicle starts to move pushed by the wind the propeller will start to rotate and not only push air back but increase the equivalent sail area. So in the initial phase the sail size increases from the vehicle back are plus the propeller blades area up to full propeller swept area.
Only small part of the wind energy is used to accelerate the Blackbird that ratio is given by the gear ratio between wheel and propeller and the propeller pitch.
So wind pushes against vehicle body and later propeller swept area and that power is split say 60% to propeller and 40% to acceleration so even more time spend charging as the acceleration is slowed then from those 60% say about 70% (assuming 70% efficient propeller) is stored but that stored energy will increase a bit the pressure differential so part of what you just stored a moment ago also starts to power the vehicle and at some point (below half wind speed according to my calculations) the pressure differential becomes more significant than the wind power.
So most of the charged energy happens well before vehicle gets to half the wind speed.

--- End quote ---

Ok, you are right.
Kleinstein:

--- Quote from: electrodacus on December 28, 2021, 06:06:55 am ---
--- Quote from: IanB on December 28, 2021, 05:56:39 am ---
But where does the stored energy in the pressure differential come from? There is no external power source to charge it up. The vehicle starts from a standstill and gets blown along by the wind like a sail, until it speeds up and goes faster than the wind. Until this point, all the wind energy is being used to make Blackbird go faster. Where does the extra energy come from to charge up a pressure bubble?

--- End quote ---

It comes from the wind while vehicle is well below wind speed. As soon as the vehicle starts to move pushed by the wind the propeller will start to rotate and not only push air back but increase the equivalent sail area. So in the initial phase the sail size increases from the vehicle back are plus the propeller blades area up to full propeller swept area.
Only small part of the wind energy is used to accelerate the Blackbird that ratio is given by the gear ratio between wheel and propeller and the propeller pitch.
So wind pushes against vehicle body and later propeller swept area and that power is split say 60% to propeller and 40% to acceleration so even more time spend charging as the acceleration is slowed then from those 60% say about 70% (assuming 70% efficient propeller) is stored but that stored energy will increase a bit the pressure differential so part of what you just stored a moment ago also starts to power the vehicle and at some point (below half wind speed according to my calculations) the pressure differential becomes more significant than the wind power.
So most of the charged energy happens well before vehicle gets to half the wind speed.

--- End quote ---
I still don't get why the pressure differential is more efficient in pushing the vehicle forward when the vehicle is above wind speed than opposed to below the wind speed.  So what slows down the conversion of energy so much that is needs quite some time from reaching the speed of the wind to reaching the maximum speed ?
Domagoj T:

--- Quote from: electrodacus on December 28, 2021, 01:37:58 am ---Well it is very relevant as the direct downwind vehicle will not travel indefinitely at 2x or 3x the wind speed but only for a limited amount of time proportional with the amount of stored energy.
Also to store that energy the vehicle will accelerate much slower when below wind speed compared to a sail vehicle that will not store anything other than kinetic energy. So Blackbird direct down wind version spends more time below wind speed to charge the energy storage (pressure differential) then uses that stored energy to accelerate above wind speed and stay a bit above that speed before returning to below wind speed.

--- End quote ---

I'm still waiting for you to respond to my wind powered toy design from page 45.
It has no storage, operates in steady state continuously and has infinite range. At no point do extended sails travel faster than the wind, but the vehicle as a whole does.
I'm waiting.
electrodacus:

--- Quote from: Kleinstein on December 28, 2021, 09:34:12 am ---I still don't get why the pressure differential is more efficient in pushing the vehicle forward when the vehicle is above wind speed than opposed to below the wind speed.  So what slows down the conversion of energy so much that is needs quite some time from reaching the speed of the wind to reaching the maximum speed ?

--- End quote ---

Pressure differential drops as it is being used up thus less power is available to accelerate and so acceleration rate drops.
Is like having a light bulb connected to a capacitor at first the pressure (voltage) is high so you get a lot of light (high acceleration rate) then as voltage (pressure) drops it will take longer and longer to use the remaining energy in the capacitor. 
electrodacus:

--- Quote from: Domagoj T on December 28, 2021, 10:26:17 am ---I'm still waiting for you to respond to my wind powered toy design from page 45.
It has no storage, operates in steady state continuously and has infinite range. At no point do extended sails travel faster than the wind, but the vehicle as a whole does.
I'm waiting.

--- End quote ---

There are two very different versions of blackbird. There is the direct downwind that works based on pressure differential energy storage and there is the direct upwind version that uses small capacity internal storage and stick slip hysteresis and that version since it always have access to wind power it can work continuously as long as there is wind.
All the wheels only toys where air is not involved are the equivalent of that direct upwind version of blackbird using small capacity energy storage device maybe charging in a few ms and then discharging triggered by stick slip hysteresis either internally in the mechanism or externally at the wheels.

The wrong equation for power either drag or generation is what made the math possible without energy storage. If the correct equation for power is used then is clear from the math that neither direct downwind faster than wind nor direct up wind at any speed is not possible without energy storage.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod