General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
Kleinstein:
--- Quote from: electrodacus on December 12, 2021, 07:11:59 pm ---Now that propeller may be 50% efficient so you will end up with 25W of thrust.
--- End quote ---
The thrust of the propeller is not measured as power, but as a force. The difference between force and power is important here. The force can be quite high, when the wind speed relative to the propeller is low (e.g. zero speed if the vehicle moves with the speed of the wind). A fixed efficiency number does not work here as this would lead to infinite force. On the other hand that same force when applied to the wheels that slow down the vehicle will create power proportional to the vehicle speed. With enough speed of the vehicle, which means enough speed of the wind, there can be more power available than needed (a constant power for a given force) to power the propeller.
The design calculation would be about like this: Chose a prop speed where is works good.
In standing air the prop needs a certain power to create enough thrust, let's say 2 x the force needed to overcome the vehicle friction. A constant friction force independent of speed is reasonable as there is no wind drag at the speed of the wind. So half that force would be available to work against the wheels and from that power drive the prop. The required wind speed for the vehicle to reach the speed of the wind would than be the required prop power divided by half the prop thrust. The quality of the prop (and the friction losses) determine the needed wind speed.
When driving at the speed of the wind, the prop sees standing air around it, and can thus produce the thrust (force) like standing in still air. The force is good to produce a power of wind speed times thrust. In the example half is needed to drive the prop and the rest is available power, e.g to accelerate or overcome friction. So you see, that it is possible to extract energy, even when going at the speed of the wind. It does not work with a simple sail, but it does with the prop to produce a force even with zero speed difference.
For the wind power available to a sail it would be proportional to (w-v)² for the force and w for the speed. So somehwat in between the formula given by electrodacus and IanB. The prop can improve on the factor (w-v)² and still produce some force at V=W. With the prop I am not sure the full area would be effectiv, but that would only be some costant factor, like a different sail shape that may also lead to a factor a bit off 0.5.
bdunham7:
--- Quote from: electrodacus on December 12, 2021, 07:11:59 pm ---25W of thrust
--- End quote ---
I don't want to rehash our much earlier discussion, but you saying this just another iteration of the fundamental misunderstanding that I was unable to get you to see then. This misunderstanding--and your statement here--is exactly where you are repeatedly going wrong. It's not a typo, lack of clarity or your self stated inability to explain things clearly. It is the fundamental error that is preventing you from seeing reality here.
electrodacus:
--- Quote from: Kleinstein on December 12, 2021, 07:59:17 pm ---
The difference between force and power is important here.
--- End quote ---
This is exactly what most seems to struggle with.
This is a wind powered vehicle and as such the most important question is what wind power is available to the vehicle (not force) but power.
Vehicle kinetic energy (and thus speed) can not increase if power is not available.
Vehicle can stay with brakes on for the entire day and significant force can act on your vehicle from the interaction with air particles / wind but that will make no change to vehicle kinetic energy / speed.
Both vehicle kinetic energy and propeller are powered by wind so you can chose the ratio of power distribution but that is about it. The main point of the propeller in this design is to store energy so that it can be used to accelerate the vehicle above wind speed.
So most important question to answer for any wind powered vehicle is what the wind power available to vehicle is?
electrodacus:
--- Quote from: fourfathom on December 12, 2021, 07:56:25 pm ---
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.
--- End quote ---
Sorry not quite sure what you are saying.
The treadmill model is not powered by wind is powered by the treadmill (as in initial condition as treadmill charges the vehicle energy storage) then that stored energy is what powers the vehicle once you let go to the vehicle.
You can call that stored energy potential energy and yes that will be correct.
electrodacus:
--- Quote from: bdunham7 on December 12, 2021, 08:27:45 pm ---
--- Quote from: electrodacus on December 12, 2021, 07:11:59 pm ---25W of thrust
--- End quote ---
I don't want to rehash our much earlier discussion, but you saying this just another iteration of the fundamental misunderstanding that I was unable to get you to see then. This misunderstanding--and your statement here--is exactly where you are repeatedly going wrong. It's not a typo, lack of clarity or your self stated inability to explain things clearly. It is the fundamental error that is preventing you from seeing reality here.
--- End quote ---
The lack of understanding is coming from you.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version