General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
fourfathom:
--- Quote from: electrodacus on September 17, 2021, 05:27:12 pm ---I say blackbird will stay above wind speed only as long as there is still stored energy so depending on vehicle construction (amount of energy that can be stored and vehicle friction losses) from a few seconds to a few minutes.
--- End quote ---
I know I'm jumping in here, but how do you reconcile your claim with the fact that Blackbird (and demonstrations with similar craft) show the craft maintains a significant multiple of the true wind speed while continuing to accelerate? The typical, and measured wind gusts come nowhere close to exceeding the craft speed as evidenced by the wind-indicator ribbons. And even if there were significant gusts, I see no energy storage system (inertia, flex, etc), especially in the tiny treadmill devices, that could maintain speed for any appreciable time.
[Yes, my question below is indeed answered in the thread]
One factor I haven't deeply studied (and may have already been answered in this thread), is how the craft gets started from an initial zero ground speed? If the propeller were symmetrical in push/pull then wouldn't the craft be driven upwind? I see two possibilities:
1) The propeller has more drag than lift when spinning "backwards", or
2) There is an initial"push" given to the craft that gets the propeller spinning fast enough to generate lift. This is certainly the case in the treadmill tests, but I don't see it in the Blackbird tests.
But even if #2 is the case, it doesn't invalidate the general principal being demonstrated.
fourfathom:
--- Quote from: electrodacus on September 17, 2021, 06:51:29 pm ---Is fairly clear that vehicle stores energy and when that is large enough the vehicle will use it to advance forward. Motion seems smoother the smaller the storage capacity is since charge discharge cycles happen faster than our brain can perceive.
Without any amount of energy storage the vehicle will not be able to move forward left to right.
--- End quote ---
I think you're just seeing stiction (https://en.wikipedia.org/wiki/Stiction). Eliminate stiction (but some amount of friction is still OK), and there will be no jumps, only steady motion.
electrodacus:
--- Quote from: fourfathom on September 17, 2021, 07:04:00 pm ---
--- Quote from: electrodacus on September 17, 2021, 05:27:12 pm ---I say blackbird will stay above wind speed only as long as there is still stored energy so depending on vehicle construction (amount of energy that can be stored and vehicle friction losses) from a few seconds to a few minutes.
--- End quote ---
I know I'm jumping in here, but how do you reconcile your claim with the fact that Blackbird (and demonstrations with similar craft) show the craft maintains a significant multiple of the true wind speed while continuing to accelerate? The typical, and measured wind gusts come nowhere close to exceeding the craft speed as evidenced by the wind-indicator ribbons. And even if there were significant gusts, I see no energy storage system (inertia, flex, etc), especially in the tiny treadmill devices, that could maintain speed for any appreciable time.
One factor I haven't deeply studied (and may have already been answered in this thread), is how the craft gets started from an initial zero ground speed? If the propeller were symmetrical in push/pull then wouldn't the craft be driven upwind? I see two possibilities:
1) The propeller has more drag than lift when spinning "backwards", or
2) There is an initial"push" given to the craft that gets the propeller spinning fast enough to generate lift. This is certainly the case in the treadmill tests, but I don't see it in the Blackbird tests.
But even if #2 is the case, it doesn't invalidate the general principal being demonstrated.
--- End quote ---
You can see the effect of how blackbird works in my slow down video version.
The only difference is that blackbird has a significantly larger energy storage relative to vehicle weight.
In my video the vehicle moves in average about 2x slower than the moving paper but that is average and there are points where vehicle is basically stationary and also moments where vehicle has same or slightly higher speed than the paper. It can be better design to exceed significantly the speed of the paper at peaks but average will always be way below the paper speed.
Blackbird works exactly the same but due to much larger energy storage you where able to see just less than a cycle in their demo so you seen the charge phase when wind speed was lower than vehicle speed and then you seen vehicle use that stored energy to accelerate above wind speed but what you where not able to see due to short duration of that test is the slowing down well below wind speed part.
In the initial phase of the blackbird acceleration the vehicle works exactly like a sail vehicle and the sail is the vehicle body including the propeller blade.
While the propeller starts to spin the equivalent sail gets larger and larger but unlike a simple sail it also pushes air backwards as propeller is used as a fan and this is what allows storing enough energy to exceed wind speed for some limited amount of time.
If I where to compete with a similar vehicle just using a round 5.3m diameter sail instead of the propeller I will win the race any time against the blackbird.
What will happen is that I will be the first to accelerate while blackbird will waste a lot of time initially to store energy and while blackbird will have higher top speed the average speed will be lower due propeller being less efficient than a sail.
So if the race is about top speed then blackbird can win but if it is about getting first to finish line then sail vehicle will win.
electrodacus:
--- Quote from: fourfathom on September 17, 2021, 07:09:09 pm ---
--- Quote from: electrodacus on September 17, 2021, 06:51:29 pm ---Is fairly clear that vehicle stores energy and when that is large enough the vehicle will use it to advance forward. Motion seems smoother the smaller the storage capacity is since charge discharge cycles happen faster than our brain can perceive.
Without any amount of energy storage the vehicle will not be able to move forward left to right.
--- End quote ---
I think you're just seeing stiction (https://en.wikipedia.org/wiki/Stiction). Eliminate stiction (but some amount of friction is still OK), and there will be no jumps, only steady motion.
--- End quote ---
You can not have steady motion from left to right. Without the energy storage part the vehicle will be unable to move from left to right as that will violate the conservation of energy.
If you have lower amount of of energy storage then this speed variation will be smaller harder to perceive but can not be eliminated.
The gear vehicle in my first video is smoother but while it has about the same gear ratio is harder to make it move from left to right so if I could have a stiffer vehicle so less energy storage there will likely be the point where energy storage is to small and vehicle will no longer be able to move at all from left to right (maybe some micro movements but in average it will move backwards or just stay in place).
Just imagine that there was no elastic energy storage in that slow down vehicle and front wheel will start to slip what then can power the back wheels if there is no energy storage and no power from the front wheels.
fourfathom:
Where is the energy storage in a lever? Where is it in a gear ratio? Energy storage is not required here.
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