4 ms·
It’s really not that complicated - wind speed is not the same as energy. The propellor is a device that pushes against the air to move forward, it takes energy
by twobitshifter 5y ago
It’s really not that complicated - wind speed is not the same as energy. The propellor is a device that pushes against the air to move forward, it takes energy from the wind, produces a force, and the force produces momentum in the vehicle. We know that a stationary turbine can continually take power from the wind. When that wind energy is stored in a battery it can power your car much faster than the wind. This just takes a shorter route and stores the energy in momentum.
See sailboats faster than wind
https://www.kqed.org/science/8503/how-do-these-boats-sail-faster-than-the-wind https://www.kqed.org/science/8503/how-do-these-boats-sail-fa...
> The America’s Cup sailboats are sleek and fast. The AC72, the type of catamaran used in this year’s race, can travel almost three times the speed of the prevailing wind. On June 18th Emirates Team New Zealand recorded a speed of 50.8 mph (44.1 knots), with a wind speed of about 18 mph (15.6 knots).
When sailing directly in line with the wind, the apparent wind in a sail is equal to the actual wind, so there’s no gain. A propellor reshapes the direction of the apparent wind to be in the same direction of the wind.
More interesting is that blackbird can go 2 times as fast as the wind running directly_into_ the wind. That ought to help understand how this is possible.
- cblconfederate 5y agoSailboats can run faster than the wind because of lift (which is not involved here), but not when running away from the wind
- 6510 5y agothe blades move sideways
- cblconfederate 5y agoif it sails away from the wind, the lift is pushing it sideways, not forward
- jjeaff 5y agoAnd if you turn a sail sideways and put it on a spindle with several other sails, you have what looks a whole lot like a propeller. So instead of sailing sideways so that your air foil sail can move sideways through the wind, the Blackbird harnesses the power of sales moving sideways through the wind while pushing the craft directly into the wind.
- mypetgoat 5y agoExactly. The article does not understand this at all.
- deleted 5y ago[deleted]
- jasonwatkinspdx 5y agoPropellers are lift devices. The lift vector is parallel to the axis of rotation. It all works the same downwind, and the crazy fast boats like the AC72s do in fact sail faster than the wind when running away from the wind.
- cblconfederate 5y agoit's confusing to talk about sailboats and this cart, because this doesn't use lift from the wind to move itself forward, it generates some lift with its propeller
- ummonk 5y agoThe more important point is conservation of energy. Since the ground is moving backwards faster than the air, you can generate more thrust against the air for the same amount of power than the amount of backwards force exerted by the ground on your wheels (since power = force * velocity). For the same reason, if traveling upwind rather than downwind, you’d need it to use the opposite mode of operation - have a turbine driving the wheels rather than wheels driving a propeller. Conservation of momentum is pretty irrelevant, since you have a giant immovable sink/source of momentum known as the Earth to push against
- strainer 5y agoI like your point about moving upwind, but think it rather relies on conservation of momentum as that insists the wheels must result in thrust in the opposite direction to the props thrust in order to change the momentum of the earth as much as the momentum of the atmosphere is changed. They must each be reduced, in order to transduce energy. If you have stored energy you can propel mass from rest, but forces can only be applied equal and opposite. It is extremely enigmatic and curious that the engine can work to synergistically blow air into the atmosphere in one direction and resist the inertia of the earth in the other direction with the same force, without expending any energy reserves in the engine. Ive not seen a model or comment or article which fully illuminates the situation, although it is well demonstrated. Perhaps there is not language or a familiar concept which yet suffices. A sane and experienced physicist lost 10k bet on its plausibility.
- koonsolo 5y agoThis does not explain the most obvious issue (or at least my most obvious issue): when going as fast as the wind, there is no wind anymore. The answer: at that point, your wheels are turning, and so you can get energy from your turning wheels, but not directly from the wind. This is how I explain it in my head: 1. When going slower than the wind, the wind pushes you forward. Propeller at this point is useless. 2. When going as fast as the wind, the wheels will drive a propeller that pushes wind back (same as an airplane) 3. When going faster than the wind, all your energy will come from the turning wheels.
- DarmokJalad1701 5y ago> 3. When going faster than the wind, all your energy will come from the turning wheels. But why do the wheels keep turning in this scenario? Is there somehow a net-force on the back-side of the vehicle from the wind?
- atoav 5y agoForward momentum of the vehicle I guess.
- atoav 5y agoI think you are spot on. IMO it is just positive feedback: the faster you go the more wind is being pushed back by the propeller. At some point the system reaches a state of saturation or equilibrium (the wheels turning no longer can speed up the propeller enough to generate more push). This equilibrium is going back and forth between phases 2 and 3 as you described it. The thing that makes it a little bit hard to reason about is that all three components (wind, wheel, propeller) are interdependent and influence each other in a more or less fluid way. To me it feels similar like reasoning about active filter topologies.