If you believe that sailboats can beat a drifting balloon downwind by zigzagging, then there is a very good explanation in the original Veritasium video that shows that this is essentially the same thing. Here's a link to the section of the video with that explanation [1]. Watch about 2 or 3 minutes starting there and it becomes clear.
A wind-powered vehicle that can travel twice as fast as the wind itself
91–100 of 263 posts
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#92Actually, knowing it works now. Even internal combustion engines have only a 40% energy efficiency in converting gas to power, the rest is lost as heat. In this case, I'm trying to think how the wind energy is lost, and it seems that some of that might be lost to the static friction in turning the wheels, and also the attached propeller. However, the energy used to turn the propeller is outputted again, as thrust. Si…
> Since the initial wind was enough to overcome the static friction
I’ve seen this idea in a couple of comments here, but I’m not sure what it means, can you elaborate?
Rolling wheels are still under static friction. They don’t become dynamic friction unless the wheels are in a skid. Unless what you’re talking about is the static friction of the oiled axle, but I assume that’s not what you meant?
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#93> The secret to Blackbird, Cavallaro explained, is that once the wind gets the vehicle going, its wheels start to turn the propeller blades — they're connected to the blades by a chain. As the vehicle speeds up, its wheels turn the propeller faster and faster. The propeller blades, in turn, act as a fan, pushing more air behind the land yacht and thrusting it forward. That's a terrible explanation - no wonder people…
I really would like a simple explanation of how that is possible. AIUI, when speed is stable, the wheels turning provide power to the propeller. But if the propeller pushes the car, which in turn makes the wheels turn faster making the car faster than the wind itself, what makes it different from perpetual motion machines? I would love a diagram showing how the power flows in this system.
Here's a simple explanation:
Let's assume that the windspeed is 10 km/hr. We all agree that the wind can push this vehicle so that it is now going 10 km/hr downwind, right? From the vehicle's point of view, the wind is now 0 km/hr. But its wheels are turning, which turns the propeller, which behaves like a fan, pushing the vehicle forward in the still air, so that it is now traveling faster than the wind.
This isn't perpetual motion, though, because eventually the thrust generated by the propeller is insufficient to overcome the additional drag created by the vehicle's faster speed.
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#94Muller’s bet reminded me of the Mysthbusters episode where they put a fan in a boat that blows on it’s own sail and moves the boat forward, which I’m pretty sure I thought wasn’t possible before it aired. https://mythresults.com/blow-your-own-sail It also reminds me of the Brayton Cycle, and turbofan/turbojet engines where the output thrust of the blades is applied to subsequent blades that turns the shaft, causing t…
Whether it's an electric fan or your own lungs, it's pulling energy from storage (a battery or your own chemical energy storage) and converting it to kinetic energy. It's no different than if you put the fan or your face in the water facing backwards to propel the ship.
That seems too dismissive of the part that’s counter-intuitive, and I think this is legitimately counter-intuitive, which is why physics professors are losing bets.
The fan on the boat is blowing air forward from behind the sail, and by it’s very presence on the boat, it applies a force to the boat that is trying to move the boat in reverse. Without the sail, the boat does move in reverse. It’s surprising that one can seemingly recover more force to push the boat forward than was already used to push the boat in reverse. IIRC Myth Busters confirmed the result but didn’t come to a detailed conclusion as to why, which could involve the shape of the hull, directions of the hull & sail relative to the fan direction, or some aerodynamics of the air blocked by the sail.
In any case, I think it’s quite different from putting the fan in the water facing backwards to propel forward.
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#95Earlier quoted context omitted.
Solar sails only absorb enough energy for tiny crafts in space, a frictionless environment. You overestimate photon flux momentum from the sun.
Solar sails indeed only absorb a tiny amount of energy, BUT per given area. They can theoretically made to be extremely large without much weight relative to the size. Which in turn allows you to accelerate even quite large payloads in a reasonable amount of time.
The mirrors are extremely heavy and solar sails are exclusively used above our atmosphere, in space, where more flux from the sun is received.
The flux even at earth orbit is a measly 47e-7 Newtons per square meter. That's 0.00016905632 pounds per square inch.
You still stand by what you just stated?
It's not feasible to do below clouds, in an environment with constant friction, air or water resistance.
The only reason you can do it in space is because that tiny slow acceleration can accumulate over a long period of time. This precludes frictional losses that would normally be incurred within atmospheres.
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#96Earlier quoted context omitted.
Solar sails indeed only absorb a tiny amount of energy, BUT per given area. They can theoretically made to be extremely large without much weight relative to the size. Which in turn allows you to accelerate even quite large payloads in a reasonable amount of time.
The mirrors are extremely heavy and solar sails are exclusively used above our atmosphere, in space, where more flux from the sun is received. The flux even at earth orbit is a measly 47e-7 Newtons per square meter. That's 0.00016905632 pounds per square inch. You still stand by what you just stated? It's not feasible to do below clouds, in an environment with constant friction, air or water resistance. The only reas…
It seems I misunderstood the thread. I somehow thought we were talking about sails in space rather than in atmo.
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#97Earlier quoted context omitted.
Any wind that's strong enough to overcome static friction would do the trick, and I can't see why 10mph wouldn't be enough. The small models that you can build at home (sorry, can't find the plans anymore; this was 10 years ago now) do not require a push to get started.
Understandable that it doesn't take much wind for a vehicle well under a pound. But what about one that weighs well over 100 pounds?
My motorcycle with me on it can weight 700-800 pounds and has a much smaller cross sectional area than blackbird. And yet it still gets pushed around sometimes by crosswinds .
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#98"A popular YouTuber filmed himself driving a wind-powered vehicle downwind faster than the wind itself. [...] Muller, the creator of the Veritasium YouTube channel, likes to break down funky science concepts for his 9.5 million subscribers." The "popular YouTuber" is Derek Muller, who studied Engineering Physics and has a PhD in Physics Education Research... It's so frustrating to see how the writer tried so hard to…
Having 9.5 million subscribers on a science-based YouTube channel is definitely more notable than having gone to university, or indeed having a PhD. I think the writer picked the correct description here.
It's a sad indictment of the times when people feel that having a million subscribers (read: being popular) is more notable than having a PhD (read: expert training) in a relevant field.
It's no wonder anti-vaccine material is so widespread. Someone famous on Youtube said it was bad, it must be true!
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#99> The secret to Blackbird, Cavallaro explained, is that once the wind gets the vehicle going, its wheels start to turn the propeller blades — they're connected to the blades by a chain. As the vehicle speeds up, its wheels turn the propeller faster and faster. The propeller blades, in turn, act as a fan, pushing more air behind the land yacht and thrusting it forward. That's a terrible explanation - no wonder people…
I really would like a simple explanation of how that is possible. AIUI, when speed is stable, the wheels turning provide power to the propeller. But if the propeller pushes the car, which in turn makes the wheels turn faster making the car faster than the wind itself, what makes it different from perpetual motion machines? I would love a diagram showing how the power flows in this system.
Where the power comes from is easy: the wind gets slowed down. Of course the nature of wind is such that you could never measure this, but the point is that when the propellor pushes against the air, that air's velocity relative to the ground is decreased while the vehicle's is increased. The vehicle leaves behind it an imperceptible region of less-energetic air and takes that energy with it.
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#100Earlier quoted context omitted.
The mirrors are extremely heavy and solar sails are exclusively used above our atmosphere, in space, where more flux from the sun is received. The flux even at earth orbit is a measly 47e-7 Newtons per square meter. That's 0.00016905632 pounds per square inch. You still stand by what you just stated? It's not feasible to do below clouds, in an environment with constant friction, air or water resistance. The only reas…
It seems I misunderstood the thread. I somehow thought we were talking about sails in space rather than in atmo.
So what were you originally referring to? Solar wind in space? I'm honestly confused now.