I ran across this demonstration a little while ago and decided it would be fun to post here, since Just Conversation is just about the most entertaining forum I’ve ever been on. It’s similar to the [THREAD=49766]plane on a treadmill[/THREAD] question, except that it is actually kind of interesting.
The question is this: Is it possible to make a wind-powered vehicle that can travel faster than the wind while moving in the downwind direction? So given a perfectly smooth and flat location on a windy day (lets say a 20 mph wind blowing in the southward direction), this vehicle could travel due south at a speed greater than 20 mph.
While you’re thinking about that, you might find this video interesting:
Does this video demonstrate a capable vehicle, or not? (or is it all just fake?)
Or it’s just an efficiently-made little vehicle. My assumption is that the propeller is powering the front wheel(s). It’s not like it’s flying faster than the wind, it’s driving into the wind, using wind power. The wind speed is not a limiting factor. Unless the assumption is something else, but it doesn’t say…
You can break this problem down to a simple bit of arithmetic:
A) How much power does the wind provide?
B) How much power does the vehicle need to accelerate?
If A > B then you’re good. Of course, the challenge is designing a very efficient vehicle. As your video shows, a passenger-less vehicle is fairly easy to propel… but that vehicle won’t scale up very easily, especially if you plan to put a person in it.
In my judgement, Jeremy only gets partial credit. There’s a lot more to a vehicle than a pair of gears.
He’s got some more explaining to do before you hand him the medal, Bob.
For the cart on the treadmill it is possible.
If they had put it on the table it wouldn’t have gone faster than the wind speed, because once it reached wind speed the propeller would have stopped turning.
Yes. So across the wind (tacking), but not directly downwind, otherwise at some point you’ll match the wind speed and begin to have wind resistance, so your propeller will still or your sell will luft.
But yeah, if you ever watch wakeboarders of windsurfers, they can use the wind to build speeds much higher than the wind speed.
LOOK EVERYBODY!! Maestro messed up his demonstration… Worse is it’s about physics
He didn’t figure out the fact that as soon as the vehicule reaches wind speed; there is no more wind power!!!
I know I’m being a jerk right now but it’s just to point that out, as it is an extremly rare (maybe unique) event : Maestro failed on a physics problem.:D:D
…So did I before the sailors gentlemen brought their wisdom around here;)
This thread needs to survive, the problem is much more interesting than the “plane on a conveyor belt” one.
Someone has to complain about that… Come on! you can’t accept this “blabla wind power blabla still air…” there must be a solution.
I appreciate your humorous post, but you’re only considering the case where the vehicle and wind are travelling in the same direction. One could design a vehicle that operates above wind speed in a crosswind. Y’know, like a sailboat?
Oh yeah, so skipping the part about “the wind going south and the vehicule going south” is the key, right?
I’m just a little offended: my post wasn’t meant to be humorous, you can’t always trust the smillies.
This is an interesting point that some other people in the thread have also pointed out (and the first thing I thought of when I saw it). Does this mean that it is impossible to go faster than the wind, since the vehicle would reach wind speed and have no more input of power? If so, the how does the vehicle in the video manage to accelerate?
The vehicle in the video is moving on a treadmill. That means that its apparent speed (how much belt it is covering in a given time) is not the same as its actual speed relative to a fixed point.
Also, the video gives no idea of what the windspeed is.
If the wind is from the north to the south at 20 miles an hour, then a dandelion seed in the wind will travel south at 20 mph. A full rigged 19th century ship might be doing half that because of the drag of the hull in the water. A modern land yacht might be doing 18 mph (for example) afterm aking a small allowance for the friction of the tyres, wheels etc.
As the speed of the “vehicle” approaches the windspeed, then the speed of the vehicle through the air decreases, tending towards zero. That is why a flag flown from a balloon always hangs downwards.
When the speed of the vehicle is exactly equal to the wind, and in exactly the same direction, then any sail or rotor will be in still air, and therefore converting (converting, not generating) no power.
However, if you take a well designed sail and go across the wind, more complex aerodynamics come into play. This means that a well designed sailing boat (or land yacht, wind surfer, etc.) can go across the wind faster than the speed of the wind - because if you are going east at 30 mph in a wind that is going south at 40 mph, you are passing through the air at 50mph.
The purpose of the treadmill is to make a nice controlled environment to test the cart, since moving at the speed as the wind over land is the same as sitting in still air while the ground moves underneath (at least from the cart’s perspective).
Keep in mind that the propeller is directly attached to the wheels, so if the cart is moving at 20mph with the wind, the propeller will still be spinning even though it is surrounded by still air.
Okay, Zzagg trolled me into believing I could actually be wrong about something, and now I actually went back and watched the rest of the videos posted by that YouTuber. I didn’t actually understand what I was watching earlier. I thought wind was powering the vehicle; other posters here may be under the same impression.
The vehicle in the video is actually self-powered; there is no wind here. The wheels are turning the propellor, which pushes against the still air, accelerating the vehicle. As I’ve said before, this isn’t a huge surprise given the vehicle is little more than wheels and a frame.
The YouTuber claims the ~10mph treadmill is used to simulate a car going 10mph with a 10mph tailwind. I can buy that… but the $10,000 question is: how does such a vehicle accelerate to 10mph without assistance from The Great Hand?
Edit: consider that, given the design of the featured vehicle, a tailwind would actually cause the vehicle to run in reverse.
Mentionning some concepts like “relative/absolute wind” could have saved you… but now I’ll never read your post like I did before. You can no longer be my physics reference.
How disapointing!
