bfilarsky

This has really been bothering me as I can normally grasp engineering principles fairly quickly. I don't believe that they are going directly down wind. The models on the treadmill had me going for a while but I believe they are misunderstanding the principles at work (on the little models). The treadmill is driving the wheels in effect what we are seeing is a gearing principle at work. That said I am not a sailor and so it may be that I simply don't understand a key principle involved somewhere. The treadmill is about the best way to demonstrate the effectiveness of this. Remember frames of reference? You can accurately calculate a physics problem from any frame of reference (so long as we're not approaching the speed of light). This is precisely why wind tunnels work - the physics don't change whether you fly the airplane through still air, or blow air over a still aircraft. The treadmill is, essentially, a wind tunnel for the car. If the car can travel up the treadmill, it can travel up an equivalent hill with a tailwind, and faster than that wind. I would prefer to see the model in a windtunnel than on a treadmill but... the wheels driving the prop in nil (relative) wind seems like it's the same on a treadmill as in a windtunnel. I think Bfil's post above is the key. How do sailing boats and ice boats travel faster than the wind when tacking? The interaction between the solid connection to the ground and the moving wind. With a boat with a static sail they need to be moving crosswind to a degree to allow the sail to continue feeling a relative wind. With the geared prop they still have a solid connection between the ground and the wind* and since the prop is moving, not static, it can still feel a relative wind in the right direction even when the craft is travelling at or above the windspeed DDW. * That's a rubbish sentence, but I'm not sure how to put it better than that. Yes, you've got the right idea.

That's the same article linked in the OP. And it is still wrong for the same reason...once the vehicle is going the same speed as the wind it can no longer be used as a source of energy because there is no relative wind. Yes, I completely understand that there is no relative wind. However, there is relative motion (of the ground) with no relative wind. Lets assume this vehicle has been moved by some other means to the speed at which there is no relative wind - say 10 mph (ie moving downwind at 10 mph with a 10 mph tailwind). The wheels are turning with a tangential velocity of 10 mph, and driving the fan through a reduction box. Now, we know the fan will create a propulsive force to push the vehicle forward. With no wind at all (meaning a 10 mph frontal relative wind), this force would necessarily be less than the force created in the opposite direction by the wheels, otherwise you would have violated the first law of thermodynamics (conservation of energy). However, with the wind, we now have a fan pushing into what it feels is static air. This will provide more force than the same fan turning with a 10 mph "headwind" Hence, it can accelerate. This does not break the first law of thermodynamics, as there is wind which powers the vehicle. Once again, while intuitively it does seem like its impossible, it does obey the laws of physics. Check out this video: http://www.youtube.com/watch?v=xHsXcHoJu-A That's the same article linked in the OP. And it is still wrong for the same reason...once the vehicle is going the same speed as the wind it can no longer be used as a source of energy because there is no relative wind. Ok, I think I get it. It isn't sustained. They are simply building momentum by using the propeller as an energy gathering system, and then transferring that energy back to forward thrust once they reach the same speed as the wind (zero relative wind). You could do the same thing with a flywheel. Use the wind to spin the flywheel up to speed and to propel the vehicle. Then when you reach the same speed as the wind, use the energy stored in the flywheel to accelerate beyond the speed of the wind. You'll be able to achieve a speed faster than the wind, but you won't be able to sustain it. Big difference. Nope. They are sustaining it. The propeller is linked directly to the wheels, its not accelerated and then bled of energy.

That's a great explanation. Problem is, they're actually doing it. http://www.wired.com/autopia/2010/06/downwind-faster-than-the-wind/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+wired%2Findex+%28Wired%3A+Index+3+%28Top+Stories+2%29%29&utm_content=Google+Feedfetcher

What you are talking about is perpetual motion. Obviously not possible to anyone with any grasp of physics. The machine they are building is NOT perpetual motion, because it does require wind. Remember that even though the air is still around the machine AT the speed of the wind, there is still a velocity over the ground. This machine can not run without wind, ie it's not perpetual motion. Look around the Internet about this. It's not a hoax.

The point of the whole question is whether you can build a machine that will sustain a downwind speed faster than the wind. IOW, if your riding a vehicle downwind, and the speed of the wind (so you feel no relative air movement), can you generate thrust from that wind, and accelerate forward. As mentioned, Ice boats, sail boats, and most things you've ever seen can not. Intuition would suggest it cannot be done period, and I would have said the same thing before seeing what they are doing to do it. Take a look at the links provided above, and you'll see how it works. Pretty weird stuff, but it works!

If you turned off gravity on a car, it wouldn't be able to do much either. Does that mean cars are powered by gravity? Sure, gliders use potential energy as an energy sink, but it isn't the source of their power. Their power comes from the wind (specifically, in the vertical direction ie updrafts, thermals, mountain waves, etc) If gravity is what "powers gliders" how can they climb? However, gliders aren't really what we're talking about, because they get their power from rising air. Sure, they can fly downwind (faster than the wind), but they are being powered by air moving up (or just turning some of that stored up potential energy into kinetic energy), not air moving horizontally like the objects of discussion.

Drag doesn't just go up as normal when approaching the sound barrier - as soon as you get close, your coefficient of drag increases exponentially due to the shock wave you begin to generate (called wave drag). You are correct about the big problem being controllability in aircraft. As an aircraft passes through its critical mach number (where supersonic flow begins on parts of the aircraft, but the total flow is not yet supersonic - usually around 0.7-.85 Mach), its center of pressure moves aft, causing a downwards pitching moment, known as mach tuck. Not really an issue with a jumper, as they create drag, not lift. I think the sound barrier can be broken from 125,000' in freefall - can't really give any math to prove it, but it sounds reasonable given the air density. Also, the point about the speed of sound in air being based on temperature only is correct. In Meters/second, It is the square root of (1.4*287*T) where T is the temperature in kelvin. At 125,000 feet (temp 246 kelvin), that gives 314 m/s, or 611 knots.

I will definitely take you up on that offer. Busy tonight though, and leaving for Mammoth tomorrow early. If you still have some by Sunday or Monday, I'll swing by!

You don't need a counter pressure bottle filler! Just get this setup... http://www.homebrewtalk.com/f35/we-no-need-no-stinking-beer-gun-24678/

Simple answer: If you were under 40 at the time of you exam, you have 3rd class privileges until the last day of the month, 5 years from the date of your medical. So if you received it August 1, 2007, its good until August 31, 2012. If you were 40 or over at the time of your exam, its only good until the last day of the month, 2 years after your exam. Preceding example, its good until August 31, 2009.

Infinity is still more!

61.89 - limitations on student pilots: (a) A student pilot may not act as pilot in command of an aircraft: (7) When the flight cannot be made with visual reference to the surface; or 91.155 requires you to be at least 1000 ft above clouds whenever you're VFR in controlled airspace (other than class B) So, unless you're a student pilot trying to fly over a solid overcast, or a certified pilot trying to fly too close to clouds in controlled airspace, I'm not quite sure what you're getting at.

If its ugly, its British. If its weird, its French. If its ugly and weird, its Russian.

I have a 3G Iphone and love it. I have my calendar on it, which syncs automatically over the internet to my google calendar, and my contacts sync over the air as well. I get my e-mail, can surf the internet, check my facebook, check snow conditions, etc all very easily. Texting is made easier with the conversation style setup - not to mention the awesomeness of being able to listen to music any time I have my phone. I think its absolutely worth it - I can't imagine going back to a regular cell phone!

Just don't tell my Mom I'm a pilot. She thinks I play Piano in a whore house

Yup. Got Mom to go on a tandem. Dad still won't go

More like cheaper version of BASE jumping...

The last clip of every episode I've seen is someone miraculously escaping something. That show is awesome, btw.

I never carry mine (iphone) while jumping, but I do while flying the jump plane. Every once in a while it wigs out a bit, just have to reset it.

Sounds like an exceedingly inefficient way to get altitude for a jump.

When all else fails... a sectional and a whiskey compass. Would more would a man need?

I third Stone Brewery!

I jumped this weekend - it was a brutal 75* F out here in San Diego. It gets down into the low 50's at night. Oh the humanity!

He jumps out at Skydive San Diego. He's a cool guy.