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shallowdeep -> RE: Advanced technology (1/7/2008 3:33:41 AM)
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quote:
ORIGINAL: eyesopened
Could you please cite your sources for this? Flying takes very little energy actually.
eyesopened,
I'll try to take a stab at this that doesn't require a background in fluid mechanics (no offense, CL) and hopefully clarifies things a bit, as the issue is complex and you actually do have a point worth examining.
In designing a transportation system you either stay on the ground and deal with friction, or lift off and deal with the force of gravity directly.* The invention of the wheel made dealing with friction a pretty good choice, and paved roads only helped. The losses from rolling friction between rubber and asphalt can vary but are around 3% of the gravitational force acting on the object.
In contrast, lifting off the ground appears to mean you have to deal with the full force of gravitation, or losses equal to 100% of the gravitational force on the object, just to stay in the air. The wing helps out with this by tackling the problem from a different angle, literally. The sky is full of air and by, essentially, angling the flow of air to create higher pressure below the wing and lower pressure above it you can get more lift than the thrust you put in. Of course you need to spend some energy redirecting the airflow, which results in drag losses. Maximizing the lift while minimizing the drag is the major goal in wing aerodynamics, which gets complex, but the efficiency can be found simply enough in the ratio of lift to drag (L/D ratio). The Boeing 767, a relatively modern design, has an L/D ratio of around 18. This means the plane has losses of about 5.6% of the gravitational force, which is actually in the same ballpark as a car. L/D ratios top out at around 60, which is actually more efficient that a wheeled car, but there are usually some undesirable tradeoffs.
Unfortunately, the single greatest factor in wing design is probably area. While a 767 can afford 283.3 square meters of it, a vehicle designed for personal transport probably can't, even when scaling the design down proportionately. Helicopter main rotors are just small wings (with angular rather than linear motion) and may offer a more reasonable expectation of a hypothetical flying car's efficiency. According to Principles of Helicopter Aerodynamics By J. Gordon Leishman, an L/D ratio of 4.5 is typical in forward flight. That's around 22% losses, which means you would need around seven times the fuel of an earth bound car of the same mass.
Of course, flight does offer some advantages for fuel economy: you can generally take a more direct, shorter route, there is no wasteful starting and stopping or idling, and the engine can be kept at a relatively constant and efficient cruising speed.
A flying car based transport system could work, at least in theory. Fuel efficiency would be on the same order of magnitude as the current transportation system and infrastructure costs could be substantially less, pushing an economics argument onto the fringes of plausibility. With that said, it's not going to happen anytime soon. Vehicle complexity would be higher and, as a result, vehicle cost would go up and reliability and safety would go down. Fuel consumption would increase, if perhaps only marginally. We would be better served improving the safety and efficiency of the existing system than developing a new and, admittedly, cooler way of killing ourselves.
What I'd personally really like to see is an autonomous road network. Handing the wheel over to computers could eliminate most of the starting/stopping/idling losses while cutting into the 40,000+ annual traffic fatalities in the US. It's a goal that isn't too far from out present technology. That is some science fiction that I'd like to see become a reality.
*A note about maglev trains, which you mentioned: Magnetic levitation trains try to get the best of both worlds. They leave the track and friction behind, but only by a very small amount, generally less than a couple of centimeters. As a result, the dynamics are quite at bit different than atmospheric flight and my imperfect understanding is that the train rides a cushion of compressed air trapped in the thin gap between it and the ground with very low drag/high efficiency. This site claims fuel economy of 2.2 liters/100 passenger-km = ~110 passenger-miles per gallon at 400 km/h and 1.1 liters/100 passenger-km = ~210 passenger-miles per gallon. By way of comparison a Boeing 767 gets around 58 passenger-miles per gallon.
quote:
ORIGINAL: eyesopened
Did not the Wright brothers use a bicycle to obtain the speed for lift?
No. While Wilbur and Oliver had a background in bicycles, their Flyer used an internal combustion engine. As I remember, it was one of the more difficult design problems as none of the existing engines provided a high enough power/mass ratio. They had to get one custom built. It wasn't until the 1970's that materials advanced to the point that a (barely) workable bicycle powered plane was built.
With regard to the original post (to which I feel somewhat obligated to respond after this digression), I don't really think the government is holding that much back. After all, they draw employees from the same talent pool as the private sector, with the same skills and knowledge. What the government does have is the ability to throw a ton of money at a problem with no requirement for monetary return. The result tends to be systems that integrate existing state of the art technology well (sometimes, at least) and some nice evolutionary improvements to the state of the art. When breakthroughs do occur, the concept - if not all the details - tend to make it out in relatively short order. I agree with samboct that the government doesn't have the competence to simultaneously use and hide the existence of a new technology for long.
-shallowdeep
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I'm still waiting for the flying car a Scholastic Reader promised me in 5 years... when I was in 3rd grade.
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