MadSci Network: Physics |
Hi Tom, Good question. Your assumption is essentially correct. Air density decreases at higher altitudes. This allows planes to fly faster while using less fuel. You are correct that there is less air, so there are limits to how high a plane can fly with a given wing area. In general, high altitude flight requires large wing area or high speed, both of which deliver greater lift. Very light, slow flying experimental aircraft such as the NASA pathfinder can fly as high as 71,000 feet at only 25 miles per hour! You can read more about it here: http://www.nasa.gov/centers/dryden/news/FactSheets/FS-034-DFRC.html On the opposite end, the SR-71 Blackbird has relatively small wings, but flies very fast at altitudes above 60,000 feet. The SR-71 replaced the older U2 which flew much slower, but had long wings like a glider. Of course, there comes a point where there is so little air that no lift is generated and no conventional plane can fly. The space shuttle is specially designed to handle these conditions. The additional benefit to flying high is the ability to fly above the weather, which makes the ride more comfortable for passengers.
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