Slashdot Mirror
Experimental Fuel-Cell Airplane's 2nd NASA Test
js7a writes "The Helios prototype, holding the sustained flight altitude record, having unsuccessfully completed its first test with a fuel cell, is almost ready for its first night flight this Thursday, Friday, or Saturday. Helios uses solar panels for flight with payloads over 600 lbs. planned for up to six months using regenerative electrolysis. What good is a plane that will fly for six months without refueling? Besides providing a UAV alternative to AWACS, they can improve internet connectivity."
I wonder about the engineering tradeoffs. The electrolyzer and the tanks add weight, compared to the other approach to nighttime operation of solar powered aircraft -- drift downward. There's quite a bit of stored energy in an airplane that's been lifted to 100,000 feet. At low speed with a good glide ratio, you can coast until dawn.
If you incur the weight penalty to stay at 100,000 feet, you get continuous radio coverage over a ~500 mile radius. If I were Indonesia and trying to deploy broadband over hundreds of islands I'd be really interested in this.
That, and thermals do go up high, but not up to 100,000 feet. And a plane that stays at 1000 feet isn't nearly as useful as one that stays at 100,000 feet.
Slope soaring would make more sense, as if you're in a coastal area is usually always windy and the wind direction is pretty well known, but this would require an appropriate slope and it's hard to gain signifigant altitude unless you have a full mountain for your slope.
You could probably keep a plane up indefinately over a constant source of massive heat like a nuclear power plant or volcano, but that wouldn't be very useful.
I believe that the world record duration for a R/C glider is 30 hours or so -- I don't know the specifics, but I'll bet this was on a slope. After 30 hours, he probably either got tired or his receiver battery died :)
The BBC are reporting on a british version designed to go higher still. http://news.bbc.co.uk/1/hi/sci/tech/3016082.stm This will go to altitudes around low earth orbit. At these heights, surely gravity is less of an issue and the night-time drift downwards would be less significant, easily made up for when the sun rises (there would be a longer day high up, too). The ground coverage at such a height would be quite dramatic, the entire UK can be covered by one 'plane. What are the hazards from weather that high? Apart from temperature exrtemes, do high winds happen?
surely gravity is less of an issue and the night-time drift downwards would be less significant
Wrong. Gravity still has about the same strength there as it does with your feet securely on the ground. The difference is only negligible. The only reason that satellites and other orbiting objects stay in space is because they are moving very quickly. Imagine this; An object traveling so fast that it is trying to leave earth orbit but it doesn't quite have enough energy to overcome the force of gravity and fly off into space. So it is constantly falling at the same rate, it's just balanced at the perfect position to not fly off and not to fall. This is called orbit. It's not because at some magic spot gravity ceases to be as effective. Look at the moon. Cheers!
I feel compelled to add another comment here. Everyone is focusing on gravity here as the big factor as to this things glide time. Although this is obviously an issue in the big picture it's not the real issue. Didn't your teachers ever show you the experiments where people would drop a feather and a bowling ball in a vacuum and they would both hit the ground at the same time? Does this mean anything to anyone. I can hear the light bulbs go off now.
The real issue is DRAG a.k.a. FRICTION. If there were no or negligible drag this thing could fly forever. It's the drag that slows it down and causes it to drift back to earth. So what do you need to increase your speed? Propellers. And what drives the propellers? Fuel cells. What these scientists are doing is trying to make their fuel cells more efficient so they can over come their ultimate enemy, drag. Just a little insight for everyone. Cheers again!
There's more to it than that. Once you're above the troposphere, you get to use the ionosphere as a reflector with much less penalty. That's how over-the-horizon radars work.
"The plane would be geostationary so would be moving at quite some speed. Significantly faster than the American plane."
From http://news.bbc.co.uk/1/hi/sci/tech/3016082.stm "At the peak altitude, Zephyr should be making three circuits every two minutes, travelling at a speed of 70 metres per second (155 mph)."
Yes the Zephyr is significantly faster than the Helios which reaches astounding speeds of 30 MPH.
However, none of this makes gravity any less significant. Please clarify your statement. After all
F=GMm/R^2 where GM/R^2 is commonly refered to as the acceleration due to gravity.
brrrrrrrrrppp 'Ey Homer...Why don't girls like me?
For serious high altitude sail plane action you need rotors...
They are basically the eddys in the wakes of mountains, and they are what is used to set altitude and duration records in gliders. I only flew in Illinois, so I never experienced one, but I had a friend in my glider club who was from Germany (a grad student at U of I) who had been at 20,000 for 6 hours in a rotor before.
I think (don't quote me) the unpowered altitude record is in the neighborhood of 60,000 ft.
"I'll have a Guinness, no wait, make that a Coors Light" -Grad student I work with, who shall remain anonymous...