Wednesday, October 19, 2005


More on Electric Aircraft

Back in August, I asked if anyone knew of any electric aircraft developments as I suspected the concept was just a "green fantasy dreamed up by bureaucrats" rather than realistic alternative to conventional aircraft propulsion. It turns out that most "electric aircraft" discussion centers around fuel cell powered aircraft, and they suffer from the same problems as fuel cell powered automobile proposals. Fuel is one of the big issues, since no one has yet perfected an acceptable way to handle and store hydrogen fuel for everyday use or a good fuel cell that can run on easily transported and stored hydrocarbon fuels. Well, the first successful civil aviation use of electric aircraft may not use fuel cells at all. Perhaps it will use batteries.

I stumbled across another electric self-launched sailplane design called the Silent 2, and this one looks ready for the market. For a normal powered airplane, you couldn't store enough energy in batteries to make it a practical power source, but it would seem to be just fine for self-launching a sailplane. I think the key factor in the success of this will be getting the purchase and operation costs (particularly battery costs) lower than the costs of a similar engine powered self-launching sailplane.

If the nano-composite thin film solar panels I discussed earlier this week can be made cheap, efficient, durable, and light enough then coating the upper surfaces of battery powered electric sailplanes would be a great application for them. A self-launching sailplane that could "refuel" itself by recharging its batteries from solar power would be useful enough for cross country flights that buyers might be willing to pay extra for it.

A normal civil aviation airplane couldn't get enough energy from the sunlight hitting its wings to make a difference, but a sailplane might. The Army hopes to get the solar panel technology cheap enough to build into shelters at power outputs of 30 Watts/lb. The battery capacity of the Silent 2 electric self-launching sailplane is around 4 kiloWatt-hours so only 34 lbs. of these solar panels, which could generate 1 kiloWatt, should be enough to completely recharge the sailplane's batteries in 4 hours. I don't see any data on what the efficiency or areal density of the solar panels are so I don't know for sure if there is enough surface area on the glider for that much solar panel. The Silent 2 has a wing area of 8.8 square meters (about 95 square feet in real units). At a typical solar power density of 1.3 kiloWatts/m^2 that would give over 11 kiloWatts of solar power falling on those wings. If these nano-composite thin film solar panels can get over 10% efficiency, then covering the Silent 2's wings with them could produce at least 1 kiloWatt of power when they are in typical sunlight. Presumably if the Army is looking at coating shelters with them, they must be durable enough that applying them to sailplane wings wouldn't create a maintenance problem. In the end it all comes down to cost, but I wouldn't be suprised to see solar powered electric self-launching sailplanes become common within a decade.

Charging the batteries by windmilling the prop might be faster on days with good thermals.
Here is an article from Design News on a solar-electric aircraft that was (remotely) piloted for 48 straight hours.
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