Science and Exploration

[Corkey]

Originally Posted by dreadgeek

The idea of a solar airplane is really beautiful! I'll have to watch the show. As far as your idea, unfortunately, it's a perpetual motion machine. Because there's friction, you will *always* lose some amount of energy whenever you do work--as it is defined in physics. For our purposes here, work means "keeping an airplane flying". No matter how efficient we make the propellers or how efficient we make the solar panels, the First and Second Laws of Thermodynamics absolutely prevent the kind of thing you're describing.

I had to think about it for a while and toss the idea around with my wife but here's why it won't work.

Let's say it takes 50 units of energy to move a plane with enough forward motion to stay flying and the solar panels generate 100 units of energy. The excess can go into the batteries. If I understand what you're describing, all 100 units from the solar panels go to the propellers but the motion of the propellers generates energy. That would mean that you would get more energy out of a system than you put in. The problem is friction.

Even if we had *perfect* solar cells that got 100% efficiency out of solar capture and could build the propeller mechanism in such a manner that it was and efficient as is possible, there would still be friction of the propellers hitting the air. That means that it takes a small bit of energy to overcome that friction. That energy is lost.

Your idea is beautiful and elegant but the universe simply doesn't allow it.

Cheers
Aj

Thanks AJ, I was also thinking that the kinetic energy only had to last until the solar cells could re power the batteries, so even if say 5% kinetic energy were lost to friction, 95% of the energy would still remain to energize the batteries/hour of darkness. Now I'm not math minded, but it seems that 5% x 9 or 10/hours of darkness would still mean that there would be at least enough left to power the batteries, and then if solar and kinetic were used in unison, would not the loss of friction be less and therefore power more of the batteries than solar alone? Sorry to be a pest but I'm so removed from physics by age it's a bit daunting.