Solar planes are not the future of flight

Read that second page, specifically the "Overall air direction is changed to go down" in the last two pictures.

You can also see this from conservation of momentum; it requires that, if something goes up, something else must come down. With an entire airplane going up, there isn't much else than air to come down (exhaust gases are another option, but then, you are talking rocket, not airplane).

And yes, gravity means the (airplane plus surrounding air) system isn't closed, but its effects only make matters worse; airplanes have to throw down air even to maintain level flight.

An airfoil (wing) creates lift when air flows from the leading edge to the trailing edge, and the speed of this airflow is one of the factors which determine how much lift is generated. A strong tailwind will interrupt the airflow, induce turbulence and decrease lift. Thus, an airfoil moving into a headwind will generate greater lift than one moving through a tailwind.

> Lighter than air solid

Is this even possible at normal temperature? I can't imagine this at 1 atm. and room temperature...

after some googl-ing:

http://en.wikipedia.org/wiki/Aerogel

The amount of lift you would get from such a small amount of helium would be practically negligible compared to the weight of the aircraft. Lighter-than-air craft require an enormous volume of helium/hydrogen - due to the cross-section of such a craft, the speed of travel is severely limited by drag. Basically, an aircraft uses wings OR lighter-than-air gas - it can't use both.

Because now you need to make the wings airtight (no, not just air-tight but helium-tight) which adds more weight than the lighter-than-air material is offsetting.

Well one of the challenges of both helium and hydrogen is that they are really very small molecules, and they tend to slip out through the smallest gaps. This is why the metalized aluminum skin was on the Hindenberg in the first place, and ultimately doomed it.

Lighter than air solids have the issue that if you remove the air from the spaces in those solids and try to hold it out you create a pressure differential that crushes them.

There was some speculation that given an ability to precisely manipulate carbon you could build dodecahedrons out of pentagons of diamond in a vacuum and then when you removed them from the vacuum they would have a net density lower than that of air, but the density ratio is so small (assuming you could build them) that you'r talking about a massive airship that ascends to maybe 3,000' on a cold day.

Exploiting air density is a challenging path that has not had much success of late.

Yes. The batteries don't generate any power, they just store it. The Solar Impulse has batteries -- that's what lets it fly at night for example. See [1] for a bit more detail on how energy is managed at night.

[1] http://solarimpulse.com/timeline/view/6437

Solar panels add weight, offer no structural support, complicate maintenance, and would need to to have their transparent cover hardened somewhat to deal with the sort of beating that an airplane skin takes. Not impossible, but you would probably see it first on the top of the fuselage rather than the wings and they would still only offer a small boost to the onboard APU. It is possible that by covering the skin the panels would also make it harder to inspect the pressure envelope that is the real skin of the plane underneath the panels for wear and fatigue. The engines are already some very efficient generators, so the panels would have to be very good to justify their added weight.