The volume in an airplane's wings is negligible insofar as providing buoyancy lift from helium is concerned. Beyond that helium is a gas that rapidly leaks out of almost any container. It is not worth the trouble.

this theory can technically work. though.... a blimp is the only way to make it efficient

I should add that the space inside the wings of most airplanes is not empty. They contain fuel tanks, structural elements, wiring, and control surface actuators. There is actually little wasted space in most wings.

Additionally, though the tail is 'empty' on most small airplanes, the stability of the airplane requires that the tail-wings provide 'down-force'--if both wings were lifting, then a conventional plane could have difficulty recovering from a stalled main-wing, as the tail would keep lifting.

Filling the fuselage with helium may provide lift (before the helium permiates the skin) but it would be lift behind the center of pressure, where it's detrimental. Also, if you relied on this lift and then it leaked, what would you do?

That's a good point, I didn't figure on leaks. Surely someone could figure that part out. Rubber bladders inside the fuselage? I dunno it's just an idea. Wings could be redesigned to change dynamics of lift. Fuel tanks don't necessarily have to be in the wings. If you need downward pressure on the tail, put the fuel in there.

It certainly is possible to make a hybrid airplane/airship which uses helium to provide a moderate percentage of the lift. They haven't caught on, but I am a supporter of the concept. I mean a moral supporter, I don't have any financial investment.

bbraden, the solution for the two-lifting surface airplane is simply go to a canard layout, which are always both lifting--if the front stalls, it just drops and the plane is flying again.

The reason you can't just move the fuel to the tail is that the back wing provides downforce, but if the front-wing stalls, then the back wing needs to 'let-up'--you can reduce aerodynamic downforce simply by moving the stick forward. You can't reduce weight downforce.

As for rubber bladders in the fuse, you're thinking that we're talking about leaks. Really we mean permiability. The molecules of He are the second smallest--only hydrogen is smaller. They sneak between the atoms of all materials. Note that mylar balloons 'last' much longer than latex balloons--some materials are better, but they all 'leak' in the permiability context.

But like Dwane says, this concept has merit and people have done good work to combine a helicopter with a balloon, but not as much for a plane. Officially called a 'hybrid airship'.

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

Unfortunately, the helicopter thing as been disastrous at least once.

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

But people are still trying it.

http://news.cnet.com/8301-11386_3-9986757-76.html

Also, we seem to be running low on helium.

http://www.bloggingstocks.com/2008/01/07/depletion-of-helium-reserves-threatens-to-ground-nasa-shuttle/