Possibly a silly question but do people fill modelling balloons with helium? Aeroplanes would look quite cool with a few pieces of cotton to anchor it, butterflies too maybe?!

I believe there is too much latex for the amount of helium that can go in them - ie they are too heavy.  Someone may correct me, but I think that 11" round is the smallest balloon that can float with helium.

But you are right, it would be cool!

5" rounds do float but not for long :-)

If you fully inflate a 350 with helium it just about floats, as Rob said a 5" round will float for a very small amount of time as well.
Maybe make the model and then hang some white balloons as clouds for the plane to float.

Helium in modelling balloons doesn't work even 646 don't float for long and take way too much hi float for it to be cost effective. Worse still the helium escapes from them due to the small molecules and they deflate quickly. When I make planes I hang them from magnets if there is a suspended ceiling