The original note, as written at the time
What if we filled large propellers with a light gas like helium in order to achieve flight at lower energy cost?
Ongoing development
Large propellers filled with a light gas such as helium, to achieve flight at lower energy cost.
The physics is favourable and it is the same trade as an airship. Buoyancy does not lift the aircraft, it removes weight, and every kilogram removed is a kilogram of lift the rotor does not have to produce. A rotor blade is a structural element that must be light anyway, and a hollow blade can be filled with something lighter than air at no structural cost.
The contribution is modest and that is the honest point. Helium gives about a kilogram of lift per cubic metre, and a blade is not a large volume. So the effect is a few percent of the rotor’s own weight — real, and much less dramatic than the idea sounds.
Where it becomes interesting is the very large rotor, where blade weight is the limiting design factor. A rotor the size of a football pitch has a structural problem that is dominated by its own mass, and a few percent there is not a few percent of the aircraft; it is a change in what can be built.
The risk is damage tolerance. A gas-filled blade that is punctured loses its lift and, worse, loses the balance of a spinning disc, which is a catastrophic failure mode. Any real version needs compartmentalisation and a leak-detection system.
Airships lost to hydrogen and survived on helium. A blade has the same trade and a much smaller margin for error.
Take the few percent, but only where a puncture cannot kill the disc.