Rob Bamber wrote:
Does the Airlander certainly use ballonets?
Yes.
Hybrid Hopes: An Inside Look At The Airlander 10 Airship | Technology content from Aviation Week
The compartments fore and aft and either side also house ballonets, or airbags, that are used for pressure control of the vehicle. The ballonets are inflated with air on the ground, reducing the volume available for the lifting gas, making it denser. Because air is also denser than the lifting gas, inflating the ballonet reduces the overall lift while deflating it increases lift. In this way, the ballonet helps to adjust the lift as required. There is also a septum diaphragm in the ballonet compartment to prevent mixing of the helium in the upper section and air in the lower part.
“As you go up in altitude, the air wants to expand and you can’t cope with trying to contain it with the strength of the hull, so the helium pushes down on the ballonets and pushes air out through valves,” says Durham. “When you come back down the helium wants to contract, so the ship would go soggy unless you push air back into the ballonets. So each ballonet has a big valve and fan in it so can vent air in and out and run the ship at a constant delta p. It’s the one system on the vehicle that’s got no parallel to any other aircraft or helicopter,” he adds.
I would have thought it more practical to use pumps to transfer helium to a high pressure storage tank when reducing buoyancy.
Rob Bamber, The Aeros Aeroscraft, uses a system along the lines of what you describe.
Technology copy - Aeros