In a hydrogen fueled plane, could part of the fuel load be allowed to become lighter than air to offset or trim for the load? This is inspired by How can adverse maneuverability due to a fuel tank behind the rear pressure bulkhead be mitigated in Airbus ZeroE hydrogen plane? .
I was thinking, with the cryogenically stored hydrogen (and required equipment) being heavy and so far away from the centre of lift of the craft in Airbus's design, it might be possible to trim the aircraft by releasing some of the hydrogen into a chamber where it can be lighter than air, reducing the nett weight of the tail, rather than trimming aerodynamically or with a movable weight.
I'm not good enough to do the math, but I suspect the size of the chamber would have to be colossal to make enough of a difference to matter - with the added bonus of being essentially an explosion waiting to happen.
Would there be a sweet spot for the average temperature of the stored hydrogen that balances the weight (including weight of corresponding equipment) against the volume of fuel stored, or is it going to be a case of coldest/most dense is the most efficient and trim needs to occur the traditional way?
The buoyancy of hydrogen is not as high as expected, and is even lower at high altitude due to the lower ratio of the density of hydrogen and air.
Just for comparison:
The tail tank of the A380 has a capacity of 23698 l.
When this tank is filled with kerosene / air / hydrogen, the content weights:
Substance           | Density [kg/l] | Weight of 23698 l [kg]
--------------------+----------------+-- -----------------
Kerosene            | 0.8            | 18958
Air (sealevel)      | 0.00122        | 28.91
Air (10km)          | 0.00041        | 9.72
Hydrogen (sealevel) | 0.00009        | 2.13
Hydrogen (10km)     | 0.00003        | 0.72

At sealevel, the lift of this tank filled with hydrogen is 28.91kg-2.13kg=26.78kg.
And at altitude, it's just 9.72kg-0.72kg=9.00kg
This is nothing compared to the 18958kg of kerosene, and shows that it makes no sense to fill any volume in an aircraft with hydrogen for trim.