Some useful data can be found in Ref. [9], regarding the GPU-3,
an engine that has been tested with both helium and hydrogen. The
optimisation of h is reported in Fig. 4a. It shows that h varies in
proportion to the rotation speed. However, the variation of h does
not perform in the same way for both helium and hydrogen. The
reduction of h is lower for the hydrogen engine than the helium
one. The engine that is pressurised with H2 feels less the effect of
the reduction in useful power, caused by the increase in pressure
losses, which are proportional to the rotation speed.