BOAC,
I thought when you were on the Bona Jet the Montgolfier brothers were hanging out in Stamford. Is this why you know about balloons
General readers,
I too tried to exercise my amateur test pilot skills (all in the interest of learning my aircraft's capabilities you understand) and see when the donk ran out of puff. In the scenario suggested by SammySU (i.e. nil wind) I VTO'd and kept going up until I didn't any more (if that makes any sense). The mighty 104 engined FRS1 got to about 2500' before it decided to stop climbing. A similar experiment in a II+ got me to a decent shade over about 4000'. I think the latter would have been more impressive but I seem to recall the temperature being pretty high.
I'm not the owner of an ETPS strain guage, so don't know if these experiments (sorry envelope explorations) would be considered valid, but it might be enough for the original poster's question for the man down the pub.