That a really interesting question. Here’s my answer:
1. Fully contained tanker (closed) – the tanker will not get lighter, because the force needed to keep the aircraft in the air will be reacted by the tanker – “for every force applied there will be an equal and opposite (reactive) force” (Newton’s 3rd law of motion) The force of the air jet will equal the mass x accel due to gravity of the aircarft – even slightly more if it is accelerating upwards.
2. Open Container – (roof removed) – if all the force of the downward airjet hits the deck of the tanker, then its the same as the above i.e. the tanker will not be lighter. As the aircraft moves away from the tanker or upwards, and the force is directed towards the sea, then the tanker will become lighter as there will be less reaction force.
Wow! Brain taxing question for a Monday morning! I’d go with when the container has it’s lid on, it wouldn’t get lighter as the jump jets are still inside of the container – they’re just applying their forces in a different way to when they were on the ground. With the lid off, I’d go with the tanker getting lighter as they will no longer be inside the container. This would be like if a plane flew over your head you wouldn’t suddenly feel a massive weight on your shoulders!
After asking around the engineers here agree with Keith and Nicola, that with a closed container it will stay the same weight. Within a closed system the weight cant change. A TV program called Mythbusters actually answered this with pigeons flying inside a lorry! The mass of the lorry didnt change whether the pigeons were flying or not.