and also, any of you lucky people who can do flips on the ground;
Meaning, not on bocks? I can answer that one. =)
and also, any of you lucky people who can do flips on the ground; How high can you jump normally? (keeping your legs straight)
aka. Sargent Jump Test or Vertical Jump Test - I have a pretty decent vertical jump, and obviously the higher yours is, the higher your back-tuck can potentially be. But to be honest, if you can jump 2ft (about 60cm) you can land a back tuck... which means most people
could land a back-tuck. I'm going with back-tucks because that's the one that best relates to a static vertical jump.
how long did it take you to learn them on the ground / how did you learn / did you/do you do any gymnastics?
The first back flip I ever did was off of a 6-7ft climbing frame in the park. It only took two attempts because after the first bail where I did a 180 twist and landed hard on the grass in a seated position I thought ouch... better land the next one. So I got straight back up, tried again and landed it. Then did a bunch more to make sure I had it. At that height it was a simple case of whipping backwards and spotting the landing. I'd say it took about half an hour.
On flat ground it was harder and took longer to land. I'd say I was trying for at least an hour and a half while my friends were in Tescos cafe. I had a friend spot me twice. Then tried solo over and over again until I landed it. This was on bark. Nice and soft. I bailed lots! lol. But when I finally got it I felt awesome! The great thing about learning outside is you learn fast! You soon become tired of landing on your face on bark or grass, so you try your hardest every time. =)
As others have said. It's a simple case of jumping straight up as high as you can (Do not whip back) and tuck your knees into your chest as tight as you can. Then at about the half way point you can spot your landing in preparation to land. Though I doubt you'll be untucking for a while. Whislt learning you'll probably need to stay tucked all the way round. In a perfect back tuck, you don't look back or whip back. Throwing your arms with your shoulders back - then bringing your knees up is enough to cause the rotation.
At that time I hadn't had any gymnastic experience. Nor did I have a trampoline, so you're already a step ahead. If you have access to a gym or a crashmat go for it! But the point of my long-winded explination was - if you don't have access to such resources don't let that hold you back. Just get a friend to spot you and go for it.
I now go to a gym and the squidgey floor is much friendlier to my even squidgier face! lol. So I do recommend it if you can get it.
The same principles apply on bocks. Only you'll eventually find whips and layouts much easier on them, than on the floor.