Chimbu,
I apologise for offending you, and I certainly defer to your vast experience of bush-flying which is more than I will ever log. However, even if you dislike and dismiss my approach, you cannot do the same with physics. Physics works the same in PNG as in SE England, and therefore so do airplanes.
You give an example where you do not climb from the runway, but you push down! Hair-raising and an entertaining tale, but not relevant to what most of us do, or this example either.
Why didn't you crash? I suspect you used the benefit of experience so your Vr was actually a lot lower than what the books said (39knts example). It was still a Vr, and if you weren't at 71% of it by half way down the runway, and if you were accelerating at a constant rate, you would not make that Vr by the end. That's not a rule I made up, and that's the rule I am pushing.
More interestingly. I also suspect you hopped off the ground well prior to published Vr, used ground effect to increase lift and reduce drag, and accelerated faster than you would have done on the ground. This is a very effective way of getting off a field which is shorter than you like. Of course, you have to get out of ground effect at some stage. You obviously do this with great effect by diving off a cliff. It doesn't always work that way, most runways don't end like that, and sometimes you cannot climb out and you hit something. Better to stop
Hey, just trying to throw out some numbers here as a safety guide. Take it or leave it.
bookworm,
Show me a light aircraft where the stopping distance is greater than the starting distance. POH figures will show that you need less runway to stop than go. So if start at zero, and you brake properly at 50% of the runway length, you will always reach zero before the end.
If somebody wants to think about ground effect then that's cool. I do not recommend you plan to use ground effect anywhere except above a runway, unless you know what you are doing and are bush-flying in PNG, where you might have no choice.
F3G,
Strap a decent airspeed indicator on it, but remember it will stop your bulldog going into warp drive. You just have one less piece of data to assist you in safe decision making. You increase your risk of running off the end of the runway if you fly a plane with an ASI not calibrated at takeoff roll speeds.