At last a topic I know something about and for the first time ever I find myself in disagreement with John T.
In my opinion the rate of onset of positive g has no effect on G-LOC which I believe depends only on the g you go to and the time you are there – indeed as Proof Reader said earlier – with but one exception that I will discuss at the end.
John T quotes two articles to support his post. The first does not mention rate of onset and even includes the following error “a G-suit squeezes the abs and legs, forcing the blood up toward your cranium to help maintain vision and consciousness”. A g-suit of course does not force the blood up it merely stops the blood descending into the parts of the body that the g-suit covers. This minor error is all the more surprising because in an earlier sentence the author states (correctly) “G-suits work by transferring a force via air-inflated bladders to the lower limbs to increase peripheral vascular resistance, thereby reducing venous pooling.”
The second article quoted by John T does discuss rates of onset (as high as 15 g/sec) but goes on to say that rates of onset are not relevant as there is a delay of 3 to 5 secs before the brain is affected by g. The relevant para being: “Very high G levels can be tolerated if they are not sustained for very long. The brain has a small reserve supply of oxygen dissolved within it. In the absence of blood flow, this will give something like three to five seconds of ongoing brain operation”.
Personally I would have thought that those times were at the top end and I have always thought in terms of 1-2 seconds before I am affected. The trouble with all this is that the effects of rate of g onset on aircrew can only be measured in flight as centrifuges cannot respond sufficiently quickly.
Now the exception I mentioned earlier. Modern aircraft can SUSTAIN levels of 6+g (which was not the case in say the Hunter era for reasons that we don’t need to get sidetracked into here). Indeed in an early Hawk you could hold 6g until you run out of fuel. Such sustained levels of g, if maintained for say 20 secs or more, allow the body chemistry to respond and it does this by increasing one's blood pressure – which of course helps. HOWEVER if following such a period of adaption you suddenly reduce the g to less than one the body kicks in at once and drops the blood pressure off a cliff. So you faint (or G-LOC for the macho). This fainting is not preceded by the normal g-induced effects of loss of peripheral vision, grey out and black out (the eyes being particularly susceptible to lack of the optimum levels of oxygen) that all occur before actual loss of consciousness in the situation where you start a pull to high g levels from normal flight. Clearly passing out without the normal warnings is very dangerous for a pilot.
If readers feel I am getting in to the nit-picking/pedantic level of comment with all this I apologise, but I do hate half truths or even 90% truths for that matter - especially on this forum.
JF