Originally Posted by
Backpacker
In addition to this, somebody mentioned that if you're sitting more or less upright, and are properly strapped in, your body can sustain 50G in the horizontal, but only 15G or so in the vertical, because your spine is relatively weak in compression. I haven't done the sums (and they would depend on the size of the horizontal and vertical crumple zone anyway) but based on this it may well be that crashing horizontally into something at stall speed is more survivable than crashing vertically into something under a partially inflated chute. (But I'll take a fully inflated chute over an unrecovered spiral dive any day!)
What forces are survivable are subject to experimentation. What forces are experienced in an airplane crash are empirical.
And you don't really want to be in a crash where the forces exceed survivable limits.
First, consider crashing at stall speed. For calculation purposes, the Cirrus SR22 stall speed is 59 knots. FYI, observable airspeeds in a Cirrus spin have been consistently 100 knots and spiral descents are often in excess of 180 knots. In comparison, under canopy the vertical descent is 17 knots (1700 fpm, 20 mph, equivalent of a 13 foot drop).
If you hit something, the energy dissipated is related by the square of the velocity. Hence, the impact at 59 knots stall speed
has 12 times more energy to dissipate. (FYI, those other speeds have 34 and 112 times more energy.)
Ah, the claim is made that the human body can withstand greater horizontal forces than vertical forces. Except, ground impacts are rarely one-dimensional and consequently the human body is subjected to significant forces in other dimensions. And there is the problem of restraints failing at higher than specified forces. The current restraint specification is 26G forward deceleration.
For me, I would much, much prefer to activate the CAPS parachute a few seconds earlier and descend vertically under a fully inflated parachute than risk a higher energy impact.
Cheers
Rick