Fly It to the Ground

"Peter Duniho" wrote:
9G deceleration is 32 feet/s/s times 9, or almost 300 feet/s/s. 1 mph
is about 1.5 f/s. To lose 50 mph (roughly 75 f/s) at 9G would take
75/300 = 1/4 second. In 1/4 second, constant deceleration from 75
f/s, you'd travel (75/2)/4 feet, or 9 feet.

No need to compute the deceleration time (intermediate result) if one
applies a little algebra (or calculus):

S = V*V/(2*a)

S = Distance traveled.
V = Initial velocity.
a = Deceleration.

The mistakes people seem to make normally come from unit conversion (e.g.
mph to fps).

Why do you need a chart?

I don't. And even if I did, it was supplied (and I have the math to
test it). But the source of the chart (which is what I asked about)
could have other background information which would be of interest.

That said, I'd think 9Gs would
be survivable vertically as well...

Not as... er... "comfortably" as horizontally.

Jose
--
"Never trust anything that can think for itself, if you can't see where
it keeps its brain." (chapter 10 of book 3 - Harry Potter).
for Email, make the obvious change in the address.

"Jose" wrote in message
t...
Not as... er... "comfortably" as horizontally.

No, of course not. I'm just pointing out that 9Gs vertical isn't all that
bad either, as crashes go.

Off the top of my head, don't the current Part 23 regulations require seats
that can sustain over 20Gs?

"Jim Logajan" wrote in message
.. .
No need to compute the deceleration time (intermediate result) if one
applies a little algebra (or calculus):

Duh. Of course, that's the whole point of algebra. However, knowing the
"full" solution makes it easier to understand, at least for some people.
That's why I posted it.

Of course, I put "full" in quotes, because I *did* leave out the calculus
step (the formula for determining distance traveled during a period of
acceleration, positive or negative).

Pete

EridanMan wrote:
FWIW, I found the actual numbers here, they were actually in the FAA
"airplane fliers handbook"

For a Constant, 9G deceleration (as I mentioned, easily survivable)-

at 50MPH, 9.4 feet
at 75MPH, 18.8 feet
at 100MPH, 37.6 feet

The complication is that it isn't the average acceleration of the
structure that matters, it is the acceleration of various body parts
that matters. Upon first impact of the airplane, the body is
experiencing no deceleration at all. Once you move forward the seat
belt begins to tighten and then you being to gradually decelerate. If
you have shoulder harnesses, you hopefully don't hit any structure of
the airplane, but you will still experience peak deceleration well above
what the airplane structure is experiencing since you started
decelerating later, but you will stop at roughly the same time as the
airplance. So the shape of the acceleration vs. time curves are vastly
different for the airplane structure than for the body inside.

The really bad part is when you don't have a shoulder harness and your
head hits the instrument panel. You will now experience an impact force
MANY times greater than 9G.


Matt

No, of course not. I'm just pointing out that 9Gs vertical isn't all that
bad either, as crashes go.

But its a question of whats more likely to give way enough to allow you
a 9G deceleration, the hard ground, or a bunch of softer stuff along
the ground

"Peter Duniho" wrote:
"Jim Logajan" wrote in message
.. .
No need to compute the deceleration time (intermediate result) if one
applies a little algebra (or calculus):

Duh. Of course, that's the whole point of algebra. However, knowing
the "full" solution makes it easier to understand, at least for some
people. That's why I posted it.

Your original post was fine and I should state that my post was intended
merely to expand on yours. I have often computed many intermediate results
for one-off computations, just as you did in your previous post. I normally
"back-up" and work out a single closed-form equation when I find myself
needing to make several computations. I suspect many people with technical
training do something similar.

"Dave" wrote in message
One of my instructors STRONGLY suggested that I "do not try to save
an aircraft that was trying to kill me"....

Hmmm.....
If the airplane doesn't get hurt, do the passengers?

The problem I have with Mr. Boatright's assumption that the field is a
better choice is that fields may not provide steady and gradual deceleration
after impact. For the same reason, it is better to land gear-up on the
runway than in the grass next to the runway. Landing on the road in Mr.
Boatright's scenario has its own problems, but for a pilot in unfamiliar
territory, the road shouldn't be ruled out automatically.

D.

"Capt.Doug" wrote in message
...
"Dave" wrote in message
One of my instructors STRONGLY suggested that I "do not try to save
an aircraft that was trying to kill me"....

Hmmm.....
If the airplane doesn't get hurt, do the passengers?

The problem I have with Mr. Boatright's assumption that the field is a
better choice is that fields may not provide steady and gradual
deceleration
after impact. For the same reason, it is better to land gear-up on the
runway than in the grass next to the runway. Landing on the road in Mr.
Boatright's scenario has its own problems, but for a pilot in unfamiliar
territory, the road shouldn't be ruled out automatically.

You've mischaracterized what I wrote. I clearly stated that you're better
off in a field if the road has vehicular traffic or if you don't know the
road to be free of wires. Those things lead to aircraft hitting the ground
out of control after the pilot stalls or collides with an obstacle. A field
is a better choice because it reduces the odds of an out of control impact.
Presumably, I should add "most of the time", because you appear to be
holding me to a standard of 100% certainty.

The idea is to work with probabilties, and you're probably better off in a
field than on a public road if survival is your primary consideration.

KB



D.

Off the top of my head, don't the current Part 23 regulations require seats
that can sustain over 20Gs?

Dunno. But I do remember a safety seminar I attended some time back
where I learned that aircraft seat belts only need to withstand about
two gs, and automotive seatbelts are required to withstand something
like seven. It is illegal to replace an aircraft seat belt with an
automotive one.

Go figure.

Jose
--
"Never trust anything that can think for itself, if you can't see where
it keeps its brain." (chapter 10 of book 3 - Harry Potter).
for Email, make the obvious change in the address.