On Aug 5, 6:07*pm, tommytoyz wrote:

Interesting points, here's a few observations:

...if it's lighter, it'll take less energy to push/pull it and
otherwise less force will be applied to it.

This here is one of the big issues encountered when designing
crashworthiness into airplanes and small, lightweight cars. Since it
takes less energy to deccelerate the lightweight vehicle, it
decelerates more quickly in an impact. The trouble with that is that
the vehicle occupants also get decelerated more quickly, and so
experience greater forces in the impact.

Of course, what has worked well for small cars are active crash
protection systems such as airbags that help distribute the
deceleration forces more evenly over the more vulnerable parts of the
body. There has been some work done to develop similar systems for
small aircraft, but I think we're a long ways away from seeing them in
gliders.

Look how robust model a/c are. They seem more crash worthy than
the real ones.

That is certainly my observation as well, but unfortunately I don't
think that it tells us much about the problem of crashworthiness of
person-carrying vehicles. At issue is that many parts of the model are
perfectly happy to resist a hundred G of deceleration or more without
breaking, and those that aren't absorb a huge amount of energy while
they break. The result is a relatively simple repair job and resumed
flight.

With person-carrying vehicles, I think you are limited to about 40g if
you don't want to hurt the occupants badly, and about 60g if you don't
want to kill them outright. What's important to keep in mind (and is
too easy to forget) is that you do not care whether the aircraft gets
broken. Really. Crunch all you want, we'll make more. In fact, you do
want the airplane to break, and break in such a way as to absorb
energy in the crushing and tearing of structure. Absorbing energy
reduces the peak and overall deceleration applied to the occupants,
and it is the occupants you really want to save, not the aircraft.

That is one of the huge issues with crashworthiness and carbon
structures. Carbon has great strength and stiffness by almost any
metric. What it doesn't do very well is absorb energy. As you load it
up towards its breaking point, it stores some energy in elastic
deformation. But then when it reaches its ultimate stress, it breaks
quickly and is is subsequently not available to absorb any more
energy.

Steel structures, on the other hand, load up and then start to
crumple, all the while absorbing huge amounts of energy in the
propagation of plastic deformation. That's why I likes me my Volvos so
much.

Obviously, the lessons of Formula and Indy car chassis design show us
that it is possible and practical to design and build crashworthiness
into carbon structures. However, the lessons seem to be to use lots
and lots of carbon, and include as much crumple volume as practical
and also to add many frangible bits such as suspension mountings to
absorb energy as they tear away. Both of those are somewhat
impractical in sailplanes.

Also of note, many auto racing classes impose a minimum weight that
allows generous margins for crashworthiness structure. We don't have
that sort of thing in soaring contests, and there hasn't been much
call for it, but there may come a day when that changes.

The trend in European sailplane crashworthiness seems to have been to
supplement the primary cockpit structure with structural elements of
very limber fibers of aramid (such as Kevlar) or advanced
polyethylenes such as Spectra or Dyneema. After the carbon gives up
the ghost, these very stretchy fibers absorb a lot of energy as they
load up into their plastic range, stretch out, and tear free of the
resin matrix. The penalty for such a system seems to be unavoidable
extra weight.

I think there is an exponential factor here - the heavier the plane's
structure is, the stronger the wing has to be, making it heavier
still, etc...and true in reverse.

Yes, that is certainly the case, no argument there.

Imagine if a structure like Diana-2 were also made of prepegs? Would
it be lighter still by a significant amount?

It could be so, but I rather doubt it is worth the effort. It could
certainly make for a stronger structure. However, much of a
sailplane's structure is bounded by stiffness considerations, not
strength. And I think that prepregs offer only a relatively modest
improvement in stiffness and I think no particular improvement in
energy absorption.

Thanks, Bob K.
http://www.hpaircraft.com/hp-24