[Apologies if this is a repeat post. Google acted like it ate the first
one I composed. Bad Google! No biscuit!]

Earlier, Eric Greenwell wrote:

Let me quibble with Bob a bit: a
"safety cockpit" is more than just more
structure to make it stronger...

Please do, Eric. We've come to expect no less from you!

But seriously, thanks for that list of basic safety items. I agree that
they all should be in any modern cockpit. And I agree that none of them
carries a substantial weight penalty. But beyond those basic elements,
crashworthiness is most effectively achieved with two basic things:

* Stuff that absorbs energy as it progressively crushes

* Space for the stuff to crush into before reaching the pilot's vital
bits

Stuff adds weight, and space adds volume, area, and drag. All of those
will have deleterious effect on perfomance.

Furthermore, I definitely disagree with the characterization of
crashworthy cockpits as simply "stronger." What you want is not
necessarily a structure that supports great loadings without failure
(that's what I think of when I hear the word "strong"). What you do
want is a structure that deflects or breaks in such a way as to
distribute an impact over the greatest amount of time possible. That
reduces G loadings to the contents of the structure.

The main point there is that the light, strong, stiff carbon fiber that
we like to build gliders out of is great for handling flight loads, but
is poor at absorbing energy. It tends to break all at once, and what's
left after that is not good at supporting any further loadings.

Modern aramids and polyethelynes (sp?) like Kevlar(tm) and Spectra(tm)
_are_ good at absorbing energy, and are also quite strong, but their
relatively low stiffness makes them much less effective at supporting
flight loads. That leaves it to the sailplane developer to arrive at
some compromise of materials. Perhaps they use the tough stuff in
greater thickness to achieve adequate stiffness. Perhaps they use a
combination of tough stuff and stiff stuff to achieve the better
properties of each. Either way, there is just plain more stuff there,
and inescapably more weight than is dictated by the basic flight and
handling loads. TANSTAAFL and all that.

And, yes, the sailplane developer is free to add wing area and to
choose docile airfoils that bring the stall speed down. However, both
of those choices will tend to have an adverse effect on performance.
And that's not a bad thing in and of itself. But as DG has discovered,
performance sells a lot better than safety does. You can make the
safest sailplane there ever was, but its performance might be so poor
that you don't sell a single example. Net safety gain for the sport:
zero. Somewhere between there and the ultimate performing thin-skinned
racing ship is a reasonable compromise. Choose wisely, and y'all be
careful now, y'hear?

Thanks again, and best regards to all

Bob "Grasshopper, why wrists say 'Hibachai?'" K.
http://www.hpaircraft.com