"Bill Daniels" bildan@comcast-dot-net wrote in message
. ..

"Peter Dohm" wrote in message
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"Bill Daniels" bildan@comcast-dot-net wrote in message
...
Weight in a glider is a double edged sword and never simple. In fact,
it can add to L/D. For example, my Nimbus 2C has an L/D max of 47:1 at
1000 pounds and 49:1 at 1433 pounds. The difference in L/D max is due
to a higher Reynolds number at the higher best L/D airspeed with the
higher weight.

That extra weight is ballast water in wing tanks. In any but the
weakest weather, that ballast dramatically increases performance. This
is shown most clearly at 100 Kts where the 1000lb GW L/D is 22.3:1 and
the 1433lb GW L/D is 31:1. But, of course, structural weight is not
jetisonable as is water ballast.

But that wasn't really my point. It was that good engineering directed
at crashworthiness is an investment in design excellence which is also
likely, but not assuredly, to increase performance. At least the two
aren't mutually exclusive. That's particularly true when the cockpit
structure is molded carbon/Kevlar which can be of any shape and might as
well be the best aerodynamic one. As near as I can determine, the
latest crashworthy cockpits don't weight any more than the old ones and
they are actually lighter than steel tubes.

Bill D

I believe that you are correct in this, and that a kevlar capsule is a
good investment.

I have not researched the matter and could be wrong; but I strongly
suspect that a many, if not most, of the dissabling leg injuries in the
old primary gliders involved easily deflected collisions rather than
"hitting a wall".

Peter
Most likely.

I know of one accident in a Schweizer 1-26A (tube and fabric) where an off
field landing resulted in a stick coming through the nose fabric severing
a leg artery. The pilot bled to death before he could get out of the
cockpit. I guess you can tell that I have no love of tube and fabric
gliders.

Bill D

This is an extreme case of the same problem that concerns me: Intrusion of
"brush" in the course of an off field landing. Obviously, in this
particular case, the shrubbery involved must have appeared to be soft enough
to be deflected by the tube and fabric structure; but the general problem
must have been nearly epidemic with the completely open seating areas of the
primary gliders.

Nothing can ever eliminate the occasional encounter with a stronger and/or
sharper than expected solid object; but a slightly flexible kevlar based
composite capsule could be a great improvement. The March racing cars used
a kevlar based capsule system about 30 years ago with considerable
success--although their earliest attempts were heavy and less than
competitive, and some developement was required.

AFAIK, kevlar has a high tensile strenght, but does not bond to the resins
in which it might be encapslated. Therefore, it will extrude under load.
That makes it a good to excellent material for safety structures and a
generally poor material for heavy load bearing structures.

Peter