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

"Ron Wanttaja" wrote in message
...
On Sun, 5 Oct 2008 13:33:05 -0600, "Bill Daniels"
bildan@comcast-dot-net
wrote:


"Ron Wanttaja" wrote in message
...
On Sun, 5 Oct 2008 09:18:21 -0600, "Bill Daniels"
bildan@comcast-dot-net
wrote:

I would claim they are directly related. A glider carefully
engineered
for great handling and occupant protection will also have a good L/D.

Bill, I don't have a dog in this hunt, but: Unless you're using a
different definition of "occupant protection" than I am, I disagree.
Adding a steel crash cage to a glider cockpit will increase occupant
protection, but has a negative effect on the aircraft weight
without improving the aerodynamic performance.

I think steel tube structures are way overrated for "occupant
protection" as
compared to carbon/Kevlar cockpits on modern composite gliders. Race
car
builders abandoned steel tube "birdcages" for carbon/kevlar tubs a long
time
ago for safety reasons. You'll never look at steel tube structure the
same
way after you've seen a folded tube sever a leg artery.

You're missing my point, Bill. Whether steel tube or kevlar, occupant
protection does not contribute to L/D, as you seemed to claim. Wrapping
an
eggshell around the pilot will give good aerodynamics, but better
occupant
protection will weigh more and hence reduce performance.

Ron Wanttaja

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