Mark Ravinski observes:
> As the prop rotates past the cooling air intakes I would suspect there
> gets to be a pressure pulse of some magnitude in the system.  I also
> suspect that this pulse when acting on the cowling might contribute to
> the stress cracking (or spider cracking) commonly seen on higher time
> installations. This is a problem on Lancairs and a lot of other planes
> using fiberglass cowling.

My 235 cowl has paint cracks around the edges of the flat upper surface
of the cowl. In flight, the area around the oil door can be seen to
vibrate with amplitudes up to about 5mm. Poking the cowl reveals the
problem - the large area that is nearly flat can act as a drumhead.
It moves easily up and down.

Looking at the design, it is clear that the monolithic layer of fiberglass
is not stiff enough to support that shape. This past weekend, I reinforced
the area behind the plenum with a strip of 0.190" (5mm) nomex core and one
ply of 5.5 oz 3K carbon.

The resulting structure is MUCH stiffer. I haven't flown with it yet,
but I expect that the vibration will be greatly reduced. Total added weight
was less than 0.5 lb.

BTW, we have read Dan Newland's suggestions concerning two-phase toughened
epoxy resins and Applied Poleramic in particular. Following his suggestions,
I called Rich Moulton at Applied Poleramic. He was very helpful and
suggested that I try their DR-5 resin with EH-102 hardener.

The epoxy worked easily and has little odor. I added just a bit of cabosil
to keep it from running into the cells of the core.
Scraps of resin that dried on my polyethylene bagging film were somewhat
flexible and did not fracture easily.
But what really blew my mind was the little hockey puck of leftover
resin in the bottom of the paper cup. After checking that it had cured
hard, I tapped it with a hammer to see how brittle it was. No damage - the
hammer just bounced. Eventually, I switched to a short-handle 5 pound
sledge, and pounded the puck firmly against a concrete hangar floor.
The hammer bounced a good 2 feet, and still did not cause any fractures.

The same tests applied to Safe-T-Poxy or any of the resins commonly
used on Lancairs would certainly have shattered them.


While fracture toughness is just one of several important properties
in a laminating resin (or structural adhesive), this has really
gotten my attention as an excellent epoxy resin formulation.
I'm looking forward to using more of this stuff.

http://www.appliedpoleramic.com/
http://www.appliedpoleramic.com/ToughenedResinSystems.htm
Phone numbers and prices are on the website.

They didn't pay me to write any of this. I've been working with composites
for a long time, and I am genuinely impressed with this product.


- bob mackey
  flying a 235
  building a 320
  103MD -at- pure-flight.com
  [use this email address - the one at the top is spam bait]