Cooling eyebrows
New Cooling Eyebrows for the A75
Modified August 28, 2013
These are the new cooling eyebrows for the Continental A75 engine on my Pietenpol, NX41CC. I've had the mounting tabs on the existing eyebrows crack repeatedly, due to metal fatigue from the merciless pulsations of propwash back into the eyebrows. I've also had fatigue cracks in several other spots on the eyebrows, so it was time to do something different.
Here's what the insides look like. Very nicely fabricated, fitted, and assembled. I found these eyebrows through Gerry Holland, a Pietenpoler from the UK. He had a beautiful pair of highly polished eyebrows on his Corben Jr. Ace and I asked where he got them. He pointed me to Falcon Aircraft, in the Czech Republic. They sell these as a set. I think I paid $300 USD for mine, postpaid.
Besides the cracking problem with the mounting tabs, I have been concerned about the height of the eyebrows on my engine. They appear to be quite tall, which leads to excessive drag and cooling. The highest oil temperature I've ever seen has been about 185F in the peak of Texas summer, so I'm happy with that, and the stated maximum oil temp is 215F. I think I can lower the eyebrows and still have adequate cooling for the engine, especially considering that the airplane will now be based in Oregon rather than in Texas.
Considerably lower profile (compare to the previous photo). The eyebrows also have rolled edges with wire stiffeners in the rolls and are one metal gauge heavier material than my present ones.
Side view, showing the nice details, and the VERY sturdy mounting tabs, which are steel rather than aluminum as my present ones are.
Here's what I've done to reduce the amount of incoming air that gets shunted straight out the back through the openings for spark plug and plug wire access. I fabricated some baffles out of inner tube rubber material and secured them in place with aluminum rings. The inner tube material is flexible enough to permit the plug wire connectors to pass through, and to fit over the slightly protruding tops of the plugs. Without doing something about these large openings, a significant amount of the incoming ram air would shoot out the back instead of turning down to cool the rear of the #1 and 2 cylinders.
Here's what the inside of the plug hole baffles looks like. I'll update this page when I get the new eyebrows fitted and installed, and again after I make test flights to see how they cool.