As noted on a previous page, the decision was made to use J-3 type "eyebrows" for engine cooling instead of a pressure cowl. These air scoops have been perfected over the past many decades and other than repairs for the occasional vibration-induced crack have proven effective for keeping little Continentals cool.
The genuine Piper eyebrows cost upward of $1000 per set and a non-PMA kit can be purchased for about $500. Either option is expensive enough to prompt exploration of alternatives.
An aircraft mechanic friend of mine alerted me to a set of J-5 eyebrows that had been removed from the plane due to service. I jumped at the chance to give them a close look and create poster board patterns with the intention of fabricating a set of eyebrows at a fraction of the cost of the "real" ones.
There is a lot going on in these things, Multiple curves and bends and tooling of joints along with interior baffles. I've noticed that no two set of eyebrows seem to be identical, all of them have been repaired and modified over the past seventy years.
Since taking the eyebrows apart would result in me being run off the premises, I had to be content with making patterns the best I could from the intact eyebrows. In less than an hour I had a set of patterns I felt would be sufficient for building my version of the scoops.
The patterns were used for building the cowl around the eyebrows.
Internet research led me to use 0.032" 3003 H14 half hard aluminum for the eyebrows. The paper patterns were transferred to the sheet stock and cut out with hand snips.
The eyebrows are secured to the engine with straps that attach to valve cover screws and this steel bracket that is secured by two cylinder base studs. The other brackets I've seen have platenuts to accept screws through the scoops but welding bolts to the bracket for metal lock nuts appealed more to me.
Here is the bracket secured by the cylinder nuts.
I consider this a rather involved exercise in tin working and is certainly not for those with limited patience. I lost count of the number of times the scoops were fitted and removed in the process of adjusting, bending and tweaking to make them conform. But even though they aren't yet finished, it is evident that we are going to have a nice set of eyebrows. I added a strap at the front of the scoop which really stiffened the assembly.
Here's looking at you, a look only a mother could love.......
But this is what we will fly until convinced some other way is better.
All the pieces of firewall forward came together for the first time this evening. Engine controls, fuel lines and gauge hookups have been in progress so it was time to see how the new eyebrows integrate with the cowl.
The cowl sides follow the eyebrows nicely.
This has been a journey into the unknown due to this being the first D.VII with this engine package. A few more details and firewall forward will be ready for final finish and assembly.
Building Log for Sam's