Saturday 28 April 2018

Titanium Firewall Part II

Once I finally got the engine back, I decided to make a firewall template out of coroplast before drilling my new titanium firewall (thanks to my friend Mathieu Gratton for the idea!).


This allowed me to try different hole locations and converge on routings for hoses, controls and wires that I was happy with. Took a lot of notes directly on the template for me to remember all details as I was trying each setup. 


I used the green tape on the coroplast to mark the location of the structural members of the fuselage. This helped to avoid holes or nutplates at a wrong location!

For the most critical elements like throttle and mixture cable holes, I placed the engine  as close as possible to its final location (the engine leveler helped a lot for that):



After taking some time to look at all this I decided to relocate the starter solenoid as well as the voltage regulator which were both mounted on the firewall before. The battery being behind the seat, it makes more sense to have the unprotected cable between the battery and starter solenoid as short as possible (I don't have a master solenoid). Also, after looking at specs from B&C aero, it is generally recommended to locate the voltage regulator behind the firewall for cooling reasons.

Once I figured out where I wanted all holes to be drilled, I transferred the final markings to the titanium firewall and drilled all I could using the press drill. Obviously some holes were too far from the edge so had to use a hand drill for these. Most of the holes were easy to drill with cobalt bits. The larger diameter holes were more of a challenge. Biggest hole was 1.0 inch. I used a unibit but it required patience since titanium is quite hard. Clamping the Ti sheet to a thick piece of wood helped. I also used cutting oil between each step and reduced the drill rpm as the hole was gradually getting bigger. Slowly but surely, I finally got there.

Last step was to install the nutplates. I also decided to rivet reinforcing brackets at two locations where more stiffness was required: behind the 802 oil valve and behind the brake fluid reservoir. I used a C-frame to make dimples and riveting (thanks to my friend Scott!)

As a comparison, here is the old SS firewall a.k.a. swiss cheese firewall:


Here's the wannabe firewall:


...and the final result:


Much less of a swiss cheese now : ) Next step is the engine installation!

Monday 16 April 2018

Titanium Firewall Part I

While the engine was gone last year, I decided to take the opportunity to make a new firewall since the old one had many unused holes that were plugged. In fact it looked more like a slice of swiss cheese rather than a firewall.

The old material was a 0.025" thick sheet of stainless steel. When browsing on the biplane forum, I saw that some builders were using titanium to save weight! Since my airplane is a little nose heavy, switching to Titanium was killing two birds with one stone.

In order to select the material thickness, some guidance exists under FAA 14 CFR 23.1191 which says:

(h) The following materials may be used in firewalls or shrouds without being tested as required by this section:

[...]

(6) Titanium sheet, 0.016 inch thick.

I therefore proceeded an ordered a sheet of 0.016" thick titanium from Titanium Joe. Great service and quick shipping. First impression: ok Titanium is VERY light.

I used the shape of the old firewall to draw and cut the titanium sheet to the desired shape. I cut the biggest parts on a heavy duty straight shear and cut to final shape with curves using a shear similar to this:

Image result for metal shear

Then I drilled the holes for the aluminum frame rivets them using a cobalt drill bit and the old firewall:


When looking at my neighbour Luc with his Pitts S-2C, I realized the lower edge of his firewall was actually bent with a large radius in order to reduce air restriction and make it easier for the warm air to get out of out of the cowl. My old setup had a sharp edge corner which obviously created a lot of turbulence and air restriction. I didn't especially had oil temperature or cylinder temperature problems, but since I may relocate my oil cooler eventually, I thought I was putting all chances on my side to make a large radius bend like on the S-2C. Easier to block air down the road rather than to unrestrict it!

In order to block the gap between the radius and the flat adjacent areas, I had to make small brackets using the leftover titanium:


Here is the result after the aluminium frame was transferred to the titanium firewall and the work on the bottom edge complete:


Next step was to locate and drill the required holes. I didn't want to blindly copy/paste the old hole locations so I prefered to wait until the engine was back before proceeding... To be continued!