Fuel System Modifications

Being still abroad for work, I worked a little bit on my new fuel system diagram. As explained in this post, I had two different issues with the previous design that made me make some changes. I will explain a little more the reasoning behind each fix.

Fix #1: To solve the fuel siphoning problem

The fix for this problem is to install a vent interconnect between the two tanks. This way, if siphoning happens following an overfill, it will only siphon until the fuel level reaches the level of the vent port on the main tank i.e. minimum fuel loss.

A downside of this is that it allows the fuel in the header tank to flow back into the main tank while inverted. To prevent this, I will add a check valve on the vent interconnect line. It is important that the check valve be vented so that air can flow in both directions. Here is a picture to better visualize the change:

 

Fix #2: To solve the potential fuel starvation issue

This problem is more important, but the fix is simpler. Instead of feeding the pump inlet directly when using the aux tank,  I will connect the aux tank to the main tank, therefore eliminating the risk of feeding air from the aux tank to the engine:

I also made a small change to the fuel level indicator: the bottom end will now be connected to the header tank purge line. This will allow me to read fuel level down to the header tank level.

Modified top level diagram

The above is how I intend to connect the lines during the rebuild. I keep an upgrade for next winter: route the EDP vent line through the firewall so that its outlet gets further away from the heat of the exhaust stack.

Anxious to get back home to work on this. Stay Tuned!

Side panel brackets

Flight tests at work being quite demanding lately, I can barely work on the Pitts. Last week-end, I had a few hours to work on the aluminium brackets that snap on the top longerons to attach the side panels.

They originally had nyloc nutplates. I decided to change them to all metal nutplates for multiple reasons:

- The rivet holes on the brackets were not dimpled, therefore the rivets were chafing with the aluminium panels

- Nyloc may not be reused if the nylon doesn't provide enough friction (AC43-13)

- All metal nutplates are smaller and lighter

Took me a few hours to drill the old rivets, dimple all rivet holes, dimple all new nutplates and squeeze all the new rivets in place. I used a hand rivet squeezer from my friend Scott both for doing the dimples and rivet squeezing. My friend Marc also helped in replacing the nutplates on the rear brackets. Thanks Scott and Marc!

Here is the new vs old nutplate comparison:

I may have to partially tap the nutplate threads as they are pretty tight. I'll see once I will try to reinstall the side panels. Here is the result once snapped on the fuse:

Will stick some anti-chafing tape on the brackets before reinstalling the panels. I'm thinking of replacing  nutplates also on the vertical braces (right of the above picture), but the access may be challenging for dimpling and riveting. 

Mission for next work session at the hangar: install the main fuel tank.

Intake pipes

Haven't been posting for a while. Business trips and the sale of the Citabria took some time. Leaving again today for work, but I could spend a few hours in the hangar yesterday to install my intake pipes.

I removed them and had them chromed back in 2014, before I discovered about the crack:

I carefully removed the old gaskets stuck on the cylinder heads with a sealant removal scraper as well as a razor blade. I used Aerograde Hylomar to seal the new gasket in place. I also changed the o-rings that seal the tube with the air distribution box. I lubricated the new o-rings with Parker lube before installation. When an o-ring looks like an onion ring, it's time to change it:

Once all 4 pipes installed:

My plan is to work on the fuel system next. Stay tuned!