Fuel System Installation #4 (of 4)

Quick post to say that today I finished the fuel system installation. I was waiting on the wings to connect the vent, drain and feed lines of the aux tank.

Here's what I see now when I get in the hangar!

Had to make new lines since I changed the routing of both the vent and drain lines. This one is the bottom half of the drain line... Pretty long to bend to what I needed.

The clear tube sections on the above picture are leftovers from my fuel sight tube (1/4 ID Tygon) I used as anti-chafing for areas close to other parts. Always better to think about this before doing the flares : )

I should be ready soon to reinstall the fuse panels...

Fuel System Installation #3 - Vents and Siphon Valve

Been working on the fuel system this week. All the fuel lines, vent lines and drain lines are done except the ones that will connect to the wing tank.

Here's an example of two vent lines that I finished today. I used tie-wraps and scrap pieces of rubber tubing as spacers between the tubes:

These two lines were originally passing just beside the fuel tank without going through the aluminium bracket. They were so long that I had to bend them every time I needed to remove/reinstall. I therefore decided to add bulkhead fittings (see top of the picture above) I found cutting the tubes in two halves is much easier to work with and also more convenient if I ever need to remove my main tank again.

Another new feature I installed is the siphon valve in the vent interconnect line between the header tank and main tank (see diagram here). It's a great idea suggested by my friend Mark Wood down in Wichita in order to avoid too much fuel being transferred back into the main tank while inverted. It is quite simple, avoids buying an expensive check valve and allows higher vent flow than a typical vented check valve. It is made by assembling two AN816-4D fittings in a AN910-1D NPT coupling with a ball in the middle. The only modification to be made is a castle shape on the bottom fitting to allow air to flow in the upright position. This is reasonably easy to do with a file. See below:

Here's what it looks like once installed on the airplane. Assembly to be secured with either Adel clamps or Koroseal lacing.

Next task is to start reconnecting wires. Stay tuned!

Fuel System Installation #2 - Header Tank

Got my flop tube back from my friend Scott. It now has a total OD of 0.900" which allows it to get in my header tank!

It took some time to reinstall the header tank back in its right place, but it's good to have it back:

I replaced the old Tygon tube by this yellow one which is fuel resistant (ordered it from US plastic).

Fuel System installation #1

Been  working on the fuel system recently. Last year I bought a new flop tube for the header tank. I decided to change it for two reasons: 1) To shorten it so that it gets closer to half the length of the tank and 2) because the old flop tube was very stiff and probably 10+ years old. Here's the comparison between the old and new flop tube once I cleaned and transferred the attachment fittings:

I used fuel lube for both NPT fittings. Once I was ready to reinstall it in the header tank... Doh!!

I gave the tube to my friend Scott for him to remove about 0.040" on the OD of the brass end with his lathe. In the meantime, I started installing the boost pump, fuel filter and some tubing in between. It feels so good to take components from the shelves back on the airplane. Here's my little progress so far:

As a followup to my post on the side panel brackets,  I finally decided to change all the nutplates on the vertical brackets as well. The access was bad on some of them and required the use of one lug nutplates (K2000-3).

Anxious to get the flop tube back from Scott and resume my fuel system installation!

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!

Miles away

It's been already more than a month since my last post and I could not make much progress since then mainly due to work and small issues with the paint job on the fuse structure. I'm currently in Salina, KS for work and still won't be able to work on the Pitts until August so I thought posting some details on my fuel system would help my readers keeping interest!

Those who read this blog from the very beginning will remember that the day I discovered the crack in the fuse structure, I was actually removing panels to make a change to my fuel system. Here are the two reasons why:

Reason #1

When I bought the airplane, the seller briefed me on a little issue with the fuel system when filling the tanks: if I overfilled, the vents would get full of fuel and siphon the whole tank on the tarmac! The only way to stop it was to quickly grab a plastic tube, connect it to the vent and blow the fuel back up... Not cool. Very not cool. I have been careful in not overfilling the tanks after it happened to me twice. I promised myself to investigate and fix this problem during winter. It turned out I had higher priority items to work on during the first winter so I decided postpone this fix until the next winter.

Reason #2

During the ferry flight from my last contest of 2014, I was in formation flight with my friend Luc in his S2C when I had sudden drops of rpm. I checked my mixture, fuel valve and turned on my boost pump and it reduced the magnitude of the drops by about 50%, but they were still more than 500 rpm drops. Although I could maintain my 7000ft altitude, I was mainly focusing on finding a suitable field with the help of Luc. At one point I realized my wing aux tank was empty but my x-feed valve was still open, so I shut it off. Immediately the drops in rpm stopped and the engine behaved normally so I decided to continue the ferry paying special attention to my fuel pressure.

A few days after getting back to my home airport, I decided to perform a little test with my fuel system: with the aux tank empty and the valve closed, I climbed to 5000ft over the airfield and then opened up the wing tank valve. My fuel pressure gradually went down until I got the same rpm drops again. My suspect was confirmed being guilty: there was a problem when the wing tanks was empty and the valve open. I didn't have this problem when the aux tank was still transferring fuel in the main; only when it got empty.

Analysis

After this test, it was already the end of the season so I only made a few practice flights before grounding my airplane and perform my investigation. I was kind of anxious to understand the reasons of both problems but with a wing tank and a header tank, the inverted/upright vent/fuel transfers are not easy to understand without actually drawing a sketch. I therefore took the time to analyze it and sketching my whole system. That's what it looked like after cleaning it up in visio:

After drawing the sketch, the reason for problem #1 became obvious: the header tank vent was the one filling up and creating a siphon when overfilling the main. Problem #2 was a bit more tricky but it seemed like air was sucked into the wobble pump inlet when the transfer valve was open.

While having the airplane teared down, I took the opportunity to give my main tank to my welder so that he adds two new ports on it: one to install a vent interconnect between the header tank and the main tank which addresses problem #1:

And another one dedicated to the aux tank fuel transfer, which I am confident will solve problem #2:

So that's about it for the story behind the changes to my fuel system. Will write a new post as soon as I start the reassembly process!