Removal of exhaust wrap

The other day I was driving to the hangar just like I routinely did for the last two years, but when I arrived, I realized I was almost at the end of my to do list (!)

I am waiting for a few small replacement parts that are not critical for first flight. I am also waiting for the availability of the local maintenance shop here in CSE4 to inspect my airplane. Strictly speaking, I could legally take-off, but having a second pair of eyes going through my work is not a luxury imho, especially since I have been closely into it for a long time.

While in waiting mode, I made a little research on the benefits of having fibreglass wrap around the exhaust. The wrap I have was installed when I bought the airplane, but it was getting very dirty and hairy.

I found this thread on the Vans Air Force forum which pointed to the Vetterman Exhaust website. Here is an extract of the website in question:

Can I wrap my exhaust with automotive type of wrap material and obtain more power? 

Any material on the exhaust will shorten it's life and failure will occur. We are using air cooled engines and one the best heat sinks on the engine is the exhaust system. The mass air flow comes in and is forced down and around the cooling fins and then is directed over the exhaust system and then exited out the bottom. If the exhaust is covered it cannot aid in the cooling process. Our tests show that any type of wrap on the exhaust makes both the cylinder head and exhaust temperature run higher. The stainless steel used (type 321) will degrade if the surface temperature is over 1250 degrees F. We also did not find a power increase of any kind by wrapping the exhaust.

More weight. Less cooling, no additional performance. Hmmm.

I also checked pictures of the Red Bull Air Race airplanes and confirmed that they don't use such exhaust wrap. I guess if it was creating any major power increase, these guys would use it!

The other reason a previous owner could have needed to use such wrap is perhaps to limit heat radiation in the bottom cowl. I never had any oil temp issues in this airplane...

I finally decided to remove the wrap, but keep a close eye at my oil temp during the first few hours of operation. 

It was a mess. At some point I had to put a mask to avoid breathing all the fiberglass dust flying around. Some guys say that exhaust wrap also traps moisture. What I found underneath tends to confirm this:

It was only surface corrosion that could easily get wiped off, but still... Here's what the whole thing looked like once removed from the airplane:

I was expecting to save about 1/2 lb, but with all the steel clamps, it was weighing close to 1.5 lb!

Finally I cleaned the exhaust using water with mild soap. This was enough to remove most of the old wrap traces. Looked nice and clean at the end.

What could I do next? Perhaps getting some dual time on a nice red S2C would be a good idea!

First run up!

The aircraft was fueled, all parts were on the airplane except the cowl. Here are the checks I made in preparation for the first engine run:

• Made sure all screws were fully secured on all panels; this thing blows
• Fuel boost pump check and wobble pump check (using fuel return line w/ purge valve)
•  The noise from both the boost pump and wobble pump was suggesting air was in the system. Took several wobble pump strokes until all cavities were filled with fuel, with no variation in noise.
• Found a small leak at the wobble pump outlet. Tightened fitting a little more and it stopped.
• Injector check: #2 cylinder was getting slightly more fuel than the others. Ordered a smaller retristrictor from Airflow Performance, but left it as is for the first run.
• Attached the oil cooler with lockwire to make sure it doesn't flop around.
• Plugged battery overnight before
• Overall inspection
• Briefed procedure in case of fire or other major engine/prop issue

The plan was then to:

• Roll the airplane outside and attach it to the car (brakes should work, but I haven't fully tested them yet)
• Put wheel chocks in
• Bring a fire extinguisher not too far
• Make a first run, checking:
• oil pressure/temp
• proper functioning of the throttle/mixture cables
• mag drop
• External inspection for leaks
• Make subsequent adjustments to idle mixture (which tipically requires many starts/stops) and check proper functioning fot he new T-30 tachometer

Sticking to the above plan worked well. The engine started quite well on the first attempt; see video below. I got great help from my dad and my friend Luc to prepare the setup and supervise the first engine run. Big thanks to you both!

We identified one oil line fitting that needed to be tightened a little more, which I did. There was also a fuel drain valve below the fuselage that was slightly leaking, but other than that, I was very pleased with the result of the first run. The new tachometer worked also very well (I used the 47k ohm resistors). We did make a few more runs to adjust the idle mixture since it was a little rich.

What a day, thanks again to my helpers to cross this great milestone!

Wing Rigging - Wire Tension Progression

Before doing the W&B, I rigged the wings using the recommended procedure in the Christen Eagle AFM.

From past experience, a biplane wing rigging is never a straightforward process. Since I was crazy enough to buy a digital torque wrench, it gives me the possibility to quite quickly and accurately read the wire tension (along with the tool from Holloway Engineering). I therefore made an experiment: record all wire tensions for every iteration of the tensioning process on one side. This way I was going to hopefully capture the converging tendencies and learn as much as possible to hopefully ease future riggings.

The following graph summarizes the results I got during the tensioning of the RHS wing wires. Each bar height is proportional to the wire tension. When a label exists above a bar, it means this wire has been adjusted compared to the last iteration. For example, on iteration 2, the fwd landing wire has been tightened by 1/2 turn. Negative values mean wire loosening. The brackets on the bottom indicate the results of the readings from the level I placed over the top wing front spar.

I could note/confirm the following trends for my aircraft:

1. When tensioning one wire in a pair, the other wire of the same pair often looses tension (see landing wires during iterations 1,2 and 3)
2. Adding tension can be done relatively gradually whereas removing tension can easily be drastic (see iteration 7)
3. Adding tension on the flying wires does not significantly add tension on the landing wires (see iteration 4)
4. Adding tension on the landing wires can have more of an effect on the flying wires (see iterations 5 and 6)
5. Building tension on the flying wires has to be done with care since it can bend the wing and cause undesireable anhedral. In this example, I caused a slight anhedral up to iteration #6, then I removed tension on the flying wires on iteration #7 to remove the anhedral. In other words, it seems to be preferable to tighten the flying wires only when there is already quite some tension on the landing wires.

After iteration #13, I switched to other side to tension the LHS wing, then when back to RHS and did small adjustments to get within tolerances. After all that, I measured the wing incidences with the rigging boards and quite happily confirmed they were within tolerance too! This seems easy to summarize, but the whole process took me around 2 days of work.

I do not pretend the above is applicable to any other biplane, but perhaps a potion of the above findings can help others : )

Next step: the first engine startup. Stay tuned!

Weight & Balance

The weighing time came. The Griffon guys (local shop in CSE4) were kind enough to lease me aviation scales over the week-end.

A little bit like when I step on a scale, I was a little apprehensive on knowing the exact weight of my airplane.

Didn't have ramps to roll it up on the scales, so with the help of my friend Scott, we removed the top cowling, lifted the airplane with a shop crane and straps attached to the engine mount, and slowly lowered it back down on the scales.

Here's what it looked like once ready to make the reading:

Before doing the tail reading, we zeroed the scale with the tail support on it. The wood blocks on the floor were there to prevent a lateral tip over of the support, but weren't touching it.

The empty weight of the airplane was a little higher than I thought, but not by much so I was quite happy with the end result. We also made a reading with myself + parachute in the airplane, as well as with some fuel in to confirm the arms of each.

It feels good to have a reliable datum to start with. I have a few ideas to save a few pounds here and there already... Big thanks to Scott for helping me with the whole process : )

Weight & Balance Preparation

Today I prepared the airplane for weight & balance. The S-1S Inspection Procedures Manual includes a section on weight & balance; here's an extract:

Since this manual is for the stock Pitts with the bungee gear, I thought the main gear axle fuselage station depicted above was probably not right for my airplane. The manual said FS0 is defined as being 50" in front of the firewall. I therefore had to retrieve the longitudinal position of the wheel hub center relative to the firewall. I could tell it was definitely more aft than just 0.44" by simply looking at it.

I raised the tail so that the top longerons are level, then I used a plumb bob to precisely locate the firewall position on the floor:

Then I used the square to mark both wheel hub centers on the floor. After making two marks from the firewall and two marks from the wheel hubs, I rolled the airplane outside to draw two lines and measured 3.00" i.e. my wheels are more than 2.5" more aft than a stock Pitts. Good thing I checked!

Next step was to clean and install the remaining parts on the airplane: panels, cowl, wing root fairings and wheel pants. This is a lot of screwdriving work, but it paid off! For the first time in two years, all the parts were back together, it felt so good; it was like if she was alive again. Wx was nice outside so I rolled her out and took a few shots:

The only unhappy of this situation today was my wall of parts:

Next step is to weigh the airplane! My plan is to weigh the following configurations:

1. Zero fuel, no pilot
2. Zero fuel, pilot + parachute
3. Full fuel, no pilot
4. Full fuel, pilot + parachute

Doing the above will confirm the arms of the fuel tanks and pilot. More to come!