My apologies for the length between posts, but now we're back!
and now back to the cooling system,
This time, we were working on the other side of bulkhead right inside the center section of the aircraft. We had to fit pipe #3. I’ve taken a picture of the shop drawings to give you an idea of what the cooling system will eventually look like!
|The Cooling System|
After getting the appropriate hardware (that is, nuts, bolts, washers, avemos, and avemo clamps), we were ready to install our pipe. Pipe # 3 is the lower one immediately left of the radiator, between the two bulkheads. This drawing will make much more sense once you see the picture of the pipe roughly fit. However, before we could fit them, we had to lubricate them so they would slide into place. To do this we used “electrical insulator compound.” Don’t let the name fool you though, it pretty much looked like Vaseline (though I’m sure it isn’t) and allowed us to fit the pipes with ease.
|Roughly fit in place|
As you can see, it’s a tight fit. The black rubber avemos indicate the joints. They will be covered later with an avemo clamp and a band-type fastener. There is a special name for the band-type fastener and I will clarify that when it is actually put on. However, when we went to fasten pipe #2 to the bulkhead (the same fashion as last week) we ran into a fair bit of trouble. Pipe #2 is to the left of the rubber avemo (at the very left edge of the picture, almost out of the frame).
|The Firewall, You can see Pipe #2 in the bottom right (barely).|
Above, you can see the firewall of our Hurricane. At the very top, you can see the coolant header tank. At the very bottom, slightly to the right, you can just make out pipe #2. Coolant will eventually run from the header tank to the radiator, through the radiator where it is cooled, then through pipe #4, #3, #2,and #1 into the engine. It will then circulate crucial components in the Merlin and keep the engine from over heating. Then the cycle repeats. If you look closely at the shop drawing, you can see the direction of the coolants flow. The small arrows next to the pipes represent the flow.
|A close work environment|
Once that was done, Ted and I tried once more to fasten pipe #2. However, this time it turned out that the flange itself was lining up with the holes in the bulkhead. Unfortunately, the flange is on Ted’s size and you can’t quite see what it looks like. Yet, if you remember from last week, or want to see what the flange looks like, you can see it in “Restoration Update #7.” We weren’t sure what the problem was so we needed Paul’s opinion. Luckily, we were able to compare the fit on our Bunny McLarty Hurricane Mk. IV. So after some consultation and discussion we figured out what the problem was.
We figured out that the problem was with the ring riveted to the bulkhead. It was placed 90 degrees out. In the above picture, you can see Paul showing us how to drill out rivets. It turns out that on the ring, there is one quadrant of it where the lip is shaved down. This lip allows for the correct amount of clearance and lets the bolt holes line up with the flange. The rivets were particularly tough to remove. You first have to drill out the head just down to the surface it mates with. Then you have to chisel it off. Then you have to punch out the remainder of the rivet. In some cases that required drilling with a smaller drill bit into the rivet. This weakens it and eventually allows you to punch it out of the hole. To save breaking up each photo with a few lines of the process went as follows:
|Knocking the head off of the rivet|
Repeat that process 8 times and we were ready to refit our plate and pipe #2. It took most of the afternoon to get those rivets out. Again, we had to be extremely careful not to oversize or ovalize the holes. Rivets will be going back into those holes and they need to remain the exact same size.
|All bolted together!|
Above, you can see the pipe reinstalled and the plate adjusted. We had to turn it 90 degrees to the left, counterclockwise, to get the shaved down portion of the lip in the correct spot. Later this week, the plate will be re-riveted. Unfortunately for the general fit, turns out Pipe #3 is out of alignment now. Despite the photo below, the pipe was quite a bit closer in fit, but it wasn’t 100 percent correct. It was just really awkward to take a picture and hold the pipe at the same time. To fix this, the pipe will be heated up at its joint and positioned so that the fit is 100 percent perfect.
At the time this was discovered, we had reached the end of the day. So that will have to wait until the next session. That’s all I have for this week on the restoration. However, we have received our first question for “Ask an AME.”
John Park, a fellow Vintage Wings member asked:
You often see people turning the propeller over by hand prior to starting an engine. Especially on radial engines. Can you turn the prop either way or only the way it operates or opposite to the way it operates? Why?
1) The propeller is pulled through by hand prior to starting to check for "hydraulic lock" (just for those who don’t know why the propeller is pulled through on radial engines).
2) Whether or not you rotate the propeller the way it runs during regular operation or not, depends.
1) “Hydraulic lock” can happen on radial engines and other inverted engines (such as the gypsy major - I believe - on the Tiger Moth). There are cylinders where their heads face towards the ground. What can happen, when the aircraft has been sitting, is that the oil from the engine can seep around the piston rings and enter the combustion chamber. Once this oil has built up in the combustion chamber, it obstructs the piston from reaching "top-dead-center" during the compression stroke. That layer of oil that has now built up between the piston and the top of the cylinder head is pretty much incompressible. The engine components aren't meant to take that kind of stress so they will give out long before that oil is compressed. When this happens, you've got hydraulic lock.
The damage that hydraulic lock can cause your engine is catastrophic. You can range from bending your connecting rods (that's the rod that drives the piston up and down) to blowing the head off the cylinder entirely. You do not want to start an engine that is hydraulically locked. If you discover hydraulic lock while pulling the prop through you must expel the built up oil before attempting to start the engine. Some newer engines have drain plugs. On others, you will have to remove the spark plug to drain the oil. You will then also have to clean the spark plug up before replacing it. Thus to prevent starting an engine that is hydraulically locked, the prop is gently pulled through by hand.
2) The reason that it depends whether you can pull the prop through in either direction is because of the vacuum pump (also called a vane pump). Generally it is good practice to pull the prop through in the normal direction of operation. However, the reason it’s a good idea to pull the prop through in the direction of operation is to protect the vacuum pump. The vacuum pump has very delicate vanes in it made of a material like carbon fiber. When you pull the prop in the wrong...or...opposite direction, you run the risk of damaging those vanes. Certain vacuum pumps strongly recommend or even prohibit you from pulling the prop through opposite of its normal rotation. This is because you run the risk of breaking those vanes in the pump. If those vanes shatter, the can also damage a number of instruments which are controlled via that vacuum pump. This is because when it is destroyed, the negative pressure (which is keeping some of your instruments active) ceases. This causes broken bits of the vanes to be sucked into the instruments themselves.
The end goal is to protect that pump and if you aren’t sure whether or not it is safe to pull the prop through opposite to its operation direction (because you don’t know what type of vacuum pump there is in the engine - or the pumps manufacture specifications), then you should not.
Hope this answers your question, thank you John!
If you've got a question about the restoration or for Ask An AME, send me an email: email@example.com
Until next time,