(May 16, 2009) -- Last Saturday, Tom Berge conducted an excellent seminar on the forward section of an RV airplane's firewall for a few members of the Twin Cities RV Builder's Group (nee Minnesota Wing of Van's Air Force.)
It was a timely session for me, since I'm looking at a beautiful Mattituck engine that's been sitting in a crate in the hangar, and am trying to figure out what should go on the firewall before I hang the engine. Oh, and who's got an engine hoist they're not using?
Here are the notes I took from the session. Feel free to share your experiences and I'll update the article:
BASIC SYSTEMS
It's impossible to come up with a step-by-step guide to installing your engine. Why? there are too many combinations. Consider these:
Engine considerations:
Is the engine experimental or certified?
Do I want fuel injection or a carburetor fuel system?
Parallel or angle valve? An angle engine is not as durable, runs hotter and is bigger.
Dynafocal or conical mount? There are also two types of Dynafocal mounts. Be sure you know which one your mount/engine requires.
Magnetos or electronic ignition? With electronic ignition, it's easier to get on the lean side of peak. The drawback is you'd need a second battery with an electronic ignition.
Vertical or horizontal induction? Horizontal takes a smoother cowling. There's no scoop on the bottom.
Propeller considerations:
Fixed-pitch wood. They're cheap and light, but if it you hit something, it's going to disappear. Because they're light, you're going to bounce around the sky "like a ping-pong ball." They'll be damaged in the rain.
Fixed-pitch metal.
Constant speed. Used to control power. Tom says he had a 160 hp with a fixed-pitch outclimb him in a hurry. You're off the ground quicker, your fuel consumption is lower. But it's more weight, more expense, and more overhauls.
Prop extensions. It depends on what kind of cowl you use.
Prop governor.
Spinner. Made of fiberglass. Depending on what sort of combination you select, they can be complicated to build.
With a "plane jane" instrument panel, a 35 amp alternator is fine, but if you have an EFIS, a 60 amp alternator will be required.
You'll also have to decide if you want/need a standby alternator.
Fuel system:
Fuel injection or carbureted
Boost pump - You'll need one with a fuel injected engine.
Gascolator - You don't need one if you use a fuel injected engine.
Fuel pressure sender.
Fuel lines. When installing lines firewall forward, they must be able to flex with the vibration of the engine.
Primer. Carbureted engines only.
Air filter system. It will increase the life of the engine.
Exhaust system:
A Vetterman exhaust system is the preferred choice. There are several versions available. One system with mufflers is quite quiet, but there's a penalty in weight. A crossover system is simple, and a four-pipe system delivers more horsepower.
Cooling
We're all running pressure plenums, it's just a matter of how the top is sealed. Even with a Sam James Holy Cowl system, you still need baffling.
Cooling is baffling in many ways, though. Tri-gear airplanes, for example, seem to run hot. "There's a lot of garbage down in the exit area" of the cooling air, Tom says.
He has a Sam James plenum on his tri-gear. "My temperatures went up... a lot," he says. "But my speed went up. I was shooting for 30-degree-lower temperatures. It went up by 17 degrees. That was a great disappointment."
He put louvers in on the side of the cowl, but that didn't change things. "That's just the way it goes," he said, adding that "no matter what, you will always want 5 more knots."
Cowling:
Van's. They look fine. They work.
Sam James' Holy Cowl. They're about $500 more and there are two types. A shortee and a standard.
Piano hinge pins. The Van's method works fine but it can be difficult to get the cowl back on without help, I noticed as watched Tom and Paul Hove put the cowls back on after taking them off for our benefit.
Cam locks. Tom prefers the hinges. They're cheaper. You can spend $400 on camlocks and still not have enough for the cowling. He says that when you're putting the cowling back on, it'll get hung up on one of the cam locks. But once in place, you can button down your cowling pretty quickly.
Pin holes. You'll get thousands of them . When he was building, Tom used Featherlite. On the bottom he used a sandable primer filler. It doesn't work well, he says.
What goes on the firewall?
The battery box
Cable/wire pass-throughs. They're expensive. "$28 apiece. I've got $375 of pass-throughs on my firewall," Tom says.
Heater box.
Gascolator.
Antennae. Tom has mounted a GPS antenna just under the cowl (see photo). "Heat has not been a problem so far," he says. He advises against putting an ELT antenna in the area, however. "The engine is the first to arrive at the scene of an accident," he notes.
Air/oil separator.
Solenoids
Pressure transducer.
Firewall considerations
BEST PRACTICES
Tie up your wires - "When I do inspections, I often see areas with 12-18 inches of wire that's unsupported. Tie them up every few inches.
Nothing loose - This includes scat tubing.
Keep the positive battery cable from touching anything. If it shorts out, it will weld and cut. "In Tucson, a guy had a rudder cable short the battery. He had to land a tailwheel airplane without a rudder. A couple of guys in a Cirrus were on approach when they needed more power but the throttle wouldn't work because it had shorted the battery cable," Tom reported.
Use Adel clamps.
Firesleeve -- At least the fuel lines. Use firesleeve on oil lines, too. Oil can burn.
Properly add washers/nuts. A lot of time, people aren't paying attention to which side of the washer is exposed to the nut.
Move the oil restrictor fitting. The 45-degree restrictor fitting can go in two locations on most engines: on the rear and on the side. Put it on the rear. "If you put it on the side, you can't get it out without pulling the engine."
Leave slack in wire bundles and control cables. "On start-up, the engine is shaking something awful."
Plan for engine mount sag. Provide for clearance on the bottom of the cowl. "It's a crap shoot how much," he said. Paul Hove, for example, reported after 140 hours, his has sagged about 3/8" .
Seal the firewall. Use Pro-Seal at the corners and at any holes.
Use an engine ground strap. If you don't, you won't be able to start the engine without a struggle..
Keep anything from touching stainless steel braids.
Double check all fittings for security. "It's a game of chance," when tightening pipe fittings. When you install them, they start to tighten about a half turn from where you want them, but that half turn won't be enough to keep it from leaking. You have to tighten it more but you risk it breaking.
Use Fuel Lube (EZ Turn) for pipe threads. But not on the flare fitting.
Paint the inside of the cowl. "I painted mine white," Tom said. "If you get oil on it, you can see it quickly and white it up. It doesn't get absorbed by the fiberglass."
Keep the area above the battery clear. See my note above.
Read accessories manuals. "I've seen people put the Lightspeed brains on the hot side of the firewall. The manual specifically says it goes on the cool side."
Take no short cuts.
WORST PRACTICES
Use snap bushings as firewall shields. "The last thing you need is a blowtorch coming at you."
Loose wires. "When I do inspections, people get sick of me saying, 'tie up the wires more.' Invest in a tie-wrap company."
Tying signal wires to plug wires. An EGT wire to spark plug wires, for example. "Use standoffs."
Anything touching stainless steel braid. "I see this a lot."
Sealant on a flare fitting. "It will leak."
The positive battery cable touching something.
Poorly fitting baffles.
Unsecured scat tubing. "It'll cut through."
READER COMMENTS AND TIPS
Jack Beck
If you are living in the frozen north where no one dares use the world "cold" unless the temperature dips below 30 degrees, you may be thinking about installing two heat muffs and running your heated air in series through them to the cabin. That was our plan and we thought far enough ahead so as to install the heat valve long before the engine. Great plan, we just did not think far enough ahead.
We installed the heat valve per the plans which call for the heat valve to receive the SCAT from the right side of the engine. What we missed early in the process was that by doing so we lengthened the amount of SCAT we needed to run and unnecessarily complicated the run of the SCAT. Here is what we would do differently. We would definately install the heat valve prior to installing the engine and plan to take the forced air for the SCAT from the right side of the engine baffle. However we would reverse the orientation of the heat valve so that it opened to receive the SCAT from the left side of the engine; then the SCAT run would have flowed much more directly from the right side of the engine baffle, to the right heat muff, to the left heat muff, to the left-facing air intake.
Martin Heller, RV-7
How about the old, 'not fully researching an issue' gotcha?
In getting ideas for planning the firewall, I copied from a well-respected builder and put the LASER control unit on the firewall forward in the same location he did. A few weeks later, the builder (now a technical counselor) helped me mount the engine and noticed my LASER location and advised. That really should be on the inside of the firewall.... I'm moving mine to get it out of the heat..
This
story appeared in the May
16, 2009 issue of the RV Builder's Hotline.
