Monday, April 26, 2010

The frustration index

In the 9+ years I've been building the RV-7A, I've tried not to set any deadlines or schedules. That gets more difficult as you near the end of a project, but it's a bad idea.

With much of the avionics for the plane completed over the winter, I've started to turn more attention to the engine installation. I've let myself think that I could have the first start of the engine some time this fall, and be flying next year sometime.

That's not going to happen.

The engine, arguably, is the most critical part of a homebuilt airplane. It is, for reasons I don't get, also the item that gets the least amount of attention in all of the instructions. As I've started working on this, it's not hard to see why so many airplanes go down. Builders are pretty much left to figure things out on their own.

There are many, many different combinations of engines, so it would be difficult, perhaps, to have a step-by-step guide for getting everything installed and hooked up properly. In addition, there are a number of different companies involved in an engine installation.

I've got an IO360 (fuel injected, 180 hp) engine with different companies producing the alternator, exhaust, fuel pump and fuel servo. And the engine itself is from Mattituck.

But when you get an engine, the thing arrives in a crate with a few small manuals. The only thing missing is a sheet of paper that says "Figure it out. Good luck."

A few weeks ago, I poured over pathetically incomplete schematics of the Airmotive fuel servo, looking for nothing more than a hint of the order of installation hardware and a torque value for the nut. I got one by way of Mattituck, but I haven't yet figured out how it was devised. It's not in any range of any of the tables for torque values that I use.

This past weekend, I tried to figure out how to install this puppy:

It's a T-fitting that comes out of the fuel pump. It has a restrictor fitting in it for the fuel pressure hose that goes to the manifold transducer, and a regular 1/4" line that goes to the fuel servo. Simple, right? Maybe for other people; not for me.

Should the fitting go in and be torqued. Should the nut be torque? We've kicked this around on Van's Air Force, and one of the problems is every answer seems to confuse a previous answer. I threaded it in to the point where I think it's tight, and notice that it dumps the fuel line just above a very hot exhaust stack. The hose that came with the engine isn't going to work. The line will somehow have to snake a different route.

So I'll have to make my own hoses. But how do I do that correctly? And how much will that cost me (A few hoses I ordered from Van's cost nearly $100. They're useless. They don't fit.)

What I need is "engine installation for dummies." Tony Bingelis' books are pretty good, but they don't fit the bill in this category. I've invited an EAA Tech Counselor to visit, but he's out of town until the middle of May.

Good building time is being wasted, but it beats putting an engine on that is likely to fail because I don't know what I'm doing.

It's a very frustrating situation. My last three visits to the hangar, I had to walk away from it. Hundreds of people have figured this out, but I can't seem to. So today I'm going to go back out, clean up the hangar, mothball what I can mothball, and walk away from the project for awhile and think about steps I can take to gain the knowledge necessary to do this better and safer than what I might otherwise do.

update 4:01 p.m. 4/26 - On the question of the part show, Gus Funnell at Van's Aicraft writes:

The fitting is installed finger tight to the desired orientation, then the nut is tightened to lock it in position. Some fuel lube to similar on the O ring is a good idea. Not sure what torque is appropriate - not much, as you don't want to completely crush the O ring. Err on the side of too little, then if it does leak when you run the boost pump, gently tighten it a little more.



  1. Bob,

    Don't give up! You're dealing with a knowledge gap, not a capability gap, and you're doing all the right things to overcome it. I live just up the road in Hopkins, and if I knew anything about building airplanes I'd lend my expertise. Since I don't all I can do is offer to buy you a beer and hangar fly a bit. Send me a message on twitter (@bradkoehn) when you're ready for that beer.

  2. That sounds frustrating. I hope you can find some experienced A&P advice, and/or advice from experienced builders. I salute your determination ( It'll come back, right? ) and look forward to hearing about your eventual first flight.

  3. One thing I can tell you is that usually the correct torque will depend on friction. If you have lube involved, that's probably going to lower the desired torque value.

    If this is an elastomeric o-ring, it should be designed so that it can't be crushed by the metal fittings. You can probably tell this by looking at it. If it's some kind of crushable metal one, then I don't know.

    In the case of the elastomeric ring that can't be crushed, if it was my project, I'd be tempted to buy a few extra fittings, set them up as they would be when installing (lubed the same, etc.), then, very carefully, tighten them until they broke or stripped, while measuring the torque. I'd probably use a torque around 50 percent of the breaking point.

    I am by no means an expert on fuel systems, or even very knowledgeable about them, but I've designed microwave connectors with o-rings and also specified torques for nuts and bolts in electronics packaging.

    If the nuts are not safety wired, I'd think about some kind of LocTite, and I'd be doing homework on what's appropriate in a fuel system.

    It's very possible, of course, that an A&P would know of some other factor.