Saturday, June 16, 2012

Flight Test Follies - Part Three

When you're inching your way through getting to know a new airplane, it's hard not to think about all those flight-test reports online in which the builder says "she flew like a dream, hands off!" And "no problems."

I think flight testing a homebuilt airplane is more complex than that and I like to think the reason we don't hear more about those less-happy-go-lucky situations is because the builder is trying to figure out what the hell is wrong with his/her baby. And maybe also kicking the cat.

N614EF has been fun to fly so far. There's still the matter of the heavy wing, but that's getting solved slowly. But, to me, I still think she's been trying to tell me that she's not entirely happy, I just haven't been able to pin it down.

As you know, if you've read this column recently, the static RPM was a little low and we struggled through timing the magneto and generally trying to figure out what's "a little low" and what's "typical for this engine/plane/prop combination."

I was just starting to accept the latter yesterday when I heard it. At least I think I heard it.

I was over the test area, and shooting a few pictures of some flash flooding below near Stanton, Minnesota, when I thought I heard a misfire. That sort of thing catches your attention real fast. But the engine smoothed right out and the engine instruments were all in the green. Maybe I was being hypersensitive.

That was yesterday morning and the rest of the flight went fine.

I made some minor adjustments in search of a lighter wing last evening and then took the plane up for a ride. The plane seemed a tad sluggish, but not overly so.

Now, the rest of this I'm pulling out of some engine data that I recorded on a laptop, but because the logging program for the Grand Rapids EIS 4000, err, stinks, it's a little incomplete. You can't actually SAVE the data to a file, so I've taken snapshots of the data.

On runup, there was a 10 RPM drop when operating on the Lightspeed electronic ignition, and about a 80-100 RPM drop when operating only on the magneto.

Accelerating down the runway, I was getting about 2298 RPM -- somewhat less than earlier in the day -- cylinder head temperatures were 359/357/363/348 and exhaust gas temperatures were 1294/1133/1112/1150.

The anomaly there is that the #1 CHT is usually the hottest -- I assumed it's because there's an air dam in front of it as specified in Van's baffle kit instructions -- and the #1 EGT is lowest; it's been a finnicky connection, and is on my list of things to rip out, reposition, and replace.

The temperature indication imbalance was the case right up to takeoff, too. The #1 EGT was more than 100 degrees lower than the rest of the cylinders. In 22 seconds during the runup, for example, the temperature (EGT) went from 1012 to 797. Then it went back up quickly. Again, I suspected the connectors were crappy.

Anyway, as I climbed, I was over 100 knots, oil temp 163, oil pressure 81, the fuel flow at about 14 gph (I should point out here that the FloScan has not been precisely calibrated, but that's probably pretty close. The manifold pressure was about 26.9 and the fuel pressure was steady at about 25.



I was about 4 miles southeast of the airport when someone called in that they were "southeast of the airport inbound" (for the love of God, people, would it kill you to indicate how FAR southeast of the airport you are?), so I increased my climb a bit.

I wasn't getting more than 2300 RPM in the climb but as I passed through 2500 AGL, it happened. The engine coughed. It got my attention. Again. This time it stayed rough.

My first reaction was to reduce RPM and think about what I'd just done to undo it. Other than climbing, I hadn't done anything. I stopped climbing. (Note: I'd also brought 20 gallons of AVGAS down from KSGS and dumped it in the tanks, but it checked out fine). The engine started running rough, but was manageable and sounded happier at reduced RPM of between 1800 and 1900 RPM. Boost pump on, fuel tank switched. No difference. Mag check. Operating. Lightspeed check. Operating.

As I immediately turned back toward the airport -- I was six miles out -- I had no trouble maintaining control and the plane was clearly able to stay aloft, but when I added power, it shuddered. I thought I heard popping noises though I couldn't confirm this. It sounded like a wing gap fairing piece of rubber might be hitting the side of the fuselage (it wasn't).

I am kicking myself that I didn't pull the alternate air source cable. If that had smoothed things out, that would've told me a LOT. And if it didn't smooth things out, I'd at least know there wasn't an obstruction around the air filter causing the woes.

I decided to stay at reduced power, entered the pattern and landed without incident. After turning off the runway, I doublechecked the ignition and both were operating. A static RPM check, however, revealed the engine could only generate 1800 RPM. I taxied back to the hangar, and shut it down.

I attempted to review all the engine data on the laptop, but it stopped recording that data right around the time the problem developed. I'm not reading much into this, however, because the monitor-to-computer setup is rinky dink and the serial-to-USB connection could've gotten loose. Plus, as I said, the EISLog program hates my Windows 7 laptop.

But I monitored those readings as the event was occurring and there was absolutely no evidence showing itself. And none of the engine monitor's alarms went off.

Pulling the top cowling did not reveal anything except a couple of drips on the bottom cowling. One point here: I've been amazed -- and a little concerned -- about how much oil the engine is NOT consuming. Since Tom Berge's test flight, I've added a half-quart of oil, resulting in oil on the belly. The engine has run for about 3-4 hours in that time, including an hour at 2400-2450 RPM on yesterday morning's flight.

Something is obviously wrong here, but what?

When he looked at the plane last weekend, Stein Bruch noticed the throttle cable was very stiff to operate. And, thinking about it, it seems to me it had gotten stiffer over the last two flights. A check earlier showed the throttle hitting the stops, but it's unclear whether this stiffness is in the cable itself, or the throttle on the fuel servo.

The other thing I noticed yesterday morning is that at 3500 feet -- when I tested it -- it takes VERY little leaning of the mixture to make the engine run rough. Very little -- less than a half inch of mixture travel. Is that a problem? I don't know, but it's different than most engines I've flown behind.

In fact, if I had to describe what the engine was doing, it would be most accurate to say "it sounds like what an engine sounds like when you lean it out too much."

The engine passed a fuel flow test last week when taken at the inlet to the fuel pump, but it appears another one is called for at the fuel servo.

Assuming all these numbers are correct, I'm going to focus on that servo, pull it off and see whether anything presents itself. Disconnected from the throttle cable (another one was already on order), does the throttle arm still move stiffly? If so, I'd guess that shows where the problem likely is. Is there some crap in there I couldn't find earlier? Is there a filter screen or anything somewhere in the servo I can pull off and check?

I also want to doublecheck how I mounted the throttle/mixture brackets. That's integrated with fuel servo installation. Should there have been another gasket in there? Did I not put it on, is there a leak there?

Air and fuel -- those are the two players in this since we know we've got good sparks and a timed ignition.

If today's probe doesn't reveal anything, I've got a problem. I've tapped out my own limited expertise, and the plane isn't at my home airport. I'll need an expert who makes housecalls.

3 comments:

  1. sticking exhaust valve, bad plug, fuel servo
    malfunction......hire an a&p

    ReplyDelete
  2. There was a blockage in the #1 fuel injector.

    ReplyDelete
  3. Stay close to the field for a couple of hours until you
    are sure.....

    ReplyDelete