Engine test flights

[Brett Buck]

I may have mentioned this as an aside before, but when you are flying the first flight after some engine adjustment/change/fix, be very careful and check it out in a safe way first. That way, if the engine unexpectedly quits, you have a chance of saving the airplane. I suggest several inside loops to start. That's the most likely place for it to crap out/flame out for inverted-mount engines. Listen very carefully to how it runs, and in particular, if there are any unexpected misfirings or the engine seems like it "goes rich", particularly at the bottoms of the loops, beware. If that sounds OK, then flip it over and try some outside rounds. You might try a hard corner each way, too.

     Be particularly careful after a glow-plug switch or testing a new plug. Sometimes, a new or different glow plug will just cause engine to quit dead, sometimes with NO warning like you flipped a switch. I have no idea why and have heard no plausible explanations. It tends to happen more with ABC/AAC engines. Many times, if you go over attach a battery and flip it, it starts like nothing happened, then will do it again.

    Fuel matters, too, and my best guess (which is poor and hand-wavy) is that the issue is somehow related to the combination of fuel and glow plug element material and some combinations are chemically questionable, and/or to not have the same catalytic action. 

    Some engines are more prone to it than others, too. I had what seemed to be identical engines from a super-high quality manufacturer. One ran fine in any condition, the other would run 30 seconds -minute on the bench, then quit like you flipped a switch. Start it back up with no changes, sounds fine, then another flameout. Same thing in the air, it would "load up" (i.e misfire severely) at the first inside loop, even worse in the second, and then it would quit in the third, time after time. Glow plug or conditions appeared to make no difference. The *only* fuel that particular engine would run reliably on was Byron "Classic", and then at astronomical (and extremely noisy) launch revs of ~12,000 rpm*. The other seemingly identical engine ran fine in the same conditions, with no issues and not fussy about fuel at all. It took me about 1 year or so of testing, etc, measuring everything you can measure, etc, and I found *no dimensional* differences at all to about .0001 in anything. Put it all back together, one engine worked, the other didn't.

   Needless to say, if you make some change, take off, jam in a hard corner to enter the wingover, and it quits, you are slow (since you just turned a hard corner), upwind, and slowing rapidly because you are vertical. The chances of saving the airplane in that situation are very low.

      So be careful.

   Additionally, after having seen the results time after time, I would categorically reject the use of Rossi plugs. They seem to have this problem time and time again, even on the ground, in all sorts of engines. Ted discovered this the hardest possible way at the NATs one year, but many others have had the same.

 So any time you are testing/changing/fixing any engine problem, be very careful for the first few maneuvers, keep the airplane alive.

      Brett

[Mike Scholtes]

That's a real puzzlement. Did you ever settle on a satisfactory explanation or theory why Thing 1 would work and Thing 2 would not? I mean, these are just simple machines and two identical copies of the same engine from a top quality maker, which you confirmed were dimensionally identical within any reasonable tolerance, should work the same, one would think. Discounting the possibility that gremlins are a real thing, there just doesn't seem to be non-magical explanation. Did the unreliable one ever become reliable?

Your main point is sure a good one. I changed to a brand new Thunderbolt this weekend and just went out and hit the pattern with no thought of testing it first. I will think twice in future. Too much time invested in these things to splat them for such a dumb reason.

[frank mccune]

     Hello Mr. Buck:

     Great post that concerning an issue that I have noticed for many years when working with engines.  In the trade, it is often summed up by the expression," PFM."  This can be interpreted as pure fxxxxxx magic.  It often happens after one has tried all known remedies.  There is no explanation as to why something will start working for no reason.  I have learned to accept this as a blessing.  Do not question why it happened but  get on with your life!  Stuff happens!


                                                                                                                             Be well,

                                                                                                                             Frank McCune

[Steve Fitton]

Matt and I went through this sort of thing Tuesday evening at Muncie.  In this case, the problem was brought on by my being in a hurry with the rebuilt fuel tank for Matt's plane.  When I finished soldering it I forgot to rinse it with acetone and then fuel, so it had plenty of flux left in the tank.  The motor quit with ZERO warning in (if I remember correctly) the last corner of the hourglass, just like a switch had been flipped.  By then I had realized my mistake, and surmised the plug had been contaminated in some way by the flux.  A new idle bar thunderbolt went in, but now the 75 seemed to stumble on hard inside corners.  Another change, this time to a thunderbolt big bore, and now the 75 seemed happy.  It took several flights for Matt to regain confidence in the engine run, and I figured every flight made sure there was less and less flux in the tank.
It could have been scary though-especially since we were down on the grass near the combat circles, and the noise level was much too high to clearly hear the 75 in Matts SV.  If it had decided to quit in an overhead the first warning Matt might have had was the propeller stopping(!)

[Paul Taylor]

To still a term from my flying buddy when the engine stops it makes "A loud quiet".  😉

[Matt Colan]

Matt and I went through this sort of thing Tuesday evening at Muncie.  In this case, the problem was brought on by my being in a hurry with the rebuilt fuel tank for Matt's plane.  When I finished soldering it I forgot to rinse it with acetone and then fuel, so it had plenty of flux left in the tank.  The motor quit with ZERO warning in (if I remember correctly) the last corner of the hourglass, just like a switch had been flipped.  By then I had realized my mistake, and surmised the plug had been contaminated in some way by the flux.  A new idle bar thunderbolt went in, but now the 75 seemed to stumble on hard inside corners.  Another change, this time to a thunderbolt big bore, and now the 75 seemed happy.  It took several flights for Matt to regain confidence in the engine run, and I figured every flight made sure there was less and less flux in the tank.
It could have been scary though-especially since we were down on the grass near the combat circles, and the noise level was much too high to clearly hear the 75 in Matts SV.  If it had decided to quit in an overhead the first warning Matt might have had was the propeller stopping(!)

Not to mention the CH-53 flying overhead, almost nothing was audible while I was trying to recreate the same thing that made the engine quit to see if it was repeatable with the new plug. Thankfully it was just the plug and the flux in the tank like Steve said.

I've had two flameouts when running a PA ever, and both times were from the result of the plug going bad.

[Tim Wescott]

That's a real puzzlement. Did you ever settle on a satisfactory explanation or theory why Thing 1 would work and Thing 2 would not? I mean, these are just simple machines and two identical copies of the same engine from a top quality maker, which you confirmed were dimensionally identical within any reasonable tolerance, should work the same, one would think. Discounting the possibility that gremlins are a real thing, there just doesn't seem to be non-magical explanation. Did the unreliable one ever become reliable?

Trying to find rational, scientific explanations to things like this can have otherwise sane and sober engineers laughing maniacally and running around in circles.  I've been part of missions to find out stuff on a level of obscurity as this, although never with engines, and you can spend man-years of fruitless effort.  You can also get a lot of false positives on the true, underlying root cause.  Even within a group of engineers, who are supposedly serious and rational science-based people, you can start getting the same sort of voodoo legends that Brett decries in stunt circles (if his work experience is like mine, that's part of the reason why he decries them).

So, find someone with a PhD in combustion technology, surround them with a couple of dozen really smart engineers and technicians from various backgrounds to question and set to tasks, and wait a year.  You may get an answer, and it may even be a correct one -- but a hell of a lot of effort will be expended.

Sometimes, unless it's for mission- or life-critical stuff, it's best to just shrug and say that it's voodoo magic, and move on.

[Eric Viglione]

Great advice Brett. Me and glow plugs have come to an understanding... change them early and often.

Joking aside, I "think" there is a plating effect on every heat/cool cycle when exposed to fuel. Kinda like "taters" only less visible and more insidious. More so with piped engines where the regulating pipe back pressure stagnates part of the exhaust cycle.  New engines, or engines that have been apart and reassembled in any fashion, new metal tanks soldered with lead or silver, all make metals and byproducts that I think plate the plug and interfere with the reaction needed to run. We all know what leaded gas does to the catalytic converter in your car, it eventually ruins the capability to react and burn off pollutants, etc.

This is why I use inexpensive plugs for break in, and chuck them after used for bench running a new engine or unknown status engine. Same thing with first few in-air flights with that engine in a new plane, plug gets chucked promptly. Also do this after installing and running new metal tanks a couple times too. Gets expensive, but I've witnessed first hand the symptoms and results. Plugs are somewhat fragile and changing them is cheaper than a new plane.

That is the other reason I go through the trouble to transport piped planes level. Sure, if you transport nose down you can flip the plane on its back and burp the engine with a little fuel to clear the oil, but that burnt oil that was in the pipe and ran into the combustion chamber is nasty acidic crap, and I'm pretty sure it does bad things to the plug, bearings and engine. I may just be a bit paranoid, but a healthy dose of paranoia can be a good thing when it comes to stunt. Another trick I do is when I land for the last flight of the day, I'll hold the plane nose up and rock the prop a few times while rocking the plane a little and the hot oil runs right out the pipe. Fitton seemed pretty surprised the fist time he saw the volume of oil from my 75 plane "taking a leak" at the end of the day. It's a pretty studly engine. Heh heh.

Brett - I know you mic'd both engines to the n'th degree, but as a hail Mary play, I'd suggest taking a cold and hot compression reading on both engines and compare them to each other. It might tell you something interesting. Not the rubber press fit gauge, but the good thread in job like the nitro r/c car guys use.

Also might not hurt to check glow plug depth to piston top clearance with both engines and pistons at TDC. I made a tool for doing this using an old drilled out glow plug and a length of music wire with the tip polished smooth so not to scratch top of piston, with a set screw tapped into the side of the glow plug shell. Put in the empty plug, bring the piston up to TDC, drop the music wire down into the hole where the element used to be until it hits the top of the piston, and lock the set screw, remove the plug and measure protrusion.  2 otherwise identical motors might still be different to each other for various unforeseen reasons.

To be honest, these days most of my engines have never even had the head off... I stopped tinkering a long time ago, once I starting using quality engines, especially new PA's from a known source. They just work, and when they don't, I send them to Randy. Used engines are always pot luck unless you know the history, so I usually figure the cost of sending to Randy for a checkup into the price I pay for a used PA... people do some odd things to their engines, even high dollar ones. heh.

EricV

[BYU]

Ooh no, not again . . .

- Clogged needle valve assy
- Burned up plug
- Tank and fuel line with gunk in them

Engine run = 4 - 2 - 0

[Warren Wagner]

Great advice Brett. Me and glow plugs have come to an understanding... change them early and often.
Also might not hurt to check glow plug depth to piston top clearance with both engines and pistons at TDC. I made a tool for doing this using an old drilled out glow plug and a length of music wire with the tip polished smooth so not to scratch top of piston, with a set screw tapped into the side of the glow plug shell. Put in the empty plug, bring the piston up to TDC, drop the music wire down into the hole where the element used to be until it hits the top of the piston, and lock the set screw, remove the plug and measure protrusion.  2 otherwise identical motors might still be different to each other for various unforeseen reasons.

EricV

Eric,

Very clever little tool.   Question:  What do you do with the information after you have
measured the 'depth to piston' ?   The stroke of various PA 75 (for example) should be
within a few thousands for a batch of engines, so the major variation should come from
head structure, I would assume.

Thanks.

Cheers.

Warren Wagner

[Eric Viglione]

Eric,

Very clever little tool.   Question:  What do you do with the information after you have
measured the 'depth to piston' ?   The stroke of various PA 75 (for example) should be
within a few thousands for a batch of engines, so the major variation should come from
head structure, I would assume.

Thanks.

Cheers.

Warren Wagner

Warren - You compare it to the one he has that run's fine and if different, and you reverse engineer the problem to the source. One thing I didn't catch from Brett is the source he got the second engine from, new or someone else claimed new, or used, etc. Some engines like PA's use a button insert, and people can put in different ones, another thing is head gaskets... I've seen the smallest one's stick together, where you think you have 1, but you have 2 or more in there. A previous owner could have milled the lip on the sleeve (not sure if even possible on a PA) Etc etc etc. Some engines shellac up from fuel additives so that makes the compression goofy. That is why I said "various unforeseen" things could be a gotcha, so I left it open ended so as to look for the actual effective clearance, and actual effective compression, regardless of how it got there, then chase the how/why later.

When Brett is baffled, you know it's a good puzzle and the basic's will have all been checked first.

All this stuff is why I now just say "No Más" and send them into Randy for his blessing... a whole lot easier and less frustrating that chasing mysteries. Heh heh.

EricV

[Brett Buck]

When Brett is baffled, you know it's a good puzzle and the basic's will have all been checked first.

All this stuff is why I now just say "No Más" and send them into Randy for his blessing... a whole lot easier and less frustrating that chasing mysteries. Heh heh.

     I wouldn't assume the former, but at least I *think* I got the basics right. And in fact, I *did* send the one engine back to the original source, where it apparently continued to have issues. I think the replacement, which worked fine, after a few gentlemanly transactions, may have finished two spots behind me at the 2006 NATs.  Engine #1 later ALSO flamed out, surprisingly and at a bad time right before 1996 NATs, which is why most of you never saw Infinity #1.

   Brett

[Ted Fancher]

    snip

 I think the replacement, which worked fine, after a few gentlemanly transactions, may have finished two spots behind me at the 2006 NATs.   

snip

   Brett

TWO spots behind you?  What'd you do finish SUPER No 1?  Number 1+?   Just checked my wall of Mr. Bridesmaid Nats plaques and there was 2006, big as you please.  Two spots behind you, huh.  Haruummpphh!

Of course, it could also be that there was a big enough point gap to allow room for several more to have slipped in betwixt and between.

Ted

[Brett Buck]

TWO spots behind you?  What'd you do finish SUPER No 1?  Number 1+?   Just checked my wall of Mr. Bridesmaid Nats plaques and there was 2006, big as you please.  Two spots behind you, huh.  Haruummpphh!

  Good thing it was a stunt contest and not a counting contest. Two spots behind is 3rd, not 2nd - it wasn't you, you had a RO-Jett 61 at the time, and this was not a RO-Jett. It was our little buddy Derek, and I think the (good) replacement engine was the one I sold to Dale many years earlier.

   You may recall the engine performance for the "bad" engine, since you were the first one to ask "why did it quit?" while I was breaking it in. Which was a pretty darn good question.

    Brett

[Dan McEntee]

   Hi Brett;
    On the "mystery flame out" engine, did you get a good magnified look at the glow plug? Just curious if you saw anything like a distorted element or anything out of the normal. And it would restart easily?  Did it tend to eat plugs? In thinking about the basics of what it takes to make a small, glow powered two stroke engine run, I would have to make a guess that something was making the element distort  and perhaps touch the side of the chamber and short it out. Only has to touch for a nanosecond or so. That would be my wild ass guess. I have some other questions and ideas, but not suree how to express them, but is related to compression, fuel flow and plug design. Too tired from a hot day at work to think much right now.
   Type at you later,
    Dan McEntee
   

[Brett Buck]

   Hi Brett;
    On the "mystery flame out" engine, did you get a good magnified look at the glow plug? Just curious if you saw anything like a distorted element or anything out of the normal. And it would restart easily?  Did it tend to eat plugs? In thinking about the basics of what it takes to make a small, glow powered two stroke engine run, I would have to make a guess that something was making the element distort  and perhaps touch the side of the chamber and short it out. Only has to touch for a nanosecond or so. That would be my wild ass guess. I have some other questions and ideas, but not suree how to express them, but is related to compression, fuel flow and plug design. Too tired from a hot day at work to think much right now.
   

      I spent many hours and sessions comparing one to the other. My first thought (after convincing myself that the engines were set up the same and had no consequential dimensional differences) was that something was plating itself to the glow plug. Confusing the issue was that this was around the time of/just after the 1995 "SIG Fuel Disaster NATs" where a bunch of people had problems and David destroyed the connecting rod in this PA40 in about 30 flights*  and the Red Max/FHS (whose fuel I was using at the time) "all castor oil is contaminated" BS. I would run the engine until it (anomalously) quit, then take out the plug, looked normal, put it back, starts immediately, runs normally until it just quits again. Take that plug out, put it in the other engine, run out the tank, put it back, 30 seconds, quit. Mayeb it doesn't like a Glo-Devil #300, put in an Enya, repeat, same story. I even violated my own rule and got every Rossi plug I had and they had at the hobby shop, even worse, particularly the "Warning, Extra Hot,  2,5 cc motori and smaller ONLY". OK, maybe it's *too hot*. Go find an ancient Glo-bee Cool, starts, runs ratty for a few seconds, quits, same thing over an over.

  Getting desperate, I dig out an even more ancient Arden "Replaceable Element" plug my dad used in 1949, the first glow plug to appear in Central Arkansas. Ran, went ratty for about 2 revs, ran some more, ratty for a second, ran some more. Completely unacceptable, but I flew it, and it would keep running but load up heavily and want to quit on insides.

   I won't down the entire list and I know this looks like a mystery for everyone to solve, but I can't tell you how many different fuel/plug/fossil fuels/paint shop materials/cooking supplies/possibly AMA-illegal "fuel additives"/variants I tried before I wound up with *BYRON* fuel, for goodness sake. Cool Power and Omega were in the first 10 percent of the things I tried. LOTS of straight synthetic from 12% to 25%. I even used some GMA "Magnum" fuel with a reported 7-8% with a now illegal (for sale) 50:1 lawn mower oil.  Many things like benzine and some other foul-smelling liquids ending in "zine". I wound up with a quasi-workable system at sea level, won a contest at Sepulveda, but at 12000 rpm and gutted out, not a CHANCE in Muncie. Even when I had the other engine and it was working as well as I could make it, it still wasn't that great, another highly-experienced competitor came to the same conclusion independently at the same time. The 40VF/pipe I bought from Alan Resinger for $125 because he considered it "worn out" worked much better, so end of problem.
     
     I am well aware that it was some engineering reason, not magic, but I gave up eventually, and I have none of these engines left (seeing as how this was 20+ years ago) and they are not commonly used any more, so I decline to throw a lot more time into the project. The one takeaway is that if your engine flames out unexpectedly, try Byron fuel. I did that at the NWR where a well-known competitor was having the same issue with a different engine, and it went away completely. He wouldn't run it after the one flight because he was afraid his "engine guru" wouldn't like it, went back to having problems.

    Brett


*Maybe the first and last failure of that type I have ever heard of from any PA engine and undoubtedly the result of whatever those clear globs we found floating in the fuel

[FLOYD CARTER]

The suggestions are valid.  It has  been my habit to fly nearly every "first flight" on an airplane, or engine just level, with any inverted flying very high to have room for recovery.

Even first flights on electric planes are done with great caution.

You can see the result of my not always following that regimen on www.flyinglines.org, under "Numerous Pieces".  I had an OS46LA quit overhead, with the P-51 reduced to scrap.

[Garf]

One thing I always do is pull the head and check how closely the plug length matches the head. It will surprize you.

[Steve Helmick]

One thing I always do is pull the head and check how closely the plug length matches the head. It will surprize you.

Yup, combustion chamber depth and glowplug recess or protrusion are both very important. I know Randy Aero sometimes machines a "heat dam" into the head...a groove around the glowplug seat...to retain heat in the plug. It appears like we now have much hotter glowplug options than 10-20 years ago.

Have never had a flame-out with either my PA .51 or OS .46VF. Can't recall ever having a glowplug problem, except for the VF once, when I bounced a landing badly and it broke the post off.    Steve

[Steve Fitton]

Was the "mystery" engine issue in the era before sealing the spraybar to the case?

[Brett Buck]

Was the "mystery" engine issue in the era before sealing the spraybar to the case?

   No.

    Brett

[Brett Buck]

One thing I always do is pull the head and check how closely the plug length matches the head. It will surprize you.

    I can see why people have endless issues with engines, don't take them apart unless you have to (i.e. there is a problem).

     But in this case, of course I measured the head depth and the plug protrusion. One of the theories was that the threads on the head were clocked differently on one head than the other, causing the idle bar to be oriented differently. That wasn't it, and, we tried otherwise identical plugs with the idle bar in different places WRT the threads and it made no difference. Perpendicular to the crankshaft, 90 degrees to the crankshaft, random angles, no idle bar at all, etc. Same thing.

   I also ran the head in all 6 (or 8, I forget) positions, no difference.

      Brett

[Steve Helmick]

No idle bar orientation problems or controversy with the Thunderbolt 4-stroke plugs. I've always run them in the PA .51 and .46VF. I've also run them in .46LA and XLS .36 with no problems.

This particular plug was discontinued by Hobby People and the replacement called the "4-stroke and Big Bore". I've never needed to get any of the new version, but wondered if they were any different in meaningful specifications. I suspect it was just a name change to sell more, but could be wrong. Since HP is defunct, it doesn't really matter, but I still am curious about that.    Steve

[phil c]

Brett- "   Be particularly careful after a glow-plug switch or testing a new plug. Sometimes, a new or different glow plug will just cause engine to quit dead, sometimes with NO warning like you flipped a switch. I have no idea why and have heard no plausible explanations. It tends to happen more with ABC/AAC engines. Many times, if you go over attach a battery and flip it, it starts like nothing happened, then will do it again.

    Fuel matters, too, and my best guess (which is poor and hand-wavy) is that the issue is somehow related to the combination of fuel and glow plug element material and some combinations are chemically questionable, and/or to not have the same catalytic action.  "

From a chemistry standpoint I think you are totally correct.  The wire in the glowplug is a catalyst and catalysts can be very fussy.  They're also subject to poisoning, mainly from dissolved metals or sulphur compounds.
Known problems- too "cold" which means there's not enough catalytic action to keep the process working.  Try a different plug or a "hotter" plug.  If the motor slows or misfires when the battery is taken off almost always means a bad plug, or sometimes one that is too cold for the engine.

Wrong thread length.  They mostly seem to come in 5,6,7 threads- or short, medium, and long, 1/4-32 thread.  Using a shorter plug than the right length exposes threads which can dampen the fire so to speak.  So can a head that is fitted too loosely(to small a plug in diameter). The small gap between the head and sleeve quenches the combustion, usually erratically.  Too loose is when the head never gets stuck.  Just right is when the head has to be aligned nearly perfectly to go in or it gets angled and locks up.

The OS A series plugs seem to all to work well for different heat ranges- #6, #7, #8, #10  going from hot to cold.  OS seems to have found the right platinum alloy.  They also make a couple 6 thread plugs in the medium and hot range labelled LC3 and LC4.

Water in the fuel will cause erratic running and unexpected engine stops.  Keep the fuel in a sealed container.  Never suck fuel from an open jug with a piece of fuel line.  A sintered fuel pick up on the fuel pickup line can work wonders keeping tiny bits out of the needle valve.

Silicon antifoam can cause trouble, more in engines run hard than in stunt though.  A hot run followed by eventually quitting rich can cause a buildup on the element.  But just the fact that it can cause a problem is enough to avoid it if at all possible.

Cheers,  Phil C