Rotax Piston Meltdown

[jnmeade]

Originally Posted by AndrewR

The head is limited to 130C. I think the limit on the barrel temperature is 200C but I have not heard of anyone measuring it.

Can you cite the reference for that, please? I see CHT 150?C and coolant exit temp 120?C in the Apr 16 operators manual. Am I looking in the wrong place?

[waterboy2110]

Originally Posted by AndrewR

Yes. You might find a smoking gun cause, but Rotax documentation suggests the situation described is enough on its own to cause detonation, if you are unlucky.

I have my doubts that the cooling shroud was a contributor. I think the head is probably the critical temperature. The head is limited to 130C. I think the limit on the barrel temperature is 200C but I have not heard of anyone measuring it.

Agreed - Using the graph and the EFIS data I would say the engine failed because it was run at less than 5200 RPM and at 30 inches of manifold. Also, the document mentions pulling in hot air - which this installation design does - basically it's in "carb heat" all the time.

I'm going to pitch the prop on the new engine and get it out of this danger zone. After that we're going to give the Keiv prop a try and see what we see.

At some point I may look into a solution to get cool air to the carbs. The engine would not have been set up by the factory to run in hot air so I expect it's running rich by this design and losing horse power.

Roger Lee from Rotax-Owner has this to say:
"They are better tuned for the engine and setup for proper cool air flow to the carbs. Using air filters on each carb under the cowl isn't a good way to go. The guys that have air filters under the cowl attached right to the carbs are flying in carb heat all the time. It makes for a rich fuel mixture. Cool or cold (depending on your definition) dry air for combustion would be better."

DanH - the X axis is RPM, Y is manifold pressure. The prop should be pitched so that it avoids the gray area on takeoff. Much the way a constant speed would be set when every this is forward. Managing these parameters on takeoff roll would seem to me to be problematic.

As mentioned earlier - my partner - a solid Rotax guy, had been dogging me to pitch the prop for higher RPM. We did at one point but the cruise suffered. If this graph was in front of me then that would have been all I needed to see.
I fell back on Vans documentation for prop pitch because that was data I could put my hands on. I regret that decision now.

[rvbuilder2002]

Investigating and evaluating failure modes can be very challenging, but finding the real answer can be made much more difficult if some info is accepted as fact and other info is ignored.

What should not be ignored is the fact that RV-12's have been flying with the same engine and propeller configuration since 2007. The first customer built RV-12's began flying in 2009.

Nor can we ignore that there are now more than 575 RV-12's flying with the prop. pitch set at a value to achieve best overall performance. The actual setting varies from one RV-12 to another because of personal preference with how the airplane is flown and what the typical operating conditions are (home airport elevation near sea level vs 5000 ft or more). That is one of the benefits of a ground adjustable propeller. The setting chosen by a particular operator may not assure operation outside of the "recommended to avoid" range of the linked chart for all portions of a particular flight, but that is the case for all of the LSA aircraft on the market that come anywhere close to achieving the maximum speed performance allowed by LSA rules. It is not possible to have a pitch set for a reasonable cruise power setting on low drag LSA airframe and fully meet Rotax's recommendation.

Is that bad? That is a question worth an answer....

In the context of the RV-12 I think the answer can be determined by looking at the fact that 575+ RV-12's have been flying (many in OAT's higher than the conditions when this engine failure occurred), using 91UL fuel, with a propeller pitch setting that only allows for ~5100 RPM during initial climb, for 11 years and 10's of thousands of hours without having a failure. Many of these RV-12's operate where it is hot a good majority of the time (South America, etc.).

I think your best chance of finding smoke is to stop looking at things that a few of the experts zero you in on, and start looking for what was different on this particular failure flight, when compared to the other 574+ RV-12's that are flying every day with the same settings and conditions.

[rv6ejguy]

A photo of the other spark plug will tell us a lot.

[DanH]

DanH - the X axis is RPM, Y is manifold pressure. The prop should be pitched so that it avoids the gray area on takeoff.

Not the Rotax graph. I was asking about the RPM graph you posted to your blog. Y is RPM, X appears to be time, and I was curious about the units. You see, detonation typically takes a while to progress to serious damage.

The physical damage to the top of the piston is largely mechanical, from chunks of broken ring and piston. There's a little splatter, but not a lot. Splatter and a sand-blasted surface is a tell-tale. It's due to the shock wave. Not much splatter, not much shock.

What I think happened here is light detonation, the kind most engines will shrug off if it isn't prolonged. The stated RPM, MP, coolant temperature, and intake air temperature are all pro-detonation, plus the front cylinders appear to have been running a little leaner than the rear pair (note the carbon difference).

However, if it overheats a plug, the failure progresses to preignition, which runs peak cylinder pressure way up. So a ring land, already at an elevated temperature and weakened a bit, breaks away. The ring, now without support, breaks next.

So why would a plug overheat a bit too easily? One reason could be the wrong heat range. It wouldn't be the first time plugs got mixed in the same engine. Another is lack of heat sink paste, which seems to be a required item in order to run the recommended heat range. The third would be a loose plug. If it's not tight, it can't transfer sufficient heat energy to the cylinder head and coolant.

It would be interesting to take a look at the missing spark plug, and the combustion chamber surface at 12 o-clock, which is why I've been asking. My supposition could be wrong, but I'll present it here prematurely in hope of seeing all the evidence.

[N456TS]

Originally Posted by rvbuilder2002

In the context of the RV-12 I think the answer can be determined by looking at the fact that 575+ RV-12's have been flying (many in OAT's higher than the conditions when this engine failure occurred), using 91UL fuel, with a propeller pitch setting that only allows for ~5100 RPM during initial climb, for 11 years and 10's of thousands of hours without having a failure. Many of these RV-12's operate where it is hot a good majority of the time (South America, etc.).

Wait, wait.... Vans is actually instructing folks to bog down their engine down to 5,100 RPM during take off. This isn't a typo? Everyone here who followed that needs to re-pitch their prop before next flight. Wow!!
Marketing specs > Engine safety.

[rvbuilder2002]

Originally Posted by N456TS

Wait, wait.... Vans is actually instructing folks to bog down their engine down to 5,100 RPM during take off. This isn't a typo? Everyone here who followed that needs to re-pitch their prop before next flight. Wow!!
Marketing specs > Engine safety.

Read my post again......

I said the pitch value people are flying with.
I didn't say the value that Van's makes them use.

And then go read the entire document.
No where does it say that if you do not 100 % meet this specification your engine will blow up.


Next I guess you need to go on all of the web sites for all the other fast LSA's and give them the same advice.

[vic syracuse]

Without throwing gas on this fire, and separately from my affiliation with Van's, I vehemently agree with Scott. I have over 1300 hours behind 912's, in a number of different types of airplanes, using different propellers (including in-flight adjustable), running both mo-gas and 100LL, since the early 90's. I have found it to be pretty much a bullet-proof engine. Early on we didn't have quite the choice of propellers we had today, and I assure you we ran some of these engines down around 5000 or lower for takeoff. They didn't blow up.

The one failure noted here in this thread is 1 in over 575 flying engines on RV-12's, or 2/10's of 1 percent. I really don't think we need to jump to conclusions that the whole fleet is at risk. There's lots of reasons why an engine can fail, and some of them are due to metallurgy or manufacturing problems, operator issues, environment, or fuel problems, and ignition problems. But I think it is pretty safe to think that the particular configuration with the heat shroud has enough time on it that if it were the problem we should have seen some other failures by now. Even if they weren't as serious as this one.

They aren't out there. The 912 in the RV-12 seems to be quite reliable. Having a lot of time behind 912's I was anxious to get the factory RV-12 so I could spend more time with it. I had flown some first flights on RV-12's and I was amazed at the cooling, compared to what I had seen over the years on other non-RV aircraft. Now, having spent time with the factory RV-12, in both hot weather and high altitudes at gross weights, I am pleasantly surprised at the performance with regards to cooling.

To me, it's not unlike the rest of the RV fleet. The majority of the fleet does not have any overheating problems. But every once in a while you will see on this web site that someone does have a problem. Usually, it gets solved. Remember, as much as these are called kits, every single one is really different. I have yet to see 2 alike in the hundreds I've inspected, except for the SLSA's of course.

I think we should see all of the evidence before jumping to conclusions. As Dan mentioned, let's take a look at the plugs, and understand any ignition anomalies before jumping to grander conclusions.

Vic

[PilotjohnS]

You said you thought it was piston meltdown, but could it be debris inside the cylinder? Just thinking out of the box.

[AndrewR]

Originally Posted by jnmeade

Can you cite the reference for that, please?

The barrel temperature is in the Installation Manual, but it's directed at people designing the cooling system rather than the general user. It also says:
NOTE: As long as the oil and coolant temperatures are within the operating limits, no cooling air ducts are necessary

The barrel temperature is not normally measured on Rotax or Lycoming. If you compare the fins on a Lycoming head vs. barrel the head seems to be the critical area. So I don't think the cooling duct is likely to have contributed. (My opinion only, I am certainly not an expert!)

Originally Posted by jnmeade

I see CHT 150?C and coolant exit temp 120?C in the Apr 16 operators manual. Am I looking in the wrong place?

CHT limit of 150C is for the 80hp 912. The 100hp 912S is 135C (maybe 130 in earlier manuals). I understand the difference is for detonation margin with the higher compression.