Cold weather flying

[FastEddieB]

Isn't there some dissimilar metal with different expansion/contraction rates at play?

Yes. But my recollection is that air cooled engines are intentionally built "loose" to allow for that.

 

Water cooled engines can be built with tighter clearances, which makes them more efficient.

 

Bear in mind, air cooled racing motorcycles routinely go from full power to no power and back again repeatedly, most often without seizing due to uneven heating and cooling.

[Tom Baker]

Modern technoligy in engine design has changed a few things. For example the piston to cylinder clearance in our 912 Rotax engines is less than .001" when new. They are able to do this because thay are using aluminum pistons in ceramic coated aluminum cylinders. The expansion and contraction rate is almost identical.

The standard for most other aircraft engines is aluminum pistons in steel cylinders. Because of the greater expansion rate of the aluminum compared to the steel they need much more clearance to prevent siezing.

As for the cold sieze I think it is something that happens in more extreme temperature conditions than most would think about riding a motorcycle.

That is why when Fast Eddie googled it they mentiond snowmobiles and two stroke engines. Snowmobiles operate in extreme cold, and the two stroke produces combustion heat with every other piston stroke instead on just one in four

[Al Downs]

Anyone ever hear of an oil temp thermostat for Rotax? Would it help? Is something like this necessary? Would it be legal? This is where I saw it. I have never heard of these.

http://www.mcp.com.au/thermostat/

[FastEddieB]

Al,

 

Do a search on this forum for "thermostat" and you'll get lots of hits, many of them relevant to your question.

 

But now that I think of it, many of those might be coolant thermostats - not sure if oil thermostats have been discussed.

[Anticept]

Had a chat with the engine guy here at A&P school. It's possible for valves to stay stuck off their seat if they are dirty and it's cold out, and it's the first flight of the day. They called it "morning sickness". He's never heard it happening in flight though.

[FastEddieB]

Well, coincidentally I recently posted about this incident to the Pilots Of America site:

 

 

About the rough running.

 

I had flown my S FL based Tiger to CT with my family.

 

On preflight I noticed a little brake fluid in a puddle under the right main. Brakes still felt firm and I wrote it off as cold weather related. It was, and never had that issue again.

 

But...

 

Taxiing out, I did notice a little engine roughness. Subtle, but there. Again, wrote it off as cold weather related, and runup and takeoff were normal.

 

But after leveling off at 6,000' over Long Island Sound, the engine suddenly got very, very rough. Nothing helped. Declared an emergency, headed towards Gabreski (?) field, circled around and landed. On exiting, there was oil all over the nosewheel fairing. I hoped I had not left the oil cap off or something equally stupid, and I hadn't. But opening the cowling revealed a bent pushrod and pushrod tube - which I still have as a souvenir. An exhaust valve had frozen, which caused the rough engine and associated backfiring*.

 

Lesson? Listen to your engine. Engine failures are usually preceded by some symptom. Lycomings can exhibit "morning sickness" as a prelude to valve problems. The Most Conservative Action is normally to shut down and investigate - very hard to do when you're all primed to go, but it could end up being a life saver someday.

 

Photo to follow...

 

 

*I know - technically after-firing

[FastEddieB]

Here's what a Lycoming O-360 may look like after a seized exhaust valve:

 

 

The source of the oil was from where the pushrod tube had been hammering against the case.

 

Fortunately, Mattituck was right nearby and I could do the parts runs in a rented car.

 

Oh, and after futzing with the repair over two days, the mechanic only wanted to charge me $50. I insisted he take more - I had assumed if I got away with anything under $1,000 I would have been happy!

[Tom Baker]

Al, the CTLS has oil and coolant thermostats. The oil thermostat is set a little lower on temp than I would like to see. I still add tape to ghe radiator.

[GlennM]

Modern technoligy in engine design has changed a few things. For example the piston to cylinder clearance in our 912 Rotax engines is less than .001" when new. They are able to do this because thay are using aluminum pistons in ceramic coated aluminum cylinders. The expansion and contraction rate is almost identical.

The standard for most other aircraft engines is aluminum pistons in steel cylinders. Because of the greater expansion rate of the aluminum compared to the steel they need much more clearance to prevent seizing

 

Correct! You get a cookie or some other treat. I don't subscribe to the whole shock cooling myth in air-cooled engines, let alone a water cooled engine like the Rotax. anyhow.

 

As for the turbochargers, the cooldown recommendation was to decrease the bearing and shaft temperatures so the oil would not coke in the bearing when the engine was shut off and the oil flow stopped. The hard coke would increase the wear on the bearing when the engine was restarted. With enough cycles, the bearing would wear out.

 

I would guess that the cylinders that crack have hot spots in their design which means they will probably crack in those areas with enough start and stop cycles. Mike Busch uses a lower maximum cylinder temperature in the engines he oversees to keep any cylinder hot spots from locally yielding material. If it is a hot spot, the local yielding and stress reversal on shutdown is the main cause of the thermal-mechanical fatigue and eventual cracking.