Do you treat your Yak the same way all year?
Think twice says Vlad Yastremski
In warm climates, it's easy to fall into this habit . Like a West Coast buddy recently, who noticed the morning air was colder than it had been ( Yep, cry for us, it got down to 37 degrees F last night) but took off without checking his tire pressures. After landing a little while later and making a rather sporty exit off the runway, he side loaded the soggy main tires enough to where the snap rings and rim popped off one of the main wheels. The tire and inner tube departed, leaving it rolling on the hub. Hurt pride, embarrassing more than costly.
Similarly, if you're flying cross-country in December, you might end the day in New Mexico, leave it on the ramp overnight with both air gauge needles nice and perky but come back to 20 bars or less in the morning.
Right off the same page in the Physics textbook.
And as for how low temperatures affect your Yak motor, lets split this little rant in to two parts: starting it and warming it up.
At temperatures lower than +5 Celsius (41 Fahrenheit) it's mandatory to preheat your Yak's motor before starting because of the damage that cold oil can do. Some use electric pads, some pipe hot air into the cowling, and you can even use a hairdryer pointed into the Oil Radiator rather than nothing at all. Back east it's not unusual for rented airplane hangars to be heated, with full service tow-outs by the FBO, but for most of us, it's just a big cold chunk of metal that's going to need a lot of warming up.
Even if it's showing warmer than 5 Celsius at your airport, one safe guide as to whether or not your motor is cold is to feel the prop. Make sure both Mags are off, brakes are on, then pull a blade for a few inches. You've turned that prop enough times over the years to notice how stiff it feels normally. If it's feeling like treacle this morning, don't just jump in and crank. In addition to the normal precautions to avoid Hydraulic Lock, follow these oil pre-heating precautions above and keep pulling blades until you feel the oil moving more freely. We'll touch on oil dilution a bit later.
In cold weather sluggish oil will cause permanent damage to the weakest link - the core of the Oil Radiator. There are two designs of bypass in our Oil Coolers, but while both re-route cold oil away from the cooling circuit (until engine heat has raised its temperature to a specified value), they can both fail. When forced into these slender-walled copper heat-exchanger tubes, it's an easy job for the cold oil to hydraulically expand them like a balloon, but permanently. Their OD increases to effectively block air flowing between the outside of the tubes, compromising the Radiator's function and will ultimately lead to fracture, leaving tell tale oil under the ground in the vicinity of the cooler. That's a pricey mistake. Preheating your oil is the preparatory step that will prevent this.
When it comes to starting, perform a thorough priming preparation and monitor oil pressure during the first few seconds. The motor catches, you set it to the normal 40-50 percent for warm up, but suddenly oil pressure is dropping to zero.. The seconds keep ticking by without improvement. What the...? So you shut it down...
In cold weather the magic number on the Oil Dipstick is 12 liters. If you have this much oil in the tank, the level will be completely covering the mechanical "flop" or pickup tube inside (which keeps oil feeding during aerobatics). With an oil level below 12 liters, some of the joints on this mechanism will be above the oil surface.
When it comes to a choice of sucking air or cold viscous oil, the oil pump will take whichever is easier. Air gets sucked into the oil system and the sensors and gauges tell you the truth.
Warming it up
What if it starts, but lopes and runs roughly for a long time?
Remember why you fitted auto plugs ? Because they are cheap. If you use a BR7 auto plug in summer, treat yourself to a set of BR6 for the winter season. The lower the number, the hotter the plug will run. Also, visit your mechanic and have them adjust the mixture a few clicks leaner. The clue to perfect mixture is that small white telltale of completely burnt oil on the trailing face of the two exhaust stacks. Expect to see oily black at the same spot if your mixture is too rich. Perfect mixture for Summer is not perfect mixture for winter.
One of the things we love about Radials is their sound. And some of us think that really slow loping idle they'll do sounds awesome (and a low idle may have some valid applications in certain flight regimes ), but the reason for the recommended idle speed is to warm it up as fast as possible. In Winter, that loping idle means the butterfly is completely shut, the vacuum created by the pistons is prodigious and fuel and oil are finding their way in through the wrong jets, through any place in the carburetor they can. You'll get blue or black smoke and so much fuel on the cylinder walls you may be compromising lubrication. But just cracking the butterfly open will stop it immediately. The warm up RPM is recommended to get T's and P's where they need to be as fast as practicable.
If you didn't do a thorough priming preparation, the motor starts for a few seconds then quits. So you sit in the cockpit, pumping and priming like a crazed weasel until pretty soon, the motor's flooded. No sign of life even after cranking through your second air tank fill. The remedy? Mags off, brakes on, throttle wide open and start pulling blades. What you are after is at least two charges of fresh air through each cylinder. The only karmic payback here is that you just performed a kind of oil dilution by flooding it. After pulling the 20th blade, you might notice the oil becoming thinner because of all the fuel you dumped into it earlier!
The Manual is the place to learn about Oil Dilution but here's the view from 30,000 feet. It's something you do before you shut down the motor the day before, so planning is required. You'll also need to know how much oil you have on board and how cold it will be overnight.The chart will tell you how many seconds to hold the momentary switch that injects fuel into your oil system. It also advises the correct rpm setting at which to perform the operation. So keep your eye on the oil pressure gauge as you do it, because you'll see the needle begin to drop due to oil thinning.
The next morning, there you are started, warmed up and ready to taxi. Once airborne in winter air though, even with the exit door firmly shut, the oil radiator will be unable to keep your oil gauge in the green, (or even in the low caution range ), without extra baffle plates over the Oil Radiator inlet. These baffle plates came in sets with different sized cut outs, marked for specific air temperatures. You may not need the entire set - or use them all year, but to install one, your Oil Radiator sheet metal housing under the right wing has clips for two wire locking pins on the lower edge and two snaps for the baffle's upper edge. Three rubber tubes are attached to the rear of these baffle plates to minimize damage to the fragile cooling fins of the radiator core.
Thanks to John Bergeson for this picture of factory baffles installed on his Yak 55/M. If you inspect the image of the 52 on skis at the top, you'll see even less opening at the entry to the Oil radiator.
And yes, that cockpit heater muff on the M14-P exhaust ring is just a liable to asphyxiate you as any other aero engine, so go the distance and install one of those visual Carbon Monoxide indicators within the pilot's field of view.
In the days of the DOSAAF, there was an entire Winterization procedure. Among other things, it included a lighter grade of engine oil, heavy grease on the uplock mechanisms (for protection against freezing & debris), lighter oil added inside actuators and cloth covering of all the wide diameter tubing associated with the oil cyclone / circuit. Without insulation these thin-walled tubes are the first place for internal condensation, that can lead to possible freezing and blockage. Your oil needs to reach 74 Celcius (165 Fahrenheit) before all the water has evaporated.
Not convinced that there's water in your oil? Try this one morning on a cold engine. Using a suitable container at the quarter-turn main drain of the Oil Tank, carefully turn it to open and just stab it quickly - and collect what comes out. (Clue: Oil floats on it)
Here's an airplane that was designed to operate in temperatures so low you'd be hard pressed to encounter them in the lower 48.
As they say in Northern Mongolia, happiness is a warm Yak - so warm yours up and go flying!