Airplane GEEK

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Cooling Things Down

By Vic Syracuse, EAA Lifetime 180848

This piece originally ran in Vic’s Checkpoints column in the November 2021 issue of EAA Sport Aviation magazine.

Solving engine cooling problems seems to be the next priority after fixing starting problems. After all, it can’t overheat unless we get it started. A quick search of any of the amateur-built forums will usually yield quite a few active threads pertaining to this topic. The solutions provided range all the way from black magic to high-probability solutions — not all of which may work on your particular aircraft — which can be confusing at times. Some of the provided solutions are just bandages that mask the real cause. As an example, one might ask why an aircraft requires a cowl flap or extra louvers for cooling when 95 percent of the rest of the models don’t.

Keep in mind that every one of these amateur-built aircraft are different, even when constructed by the same builder. Sometimes it will take actual eyes on the aircraft to identify the difference, but there are some things that usually can be applied across all of them.

There are also lots of opinions on cooling solutions for aircraft, so I don’t intend this column to be the ultimate go-to guide or bible. I am going to share with you the top four culprits that I have discovered to be the primary causes of overheating in our piston-powered aircraft. This experience comes from over 40 years of flying amateur-built aircraft, inspecting more than 2,000 of them, and regularly maintaining a pretty-good-sized customer base that allows us to see the effects of changes year over year.

In rank order, the four primary causes of engine overheating are engine ignition timing, fuel flow, cooling air flow, and operating practices. Let’s go through them individually.

Engine Ignition Timing

Piston engines have a recommended fixed base ignition timing. It is usually set to occur at some point before top dead center (BTDC). Most Lycomings have a value of 20 or 25 degrees. That can vary somewhat when engines are modified with higher compression pistons, or turbocharging, but let’s assume stock engines for all of the following explanation. The timing is set so as to not cause detonation (assuming the proper octane fuel is used) at maximum power settings, such as takeoff, and to keep temperatures in line with the manufacturer’s recommendation. Ideally, we would set the timing much closer to TDC for starting, and then advance it continually as the aircraft climbs to higher altitudes. Guess what? This is exactly the function that is performed by electronic ignitions. It explains why the engine usually starts easier with them.

It is important that this base timing is set correctly, as having it advanced just a few degrees can cause rapid overheating of the cylinder head temperatures (CHT) at high power settings. Most recently, we had an RV-7 come to our shop that had a newly installed Lycoming with two magnetos. The owners complained that they were seeing cylinder No. 3 CHTs of 471 degrees on climb-out, which is way too hot and noticeably different than their prior engine. Just a quick note on this for those of you who are changing out your engines: Usually, old, worn-out engines aren’t producing as much power at the end of their life and tend to run cooler. Changing out to a new engine that still needs to be broken in and is producing more power will cause a higher CHT reading, but it should still be within reason. More about that later.

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A quick look at the engine installation showed most of it to be normal, with new silicone baffling material. Although, the baffling was quite tight behind cylinder No. 3. Since experience has shown ignition timing to be the primary cause of high engine temps, we immediately checked it. Yep — there it was, sitting right at 28 degrees BTDC, which is way too advanced for Lycoming engines.

Some of you are probably wondering why the timing on the RV-7’s new engine was at 28 degrees. It had been installed by a reputable shop, so we called it to discuss. The shop informed us that the timing had been set at 25 degrees. However, the shop had been seeing up to 3 degrees of timing drift with a certain manufacturer’s magnetos and replacement contact points after about 25 hours. Interestingly enough, I had just performed the 500-hour service bulletin inspection on the Slick magneto in my RV-10 about 35 hours ago, using the same parts. Out of curiosity, I decided to check mine. I had been seeing a little higher CHTs on climb-out lately but had attributed it to the 90-plus degree heat. Sure enough, mine had drifted 2 degrees! So, for those of you who might be installing new mags or performing service bulletins, recheck the timing after 25-35 hours.

Another experience really drove home the point on how the wrong ignition timing can quickly cause alarmingly high CHTs. I was performing the test flight of a new RV-10 with one magneto and one electronic ignition system. As many of you know, the key to new engines is to keep the number of ground runs low and keep CHTs less than 300 degrees. This was done, and everything looked normal. Takeoff power was applied. I always glance at the engine gauges when the throttle is full in and then again when I’m about 100 feet in the air. This time, at 100 feet, the CHTs were already at 475 degrees Fahrenheit. There was not enough runway left to abort, but I had already mentally prepared for an option. I immediately reduced the power to as little as possible and nursed the airplane back around the flat farmland for a landing, with the CHTs luckily rapidly declining. We rechecked everything on the ground, including configuration of the CHTs, and all looked normal. The decision was made to try again, and the same results were there at 100 feet. This time, I quickly turned off the electronic ignition and the CHTs immediately started decreasing. I reduced power a little but continued the climb to a safe altitude. Everything was running okay, so I proceeded to break the engine in for about an hour. The last thing I did was turn the electronic ignition back on with a reduced power setting. The CHTs immediately started climbing again, verifying my suspicion that it was ignition timing. The owner later confirmed that he had misdrilled the holes on the back of the flywheel for the timing magnets, causing the timing to be set at 40 degrees instead of 25!

Fuel Flow

Let’s move on to culprit number two — fuel flow. The engine needs the proper amount of fuel at high power settings to cool properly. On carbureted engines, the carburetor has an enrichment valve when the throttle is in the fully open position to provide the extra fuel. This is where the wrong operational practices can have a negative impact. Carbureted engines should be flown to cruise altitude with the throttle fully open. This is normal practice with fixed-pitch propellers, remember? Then someone added a constant-speed propeller, which requires a manifold pressure gauge, and was taught that “25-squared” was the proper climb setting. Not so with a carbureted engine. Reducing the throttle will not allow the enrichment valve to open and provide the extra cooling fuel. (By the way, be sure to check that your throttle and mixture controls are hitting the stops at full power.)

Now there still is a possibility with a carbureted engine that you are not getting the proper fuel flow at full power settings. You should look up the fuel flow charts provided by the manufacturer for your particular engine. However, you can’t take a carbureted engine from another aircraft, such as a Piper Archer, which cruises at 120 knots, and expect it to work on your RV-7, which cruises 40 knots faster. The extra ram air pressure will cause the carburetor to run leaner. The carburetor will require a larger jet, or you can carefully drill out the jet to a larger size.

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Injected engines are set up a little differently with regard to fuel flow. First, they also have a main jet in the fuel servo body that usually determines maximum fuel flow. The fuel servos are a little more complicated. They are adjusted to set the proper mixture across the entire range of the throttle, and this is usually done at the factory or a service center. The third piece of an injected engine that determines the proper fuel to each individual cylinder is the injector, which can be adjusted by using various sizes of inserts to balance the fuel for each cylinder.

Here’s a quick test to determine if your engine has the right range of fuel mixture. It works for both carbureted and injected engines. Climb to an altitude where you can set power for about 65-70 percent of power. Note the exhaust gas temperatures (EGTs) at full rich. Slowly lean until the engine starts to run rough, and then note the EGTs again. You should have a difference of 175-250 degrees from rich to lean. If not, you are either too lean or too rich.

Cooling Air

The next contributor to high CHTs is cooling air. Most of our engine installations are downdraft cooled, meaning the air comes in the top and it exits out the bottom. Much like the wing itself, there has to be an area of high pressure and an area of low pressure for this to work properly. The top of the engine is the high-pressure area, and the bottom is the low-pressure area. To make this happen, the lower area exit needs to be larger than the upper area intake. Also, there should be minimal flow from the top to the bottom except through the cylinders. If you have leakage from the top to the bottom — such as inadequate sealing of the baffling to the case with RTV silicone or too many “cooling tubes” for accessories — you will pressurize the lower cowl, and the air will not flow properly. The baffling needs to flow so it can’t be too tight around the cylinder heads. A trick is to put washers in between the baffling and the cylinder heads on the aft cylinders (see picture). The biggest offense we see here is having the top baffling “Swiss cheesed” with large cooling tubes pointing down to accessories in the lower cowling, such as alternators, magnetos, and fuel pumps. Small 5/8-inch blast tubes are okay when used in moderation. However, it’s very important that the exit area on the bottom of the cowling is properly sized.

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Operational Practices

The last item has to do with operational practices. As an example, I asked at what speed the RV-7 owners were seeing the high cylinder head temps. I cringed when I was told 75 knots on climb-out! This is really hard on a brand-new engine that is going to run hotter until it is broken in. A good climb speed for cooling is approximately halfway between the stall speed and cruise speed. That speed usually gives good over-the-nose visibility as well. In the case of RVs, usually 100-110 knots indicated airspeed works well. High-performance aircraft with high cruising speeds are set up to cool properly at cruise, where the ram air pressure is the highest. Consequently, flying at slower airspeeds and higher power settings will normally yield higher engine temps, but they should still be within the normal range.

Normal range means pay attention to the manufacturer’s recommendation. As an example, it is normal to see CHTs of 435 degrees on Lycomings during the break-in while climbing at high power settings. Don’t panic and start pulling the power back. It’s important for break-in to have some heat and high pressure to properly seat the rings. Usually, you will only see the CHTs that high for the first flight, or maybe a couple of flights after that. They should slowly decrease as you build time on the engine. On 90-plus degree days, I might see 395 degrees Fahrenheit on climb-out after a fuel stop, at gross weight and a climb to 10,000 feet with my RV-10. The hottest temperatures are seen below 5,000 feet. They start to run cooler above 5,000 feet, and the oil temperature never exceeds 193 degrees Fahrenheit. I climb at full throttle and 2500 rpm all the way to cruise. Everything is stock, with one magneto and one electronic ignition. With almost 800 hours on the Thunderbolt engine, I add about 2 quarts between oil changes and change the oil every 35 hours.

I’ve broken in dozens of engines the same way, have run them the same way over the years, and see the same results, so I am happy. I hope the hints I’ve provided here will help those of you with cooling issues and keep your fun factor alive.

Vic Syracuse, EAA Lifetime 180848 and chair of EAA’s Homebuilt Aircraft Council, is a commercial pilot, A&P/IA, DAR, and EAA flight advisor and technical counselor. He has built 11 aircraft and has logged more 9,500 hours in 72 different types. Vic also founded Base Leg Aviation and volunteers as a Young Eagles pilot and an Angel Flight pilot.


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