Engine Break-In. Part 3. Flight Tests
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BMEP: Brake Mean Effective Pressure is generated within the combustion chamber when the engine is running. The higher the power setting, the higher the BMEP, the lower the power setting, the lower the BMEP. It's that simple. High BMEP pushes the rings against the wall. A very low BMEP will cause the rings to chatter against the cylinder wall. If this happens, you'll have to tear down and start over with freshly honed cylinders and new rings.
BMEP pushes the piston rings outward against the honed cylinder walls. The surface temperature at the piston ring face and cylinder wall interface will be greater with high BMEP than with low BMEP due to the friction caused by the rings pushing harder against the cylinder walls.
Too much heat at a low or medium BMEP will cause the lubricating oil to break down and glaze the cylinder wall surface; the rings won't be able to push hard enough against the cylinder wall to seat the ring. This glaze will prevent any further seating of the piston rings. If glazing is allowed to happen, break-in will never occur.
Ideally: We want a process that heats (high BMEP) and cools (low BMEP) the cylinders in a way that achieves the best break-in of the cylinders. The following flight pattern, if followed, will create the ideal heating and cooling cycles to quickly break-in your engine.
I recommend you break-in your new engine (or cylinders) with AeroShell 15W-50. I've been doing it this way since 1991.
Note: Density altitudes in excess of 5,000 feet are not recommended.
Maintain 75% (or greater) power during all climb and cruise conditions.
• Percent power may best be determined with a manifold absolute pressure gauge or fuel flow using
• Percent Power = (displayed) gals/hr * 6lbs/gal * 2.365 hp-hr/lb
The Racetrack
When I break-in a new engine, I like to stay fairly close to the airport. I fly, what I call, a rectangular racetrack about 5 miles wide and 7 miles long. The last leg keeps me on a heading for a straight in to the runway. WOT: Wide-open-throttle
Initial Climb: My initial climb at 110 knots (Tiger) gets me to my test altitude (approx. 3000 AGL) and heading into the wind.
This long slow climb creates high BMEP without overheating the engine.
Leg 1: For the 1st leg (6 to 10 minutes), I'll turn 90 degrees (I usually turn left) and fly WOT straight and level at 3000 AGL
Flying straight and level at WOT allows the engine generate high BMEP. Watch CHTs.
Leg 2: For the 2nd leg (4 to 6 minutes), I'll again make another 90 degree left heading change and descend. We'll cool the engine.
I'll make a gradual 1000 feet descent to 2000 AGL. Again, keep the manifold pressure as high (high RPM, high BMEP) as practical.
Leg 3: For the 3rd leg (6 to 10 minutes), I make another 90 degree heading change and fly straight and level at 2000 AGL at WOT.
Flying straight and level at WOT keeps the engine to cooler while making high BMEP. Watch CHTs.
Leg 4: For the 4th leg (4 to 6 minutes), I make another 90 degree heading change. Two options.
1. I can make a long descent to the airport since I will be positioned if I need to land, or
2. I can make a long, gradual, climb at WOT and climb 1000 feet to put me back at 3000 MSL and repeat the test legs.
Repeat these legs for about 2 hours. Descend and land. I recommend you do this 2 more times for a total of 6 hours.
Prior to Flight Test
• Verify oil is at 7 quarts. Expect to puke some all over the bottom of your plane.
• Start engine. Do not let it IDLE.
• Do Not delay in getting to the departure end of the runway.
-- Enroute to the departure end of the runway, do the following:
a. Perform normal preflight run up in accordance with the engine and
aircraft operator's manual
b. Watch oil temperature. It must be greater than 100 degrees F before take-off
-- Ideally, you have a an engine monitor capable of monitoring ALL engine functions
• Watch cylinder head temperatures (CHTs). Watch for anything not considered normal.
• Watch exhaust gas temperatures (EGTs). Watch for anything not considered normal.
• Once oil temperature is greater than 100°F and no anomalies, you're ready to go.
Initial Climb-out at FULL POWER:
• Begin the initial Full Power climb to 3000 AGL at a minimum of Vy (for your altitude) plus 20 knots at full rich mixture
-- Do the following:
a. Climb at approximately 250 feet per minute. Approximately 12 minutes.
-- Watch CHTs on your JPI EDM engine analyzer.
-- Keep CHTs below 450°F. If the get to 435°F, lower the nose. If they get to 450°F, pull on carb heat.
b. Watch oil temperature. Keep it under 230 at all times.
-- Ideally, you have a an engine monitor capable of monitoring ALL engine functions
• Watch cylinder head temperatures (CHTs). Watch for anything not considered normal.
You should be spring-loaded to abort this takeoff if you notice any anomaly.
Continue only if everything seems just right.
Leg 1: (6 to 10 minute straight and level flight). This is where you let your engine cool after a long slow climb.
• After you've reached 3000 AGL
-- Do the following:
a. Make a 90 degree turn (I make left turns so all of the references will be left turns).
-- Establish a cruise condition at greater than 75 % power.
-- Keep CHTs below 435°F, preferably between 400°F to 425°F.
b. Monitor RPM, fuel flow, fuel pressure, oil pressure, oil temperature, CHT and EGT (if possible).
• After 5 to 8 minutes at 75% cruise condition, slowly reduce power to approximately 65% power. Check engine gauges.
• After 1 to 2 minutes in this cruise condition, go to Leg 2.
Leg 2: (4 to 6 minute reduced power descent). This leg will further cool the engine. The reduced power will reduce BMEP and cylinder heat.
• After you've established 65% power
-- Do the following:
a. Make a 90 degree left turn.
-- Make a long, reduced power, descent to 1000 feet below your Leg 1 cruise altitude (2000 feet AGL).
-- You'll be descending at approximately 250 feet per minute or less.
b. Keep the manifold pressure as high as practical.
-- Remember: Very low manifold pressure will cause the rings to chatter.
• After 4 to 6 minutes in this cruise descent condition, slowly increase power to greater than 75% power. Check engine gauges.
Leg 3: (6 minute Full Power acceleration). Now it's time to make some heat.
• After you've established 75% power or greater
-- Do the following:
a. Make a 90 degree left turn.
-- Establish straight and level flight at 2000 feet AGL.
b. Keep the manifold pressure as high as possible.
-- Lean the mixture to peak rpm.
-- Keep CHTs below 450°F, preferably between 425°F to 445°F.
• After 6 to 10 minutes in this high power cruise condition, check engine gauges and get ready for the next leg.
Less than 2 hours: If you've been flying for less than two hours, Go to Leg 4.
More than 2 hours: If you've been flying for more than two hours, do the following.
• If you've flown for more than 2 hours, you should setup for a long, reduced power, (do not pull power off quickly) descent to land.
-- Reduce power to between 60% and 65% power
-- Avoid too low of a manifold pressure.
-- Make a 90 degree left turn. You should be on runway heading or close to it.
-- If possible, extend flaps for added drag and descend under power.
-- Do not reduce altitude too rapidly or the engine temperature may drop too quickly.
Leg 4: (4 minute Full Power climb). Now it's time to seat the rings.
• After you've established 75% power or greater
-- Do the following:
a. Make a 90 degree left turn.
-- Establish a Full Power climb at Vy plus 10 knots to 3000 feet AGL.
-- You'll be climbing at approximately 250 feet per minute.
b. Watch CHTs on your JPI EDM engine analyzer.
-- Keep CHTs below 450°F. If the get to 435°F, lower the nose. If they get to 450°F, pull on carb heat.
c. Watch oil temperature. Keep it under 230 at all times.
-- Ideally, you have a an engine monitor capable of monitoring ALL engine functions
• After 4 minutes in this high power climb condition, check engine gauges and get ready for the next leg.
a. At this point you've flown the racetrack. You know what it's like.
b. Go back to Leg 1 and repeat these flight test legs