Aerobatics during engine break-in

Ed Haywood

Ed Haywood

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Big Ed
How many hours should be on a new/overhauled Lycoming before doing acro?
 
According to Lychoming it takes 50 hours to fully break in the cylinders. A lot of mechanics say less.
 
I'd be more concerned that the oil drain plug doesn't fall out. Happened to a friend of mine at around 25 hours or so during aerobatic instruction in a brand new Decathlon. Lost all the oil. Some folks don't like the phrase old mechanics use when they "tightened the engine" during an inspection. I like to see it in the logbooks. :)
 
RyanShort1 said:
Do the aerobatics involve a lot of power changes? Look at the break-in requirements - carefully.
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Not in a Super Decathlon. I set power to full throttle and 2550 RPM and don't touch except for spins. Easy enough to avoid spins for the first 50 hours. A fixed pitch prop might be different, but I lack the experience to say, having only flown acro with a CS prop.

50 hours is an awful lot of straight and level flight. I could probably justify 25 hours, which I believe is the period where mineral oil is required. I'll talk to the engine shop and see what they say. The guy doing my engine is very solid and has a lot of experience with aerobatic birds, so he'll know for sure. I was just trying to get an idea of how long my withdrawal symptoms are going to last.
 
Typically when we’ve done break in stuff we do the first 5 hours over the airfield and then do 5-10 hours trying to keep it over other airfields cross-country style.
 
AKBill said:
Off topic, but what are the normal g forces you see during acrobatic maneuvers? I see the Decathlon is rated for +6 to -5 g's.
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Depends on what maneuvers you fly and how you fly them. Most basic aerobatic maneuvers can be done smoothly with 3.5G. When you get into competition, you often pull harder to get a consistent radius or lengthen a line. I usually pull 4 to 4.5G in Sportsman. The higher categories pull more. Physically I am fine to 5G, and do not notice 4 or less after the first pull in a sequence.

Sometimes a sequence is designed poorly and requires more G to fly. The current Sportsman sequence has a Split S with a 2/4 roll at the top. That maneuver is very difficult to fly in an older Super D with a slow roll rate. I sometimes approach 5G at the bottom, which IMO should be unnecessary. My profile pic was taken after that maneuver. I will be glad when I do not need to fly that one anymore.

One misconception among newer aerobatic pilots is that lower G's are "gentle" and thus safer. Loading up the G's with an aggressive pull is the best way to prevent speed buildup while pointing downhill. A hard pull while pointed straight down at 130mph with full power will keep airspeed at or below 160mph, the top of the green arc in a Super D. It is very hard to break the airplane at that speed. A "gentle" pull in the same situation can allow airspeed to approach or exceed Vne at 200mph, where you have a lot better chance of damaging the airframe.
 
When I replaced the engines in the flight schools airplanes, I did the initial break-in flights myself. 2.5 hours or so, according to that Lyc document I posted. Then I forbade any circuit or upper-air work (stalls, spins, forced approaches, etc.) for the next 15 hours. Cross-country work only. After that they could use the airplane for anything. That made sure that the rings were fully seated, and those engines all went to TBO with compressions in the high 70s, no metal, and very low oil consumption. There was one exception: They pestered me to let them use one for circuits when it had only about six or eight hours on the engine. All the other airplanes were booked solid. They bugged me so much I finally relented, and that engine used considerable oil its whole life. The cylinders got glazed just enough before the rings were fully seated to do that.

Never baby a freshly-overhauled engine. You're not doing it any favors.
 
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