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Old 07-30-2016, 12:27 PM
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Toobuilder Toobuilder is offline
Join Date: Jul 2009
Location: Mojave
Posts: 4,859

So here we are several months after my last "flight test" and I wanted to share some results of a flight yesterday to Sedona, AZ.

Recently, I have been "addicted" to seeing the magic 200 KTAS on my EFIS and as a result I'm running a little less efficiently than I could be at peak EGT and 12 GPH. Prior testing has driven me to set my CPI "LOP switch" at 3 degrees advance, so I decided to play with some "what if" scenarios on my 1.6 hour flight yesterday.

Cruise at peak EGT, 9500 MSL and a sweltering 70 degrees OAT resulted in 12 GPH and just at 200 KTAS. In this rich condition I run without the LOP switch advance active (30 degrees advance). Well stabilized, I advanced the timing to 33 with the LOP switch and within a minute or two I saw zero change in TAS, but the CHT settled in 8 degrees hotter and the oil temp increased a couple of degrees. This is expected behavior based upn my earlier flight test. If you are rich, too much advance only drives more temperature into the engine. after flying in this condition for about 10 minutes, I deactivated the LOP advance to see if the temps would drop to their prior levels. Almost immediately after pulling the advance out, the temps started downhill, settling in at their prior levels after just a few minutes. Ok, that result was confirmed as repeatable.

Next test was LOP cruise. As much as I hated to give up my 200KTAS, I dialed the mixture back to 10 GPH and the speed and temps plummeted, eventually settling in at 188 KTAS and a full 20 degrees cooler on CHT and close to 10 on oil temp. I flew for close to 10 minutes in this configuration to establish stability. Without touching anything else, I activated the CPI LOP advance switch taking the ignition from 30 to 33 degrees. The EGT dropped immediately, and the CHT started climbing. Within 5 minutes, the TAS climbed from 188 to 191 knots and stabilized, with the CHT finally settling in about 8 degrees warmer than before (still a very comfortable 375). I flew the remainder of the mission in this condition and it remained stable for the next 30 minutes until the TOD into Sedona.

So bottom line is even with a new season and new temps, my initial flight testing has been repeated: the optimal ignition advance requrements are highly dependant on the mixture and operating environment of the engine.

I'm even more convinced than before- one ignition advance schedule does NOT fit all.
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
Harmon Rocket II -SDS EFI
1940 Taylorcraft BL-65
1984 L39C

Last edited by Toobuilder : 07-30-2016 at 01:39 PM.
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