Leaning in flight - is it better, really?

[Red Leader]

Good morning guys! I have a 1981 M20k with the GB engine. We all know this model engine is prone to getting hot fast if you don't watch the temps, even with the intercooler and Merlyn (which I have)  Usually I run LOP and watch my pressures and temperatures - running about 29" and keeping it under 400 degrees. At these settings, I don't get the of speed our model is known for but I do get good fuel burn (9 - 9.5gps). On my last trip, I was running a little late on the return leg so I ran it ROP. I was burning around 11gph, going noticeably faster while not seeing the high cylinder temperatures I notice on my typical flights. Other than fuel burn (and higher speed) is there anything I am doing wrong by running it ROP? 

[Pinecone]

No, ROP is fine.  I run ROP when I need the speed.  Just make sure you are at least 100 - 125 degrees ROP.

But you can also run a higher manifold pressure to get a higher % power when still LOP.   I run my -SB at 29.5/2300 and 10.1 GPH for 63 - 64% power.

For a Continental, Mike Busch (and others) recommend keeping CHTs under 380.

[Falcon Man]

Previous experience with my K models operated in cruise at peak or 25F LOP, TIT ~ 1550F:

1) 231 w/ LB engine, intercooler and auto wastegate, cruise 2200 rpm and MP to keep CHT's less than 350F, lean test showed < 0.5 gph spread, cylinders lasted ~1500 hrs. The cowl flaps had to be at least partially open almost all the time in cruise.

2) 262 w/ MB engine operated same parameters, cylinders never > 350F, lean test showed 0.3 gph spread, I had to modify the cylinder baffles on # 3 & 5 to lower CHT on # 3 (5 was very cool and 3 was too hot w/ stock baffling). In the cooler times the cowl flap was usually closed, sometimes trailing open in the hotter times. This engine went ~ 700 hrs past TBO.

The speed loss due to open cowl in cruise was just a couple of knots TAS.

My goal was to keep the engine out of the Red zone.

 

 

 

 

 

 

 

 

o keep the engine out of the RED box.

[Ragsf15e]

13 hours ago, Red Leader said:

Good morning guys! I have a 1981 M20k with the GB engine. We all know this model engine is prone to getting hot fast if you don't watch the temps, even with the intercooler and Merlyn (which I have)  Usually I run LOP and watch my pressures and temperatures - running about 29" and keeping it under 400 degrees. At these settings, I don't get the of speed our model is known for but I do get good fuel burn (9 - 9.5gps). On my last trip, I was running a little late on the return leg so I ran it ROP. I was burning around 11gph, going noticeably faster while not seeing the high cylinder temperatures I notice on my typical flights. Other than fuel burn (and higher speed) is there anything I am doing wrong by running it ROP? 

It’s interesting to me that you say you were slower and hotter LOP… if you ran the same power lop vs rop, you should see roughly the same speed, but lower CHTs when lop.  If you’re running higher power ROP (to get the more speed), then you’d likely see higher CHTs.  Exactly what mp, rpm and ff do you use lop vs your power setting rop?

[PeteMc]

15 hours ago, Red Leader said:

Usually I run LOP and watch my pressures and temperatures - running about 29" and keeping it under 400 degrees.

I had a -GB engine for a few years when I first got my plane and then went to the -LB.  But from what I remember of the -GB, if you're only running at 29" and you're having problems keeping the cylinder below 400 degrees, something seems off.  Is this from the single factory cylinder temp probe or do you have a JPI or some other engine monitor?  With a stock -GB at 9.6g/hr, 31.8" and 2200 RPM, you'd be at 65% power (Best Economy).  You're below that and the Intercooler should be bring your temps down below the stock engine.  An even at a higher RPM with that fuel flow, you're even lower than 65%, so again, the 400 degrees seems strange.   

And if you've been going to the same mechanic for years, maybe you want to have someone one else take a look.  For starters, I'd be checking to see if the temperature probe(s) are giving you the correct reading.  Also, is the baffling sealing correctly or is there just no air going through the cooling fins.  And I guess it could also be your Tach or MP gauge that are off.  I'd say to check the Fuel Flow too, but you'd probably notice when you filled up if the 9.6g/hr was off.  

 

 

[Red Leader]

Good points, all. My engine monitor is a new Insight G4 installed a couple of years ago. I took a picture of the instruments on that flight and just looked again - ROP I was at 11k feet, 30" and 2350rpm. The engine just loved that setting and I was faster by about 5 knots indicated. Still running close to 400 on 5 & 6 but lower on 3 & 4 and lowest on 5 & 6. The temperature spread was about 100-degrees (between #1 and #5) and my fuel flow indicated a solid 10.2gph. Not a big difference but noticeable. I don't remember things running that smoothly when operating LOP. In fact, usually when lean, the engine would stumble several times during cruise. On shorter trips when I lean, it might not stumble at all, but the fact that I feel it when LOP and can also feel the reduction in speed when leaning suggests I may not be doing something right. I am taking another short trip tomorrow and will play with it a little more.

[Falcon Man]

It is common for the GB, LB and some MB engines to run slightly rough when LOP. My LB engine didn't stumble at peak or slightly below. I tried all sorts of RPM/MP settings and it didn't make any difference.

[Ragsf15e]

2 hours ago, Red Leader said:

Good points, all. My engine monitor is a new Insight G4 installed a couple of years ago. I took a picture of the instruments on that flight and just looked again - ROP I was at 11k feet, 30" and 2350rpm. The engine just loved that setting and I was faster by about 5 knots indicated. Still running close to 400 on 5 & 6 but lower on 3 & 4 and lowest on 5 & 6. The temperature spread was about 100-degrees (between #1 and #5) and my fuel flow indicated a solid 10.2gph. Not a big difference but noticeable. I don't remember things running that smoothly when operating LOP. In fact, usually when lean, the engine would stumble several times during cruise. On shorter trips when I lean, it might not stumble at all, but the fact that I feel it when LOP and can also feel the reduction in speed when leaning suggests I may not be doing something right. I am taking another short trip tomorrow and will play with it a little more.

I suspect you’re a little too much LOP which is causing the stumbling.  Id recommend starting LOP to try a setting that will get you 65% so you can’t hurt anything and then play with it from there.  You just need the FF for your engine.  As long as you’re on the lean side of peak,  FF is the only determinant of power.  So your engine, 65% is 210hp x .65% = 136.5.  Divided by 13.7 gives you a ff of ~10gph.  Try that ff at say 27/2300 or so maybe 28/2300 if it will run smoothly.  set the mp/rpm, then big mix pull to 10, then reset the others back to 27/2300 or 28/2300 as required.
Then slowly richen until peak to see how far lean of peak you were (and then set back to 10gph).  At 65%, you can shoot for closer to peak, maybe 20-40degrees.  If it’s too lean, pull throttle back just a little as that’s what is controlling the air into the cylinders.

you should see much cooler cylinders.

[Falcon Man]

Have you performed a GAMI lean test to see what your FF difference is for the injectors? 100 degree CHT spread between cylinders is very high.

[Red Leader]

Don't have Gami's but have been considering them. At my last annual I had several fuel flow tests performed and the output of the injectors was almost exactly the same.

[Pinecone]

The goal is not to have the injector outputs exactly the same, but to have each one matched to the airflow of THAT cylinder.

But, do the GAMI lean test.   They won't sell me a set of GAMijectors.  They say my engine/injectors won't benefit from them.