Engine Sweet Spot

[N941WR]

Originally Posted by scsmith

I absolutely agree about Deakin's articles, based on a lot of solid testing and experience, as well as sound science. But it is primarily directed at injected engines for which the injectors have been adjusted for narrow "GAMI spread." If Dayton has a carbureted engine, he may be able to get slightly LOP, but probably can not follow Deakin's methods.
...

Steve, Dayton has dual P-mags that have been tuned with his EIC on his O-360. Same setup as I have, only he has a CS prop and I have a FP prop.

The P-mags will allow the O-360 to run LoP very smoothly. I typically run 50? LoP, sometimes higher. (I'm looking at a picture of me cruising at 45% power, 143 knots true, burning 5.6 GPH at 7300' DA.

360's seem to have a better intake plenum than the O-290 and 320 as I could not run LoP with my 290 and some of our customers report that they cannot run LoP with their O-320's.

[Eddie P]

Originally Posted by tgmillso

Hi Dayton,

There's a series of articles by John Deakin called the Pelicans Perch that is absolutely fundamental for understanding where and more importantly why you should run your engine at a particular setting.

https://www.avweb.com/flight-safety/...s-perch-index/

I would say the best place for you to start is article #63, then #64, #65, #66, but then again they're all worth reading and some more than once.
In the 20+ years I have been flying aircraft, this series of articles is unquestionable the most valuable assembly of literature I have discovered. Operating under this modus operandi will ensure your engine runs cleaner and your wallet remains fuller.

Regards,

Tom
RV-7
IO-360M1B

Thanks for that post Tom! I read all of the engine articles and will be referring to them a few more times for sure. I have to say Deakin?s method to descend using mixture and RPM to manage power is new to me and interesting to say the least. Very good reading.

[humptybump]

Rather than look at the science of an operational sweet spot, I take a gut feeling.

Personally, every airplane I’ve flown feels best at some combination of throttle, mixture, altitude, etc.

My current RV-8 has a carbureted O360 with a custom air box and dual magnetos and a fixed pitch propeller - a [mostly] basic setup.

If feels good at about 2550 RPM and 160kts (+/- a little).

As you can tell, it’s not the fastest possible setup. However, given the airframe, engine, and propeller combination, this is where it feels good to me. It can go faster but feels like it’s working harder than it needs to. It can fly slower with lower fuel burn but that feels like it’s loafing.

I suspect, another pilot flying my RV-8 may well feel it is operating best at some other combination of settings. I say, do what feels right

[M McGraw]

Originally Posted by humptybump

Rather than look at the science of an operational sweet spot, I take a gut feeling.

Personally, every airplane I?ve flown feels best at some combination of throttle, mixture, altitude, etc.

My current RV-8 has a carbureted O360 with a custom air box and dual magnetos and a fixed pitch propeller - a [mostly] basic setup.

If feels good at about 2550 RPM and 160kts (+/- a little).

As you can tell, it?s not the fastest possible setup. However, given the airframe, engine, and propeller combination, this is where it feels good to me. It can go faster but feels like it?s working harder than it needs to. It can fly slower with lower fuel burn but that feels like it?s loafing.

I suspect, another pilot flying my RV-8 may well feel it is operating best at some other combination of settings. I say, do what feels right

I agree with this. I tested a whole bunch of stuff to get a feel for my plane after all is said and done I just like certain altitudes and I basically run at about 9.0GPH@2400RPM and let the speed fall out. Sometimes I choose 8.5 and sometimes 9.5, but basically if just feels comfortable.

[tgmillso]

No worries Eddie.

Running 2450rpm, 11gph and 171kts used to feel right to me, because running ROP was all I had ever been taught, but since become enlightened regarding the issue of ROP vs LOP, what feels right is now 2350rpm, 22in MAP LOP and 163kts for 7.5gph. Sacrificing 4.7% of speed for a 31.8% reduction in fuel burn just makes sense and now extended periods or ROP operation feels dang wrong.
Part of feeling right is often rooted in understanding, which is what the OP had reached out to obtain. Those Deakin articles were a prime part of the pathway to getting me to that new point of comfort.

T.

[gereed75]

Dayton. One of the things I do not see mentioned is the throttle position. I, like almost everyone with a CS prop, always run WOT in cruise. This is the basis of everything else. From there power is adjusted using RPM and mixture. The WOT is often a surprise to those used to running a constant pitch prop.

Personally, I use WOT, 2400 rpm and about 125 ROP EGT or best power. I am generally not concerned about LOP. The fuel savings is not that important to me. I built a fast airplane to fly fast. I find that my Hartzell BA is nearly as fast at 2400 as it is at 2700, only about 2-3 knots difference, and yes 2400 “feels” right. I fly at 10500, -11500 and I think I get 24 miles to a gallon last time I checked IIRC

And yes understanding the Deakin articles are essential.

[DanH]

Let's assume good reciprocating and rotational balance, mechanical issues not subject to inflight control. The two smoothness factors which may be influenced by the pilot are cycle-to-cycle variation and resonant vibration.

Not every cylinder charge lights and burns at the same rate. There are multiple factors at work here. The mixed fuel and air presented to the spark plug at the desired moment of ignition may not be homogeneous, being either too rich or too lean in the immediate vicinity of the spark, so initial inflammation may be slow. The overall mixture may be very lean, both more difficult to light and slower to burn. The spark duration lasts for a finite time, through some range of crankshaft rotation, and may achieve ignition at some point later than the planned initial spark discharge. And of course sometimes a cylinder fails to light at all, a misfire. Regardless of root cause, peak pressure and the point of peak pressure vary from optimum, and cylinders make an uneven contribution to torque. We call it roughness, and fix it with the mixture knob.

Resonant vibration happens when a periodic driving force is applied to an object, and the period frequency is at or near the object's natural frequency. When the driving frequency is within a range of (caution, rough example) 0.9 to 1.1 times the natural frequency, the object vibrates powerfully, with the peak amplitude happening at Fd = Fn. In theory, amplitude can reach infinity, and doesn't only because the world is full of damping factors.

Probably the most significant pairing in the context of "engine sweet spot" is one in which the prop blades are the resonating victim. It's the basis of the "Don't Operate Here" RPM and MP combinations.

Less appreciated is the fact that our airplanes are full of structures, all of which have a natural frequency. Any of them with a natural frequency which happens to match one of the multiple driving frequencies emanating from the flailing and whirling parts up front may vibrate, some with considerable amplitude. Here's the takeaway; it is possible to have a very smooth running engine, and still have an irritating vibration notable in the cockpit. Although the powerplant is vibrating at low amplitude, the magic of resonant amplification can result in an airframe component with high amplitude vibes.
The pilot's perception, based on what he feels, can be misleading. He may make a power change (notably RPM in this case) until the vibe subsides, when the real cure may be to add some damping to an airframe component, or change its natural frequency.

Originally Posted by N941WR

Steve, Dayton has dual P-mags that have been tuned with his EIC on his O-360. Same setup as I have, only he has a CS prop and I have a FP prop. The P-mags will allow the O-360 to run LoP very smoothly.

Two points please.

Tom and Steve mentioned Deakin articles. Advanced timing for LOP is not within that school of operating thought.

In the fixed timing approach, peak pressure is purposely moved further from TDC by slowing combustion speed with lean mixture, then an inch of MP is added to compensate. Easy to do in your turbocharged whatever.

With variable timing, we add advance, moving peak pressure back to where it was at best power mixture, ballpark being 15 ATDC.

With the former, peak pressure is lower near TDC and a bit higher near exhaust valve opening. With the latter, peak pressure is high near TDC and lower at valve opening. Nothing wrong with either approach, but they are quite different. You can see it on the gauges. Fixed timing slow burn is low CHT and higher EGT. Advanced timing to compensate for slow burn is high CHT and lower EGT.

Second, timing advance is not required to run smoothly when LOP. The ability to light a lean mixture is a function of healthy spark across a wide plug gap, not timing. It can be done with mags and narrow gaps, but in our world, it means an electronic ignition. However, we should not conflate the typical EI's two entirely separate features, (a) high spark energy and (b) spark timing.

Break

Dayton mentioned 8500 alt, 2500 rpm 20" MP and 9.5gph. That should be around peak, rather than significantly LOP. Nothing wrong with that either.

[Larry DeCamp]

For teaching us what many are capable of but not investing the effort to do so,,,,THANKS DAN

[gereed75]

Wow Dan, that is a lot of thought going there. Reminds me of the time I was sitting in a gondola with a ski instructor checking out two fledgling instructors. Head guy asked “ what are the five techniques for executing a turn”? The two guys proceeded into a 5 minute dissertation on turn techniques. As they finished, the gondola doors opened and as I exited I looked back and said “ ya, and don’t forget to follow the tips of the skis”

Set WOT,, pick an allowable RPM you like, set EGT to best power or LOP, sit back, enjoy View.

[Tom @ N269CP]

I like flying my 8 with O-360 just a tad below WOT (to close enrichment circuit) at 2280 rpm, 7.2 gph and 182 mph TAS. Typically at 19-20 inHg MP (12,500 - 10,500 ft MSL). CHTs circa 320 F, EGTs circa 1380F. Smooth, quiet and plenty fast for my needs.