Cruise Performance - Efficiency Thread

[Bcone1381]

Group: This is me thinking/observing cruise performance....Cruising at any altitude.

• Whats best engine setting for cruise?
  • Define your goal.
• Max efficiency.
  • Do you want to fly slow?
• No
  • Going fast requires more Fuel Flow. How much fuel do you want to burn at this altitude?
• Lets say 8 gph today.
  • Then Experiment! Try different RPM's, MP & mixtures at 8 gph. Then discover the setting that maximizes TAS while staying with in the boundaries of the Lycoming power chart (Max 5" over square)?
• That chart lets me go down to 1800 RPM. For example, 5" over square is 23"/1800.
  • Thats what I mean.

__________________
One Observation and a few questions.

• The other day I had a short flight at 2500'. At LOP & at 5.4 gph I improved my cruise speed by 3% by slowing the RPM from 2200 to 1900.
• LOP Question....
  • Is the LOP calculation that my EI Engine Monitor that I did at 2200 RPM still valid if I set 1900? Do I need to re-sweep the mixture to re-find Peak EGT?
  • If my EGT stays about the same, is my LOP calculation is still accurate?
• Oil Consumption
  • I think my oil consumption goes up when I slow down the RPM to 1900 or 1800 rpm.
  • Max Efficiency may (key word) have a cost of increased oil consumption...needs more research.
• Do others see higher Oil Consumption at slower RPM's in parallel valve Lycoming engines?
• CHT's and Oil temps drop when slowing RPM down this much.
• Is it worth 3%?

Tell us why you use your "go to" power settings, what you know, curiosity you have, or what you don't know and want to learn. Personal experiences are important. I want to know both what you set along with why you choose it.

[U2fletch]

What technique do you use to get to LOP at 2500’? Mike Busch recommends the “quick pull” in this regime to quickly get through the knock zone. Usually when I go low, it’s a short flight, so I accept the higher fuel flow and just go fast. That’s an excellent question about RPM affecting your LOP fuel flow numbers. I’ve been listening to the AOPA podcast “Ask the A&P’s” and there is a lot of discussion there about how to go LOP, but I don’t recall discussion of really rolling your RPM back.

[jaredyates]

I can tell you what I've come to believe about engine operation, whatever it's worth. Consider it as one data point at least.

I think for this discussion to be productive, we have to do a little sorting. What is useful for a fuel injected engine will not necessarily apply to a carb, and likewise with 4-cylinder vs 6-cylinder.

Lots of 4-cylinders have RPM ranges that we need to avoid due to harmonics. When I had our 4-cylinder plane with a carb, I typically set cruise RPM to 2500, and goof-around RPM to 2200. The lower RPM made less noise in the cabin, but it limited my ability to extract power from the engine in cruise. If I was in a hurry I would run higher MP at 2500 RPM, maybe in the realm of 10gph, but more typically around 8gph. The carbureted engines sometimes run better a little shy of wide open throttle because the carbs have an enrichening circuit that kicks in at full-open. Also, some folks have found that a slightly closed throttle will create turbulence which helps with fuel distribution, both for carbs and FI. I found that our old engine ran a little smoother and the temps were a little more even with a slight reduction from WOT. Occasionally when I was in a big hurry or had a strong headwind, I might be at WOT and 2500 RPM or even 2200 RPM, but that 300 RPM difference really limits the engine's output. At low altitudes this might be 26-28" indicated MAP. I have never observed any oversquare limits and also don't believe in the great pumpkin.

For our carbureted 6-cylinder, my favorite cross country RPM is 2300. For local goofing around, that may be 1900-2000. Then I set 10-12 gph depending on mission and conditions for cruise. For non-cruise, I might be back around 8gph. I was flying this morning and set 2300 RPM and 23", and that was showing 60% power as calculated by the Dynon, and things were happy there. I don't find myself cruising at WOT with the bigger engine because that's just more HP than we need for efficient cruise. I'm sure there is an altitude high enough where WOT would become necessary, but I don't carry oxygen and rarely need to go much over 10,000 feet.

For just about any normally-aspirated engine, I find that there is a sweet spot around 8000' where the TAS advantage is noticeable but the engine's loss of MAP is still acceptable. If the winds or terrain require higher altitudes, they are available of course, but then it takes a lot more throttle to get the same MAP because the engine output diminishes more noticeably. With the 4-cylinder we were needing to run wide open at those altitudes.

As for the mixture, as soon as I level off, regardless of the length of flight (except in the pattern), I make a coarse adjustment in MP, then set the RPM, then fine tune the MP, then pull (not twist) the mixture back to the point of roughness, then enrichen it just enough to make the roughness go away. With the electric ignition, this is a very subtle point. If I'm going to be climbing really high, I might make a mixture reduction in the climb. The strategy here is to note the EGT at pattern altitude, and then dial the knob to match that EGT until the top of climb. I would only ever fly around full rich when doing pattern work, just because the fuel savings aren't worth the risk of forgetting it for a go-around. This is when I'm in "10 landings per hour" mode and the prop and mixture both stay full forward.

I'm not positive about the theory, but I would think the mixture needs to be revisited any time the RPM or MAP are adjusted very much, especially with the carb. Maybe this is not the case with fuel injection?

Brooks, how much of an oil consumption difference did you see, and how confident are you in your measurement protocol? I find measuring oil quantity precisely to be really hard.

Turbos would of course be an entirely different discussion and strategy.

[svyolo]

Brooks;

There are a huge number of variables like Jared said. A smooth running engine might be #1. When you change RPM and MP, you are also changing what optimal spark timing would be, which is a MUCH bigger deal than most realize. If you are running MAGS or EI in fixed 25 deg mode, running oversquare (high MP/ low RPM) helps with efficiency also because that combo likes a ignition advance closer to 25 degrees.

There are lots of resources for this info, but unfortunately most are very incomplete or only cover one variable.

Things that "want" more ignition advance to be optimal:

High rpm
Low MP
LOP

If you are getting better efficiency running oversquare, especially LOP, that power setting and your ignition advance are probably closer to optimum.

I have a LOP toggle switch on my EFI. I can adjust how much fuel to "lean" in %, AND how much ignition advance to add. I have been using 4 degrees. If I set it to 0 degrees, I lose a few knots without changing anything else.

There is a lot of documentation about running over/undersquare. Almost none of them talk about how that effects optimal ignition timing, as when they were written almost all aircraft engines ran fixed tiiming.

I think Lycoming p/v engine docs allow oversquare up to 5 or 6 inches, ie 25 inches/2000 rpm. These docs should all assume fixed ignition timing, as that was all that was available when written.

I have heard PMAGs can be very aggressively timed when run in "variable" mode. That means lots of advance already.

I don't recall what ignition you are running. I have not looked into Surefly at all.

[Battson]

Group: This is me thinking/observing cruise performance....Cruising at any altitude.[*]The other day I had a short flight at 2500'. At LOP & at 5.4 gph I improved my cruise speed by 3% by slowing the RPM from 2200 to 1900.[*]LOP Question....

• Is the LOP calculation that my EI Engine Monitor that I did at 2200 RPM still valid if I set 1900? Do I need to re-sweep the mixture to re-find Peak EGT?
• If my EGT stays about the same, is my LOP calculation is still accurate?

[*]Oil Consumption

• I think my oil consumption goes up when I slow down the RPM to 1900 or 1800 rpm.
• Max Efficiency may (key word) have a cost of increased oil consumption...needs more research.

[*]Do others see higher Oil Consumption at slower RPM's in parallel valve Lycoming engines?[*]CHT's and Oil temps drop when slowing RPM down this much.[*]Is it worth 3%?[/LIST]Tell us why you use your "go to" power settings, what you know, curiosity you have, or what you don't know and want to learn. Personal experiences are important. I want to know both what you set along with why you choose it.

Well, quite a stream of questions there Brooks!

I've been running solely lean of peak for the last 13 years / 1200 hours and the engine looks great inside, and runs like new. So I feel confident to share a select few remarks:

Efficiency
- Efficiency is undefined in engineering terms, so let's say it means distance travelled or airspeed achieved per unit of fuel burnt. If you're looking for stoichiometric efficiency, that's different.
- Flying slower is more efficient, down until you reach something impractical like 70-80 KIAS, when efficiency starts to get worse again.
- Lower RPM generates less friction and less drag, so is measurably more efficient and the engine always runs cooler, assuming the cooling airflow is sufficient.
- Therefore, "oversquare" power settings (RPM number /100 being less than MAP number) is generally going to result in better efficiency and a happier, cooler engine. i.e. 26"Hg and 2,200 is better than 24" and 2400.

Lean of Peak
- Operating lean of peak increases efficiency dramatically. Personally, I lean immediately after take-off during the climb out, but that took practice to achieve. I remain lean until the pre-landing checks (unless I require full power climb, which is very rare). I do this on every flight to increase efficiency and range, lower costs.

To your questions specifically:

- Reducing RPM increases the risk of running LOP, by narrowing the margin over detonation.
- Changing MAP or RPM, without adjusting the fuel mixture, changes your relative position to peak EGT, and therefore risk level
- At different power settings, a fixed EGT is almost certainly not the same relative position, to peak EGT
- As per the efficiency comment, lowering RPM or MAP reduces speed and increases efficiency when running LOP, in the same way

Oil Consumption
- I have never noticed a change in oil consumption based on LOP.
- I can't think of a good reason why it should be worse, at lower RPM, if anything it should be better... (all else being equal).

[ToddJ]

I read somewhere that the 2% on the horizontal stab becomes drag at cruise and is really useful for landings only. Wondering if making that adjustable during flight is feasible.

[jaredyates]

I read somewhere that the 2% on the horizontal stab becomes drag at cruise and is really useful for landings only. Wondering if making that adjustable during flight is feasible.

I see two separate questions here: is it possible to make the incidence angle adjustable in flight, and would doing so improve cruise efficiency?

First, while anything is possible, because our stab is externally braced, this would be a very major change and structural redesign that would require lots of engineering and testing. It can be ok to exit the "expression of builder preference" realm to enter the "airplane designer" realm, as long as the exit is intentional and supported by adequate skills and knowledge, but this would definitely be a bigaq excursion.

As to whether or not it would improve efficiency, I'm wondering if something might have gotten lost in the discussion, or perhaps I have something new to learn about this. As I understand it, I don't think there is much to be gained here. The horizontal stab/elevator is still doing something important for us in cruise. How much work depends on speed and CG. At an aft CG, the work is less. At a forward cg and slow speed, the work is more. With our tail system as-designed, there is only one CG+airspeed combination that we can tune for, where our elevator will be in trail with the horizontal stab. As I understand it, this is the minimum drag.

Most of us select cruise speed at a somewhat-loaded CG for this tuning. If today's actual CG varies broadly from the CG that the incidence angle is set for, then the elevator will be disolaced in cruise, reducing efficiency. Maybe on the Five this becomes more noticeable than on our plane?

[Battson]

As to whether or not it would improve efficiency, I'm wondering if something might have gotten lost in the discussion, or perhaps I have something new to learn about this. As I understand it, I don't think there is much to be gained here. The horizontal stab/elevator is still doing something important for us in cruise. How much work depends on speed and CG. At an aft CG, the work is less. At a forward cg and slow speed, the work is more. With our tail system as-designed, there is only one CG+airspeed combination that we can tune for, where our elevator will be in trail with the horizontal stab. As I understand it, this is the minimum drag.

I've done quite a bit of testing on this, over a long period of time.

I was unable to find a clear trend, whereby CG location and therefore load on the tailplane change cruise efficiency or speed in a consistent or measurable way.

In fact, I find the ambient conditions have a greater impact, and drown out any potential minor yet measurable savings.

For context, my stabiliser is set at 2 degrees, not 4.

[TimTall]

I've been paying attention to this for a while now too and there seems to be very little difference in cruise efficiency or speed with different weight and CG positions. The elevator is in almost perfect trail with the horizontal stab at any CG position while in cruise.

My A model has the airfoil ribs in the horizontal stab and it's set to 2 degrees.

I'm not sure what the trim tabs difference is between forward and aft loading conditions. I can't see it out the window and a microscopic movement at the control wheel has a massive influence on pitch attitude.

[Bcone1381]

Regarding rigging:

I've done quite a bit of testing on this, over a long period of time.

I did not mess with H-stab incidence. I did have measurable effects connected to aileron rigging. I reflexed them some to improve adversity yaw and lighten up the roll force. I loss cruise speed...maybe 3 kts...with a more assertive aileron reflex. I'd guess 1 inch was assertive. I'd guess now they are 1/2" reflexed. I have my flaps reflexed as much as I can without having contact with the rear spar when they are up.

[Battson]

Very interesting, thanks for sharing. I have mine reflexed too, probably an inch at the ailerons, but I set my trailing edge differently (down about an inch to increase camber), so results may vary

[Bcone1381]

Oil Consumption: Its a feel I have. I have no good data to provide, and I was trying to gather more data from the Group to validate/dismiss my feeling. To get good data from my engine will take 6 months. I'm not sure I'm committed to that.

My Engine:

• PV Lycoming IO-360 with Bendix FI, Surefly Mags.
• The Mags are set to fire at 25 BTDC and using RPM + MP they will advance up to about 35 BTDC.
• I am very impressed with the efficiency of this configuration.
• Lycoming Power Chart approves up to 5"over square at RPMs above 1800.
• Hartzell does not prohibit any RPM areaa, yet EI is not tested. I avoid 2000 - 2200 RPM based on other engine limitations.
• When I go LOP, I pull the mixture until I feel a healthy power change then approach Peak EGT from the lean side.

I'll go through the above posts more carefully later. It/s nice out and Its time to go flying.

One last data point. 9500', 140 mph at 7.1 gph. Wow!
Screenshot 2026-04-23 at 10.07.19 AM.png Screenshot 2026-04-23 at 10.07.28 AM.png Screenshot 2026-04-23 at 10.07.08 AM.png

[Nev]

One thing worth keeping in mind when testing any rigging changes is the effect of ambient conditions on IAS.

Variations in temperature and pressure will change density altitude, which in turn affects indicated airspeed. So if you’re seeing a difference in IAS, it may be due to changing conditions affecting IAS, rather than the rigging itself affecting actual fuel efficiency.

Ideally, try to keep conditions as consistent as possible between test runs, or account for density altitude when comparing results.

[Battson]

My Engine:[*]Lycoming Power Chart approves up to 5"over square at RPMs above 1800.

That's interesting, I think that's different to the -540 power chart.
I recall the -540 allows up to 7" above the RPM /100, so in theory you can run at 29" and 2200 if you wanted to... not sure I would.
I cruise at 27" and 2300 pretty regularly, if I am in a hurry, also LOP.

[rodsmith]

Living in the vertical half of Colorado, I get very little opportunity to test over square power combinations. Even if cruising along the local valleys, 7500' is the lowest that provides adequate terrain clearance. I only see about 23.5" mp at that altitude. Throttle management is simple. Advance to the stop on takeoff and leave it there until a couple miles from entering the landing pattern. Even on a short 30 mile flight I set the mixture lean of peak. The Garmin EMS seems to require slow leaning to catch the peaks for all the cylinders. It is my understanding that as long as you are under 70 percent power there is no danger of detonation. I am generally around 50 percent power when leaning. My normal cruise is 2200 rpm and whatever MP is available. There have been just a few flights where I have been in a hurry and set rpm at 2400. Depending on altitude I see true speeds of 122-125 kts, with a fuel flow of 10 to 11 gph. In Brooks example at 9500', I would be at the same speed, but burning an additional 3 gal/hr. I have good fuel distribution to the cylinders at WOT. LOP the cylinders are within 20 deg of each other measured lean of peak. From my home field at 6500' I takeoff with the mixture out about 1 1/4". Initial climb out the EGTs are about 1300 which is about 100 deg below peak and best power. At pattern altitude I reduce rpm to 2500 for climb out and start leaning further to maintain 1300 deg. . I don't lean to LOP until established at cruise altitude.