Flight Ops

Some Suggestions For More Efficient Engine Cruise Power Settings

Bob Kromer
MAPA Executive
Director & Former Engineering Test Pilot

Ed. Note: Last month, we made some suggestions for more efficient climb power settings. This month, let's look at some power setting ideas that might help you fly your airplane a little more efficiently and consistently in level cruise flight. These concepts come from my background as an engineering test pilot at Mooney and Cessna doing a variety of developmental and certification flight tests. This article is not meant to sound like I am a "know it all." The ideas presented are only for your consideration. You may be perfectly happy flying the way you are. But give some thought to the ideas presented in this article and give them a try on your next trip or two. Most of the suggestions and "rules" we hear about flying our airplanes are based on rumor and outdated technology. The ideas in this series of articles are based on sound engineering principles and lots of engineering flight test data. I think you'll find your airplane will give you more consistent performance if you try these cruise techniques.

Introduction
First of all, thanks for the positive comments regarding last month's article on more efficient climb power settings. From the feedback, it appears that a lot of you desire more operational information like this, so we will continue to bring it to you through these series of articles. In these articles. I am attempting to bring to you information based on sound engineering data and extensive engineering flight testing I did at Mooney several years ago during my career with the factory. With that said, this month let's take a look at some suggestions on engine power management and settings in level cruise for the most efficient way to cross the country in our Mooney.

I. Normally Aspirated (Non-Turbocharged) Power Settings for Cruise

So here we are, leveling off at our assigned (or desired) cruise altitude after climbing using the techniques described in last month's article. Our goal is to now transition as quickly and efficiently as possible into level cruise flight and to set the best power setting we can for extended cross country flying at our assigned or chosen altitude.

Accelerate First
As we level off in our normally aspirated Mooney, the first reaction we have is to do something right away. Don't. Just let the nose come down to level flight and allow the airplane to accelerate to cruise speed. Keep the climb power setting and keep the cowl flaps open for two minutes or so as the airspeed increases and stabilizes. Too many Mooney pilots like to begin reducing power immediately upon reaching cruise altitude. This really extends the time required for the airplane to accelerate and reach a stabilized cruise speed. So for the first two minutes or so after reaching cruise altitude--don't touch anything. Keep the power up. And leaving the cowl flaps open for a little bit eliminates the "hot spots" that developed under the cowling during the climb and helps the accessories (fuel pump, mags, vacuum pump) cool down quicker after they got hot in the climb. So, we've not touched anything for the last two minutes after reaching our desired cruise altitude. Now it's time to reset the engine for cruise. Here are my recommendations for setting cruise power In a normally aspirated Mooney.

Keep the Throttle Full Open
Remember, a normally aspirated engine operates most efficiently with the throttle full open. With the throttle in the full open position, the cylinders are getting the most volume and most even distribution of induction air possible. Engineers "tune" induction systems assuming a fully open throttle. Anytime you operate a normally aspirated engine at part throttle settings, you are adding a physical restriction in the induction airflow (a partially closed throttle valve). This restricts the amount of induction air flow and also makes it turbulent inside the induction system--not good for efficient engine operation.

In any normally aspirated (non-turbo) Mooney, the best cruise power setting is full throttle, 2500 RPM, Peak EGT +50 degrees rich, cowl flaps closed. Only the M20S Eagle whould be cruised at 2400 RPM.

So keep the throttle full open. The only time to back off the throttle in cruise in a normally aspirated engine would be 1) if you are flying below 3000 feet, where a full throttle would give some pretty high manifold pressures or 2) you're just out flying around to sightsee, where reduced power and slower speeds are called for. But for most cruise conditions where, you want to cross the ground quickly and in the most efficient manner, keep the throttle full open. And take whatever manifold pressure you get at full throttle. And for those airplanes with the ram air system, by all means open it for some added manifold pressure with the throttle full open. The increase in manifold pressure can be as much as 1" Hg, or about 10 horsepower. But only use the ram air after the throttle is full open and only use it when you are flying in clean air. The ram air system is nothing more than a bypass of the induction air filter, so anytime you open it you are venting unfiltered air directly into the engine. Good for power, but bad if you're flying in dust or heaven forbid a sandstorm.

For the M20K (231), try cruising at 31" MP, 2500 RPM, Peak TIT + 50 degrees rich, cowl flaps closed.

Prop RPM at 2500 (2400 in the Eagle Only)
Except for the new M205 Eagle, for the normally aspirated (non-turbo) Mooneys we fly, the best RPM to cruise is 2500. Period. For the engine/prop combination in the Eagle, the best cruise RPM is 2400. You can talk about all kinds of reasons why 2400, 2300, or 2200 might he better, but from a pure technical standpoint, cruising at lower RPM settings makes no sense at all. By selecting cruise RPM lower than 2500 (2400 in the Eagle) you are simply giving away power and better performance for nothing in return. Think the engine will last longer at lower RPM settings? Forget it. TBO is set assuming maximum continuous power (that's max rated RPM). Think the engine will run cooler at lower RPM? Nope. As you increase the spread between MP (high) and RPM (low), internal engine operating pressures go up slightly, making the engine run just about as warm as it would at a higher RPM setting. And I have yet to fly a Mooney that is as smooth at 2200 or 2300 RPM as it is at 2500. So why do it? So try cruising at 2500 RPM. If you have an Eagle,2400. Everything else is giving away performance (our most valuable Mooney asset) for nothing in return.

Three Choices of Mixture Settings
I think you really have three choices of mixture settings with a normally aspirated engine. They are 1) 50 degrees rich of peak EGT (Exhaust Gas Temperature), 2) peak EGT, 3) 50 degrees lean of peak EGT. My choice? 50 degrees rich of peak EGT. All the time. It's the best combination, in my opinion, of best power mixture (100 degrees rich of peak EGT) and best economy mixture (peak EGT). Some pilots like to fly at peak EGT --that's okay, you save a little gas but loose some performance. Lean of peak EGT is okay too, but only if you're flying a fuel-injected, normally aspirated engine and have a good set of balanced fuel injectors installed. Operating lean of peak EGT is impossible if your fuel injectors aren't balanced to "squirt"equal amounts of fuel into each cylinder -- one or two cylinders are going to get too lean before the others, resulting in a very rough running engine.

For the M20K (252), 28" MP, 2500 RPM, Peak TIT +50 degrees rich, cowl flaps closed works well at all altitudes.

For engines with a carburetor, forget lean of peak EGT operations. Carburetors are very poor fuel distributors. I haven't seen many carbureted engines that could be operated lean of peak EGT with any level of smoothness. The engine feels like it's coming out of the cowling. A final word on leaning -- take the time to find a true peak EGT value when you are setting the mixture for cruise. Most pilots don't have the patience to find a good peak EGT reference value. There is considerable lag between the EGT gage indications and the movement of the mixture control--it simply takes a few seconds for the change in the exhaust gas temperature to register on the gage. So, be patient when establishing peak EGT. Lean a little, wait for the EGT to register, lean a little more, wait, etc. Finding a true and accurate peak EGT reference point is critical to precise engine operation and proper cruise mixture setting.

Cowl Flaps Closed
In all but Ihe most extreme heat, level cruise flight in a normally aspirated Mooney should always be done with the cowl flaps full closed. Normally aspirated Mooneys cool well in level cruise. So keep those cowl flaps tucked up tight in level flight. If you're having to trail open the cowl flaps in you're normally aspirated Mooney to keep that engine cool in cruise, something is wrong with your engine.

II.Turbocharged Power Settings for Cruise
With turbocharging, things get a little more complicated in setting cruise power and managing the engine. You really can't hurt or damage a normally aspirated engine, no matter what you do. But a turbocharged engine can be damaged by mis-managing the power controls. Heat is our biggest enemy with a turbocharged engine. So is overly aggressive leaning at high power. With that said, here are some recommendations for managing a turbocharged Mooney in level cruise flight.

Accelerate First
Same as with the normally aspirated engine, when we reach our cruising altitude just let the airplaine accelerate out to cruise speed with the engine power and cowl flaps set as they were for the climb. Our tendenancy is to immediately start reducing power from climb to cruise, but let the airplane accelerate for a couple of minutes before you touch anything. And leave the cowl flaps in the position they were in for the climb. The extra couple of minutes of increased airflow through the engine compartment. with the cowl flaps open will help cool the engine and accessories that probably got pretty hot in the climb.

Throttle (Manifold Pressure) Settings
A couple of minutes after leveling off at cruise altitude, it's time to reconfigure the engine for cruise.Obviously, we can't cruise at full throttle like we did with them normally aspirated engine. With turbocharging, full throttle would result in power settings too high for extended cross-country flying. So what do we do? I can tell you what I do. From a review of the power charts for the three models of turbocharged Mooneys I fly, I have picked one cruise manifold pressure setting for each model.

For the TLS/Bravo, a good cruise power setting is 30" MP, 2400 RPM, Peak TIT + 50 degrees rich, cowl flaps closed.

And that's the manifold pressure setting I use for cruise all the time. Forget the awkward and tedious exercise of correcting the manifold pressure for differences in OAT, differences in altitude. etc. That stuff drives me crazy. I just pick a manifold pressure that gives me good performance but is well within the operating parameters of the engine and I fly there all the time. With that said, here are the manifold pressure values I use all the time when flying each of the turbocharged Mooneys listed below in level cruise flight (Note: these numbers assume a stock airplane with no aftermarket mods such as intercoolers and/or wastegates):

M2OK "231" 31" Hg
M2OK "252" 28" Hg
M2OM "TLS/Bravo" 30" Hg

Don't care what altitude I'm flying at or what the OAT is, day in and day out these numbers are where I'm going to set the manifold pressure in the turbocharged Mooneys I fly. I know they're slightly conservative in some cases, but in general I get excellent airplane performance with these settings with acceptable fuel economy. The engine is running cool and smooth. And I'm not trying to calculate exact manifold pressures to the nearest .1" all the time from the power charts--a silly waste of time. I think you'll find these manifold pressure values will serve you well, just as they do for me.

Prop PRM at 2500 (2400 for TLS/Bravo)
Absolutely, positively, run the cruise RPM at either 2500 (M2OK) or 2400 (M2OM) in level cruise flight. None of this 2300 or 2200 RPM stuff with a turbocharged engine. Try to avoid a combination of high manifold pressure and low RPM in a turbocharged engine, especially at altitudes above 18,000 feet. High MP/low RPM makes the turbocharger work harder (spin faster), increases internal operating pressures and temperatures and is the worst case for detonation. So keep the RPM up. The perfect setting for the 231 and the 252 is 2500 RPM. For the TLS/Bravo, it's 2400. Anything else is off the peak efficiency curve.

Mixture Setting 50 Degrees Rich of Peak TIT (Turbine Inlet Temperature)
There is lots of controversy here.Some pilots and mechanics swear that running at best power mixture (100 degrees rich of peak TIT) is the only safe place to be. Peak TIT is approved in the power charts, but lots of people say you really shouldn't be there. Then there's the new idea of running a turbocharged engine lean of peak TIT. George Braly of GAMI swears that this is the place to be with a set of balanced fuel injectors installed. He says the engine runs cooler 50 degrees lean of peak TIT than 50 degrees rich of peak and has the test cell data to prove it. With all that said and considered, this former test pilot flies turbocharged Mooneys at 50 degrees rich of peak TIT. Like the normally aspirated Mooneys, it is my professional opinion that this mixture setting is a perfect compromise between best power (100 degrees rich of peak) and best economy (peak) mixture settings. I have never seen an engine over-temp in level cruise using 50 degrees rich of peak, even the notoriously hot running -GB engines in the early 231's. I might be using a little more fuel at 50 degrees rich of peak TIT than at actual peak, but come on-- what's an extra gallon an hour of burn cost compared to that $60,000 engine I'm flying behind? Saving a few dollars of extra fuel burn by operating at peak TIT or even lean of peak TIT turbocharged engine just isn't worth it to me. Replacing a turbocharged engine costs too much.
You can listen to GAMI and fly lean of peak TIT. They do have some compelling data and the engineering competence to back it up. Or you can fly very conservative 100 degrees rich of peak like some suggest. But for me, I think one is way too aggressive and The other way too conservative. So I pick and suggest 50 degrees rich of peak TIT. From sitting in the cockpit in engineering prototypes for many hours and watching the instrumentation, I know your engine will he happy there.

Cowl Flaps Closed
With the power settings as detailed above, you should be able to fully close the cowl flaps in cruise in any turbocharged Mooney and still keep the cylinder and oil temperatures in the green. The only times I've had to trail open the cowl flaps in level cruise in a turbocharged Mooney was in the 231 at very high altitudes (above 20,000 feet) on very hot days. This was dictated by cylinder head temps nearing redline. So, always monitor the cylinder head and oil temps in level cruise and keep them in the green with the cowl flaps. But you'll find that in all but the most extreme conditions, the temps can be held in the green arc with the cowl flaps closed.

Summary
The chart below summarizes some suggested cruise power settings for the Mooneys we fly. Give these settings a try on your next trip and see how they work for you. I welcome your response and opinions. But I think that for the flying we normally do, these cruise power settings will give you the optimum combination of performance and economy. We Mooney pilots are speed and economy freaks. I know I am. These cruise power settings and techniques have served me well over the years and have always extracted a superb combination of performance, economy, and low maintenance requirements from the Mooneys I have flown. I think you'll find they will serve you well also.