Advice and techniques for running at reduced power settings

[Green Goggles]

I started flying about 4 years ago, and have logged all of my 200 hours in an M20C Mooney.
I have the carburated, 180-horse Lycoming engine.


For all of those 200 hours, I operated with engine RPM between 2300 and 2500- the recommended operating range according to my owner's manual. (Generally I was going somewhere and wanted to get there.) 2000-2250 is the Red Arc, denoting no continuous operations.


Recently, it seems I am never in a hurry, just sight seeing. And I want to burn less fuel. So what do I need to know to safely operate below that range?
Is there anything to be aware of regarding mixture settings, carb heat, or the reduced cylinder temperatures?


My owners manual gives powers settings for 1800 RPM, noting 16.9, 16.8, and 16.6 inches at 5,000 ft, 7500 ft, and 10,000 ft respectively, but that's it.
Do I need to reduce power to 16 inches to safely buzz around at 1800 RPM? Am I over thinking this?


Any thoughts or advice are appreciated!

[WLIU]

For what you are doing, there is no advantage to running at low RPM. I believe that you are looking for low fuel consumption and you can get that at more typical RPMs.

Lycoming will be happy to sell you an operating manual for the O-360 engine that contains a TON more operating info than you will find in the Mooney airframe manual. I highly recommend that you get it.

I will suggest that you can happily run around at 2300 RPM with the manifold pressure set for 55% power with the mixture pulled way out to get low fuel burn at moderate speed. You could run 18" - 20" MP with lots of leaning if you want.

So the next question is whether your airplane has an Insight or JPI or EI engine analyzer. With the temps for every cylinder you can really tell what the mixture control is doing for you and you can get your lowest fuel consumption.

At low RPM, the airplane won't accelerate if you discover that you need to maneuver and you can hurt the engine by applying too much manifold pressure. Low RPM, low MP, cruise is usually done by the ferry guys trying to get the absolute max miles per gallon so that they can cross an ocean.

Hope this info helps,

Wes
N78PS

[Green Goggles]

That all makes sense and is very helpful. Thanks! 2552

FWIW, I don't have any type of engine analyzer in my airplane. I have one overall cylinder temperature guage, not individual readings.

[WLIU]

I will suggest that a GEM or a JPI is a better investment than the latest sexy mode S transponder. With a light bar for every cylinder you can see exactly what is going on in your engine and lean very reliably. I own two Insight GEM 602's (two airplanes). If you foul a plug on startup and your engine runs rough when you do your mag check, you can see exactly which cylinder and which plug. Takes the sweat out of trouble shooting. Lets you get the best fuel consumption at any power setting.

Best of luck,

Wes
N78PS

[Bill Greenwood]

I owned a 1970 M20C.

The prohibition of between 2000 and 2250 is probably due to some harmful vibration either prop or engine or the combination of both.

As for as hurting the engine with too low of rpm if you need to add power, that is a partial truth. If you have to add any significant amount of power, like a go around, THE PROPER PROCEDURE IS FIRST PUSH UP THE PROP RPM TO FULL AND PROBABLY THE MIXTURE TO FULL IF AT LOW ALTITUDE AND ONLY THEN ADD THE THROTTLE.
So going to more power lead with the rpm and mixture, then add throttle.
The reverse is true when needing less power, bring throttle back as needed, then prop rpm, then set mixture leaner as needed for less power.
It only takes a few seconds to lead with prop and mixture when adding power and even on a go around you can come in with a smooth addition of power and not need to slam the throttle open.
It is one reason I like the Mooney style pedestal with the three levers better than the Cessna style push pull knobs; it is easier to make changes with one hand for all three levers.

It might be nice to have an engine analyzer, but I think the 4 cylinder carbureted engine is a simple and reliable one as it is.
My injected Lyc IO 360 in my next Mooney 201 went to 2000 hr tbo, used a lot of oil but kept running.

By the way if anyone tells you those Mooneys are hard to land is almost surely not much of a Mooney pilot and probably trying to land too fast and/or with power on.

[Green Goggles]

Many good thoughts there, Bill.
Thank you.



By the way if anyone tells you those Mooneys are hard to land is almost surely not much of a Mooney pilot and probably trying to land too fast and/or with power on.

I did all my flight training in my Mooney, and it is the only plane I've ever flown.
So... even if they are difficult to land, I wouldn't know any better! http://images.corvetteforum.com/images/smilies/lol.gif

[kscessnadriver]

I will suggest that a GEM or a JPI is a better investment than the latest sexy mode S transponder. With a light bar for every cylinder you can see exactly what is going on in your engine and lean very reliably. I own two Insight GEM 602's (two airplanes). If you foul a plug on startup and your engine runs rough when you do your mag check, you can see exactly which cylinder and which plug. Takes the sweat out of trouble shooting. Lets you get the best fuel consumption at any power setting.

I could see this talk of an engine monitor if he had a fuel injected engine, which could likely be ran lean of peak fairly easily. However, I'm not sure that an engine monitor is really a worth while investment on a carburated engine, where running LOP is going to be a challenge no matter what you do.

[WLIU]

The reason for the red arc on the tach is indeed due to destructive resonance between the 4 cylinder engine and the prop. Hartzell does a bunch of testing and in that RPM range the blades apparently are fatiguing at a rate that greatly shortens their fatigue life. It apparently has to do with resonance between the prop and crank, etc.

An engine monitor is just as valuable with a carbureted engine, but if you do not need to worry about that last gallon-per-hour, or when the engine burps you just drop your ship off at the shop and wait for the bill, you don't need one. I can report that as soon as I installed my first one, fired up, and started seeing what it told me, I had an "A-HAH!" moment and never looked back. But I fly airplanes with limited fuel supply and when going cross country always have to optimize my fuel burn. If you fly an airplane where low fuel means that you still have 2 hours on board, then you don't need precise info on your engine performance.

Fly safe,

Wes
N78PS

[Bill Greenwood]

An engine monitor would be nice to have, just like leather seats and a new Porsche to drive to the airport. But if you run that O-360 engine carefully , it should go to TBO, either 1800 or 2000 hours with or without the monitor.

If you have nothing to see egt, then once at reduced cruise power, probably at 2000 rpm, you slowlly lean until the mp just starts to drop or the engine seems a little rougher, then enrich just a bit. If you have an egt gauge you can see the temp rise as you lean, then peak, then fall and go a bit, say 50* cooler than peak.
I think you will find that this setting will be close no matter if you are leaning with no gauge or just egt or a GEM which gives egt and cyl head temp for each cylinder.

I am not sure what a "burp" is in this engine, might be that the pilot had the Taco Bell giant breakfast burrito, but it is unlikely that you are going to have a sudden engine disaster in a new or good OH Lyc O-360 that is flown at less than 75% power for cruise, and properly cared for. The is not an engine overheating problem in these Mooneys like in some tightly cowled planes and he doesn;t have a turbo or even fuel injector to deal with. Just a relatively simple and understressed engine.

One big advantage of a Mooney is range. The M 20 c Ranger might cruise around 150 mph at 7500 feet, and will likley go for 5 hours or so, I can't recall exactly.
My 201 had 64 gallons of fuel, and burned less than 10 gal per hour in cruise, thus fuel was not something you had to worry about, that is, of course if you used good common sense and filled up the tanks after each flight. I know I went from Colo to Austin once non stop, safely, but these days the pilot needs more stops. I recall heading directly to the mens room upon landing at Mueller.

[Hank]

Goggles,

My Owners Manual recommends 20"/1950 RPM for low-speed loitering and flightseeing. But I don't like the way the plane handles there. I often run 2300 down low, and put the throttle wherever I need it airspeed-wise. Food runs to the next county are 23/2300; flightseeing can be anywhere from 16-20" & 2300.

Bill, my C holds 52 gallons, burns a pretty steady 9 gph [good for 5½ hours], and won't run LOP without lots of vibration. 201's do it nicely if your injectors are balanced, but carbs are difficult to get there even with slightly reduced throttle and partial carb heat. I probably average ~140 knots groundspeed, indicating around 140 mph at 6000-8000 msl.

[Green Goggles]

Goggles,

My Owners Manual recommends 20"/1950 RPM for low-speed loitering and flightseeing. But I don't like the way the plane handles there. I often run 2300 down low, and put the throttle wherever I need it airspeed-wise. Food runs to the next county are 23/2300; flightseeing can be anywhere from 16-20" & 2300.

Bill, my C holds 52 gallons, burns a pretty steady 9 gph [good for 5½ hours], and won't run LOP without lots of vibration. 201's do it nicely if your injectors are balanced, but carbs are difficult to get there even with slightly reduced throttle and partial carb heat. I probably average ~140 knots groundspeed, indicating around 140 mph at 6000-8000 msl.

Awesome info, Hank, thank you. I always like to hear how other Mooney owners operate their aircraft.
I am going to begin doing some expermentation with different settings and see what works for me.

My C also holds 52 gallons. I do all of my flight planning on 10 gal/hour.
My bladder has never allowed me a flight much longer than 3 hours, and I suspect it never will.

[Bill Greenwood]

Although I now own a Be 36TC and like it, I still have a soft spot for Mooneys. My first plane was the "70 M20C Ranger, and I just felt like I had a better or more advanced plane for my needs then than something like a C 182 which is another very popular plane in my area.
I only had the C for a few years and moved up to the "77 M20J, 201, for the better avionics and a little more climb and speed. It had great King KX 175 B radios which are great and reliable and simple to operate. The KX 155s in my Bonanza are good to , but not quite so obvious to operate.

My normal practice is to refuel after a flight, before leaving the plane. It saves the time and trouble of doing it right before your next flight and takes away the temptation to take off with less than full fuel if there is some delay in getting topped up when you get ready to go.
So flying the 201, you come in and top off. The you really don't need to worry about fuel range next time on any normal flight. It burns less than 10 gal per hour and holds 64 gal. That is sure a nice asset to have. My cruise flight planning was 155 knots ,as I recall, and often would work out a little faster.

I have mostly flown 4 airplanes in my time. Range in time for each was, about 6 hours, about 5 hours, about 3:45 and 2:10 til empty. There is some careful planning and a bit of edge of seat flying when you get planes with shorter range.
I think the real comfort level is 3 hours.

The worst airplane that I know of for lack of fuel range is a Polikarpov, the little Russian open cockpit WWII fighter. It has only about an hour of fuel. One year Dave Morse was flying one at Midland CAF. The photos guys wanted to get some shots of this new and rare bird. Normally you'd go outside the airport area and maybe spend a half hour or 45 min in formation getting such shots, then come back to land. No big deal, but for this plane you really had to plan and be carefull. It was a quick 5 min flight to the photo area, perhaps 30 min max for pictures and then straight back to land, no delays can be done.

Hank, if you are indicating 140 mph at 8000 feet that is pretty fast. If you use the rule of thumb of 2% more true airspeed for each 1000 feet of altitude above sea level, then that would be an additional 16 mph so your true is 156 mph, which seems pretty good for a C.
If you are wondering, a good 201 really will go 201 moh in level flight, I have done it, but that is full power at about 7500 feet, not normal cruise. And I have had mine over 19,000 feet when light, did lot's of flights at 15,000 or so to get over the mountains.

[Hank]

Bill--

Here's a photo indicating 135 mph at 11,000 msl. Just don't look at the groundspeed visible on the GPS, it's too depressing.

Well, the system won't let me attach. The window comes up blank, then I refresh it and have to sign in again, and it goes blank. Repeated twice, no luck.

Indicated airspeed = 135 mph [~165 mph true]
Altitude = 11,060 msl [setting ~29.98]
VSI = <50 fpm descent
GPS groundspeed = 114 knots [~131 mph]

[Bill Greenwood]

Hank, if you have an indicated airspeed of 135 at a pressure altitude of 11,000 that seems pretty fast to me for a C. Using the standard of 2% more of true airspeed per 1000 above sea level thus add 22 mph and you have 157 mph true.
That's not counting temp correction . Standard temp at sea level is 59* F or 15*C. At a standard lapse rate of 3 1/2* F per 1000 feet that is 39* so if you were warmer than 20* at 11,000 you have a true airspeed that is even a little bit faster than 157.
What was your power setting then, and or % power?

[Hank]

Bill--

Double-check your math. 11,000 msl = 22%; 135 x 1.22 = 164.7 mph.

OAT was 63ºF at the time, and I've forgotten how to compensate for that in speed. The far side of the panel is not in the photo, but it was most likely 2500 RPM and as much MP as I could get. I typically run WOT in the climb, then pull back enough to make the needle move to create a little mixing turbulence in the carb before I start to lean.

My Owner's Manual says I should have 20.2" available at 10,000 msl, 23ºF, giving 71% power at 2500 [so 63º is 4% less, or 67% power] with speed between 161 and 164 mph depending on weight [gross = 161; 375 below gross = 164]. Summer in the South is warm even at altitude; I was coming back to WV from the beach at the beginning of August.

I'm getting book speed, or durn close to it. Guess the "speed loss" from the 3-blade prop is canceled out by the 201 windshield. Is it not normal to achieve book performance? It also says 9.0 gph at that altitude/power setting. Pulling back to 20.0" costs 1 mph at both weights and reduces fuel flow to 8.8 gph. My block average for five years is right at 9, so I'm doing something right.

"Life is too short to fly slow." At least when I'm going somewhere; slow flight and flightseeing both have their places, too. I bought this plane to go places, and I've gone south to Miami, east to the ocean and west to Yellowstone; still working on the "north" part.

But the OP was asking about low power, as in flightseeing. I use 2300 RPM, set power as needed for the speed you want. Recently, 16-18"/2300 kept me right at the top of the white arc, giving people plane rides. I took 15 people up on 8 rides that day--climb out at WOT/Max RPM, pull throttle way back, set 2300, trim trim trim, adjust throttle for ~115 mph indicated at 1000-1200 agl. The handling isn't too sluggish, and I fit in with the Cessnas doing the same thing. but then again, I was pulling back before leveling off, too.

It's a versatile plane, learn to fly the whole envelope. Lots and lots and lots of fun! Even more fun to share the experience!

[Bill Greenwood]

Hank, I think your math is better than my mental calculation. I just took 2% times 11 and came out with plus 22 mph when it should have been plus 22%, or as you say 164.7 mph.

Now as far as a correction for non standard temp, the warmer the temp where you are actually flying, then the faster your true airspeed. Your 63* is much warmer than I think standard of about 20* at 11,000 feet so you are actually faster than 164 mph.
I thought that was pretty fast for a M20 C from what I recall,but you didn't say before that you had the 201 windscreen which I'd guess will add about 5 mph.

I don't have my E6B with me now to calculate it.

[Hank]

Bill--

Standard temp at 10,000 is 23ºF; subtract 3 for another thousand feet makes it 20ºF like you said, or 43ºF above standard. My whiz-wheel is at home, but about all I use it for anymore is figuring speed correction for winds. I'll have to get it out and play.

Do you agree with many people who say the 3-blade losed 3-5 mph, just like the 201 windshield adds it? I always figured they canceled each other out.

Hmmm, still can't attach a picture. "Manage Attachments" brings up an empty, blank window.