How concerned should I be about throttling back for lower fuel burn?

MarkH

MarkH

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MarkH
I decided to buy a plane to travel and build time. I was shopping for a Cessna 150, but I found an Grumman AA1C with an O-320 and bought that instead.

One of the things I was looking forward to was building time 5gph in the Cessna, I recently flipped through a manual for an AA1B and noticed that it shows a power setting for cruising at 52% power on 4.5 gph. In theory, my O-320 should get similar fuel burn at similar power outputs (approx. 56hp), but it would require throttling back to 37% power and most performance charts do not go below 50% power.

Are there any risks to throttling that far back in cruise? Is there anything I should watch out for if I were to try this?
 
Is there a POH limitation against it?

Otherwise, if an engine is getting an appropriately combustible mixture of fuel and air it should run.. I've seen some charts go as low as 45% power.. but two things.. most planes aren't going to be in an envelope where flying much below 50% makes sense, so why would a POH go that low? and B, while a combustible mixture may burn these engines are designed to operate at certain temperatures, pressures, and extended power settings.. if 37% power doesn't check those boxes off that could reduce engine life in other areas..

Is the fuel burn that big of a factor? You'r talking about a couple dollars per hour. Probably easier to go at the lowest POH figures
 
First off good choice on aircraft

Second 37%
No on so many fronts
 
Tantalum said:
Is there a POH limitation against it?

Otherwise, if an engine is getting an appropriately combustible mixture of fuel and air it should run.. I've seen some charts go as low as 45% power.. but two things.. most planes aren't going to be in an envelope where flying much below 50% makes sense, so why would a POH go that low? and B, while a combustible mixture may burn these engines are designed to operate at certain temperatures, pressures, and extended power settings.. if 37% power doesn't check those boxes off that could reduce engine life in other areas..

Is the fuel burn that big of a factor? You'r talking about a couple dollars per hour. Probably easier to go at the lowest POH figures
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The O-320 was installed under STC. The POH won't have tables for his engine.

To the OP, I'm not sure why you bought a fast plane to go slow. It's like buying a steak and wishing it were a salad.
 
aftCG said:
The O-320 was installed under STC. The POH won't have tables for his engine.

To the OP, I'm not sure why you bought a fast plane to go slow. It's like buying a steak and wishing it were a salad.
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A) lol on the steak
B) wouldn't there be a supplement for the engine?
 
Tantalum said:
Is there a POH limitation against it?

Otherwise, if an engine is getting an appropriately combustible mixture of fuel and air it should run.. I've seen some charts go as low as 45% power.. but two things.. most planes aren't going to be in an envelope where flying much below 50% makes sense, so why would a POH go that low? and B, while a combustible mixture may burn these engines are designed to operate at certain temperatures, pressures, and extended power settings.. if 37% power doesn't check those boxes off that could reduce engine life in other areas..

Is the fuel burn that big of a factor? You'r talking about a couple dollars per hour. Probably easier to go at the lowest POH figures
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In reality, no. Theoretically, I could see some advantages of having more time in the air before I have to land to refuel on long flying days as I build time toward my commercial.

The POH numbers don't say anything about the 150hp engine (for obvious reasons), and the STC simple advises that the Green Arc on the Tach should be 1700 to 2510 RPM.

The O-320 operator's manual shows 1900rpm at sea level outputting approx 50hp.

While I can't find anything to suggest it is not safe to run there, I am still concerned about a Cessna 175 situation (many blame the GO-300's not making TBO on running the engines at too low of an RPM due to the tach reading engine RPM instead of prop RPM).
 
aftCG said:
...
To the OP, I'm not sure why you bought a fast plane to go slow. It's like buying a steak and wishing it were a salad.
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Sometimes you don't want to go fast. Doesn't mean you don't want the ability to go fast when you want to. I owned a C185 for 20 years. Was often called upon to do flight seeing and wildlife viewing trips for visitors to Alaska. Used 17 in. MP and 2250 rpm. There was no need to go 130 kts and burn 16 gal/hr if your mission is to see the sights (or just build time). Fast airplanes can fly slower on less fuel. Slow airplanes are slow airplanes. It would be nice to have an airplane suited exactly for each day's mission, but many of us can't afford several airplanes. :)
 
aftCG said:
The O-320 was installed under STC. The POH won't have tables for his engine.

To the OP, I'm not sure why you bought a fast plane to go slow. It's like buying a steak and wishing it were a salad.
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I want the option of going fast, but running 130Kts at 9gph does not make much sense when I am flying in circles to build time after work. I'm looking forward to being able to make a 400nm run for lunch, and come back on a Saturday.

Right now, more than miles I need time, and I am looking for the cheapest, safest, most fun way to get there.
 
Just to clarify, my order of priorities is:

1)Safest
2)Most Fun
3)Cheapest

Please don't infer from the last sentence of my previous post that there is any other order.
 
I recently sold my AA1B with the O-235. You made the right choice on going with the O-320. While I never flew one with the O-320, the O-235 was very lethargic
 
Whatever you like within the POH but I think most engine data says they like being run a bit harder than 37% power.

Would fly it around all day at 65-75% power if I felt like going slower, no worries at all. Assuming your POH is happy with that, of course.

At our altitudes that’s often WOT anyway... lol.
 
jd21476 said:
I recently sold my AA1B with the O-235. You made the right choice on going with the O-320. While I never flew one with the O-320, the O-235 was very lethargic
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It's an amazing plane. I have never flown an O-235, but I love the 1000+fpm climbs, the ability to take off in less distance than I land and I am really looking forward to being able to go a bit faster in cross countries.

My biggest challenge I have found is that it's still pulling a bit at idle, and that is making it a little challenging to land short. But, I only have 7 hours in it and I know I will improve with practice.

In all honesty, this was my ideal plane, but I did not think I could afford it. I feel really lucky to have found one that I could.
 
Can it even maintain altitude at that power setting?

From my training, I can’t remember very many hours of level cruise flying until cross-countries (i.e., not a majority of the time).

Just spend your money burning gas and learning how to best fly your plane.

Have fun!!
 
If being safe is higher on your list than cheap, then it would be better to spend your time learning or honing skills rather than flying in cruise as slow as possible. Time may be what matters to employers, but only because there’s not an easier metric to gauge experience. 1500 hours flying in cruise does not provide the same experience as 1500 hours where you try new things and go new places. 1500 hours flying the same hour over and over. Ew, for many reasons.
 
Back when I had my O-320 powered Warrior I calculated fuel burn at 7.25 GPH. This was at a reasonable, ‘get there’ power setting, not max, but not max conserve either.

Even with my current engine, IO-390, I come back a good bit if not trying to get anywhere. It still does take fuel & $$ to stay in the air though.

If you really want cheap flight time, start up, taxi to the run-up, sit for an hour, then do a short flight, repeat on the way back to tiedown.
 
37% is pretty low, if it will fly that way without having to constantly add power to fight down drafts go ahead. I know my AA1 didn’t like to fly bellow about 60% power. If I hit turbulence or even a slight down draft it just wouldn’t have to power and lift to fight it.

The majority of the time I fly the Venture I’m at 50% power or less but that’s still 175 knots so different story.
 
How will you make sure to burn off all the gunk from your spark plugs at 37%? Keep those temps up and in the comfort zone of 300-380, stay at 55%. I can't find it but there's a fantastic YouTube video all about how to run your engine.
 
I don't know if flying at ultra low power settings will damage the engine, but I do know that the joy of flying a Grumman will be lost. :(
 
I'm a fan of throttling back (more so than is "normal") for fuel economy. I'm not in any rush to get somewhere. I fly The Husband's 172 with O-360.
One day, we were flying together and had a conversation like this:
"Kath, do you always fly at only 2100 RPM?"
"Yeah, I do that a lot... it really conserves fuel!"
"Well, you're *supposed* to cruise in the green arc. The engine's designed for the green arc."
"Honey... I watched *you* dork around at 1800 RPM for over an hour, just last week!"
(a long pause)
"Well, if you're going to throttle back, just go high-throttle for a while at some point during the flight, just to get it nice and hot in there."
So there ya go. :)
 
I have a 68 Cardinal, it has the O-320 so the Power settings should be the same, I would think that the AA1C would be a little faster but fuel burn and power setting should be the same. this is from my POH
 

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If the OP can't afford to feed an 0320 he doesn't have the financial stamina for aviation. The gas burned in that power plant is the least of an airplane owner's expenses.
 
MarkH said:
. . . I am flying in circles to build time after work. . . . Right now, more than miles I need time, and I am looking for the cheapest, safest, most fun way to get there.
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Why are you building time? If it's for a flying job somewhere, I'll say that miles (seeing stuff and using airmanship) may be more important than flying circles around your home airport.

You know, the old "one hour a thousand times" trope.
 
Best way to save fuel and keep plugs clean is to lean according to Lycoming's instructions for normally aspirated engines: lean to stumble; enrichen to smoothness.

My O-320-D3G (160 HP) in a Warrior II, typically burns about 7 gph at 2350 RPM yielding 105 kts IAS. I always beat the POH numbers for fuel burn and this is with a calibrated tach.

Based on the POH charts I extrapolated the following chart:
https://www.dropbox.com/s/mp15vure923ir27/Engine.RPM.DA.to.Power.O-320-D3G.xlsx?dl=1

You will notice there are rpm and DA settings which produce less than or equal to 40% power. For example:
2000 rpm at any density altitude above 1000 MSL. Not sure at what altitude the plane will stop flying.
2100 rpm at any DA above 3000 MSL.

I have flown the Warrior at 2000 rpm at 1000 MSL in the pattern of my home drome.
 
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With an O-320 you will burn about 8 gph more or less at around 70% power, which is a typical cruise setting. In an AA1X, with 22 gal of usable fuel, that's pretty short legged. At 55% power you should be getting around 6 gph, like the stock O-235 at cruise power, good for 3+40 endurance. Even with the stock engine, the AA1X is a pretty short legged airplane, but really fun to fly. I can't imagine you will hurt the engine flying at 55% power if so desired. Just use the red knob properly so you don't run out of fuel.
 
Grum.Man said:
37% is pretty low, if it will fly that way without having to constantly add power to fight down drafts go ahead. I know my AA1 didn’t like to fly bellow about 60% power. If I hit turbulence or even a slight down draft it just wouldn’t have to power and lift to fight it.

The majority of the time I fly the Venture I’m at 50% power or less but that’s still 175 knots so different story.
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That's not actually a different story. From a physics perspective, you are powering back an overpowered airplane to cruise at your desired cruise speed. It's the same thing I want to do, just the numbers are bigger.

What engine is in the Venture? How much time have you put on it? How much of that was at the low power cruise? Have you seen any anomalies like fouled plugs?

60% power on a 108hp O-235 is 43% power on my O-320.
 
You could set it up for endurance flight. That gives the most time in the air for fuel burned, which is what you should have learned in the PPL. Getting lost and needing time to find your location without running out of fuel or getting even more lost. Or orbiting an airport waiting for the wreck on the runway to be cleared. Or waiting for the ground fog to burn off. If you lean it aggressively it won't foul up, and the engine will stay plenty warm due to decreased cooling flow and the high AoA.

If your engine is still pulling at idle (post #14) you should get the idle RPM checked.
 
Dan Thomas said:
You could set it up for endurance flight. That gives the most time in the air for fuel burned, which is what you should have learned in the PPL. Getting lost and needing time to find your location without running out of fuel or getting even more lost. Or orbiting an airport waiting for the wreck on the runway to be cleared. Or waiting for the ground fog to burn off. If you lean it aggressively it won't foul up, and the engine will stay plenty warm due to decreased cooling flow and the high AoA.

If your engine is still pulling at idle (post #14) you should get the idle RPM checked.
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Its getting checked today, I'm having the mechanic inspect the throttle and cable because it seems to idle 20% higher when warm.

On the 172s I trained in, I didn't really worry about it, mostly because I would start descending before I throttled back to the power settings that were not in the POH.
I feel like the moral of this thread is that I am overthinking this, it seems if the engine is not running rough, it should not be a problem.
 
One point that @Domenick touched on. Everyone has been saying "throttle back". Are you leaning the engine to match your power setting? If you've only been flying rentals, at wet rates, leaning often doesn't come into the picture. As he said, lean to a stumble and richen to smooth. 65% power, leaned properly will get you a decent gal/hour rate and enough speed to get places.
 
steingar said:
If the OP can't afford to feed an 0320 he doesn't have the financial stamina for aviation. The gas burned in that power plant is the least of an airplane owner's expenses.
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Just cuz u don’t want to burn it doesn’t mean ya don’t have it... being “cheap” is not always a trait of the destitute...
 
edo2000 said:
Sometimes you don't want to go fast. Doesn't mean you don't want the ability to go fast when you want to. I owned a C185 for 20 years. Was often called upon to do flight seeing and wildlife viewing trips for visitors to Alaska. Used 17 in. MP and 2250 rpm. There was no need to go 130 kts and burn 16 gal/hr if your mission is to see the sights (or just build time). Fast airplanes can fly slower on less fuel. Slow airplanes are slow airplanes. It would be nice to have an airplane suited exactly for each day's mission, but many of us can't afford several airplanes. :)
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Well I did have more in my post but deleted it since I just wanted point out why he won't have power tables for this engine application, and to poke a bit of fun at the OP.
So here goes:
My Citabria has an STC for the O-320 (from the O-325). Even though the pamphlet that serves as the POH/AFM is scant, it does have power tables for the O-320 because the factory produced that variation.

That said, the extra power in my case is great at getting the plane off the ground and climbing very well but you have to burn a lot of gas to get much more top end speed out of it. As such, I do commonly fly at 2200 rpm, which gives me 90mph - and when leaned half way properly burns a documented 4-4.5 gal/hr. Running at 2500 rpm only gives me 105 mph for almost double the fuel burn. I'm not bulking up my log book at this point but I do get in an airplane to enjoy the flying part, so the OP's intent doesn't strike me as insanity. At 90 mph my plane is much quieter and passengers even comment on how nice the ride is. I can and do have the option of using all the power available to fight a stiff headwind, beat some weather/darkness or get over the mountains before it gets rough. It's nice to have.

As for operating the O-320 below the green arc, SOP for instrument conditions in the C172 when I got my rating was 1900rpm which produced 80 knots in level flight and we used that for holds and most things on the approach plate. Just one of the (many) reasons was that you could go missed by just applying power and not even need to trim. We flew for hours that way, no problem.

Back to the OP's situation. You bought a stubby wing plane. I've read that they're a scream with the 150hp STC and they have been on my short list of planes before. There is most certainly a reduced power setting, like 2200 or 2300 which will give you a very efficient cruise without wallowing around in the sky. Go find out what RPM is required to hit the book speed for your 4.5 gal/hr figure and work from there.
 
Salty said:
1500 hours flying in cruise does not provide the same experience as 1500 hours where you try new things and go new places. 1500 hours flying the same hour over and over. Ew, for many reasons.
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...or as I have pointed out to some blowhards: There is a difference between 1500 hours of experience and 1 hour of experience repeated 1500 times. :)
 
MarkH said:
more than miles I need time
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MarkH said:
flying in circles
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^don't do that

If you're nervous to leave the safety and comfort of the 51nm radius around your home airport you need to work on that first before going to the same airport 50.1 nm away dozens of times. You will learn a lot more planning and executing a longer leg.. especially if you are trying to build time to do this as a career.

I also bet you might actually save overall money.. that 300 nm flight, back and forth, is the equivalent of 12 little 50 mile hops.. but in those 12 hops you are doing run ups, taxing, idling on the ground, etc... how much gas are you burning just sitting on the ground? If they assume 2 gallons for ground taxi time, then in 12 flights you've burned 24 gallons to go NO WHERE, or over $100 to run your engine on the ground.. think about **that**!
 
Depends on the engine and airframe installations. Some engines have limited ranges for harmonics.
Is that RPM in the green on the tach? What does the POH say for recommended cruise settings?

Is the RPM sufficient to provide good oil pressure? CHT and OilT staying healthy at that power, speed, and pitch configuration?

The most concerning thing is to check if your oil is running too cool. The bearings, cylinder pressures etc all like running at low power but unless you’re getting around 70c, the oil won’t evaporate the moisture from your engine.

Nonetheless, trying to run an O-320 like an O-200 is kind of obtuse, but each to their own.
 
MarkH said:
I want the option of going fast, but running 130Kts at 9gph does not make much sense when I am flying in circles to build time after work. I'm looking forward to being able to make a 400nm run for lunch, and come back on a Saturday.

Right now, more than miles I need time, and I am looking for the cheapest, safest, most fun way to get there.
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Don’t fly in circles, fly somewhere. Flying in circles will get old quickly and it’s not very productive.
 
MarkH said:
My biggest challenge I have found is that it's still pulling a bit at idle, and that is making it a little challenging to land short. But, I only have 7 hours in it and I know I will improve with practice.
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Don't be afraid to go real low on the idle speed. Mine's set at about 600 rpm, where the engine's just barely staying lit (composite FP prop doesn't have the flywheel effect of a metal equivalent).

There are lots of stories about maiden flights of RV-9s with a too-high idle making three or four too-high approaches before landing successfully!
 
smv said:
...or as I have pointed out to some blowhards: There is a difference between 1500 hours of experience and 1 hour of experience repeated 1500 times. :)
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Yeah, I was so guilty at the latter that I stopped logging those hours. (Now I'm digging through paperwork to add 'em back in! Just because; no desire to fly professionally.)
 
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