Lycoming O-320 Engine Performance

mcoflyer

mcoflyer

Pre-takeoff checklist
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mcoflyer
I've been flying a C172 and was looking over the POH and trying to gather some further understanding of how to get the most endurance out of the airplane. Endurance meaning...lowest fuel flow.

Looking through the POH I found the engine performance table. It shows the engine to be most efficient at 6,000 ft (pressure altitude) while at 2,100 rpm. It shows a burn rate of 5.2gph.

These numbers are however based on a weight of 2,300 pounds. When I fly alone and with full tanks the aircraft weight at departure is about 1,950 pounds. Over 15% lighter than the 2,300 pounds listed in the POH. Obviously as time goes by the airplane also becomes lighter. By the time I have 10 gallons left i'm down to under 1,800 pounds.

Does anyone know where I can find the performance numbers that also include the weight of the aircraft? How much more efficient should I expect the engine to be when it's 20% lighter?
 
I think you'll pick up a couple knots at the lower weight, but fuel flow will still be the same. POH's are great, but real world flying is going to give you your answer, not every airplane flies the same, some run better leaner or richer than others. At 2100 RPM you aren't going anywhere fast! ;)
 
My point isn't to get anywhere fast. It's to go somewhere, build some time in the meantime, and save a few bucks. I don't care if i'm going 90, 100, or 110.
 
Fuel flow is usually dependent on the Rpm setting as a generalization. Don,t think you will get much better fuel savings. Would suggest you run your own numbers.
 
You need something like a JPI which can give you real time temperatures and fuel flows.
 
Definition of endurance is max time aloft on your fuel. You can figure it out anytime you want by reducing the power 100 RPM at a time, and resetting the trim after the airspeed has stabilized in level flight. The nose will be higher each time you reduce the power. Once you get to a point where the airplane won't maintain altitude after the power has been reduced, you've gone too far. Go back to the last power setting that maintained altitude, trim, and lean as much as you can.

In a 172 with a couple of guys aboard it will be around 1900 RPM.

Dan
 
I think a good point was brought up. Even though the POH might suggest one thing, in reality it could be somewhat different.

I went up the other day for 1.3 (only 1.0 on the tach) and did some stalls, steep turns, and a couple of touch & go's and I burned just under 6 gallons. I was kind of shocked to learn that.
 
Doc Bruce put up a study on how to calculate best endurance. It was an interesting article.

All I remember is that best performance approximate the best climb speed of most aircraft within 1-2 knots.

So if your best climb in a 172 is about 75 knots or so then its probably 77 knots for best endurance.
 
I can't answer your question exactly but can give you my related experience. I fly two airplanes and try to get the lowest fuel burn I can, unless I'm trying to get somewhere which is not often.

I fly a Cessna 140 with a C-90. When it is full of fuel I find it needs about 1800-1900 rpm to maintain a comfortable angle of flight at about 70mph. It will maintain altitude at 55mph at 1600 rpm but it's really mushing and not a good view out the front. After fuel burn off when it's real light 1600-1700 rpm is okay. This is in smooth air. If it's bumpy about 2000rpm is ideal. Doing this I routinely get about 3gph.

I also fly two IO-320s the same way. On a PA-30. I have a theory since there's two of them you can operate them at a lower power setting than just having one to keep the airplane up. When full of fuel, 2000-2100rpm and 17" is good and indicates 120mph. When it's light, 2000rpm and 15" yields 110mph comfortably. I've consistently gotten less than 9gph doing this, mostly around 8.6 - 8.7gph.

This is with a lot of takeoffs and landings. I'm sure if you go on a nonstop flight from full to empty you can better these numbers but I can't sit that long. I haven't been able to find much data about flying best endurance, most of the stuff out there is for best efficiency (mpg). I'm not sure that flying high gets you anything, the airplane is more efficient for cruise but if you're just trying to operate at low power you can keep the power setting low and the wing benefits from the denser air to be able to fly slower.

It would be nice to have an analyzer/fuel flow but this is just with standard instruments.

Good luck.
 
mcoflyer said:
I've been flying a C172 and was looking over the POH and trying to gather some further understanding of how to get the most endurance out of the airplane. Endurance meaning...lowest fuel flow.
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Well, for maximum endurance and minimum fuel flow, pull the red knob all the way back. ;)

However, assuming you want to keep flying the longest amount of time for the least possible fuel, fly at Vg. (L/d)max is what you're looking for, and you should find it at Vg.

Remember to adjust for weight - sqrt(actual weight/max gross) * published Vg. So, for example, you're in a 2550 MGW airplane with a Vg of 75 knots but your actual weight is 2300 pounds, it'd be sqrt(2300/2550)*75 = 71.23 knots.
 
Ok...that's very interesting.

So if Vg on my airplane is 65 knots at 2,300...then with just me at 1,950 Vg should be 59.85. Wow, that's quite a bit slower than I thought. I was thinking that if I went about 80-85 that would be about as good as it goes.

Next time I go up i'm going to try a few things.
 
mcoflyer said:
Ok...that's very interesting.

So if Vg on my airplane is 65 knots at 2,300...then with just me at 1,950 Vg should be 59.85. Wow, that's quite a bit slower than I thought. I was thinking that if I went about 80-85 that would be about as good as it goes.

Next time I go up i'm going to try a few things.
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That's getting into slow flight territory. It's not best endurance.

I think. It's been awhile since I did it in a 172.

Dan
 
I might be confused but it seems when I figured it for the Cherokee it may well have been Vg for longest flight aloft and Climb speed was about what longest distance endurance was. So it would be 80 mph for best time aloft and 89 mph for farthest distance.
 
mcoflyer said:
My point isn't to get anywhere fast. It's to go somewhere, build some time in the meantime, and save a few bucks. I don't care if i'm going 90, 100, or 110.
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is there a 150 or 152 you can get your hands on? They are much cheaper per hour to fly than most planes.
 
I'm 230 pounds and 6'1". The cabin of the 152 is just too small. Not to mention...it's hard being able to even get a checkout at a flight school or flying club in it since me and an instructor are pretty close to being at gross weight without any fuel. LOL!
 
That is why I limited my plane search to 172/Cherokee with 930 useful load. Burns 2 more gallons per hour but then I could carry 3 adults and enough fuel and still go some place. People say you cannot take more than 2 adults but that is an exaggeration. I took two new college graduates 1600 miles to Miami, me in the front seat and them in the back seats.



mcoflyer said:
I'm 230 pounds and 6'1". The cabin of the 152 is just too small. Not to mention...it's hard being able to even get a checkout at a flight school or flying club in it since me and an instructor are pretty close to being at gross weight without any fuel. LOL!
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I like the C172. Good airplane and can usually get 3 adult males onboard with fuel. I can only get 4 people if there's 2 women usually. Plus, we don't have any C152s in our club.
 
mcoflyer said:
Ok...that's very interesting.

So if Vg on my airplane is 65 knots at 2,300...then with just me at 1,950 Vg should be 59.85. Wow, that's quite a bit slower than I thought. I was thinking that if I went about 80-85 that would be about as good as it goes.

Next time I go up i'm going to try a few things.
Click to expand...

Take a look at this graph, it's not for the 172, but you get the idea.

http://www.mountainflying.com/Pages/articles/images/drag_curve.jpg

The drag curve is not symmetrical and cheating on the plus side of the drag curve doesn't cost much in induced drag, while it reduces parasitic drag slightly. Take a 10% increase from point C and look at the added drag to go 88 as opposed to 80(based on this graph) you can see that a 10% improvement in speed costs maybe 2% in added total drag, for a total increase in power required of about 12%.

Using your calculated L/d max of 60knots, if you went 66knots, it would use about 12% more fuel for a 10% increase in speed. That's why most of us don't fly around at max L/d and we generally give up some economy for speed.

Also, it gets more complicated when you start figuring in the thermodynamic efficiency of your engine. As the engine is reduced from it's max power setting of 2700RPM(or thereabouts, I'm not going to look up the curve), it becomes less efficient due to various things like pumping loss, timing and cam profile. Part of what you gained by slowing down is lost when the engine is not working in it's 'sweet' spot. However, this curve as well is optimized as a modified parabola and setting a power level of 2000 or 2100 is not going to cost much in thermodynamic losses.

So, without doing a lot of calcs to get the last drop of fuel I would just add 8% to the reduced weight max L/d and fly that with the mixture control back as far as it will go without causing engine stumble.
 
docmirror said:
Also, it gets more complicated when you start figuring in the thermodynamic efficiency of your engine. As the engine is reduced from it's max power setting of 2700RPM(or thereabouts, I'm not going to look up the curve), it becomes less efficient due to various things like pumping loss, timing and cam profile. Part of what you gained by slowing down is lost when the engine is not working in it's 'sweet' spot. However, this curve as well is optimized as a modified parabola and setting a power level of 2000 or 2100 is not going to cost much in thermodynamic losses.
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Prop efficiency also goes up with reduced RPM.
 
kgruber said:
Prop efficiency also goes up with reduced RPM.
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Yes, and that is a fairly sharp curve as I recall. Min BSFC should be around 2150, and I'm betting the prop curve peaks very near that as well.
 
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