Understanding the Lycoming Performance Chart

[mlwynn]

Hi all,

I have been trying to ascertain a safe cruising power configuration and also out how to get my GRT Hx to accurately read %HP. I have a Lycoming IO360M1B, Hartzell constant speed prop. I am using the Lycoming chart labeled Figure 3-26 in their manual. I would post if I could figure out how to do so.

The chart on the left says sea level performance. At 2700 RPM and 29" MP, you get 180 HP. If I go to 2300 RPM and 23" MP, the read off is 109 HP. I would assume that means I am developing 109/180=61% power at sea level.

Going to the other chart, altitude performance, 2300 RPM and 23" at 6500 feet reads 126 hp which should be 126/180=70% power.

I entered the numbers from the GRT chart for 0-360 engines and tried again with the numbers derived from the Lycoming chart for my engine model. Either way, I get an indication of 90+% power from the EFIS at 2300/23".

I am trying to figure out if I am reading these charts correctly. If so, cruising at 23 squared is pretty safe for my engine. It seems quite happy there and is giving me about 145 kt IAS, close to 160 TAS. If 2300/23" is really 93% power, that seems like a pretty high power setting for sustained cruise.

Can anyone tell me if I am in the right ballpark or is there some part of reading these charts that eludes me.

[bruceg]

Don't know about the GRT system but I can say you are getting the correct numbers from the Lycoming chart. And those numbers are based on using "Best Power" mixture. If you decide to run lean of peak, that power chart will not be applicable.

[BobTurner]

The GRT algorythm is strange imho. For the 'altitude' data they do not want HP nor percent power, but rather the HP difference from the sea level chart. e.g., for the data posted, you need to enter 126-109 = 17
You need a bunch of data points at 6500', and more data at higher altitudes. Just follow the examples in the manual.
As stated, the GRT makes no changes to the numbers for different mixtures. Not much change best power to peak egt; significant fall-off if you run LOP. You can estimate the LOP correction - pretty much proportional to fuel flow.

[Isaac]

Kevin Horton wrote an article in the March 2016 issue of KitPlanes "Determining Engine Power". At the end of the article, there some download links to several MS Excel Spreadsheets for calculation of % power that Kevin created.

Lycoming O-360-A-C, Power Chart Spreadsheet

www.kilohotel.com/rv8/rvlinks/o360apwr.xls

Lycoming IO-360-A, -c, -D, -J, -K & AIO-360 Power Chart Spreadsheet

www.kilohotel.com/rv8/rvlinks/io360apwr.xls

The engine in my RV6 is an O-360-A1A with AFP fuel injection. I put the standard values that GRT provides for the O-360 engine into my GRT Sport HS EFIS, went flying and recorded data at multiple altitudes and power settings. I found that the GRT EFIS displayed % power matched very tightly with the offline spreadsheet values that Kevin's spread sheet calculated for the same conditions. For me having the spreadsheet calcs and the GRT EFIS displayed values agree was confirmation that the EFIS was displaying accurate % power data.

Like Bob and Bruce commented, the Lycoming charts (and the GRT calculated vales) are based on using mixture settings for best power, so if you aren't setting the mixture for best power (i.e. running LOP) the EFIS displayed values won't be perfect (displayed with be higher that actual), but plenty good enough for letting you know where the engine is operating.

[BillL]

These engines (NA) are mass flow machines. This means that the primary effects are RPM, and manifold pressure at WOT. Given that volumetric efficiency will improve with slower speeds, the lines not the chart are not straight, but if you just proportion rpm & MAP you will be close on % power. This way you can check your read out.

In addition to air density and volumetric effects, humidity, oil temperatures and certainly A/F will affect real power. I would rank them as A/F, oil temp, and humidity in order of significance.

We are neglecting discussion of intake and exhaust restrictions.

If you unprotect Kevin's sheet, you will see some +20F and -20F effects. I think they are not correct according to inlet air density effects, so be careful.

IMO - a % power readout is a novelty.

[rzbill]

I found that the O-360 numbers available on the GRT site did not match the IO-360-M1B at all.
They gave artificially high power % readout compared to the actual (assuming the Lyc charts are accurate, which I do). They may be fine for an O-360. I have not bothered to check them against an O-360 Lyc table.

Created my own -M1B table.

[Kevin Horton]

Originally Posted by BillL

If you unprotect Kevin's sheet, you will see some +20F and -20F effects. I think they are not correct according to inlet air density effects, so be careful.

Bill - what is your concern with the temperature corrections? As near as I can tell, the corrections on the spreadsheet match the corrections specified on the Lycoming power charts.

I do agree that it is counterintuitive that the effect of temperature on power is an inverse square root effect. I would have expected power to vary with the density, i.e. be inversely proportional to temperature. But, this surprising characteristic was observed in dyno testing by NACA and many other researchers in early aviation, and was published in their reports. E.g. NACA Report 190 - Correcting Horsepower Measurements to a Standard Temperature, 1925

[BobTurner]

Originally Posted by Kevin Horton

I do agree that it is counterintuitive that the effect of temperature on power is an inverse square root effect. I would have expected power to vary with the density, i.e. be inversely proportional to temperature. But, this surprising characteristic was observed in dyno testing by NACA and many other researchers in early aviation, and was published in their reports. E.g. NACA Report 190 - Correcting Horsepower Measurements to a Standard Temperature, 1925

Kevin, the inverse square root proportionality is a bit subtle, but not surprising. In a very slow turning engine the power varies with density, as you expect. But as the RPMs go up, another factor is just how much fuel-air can flow thru the intake port in the short time it's open. The air tends to flow at the speed of sound, which is proportional to the square root of temperature. The two factors together give you an inverse square root dependence, which is the model the charts usually use. For 5000 RPM car engines this model works well. For 2300 RPM aircraft engines in the real world, they are sort of in-between the slow and fast turning models. A number of overhaulers with dynos have noted that their 'summer engines' put out less model-corrected power than 'winter engines'. It's just that the fast-turning model is not 100% correct at our modest RPMs.

[BobTurner]

Michael, I will be at the airport next week to do my annual condition inspection. I'll try to remember to copy all my power settings and send them to you. I'll bet that if you take 2/3 of my numbers (a 540 is really just a 360 with 3/2 more cylinders!) your GRT will display correctly.
Bob

[Kevin Horton]

Originally Posted by BobTurner

Kevin, the inverse square root proportionality is a bit subtle, but not surprising. In a very slow turning engine the power varies with density, as you expect. But as the RPMs go up, another factor is just how much fuel-air can flow thru the intake port in the short time it's open. The air tends to flow at the speed of sound, which is proportional to the square root of temperature. The two factors together give you an inverse square root dependence, which is the model the charts usually use. For 5000 RPM car engines this model works well. For 2300 RPM aircraft engines in the real world, they are sort of in-between the slow and fast turning models. A number of overhaulers with dynos have noted that their 'summer engines' put out less model-corrected power than 'winter engines'. It's just that the fast-turning model is not 100% correct at our modest RPMs.

The investigations in the 1920s found that the inverse square root variation of power with temperature held true even at rpm less than 2000.

I wonder what relationship of power vs temperature would be required to make the 'winter engines' and 'summer engines' be equal?