What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Understanding the Lycoming Performance Chart

mlwynn

Well Known Member
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.
 
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.
 
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.
 
RE: Lyc performance - % Power - GRT

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

I think the answer to your question is that now you're getting into a level of detail that is beyond a simple model. e.g., the 10% HP gain of angle valved 360's over straight valved 360's is, partly, due to improved air flow thru the intake.
 
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

I reviewed the sheet again and read the reference. The first time I looked the 20 F changes just seemed small relative to density and square root is small. I still don't understand why it would be so little, in spite of the orifice reference. Thinking about IMEP and FMEP, there are certainly some adverse effects of elevated temps for efficiency. On the positive, Lycs are a bit large bore to stroke ratio yielding more intake valve area. If memory serves, (less these days) testing/analysis of intakes indicate that sonic velocity is only achieved as the valve begins to open, so the flow is not choked. So, I don't have a better alternative. :eek: Maybe this is good fodder for a social discussion.

Thanks for the reference.
 
Progress in Understanding

Thank you all for this discussion. I usually read Kitplanes cover to cover. Don't know how I missed Kevin's excellent article, but thanks to Kevin for the article and thanks you guys for pointing it out. This is starting to make a lot more sense.

Just to clarify--this being a really dumb question--but I am assuming that %power is the percent of max power at sea level. In the case of my IO360M1B, that is 180 HP. Operating at 6500 feet at 2300 RPM and 23 inches of MAP is about 126 HP. So I would assume that means I am operating at 126/180 or 70%. As opposed to 100% of the power available.

Hence, my engine should be pretty happy at that power setting for a long cruise.

Kevin makes several good points in his article about how EFIS software calculates percent power. The article on GRT's website about power calculations and specific fuel consumption made it seem like an easy measurement to set up. It looks like I may be expecting more accuracy from the instrument than is realistic. In any event, I am going to try to get some more accurate numbers for my engine and see if I can get a %HP reading on my EFIC that is closer to what the performance charts suggest it should be.
 
Aren't there enough Lyc's flying that a sufficient data base exists so that, with an accurate flow meter, you can work HP backwards from BSFC ???
 
Aren't there enough Lyc's flying that a sufficient data base exists so that, with an accurate flow meter, you can work HP backwards from BSFC ???

Depends on what kind of accuracy you want. Rich of peak, the engine is air-limited, and you will see only small power changes with substantial fuel flow changes. Lean of peak this method should work, as long as all cylinders are well matched.
 
I discussed this with Kevin back in Feb. and put forth some additional thoughts. Exhaust back pressure changes with altitude has a significant effect on hp and as far as I know, only dynos with altitude cells can measure this accurately.

Ignition timing has quite an effect too, especially at leaner AFRs as we've seen from some flight testing with EIs. Remember, the Lycoming charts assume fixed timing.

I agree with Bill's comment that glass %hp display accuracy should be viewed as approximate given that these parameters may not accounted for.

The Lycoming charts also don't take into account variable ignition timing, exhaust backpressure/ aftermarket exhausts, port mods, or CR increases which many people are doing these days.

In my dyno days, hp was pretty proportional to charge density but most of that was on FI engines or with proper carbs attached to proper IR manifolds. I believe a lot of carbed aircraft induction systems might not perform very well at colder temps due to poor mixture distribution. This might explain the early test results.
 
If you are concerned about detonation, Don Rivera at Airflow Performance informed me that detonation can’t occur at 24 inches of manifold pressure and below. I asked him where that number came from and he replied Lycoming detonation charts.
 
I spent some time on this putting it in my GRT, and now I basically ignore it. I thought this would be essential information to have while flying, but turns out it's not.

The main engine parameters I look at when flying are RPM and fuel flow. MP and % power are interesting, but not important for the type of flying I do (WOT all the time). :D
 
Back
Top