speyers

speyers

Well Known Member
How is percentage of power calculated? When I look in POH's I can usually find some nice power chart for a given altitude along with manifold pressure and rpm. The charts usually give a fuel flow and percent horsepower. This is all well and good but what happens when I don't want to run my engine at "best power" 50 degrees ROP where the highest CHT's and inner cylinder combustion pressures are? For example, I know if I stay with the SAME manifold pressure and rpm but go 50 degrees LOP instead of 50% ROP, my percentage of power is less. How can I find out then what my engine is really producing?
While I was out flying for my day job today, I had 30 inches with 2300 RPM and a fuel flow of 13.0 gph (TSIO-520 in a C-206). The POH says at 30/23 I should be burning 15 gph with 73% power, but my Fuel Flow is 2 gph less than that so I can't be at 73% power. So the question is, what is my true percentage of power?
On a second note, is percentage of HP directly related to fuel flow? IE, will 15 gph always be 73% power no matter what manifold pressure, RPM, and mixture setting? I know this is not true at the extreme ends because if you turn the mags off you have ZERO power while still 15 gph! Also, I would imagine with 36 inches of manifold pressure and 2700 RPM at 15gph it might not be rich enough to even run the engine, or would be so lean that you would not be anywhere close to the fabled 73% power.
Comments and more brain scratching is welcome.
-David
RV-8 in process...
 
% Power while LOP

David as I remember, % power is directly proportional to FF while LOP calculated by this formula (14.95 * FF)/Max rated HP. eg, (14.95 * 7.5gph)/180 = 62.2% You must be LOP for this formula to apply.

Gerry
 
You can go nuts trying to set % HP with MP and RPM. There are too many variables.

I use fuel flow and BSFC and a spread sheet. (base specific fuel consumption)

For example:

Barrett Precision Engines ran my engine on a dyno and the paper work states it has BSFC of .51. What that means is .51 pounds of fuel per HP per hour.

So if fuel weighs 6 pounds per gallon, the engine burns 91.8 pounds of fuel per hour (15.3 gph) producing 180 HP. (actually, the burn is slightly higher because the dyno indicated the engine produces 187 HP)

The spread sheet will calculate the % of power for any burn value. And from that info you can build a chart to carry in the airplane; i.e., 11.5 gph = 75%, etc, etc.

I assume this is reasonably valid when leaned to best power which is about 100F rich of peak. Running LOP the fuel flow is less and so is power so the numbers are still in the ball park.

I believe the technique is valid at any altitude/temperature. Air speed will vary due to drag affected by air density, the higher you go, the better is TAS. I see this as TAS is about 10 knots faster at 8500' than at 2000' with the same burn.

Theoretically, max power at 8000' is about 75%. A WOT run at 8000 the burn comes in at about 11.5 gph, so the technique is valid although I don't lean very aggressively, very long at that power setting.
 
In my Lycoming curve #13264-A for the O-235 there are two slopes for the FF vs HP that change at 75% power. The one below 75% has an SFC of 0.446 lb/hr, 13.23 HP-HR/gal at 5.9 lb/gal, and the one above, 0.470 SFC, 12.55 HP-HR/gal.
Curve # 13380 for the O-320-D has 0.425 SFC, 13.88 HP-HR/gal below 75% and 0.50 SFC, 11.8 HP-HR/gal above 75%.
There is a reference chart that shows SFC, power, CHT, and EGT vs Air-Fuel ratio printed on P.37 of the Lycoming Flyer Key Reprints and on Kas Thomas' Aircraft Engine Operating Guide. P. 137 which relates all of these parameters on one graph.
 
49clipper

This has been a question for me also. My engine analyser manufacturer tells me he determines % HP by FF alone. But, if I go to my trusty 0-320-D1A power chart, you need to know Manifold pressure to determine % HP. If I criuse along at 240rpm at a specific altitude, and start to lean, I immediately see the % hp go down, is it really. The analyser only knows that the FF dropped, what about the other factors. I do not in my mind, concieve how the % percent could be determined by FF alone. It makes a difference because I don't want to aggressively lean about 75% power.
Jim
 
speyers said:
How is percentage of power calculated?
Click to expand...
I use the chart on page 3-12 of http://www.aerosportpower.com/docs/io-320-opsmansec3-boperatinginstcharts.pdf which gives HP for Best Power and also Best Economy (Peak EGT) using FF and rpm. Had a short discussion on this a few days ago. See posts 13,14 and 15 http://www.vansairforce.com/community/showthread.php?t=63447&page=2

ghatch said:
David as I remember, % power is directly proportional to FF while LOP calculated by this formula (14.95 * FF)/Max rated HP. eg, (14.95 * 7.5gph)/180 = 62.2% You must be LOP for this formula to apply.

Gerry
Click to expand...
I don't really understand this formula as it takes no account of rpm. Pumping and frictional losses increase with rpm and thus increase SFC. So surely HP at say, 7 g/h, LOP and 2,000 rpm would be significantly more than HP at 7 g/h, LOP and 2,700 rpm.
At Best Economy (Peak EGT) Lycomings Chart for the I0-320 on Page 3-12 shows 87 hp at 7 g/h and 2,700 rpm and 102 hp at 7 g/h and 2,000 rpm. Why would a similar HP difference not also apply LOP?
The chart referenced above on page 3-12 can be used to calculate HP at Peak EGT but I can't see why it could not also be used to give a close approximation to HP when LOP. Looking at a different chart on Page 3-8 of the document referenced above, it seems that SFC is pretty much the same between peak and about 50 degress LOP. The chart on page 3-12 gives HP based on SFC at Best Economy (Peak) so why not use it to give HP for LOP given that the SCF when up to 50 degrees LOP is similar?

Fin 9A
 
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I agree with Jim, how can percent power simply be a function of fuel flow? I could see this being the case at optimum (or at least consistent) EGT, but I can easily run my engine rich and pour unburned fuel out the exhaust while getting no more power than I was getting at optimum EGT.

greg
 
ghatch said:
David as I remember, % power is directly proportional to FF while LOP calculated by this formula (14.95 * FF)/Max rated HP. eg, (14.95 * 7.5gph)/180 = 62.2% You must be LOP for this formula to apply.

Gerry
Click to expand...

Gerry, thanks for passing that info along. I did some more e-mailing and found out the following...

When LOP, and LOP ONLY, HP is dependent on FUEL FLOW only. There is excess air, which does not burn.

A very accurate method of computing HP is to simply multiply FF by a constant. That constant is:

14.9 for 8.5:1 compression ratio engines, and
13.7 for 7.5:1 compression ratio engines.

Manifold pressure and RPM are no factor, IF LOP.

An example would be the following, IO-360 capable of 200 HP max. Cruise FF of 11 gph lean of peak. 14.9 multiplied by 11 is 164. Which is 82% of 200... thus you would be running roughly 82% power.

I am assuming the difference in "constant" is a result of different BSFC's. (makes sense that lower compression is less efficient)
The point made about RPM and friction lost/gained due to high/low RPM is also valid. I guess the only really good way to measure percent HP would be with a torque gauge like on PT-6 engines.
-David
 
Greg Arehart said:
I agree with Jim, how can percent power simply be a function of fuel flow? I could see this being the case at optimum (or at least consistent) EGT, but I can easily run my engine rich and pour unburned fuel out the exhaust while getting no more power than I was getting at optimum EGT.

greg
Click to expand...

Greg,

Not sure who you are responding to but I did qualify the technique by saying it appears to work with best power mixture - 100F ROP - and appears to work at LOP also.

Beyond all that, none of this is an exact science and one can easily get into meaningless nit picking. Unless the engine in instrumented as it is on a dyno, we don't know exactly what is going on all the time. You come up with a method to determine engine performance based on your needs and information available, and move on.

For me, 8 gph yields 153 KTAS and it works (about 25F LOP). One can call it "52.29%" power if it feels good, but it really doesn't mean thing. What is important is being able to predict accurately the amount of fuel required for a given mission, when engine performance permits leaning, and preserving engine life. Yes, you can go LOP at more than 75% power but it is impossible to do it and not go through the red box.

So what is the magic number for 75% power or less for leaning purposes at any altitude? Pull throttle back to 22.4" of manifold pressure. RPM, fuel flow don't matter one hoot. You can not get more than 75% at 22.4" MP.

That assumes the original question about percent of power is motivated by the need to know when it is ok to lean. That is a very valid question. Fuel flow can not be used to determine when it is safe to lean, and that needs to be stated.
 
David,

No offense meant. I'm just trying to figure this out myself - I agree that this looks like a very inexact science. My original thought was that (for my FP setup) percent power should be directly related to RPM, no matter the fuel flow. But now I'm not very sure that is correct either.....

cheers,
greg
 
There was a long discussion on this here.

================
The formula I ended up with is as follows:

Lycoming O-320 B and D series Horsepower Formula (Rich Mixture)

h=(H-((R-r)*(5.58-0.125*(M-m))/100 + 7.35*(M-m)) + 2.0*pa/1000)*sqrt((519-3.58*pa/1000)/(460+ta))

h% = h/H*100

where

Constants:

H= maximum sea level horsepower at maximum manifold pressure (from manufacturer?s data)
R= maximum sea level RPM at maximum manifold pressure (from manufacturer?s data)
M= maximum sea level manifold pressure at rated RPM (from manufacturer?s data)

For Lycoming O-320 -B and -D series:

H= 160
R= 2600 (Sensenich prop limit)
M= 28.6

Variables (inputs)

r= actual RPM
m= actual manifold pressure
pa= actual pressure altitude (altimeter set to 29.92 inHg)
ta = actual air inlet temperature at pressure altitude

Outputs

h = calculated horsepower
h%= calculated percent horsepower

=============
Note that the above formula works for rich (excessive fuel) operation, fixed or constant speed props.

For Lean of Peak operation, use the BSFC from the Lycoming chart (reprinted on the Aero Sport link).

Vern
 
49clipper

Interesting.
So much for that little % of power digit on my engine monitor. From everything I studied (lyc power charts & LOP ops), I knew % power had to at least involve MP. My cylinder temps remain nice and cool (320-355) in cruise, so I think I am not doing any damage. I try and run near peak on the hottest cylinder but the other three are grouped at 80f less. I know, in the red zone, but nice and cool.
Jim
 
Bump bump

the weather finally cleared up again to allow some flying a few days ago. I don't think my engine is fully broke in yet, so was going to run it hard. I also wanted to do a fuel balance check by checking where each cylinder peaks. But to do that I need to make sure I don't spend time in the "red box". I couldn't climb high due to high winds beating me up. So I thought Id throttle back to 70 or 75% power.

ANYWAY: I am struggling with trying to figure out percent power. My Dynon has a readout but don't know if it's accurate. I had the power set at 23.7" 2350rpm, 5000' DA and varied the FF 11gph down to 10 gph where it got rough. I did the big pull and was able to run smooth in the 8.4gph range. I'm thinking I may have had a momentary detonation around 10gph, as this was about the time some cylinders were peaking.

IO-360(parallel valve) 9:1 comp. AFP injection, Emag/Pmag. So what power am I running? or how does a simpleton figure it out quickly while flying along? Anyone have some numbers to help me figure this out.
 
If you got the SFC down to about 0.43, then you would be getting about 13.7 HP/gph and if your 8.4 gph FF is accurate that would give 115 HP. At .45 SFC it would be about 110 HP. Why do you care? The Dynon is just an estimate.
 
For the simple folks here (if only me), can you take the percentage of MP/29.9 and that is the maximum power you would have?

Example, at 11,500 pressure altitude, that would be (29.9-11.5)/29.9 or 18.4/29.9 or 61.5%
 
hydroguy2 said:
the weather finally cleared up again to allow some flying a few days ago. I don't think my engine is fully broke in yet, so was going to run it hard. I also wanted to do a fuel balance check by checking where each cylinder peaks. But to do that I need to make sure I don't spend time in the "red box". I couldn't climb high due to high winds beating me up. So I thought Id throttle back to 70 or 75% power.

ANYWAY: I am struggling with trying to figure out percent power. My Dynon has a readout but don't know if it's accurate. I had the power set at 23.7" 2350rpm, 5000' DA and varied the FF 11gph down to 10 gph where it got rough. I did the big pull and was able to run smooth in the 8.4gph range. I'm thinking I may have had a momentary detonation around 10gph, as this was about the time some cylinders were peaking.

IO-360(parallel valve) 9:1 comp. AFP injection, Emag/Pmag. So what power am I running? or how does a simpleton figure it out quickly while flying along? Anyone have some numbers to help me figure this out.
Click to expand...

You're not going to get detonation burning 100LL on a 9 to 1 engine at 23.7 inches unless you had over 40 degrees of timing advance. Even then, very unlikely.

As Paul said, the glass is displaying a close estimate of % power. There was a similar discussion a few months back. The glass does not sense enough variables to give a super accurate reading under all conditions.
 
Not a good formula

MauiLvrs said:
(RPM/max RPM) * (MP/Atm P) * Rated HP = HP
*
(HP / Rated HP) * 100 = % power
Click to expand...

This makes a lot of sense to me. However, when you check the slope of the lines on the engine manufacturer's power graphs you see that they are pretty straight; this formula will not give you a straight line. The slopes of the lines (for RPM or for MP) will tend to give you constant increments of power for uniform increments of, for example, MP. Try it.
 
hydroguy2 said:
I had the power set at 23.7" 2350rpm, 5000' DA and varied the FF 11gph down to 10 gph where it got rough. I did the big pull and was able to run smooth in the 8.4gph range. I'm thinking I may have had a momentary detonation around 10gph, as this was about the time some cylinders were peaking.
Click to expand...
Caveat. I am no expert and I am happy to be corrected :)

My reading of Lycoming's Part Throttle Fuel Consumption graph for a standard I0 360 suggests that the most fuel the engine is capable of consuming at peak EGT and 2,350 rpm would around 9 g/h which would produce about 130 hp or 72%. If this is correct, then at 10 g/h you were either on the rich side of peak or perhaps your FF gauge is not accurate? However the situation is more complicated because your engine with higher compression, EI and the more efficient Vans exhaust is not standard so it may be possible that it is able to consume more fuel and produce more power than a standard engine at 2350 rpm and peak.

For a standard I0-360, 65% power occurs at 2350 rpm and peak EGT with a FF around 8.5g/h. So assuming you want to keep below 65% power at peak to stay out of the red box make sure that at peak and 2350 rpm your FF is not above about 8.5 g/h.
Fin
9A
 
Thanks everybody, I've got more testing to do. I guess the actual number doesn't metter so much, I'm just trying to make sure I'm not frying my motor. My fuel flow seems very close and but my peak EGT occur around 10gph.

Need some decent weather to go longer and harder.:D

Also I'm new at these speeds and RV's. (too much citabria and 172 time) 170-175 is solid, but above 180-185kts while stable seems a little sensitive to bumps and stick input. anyone(new rv'rs) else feel this way?
 
The information you request "how is percent of power calculated" is contained in the Sea Level and Altitude Performance Charts contained in the Engine Operators Manual provided by every type certificated engine. It is a standard format that contains a wealth of information. The information can be accessed easily while in flight without any mathematical manipulation.

As a side-note of possible interest:

You can go nuts trying to set % HP with MP and RPM. There are too many variables.

Given a constant MP and RPM; horsepower (and fuel burn) increase with altitude due to reduction of back-pressure in the exhaust. This is shown in the 'Altitude Performance Charts" as an upward sloping line (dotted line inside green circle). Of course there is also a temperature correction - also included in chart.

altitudePerformanceChart.jpg
 
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