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Calculating Horsepower, % power

Cafe

Read all the the reports. Once some time ago and again with this thread. Definitely going to get cockpit controls - to disable the manifold pressure control of timing in peak to rich modes. Lycoming got it pretty close with fixed timing and magnetos! George - thanks for the reminders to go have another look. Should have some comparative test results in a week or three.
 
**Lycoming got it pretty close with fixed timing and magnetos!**


1) That is a VERY important thing to remember.

2) See #1. <g>


Note:
The problem with the fixed timing issue is that it is too far advanced at takeoff power already, so an *optimized* map, set for HP optimization, will RETARD the timing at very high power settings.

Walter
 
formula for horsepower

OK folks, here is a power formula I derived for a Lycoming B or D series O-320 engine.

I did some curve fitting and verification. Accuracy seems to be within 1HP over altitude, MAP, RPM and air temperature. This is based on Lycoming's data only, not in-flight analysis.

I provide this with the standard caveats... try it yourself, but don't trust me to get it right! In other words, if you find errors or improvements, let us know.

This is for RICH mixtures only (excess fuel). Walter has provided us the formula for lean mixtures based on fuel flow.

From this, you can build your own spreadsheet or program it into a programmable calculator for in-flight use. It compensates for both pressure altitude and outside air temperature at altitude.

I've got some spreadsheets, but I'll be out of town for a few days and don't have time to clean them up.

Have fun.

Vern Little
==============================

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
 
This formula suggests that the O-320 B and D series will produce 160 hp at 2600 rpm. Is this correct? I would have expected 160 hp at 2700 rpm, and a bit less at 2600 rpm.

I don't have the power chart for these engines at hand, so I can't check myself.
 
Kevin Horton said:
This formula suggests that the O-320 B and D series will produce 160 hp at 2600 rpm. Is this correct? I would have expected 160 hp at 2700 rpm, and a bit less at 2600 rpm.

I don't have the power chart for these engines at hand, so I can't check myself.


Hi Kevin, that's exactly what the charts imply. The Lycoming charts are quite entertaining (and sometimes contradictory).


Vern
 
reviving an old thread

I am reviving this very old thread (started in 2006) due to the interesting discussion concerning %HP calculations. I would be interested to know if there were any further ideas coming from Kevin or Vern concerning the last posts by each of them. I am not sure if Walter is still around, I know George no longer frequents this forum but others may still be here. Would there be any consensus that the formulas presented in this thread are usable formulas for attempting to calculate %HP, etc.? I have been playing around with Alan Adamson's spreadsheet he posted in this thread. Alan, if you are still around this forum, have you done any tweaking on this or other spreadsheets?

I would be very interested in hearing thoughts of many of those who posted after 7+ years since this thread was last discussed.
 
Lycoming paper

I have an old Lycoming paper I got several years ago, can't recall where from. It permits determining HP from fuel flow. If I recall correctly, for fixed pitch prop fuel flow at peak exhaust temperature is 85% of max HP at the same RPM and manifold pressure. You then determine fuel flow at max HP and reference a compression ratio table to determine pounds of fuel per horsepower per hour. From these numbers, HP can be calculated. For example, 8 GPH X 6 pounds per gallon divided by 0.4 pounds per HP per hour equals 120 HP. 120 divided by 180 equals 66.66% HP. There is also a method in the paper for constant speed props. I would be happy to fax or mail a copy. The copy quality is poor.
 
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Performance of Light Aircraft

Hello chaps,

In the book by John T Lowry: Performance of Light Aircraft, Chapter 5 deals with an analysis of power available as a function of Altitude, MAP & RPM. It only requires that you know the sea level standard day rated power or torque and the associated engine speed and are prepared to make a couple of reasonable assumptions. It is a pretty good read and understandable if you are comfortable with standard Lycoming power charts and some light mathematics. John's method for determining power available forms part of the basis for what he calls his 'bootstrap approach' to determining light aircraft performance.

Regards

Nigel
 
Mike Busch webinar ..

Glad to se this thread get revived. I've been noodling over this very issue for a few weeks now. The RPM and MAP data input to the GRT setup table of course doesn't consider mixture, thus, when I look at the % power dial in flight, I never know if it's accurate, close, or way off.

Not long ago Mike Busch repeated the "fuel flo x constant = HP", which is just a tad better than rule of thumb. I recall the constant for an O-320 was 14.9. I also recall it was with mixture set for best power. That constant comes from the PPH burn data from Lyco, similar to what Brad just said.
When I compare that to the fuel burn data I get from my IO-320, it doesn't jive very well.
 
Gent's I am please to advise Walter is still around. So are John and George.

To clear up the % power issue, the following formula are for ROP and LOP operations. Remember that on the lean side of peak the fuel flow determines power and that will vary with compression ratio.


When ROP, mass airflow determines HP, therefore, %Hp.
( %Hp = (100-(((Max RPM-RPM)/100)*2.5+(Max MAP-MAP)*3.5))/100

When LOP, FF determines Hp.
For std CR engines: 14.9 X FF = Hp produced.

Fuel Flow LOP
Multiplier = 1.1604 * Comp. Ratio + 5.030

7.5:1 pistons 13.7HP/USG
8.1: pistons 14.43HP/USG
8.5:1 pistons 14.9HP/USG
9.0:1 pistons 15.47HP/USG
10:1 pistons 16.64HP/USG
 
nice! equation just what I really needed.

thanks Dave,
I tried plugging my numbers into your equation; after about 10 tries of getting 2% horsepower, I pasted it into an online calculator....
..presto! I gained 63% power!! :)

...and this, after only 2 years of college, math, calculus, beer, physics, beer, statics, beer....hmmmm, seeing a trend here eh?
 
Dave,

Can you point me toward a derivation of the ROP formula? I'm curious how it came about, and the approximations involved (e.g., we know that power falls a bit (maybe 4%) leaning from best power mixture to peak EGT, yet the ROP formula doesn't reproduce that.

Bob
 
I have an old Lycoming paper I got several years ago, can't recall where from. It permits determining HP from fuel flow. If I recall correctly, for fixed pitch prop fuel flow at peak exhaust temperature is 85% of max HP at the same RPM and manifold pressure. You then determine fuel flow at max HP and reference a compression ratio table to determine pounds of fuel per horsepower per hour. From these numbers, HP can be calculated. For example, 8 GPH X 6 pounds per gallon divided by 0.4 pounds per HP per hour equals 120 HP. 120 divided by 180 equals 66.66% HP. There is also a method in the paper for constant speed props. I would be happy to fax or mail a copy. The copy quality is poor.

I have what must be the same paper. I posted that Lycoming power from fuel flow document as a PDF on my site.

I also created an OpenOffice (or NeoOffice if you use OS X) spreadsheet that does the calculation described in the document. No, I don’t have an Excel version, as I would have to convert the calculations from OpenOffice Basic to whatever Excel uses today. I don’t own a copy of Excel, so I’ve never been motivated to do that.

If you prefer working with python, the lycoming_power module in my AeroCalc module also does this calculation.
 
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This formula suggests that the O-320 B and D series will produce 160 hp at 2600 rpm. Is this correct? I would have expected 160 hp at 2700 rpm, and a bit less at 2600 rpm.

I don't have the power chart for these engines at hand, so I can't check myself.

I looked at a recent edition of the Operator's Manual and in Fig. 3-5 for the O-320-B and D, it shows 160 hp at about 2,710 rpm. The specs page says 160 hp at 2,700 rpm.

Dave
 
Thank you Kevin

Yes i think i may have pulled the document from your website. I found it very interesting and used it as my primary method of determining where my various percent power settings are. It seems to correlate well with expected results.
 
This is all interesting, but please tell me what the "real world' usefulness of percent power is.
 
my use

I use the percent power to repeat past performance. I know what percent power we usually run at, so I go to that setting. I know it is not exactly accurate, but I do not really care, just that it is the same power level I used last time I ran at that percent power.
 
This is all interesting, but please tell me what the "real world' usefulness of percent power is.

Not a lot really. WOTLOPSOP is the simple example, although if you know roughly what percentage of power you are generating you can select the appropriate amount in dF ROP or LOP to set.

The simple formula I quoted previously is derived based on mass airflow for ROP operations, and as has been spotted by someone at the very rich end of the scale, there is a 1% drop off on say a IO550 with 29.5GPH set up, but really....who cares, can tell or even needs to know anything more detailed than the formula presented. A couple of % between friends is nothing.

For rule of thumb mental maths calculations use a simple %RPM x % of MP, in other words 27" is 90% MP and 2450 RPM is near enough 90% RPM. 0.9 x 0.9=0.81 or near enough 80% power. So set either 200dF ROP or more than 60dF LOP and you are good to go.

That is the only useful use.

________________________________________
Red Box = No Fly Zone
? At and below about 60% power, there is no red box. Put the mixture wherever you want it.
? At about 65% power or so, 100?F ROP to Peak.
? At about 70%, 125?F ROP to 25?F LOP.
? At about 75%, 180?F ROP to 40?F LOP.
? At about 80%, 200?F ROP to 60?F LOP.
________________________________________



________________________________________
Outside the Box
? At 65% power, use richer than 100 ROP, or leaner than peak EGT.
? At 70%, use richer than 125?F ROP, or leaner than 25?F LOP.
? At 75%, use richer than 180?F ROP, or leaner than 40?F LOP.
? At 80%, use richer than 200?F ROP, or leaner than 60?F LOP.
________________________________________
 
I'd say the most important use is if you follow Lycoming's advice to not lean agressively above 75%.

Bob,

I would disagree completely. Above 75% is exactly where leaning aggressively is exactly what you should be doing.

Unless you need the few extra HP why clog up your engine, waste fuel, increase the particulates in your oil(rings) etc when you could be running 40dF LOP or 60-80dF LOP at above 80% power.

These engines LOVE running at very high powers LOP. They are no different to the old R3350's on the piston airliners in this regard. If you could physically do it in your RV you would take off at 100% 90dF LOP. Now there are some folk who could, if they knew how to. There are some Reno air racers who achieve a legitimate and fair by the rules unfair advantage this way. And no do not ask anything more....I will not answer. ;)

So have a look at this cylinder....how many hours? 10, 20...50 maybe? 500 I think.
IMG_6270_zps8a2d9ae7.jpg


And here is a photo of high power LOP...look at the numbers, what is not to like about that :)
80LOP_zps6e22f7f3.jpg


Remember why the Lycoming data was published like it was, they could not give you the other side of the graph in many cases, and still can't, so they gave basic instructions on how to live safely on the other side of the graph only. It was not that you could not do it.

Their new ie2 engine runs that way....so they know they can do it.
 
I agree. Notice I said, "if you choose to follow Lycoming's advice". (There are still some non believers in LOP!).
I also think that at higher power settings there is a higher penalty if you don't know what you're doing.
 
Brad, as best we can tell, in all the literature and on the dyno test stand, there is no way you can get detonation when LOP, or not in any N/A engine, and even in a turbocharged engine if run LOP properly.

When I am there next, which will be quite often in the near future doing reall serious detonation testing, I will have a go. At 30" 2500 and 10dF LOP I think on conforming fuel there will be none. And we will have to abuse it a little.

The most detonation prone mixture is one around 50dF ROP. Does that number sound familiar.

Have your CHTs at 370 in a NA engine and LOP......not a chance.

Having said that, it may still not be optimal for the engine. If you are LOP, and not far enough LOP say high power 85% and only 10dF LOP and your cooling system is good, or you are in a cold climate, you could consider this abusive. Not going to crater your engine in any time soon, but the concept of stress and temperature in combination being the enemy of longevity plays its hand. So just because you have forced the CHT down by being in Alaska in winter, and abusing the engine, I would say that is sub optimal long term would you not?

If you did the same thing at 40-50 ROP and asked me which would I prefer of the two, it would be a no brainer as the ICP's and CHT's would all be lower. If you were conducting ops requiring high power and ROP, then 200+ or full rich is the answer. If not throttle hacking, then 80dF LOP.

The APS Landmarks graph paints a good picture.
Landmarksgraph_zpsbfb07cbb.gif



Bob, I will take that as you meant to say "Choose" and I do agree with you :), the better way to say it would be
I'd say the most important use is if you follow Lycoming's advice to not lean insufficiently enough above 75%.
 
Solid v Dashed Brown lines

What is the difference between the solid and dashed Brown lines for 1/BSFC?.

Cheers

Nigel
 
BSFC is different at high power settings and low power settings.

Serendipity strikes too......the Higher power BSFC also tracks the red box edges and say 60-90LOP and low power at 10-20LOP.

This and much much more is revealed at an APS class. Lots of a-hah moments. :)

You should try one!
 
I thought it would be in this thread, but could not find it. Somewhere I ran across a spreadsheet created by some clever and diligent individual, which would take as input your engine type and a set of operating parameters and generate the classical Lycoming type graph with the A,B,C,etc. points and lines, culminating with HP. I can't find it! Anyone know where this is?

Thanks!
 
I have an app on my ipad called AircraftPower that does it all.

That's it, that's it!! Thanks! I thought I had it but kept looking on my PC and could not find it - Forgot that it was on the iPad! This is what happens when you get old!

Perfect, thanks!
 
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