Percent power

speyers · #1 10-12-2010, 09:11 PM

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...

terrye · #2 10-12-2010, 10:29 PM

Have a look at some engine power charts, for example:
http://www.aerosportpower.com/referencelibrary.htm
Then look under Engine Flight Charts for the specific engine you have.

ghatch · #3 10-13-2010, 06:35 AM

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

David-aviator · #4 10-13-2010, 06:42 AM

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.

elippse · #5 10-13-2010, 11:10 AM

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 · #6 10-13-2010, 12:56 PM

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

Finley Atherton · #7 10-13-2010, 04:08 PM

Quote:

Originally Posted by speyers

How is percentage of power calculated?

I use the chart on page 3-12 of http://www.aerosportpower.com/docs/i...instcharts.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/communit...t=63447&page=2

Quote:

Originally Posted by ghatch

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

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

Greg Arehart · #8 10-13-2010, 09:56 PM

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

MauiLvrs · #9 10-13-2010, 10:40 PM

(RPM/max RPM) * (MP/Atm P) * Rated HP = HP
*
(HP / Rated HP) * 100 = % power

speyers · #10 10-13-2010, 11:36 PM

Quote:

Originally Posted by ghatch

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

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