Percent of Power

Coming up with a power chart to compute percent of power for your RV as been a question lately. These numbers are normally computed by an aircraft manufacturer for a specific airplane and prop. For the most part, us experimental types can ball park it, but it would be nice to nail it down. I recall hearing of a simple formula to compute percent of power in an airplane with a fixed pitch prop, if you also have a manifold gauge....guess it's the same for a constant speed prop. Can anyone provide insight?
Dave Dollarhide

Some people have said that you can take the manifold pressure in inches, and add the rpm in hundreds. If the total is 48, that is 75% power. If it is 45, that is 65% power. If it is 42, that is 55% power. This would be very wonderful, if it worked.

But, things are not so simple in the real world. If you dig into the Lycoming power charts, you find that for a given manifold pressure and rpm, the power produced goes up with altitude. The simple formula above doesn't take altitude into account.

I fired a few numbers into my IO-360 power spreadsheet. The rule of 48 isn't too far off the mark at altitude (i.e. near the highest altitude at which you can reasonably expect to get a given percent power). It is quite inaccurate at lower altitudes. For example, using 48 (hundreds value of RPM, plus MP = 48), I get percent powers that range from 68% to 74%. 45 gives me between 54% and 65%, and 42 gives me between 45% and 55%. The rule is worst at sea level, and gets better as the altitude increases.

The rule of 48 is not very good. It is about as accurate as saying that on average, the sun's position is in the south (for us northern hemisphere folks). So if you head for the sun you'll be going south. We wouldn't propose that as a means of navigation, and we shouldn't propose the rule of 48 as a substitute for a power setting chart.

If you have an O-360-A series engine, and an MP gauge, I created an Excel spreadsheet that replicates the Lycoming power chart:

O-360-A power spreadsheet

If you have an angle-valve IO-360 engine, you want:

IO-360 power spreadsheet

You could produce your own power charts using those spreadsheets. With a FP prop, it is relatively easy to set a desired MP. The rpm will vary with your speed. You could create a chart that had sections for various altitudes of interest. Each section would have a table with MP and rpm values, and the resulting power.

Or, you could use the O-360 power chart that Larry Pardue created, using my spreadsheet:

Larry Pardue's power chart

percent power

Using the Lycoming O-320 manual, I developed a power spreadsheet.

It all boils down to:

75%: MAP + RPM/100 = 47
65%: MAP + RPM/100 = 44

These are accurate at cruise RPMs (2300-2700 RPM) to within 2 to 3 percent (more error at lower RPM)

I hope this is useful.

Vern Little
RV-9A... final inspection complete

Vern and Larry,
Thanks for the great info. For my fixed pitch RV-4, I'll use your info to build a simple chart for a panel placard with altitude and RPM for 75%. 65% info is nice to have for range planning, but 75% numbers will tell me when I can lean.
Dave

Effect of fuel flow?

Should fuel flow enter into the power equation?

It seems that when I lean to 50F LOP at a power setting of 20.5"/2350 (approx 65% power), I see a fuel flow of 7.2GPH and an IAS of 140KIAS at 4500' P.A.

Then if I enrichen to 100F ROP I see 8.8 GPH and 147KIAS with no perceptible change in MP or RPM.

Ben
-6A
O-360 A1A with Airflow Performance FI
Hartzel C/S

I'm curious. Why does the Lycoming produce a higher percentage of its rated hp at higher altitudes, when flying in standard conditions with constant rpm and mp?

Back pressure

Lack of exhaust back pressure.
Pete.

Correction

Of course I should have said REDUCED exhaust back pressure.
Pete.

Ben Beaird said:

Should fuel flow enter into the power equation?

Click to expand...

Yes, fuel flow has a big effect on the amount of power produced, as your data shows.

The Lycoming power charts are only valid if the mixture is leaned to give best power. The Lycoming Operator's Manual explains what leaning technique you need to get best power. The Lycoming Operator's Manual also discusses leaning further to a "best economy" mixture, and provides an estimate on how much lower the power will be at that mixture (it is in Figure 3-1 in my O-360 manual).

I guess I'll have to dissagree with the notion that any engine increases power at higher altitudes. Whether recip or jet, power is lost with altitude. Loss of exhaust pressure may be a factor, but has to be small....don't understand that. For example, see the typical Lycoming power chart below (captions don't line up):

Altitude RPM Percent of H. P Endurance on 59 gals. fuel

2500 2550 75% 4.8 hours
3500 2575 75% 4.8 hours
4500 2600 75% 4.8 hours
5500 2625 75% 4.8 hours
6500 2650 75% 4.8 hours
7500 2675 75% 4.8 hours

From the Lycoming discussion below for fixed pitch propellers, it seens to me that using my fuel flow gauge, and setting 9.8GPH on my O-360 will give me 75% power. I just don't understand the "propeller load chart" as mentioned....will have to look further.

"The effect the propeller has on engine operation and on aircraft performance is quite significant. Based on questions which have been asked by aircraft owners and from experience gained at the Textron Lycoming service hangar, there are several areas of propeller related information which may be of interest.

Aircraft equipped with a fixed pitch propeller will usually have static RPM (full throttle with aircraft standing still) limitations and full power in flight RPM limitations spelled out in the Pilot?s Operating Handbook. If static RPM is below the minimum specified, the engine could be low in power. However, experience has shown that this is not always true. Faulty induction air systems and/or faulty exhaust systems have been shown to contribute to indications of low power. A propeller which is ever so slightly less than perfect may cause the static RPM to be outside the designated full throttle static RPM zone. In addition to these other factors, it is not unusual to find a tachometer which is inaccurate. If an incorrect static RPM reading is observed during the engine check, any one or all of these components could be at fault. The tachometer may be the easiest to check as there are hand-held devices that quickly give an RPM reading that will verify the accuracy of the standard aircraft instrument. Knowing the accuracy limits of the aircraft tachometer may eliminate the need for further examination of the engine and propeller, or it may confirm the need for further troubleshooting. In any case, consider each component of the system before blaming low static RPM reading on one of them.

Another aspect of operation with a fixed pitch propeller came in the form of a question from a Lycoming engine owner. He indicated that the propeller provided by the airframe manufacturer had been exchanged for a cruise propeller. (This exchange should only be done with FAA approval.) With the new cruise propeller is use, an increase in fuel usage was soon apparent. Operating costs increased and an explanation was requested.

It is well known that the amount of horsepower taken from an engine will have a direct relationship to the amount of fuel used. Therefore, it can be deduced that use of the cruise propeller increased the horsepower requirement. This deduction deserves some additional explanation.

As an example, the standard propeller supplied with an aircraft may allow the engine to develop 180 horsepower at 2700 RPM at full throttle, in flight at sea level, with a standard temperature. The Lycoming 0-360-A Series normally aspirated engine illustrates this example.

Next, let us assume that this same engine/propeller combination is operated at 75% power with a "best economy" fuel air mixture setting. Again, assume sea level and standard temperature to simplify and standardize the discussion. Seventy-five percent power will require about 2450 RPM with a brake specific fuel consumption of .435 pounds per brake horsepower hour. Also, 75% of the 180 rated horsepower is equal to 135 horsepower. Fuel usage at this power and mixture setting will be 58.7 pounds per hour or 9.8 gallons per hour. The mathematics to arrive at this fuel usage are simple:

180 HP X 75% of power = 135 HP

135 HP X .435 BSFC = 58.7 lbs. of fuel

58.7 lbs. of fuel / 6 lbs. per gal. = 9.8 gal. per hour

Having made some assessments about what can happen with a standard propeller, now we will try to see what happens when a cruise propeller is installed in place of the original. The first thing we must know about the cruise propeller is that it has more pitch than the standard propeller. This means it will take a bigger "bite" of air than the original propeller with each revolution. This bigger bite of air will have an effect on aircraft performance and on how the engine may be operated.

Taking a bigger bite of air increases the resistance to the turning propeller. Perhaps it may be easiest to imagine what happens by considering your hand when held in the air stream outside a moving automobile with the palm forward as compared to having the side of the hand forward. Because of this increased resistance, the static RPM will be lower than with the original propeller. The same thing will be true when full throttle, in flight RPM, is compared to that of the standard propeller at a similar altitude and temperature. This will reduce takeoff performance of any aircraft. Using the earlier example, the engine was rated at 180 horsepower at full throttle and 2700 RPM. Now, in spite of applying full throttle, the increased resistance reduces the maximum attainable RPM to something less than 2700. As a result of not developing the rated 2700 RPM, the engine also will not develop the power for which it was rated. Since maximum power is less than full rated, aircraft performance will suffer. This should be considered before a fixed pitch propeller is chosen or exchanged for a different model.

At this point we must return to the original question. Why does the engine require more fuel with the cruise propeller? It is an accepted fact that the cruise propeller is more efficient for cruise operation, so it would not be unusual to follow this line of thinking. Seventy-five percent of rated power, using the original propeller at sea level and standard temperature, required a throttle setting to achieve 2450 RPM. Therefore, without more thoughtful consideration, it seems logical that the cruise propeller might also be set for 2450 RPM when 75% power is desired. Of course there is an increase in performance, but this can be attributed to the more efficient cruise propeller. Next comes the realization that the improved cruise performance isn?t all efficiency. Instead of 9.8 gallons of fuel, the engine is now using a greater amount of fuel per hour. For purposes of this illustration, let us assume that the number is 11 GPH. By reversing the mathematics used earlier, it is possible to estimate the horsepower and percentage of power actually being used as a result of operating the cruise prop at 2450 RPM with a best economy fuel air mixture.

11 GPH X 6 lbs. per gallon = 66 pounds

66 pounds / .435 BSFC = 151.7 horsepower

151.7 HP / 180 rated HP = 84.3% of power

Assuming a fuel usage of 11 gallons per hour for this problem provides a reasonably realistic example of the change that a different fixed pitch propeller might create. It also illustrates the need for pilots to change their habits when a propeller is changed. In addition to the change of habits, the discussion shows a real need to reevaluate the takeoff, climb, and cruise performance of an aircraft if the fixed pitch propeller is changed for a different model.

Another very important point concerns leaning. Remember that Lycoming recommends leaning to best economy only at 75% of rated horsepower or less. It is very possible that leaning to roughness or to peak on the EGT gage could cause serious damage if the engine is actually producing more than 75% of rated horsepower as shown in this illustration.

With this information as background, it is easy to see that setting a desired power with a fixed pitch propeller can only be accomplished if the pilot has a chart that applies to the specific aircraft/engine/propeller combination. Although the power chart for a new aircraft may come from data obtained by test flying with a calibrated torque meter, a fairly accurate chart can be derived for any fixed pitch propeller and engine combination. Briefly, this is done by finding the maximum available RPM at any particular altitude and applying data from the propeller load curve.

To conclude, the purpose of this article is to make readers more aware of some operational aspects of the fixed pitch propeller. Usually it is only necessary to accept the material provided by the airframe manufacturer and to use the engine/propeller as directed. If a propeller change is made, or on those rare occasions when we question the power available to the propeller, the material presented here could prove to be helpful."

Dave Dollarhide said:

I guess I'll have to dissagree with the notion that any engine increases power at higher altitudes. Whether recip or jet, power is lost with altitude.

Click to expand...

Dave, read my original post carefully. I said "If you dig into the Lycoming power charts, you find that for a given manifold pressure and rpm, the power produced goes up with altitude."

If you push the throttle fully forward, the amount of manifold pressure will decrease with altitude, and the power will decrease too.

But, if you use the throttle and prop controls to keep the same manifold pressure and rpm at different altitudes, the power will increase as the altitude increases. The lower exhaust back pressure as the altitude increases helps the engine breath better.

Altitude RPM Percent of H. P Endurance on 59 gals. fuel

2500 2550 75% 4.8 hours
3500 2575 75% 4.8 hours
4500 2600 75% 4.8 hours
5500 2625 75% 4.8 hours
6500 2650 75% 4.8 hours
7500 2675 75% 4.8 hours

From the Lycoming discussion below for fixed pitch propellers, it seens to me that using my fuel flow gauge, and setting 9.8GPH on my O-360 will give me 75% power. I just don't understand the "propeller load chart" as mentioned....will have to look further.

Click to expand...

Yes, setting 9.8 GPH fuel flow will give you 75% power, if you are leaned to the mixture for best power. That is a very big if.

Edit - hit the submit button too soon.

Kevin Horton said:

Some people have said that you can take the manifold pressure in inches, and add the rpm in hundreds. If the total is 48, that is 75% power. If it is 45, that is 65% power. If it is 42, that is 55% power. This would be very wonderful, if it worked.

But, things are not so simple in the real world. If you dig into the Lycoming power charts, you find that for a given manifold pressure and rpm, the power produced goes up with altitude. The simple formula above doesn't take altitude into account.

I fired a few numbers into my IO-360 power spreadsheet. The rule of 48 isn't too far off the mark at altitude (i.e. near the highest altitude at which you can reasonably expect to get a given percent power). It is quite inaccurate at lower altitudes. For example, using 48 (hundreds value of RPM, plus MP = 48), I get percent powers that range from 68% to 74%. 45 gives me between 54% and 65%, and 42 gives me between 45% and 55%. The rule is worst at sea level, and gets better as the altitude increases.

The rule of 48 is not very good. It is about as accurate as saying that on average, the sun's position is in the south (for us northern hemisphere folks). So if you head for the sun you'll be going south. We wouldn't propose that as a means of navigation, and we shouldn't propose the rule of 48 as a substitute for a power setting chart.

If you have an O-360-A series engine, and an MP gauge, I created an Excel spreadsheet that replicates the Lycoming power chart:

O-360-A power spreadsheet

If you have an angle-valve IO-360 engine, you want:

IO-360 power spreadsheet

You could produce your own power charts using those spreadsheets. With a FP prop, it is relatively easy to set a desired MP. The rpm will vary with your speed. You could create a chart that had sections for various altitudes of interest. Each section would have a table with MP and rpm values, and the resulting power.

Or, you could use the O-360 power chart that Larry Pardue created, using my spreadsheet:

Larry Pardue's power chart

Click to expand...

I know I'm reviving a 13 year old thread but I thought it was an interesting discussion and wanted to update the link for future readers.

The spreadsheets are here now Link.

Good stuff by Kevin !!!

I used his information for generating a power-speed curve with temperature, altitude, and GW as variables. Used it for my RV7 and a friends RV10. It works like a charm!

BTW - the only engine point at which one can truly calculate power is at peak EGT.

BillL said:

BTW - the only engine point at which one can truly calculate power is at peak EGT.

Click to expand...

Lycoming power ratings are at peak EGT?

rv6ejguy said:

Lycoming power ratings are at peak EGT?

Click to expand...

No, read the comment carefully - when flying and trying to get/calculate the most accurate power of the engine, this is the only place one really knows the A/F. Then using friction curves and a lycoming chart for determining IMEP, based on ISFC, one can determine the BHP to the prop. This is according to the Lycoming document from their performance guys.

This is why we use ROP/LOP and a temp delta referencing Peak EGT. I found it a waste of time to even attempt to use a non peak EGT data point as the data became too variable. Especially at lean settings, I burned a lot of fuel learning this.

For general percent power calculations, I use the G3X readout but it is just not that accurate. Good as any for an estimate, though.

BillL said:

No, read the comment carefully - when flying and trying to get/calculate the most accurate power of the engine, this is the only place one really knows the A/F.

Click to expand...

Agree with that.