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Thread: Percent Power Calculation

  1. #1

    Percent Power Calculation

    What is the equation that the AFS software uses to calculate percent power?

    I am having trouble converting from my engine operating manual charts to the tabular data required by the software, as there is no "55%" or "75%" MAP info on these charts.

    Thanks!
    Highest Regards,

    Noah F
    RV-7A
    Rhode Island

  2. #2

    Power Redux with altitude

    Looking at the operators manual charts, I am still unclear about how to get the 2nd data set the AFS requires for input, which is HP reduction with altitude. The chart, shows several possible reductions at each altitude, depending on RPM & manifold pressure. So how does one get this info?

    Also I would like to know what error is associated with the % Power number that the AFS reports. I need the actual equation used in order to calculate this error.
    Highest Regards,

    Noah F
    RV-7A
    Rhode Island

  3. #3
    I will try to attach some good power tables, it it will not attach, send me your e mail and I will send you the file.

    Regards,

    Gary Brown ATP A&P IA CFI DAR gdbrown@minetfiber.com

  4. #4
    Hi Gary, I've got the charts:


    The question I have is how to get the data for HP reduction vs. altitude off the chart.

    The numbers I get seem to be an order of magnitude greater than the numbers that AFS publishes in their samples, which is why I am concerned.

    More than that, though, is the error associated with the number presented on the AFS display for % power, since we all know that running LOP at greater than 75% power can damage your engine. Is the error in AFS' number 5%? 15%? 25%? I would like to know the equation that AFS uses in their softwareto display %power so I can compare this with the operator's manual chart and thus the error and know whether I should trust AFS' reported number.

    So what say you, AFS?
    Highest Regards,

    Noah F
    RV-7A
    Rhode Island

  5. #5
    Hi Noah,

    The horsepower calculation is a lot more complicated than meets the eye. First, you cannot use it for any lean of peak operation. Notice that the box on the upper right assumes maximum power mixture. As soon as your run rich or lean of max power, this table goes out the window. If you have electronic ignition, this chart goes out the window. We want to make sure all of our customers understand that the horsepower calculation displayed on the EFIS is for informational purposes only. It is a rough number. It's still very useful information for basic cruise data, but that's it. Here is an example, notice the difference between 10,000ft and 2,000ft is about 8HP for a given setting. 8HP out of 180HP is about 4%. A 4% difference in HP is very small. You could have that error with one or two turns of your mixture knob. I'm not a LOP expert, but I understand the best method is to use EGT temps, fuel flow, RPM, MP, and OAT. I've been told that GAMI fine tunes fuel flow in the injectors so they can accurately use that for calculating HP.

    Our intention is to provide a basic HP readout for normal operations. Our official recommendation is to use the stock O-360 chart in our manual (and slightly tweaking for your engine if you have the charts) which will give you that info. I apologize that we don't have a more accurate way of doing it. It can get really complicated really fast and the difficult part for most customers is getting the actual charts for their engine. Adding Pmags and other power/mixture altering devices really throws things off. At one time we used fuel flow to calculate HP, but found it was error prone and a majority of our customers didn't like it.
    ---
    Trevor Conroy
    Technical Support
    Advanced Flight Systems
    Support@Advanced-Flight-Systems.com

  6. #6
    add .17" MP for each degree above ISA

  7. #7
    When I run often at 9500'; I run full throttle, 2300 RPM; lean to peak, and the ff is about 7.5 gph with 180HP fuel injected/P-mags. this renders a true airspeed of 186mph. Very consistant at this setting.

  8. #8
    Thanks Gary. Your airline pilot secrets are always appreciated!
    ---
    Trevor Conroy
    Technical Support
    Advanced Flight Systems
    Support@Advanced-Flight-Systems.com

  9. #9
    I must still be missing something regarding the chart data. You say “notice the difference between 10,000 ft and 2,000 ft is about 8HP for a given setting”. If I start at 2000 feet on the right side of the chart and look at the intersection with the 2700 rpm line, I read 187HP in the middle of the chart. If I go to the 10,000 ft vertical line and again move up to the 2700 rpm line, I now read 140HP. This is a reduction of (187-140) 47HP, not the 8HP that you quote, if I am interpreting the chart correctly.

    Further, the AFS manual, on page 58, states that for an O-360, the delta HP with altitude at 8000 ft is 9.1 HP. We all know the rule of thumb that normally aspirated engines cannot exceed 75% power above 8,000 ft. For an O-360 rated at 180HP, this would be a 25% reduction, or 45HP, not the 9.1 HP in the manual example. The chart I posted above for an IO-360 A model supports this as well; maximum power at 8000 ft. is indicated as 150 hp, reduced 25% from a sea level maximum of 200HP.

    So I think I must be missing something, can you please explain your numbers? How did you get them?
    Highest Regards,

    Noah F
    RV-7A
    Rhode Island

  10. #10
    I can only agree with Noah. The delta horsepower on page 58 is way off. The MAP/RPM values corresponding to 55% and 75% power seem to be accurate. The presentation is however unfortunate and confusing as the values on the right hand side of the table have nothing to do with those on the left. For example it is obvious that you will never see 23.9" MAP at 10 000ft with a normally aspirated engine hence you will never be able to get 75% power from your engine. This obviously makes the Delta HP of 11.4 look ridiculous.
    Gary, I am somewhat confused about "add 1.7" MP for each degree above ISA". Surely you are going to see less manifold pressure as the temperature goes up, density altitude increases and the amount of power your engine can produce decreases.

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