They don't call it a rule of thumb for nothing
RV8iator said:
I found it on Randy Lervold's site in a quote from Van. (
www.romeolima.com/RV8/)
I was just wondering if anybody had found similar numbers on the high-tech engine monitors.
Randy is the man and what he says is always well thought out.
Running thru other combos or RPM and MAP, comparing them to the real power charts, it seems the "rule" is a little too crude to be of real use.
75% power table (generic)
RPM v Map
2500......2600.......Altitude
23.6.......23.2.......0
23.3.......22.9.......1000
23.0.......22.7.......2000
22.8.......22.4.......3000
22.5.......
22.1.......4000
22.2.......21.9.......5000
22.0.......21.7.......6000
21.7.......21.4.......7000
21.5.......21.2.......8000
Now plugging in RPM and MAP into the Rule of 48.
RPM v. "Rule of 48"
2500......2600......Altitude
48.6.......49.2.......0
48.3.......48.9.......1000
48.0.......48.7.......2000
47.8.......48.4.......3000
47.5.......
48.1.......4000
47.2.......47.9.......5000
47.0.......47.7.......6000
46.7.......47.4.......7000
46.5.......47.2........8000
From the above you see at 2,600@22" on std day at about 4,500 alt will give you "48" and 75" power.
The rule gave values of approx 47 to 49. At lower RPMs it seems the spread shifts down a little more (46 to 48).
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How do engine monitors figure it out?
I can't speak for all of them, but many use a look up table based on tables and pwr curves from Lyc (see below). From the table of data its interpolated or averaged. Some also use a mathematical equation ** to estimate the precentage. Because they measure RPM/MAP/Temp and Alt, you should get close. However there are always "assumptions".
** Engineers know how to take numbers/data (a curve) and back fit that data into a mathematical
equation.
Its called curve fitting. Usually the equation can take any form, such as 2nd order or higher polynomial or logarithmic equation. The rule of 48 is a "first order" equation with 2 out of 4 unknowns in the mix. It will not be accurate or "fit the curves" very well except in a very narrow range or RPM/MAP.
Engine monitor with RPM, MAP, ALT and Temp input can read power fairly accurately. We have two prime unknowns, MAP and RPM. The third is density altitude (pressure alt + temp). We can make assumptions about MAP/RPM and temp/altitude to simplify but accuracy goes out the window.
It would be better to have torque and RPM, but we don't have torque gauges on little planes. Turbine or turbo-props do have torque gauges to measure engine power.
My simple solution is a laminated card with some known combos (like the generic 75% power table above. However that is for standard temp and needs a little fudge factor for hot/cold temps. Taking a clue from Lycoming, for every 10F colder than standard power increases by 1%.)
Look at different AFM's for certified planes to see how they do it; you can see the different approaches. Most people don't want to crunch a log or polynomial to get an answer, so the look up table is handy. AFM's for Cessna or Piper have simple tables with simplifying assumptions as well. However the will not work directly for a RV. You have to make up your own. That is why a MAP gauge is so critical to knowing power, even with a fixed pitch prop.
What the AFM's do, through flight test, with a MAP gauge, is come up with the data and turn them into power v rpm tables, usually normalized to std day. However manufactures who sell fixed pitch prop planes, take the MAP gauge out when they certify it and sell it. Why? The FAR's tell them its not required and it simplifies pilot operations (one less gauge to look at). Also it simplifys the manufacturing, lowers cost. They can do it because there's some relation to RPM verses MAP with fixed pitch props (+/- some variation).
Unfortunately its a BIG simplification and lowers your ability to estimate the true power you are making. With a RV we have so many variations in engine and props that the airframe installation factor comes in, so using Cessna data on a RV or even between RV's is not going to work great. The MAP gauge takes the most of the guess out.
MAP is needed even for fixed pitch prop planes. The charts made for fixed pitched Cessna's and Pipers are approximate based on specific flight testing, however even a MAP gauge would still be useful regardless of prop.
The reason a C-172 or Cherokee 180 tables don't work for RV's directly is for many reasons: airframe drag, prop design, induction design and exhaust design. All these factors affect MAP and power. The more airframe drag with a different prop means the prop is loaded differently and there for the "load" (power) on the engine is different and the MAP is different.
At the bottom of AFM charts, in small print, are "correction factors" or "fudge factors" for high temp or low temp. Many ways to estimate power, but the key word is estimate. With out torque we are gussing a little even with RPM and MAP.
However ALL planes, especially experimental ones should have a MAP gauge, even with a fixed prop.
It's a little more complicated than that:(click me once or twice)
NOTE: The chart above does not really work for RV's because of our ability to pull more MAP at higher altitudes. This is due to the better air box and faster speeds we fly at (ie ram air pressure). Still MAP, RPM, Alt and Temp relations to power still hold true, with a small error. The only TRUE measure of power is Torque and RPM by definition.
