Accuracy of "rule of 48"

randylervold · #1 10-07-2007, 04:54 PM

I've been using the "rule of 48" to determine power settings for some time now. I do so because I can't find another method that works, and because both Dick and Jerry VanGrunsven (seemingly reliable sources) recommended it to me. Lycoming power charts are notoriously hard to read, and also don't take into account the ram air manifold pressure boost we get the Van's intake scoop and therefore throw off the tables.

For those unfamiliar with the "rule" it goes like this... take your rpm in 100s and your manifold pressure in inches and add them together. A sum of 48 equals 75% power (such as 24" MAP and 2400 rpm), each reduction of 3 is a reduction of 10% power, so 45 would be 65% and 42 would be 55%. I put together a table listing all the combinations and settings in between that I used in the cockpit and it is a handy tool for cruise power management.

All that said, does anyone know how accurate this is? Any empirical data to compare with it?

Frank · #2 10-07-2007, 05:33 PM

Don't know how specifically accurate it is but do know it assumes a normally aspirated engine. i.e. non turbo-charged or normalized. FYI, it reflects an old rule of thumb used by round engine drivers back before the current flock of kerosene burner drivers. Most of the people I've heard it from are pushing their 70's (or more) and earned their flight time during the 40's, 50's and 60's.
I think its an easy rule to remember and apply. I use it all the time with no ill effect!

GrayHawk · #3 10-07-2007, 06:04 PM

Quote:

Originally Posted by randylervold

I've been using the "rule of 48" to determine power settings for some time now. I do so because I can't find another method that works, and because both Dick and Jerry VanGrunsven (seemingly reliable sources) recommended it to me. Lycoming power charts are notoriously hard to read, and also don't take into account the ram air manifold pressure boost we get the Van's intake scoop and therefore throw off the tables.

For those unfamiliar with the "rule" it goes like this... take your rpm in 100s and your manifold pressure in inches and add them together. A sum of 48 equals 75% power (such as 24" MAP and 2400 rpm), each reduction of 3 is a reduction of 10% power, so 45 would be 65% and 42 would be 55%. I put together a table listing all the combinations and settings in between that I used in the cockpit and it is a handy tool for cruise power management.

All that said, does anyone know how accurate this is? Any empirical data to compare with it?

Randy,
Would not the RAM air boost be accounted for in the manifold pressure part of the approximation?

That said, I have one of the 'power wheels' and would expect that it does not account for the ram air boost. I also have an EI FP-5L fuel computer that approximates % HP without manifold pressure (fuel flow, corrected by EGT)

Summary: I'm interested in this also.

WildBill · #4 10-07-2007, 06:39 PM

Perhaps you could work a way to figure this out using the PLANK formula or other derivations?

IHP = PLANK/33,000

P - indicated mean effective pressure

L - length of stroke (ft)

A - area of piston head in square inches

N - rpm / 2

K - number of cylinders

-----------------------------------
bmep (brake mean effective pressure) = (33000 * bhp) / LAN (see above)

----------------------------------------------------
bhp (brake hp)= (bmep * rpm) / k (k factor of engine ?, might try constant of 792000)

Hope I got that right. 'Aircraft Powerplants' by Bent/McKinley has much more info.

randylervold · #5 10-07-2007, 06:47 PM

Quote:

Originally Posted by GrayHawk

Randy,
Would not the RAM air boost be accounted for in the manifold pressure part of the approximation?

That said, I have one of the 'power wheels' and would expect that it does not account for the ram air boost. I also have an EI FP-5L fuel computer that approximates % HP without manifold pressure (fuel flow, corrected by EGT)

Yes, ram MAP boost WOULD be accounted for in the rule of 48, but not the Lycoming power charts -- they assume more/less ambient air pressure at a given altitude.

As to the various engine monitors, most simply measure fuel flow which I don't understand how that can be accurate when you can vary your fuel flow by 2-3 gph simply by leaning at the same MAP and rpm. Also, the UBG-16 doesn't even know what engine you have: O-320, O-360, O-540.

lostpilot28 · #6 10-07-2007, 07:08 PM

Hi guys,
Does the same thing apply for more than 75% power? For example, 2700 RPMs + 24 MAP = 51...therefore, 85% horsepower?

I have never used a MAP guage, so if my question doesn't make sense, forgive me. I do anticipate using one when my RV is flying (someday), so this is of interest to me. Thanks!

gmcjetpilot · #7 10-07-2007, 08:21 PM

Quote:

Originally Posted by lostpilot28

Hi guys,
Does the same thing apply for more than 75% power? For example, 2700 RPMs + 24 MAP = 51...therefore, 85% horsepower?

I have never used a MAP gauge, so if my question doesn't make sense, forgive me. I do anticipate using one when my RV is flying (someday), so this is of interest to me. Thanks!

The rule48 seems to work just as well above 75% as below, with errors. Like any rule of thumb it tends to be accurate over a certain range and conditions, than accuracy falls off. I ran some numbers for different RPM/MAP at standard temp and sea level.

Rule-48 is less accurate when RPM/MAP miss match, e.g., not squared, high RPM & low map.

Example 25(000) rpm + 21.5" = 46.5 : or 70%. However from Lyc at sea level, std day temp, power is only 67%. So rule48, is high. When MAP is lower rule48 seems to predict high; however if we're talking 8,000 ft, std day (30.5F) with the same 25/21.5 RPM/MAP, power is 75%. So rule48 would predict low. You can see altitude can have a big affect on power. Non standard temp will also affect power. The rule48 accuracy varies since it does not account for temp or altitude. On the other hand if I was at low altitude and want about 70% power, I'd set 23-square or 23/22" not 25/21.5". The rule48 predicts 23/23 as 68.3% which is a close match with Lyc data, 68.7%. However if you are flying 23/23 at 4000 feet that is 73.5% power. The rule is off by 5%. That is a lot. But if its a hot day than actual power drops a percent or two and the rule is a closer match. A rule with out altitude or temp adjustment can only be accurate in a narrow range of conditions. Is 2% or 3% off critical? No unless you assume you are at 75% and lean when you are at 78% power.

The opposite of the above, is when MAP is higher than RPM, called "over square". Rule48 tends to predict power is lower than actual when "over square". Since we don't do a lot of over square (at least more than 1 or 1.5" map over rpm) and MAP drops as we climb, this RPM/MAP combo is less likely. Nothing wrong with over-square by the way; actually it's an efficient way to fly, say 2,400 rpm/ 25 map. When you take off at 2,700rpm/29" you are over square right.

I looked at 65% to 100% power, "Rule48" error ranged between -2.5% to +4.6%, with a median of +/-0.8%. For square power settings (rpm & map equal), its within +0.6% (std say/sea level).

However in real cold temps and/or "over square", the rule can under predict power (engine making more power than rule48 calc). Temp & altitude are big factors on actual power, for a given RPM/MAP, where power can vary by more than +/-5% in theory. Just keep that in mind. On a cold winter day, at 4,000 ft and over square, the rule48 could be low by several percent. You just don't want to lean over 75% power.

Bottom line its a fairly accurate "rule", works above or below 75% and accurate within a percent or two typically, but worse case could be off as much as +/-5 percent or more.

Kevin Horton · #8 10-08-2007, 05:54 AM

Quote:

Originally Posted by randylervold

All that said, does anyone know how accurate this is? Any empirical data to compare with it?

Well, you could compare it against the Lycoming power charts. They, if anyone, should have a pretty good idea how the power produced varies with rpm, MP and altitude.

If you look at the Lycoming power charts, you will quite quickly see that for a given rpm and MP, the power varies quite a bit with altitude. For example, looking at the IO-360-M1B power chart, I see that 2400 rpm and 24" makes 67% at sea level, and 75% power at 5300 ft.

There is also significant variation in power vs rpm and MP between different model engines. Looking at the O-360 power chart, I see that 2400 rpm and 24" makes 75% at sea level, and 82% at 5100 ft.

Summary - the rule of 48 is not very accurate.

plehrke · #9 10-08-2007, 06:55 AM

I bought a flight performance recorder from Pegasus Technology 5 years ago that does a great job calculating % power. I am not sure the company still exists but there website is still online.

http://www.flightperformance.com/fli...e_recorder.htm

It uses MP, RPM, OAT, Density altitude and fuel flow to calculate % power. I actually think it uses look up tables. When I ordered it, I specified my engine (IO-360-B2B). The case has a tag indicating the engine model that the software has hard coded in.

I have had great results from this instrument as all the data is recorded every 3 seconds including gps position so you can play back the entire flight including what your flight instruments (T&B, VS, AS, DG) all indicate. I can overlay it over Google Earth.

mgomez · #10 10-08-2007, 07:14 AM

I took the data sheet for a Lycoming O-360-L2A, which is rated at 160 HP @ 2400 RPM, or 180 HP @ 2700 RPM.

It has a plot of power as a function of MP, for various RPM. I found that the following combinations yield 135 HP (i.e. 75% of 180 HP) at sea level. Note that I'm not saying all of these combinations are recommended.

RPM MP
-----------
2700 23 (sum is 50)
2500 24 (sum is 49)
2400 26 (sum is 50)
2000 29 (sum is 49)

For 55% power, or 99 HP:

RPM MP
-----------
2700 18.5 (sum is 45.5)
2500 20.5 (sum is 45.5)
2400 21 (sum is 45)
2000 23 (sum is 43)