randylervold

randylervold

moderator
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?
 
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!
 
randylervold said:
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?
Click to expand...
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.
 
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.
 
GrayHawk said:
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)
Click to expand...

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.
 
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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!
 
Here's my analysis

lostpilot28 said:
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!
Click to expand...
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.
 
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randylervold said:
All that said, does anyone know how accurate this is? Any empirical data to compare with it?
Click to expand...
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.
 
Flight Performance Recorder

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/flight_performance_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.
 
49 or 50 seems to work better...

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)
 
Can someone explain to me, the average joe bag of donuts RV guy, why % of power is important? My nifty engine monitor displays it, but I have never decisioned based on it.

So there I am, whiz banging along at 7k', and my engine monitor says 70% power. Why do I care? Am I supposed to say to myself, oh geeze, I really want to be at 65% power? I have so many things to care about on the monitor, like FF, RPM, MP, EGT's, CHT's and so forth for all good reasons which I can easily articulate, why on earth would I care about % of power?
:confused:
 
It's supposed to save labor

Kahuna said:
So there I am, whiz banging along at 7k', and my engine monitor says 70% power. Why do I care? Am I supposed to say to myself, oh geeze, I really want to be at 65% power?
Click to expand...

The way I use those tables and MP/RPM combinations is to save labor. I planned the flight based on (say) 130 KTAS and 10 GPH...that's what I told ATC. I get to the top of the climb, I set the MP/RPM combination I want to cruise at, and forget about it. I don't have to wait until the airspeed stabilizes to find out that I set the wrong power.
 
Kahuna said:
Can someone explain to me, the average joe bag of donuts RV guy, why % of power is important? My nifty engine monitor displays it, but I have never decisioned based on it.

So there I am, whiz banging along at 7k', and my engine monitor says 70% power. Why do I care? Am I supposed to say to myself, oh geeze, I really want to be at 65% power? I have so many things to care about on the monitor, like FF, RPM, MP, EGT's, CHT's and so forth for all good reasons which I can easily articulate, why on earth would I care about % of power?
:confused:
Click to expand...

Because then if you find that you're only truing 173 kts instead of 173.5 kts, you'll know that your EGT was off by 2.5 degrees! Kahuna, you've got to keep your head down there in that cockpit, and worry about those important things!
 
Kahuna said:
Can someone explain to me, the average joe bag of donuts RV guy, why % of power is important? My nifty engine monitor displays it, but I have never decisioned based on it.

So there I am, whiz banging along at 7k', and my engine monitor says 70% power. Why do I care? Am I supposed to say to myself, oh geeze, I really want to be at 65% power? I have so many things to care about on the monitor, like FF, RPM, MP, EGT's, CHT's and so forth for all good reasons which I can easily articulate, why on earth would I care about % of power?
:confused:
Click to expand...
In my case, it is mostly to learn as much as I can about the engine, so as to minimize failure and increase it's longevity. Maybe that's as simple as keeping the CHTs within a certain range. I admit I don't look at the % HP that much. Just curoious & want to learn as much as I can.
 
I suspect Lycoming's charts have been compiled with the use of altitude cell dyno. Power increases with altitude at the same MAP and rpm because reduced exhaust back pressure reduces charge dilution during valve overlap. This is even more prevalent on turbocharged engines. Also in play is throttle blade position and its effect on pumping losses, not to mention mixture distribution on carbed engines.

None of the other charts or electronic methods I've seen take humidity into account either which can affect power by nearly 10% between 0% and 100% so I think you guys are wasting your time here. Exhaust system variations, AFR, EI ignition timing all have major effects on engine hp. In short, many of the important variables affecting hp are not accounted for in most of these calculations. Therefore the result is inaccurate. If you want to really know what your engine puts out, you need to stick it on a dyno within an altitude cell. Count on $$$$$$.

While technically interesting, I'm with Kuhana on this. Who cares. Need more speed, push the MAP or rpm up- or get a turbo:rolleyes:.
 
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I spent a lot of time calibrating my engine performance (search threads).

There are really two operating regimes on your engine-- excess fuel (rich) and excess air (lean).

When you have excess fuel (rich), the MAP/RPM charts come into play for determining your horsepower. MAP/RPM is an indirect measurement of the volume of air being pumped through the engine. Since you have excess fuel, its the airflow that determines HP.

When you have excess air (lean), then it's fuel flow that determines HP, based on brake specific fuel consumption.

What does this mean? If you have an engine monitor that displays percent HP, you need to know the algorithm that the manufacturer uses. If it's just MAP/RPM, then it will not be accurate when running lean.

I have two tables: percent HP based on MAP/RPM and percent HP based on fuel flow for lean operations.

With a fixed pitched prop, a lot of this is academic anyway. You'll soon get to know your rule of thumb settings below 8000'. The rule of 48 is ok for this.
 
Kahuna said:
Can someone explain to me, the average joe bag of donuts RV guy, why % of power is important? My nifty engine monitor displays it, but I have never decisioned based on it.

So there I am, whiz banging along at 7k', and my engine monitor says 70% power. Why do I care? Am I supposed to say to myself, oh geeze, I really want to be at 65% power? I have so many things to care about on the monitor, like FF, RPM, MP, EGT's, CHT's and so forth for all good reasons which I can easily articulate, why on earth would I care about % of power?
:confused:
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Kahuna, the main reason is simply to know whether I am below 75% or not and can therefore lean aggressively. In this day and age of high fuel prices I'm all over that mixture lever. I would think electronic ignition, which most of us have, would make mixture management above 75% power even more important. Said differently, you don't want to operate in the "red zone" which is high power and anywhere near peak EGT... supposedly hard on the engine, again I think exacerbated by electronic ignition.

Also, probably just me, but I like to be as aware as I can as to cruise power settings. Granted, it is not as important as many other bits of data, but nice to keep a rough mental average of where I am spending my time %-wise.
 
randylervold said:
Kahuna, the main reason is simply to know whether I am below 75% or not and can therefore lean aggressively. In this day and age of high fuel prices I'm all over that mixture lever. I would think electronic ignition, which most of us have, would make mixture management above 75% power even more important. Said differently, you don't want to operate in the "red zone" which is high power and anywhere near peak EGT... supposedly hard on the engine, again I think exacerbated by electronic ignition.

Also, probably just me, but I like to be as aware as I can as to cruise power settings. Granted, it is not as important as many other bits of data, but nice to keep a rough mental average of where I am spending my time %-wise.
Click to expand...

Leaning to/from or using % power for a reference as to when you can begin aggressive leaning is not a good reference IMHO. As an example, you could be at 25" and 2100rpm, showing less than 75% power but have a real detonation problem. Aggressive leaning, according to my reads on the subject, should only happen below 24"MP, that is the reference. Again we are talking our normally aspirated lycos here.

Oh ****, I have hijacked your thread. Sorry bout that. I hate it when people do that, ESPECIALLY ME!
 
Another way (need fuel flow and EGT)

Assuming the Lycoming and Superior are very similar in fuel efficiency (not volumetric efficiency), the Superior specification for SFC is:
75 Deg F rich of peak = 0.50
peak = .43
Using GAMI we can estimate LOP at .40 - .38.

Using the slightly innacurate 6 pounds per gallon...

(Fuel flow x 6) / SFC = HP

Examples:
10 gph x 6 lb/gal / .50 = 120 HP. And then, 120/180 = 67%

9 gph x 6 lb/gal / .40 = 135 HP. And then, 135/180 = 75%

My GRT shows these calculations to be pretty accurate. Of course, altitude is not factored in, but you can just add a little as you go up.

You can quickly construct a little table to carry in the airplane.
 
So true!!!

Kahuna said:
Can someone explain to me, the average joe bag of donuts RV guy, why % of power is important? My nifty engine monitor displays it, but I have never decisioned based on it.

So there I am, whiz banging along at 7k', and my engine monitor says 70% power. Why do I care? Am I supposed to say to myself, oh geeze, I really want to be at 65% power? I have so many things to care about on the monitor, like FF, RPM, MP, EGT's, CHT's and so forth for all good reasons which I can easily articulate, why on earth would I care about % of power?
:confused:
Click to expand...

So true. I don't care what the power % is showing. I hate the phrase but, "it is what it is." I use the AF2500 and do a LOP check every 30 minutes. I've been flying at 15,500 showing 55% power, LOP to 5.4 gph and cruising at 176kts ground speed. Nothing I can do about the % of power that is going make any real difference. To me the LOP setting with proper temperature monitoring is key.

Way to go Kahuna. We'll probably get a 3 page response from the resident expert on everything on why it is important:D
 
Wide open throttle (as if there's variation on THAT theme?!), set the RPM to something you're happy with (I use 2280), and lean to ~50 LOP. Turn on the tunes. Set your alarm to wake you up 15 minutes out. You mean folks do it differently than that? :rolleyes:
 
What I meant to say was..."rule of 48" is GREAT(!) if you have no other instrumentation other than MP & RPM. But we typically do...
 
Because then if you find that you're only truing 173 kts instead of 173.5 kts, you'll

xxxxxxxx
LMAO
 
dan said:
Wide open throttle (as if there's variation on THAT theme?!), set the RPM to something you're happy with (I use 2280), and lean to ~50 LOP. Turn on the tunes. Set your alarm to wake you up 15 minutes out. You mean folks do it differently than that? :rolleyes:
Click to expand...

And just how many hours do you expect to get out of your engine treating it that way? I'd guess maybe 3000 or so before I'd start thinking about TBO!:D
 
I've been using it also but after some time with the lyc charts changed over to using 49-46-43. This seemed to come closer for the 0-320.

Bryan
 
Best idea yet

Bryan Wood said:
I've been using it also but after some time with the lyc charts changed over to using 49-46-43. This seemed to come closer for the 0-320.

Bryan
Click to expand...
Bryan, good idea, that's what I need to do, study the Lyc charts for my engine and make some adjustments to my rule-of-48 based cockpit reference card accordingly. This only matters at the lower altitudes anyway where you can develop enough MAP to get above 75%. Those adjustments to a straight rule-of-48 computation will make it more accurate, which in spite of certain comments (Kahuna, Darwin, Checkoway), makes me happy! ;-)

Not that it matters to you guys, but it will let me put more accurate fuel burn data table in my POH. I know you all have this data collected for your planes, right?
 
I was working through some performance numbers tonight and reviewed this thread. If you would like an equation instead of the "rule of thumb" try this:

PWR% = 3.33(mp+RPM/100) - 84.84

I know the rule is not exact, but if you're recording data in a spreadsheet this equation makes it easy to estimate power.
 
<Posted from an earlier thread on percent horsepower>

Curve-fitted formula. Works ONLY for fuel-rich mixture (excess fuel). For lean mixture, use BSFC from manufacturer's data.

==========================
Lycoming O-320 B and D series Horsepower Formula (Rich Mixture)

h=(H-((R-r)*(5.58-0.125*(M-m))/100 + 7.35*(M-m)) + 2.0*pa/1000)*sqrt((519-3.58*pa/1000)/(460+ta))

h% = h/H*100

where

Constants:

H= maximum sea level horsepower at maximum manifold pressure (from manufacturer?s data)
R= maximum sea level RPM at maximum manifold pressure (from manufacturer?s data)
M= maximum sea level manifold pressure at rated RPM (from manufacturer?s data)

For Lycoming O-320 -B and -D series:

H= 160
R= 2600 (Sensenich prop limit)
M= 28.6

Variables (inputs)

r= actual RPM
m= actual manifold pressure
pa= actual pressure altitude (altimeter set to 29.92 inHg)
ta = actual air inlet temperature at pressure altitude

Outputs

h = calculated horsepower
h%= calculated percent horsepower

You can run your own numbers and see how they fit for yourself.

Vern
 
If you have a carburetor, your sea-level power is based on 28.5" MAP rather than 29.92" due to about 1.4" carb pressure drop. With fuel injection, it's probably more like 0.5" drop so use 29.4". You can determine your actual drop by seeing your full-throttle MAP and differencing it with static pressure. You then use your actual rpm divided by your sea-level rated rpm. Finally, you can correct for density by 59F / OAT. So estimated power %= (100 x MAP x rpm x 59) / (SLMAP x SLrpm x OAT). Another way, if you have fuel flow, is FF x 11.8 at best power mixture; this is from 0.5 SFC, 5.9 lb / gal.
 
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