(Puts on large nerd hat)I'm not a real pilot so this subject has always confused me a bit. Allot of planes don't have an easy to find POH and I have been trying to figure out the correct way to judge power settings when it's possible to exceed 75% power on fixed pitch props.I know you can judge the hourse power by the RPM however as altitude increases the scale to power vs RPM changes.So lets say I am in the Carenado Cessna 152 II. It lists a max RPM of 2550 and a 110 BHP engine.Now lets assume I can produce 2550 RPMs at sea level with full throttle. If I wanted to calculate 75% of my HP for cruise flight it would look something like this: (2550³ * .75)^1/3=2316.828756 or 2317 RPMSo now if I want to figure out my HP at 75% that would look like: 110(2316.828756/2550)³ = 82.50000002 HPI can check the HP by: (2550³(82.50000002/110))^1/3 = 2316.828756So I would then assume (I know bad word) that any altitude where I am unable to reach 2316 RPM would be below 75% and safe to run at full throttle while anything above 2316 RPM would be over 75% and I would need to reduce power untill below 2316 RPM. The math should allow me to create my own power chart that I can apply the curve to pressure altitude. correct?³ = cubed^1/3 = cubed rootPTHP = part-throttle hpFTHP = full throttle hpPTRPM = part-throttle rpmFTRPM = full throttle rpm PTHP = FTHP (PTRPM /FTRPM )³ 75% HPPTRPM = (FTRPM³(PTHP/FTHP))^1/3 or 75% RPMPTRPM = (FTRPM³ x .75)^1/3 or 75% RPM or 2316 RPM on the C152 II @ sea level & standard tempPTRPM = (FTRMP³ x .65)^1/3 or 65% RPM or 2208 RPM on the C152 II @ sea level & standard tempI know the math works however I'm not sure if I am understanding it properly.(Removes large nerd hat)

I just did a small a small test flight in the carenado C152II. 29.92 alt, 15^oC 2000ftFTRPM= 2545 so (2545³x .75)^1/3 = 2312110(2312/2550)3 = 81.98523259 PTHP(2550³(81.98523259/110))^1/3 = 23122312 RPM should be around 75% Below is a URL to a performance chart I found on the C152II and as you can see a standard temp cruse setting at 2300 RPM 66% BHP 96 KTAS 5.4 GHP I didn't measure my GPH however 2300 RPM VS 2312 is about right. My Airspeed was 96KIAS and should be about right since I am only at 2000ft. However the math shows 82HP while the performance chart reads 66BHP does that seem correct? (I did not lean the mixture since I was at 2000ft)C152 Performance chart

Interesting formulas..In general, an engine's HP is a function of RPM and TORQUE. I don't remember the exact formula.. there's a constant thrown in there.. but aside from trying to calculate a complete torque-curve.. it's RPM and TORQUE.For airplane engines, we have a built-in limitation. Their rated HP is at a maximum, useable RPM. If you were to stick a 110HP 0-235 on a dyno; you'd find that it can produce much more than 110HP, probalby at well over 3000RPM. Maximum RPM ratings have to do with engine life, and propeller limitations, and heaven knows what else. That would be a forum discussion unto itself.ANYway.. for piston airplanes, power is a function of RPM and MP (manifold pressure). For the sake of this discussion, we'll assume that a wide-open throttle allows the engine to avail 100% of atmospheric-pressure as MP. That would be 29.92" of MP at sea-level on a 'standard' day. AS you noted; the HP vs RPM scale changes with altitude, because gains in altitude, lower the available MP. That 29.92" of MP for a wide-open throttle at sea-level, will be aprox' 1" less per 1000 feet of altitude. At 6000msl that wide-open throttle will only get you ~24" of MP. At 8000msl, you'd be lucky to get 22" of MP with a wide-open throttle. Basically, gains in altitude have the same, proportional effect as closing the throttle.Here's where we see the main disadvantage of a fixed-pitch-prop. Even though MP is greatly reduced by altitude, you can still get high RPMs. The only way for the pilot to reduce RPM at cruise (for engine wear and fuel economy), is to pull back on the throttle, and reduce the MP even further. A constant-speed propeller allows you to leave the throttle open, while reducing RPMs.The point is (per your 75% question), for a normally-aspirated, piston airplane; RPM alone does not equate to power.. and even at cruise altitudes, full-throttle will yield near (if not over) red-line RPM while the engine isn't delivering anything near 100% power.Since the only control over RPM for the pilot (in level-cruise), for a fixed-pitch-prop, is the throttle; the percentage of power vs RPM chart has to account not only for the atmospheric loss of MP at/by altitude, but also for the further reduction in MP when you reduce RPM by the throttle.

I do understand the relationship of MP with RPMs and pressure altitude, however if we jump back to my math a second I think you will see that it depicts the exact relationship you stated in your post. Even though you are dealing with a curve that changes with altitude and temp If you stick with 29.92 and 15⁰C you have fixed the curve to a specific location for the test. Lets take your 8000ft altitude example. If the math is right and we are dealing with 29.92 altimiter 15⁰C at sea level we now have a fixed rate of change in pressure altitude and temp. If eigther temp or altimiter reading change (Beyond the expected norm) then the curve has also changed making the calculation not work. Again with the 152 at 8000ft and 110HP I would expect the value to change with the data. I'll give it a shot in FSX. Might take me a while to get to 8000ft though. :)Here is a link to where I found the math. It might explain it better than I can.How To Determine The Part-Throttle RPM of a Fixed Pitch Propeller At a Given Horsepower by Stan Hall

I do understand the relationship of MP with RPMs and pressure altitude, however if we jump back to my math a second I think you will see that it depicts the exact relationship you stated in your post. Even though you are dealing with a curve that changes with altitude and temp If you stick with 29.92 and 15⁰C you have fixed the curve to a specific location for the test. Lets take your 8000ft altitude example. If the math is right and we are dealing with 29.92 altimiter 15⁰C at sea level we now have a fixed rate of change in pressure altitude and temp. If eigther temp or altimiter reading change (Beyond the expected norm) then the curve has also changed making the calculation not work. Again with the 152 at 8000ft and 110HP I would expect the value to change with the data. I'll give it a shot in FSX. Might take me a while to get to 8000ft though. :)

Now you're confusing me.. We can't have temp and altimeter variables in this test. All we're conscerned about, is the loss in MP with altidude. That temps and atmospheric pressure decrease with alitutde is a given. We have to treat weather as a constant. A 29.92 altimeter at sea-level, is a 29.92 altimeter at 8000msl.

Now you're confusing me.. We can't have temp and altimeter variables in this test. All we're conscerned about, is the loss in MP with altidude. That temps and atmospheric pressure decrease with alitutde is a given. We have to treat weather as a constant. A 29.92 altimeter at sea-level, is a 29.92 altimeter at 8000msl.

That's exactly what I said. If you make altimiter and temp a constant then you can apply the math to build a curve for power. By setting the altimiter and temp you know an exact value that will change as altitude increases.8000pa(pressure altitude) is 8000feet As you increase in altitude you have a perdictible change in temp (2⁰for every 1000 feet if I remember correctly) The math is only an equasion to find 75% of the power being used. So if you increase altitude untill 75% can no longer be reached at 2550 RPM you have found a specific point in the curve and should be able to make you power graph for 75% power at 2550 RPM. You can then do the same for 65% and say 45%. Once you have your graph then you should be able to to make range and fuel calculations based on weight and anything else you might need.

Let me put it another way. Lets say your flying at 4000ft. Reduce power untill you can no longer maintain 4000ft and mark down the RPM. This would be the low end of you scale before stall speed. Now increast to 100% RPM at 4000 feet lets say you can only get 2300 RPM anything above 4000ft at full throtle would have deminishing returns untill you can no longer climb. Again this is your stall speed.The deminishing returns would be perdictable as long as sea level stays at 29.92 and 15⁰C and give you a usable power scale.

That article confuses me even further.. :( His first paragraph:

To get the most life from an aircraft engine while at the same time getting acceptable airplane performance the engine manufacturers commonly recommend their engines not be operated continuously over 75% of their full throttle rpm. We have thus come to accept this rpm as a standard "cruising" rpm. The problem is, how does the pilot of an airplane equipped with a fixed pitch prop know when he is pulling 75% since the only power-related instrument he has is the tachometer and the tach doesn't tell him directly?.

He's mixing power and RPM here. You've both correctly pointed out that RPM and HP are not linear (I think that's what he meant by engine-RPM and prop-RPM not being linear..as in changes in RPM do not yield linear changes in THRUST because obviously, engine and prop RPM are linear), so it's a given that 75% of FTRPM will equal 75% power at only ONE altitude (confirmed by any airplane's performance chart). You cannot use PTRPM and aim for a 75% setting for both of them.. right ?You can try to fly at 75% power.. or 75% FTRPM. It will take more than 75% of FTRPM to get 75% power as altitude increases.Is this excersize about calculating one against the other ? i.e.. figuring out the RPM needed for a target power percentage, and varying altitudes ? .. or vice-versa ?I'm not trying to be difficult.. I'm just not getting the point to this. And he further confuses me with this sentence:

Stick a little piece of tape on your tachometer at that rpm, and "cruise" at that power setting. In fact, stick several pieces of tape there, each at one of your chosen power settings. Now you can fly, knowing whenever the tach needle is on one of the marks, what percentage power you're pulling.

How can that be true, when we know that the percentage of power for a set RPM (tape mark) changes with altitude ?The only sense I can make of it, is that his chart can be used for an RPM that will yield a percentage of available power (which again is a function of altitude), but not an actual percentage of rated power. Again confirmable by any performance chart where you can see that 100% of FTRPM yields a power percentage reduced by altitude. I'm not sure where knowing what percentage of a percentage of rated power your using, would be useful.. unless you only fly on standard days at the same altitude.Maybe I need to re-read that article ...

Let me put it another way. Lets say your flying at 4000ft. Reduce power untill you can no longer maintain 4000ft and mark down the RPM. This would be the low end of you scale before stall speed. Now increast to 100% RPM at 4000 feet lets say you can only get 2300 RPM anything above 4000ft at full throtle would have deminishing returns untill you can no longer climb. Again this is your stall speed.The deminishing returns would be perdictable as long as sea level stays at 29.92 and 15⁰C and give you a usable power scale.

It's been a while since I flew a fixed-pitch-prop above 4000msl.. but I'm pretty sure full throttle (level flight)can get you up to (if not past) red-line RPM.. and I'm pretty sure that's the case at even higher altitudes. The same, thinner air that reduces MP, also allows the prop to maitain high RPMs. That's the main short-coming for a fixed-pitch-prop. In order to bring RPMS down to say 75% of FTRPM, you have to FURTHER reduce MP by closing the throttle.

It's been a while since I flew a fixed-pitch-prop above 4000msl.. but I'm pretty sure full throttle (level flight)can get you up to (if not past) red-line RPM.. and I'm pretty sure that's the case at even higher altitudes. The same, thinner air that reduces MP, also allows the prop to maitain high RPMs. That's the main short-coming for a fixed-pitch-prop. In order to bring RPMS done to say 75% of FTRPM, you have to FURTHER reduce MP by closing the throttle.

You are correct I was using that as an example. I'm glad I wasn't the only one confused by his article. The math seems to work like I originaly said however it somewhat does or does not make sense when I apply the values to other situations. I think what he is calculating is a percent of potential power besed on RPM and BHP. However as my test in flight sim showed even though I was at 2000ft with 2545 RPM my HP was 82 and his calculation seems to show 75% of 2545 RPM @ 2000ft or 2312 RPM. However his math does seem right when considering potential power and as I said could be used to plot an accurate power graph as long as you can assume 29.92 and 15⁰C. My goal would be to create a chart similar to the one I was testing against for planes I don't have charts on. You are still dealing with some percentage between 0% and 100% of the total power and then scaling the data based on pressure altitude, temp, weight, and mixture. You could still plot a curve for power by finding a low and high value then use his math to obtain a default value to compare each point on the graph.

It's been a while since I flew a fixed-pitch-prop above 4000msl.. but I'm pretty sure full throttle (level flight)can get you up to (if not past) red-line RPM.. and I'm pretty sure that's the case at even higher altitudes. The same, thinner air that reduces MP, also allows the prop to maitain high RPMs. That's the main short-coming for a fixed-pitch-prop. In order to bring RPMS down to say 75% of FTRPM, you have to FURTHER reduce MP by closing the throttle.

I fly a C172, a fixed pitch prop. When I calculate cruise fuel burn and sector times I look at the POH ( a required item on board, so it's always available), look at the cruise tables and pick my altitude that I have already planned ( in the Denver area here, aroumd 5,000' is 0' AGL), which will give me a selection of RPM's (these are correlated to HP for the altitude that I have selected), and each RPM will give me a specific TAS and a fuel burn (gallons/pounds per hour). I then calculate IAS from TAS for the altitude I am at to confirm that the engine actually is developing the energy that it's meant to. Works fine, even in instrument flight where you need to calculate sector times fairly accurately, and even make mandatory reports if your actual sector times are displaced from your planned ones (within a limit of tolerance). Of course, winds factor into this as well....But I have never done the math that is shown in this post, nor would I want to, the goal is to safely fly the plane. Is the OP asking the question related to FSX airplane design? I would like to help with an answer but am having trouble understanding the question....Thanks, Bruce.

I fly a C172, a fixed pitch prop. When I calculate cruise fuel burn and sector times I look at the POH ( a required item on board, so it's always available), look at the cruise tables and pick my altitude that I have already planned ( in the Denver area here, aroumd 5,000' is 0' AGL), which will give me a selection of RPM's (these are correlated to HP for the altitude that I have selected), and each RPM will give me a specific TAS and a fuel burn (gallons/pounds per hour). I then calculate IAS from TAS for the altitude I am at to confirm that the engine actually is developing the energy that it's meant to. Works fine, even in instrument flight where you need to calculate sector times fairly accurately, and even make mandatory reports if your actual sector times are displaced from your planned ones (within a limit of tolerance). Of course, winds factor into this as well....But I have never done the math that is shown in this post, nor would I want to, the goal is to safely fly the plane. Is the OP asking the question related to FSX airplane design? I would like to help with an answer but am having trouble understanding the question....Thanks, Bruce.

I guess my question is can I make an accurate power chart from the math for FSX planes (Not real ones) that I can't find a POH for online. I have many planes that are mostly accurate when following a POH however if I buy a POH for every plane that has no data I would be spending allot of money to fly safe in a simulated plane that will normaly be less accurate then a true POH for a real plane.I know in a real plane even more factors come into play like engine ware and thing like that. If you look at the carenado C152II that I use in my example there is almost no performance data to accurately plan and fly in a realistic way and isn't that the point of flying in the simulator? Why pick up bad habbits by flying wrong if I do ever decide to fly a real plane.

However his math does seem right when considering potential power and as I said could be used to plot an accurate power graph as long as you can assume 29.92 and 150C.

The atmospheric pressure at sea-level, and ambient temp at sea-level, are arbitrary. His chart still does not account for different altitudes (pressure altitudes).The best I can get out of his math; is that it allows for a conversion constant between RPM and thrust (i.e. a 25% change in RPM does not net a 25% reduction in thrust).. and even that seems wrong, because I doubt that would be a straight line in his graph.Or.. I just don't understand his premise..

I guess my question is can I make an accurate power chart from the math for FSX planes (Not real ones) that I can't find a POH for online. I have many planes that are mostly accurate when following a POH however if I buy a POH for every plane that has no data I would be spending allot of money to fly safe in a simulated plane that will normaly be less accurate then a true POH for a real plane.I know in a real plane even more factors come into play like engine ware and thing like that. If you look at the carenado C152II that I use in my example there is almost no performance data to accurately plan and fly in a realistic way and isn't that the point of flying in the simulator? Why pick up bad habbits by flying wrong if I do ever decide to fly a real plane.

What I've learned over the years.. is that performance charts (percentge of power), are more engine-based, not airplane-based. Of course the prop comes into play.. but an IO-360 in a Mooney, will have a near identical chart to another aircraft using an IO-360 and similar prop. So (assuming the models are accurate) for simming purposes.. you can transpose data.