Engine power output and adjusting it.

[anthony31]

I am starting on the FDE for a new aircraft and I have entered all the aircraft specs into the aircraft.cfg.

The engine is a Rotax which produces 100hp at 5800 rpm (i have the specs sheets for the engine output vs rpm) with a fixed prop.

The problem is that when I enter the "piston_engine" data the best I can get is 75hp at 6000rpm. I have tried adjusting every parameter in the piston_engine data including power_scalar but the only one that seems to change this fundamental fact of 75hp at 6000rpm is engine displacement. Everything else, even the power_scalar, only seems to allow the engine to rev higher and produce more power that way rather than produce more power at 6000rpm.

Messing about (using aam) with tables 508 and 509 in the air file also seem to adjust only how high (or low) the engine will rev and no matter what I adjust I always end up with 75hp at 6000rpm.

The only thing that seems to work is adjusting table 512, prop power required coefficient. By adjusting the curve upwards I am able to produce more power.

The question is, am I going about this the right way? Is using table 512 to get the required power output from the engine the right thing to do?

Thanks in advance for any help.

[Brett_Henderson]

Try using the standard air-file (un-modifed)... and use realistic numbers in the cfg file.. and THEN allow for the gear-reduction (I believe it's in the Propeller paragraph) combined with adjusting the fixed-pitch.

[fliger747]

Compression ratio will also adjust the delivered HP. I suggest looking at the engine parameters with AFSD which will show both the output and friction HP values. It may be that you can reduce the friction values either in the cfg or air files and change the max delivered output.

Worth a try: T

[Shane Olguin]

You can get more power by reducing the friction at the rated RPM. Go to table 509. The last value on the table will be your max RPM. X is the RPM and Y is the amount of friction applied. Gradually reduce the fricton value until you get the sort of horsepower you want at your rated RPM.

Lemme know if that helps.

[Brett_Henderson]

You really don't want to change much in the air file. If you start "cheating" the physics, ala artificially low friction.. you might as well just crank up the power scalars (prop and/or engine) in the cfg file.

The delivered power in the MSFS algorithm (assuming realistic parameters), is a function of manifold pressure and PROP rpm. It gets tricky with these Rotax type engines, because they have gear reduction between the engine and the prop. A Rotax red-lines at well over 5000rpm, where a Lycoming/Continental redlines at ~2800rpm.

A normal GA prop is in the 6' diameter range, so you can't have the prop going much over 2800rpm, else the blade-tips go supersonic, and the prop becaomes very inneficient. ... hence the gear reduction for engines that run well over 2800rpm.

So what you have to do, is plug the gear-reduction into the cfg file, and carefully experiment with blade pitch.

Also.. most of the data you'd change in the air file, is over-ridden by the cfg file, anyway.

[fliger747]

There ae friction/RPM eficency and torque curves in the .air file (501-508-509) which can be tweaked to adjust the power deliverd vrs RPM or MP to match known values. However you might also consider the mechanical efficency and friction scalars in the .cfg file:

max_rpm_mechanical_efficiency_scalar = 1
idle_rpm_mechanical_efficiency_scalar = 1
max_rpm_friction_scalar = 1
idle_rpm_friction_scalar = 1

Hard to guess at the exact issues, but looking at the friction and output values for the engine, as well as the delivered thrust and prop efficency values using a utility such as AFSD. With a fixed prop the prop tables may have a significant effect on the loading of the engine.

Cheers: T

[Ivan]

I believe Shane Olguin has a pretty good method with editing record 509. Besides the friction table, there is also an efficiency table (508) that can be adjusted. I believe the reduction gear ratio is specified in record 510 and also in record 505.

I don't know if the Rotax is supercharged, but you can adjust how quickly the power falls off at altitude by balancing the values in 508 and 509. There are many pairs of values that produce the same power at sea level and up to the critical altitude, but these pairs don't produce the same power past the critical altitude on a supercharged engine.

BTW, Record 512 needs to be reasonable for your engine / propeller combination. Sparks can easily explain it better than I can, but I see this table as describing the relationship of how the engine power / reduction matches the propeller that is installed. It doesn't really describe either the engine or the propeller but is more a description of the match between the two.

Hope that helps.
- Ivan.

[fliger747]

One thing to note in the editing of the engine RPM Friction/efficency tables is the lower end values will effect the startup torque required. To high a value and you might have to tweak the starter torque up, too low a value the prop may continue to spin on shutdown.

Also the Rotax, being a normaly aspitated engine with a fixed pitch prop... will have it's critical altitude at sea level. The prop tables (which are not as obvious) will have a big effect on the RPM's developed at various throttle settings and on the thrust delivered.

Have fun.... T

[Ivan]

Hi Brett_Henderson,
Do you happen to know the actual formula used by MSFS to calculate engine power? It would save me a lot of experimentation if I could use a formula to plot the power curves. I would imagine it has more to do with the Engine RPM rather than the Propeller RPM.

Hi Fliger747,
There are enough values in the tables (thought barely so) to alter the high rpm effects without really affecting the low RPM values. I plot the values on a spreadsheet when I rework these tables to make sure the curves look reasonable. The funny thing is that often if I try to match the output at lower RPM and manifold pressures as from a Specific Engine Flight Chart, often at full manifold pressure, the HP will peak at a lower RPM than maximum. I have run into this situation a bunch of times: most recently with the B-25C and A6M2 I am working on.

- Ivan.

[Brett_Henderson]

Hi Brett_Henderson,
Do you happen to know the actual formula used by MSFS to calculate engine power? It would save me a lot of experimentation if I could use a formula to plot the power curves. I would imagine it has more to do with the Engine RPM rather than the Propeller RPM.

No, alas I do not.. but it has to be some combo of MP and RPM..

On that note.. engine RPM and prop RPM are the same thing. The prop and engine are rigidly, mechanically linked.. even if by gear reduction like a Rotax or a big radial. As engine RPM changes, prop RPM changes by the same percentage.. always.

Like I've tried to point out.. if you're going to allow yourself to manipulate things like frictions in the air file, you might as well just use the power and thrust scalars.

I'd work with the gear-reduction, prop-MOI, and blade pitch in the cfg, as a first step.. and THEN "slightly" tweak the air-file. Starting in the air file is like pretending you have magical motor oil that can let you defy friction..

[Ivan]

Hi Brett_Henderson,

.... or big inlines.....

I play mostly with CFS1, so there isn't the CFG Parameters to manipulate. Messing with the AIR file is the ONLY way with the earlier sims. There are a lot more factors than just Manifold Pressure and RPM such as Compression, Displacement, Efficiency (508), Friction Loss (509), Supercharger values, and probably a bunch more that I either don't know about or can't recall at the moment.

I chose to distinguish between Engine RPM and Propeller RPM even though they are related because there are lots of more or less identical engines that differ in reduction gear ratios and propeller sizes. When you plug the RPM intio your equation, it will be the engine RPM and not the propeller RPM.

If you are starting with a given engine, you should use the REAL reduction gear and propeller MOI and blade pitch rather than tweaking those values to get a particular effect. I believe that most the factors you mentioned can limit RPM and thus horsepower generated at a particular speed, but don't really affect HP otherwise. I don't really see how prop MOI actually affects HP at all though it would affect how fast the prop spins up.

As I see it, the efficiency and friction tables are the correct means of adjustment because they are a summary of the cumulative effects of engine features which are not otherwise described. Perhaps your engine has cams, headers or a tuned runner intake that produces a torque peak at a particular RPM. I don't see how you can duplicate that effect without tweaking tables 508 and 509.

- Ivan.

[fliger747]

In many real aircraft engines max HP may not be developed at max RPM. There were many difference between the R2800 B and C engines. One significant one was that they were able to increase the max RPM from 2700 to 2800. At max RPM's the amount of HP lost through slinging oil about the crankcase was something on the order of 400 HP. by carefully redesigning the oil scavenging system they were able to reduce much of this loss and operate effeciently at both higher RPM and also deliver more shaft HP. I am currently working on the R2800 22W and 34W engines for the various SOH Tigercats.

It would appear that FS derives HP by taking the displacement, Compression ratio and friction values vrs RPM. For instance delivered HP can be adjusted by varying the compression ratio. Usually starting with the book figures for any given engine will give up pretty close values. I don't know anything at all about the ROTAX engines. Gas powered Aircraft engines tend to have fairly low compression ratios vrs automotive applications. Part of the reason for this has to do with the desireability of relaiability at fairly high power values, maintained for quite some time. Auto engines are by comparison usually run at low % of max power for most of their duty cycle.

Generally not too much varation from standard book engine values are required to at least touch the performance curves at several points. That you are dealing with a fixed prop makes things more complex rather than simpler. The prop loadings continuously and variably effect engine loading and RPM, dependng on altitude, temperature, airspeed and throttle setting. Way to many things changing each other!!

Good Luck: T

[Brett_Henderson]

Way to many things changing each other!!

That's the problem we deal with.. Not that that itself is a bad thing.. we'd want all the variables to be in play, and interact with each other.. but the MSFS algorithms aren't that accurate to begin with.

When you start adjusting things like internal engine friction, and compression ratios; you're messing with the foundation of an already suspect building. I long ago gave up on thinking that careful research and accurate numbers will result in accurate results... and so did MSFS.. that's why there's an entire paragraph in the cfg file named [Flight_Tuning].

I did not know that CFS has no cfg file.. so you're forced to tweak within the air-file.. and I'll defer to those with CFS tweaking experience. But you're still dealing within the limitations of the MSFS "engine" in general... So you have to decide what your goal is...

Do you want the engine itself (ala testing with a guage to read % of power), to have a realistic power curve ? Or do you want the model in flight to have realistic performance ? In MSFS (as opposed to CFS), the further away you get from a 180hp, light single; the more impossible those goals become... and the more cheating is required. I'd imagine it's similar in CFS.. and with no cfg file to play with.. you really do end up opening cans of worms while chasing your own tail.

What I've found in MSFS, is to set the internal stuff at real numbers.. and then work from the outside in. When you work from the inside out, it's like trying to change the appearance of a house via the foundation.

If there are adjustments (cheats) for just power and thrust.. work with those before trying to get the power accurate from inside the engine... and then in an attempt to get actual in-flight performance accurate, work with parasitic/induced drag.. play with gear reduction ratios and prop pitches, rather than frictions and compressions.. then, eventually you'll have to custom calibrate guages and gauge bitmaps for in-cockpit realism.

No matter how you cut it.. you're gonna cheat somewhere.. I just think it's easier working from the outside, in.

[Milton Shupe]

Just another thought here ...

Can you develop the air file in FS9 or 8 using Jerry's Airwrench (free version for single engine).

Then use the output air file in CFS, or the tables that are compatible.

[fliger747]

Indeed you might give Airwrench a try as the free default version is for fixed pitch props.

Perhaps not so useful an approach for the project under consideration... but the approach I use is to get the HP and Thrust right and then adjust the drag to match the known performance points (as best as possible). For turbocharged engines this works fairly well, but supercharged engines with an accessory stage (perhaps with high and low speeds) MS cannot make the zig zags without some sophisticated gauge work.

Airwrench will change EVERYTHING in your files, If this isn't totally satisfactory you can take the adjusted pieces (ie. engine etc) and transfer them into you main air/cfg files.

Good luck! T

[Ivan]

Hi Fliger747,
Do you have actual data for HP versus RPM for the R-2800 engines? I am curious as to where the peak HP is reached if it isn't at max RPM. I know this happens a lot with Automobile engines. It is also a good exercise to see if I can edit friction and efficiency to reproduce a given graph. At this point most of the data I have is from SEFCs from various manuals.

Hi Brett_Henderson,
I am not really sure what you mean about the MSFS algorithms being inaccurate. Other than being unable to represent multi-speed superchargers and their shift points, I don't see a big problem. I figure that as long as the power output at a given manifold pressure, RPM and altitude can be approximated, there really isn't an issue.

The oil slinging issue that Fliger747 described is basically an increase in friction at a certain RPM. If we reproduce that by increasing the friction in our flightsim "engine", how is that cheating? On the next model engine, if we installed a windage tray and reduced this power loss, what is wrong with adjusting the friction table to take this into account?

In real engines, there often is a major difference in efficiency at various speeds. With a cam with lots of overlap, you may have very poor efficiency and torque at low speeds and pretty good power and torque at high speeds. You can even exceed a 100% volumetric efficiency in certain RPM ranges if the harmonics in the intake and exhaust are just right. What is wrong with simulating these features with the efficiency table?

- Ivan.

[Brett_Henderson]

If the algorithms were accurate, we wouldn't be having ths discussion

Think about it .. The designer provides displacement, number of cylinders, and compression.. and THEN is asked to supply a HP figure ..

Bore and stroke (biggest factors for a power curve) aren't even used... :isadizzy:

Like the flight model that won't stall accurately.. the engine model is very crude.. To think that all these tables have predictable, realistic effect when you start tinkering with them, is naive. The crude engine model works best if left alone, and the tinkering is best done "outside" of it..

If you really want to play with torque and power curves,,, you're best off designing gauges that read environment variables, and send performance paramters back.

[fliger747]

Ivan:

Of course the various R2800 models, especially the B and C models have a lot of difference in HP vrs RPM/MP etc. Each instalation varied in intake system and super/turbocharging. I am on the road (VHHH) at the moment and have no references available. I did see some engine data on the net recently for the F4U installation. I know we also had this info available for the A26 project. For the 22 and 34W engines used in the F7F I hope to have the info available soon, presuming the ordered pilots handbook arrives.

Bore/stroke are included to some extent in the calculations as displacement and compreesion ratios are considered. Most classic air cooled piston aircraft engines have similar bore/stroke ratios, close enough to allow a basic calculation that can be tweaked.

A certain degree of precision is possible, but only a certain degree. It is possible that the variation in performace between in service aircraft may be as great as our ability to approach the "book figures" America's Hundred Thousand is an interesting work in that where available both Factory and service test data are compared. The difference can be significant!

Cheers: T

[Brett_Henderson]

Bore/stroke are included to some extent in the calculations as displacement and compreesion ratios are considered. Most classic air cooled piston aircraft engines have similar bore/stroke ratios, close enough to allow a basic calculation that can be tweaked.

Agreed.. However.. displacement is the geometric volume of an imaginary cylinder; defined by bore/stroke. And compression-ratio is the ratio between the volumes "above" the piston; at the top and bottom of the stroke. It's determined by the piston size/shape... ie. you can raise compression-ratio by swapping in "high compression pistons", without changing displacement.

If we assume that the MSFS "engine" uses a generic bore/stroke so that displacement can define the bore/stroke, then we're admitting THE most important set of variable for a specific power-curve, aren't variables at all.

If you try to shape a curve "internally", you're assuming that the MSFS "engine" actually (and acccurately) takes those type of variables into consideration.. It doesn't. Sure, things will change as you play with that stuff, but not realistically. The model isn't that sophisticated.

If your end goal is realistic, in-flight perfromance.. you're better off leaving the guts alone, and working with prop-pitch, gear-reduction, and the brute force, power/thrust scalars. All you'll really "see", or even be able to document; are airspeeds per power-settings. You can sit down and make a chart, documenting airspeed by power-setting, but a rate of change in airpseed over a range of power settings (the curve) would be next to impossible to document.. It's great mental gymnastics to attempt, but not really achievable. It's all kinda like groping in the dark. I've found that you do most of it form "outside" the engine, you aren't chasing as many tails.

Now again.. it might be different for CFS, but I got a feeling they're the same "engines".. and these are just my experiences.. not meant to be argumentative.. I'll be looking forward to the results :salute:

[anthony31]

Thank you gentlemen for your input on this subject. You have certainly given me some good places to start looking.

I think I need to do some more tinkering to find out what does what especially in the .cfg file.

I have filled out the .cfg file with all the data for the engine and prop (compression ratio, capacity, gear reduction, prop size (this can have a big effect on the engine!) ) etc etc.

The Rotax is a 82.6 cu in 4 cylinder / 4 stroke engine producing 100hp at 5800 rpm and about 90hp at 5200 rpm (cruise speed), compression ratio 10.5 and an inbuilt gear reduction of 2.43.

As it isn't turbocharged am I correct in assuming that the manifold pressure values are not used?

[stansdds]

If it has neither a turbocharger nor a supercharger, then the manifold pressure would actually be a vacuum, just like most gasoline powered passenger car engines. Turbochargers and superchargers both force air into the manifold, changing manifold pressure condition from a vacuum (negative pressure) to a positive pressure.

[Brett_Henderson]

If it has neither a turbocharger nor a supercharger, then the manifold pressure would actually be a vacuum, just like most gasoline powered passenger car engines. Turbochargers and superchargers both force air into the manifold, changing manifold pressure condition from a vacuum (negative pressure) to a positive pressure.

It's never really a vacuum (zero pressure), but it's always less than atmospheric (sans turbo/super charging), except when the engine isn't running. If your MP pressure gauge were more accurate, you could use it to set your altimeter, before starting the engine (adjusting for field elevation)... because it's really just a barometer for the inside of the intake manifold

[Brett_Henderson]

Side note on the Rotax.. My very first freeware model was a Europa-XS (FS2002). I gave up on trying to make it work.. and just put a 110 HP Lycoming in there.. And oddly enough, some years later, our club bought a Liberty-XL2 (essentially an factory built Europa), with an IO-240 in it.


Edit: so.. you could just us a standard engine (down-size and de-rate the engine out of the default C172), and then customize the tachometer to display the Rotax-like RPMs..

[Ivan]

I will try to build the Rotax engine as an exercise, but I need a couple more details:
What is the cruise and maximum manifold pressures?
What is the prop diameter and reduction gear ratio?
What is the pitch of the prop and is there a measured static thrust at a given power setting?
Is there a torque or HP to RPM graph that I can try to match?

- Ivan.

[Brett_Henderson]

I will try to build the Rotax engine as an exercise, but I need a couple more details:
What is the cruise and maximum manifold pressures?
What is the prop diameter and reduction gear ratio?
What is the pitch of the prop and is there a measured static thrust at a given power setting?
Is there a torque or HP to RPM graph that I can try to match?

- Ivan.

Manifold pressure is weather and altitude dependent.. I.E.. if you're at sea-level, and the atmospheric pressure is 29.92, your theoretical MP limit is 29.92 (with a theoretically perfect induction system). As you climb the available MP decreases. As for what YOU set MP at.. it's a non-issue with a fixed-pitch prop. The throttle is your only means for adjusting RPM, so you never get the most out of the available, atmospheric pressure. With a constant-speed prop you'd have the throttle wide-open at cruise altitudes, and would pull the RPMs back with the prop-control. You lose about an inch per 1000 feet, so at 8,000, a wide-open throttle would only get you ~22 inches of MP.

Just use published numbers for the prop diameter (double-checking so that a prop-strike doesn't happen with hard braking).

The gear-reduction is published too, but trust me, you'll be tinkering with that.

Same for prop-pitch..

Trying to match a realistic power curve (via a chart) is admirable, but it'll be problematic when you try to get realistic performance out of an engine that lives up to that graph (if you could even document that it does)(see the testing post itn the other thrust thread)