realistic leaning of piston aircraft: an approach to finally get there

[AndiKunzi]

I consider P3D an excellent platform for serious training. However, there is still one huge bug: leaning of piston engines, especially of fuel injected and turbocharged engines.
For Lockheed Martin, it should be easy to fix this major bug. This should have priority to (constantly improved) visual effects, ATC and other details not essential for serious training. Leaning is! Most pilots do meet fuel-injected or turbocharged aircraft in flight training and or career.

I am a real world pilot (CPL, SEP + MEP, IR) and a mechanical engineer. I try to post a brief summary of what the sim is doing and how that can be simulated correctly.

Leaning is an essential part of operating a piston engine aircraft. Leaning for fuel injected engines and for turbocharged engines is different than for carburetor engines.

At non-regulated carburetor engines, air flow at the regulating venturi is at ambient flight altitude pressure. Fuel flow is proportional to the air speed in the venturi. This is well simulated in P3d.

At turbocharged carburetor engines, air arrives at the regulating venturi at a higher pressure than ambient pressure. Venturi is after turbo, but before throttle.

At all fuel injected engines, irrelevant of turbocharged or normally aspirated, fuel flow is regulated by air mass flow. Fuel-air-ratio is constant with changes in altitude and power at a given position of the mixture lever.

abbreviations:
FAR = fuel-air-ratio
MP = manifold pressure [in hg]
AP = ambient pressure [in HG]
FF = fuel flow [GAL/h]
f1(), f2() = simulator specific arithmetic functions or tables, algorithm not exactly known
k1 = simulator specific constant
shp = engine shaft horse power [hp]


Sim vs. reality:


1. What P3D is doing:

P3D is adjusting the FAR with altitude, which is wrong for turbocharged and also for fuel injected engines.
FAR = 0,0873 * f1(mixture position) * (1013 hPa / AP); this is correct for carburetor engines
FF = f1(shp); FF about proportional to shp; this is incorrect for rich of peak, especially for take-off power = full rich (failure: about factor of 1.35)
Full rich fuel flows are way too low for all piston aircraft.


2. What P3D should do:

2.1 fuel injected engines:
FF is regulated by air mass flow, thus altitude compensated.
FAR = 0,1200 * f2(mixture position); full rich must lead to much higher fuel flows, about 0.10 GPH/hp (normal aspirated) to 0.11 (turbocharged) (see handbooks)
FF = k1 * FAR * (MP/MP max) * (RPM/RPM max); FF cannot be calculated based on shp

2.2 non-regulated carburetor engines, turbocharged:
FAR = 0,1200 * f2(mixture position) * (1013 hPa / MP); AP substituted by MP
FF = k1 * FAR * (MP/MP max) * (RPM/RPM max)

2.3 non-regulated carburetor engines, normally aspirated:
FAR = 0,1200 * f2(mixture position) * (1013 hPa / AP); despite full rich, the FAR formula is identic to present P3D
FF = k1 * FAR * (MP/MP max) * (RPM/RPM max)

Table 507 in the xxx.air would have to be amended for about 3 % power loss at full rich / take-off power compared to best power setting, with take-off power being rated power.
The rich setting at take-off is to prevent detonation and is necessary for cooling. Rated power is achieved at full rich mixture.

Sources:
Operation Manuals of Continental Aero and Lycoming. AFM / POH of PA34-III, C303, C340A, C421C. Copyright, thus I can't offer them for download.
Further information for a general overview: http://okigihan.blogspot.com/p/fuel-inj ... stems.html
(And wikipedia, of course.)


3. How to not ground the whole fleet of aircraft?

Implement additional fuel metering types 3 and 4 in aircraft.cfg:
fuel_metering_type=4 //0=Fuel Injected, 1=Gravity Carburetor, 2=Aerobatic Carburetor, 3=realistic Carbureter, 4=realistic Fuel Injected
Use the new formulas (above) only for the realistic types 3 and 4. Thus, old planes will still work, but can be easily adapted to the new formulas.

I think that for the product claim of P3D, Lockheed Martin should not accept substantial bugs in simulation.
If you should have any questions, please let me know.

[funkyaz77]

+1 Andy, I hope LM listen to your detailed notes, again its all about flight dynamics and modelling.

Blessings

Duncan Odgers

[sibbe83]

I totally agree. This bug has been in P3D for as long as I can remember. And one of the reasons I only fly A2A GA airplanes. Simply because they have their own external physics and engine simulation which bypasses P3D's own, which is quite buggy with several issues. This bug along with several others should be fixed before introducing any new features in P3D. You also have the RPM/MP bug, which is still present after so many years.

We need a realistic simulation. I've already contributed here with my list of buggy and faulty simulation in P3D, so I'm not going into details. My hope is that this will get fixed and improved.

Jan

[Tim-HH]

I second that request!

A professional flight simulator like P3D should provide a realistic leaning of piston engines. The requirement to constantly lean turbocharged engines during climb is a very old bug from the MSFS codebase and should be ironed out quickly. It is an important requirement for every simulator that these core parts of a simulation are correctly modeled. Especially if the simulator is used for professional flight training.

Greetings
Tim

[Rob McCarthy]

Hello Everyone,

We'll investigate this more on our end and get back to you with more information.

Regards,
Rob McCarthy

[funkyaz77]

Thanks Rob for listening to the community, proper private pilots and qualified engineers and hopefully finally getting somewhere to improving and fixing current flight dynamics modelling!!

Kind regards

Duncan Odgers PPL