Piston engine variables

[paulopp]

Greetings everybody,

I intended to install a two-stage supercharger to JBK's DC-4/C-54. For the appropriate engine data I consulted the TCDS (Type Certificate Data Sheets) hosted by the FAA for type ratings and compressor ratios. However, when coding the piston engine variables in the aircraft.cfg I wondered about how to implement them correctly after consulting the FS9 SDK. What worries me specifically are the variables "turbocharged=", "max_design_mp=" and "critical_altitude=" in conjunction. As far as I understand it the boolean for the turbocharger determines if the max_design_mp is maintained up to the critical_altitude, from which on the manifold pressure will decrease with gaining altitude. So far, so easy. JBK's DC-4-1009 is encoded by FSAviator with a design_mp of 50 in.hg and a critical_altitude of 9500 feet as an "as if" figure simulating the supercharger already in place. I took these figures as TOGA ratings. That means that up to 9500 feet the engine would - in plain theory of course ignoring engine wear - produce steadily 50 in.hg mp in TOGA configuration, and it actually does so in simulation! Now, when looking up the engine data for the default R-2000-11M2 which only has a low blower in real life, I find that in TOGA mode the 50 mp are only achieved at sea level whereas it is 49.5 at 1000 feet (see TC 5E-5).

Now, am I correct or mistaken in my assumption, that to "reallistically" implement those TC data for a two-stage supercharged, say, R-2800-11M3 (as -11M2 but with high impeller gear) in FS I encode them as follows in the piston engine data section of the cfg:

turbocharged=1
max_design_mp=50 //49.5 at s.l.
critical_altitude=1000 //or less to account for the .5 drop in TOGA mode
emergency_boost_type=1
emergency_boost_offset=0
emergency_boost_gain_offset=0.3315 // (9.52/7.15)-1

I compared the cfg with those of the default DC-3 and PILOT'S B314. The Austrians e.g. have rated their Clipper's GR-2600-A2 radials with a critical_altidude of 0 which means, that mp will drop instantaneously as altitude is gained even with full throttle applied. This is something I would rather expect than a critical_altitude of 7000 feet upon which the DC-3's radials steadily deliver 47 (48) in.hg MP.

The expected "simple" task of adding a hi-level supercharger to the DC-4 now has led me to the question of how one does realistically implement real-world piston engine data within MFSF? Thank you in advance for all your much appreciated advice!

Kind regards,
paulopp

[Defender]

Hi,

Like you, I'll be interested to see any input from others more knowledgeable but what I've gathered so far over the years is that MSFS can't replicate the behaviour of most big prop engines because it's set up for turbocharged engines (like the B377's engines), and not supercharged engines (like nearly every other big prop). Consequently setting critical altitude is possibly trial and error and each developer may have a different approach to aircraft.cfg settings as well as operating instructions to the user. 

I think I'm correct in saying that MAP will always drop with pressure altitude but the BHP (and therefore BMEP) should remain constant up to critical altitude. If you have any of Manfred Jahn's Connies you will see they include a "Status Gauge" which shows BHP. Many of its functions, including BHP, seem to work in other aircraft with the Wright R3350 engine. Now, it will not work accurately with the R2000 because that engine will have a different BMEP constant, but if you note what BHP you get when you run up TO or METO power at sea level ISA using the published RPM and MAP, then see what altitude you get to before the BHP drops, that might help you play around with the effect of different critical altitude settings.

As I said, I'll be interested if anyone can better explain MSFS critical altitude variable settings.

Bill   


   

[Tom/CalClassic]

Hi,

The way that FSAviator told me to set up the high/low superchargers.

1. As you say above, he has the plane set up so the high supercharger is already used at high altitudes - the only thing you will be doing in this conversion is to activate the LOW supercharger setting (that will have limited power as you climb). So adding this feature should NOT change the behavior of the plane at high altitudes.

2. Because of this fact, the result you should end up with will be equal performance at high altitude at high supercharger, with the added feature of limited power at low supercharger.

3. Using the existing FD (leave all lines in the engine section as they were originally), find the highest climb band in the reference file and get yourself stabilized in flight within that altitude band (use an altitude that you would typically cruise at in high supercharger).

4. Apply the mandated climb RPM.

5. Apply Full Throttle (yes, full throttle)

6. Measure the fuel flow.

7. Save a Flight so you can use this later.

8. Now edit the line to: emergency_boost_gain_offset=0.3315 // (9.52/7.15)-1

9. Edit the line to: emergency_boost_type=1

10. You can estimate a new (lower) critical_altitude line, but we will need to fine tune that.

11. Save the cfg file.

12. Reload the Flight you saved in step 7 to reload all variables and the new cfg file.

13. Set the superchargers to HI.

14. Check the RPM and that you are at full throttle, and measure the fuel flow. If it is not the same as in the first test:
a. Change the critical_altitude value and save the cfg file.
b. Reload the Flight
c. Measure the fuel flow.

15. Repeat step 14 until the fuel flow is the same as it was in the original FD.

BTW, horsepower in FS in piston engines is 2 * fuel flow / fuel_flow_scalar. Now you have a plane that will cruise the same at high altitudes (i.e. develops the same horsepower).

Note that the resulting critical_altitude value will probably be lower than in real life, since you are compensating for FSAviator boosting power at high altitudes to simulate the high supercharger setting.

Hope this helps,

[paulopp]

Thanks for the reply. So, this is a kind of putting-the-horse-before-the-cart process. Well, in the absence of an sufficiently accurate MS s'chrg engine simulation there seems so be no alternative than to fine-tune engine settings as laid out. I will try that, thank you, both, for your effort.