View Full Version : Engines, piston, other than normally aspirated.

January 25th, 2011, 11:22
A place for some discussion about how FS models piston aircraft engines. With FSX some degree of modeling of supercharged planes is apparently possible. I haven't seen anyone put together a good working model as yet that mimics how the WWII era aircraft with multistage blowers using this system without extensive gauge work.

FS does a pretty god job of modeling a turbocharged engine with automatic controls. to set this up about all is necessary is to get the sl HP and mp correct and set the critical altitude in the CFG. File. The HP delivered as altitude increases is pretty constant as altitude increases up to the critical altitude. For the real world aircraft the max boost available was determined by such things as turbine speeds and temperatures.

For the supercharged engine, typically there was a basic supercharger stage and an auxiliary blower that could have several gearing speeds to provide additional boost as Tyne aircraft climbed. FS does not model this well. At sl the engines were run with just the primary blower, perhaps up to 5000 ft or so, at which it was advantageous to shift into low blower, which could maintain rated MP up to some altitude, say 12,000 ft for discussion. Above that high blower could keep the max MP to some altitude in the low-mid twenties. The aux stages used power from the engine, soot was best to not shift to a higher and more power robbing stage till necessary.

The supercharged engine produces a power vrs altitude curve which is a zig zag. So far the best compromise for supercharged engines has to been to assign them a turbocharged status and get a best fit for the points of the zig zag, at least anchoring e power/speed points at sl and critical altitudes. This does generally give reasonably accurate speeds for the aircraft. Some have gone with the idea of simply starting out with a huge possible MP value at sl which will diminish to rated HP at critical altitude. This depends on the user to manage the Mp to a reasonable value at lower altitudes. However the engines did not work like this, some FS planes have developed 4-5,000 HP out of a 2000hp engine at sl using this system. It is not how the engines worked.

If someone has figured out how to make the FS system work it is a good place to start the discussion.


January 25th, 2011, 17:11
All I really know about this stuff, is that super-charging is a mechanically driven compressor, and turbo-charging is an exhaust gas driven compressor. The advantage to supercharging, is that there is no "turbo-lag"; the time period where you wait on the exhaust driven turbine to spool-up and drive the compressor turbine. The lag is modeled pretty well. The advantage to a turbo-charger, is that it's more efficient, once it spools up. It's not a mechanical load on the engine, like a super-charger, and it actually avails wasted energy that normally just goes out the exhast plumbing.

I think I can visualize a multi-stage super-charger.. kinda like two mechanical compressors, where on can be left in "neutral" until some sort of clutch engages it ?

I know that combinations of super and turbo charging were used. The A2A 377 models it on those big R4360 radials.. I'm just not certain if it's realistic. From what I can make of it, a super-charger is at work on takeoff, and the the pilot contols waste-gates on a turbo-charger to "blend" that charging as altitude increases. It will definately punish you if you don't use that option wisely.. they get too hot if you over-use them, and rob HP if you under-use them. I'll have to confess that I turn that task over to the virtual co-pilot... there's just too much going on.

As for ways to achieve realistic, complex-charged MP ? I'm sure it can be done. Just use an invisible XML gauge that reads MP/altitude/RPM, and forces an adjusted MP.

This should be an interesting discussion.

January 25th, 2011, 21:57
Generally. A clutch Was used to shift the aux blower in and also to shift it to high speed gearing at altitude. For the R2800 a Full 400 HP was swallowed up by high blower. This is the reason the HP output of a 2000 HP engine was only 1600 at altitude with the same sl mp, the blower was sucking away power.

I have some recollection that someone did a shiftable blower for a typhoon for FS9 some time back. Unfortunately every time someone does this sort of thing it is a one off gauge.

It would seem off hand that the turbo would ve the way to go, but they had a lot of weight and complexity and were actualy at a disadvantage low down. The USN tried a turbo in the f4U-3 but did not follow up on the project. Combat in the pacific was seldom over 25,000 ft.

Notable that only supercharged radials are in service today, the KISS principle won out in the longevity race.


February 2nd, 2011, 12:00
For anyone who is interested, a few words as to how I set up an engine.

First the max RPM, displacement (per cyl x # cyls), compression ratio, go in the .cfg file. The specify Max MP, critical altitude (which for a supercharged engine will be the high blower alt) and minimum MP. The HP entered value really has no effect on HP but affects the sound levels.

Props, derive the prop table using whatever methods you use. I often start out by stealing one from some previous plane I have done, such as on of the Corsairs and later modify as necessary. In the cfg file max, min pitch values are specified, for most WWII type fighters 15-65 deg usually is a decent range, and for a multi, feather pitch is specified, usually about 88.5 deg. Low speed theory seems to work well is specified at 80 knots. The planes I am considering have constant speed props, the constant speed range should start somewhere around 1400 RPM.

Get this all running and then comes a more mysterious part, lots of testing. I use AFSD, from Herve Sors (Google) to test HP and thrust. The critical part of arriving at the proper relationship between MP and HP is adjusting the 509 table, which governs internal friction vrs RPM. Changing these values (I use Aircraft Airfile Manager) determines the HP delivered at a given MP RPM combo. Too much HP, ya need more friction etc. The values at the bottom end determine how the engine spools down. They may also affect required starter torque and idle RPM.

A turbocharged engine, such as the installation in the P-47 D or the P61-C, E is actually easier to model. Both of these planes were fitted with an automatic control that provided overboost protection and regulated the throttle plates and waste gate to maintain a desired boost related to throttle position. It was not generally necessary to keep adjusting the throttle position to maintain a selected MP with altitude change. FS works like this, fortunatly if the critical altitude is properly specified. I fly these planes with automixture turned on, as the auto rich, auto lean system was used almost all of the time.

Lots of small items to tune, specific fuel consumption, Oil P values, CHT etc.

Get the HP right, then all that is required is to set the drag to get the speed correct. Two things here, CDO in the Air file and mach drag rise for faster planes at altitude.


February 9th, 2011, 10:46
A few more thoughts:

Supercharged engines with multiple stages and or gear ratios used more shaft power at altitude. For the R2800 high blower sucked up some 400 HP! So a nominal 2000 hp engine at Sl delivered only about 1600 HP at say 22,000 ft in high blower.

However FS, using the turbo model to simulate supercharging, as described earlier will have a constant HP output to the critical alt. Examination of speeds, both SL and critical altitude usually will reveal an almost correct ratio of MP/RPM's to true speed delivered for the altitude. In a supercharged aircraft, the stack gasses, especially if the collector/augmentor is well designed, will contribute quite a bit of thrust at altitude, sometimes as much as equivelent to 400 hp/engine at full throttle. So in actuality we are often not far off the mark, if by accident.

Of course in final totality one must filter in the mach drag rise for the faster aircraft. The ramping up and relative delivered drag rise values must be in proper proportion to the overall form and induced drag. The check on this is whether or not the SL and crit altitude speeds both hit hte mark. The point of the rise can be used to tune aircraft SL vrs speeds at alt, but needs to be done in a manner that reflects the actual aircraft capability. Lots of experimentation!!

Cheers: T

June 24th, 2011, 19:55
Notable that only supercharged radials are in service today, the KISS principle won out in the longevity race.


I believe the main reason why Turbos are disabled on surviving Warbirds is because they had a tendency to catch fire every once in a while. Under Combat conditions, this was an accepted risk with the consequence being for the pilot to abandon the aircraft. There was no other solution for a turbo fire!

These days, only mock, scripted "Combat" is flown and safety trumps all. There is no need to risk a veteran Warbird just to have a little extra power at altitudes where spectators can't see them. Besides, even a P-47 with a Turbocharger also has a regular engine driven supercharger.

By my understanding of Turbocharger versus Supercharger in an AIR file, a Turbo maintains a constant power output up to the critical altitude. A Supercharger will increase power slightly from SL to low altitude.

- Ivan.