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CrashTronic

[XPLANE] DC-6 Supercharger inquiries

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Good morning/afternoon all,

I was previously a DC-6 owner in FSX:SE and now the same in XPLANE 11. I know the DC-6 was developed for 10 but I accepted the risk and so far have been loving it. A quick question. I noted somewhere, I can't find it now after searching, a reference to the supercharger switches not really doing much in xplane. I encountered the same issue, a slight blip in MAP but ultimately it returned to the same value. That behavior is vastly different to the FSX:SE experience and I was wondering if this was an XP11 or an XP thing in general.

Quote

Engines with mechanically-driven centrifugal superchargers (as used in most Warbirds) have to be approximated by selecting a critical altitude and maximum power combination the real engine was capable of. Below critical altitude, the simulated engine will develop a bit too much power since the power loss due to throttling back is not considered.

...

Two-speed (or three-speed) superchargers are difficult to simulate correctly because they have two (or three) different combinations of critical altitude and maximum power. Select the high critical altitude/low maximum power combination to get the top speed at best altitude correct, or select the low critical altitude/high maximum power combination to get speed at sea level and take-off performance correct. Or select anything between for a compromise.

http://xplane.wikia.com/wiki/Engine_Specs

Additionally I noticed that the BMEP values under the same atmospheric conditions are a bit different between the 2 sim versions, not that one is more right than the other, because I honestly don't have data to make that conclusion. What might cause the discrepancy?

Thanks for your attention to this post,

 

Mike

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Mike: The PMDG DC6 for XPL is for 10 only.  I assume that now that the P3D version is out and running they will now go back and revise the XPL version up to 11.  In your defense, the product page was written before 11 and still says 10.45+ but as you know the XPL developer tends to make changes to the core of XPL with every major release, which is what has happened.

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Thanks Dan. I was wondering about that and I think XP may become my primary sim with how nice it is and I would thoroughly enjoy an update to it. The CRJ may be the cutie at work irl but the DC-6 is my true love back home. :laugh:

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I've had an interesting revelation regarding BMEP, and MAP. This may end up as a string of consciousness so hang on.

Hypothesis: Something isn't right with the engines supercharger high/low vs BMEP

Definitions

1)Mean Effective pressure mathematical definition: 2Pi*torque*number of revolutions per power stroke/displacement

The result of the above is for a given engine the only criteria for MEP or BMEP is the amount of torque the engine crankshaft is stressed to. In the Prop world this is a function of induced and parasite drag on the prop vs amount of power produced by the engine. From the pilot's perspective regarding an operating engine this means there is a proportional relationship between throttle, blade angle and torque on a given engine.

 

I tested the DC-6 and found that the reported BMEP numbers are correct but the problem is elsewhere in the engine sim dealing with the MAP and the supercharger setting. going from Low to High appears to only provide a manifold pressure boost with no corresponding fuel flows to increase power. The effect is the MAP is raised but the power output remains the same requiring a lower throttle to maintain the required MAP to keep from damaging the engine but as you do so, fuel flows are reduced, power is then lessened and at any given MAP you end up with less power at the same MAP in the High blower than you do in the Low blower

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I can confirm there is something different between XP10 and XP11 that handles the engine differently. I have posts out to find out what it might be.

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Indeed, I can't fly long distances at very high altitudes anymore in XP11. Please devs, give us a hand, at least a hotfix, I don't care right now about PBR but the plane to flight accordingly... thanks,

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Posted (edited)

So I've tested pretty extensively in XP10/11 and while not particularly helpful, it appears to be either the new prop physics for some reason or the fuel leaning in the carbs or the code dealing with supercharger switches. I am unable to locate a detailed change log for XP11 aside from the release notes so I really can't go further in the research.

If this where a real aircraft I would say the effect appears to be the result of one of the following:

the carburetor being located before the supercharger but the MAP gauge probe  being located after the supercharger.

======CARB====Supercharger=========MAP======ENGINE

or

possibly more accurately it is like the carburetor is only a throttle body and fuel is being metered at the carburetor but mixed with air aft of the supercharger. This would be most synonymous with a fuel injected system metering fuel using mass flow data from before the supercharger, not after.

======CARB====Supercharger==(fuel)===MAP======ENGINE

 

which is contrary to the actual manner the system should be operating in, again, not through any direct errors made by PMDG but due to a shift in the X-Plane 11 code revision from 10.

 

pp31_singstage.jpg

 

Thanks,

 

Mike

Edited by CrashTronic
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Evidently I can no longer edit posts or I am having an error between monitor and keyboard

Correction:

Quote

the carburetor being located before the supercharger but the MAP gauge probe  being located after the supercharger.

======CARB====Supercharger=========MAP======ENGINE

is exactly the way as in the image at the bottom of the above post. So it clearly can not be an issue with my hypothesis as the quoted configuration

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My understanding of the system is that the Carburetor meters fuel flow based on sensed airflow through the carburetor. Pressurized and metered fuel is then directly injected into the inlet of the supercharger impeller. At this point, the impeller mixes the fuel with the air charge, while accelerating the combination. The accelerated fuel/air enters the diffuser, where velocity is traded for ambient pressure where the indicated Manifold Air Pressure is measured.

I believe the proper configuration is ======CARB===(fuel)===Supercharger===MAP======ENGINE

In your scenario, a MAP increase with no increase of FF means that the engine is now running substantially lean compared to before the blower switch.

Robert Toten.

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Hey Robert,

You are correct. the fuel is injected in the Carb which is why in the other examples I don't denote the fuel location. And it does appear the engine is running more lean, but in the sense of the weird dynamics of perfect sim combustion. There is no longer a stoichiometric balance between fuel and air so the excess air is ported out the exhaust but the same power is produced. Rather than the usually observed scenario of, same air and reduce the fuel which reduces power. The same potential is being injected into the cylinders but there is now an excess of air which is not being used for combustion.

Which is why it feels like a fuel metering issue in the real world sense but in the coding sense is probably something else

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I got up around 20k and turned on the hi blowers and did in fact see a rise in hp, but it also boosted my MAP, which I expected.  Once I lowered it my hp did seem to be lower than in low blower.  Another thing I noticed, according to the charts for cruise, at 20k you can achieve 1200 bhp with 2300 rpm and 147 BMEP, however in order to achieve that bmep you have to push the MAP all the way to the redline.    I can't see any settings in Plane maker that seem to be able to fix any of this

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Good to know. Since the aircraft was made for X-Plane 10 we weren't guaranteed functionality in 11. I can't locate a detailed change log regarding the new changes and as an additional complexity the supercharger code is probably in the Pmdg dll. 

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Yeah I've scoured plane maker and can't find any settings regarding a supercharger.   Looking around for info about superchargers in XP I found something that said they don't model superchargers per se, you just have to play around with the critical altitude.  So yes the SC operation must be controlled by plugin. I was able to make adjustments in plane maker to get much more accurate fuel flow rates in xp11,  didn't have time to see if this improved supercharger behavior though.  

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Cool if you get a chance, post what changes you made and the effect.

 

Thanks!

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Well adjusting the SFC didn't have any effect on the supercharger.  The same behavior remains, boost to MAP but no change in HP output of the engine.   At least the fuel flow values are a little more accurate now though.  The numbers I'm using are .850 for max power and .425 for half power.  Same values for hi and lo altitude.  

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