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Meredith Effect - Is this modeled?


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No idea if its modeled and never heard that name used for the effect

 

I came across it before, when reading about the work done to reduce induced drag from both radiators and radial engines.

 

I seem to remember the NACA cowls used on radials employed a similar phenomena to reduce the drag on radials, which were a revelation at the time.

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This effect is not specially modelled... it appearas naturally from the radiator model.

 

The cooler can not only reduce it's drag for zero but under certain conditions can add thrust. But the conditions are - full power, max speed and closed scoop.

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Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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  • 3 years later...
This effect is not specially modelled... it appearas naturally from the radiator model.

 

The cooler can not only reduce it's drag for zero but under certain conditions can add thrust. But the conditions are - full power, max speed and closed scoop.

 

P-51D_15342_Shutter_Calibrations.thumb.png.60b9ac2b1be87f2a3ba652ada85fd059.png

What do you make of this? In P-51 ingame I reach my top speed with the coolant and oil flaps closed, but that doesn't follow the chart.

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  • 6 months later...
  • 7 months later...

If the effect is modeled, why does DCS then not follow the chart I posted above?

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1 hour ago, Magic Zach said:

If the effect is modeled, why does DCS then not follow the chart I posted above?

I don't see any reference in the charts, but anyway, did you match ISA conditions and everything? Indicated airspeed usually doesn't match charts at all unless everything is in the right atmospheric conditions.

 

S!

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  • 4 weeks later...
On 6/22/2021 at 1:38 PM, Ala13_ManOWar said:

I don't see any reference in the charts, but anyway, did you match ISA conditions and everything? Indicated airspeed usually doesn't match charts at all unless everything is in the right atmospheric conditions.

 

S!

I have matched the aircraft conditions as listed on the chart ("clean" is with empty bomb racks).  2700RPM, 46".  And while not written on this paper, the aircraft weight was 9760lbs, which I replicated as closely as I could within DCS.  However, I'm unable to get the weight of the bomb racks (though if they're like the A8's bomb racks, in DCS they're weightless).  Weather conditions to no crosswinds, 29.92Hg, 15C.

The chart lists the coolant shutter's recorded position at 0%, 30.6%, 63.9%, and 100% open.  In order, the temperature for each position from the chart is: 130C, 99C, 80C, 72C.
Within DCS, at the same shutter positions, our coolant temperatures are: 116C, 99C, 88C, 84C.  The temperature range is significantly smaller, top and bottom.

The chart lists the oil shutter's recorded position at 0%, 36.1%, 72.2%, and 100% open.  In order, the temperature for each position from the chart is: 90C, 60C, 50C, 50C.
Within DCS, at the same shutter positions, our oil temperatures are: 90C, 65C, 58C, 56C.  Below 90C, the oil temperature is roughly 6C over the line it should follow.

Speeds are a bit harder, since in the chart they do not list what the setting for the momentarily ignored shutter is.  So, I'm going to assume that while the coolant shutter vs speed were recorded, the oil shutter was left in auto.  And while the oil shutter vs speed was recorded, the coolant shutter was left in auto.
In the chart, for the coolant shutter at 0, 30.6%, 63.9% and 100% open, the IAS speeds (reminder, at 1500ft) recorded in order are: 295MPH, 303.5MPH, 302.5MPH, 293.5MPH.
Within DCS, at the same shutter positions, our recorded speeds will be: 316MPH, 313MPH, 308MPH, 299MPH.  Of significant note here, is that not only the speeds don't match...but if graphed, it's not concave down.  As the shutter is closed more and more, the speed increases more and more as well.  This isn't a reflection of the behavior on the chart.

In the chart, for the oil shutter at 0, 36.1%, 72.2%, and 100% open, the IAS speeds recorded in order are: 301MPH, 299MPH, 296MPH, 294MPH.
Within DCS, at the same shutter positions, our recorded speeds will be: 317.6MPH, 313MPH, 313MPH, 312MPH.  It's evident that while the speed is fast, but also notice that the net drag from the shutter seemingly flattens out once the oil shutter is 1/3 open.


Edited by Magic Zach
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  • 2 months later...

The DCS P-51 data in this post has been found to be incorrect, and has been corrected here

For easy visual comparison, I charted out these results in a similar manner used in the chart here.
Also, here is verbatim the conditions of the original graph:
As before, the conditions were all matched when gathering the raw values from DCS.
 

Quote

E.   Cooling Shutter Tests
       All tests were flown with oil and coolant flaps automatically controlled by thermostats set to maintain a coolant temperature of 107°C and oil temperature of 70°C. All performance was therefore corrected to flap positions which would maintain these temperatures on a standard day.
       The effect of oil and coolant flap positions on indicated airspeed is shown in Figure 9.
       F.   All data has been reduced to NACA standard atmospheric conditions. Performance curves are presented as Appendix to this report. A normal rated power curve (2700 RPM and 46" Hg) was flown to establish a standard weight at various altitudes. Performance data is corrected to this weight, as shown in Figure 2, less an allowance of 60 pounds of fuel for warm up and take-off.


unknown.pngunknown.png

Airspeed was taken as Indicated Airspeed.  To match the real graph as linked above, the orientation of the oil shutter was inverted (360 is open, 0 is closed).  While the coolant shutter has position 0 as open, and 360 as closed.  I can't say why they did it this way, but I matched that as well (but limits in Google Sheets prevent me from doing it properly).  Regardless, the lines are still representative if you look at their outputs on the left or right.  Left side of the graph is the open position, right side of the graph is the closed position.

Things to note:
-The drag curve model for the coolant shutter within DCS nearly linearly drops in proportion to the coolant shutter's position.  If you look at the relationship between drag and coolant shutter position in the real aircraft (ser 44-15342), the drag is minimized at the point when the coolant shutter is roughly 1/3 open.  This shutter position vs drag curve is not represented within DCS, and DCS assumes the more closed the shutter is, the less drag will continually be generated.
-The sheer range of control the coolant shutter has over the coolant temperature is much larger.  The temperature minimum and maximum both feet comfortably within the minimum and maximum attainable temperatures of the real aircraft
-The drag curve model for the oil shutter is nearly identical between DCS and the real thing, despite what may initially appear on the graph.  If you were to take the average slope and curve of the speed drop between the DCS P-51, and P-51 44-15342, they will very closely follow each other.  The difference here is just that our 51 is simply faster overall.  This is an issue that is for another topic, another day.  
-The temperature range between the two needs adjusting, while not nearly as significantly as with coolant.  While both our Mustang and the real Mustang will peak at the same temperature, the real Mustang can attain a lower oil temperature when it requires.  As significant as 7C difference at most at their respective minimums.
-The oil temperature of the real Mustang will flatten out once 72+% open.  Past nearly 3/4 open, the oil temperature does not change.


Edited by Magic Zach
typo
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9 hours ago, Magic Zach said:

For easy visual comparison, I charted out these results in a similar manner used in the chart here.
Also, here is verbatim the conditions of the original graph:
As before, the conditions were all matched when gathering the raw values from DCS.
 


unknown.pngunknown.png

Airspeed was taken as Indicated Airspeed.  To match the real graph as linked above, the orientation of the oil shutter was inverted (360 is open, 0 is closed).  While the coolant shutter has position 0 as open, and 360 as closed.  I can't say why they did it this way, but I matched that as well (but limits in Google Sheets prevent me from doing it properly).  Regardless, the lines are still representative if you look at their outputs on the left or right.  Left side of the graph is the open position, right side of the graph is the closed position.

Things to note:
-The drag curve model for the coolant shutter within DCS nearly linearly drops in proportion to the coolant shutter's position.  If you look at the relationship between drag and coolant shutter position in the real aircraft (ser 44-15342), the drag is minimized at the point when the coolant shutter is roughly 1/3 open.  This shutter position vs drag curve is not represented within DCS, and DCS assumes the more closed the shutter is, the less drag will continually be generated.
-The sheer range of control the coolant shutter has over the coolant temperature is much larger.  The temperature minimum and maximum both feet comfortably within the minimum and maximum attainable temperatures of the real aircraft
-The drag curve model for the oil shutter is nearly identical between DCS and the real thing, despite what may initially appear on the graph.  If you were to take the average slope and curve of the speed drop between the DCS P-51, and P-51 44-15342, they will very closely follow each other.  The difference here is just that our 51 is simply faster overall.  This is an issue that is for another topic, another day.  
-The temperature range between the two needs adjusting, while not nearly as significantly as with coolant.  While both our Mustang and the real Mustang will peak at the same temperature, the real Mustang can attain a lower oil temperature when it requires.  As significant as 7C difference at most at their respective minimums.
-The oil temperature of the real Mustang will flatten out once 72+% open.  Past nearly 3/4 open, the oil temperature does not change.

 

The drag is not minimized at the certain coolant shutter position. The fundamental dependance is monotone. The modal character of speed curve vs coolant shutter position is due to the common airflow pass for engine cooler and aftercooler radiators. As the shutter is closed, the drag is minimal (or even slightly negative) but the air charge temperature rises significantly decreasing engine power. The last factor is the most uncertain regardles references to fine tune the whole system (prop thrust, exhaust thrust and radiators drag), because they are cross-dependant.
Anyway, as you can see, the oil radiator gives the same results as RL test (the small constant difference is due to different coolant shutter position, for example), and the curve for coolant radiator is not monotone due to power loss.

P.S. As you can see the temperature range for the cooler is a bit lower than in the test, so the speed curve has less droop.

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Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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11 hours ago, Yo-Yo said:

The drag is not minimized at the certain coolant shutter position. The fundamental dependance is monotone. The modal character of speed curve vs coolant shutter position is due to the common airflow pass for engine cooler and aftercooler radiators. As the shutter is closed, the drag is minimal (or even slightly negative) but the air charge temperature rises significantly decreasing engine power. The last factor is the most uncertain regardles references to fine tune the whole system (prop thrust, exhaust thrust and radiators drag), because they are cross-dependant.
Anyway, as you can see, the oil radiator gives the same results as RL test (the small constant difference is due to different coolant shutter position, for example), and the curve for coolant radiator is not monotone due to power loss.

P.S. As you can see the temperature range for the cooler is a bit lower than in the test, so the speed curve has less droop.

Quote

but the air charge temperature rises significantly decreasing engine power.

I'm having trouble getting the wording in this part.  What do you mean by "air charge temperature"?  Do you mean the air temperature leaving the coolant shutter?  How would the increased air temperature here decrease horsepower output from the engine?  Is it through the rising coolant temperatures?  You mean a hotter engine is less efficient?

 

Quote

P.S. As you can see the temperature range for the cooler is a bit lower than in the test, so the speed curve has less droop.

Could this be fixed within the DCS model?

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49 minutes ago, Magic Zach said:

I'm having trouble getting the wording in this part.  What do you mean by "air charge temperature"?  Do you mean the air temperature leaving the coolant shutter?  How would the increased air temperature here decrease horsepower output from the engine?  Is it through the rising coolant temperatures?  You mean a hotter engine is less efficient?
 

Do you know what aftercooler does for air charge?

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Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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"Air charge" is often described as air/fuel volume/density which goes in to combustion chamber.

So by colling air/fuel charge more, you get higher power out of engine.

I totally understand this.

I did test by my self as well, and DCS result don't quite match tests which @Magic Zach provided.

 


Edited by grafspee
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21 hours ago, Yo-Yo said:

Do you know what aftercooler does for air charge?

Except that the airflow coming off the oil cooler exits by it's own ducting without ever touching the radiator cooling air-stream.

And the supercharger charge air is never routed through the coolant radiator. The radiator air scoop positions should have basically nothing to do with charge temperature; only engine temperature and coolant temperature should have anything to do with it; the radiator scoop position is only very indirectly involved (in that it impacts coolant temperature).  The intercooler itself is part of the engine accessories and the chin scoop, not the belly scoop.

I don't think a 8 C difference in temperatures and 19 mph difference in speed at the extreme ends of oil radiator positioning compared to the flown test data, are quite "the same results as RL test" though.

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The current DCS implementation actually feels like it limits the extent to which the radiator doors can be opened: at max open, it allows higher speed than the real aircraft, but also less effective cooling.  This limits the aircraft's abilities in a low speed/ high engine power scenario.  At low speeds, drag is less a factor, but cooling is critical, as it is the limiting factor on performance in a hard-turning low speed turn fight: less cooling means that the engine oil will rapidly overheat unless the pilot reduces engine power (which, in turn, reduces sustained turn performance).  This means that the apparently artificially narrowed scope of radiator door positioning in DCS strongly negatively impacts P-51 turn-fighting capacity. 

Yes, I know, P-51 is not a classical "turn fighter", but there are situations where that performance is necessary to finish an opponent.  And beyond that, the current implementation isn't quite true to the real aircraft at the edges of it's envelope, which should be reason enough to want to tune it.

As an aside: the changes to the forum here are a bit oppressive; you can't edit or delete posts whatsoever, and it takes 10 minutes of "saving" for them to post in the first place.  Not great.

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2 hours ago, OutOnTheOP said:

Except that the airflow coming off the oil cooler exits by it's own ducting without ever touching the radiator cooling air-stream.

And the supercharger charge air is never routed through the coolant radiator. The radiator air scoop positions should have basically nothing to do with charge temperature; only engine temperature and coolant temperature should have anything to do with it; the radiator scoop position is only very indirectly involved (in that it impacts coolant temperature).  The intercooler itself is part of the engine accessories and the chin scoop, not the belly scoop.

I don't think a 8 C difference in temperatures and 19 mph difference in speed at the extreme ends of oil radiator positioning compared to the flown test data, are quite "the same results as RL test" though.

Please, take a closer look and more carefully, how the aftercooler works...

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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On 10/3/2021 at 10:48 AM, OutOnTheOP said:

As an aside: the changes to the forum here are a bit oppressive; you can't edit or delete posts whatsoever, and it takes 10 minutes of "saving" for them to post in the first place.  Not great.

You should be able to delete your own posts now, editing your own posts has always been allowed and I don't know what you mean about 10 minutes to save. 

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3 minutes ago, Enduro14 said:

Oh god not this again, what is it about fall and the Fuel debates and This silly Meredith myth

Meredith is not a myth, the effect is very common, just a ram-jet. It can not give a significant thrust, though, but can reduce cooling losses. For sure, it's a matter of high speed, where the air passage area can be reduced significantly forming a nozzle to use added enthalpy.

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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