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POH power charts vs sim and other questions

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I'm trying to set up my DC-6 according to the power charts in the POH pp307.
However I can't get to a point where the instruments match the data from the tables, although I'm not 100% sure I'm reading those right.
 
For instance if I try to set up a 1500BHP climb the POH p307 suggests: LOW 2400, BMEP of 177 and MAP of 38 @6000(-20°C) with a fuel flow of 1040.
But when I set the manifold pressure to 38 in flight I get about 136 BMEP and a fuel flow of 580 - with BMEP set to 177 I get a fuel flow of 780 and a MAP of 46.
I never reach that 1040 fuel flow, unless I open the throttle to dry take off.
 
Similarly a 1000 BHP cruise @6000 according to the table on p310 is achieved with LOW 1850, BMEP 153, MAP 31.9 and a fuel flow of 436.
In flight I can get to MAP 32, BMEP 128 and a FF of 410  or  MAP 36.5, BMEP 152, and FF 490.
 
Those numbers are a bit closer but still leave me wondering what numbers I should be aiming for. 

 

 

Also the chart on p287 mentions 2BMEP and 12 BMEP drops - I presume those are for manual leaning? Are those applicable at all in the simulation with auto lean?

 

Any insight or information on this topic is highly appreciated.

 

 

 
- completely unrelated topic about reversing

 

Recently I needed to push back the plane from a gate, so after starting up I pulled the throttle back to idle engaged the reverse pitch selector and the propeller reverse lights came on. Moving the throttles further back did spin up the engines however the plane started to move forwards.

Shouldn't it be moving backwards in that case or am I doing something wrong here?

 

 

 

- other POH vs sim stuff

 

It appears the three heater backfire lights on the heater control panel (p108) along with the cabin pressure change limit control (p129) and the static selector valves are missing in the sim.

 

TIA

Martin Peters

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For instance if I try to set up a 1500BHP climb the POH p307 suggests: LOW 2400, BMEP of 177 and MAP of 38 @6000(-20°C) with a fuel flow of 1040.

But when I set the manifold pressure to 38 in flight I get about 136 BMEP and a fuel flow of 580 - with BMEP set to 177 I get a fuel flow of 780 and a MAP of 46.

I never reach that 1040 fuel flow, unless I open the throttle to dry take off.

 

Similarly a 1000 BHP cruise @6000 according to the table on p310 is achieved with LOW 1850, BMEP 153, MAP 31.9 and a fuel flow of 436.

In flight I can get to MAP 32, BMEP 128 and a FF of 410 or MAP 36.5, BMEP 152, and FF 490.

 

These tables  are from actual line pilots and are provided to establish power settings using RPM/MP.  If BMEP is not matching then it is because your power is not 1500 HP due to variations in RPM or MP.  The relationship is BMEP = (BHP * 283)/RMP.  As for fuel flows, these were very difficult to set up in XPlane to cover the entire operating range and are probably a little low at high power.  We did run a lot of speed/power cruise trials to try to ensure cruise power fuel flows were close to book.  Realistically, no engine is going to be at book value. 

 

I recommend that if you use the RPM/MP tables your focus is on those two variables rather than BMEP.  Conversely, using the original Douglas tables that provide RPM/BMEP then those are priority. Either way works.

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Thank you, I think I'm going to stick to the Douglas tables then - less numbers to worry about.  :wink:

 

Regarding those tables I have one more question: What does the LIV L/D marker in the tables on pages 290/291 stand for?

 

 

Also any idea what is up with the plane moving forward while the props are in reverse pitch?

 

 

Regards

Martin Peters

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What does the LIV L/D marker in the tables on pages 290/291 stand for?

 

I asked that during beta and never got an answer. I then moved on and forgot about it.  I recall looking at it trying to decipher a lift/drag meaning out of it but I couldn't connect dots.

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Thank you, I think I'm going to stick to the Douglas tables then - less numbers to worry about.  :wink:

 

Regarding those tables I have one more question: What does the LIV L/D marker in the tables on pages 290/291 stand for?

 

 

Also any idea what is up with the plane moving forward while the props are in reverse pitch?

 

 

Regards

Martin Peters

Reverse pitch only works if you map your throttle to a quadrant or slider. Buttons and keyboards are bugged and cause the prop pitch to go back to 0 degrees, thus making you go forward. Even with the lights on. Using the afor mentioned throttle controllers will make your reverse pitch work.

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Reverse pitch only works if you map your throttle to a quadrant or slider. Buttons and keyboards are bugged and cause the prop pitch to go back to 0 degrees, thus making you go forward. Even with the lights on. Using the afor mentioned throttle controllers will make your reverse pitch work.

Everything works fine if you use the assignment PMDG provided rather than the default X-Plane control. It's described in the introduction manual. I have a simple single throttle set up and my DC-6 powers back perfectly well.

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Make sure the throttles are right back at idle before attempting to operate the lever. If they are forward of idle, the gate is locked and won't move, even when clicked.

 

Rob Smith.

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Wrong and wrong, the bug was proven in another post, there is NO control over reverse props with the keyboard assignments or buttons. I am not an idiot, I WAS using the provided functions by PMDG.

 

Your reverse prop works because your using a single throttle, you obviously did not read what I wrote. Even moving the throttles inside the VC causes the reverse prop to fail. THIS IS A BUG THAT PMDG NEEDS TO ADDRESS.

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THIS IS A BUG THAT PMDG NEEDS TO ADDRESS.

No need to shout... I assume you have documented this bug by submitting a trouble ticket. This is only way to ensure bug is validated and put in their bug tracking system.

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No need to shout... I assume you have documented this bug by submitting a trouble ticket. This is only way to ensure bug is validated and put in their bug tracking system.

Not shouting. Just making sure that it is observed.

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Not shouting. Just making sure that it is observed.

Online it's shouting, and no more acceptable than shouting at people in real life. The only way to get your view observed by PMDG would be to open a support ticket, as Dan suggested.

 

I don't see the point in paying $70 for an addon then using the keyboard to control the engines, but it's your choice I suppose.

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Online it's shouting, and no more acceptable than shouting at people in real life. The only way to get your view observed by PMDG would be to open a support ticket, as Dan suggested.

 

I don't see the point in paying $70 for an addon then using the keyboard to control the engines, but it's your choice I suppose.

Excuse me, but how I wish to control the engines isn't your concern. Justifying a broken system because I chose to use keyboard commands isn't correct either.

Also, assuming that I was "Shouting" is YOUR opinion. Not the fact.

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Excuse me, but how I wish to control the engines isn't your concern. Justifying a broken system because I chose to use keyboard commands isn't correct either.

Also, assuming that I was "Shouting" is YOUR opinion. Not the fact.

When you type in all caps online that is shouting by definition, it's not an assumption. If you don't believe me try googling it.

 

I wasn't justifying a broken system (if it is broken), just politely wondering why you would use the keys. I suppose hoping for a polite response was too optimistic. You seem to be too busy telling people they are wrong to listen.

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When you type in all caps online that is shouting by definition, it's not an assumption. If you don't believe me try googling it.

 

I wasn't justifying a broken system (if it is broken), just politely wondering why you would use the keys. I suppose hoping for a polite response was too optimistic. You seem to be too busy telling people they are wrong to listen.

You started all this because you did not want to read ALL of the information that has been provided thusfar

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So, still I'm a little concerned about the correct power settings when i.e. in cruise. Currently I'm using Simbrief as flightplanning, and I'm not sure if Simbrief is calculating power and speed correct, or simply I'm doing something wrong. Hopefully someone can help me with that. I can state that winds are in an acceptable range in the simulator compared to the Simbrief flightplan when I tested this.

 

So I show my problem here with a flightplan example (main problem here is that my speed is too low according to Simbrief):

* According to Simbrief flightplan, power setting will be 1100bhp, and cruise is FL120.

 

I am finding that the correct chart is on page 309 where at FL120:

* MP should be set to 33.3 at carburator temp of 20 C degrees

* RPM should be at 2200

* Gross weight around 74000lbs

 

When cruising at this flightlevel with these settings, my groundspeed was only 205 knots, and IAS about 200-210 knots. Calculated pressure altitude was around 10500 feet. According to table at page 291 in POH, I should have a TAS of 250 (or if calculating it manually 254 knots, 1.21*210, where 21 comes from 2% extra TAS for each 1000ft of density altitude). I did have an approximate headwind of 38 knots, according to flightplan. TAS minus headwind in this case gives a groundspeed of 216 knots.

 

But the problem here is, according to Simbrief, I should have a GS of around 240 knots, 35 knots more than I actually have. So if Simbrief does not calculate GS and overall speed/power correctly, what is the best way to calculate it manually?

 

Is the correct way to calculate GS before flight:

Approximate winds 335/44

Flightheading 306 degrees

Above gives a headwind of cos(29)*44 = 38 knots

ISA is M17 which gives density altitude around 10500 feet at FL120

Cruising speed at 10500 density altitude, and a grossweight of 74000lbs, should be IAS 210 knots according to table in POH at page 291 (1100bhp).

TAS at 0 wind at density altitude 10500 is 254 knots (2% increase of IAS for every 1000 feet, 1.21*210).

GS will then be 216 knots (TAS minus headwind, 254 - 38).

 

Why am complicating things so much here? First, I want know how long the flight will be, so I know how much fuel I need to put in. Also it is good to file the correct speeds when flying with IVAO/VATSIM. And at last, I want to get home in time. ;)

 

Edit: It seems the chart at page 291 do notcoincide with the charts at pages 300-312????

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Keep it simple Gosta.

 

The only thing you are assured of is the TAS will be about +/- 5% of the book value.  You file the approximate TAS and don't worry about GS. 

As for flight planning, your best guess is based on wind forecasts. Actual winds do vary sometimes significantly.  This is why you always include a contingency amount of fuel, and ensure you have at least 30-45 min of extra fuel planned.

 

Cowl flaps can subtract several knots per degree.

 

Most trips in the DC-6 are only a few hours so you can easily use an average wind component.

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Thanks, I had the cowl flaps set to 0 degrees. I now read in the POH manual that each degree affect about 3 knots speed. So is the speed change affected for each degree of each cowl flap, or each degree for all cowl flaps?

 

I.e. if adjust all the cowl flaps from 0 degrees to -4 degrees, will the speed increase:

* 3x4 = 12 knots

or

* 3*4*4 = 48 knots

 

And also I wonder why the charts at pages 290-293 differs from the charts at pages 300-312.

The later charts seems to be based on:

* Pressure altitude.

* Carburator temperature

* Manifold pressure

* RPM

 

while the charts at 290-293 is based on:

* Density altitude

* BMEP

* RPM

 

I'm not sure which one to use. The later charts at page 300-312 disregards the density altitude, while the charts at 290-293 include density altitude. But they do disregard the carburetor temperature, which I then do not know which temperature to set.

 

I.e. if flying a cruising profile of 1100bhp at FL120 (let us say a density altitude of 10000ft):

 

The first charts would give:

* RPM = 2030

* BMEP = 153

 

The later charts would give:

* RPM = 2200

* MP = 33.3

I'm not sure what BMEP drop means in these charts.

 

Anyway, what I do not understand here why the RPM is much different. I guess with first charts you will run with lower RPM:s but with a higher MP (BMEP higher = MP higher), while with the later charts you are running with higher RPM but with lower BMEP/MP.

 

In the case of the first charts, how do I know which carburator temp is recommended?

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These tables  are from actual line pilots and are provided to establish power settings using RPM/MP.  If BMEP is not matching then it is because your power is not 1500 HP due to variations in RPM or MP.  The relationship is BMEP = (BHP * 283)/RMP.  As for fuel flows, these were very difficult to set up in XPlane to cover the entire operating range and are probably a little low at high power.  We did run a lot of speed/power cruise trials to try to ensure cruise power fuel flows were close to book.  Realistically, no engine is going to be at book value. 

 

I recommend that if you use the RPM/MP tables your focus is on those two variables rather than BMEP.  Conversely, using the original Douglas tables that provide RPM/BMEP then those are priority. Either way works.

I am confused with the table numbers too. I tried to simulate exactly what Martin did. And I have same numbers in X-Plane as he has. Reading the post I can not understand now. So does it mean that the aircraft is not modeled close to the numbers and we should completely ignore the tables in POH? Or I can set RPM/MP using tables like the one on pp 307 and ignore BMEP and fuel flow believing that I set up the correct BHP? I want to say that the aircraft is of rather high price. And If I am paying the price I think I should rely on the docs coming with it. Especially when PMDG stands for Precision Manuals.

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Remember you are flying a old airplane with engines that have been repeatedly rebuilt, and that is what PMDG modeled.  The POH was published by Douglas when the aircraft was new.  The supplemental tables are provided by a major operator of the aircraft using it today.  You choose.  Either select the POH tables that set power using RPM/BMEP or the supplemental tables using RPM/MP. There is not a wrong answer.

 

This plane is rarely flown as high as 10,000.  It is much more economical to stay around 8000.

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Remember you are flying a old airplane with engines that have been repeatedly rebuilt, and that is what PMDG modeled.  The POH was published by Douglas when the aircraft was new.  The supplemental tables are provided by a major operator of the aircraft using it today.  You choose.  Either select the POH tables that set power using RPM/BMEP or the supplemental tables using RPM/MP. There is not a wrong answer.

 

This plane is rarely flown as high as 10,000.  It is much more economical to stay around 8000.

Thank you for the quick answer. I can undastand it is an old aircraft. And I love it! And I have pfpx that can create the flight plan for me and calculate fuel. All is great. But what I am trying to do is to fly the bird like pilots did the old days, calculating fuel amd route themselves and flying with vor/adf navaids (for some reasons they didn't have gps navigators installed :) ) only. I think the best answer for my case is to recreate tables that would be working for the aircraft myself, flying with different weights, temperatures, rpms, etc. If anyone already has it done you are welcome to share it with me :)

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Either POH or supplemental tables will work for you.   Just don't expect 0.3% error... be realistic.  There are 5% differences between aircraft flown by the same operator. Some need aileron trim and others don't.

 

As already mentioned, plan your flights around 6000-8000 feet. That's where these birds spend most their time. 

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According to the topic chart I've got another question.

We can set the same BHP with different RPM/BMEP settings. For example, if we take into account the mentioned formula
PMEP = BHP * 283 / RPM, we can set BHP=800 with
 
RPM=1600, BMEP=140   (fuel flow ~440 )
RPM=1800, BMEP=125   (fuel flow ~420 )
RPM=2000, BMEP=115   (fuel flow ~418 )
RPM=2300, BMEP=100   (fuel flow ~410 )
RPM=2500, BMEP=90    (fuel flow ~400 )
RPM=2700, BMEP=85    (fuel flow ~400 )
 
 
The fuel flow is measured while flying at standard temperature, FL050 (I don't think my altitude makes any difference here, rather then the speed I am flying). I don't have the exact numbers for manifold pressures, but they were changing from 30" to 40" with these settings.
 
My question is: if all the parameters are within the allowed limits, why I should stay with lower RPMs
as all the charts do when I can see that with higher RPMs I spend less fuel?

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Did you check your TAS at those different settings?  I would expect it to vary with prop RPM, and this directly impacts your fuel used for a given distance.

 

Also, running low RPM and high MP is one sure way to buy an engine overhaul, which is why prop pilots like to stick to the tables.

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Did you check your TAS at those different settings?  I would expect it to vary with prop RPM, and this directly impacts your fuel used for a given distance.

 

Also, running low RPM and high MP is one sure way to buy an engine overhaul, which is why prop pilots like to stick to the tables.

Nope, I didn't check TAS. :nea:  I could not expect that for the same horse power the speed is going to vary. Gonna check this evening.
But if the speed is different then I am totally confused. Cause I always thought that
BHP uniquely identifies the aircraft thrust. And when the environment stays the same (temp, flight level, density,...) and the weight does not change
then the speed increases/decrees only when thrust changes. If I see that the speed varies it means that for the same BHP I have varying thrust...  :blink:

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I could be misinterpreting what you're saying, but...

 

You can have a high BHP engine running at rated power, but if you don't have a prop attached to it, you're not going anywhere. So, for a fixed BHP, you're definitely going to see varying thrust based on the prop angle being driven. This is shown in the prop RPM and torque (indirectly shown on the BMEP gauges). Lower RPM takes a larger bite of air, but also increases torque (why BMEP increases when you pull the props back).

 

Full names in the forum, please - first and last.

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