POH power charts vs sim and other questions

[downscc 1,327]

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.

[Vantskruv 11]

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?

[uvuch 1]

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.

[downscc 1,327]

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.

[uvuch 1]

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

[downscc 1,327]

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. 

[uvuch 1]

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?

[downscc 1,327]

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.

[uvuch 1]

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.   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:

[scandinavian13 5,192]

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.

[uvuch 1]

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.

I see. yes, It really makes sense.  :smile: Thank you! and ... my name should be in the forum now.

[scandinavian13 5,192]

I see. yes, It really makes sense.  :smile: Thank you! and ... my name should be in the forum now.

 

It is. Thank you.