TAS on cruise power charts & BMEP drop

[TheFinn88 13]

Hi

I'm a litte bit confused because of the power-setting charts provided in the PMDG manual. Lets take an example:

The cruise altitude will be at 17'000ft, with OAT at -20°C and GW at 80'000lbs. According to the "Speed Power Table" on page 285, this results in a density altitude of 16'000 feet, a power-setting of 1100 BHP and a TAS of 247 knots. Now when I go to the "Level Flight Cruise Chart / 1100 Toruqemeter Horsepower" on page 291, I get a different TAS when entering the chart at the density-altitude of 16'000 feet and GW between 87'500 to 77'500 lbs. This chart states 259 mph (as the TAS on this chart are published as statute miles per hour), convertet to knots I get 225. That's a difference of 22 knots.

I don't know which number I should use for flight-planning. Do you guys use the TAS on the "Torquemeter Horsepower" charts on pages 290 to 293, or do you use the TAS out of the "Speed Power Table" found on page 285?

And my second question is about the BMEP drop. There are some mentions in the manual about a BMEP drop (e.g. on the "Power Curve" chart on page 287). What exactly is a "2" or "12" BMEP drop? How do I get a "2" or "12" BMEP drop? Is this used when leaning the engines manually?

[scandinavian13 5,192]

I'll leave the details up to one of the more experienced DC-6 guys, but do keep in mind that these manuals were made before the days of electronic data collection, FADEC, and other high-accuracy devices.

In fact, Ernest Gann (famed author and aviator) actually referenced manuals somewhat derisively a few times in his books: the manuals were great, in theory, but they knew how to extract max performance from the aircraft, using entirely different settings.

[Ramjett 27]

A little off the topic, but since Kyle brought him up, one of the chapters in his book "Fate is The Hunter" begins with a quote something along the lines of "A rulebook will not cushion the sudden impact of metal and stone".  I always found that a good "course" to follow.

Randy

[downscc 1,327]

2 hours ago, TheFinn88 said:

Hi

I'm a litte bit confused because of the power-setting charts provided in the PMDG manual. Lets take an example:

The cruise altitude will be at 17'000ft, with OAT at -20°C and GW at 80'000lbs. According to the "Speed Power Table" on page 285, this results in a density altitude of 16'000 feet, a power-setting of 1100 BHP and a TAS of 247 knots. Now when I go to the "Level Flight Cruise Chart / 1100 Toruqemeter Horsepower" on page 291, I get a different TAS when entering the chart at the density-altitude of 16'000 feet and GW between 87'500 to 77'500 lbs. This chart states 259 mph (as the TAS on this chart are published as statute miles per hour), convertet to knots I get 225. That's a difference of 22 knots.

I don't know which number I should use for flight-planning. Do you guys use the TAS on the "Torquemeter Horsepower" charts on pages 290 to 293, or do you use the TAS out of the "Speed Power Table" found on page 285?

And my second question is about the BMEP drop. There are some mentions in the manual about a BMEP drop (e.g. on the "Power Curve" chart on page 287). What exactly is a "2" or "12" BMEP drop? How do I get a "2" or "12" BMEP drop? Is this used when leaning the engines manually?

Your cruise altitude is only the pressure altitude (required for conversion to density altitude) if the local altimeter setting is 29.92 inHg.   But for illustration let's assume this is true and I get the same number from the table of 247 KTAS.

I know the footnotes on the original charts says TAS is MPH but because the airspeed indicator in this aircraft is calibrated in knots all performance tables are providing speed in knots.  So now you are comparing 256 KTAS from the second table with 247 KTAS from the first, and since in the first case the value is for a specific weight then one can see how this relates to the second value which is an average value for a range of weights.

In practice, I rarely fly higher than I need to for weather or terrain.  This isn't a jet, higher uses more fuel. Second, I usually pick 1100 BHP table for my power settings and use the BMEP gauge.  There are known errors in the MP gauge so the tables that give power settings for rpm/mp are not of much use to me.  Third, operators such as Everts Air in Alaska that operate a number of DC6 will actually use 800 BHP power settings and many cargo haulers have had pressurization ripped out to lower costs and increase airframe time.

The BMEP drop is used for leaning, but I always always use auto rich or auto lean.  There is nothing to be gained by trying to decrease your fuel burn by a few pounds per hour with those rare and expensive engines. Just plan on 500 lb/hr/engine or 2000 lb/hr of flight plus contingency and your fuel planning is done.

[TheFinn88 13]

Thanks for your informations. I'm in the air right now, cruising at 17'000ft and power settings according to the 1100 BHP chart (2230RPM@139BMEP). My GTN650 tells me, that the density altitude is 16'200 feet, and that I have a TAS of 244 knots. And when the speeds on these charts are indeed in knots , then thats pretty spot on! According to the chart, I should get a TAS of 246, which is just 2 knots off. Also KIAS is spot on and my fuel-flow needles are right where they should be.

I actually made myself an excel-sheet, which computes the fuel required for a certain flight (taking take-off, climb, cruise, descend, average wind, fuel-flow, etc. into account). On my last 4 hour flight, my fuel calculations were just 200 lbs off (and that because I havn't added up the long approach path to my flight distance). And I'm only flying at high altitudes on my ferry-flights from Alaska to Europe, in order to stay above most of the bad weather. As soon as I get "home", I'll also fly at lower altitudes. 

[randomTOTEN 13]

You guys are certainly free to fly her as you like, but this aircraft doesn't have to be relegated to such low cruising altitudes. A couple days ago I flew MDW-LAX with an initial cruise altitude of 16,000'. Hours later and thousands of pounds lighter I began the TOD from FL240. She handles just great at that altitude, speeding along in thin air with a pressurized cabin and high blower keeping the engines producing 1100BHP. Give it a try some time. You will burn more fuel, but TBO costs more.

No, it's not a jet aircraft, but it's not an unpressurized Cessna either.

Robert Toten

[TheFinn88 13]

6 hours ago, randomTOTEN said:

You guys are certainly free to fly her as you like, but this aircraft doesn't have to be relegated to such low cruising altitudes. A couple days ago I flew MDW-LAX with an initial cruise altitude of 16,000'. Hours later and thousands of pounds lighter I began the TOD from FL240. She handles just great at that altitude, speeding along in thin air with a pressurized cabin and high blower keeping the engines producing 1100BHP. Give it a try some time. You will burn more fuel, but TBO costs more.

No, it's not a jet aircraft, but it's not an unpressurized Cessna either.

Robert Toten

You're so right! My preferred cruise altitudes for longer flights are between 17'000 and 19'000 feet. Just had a look into the performance charts yesterday and made some calculations. My last flight would have took about 20 minutes longer at a lower flight level (7000ft @ 500lbs/hr instead of 17'000ft @ 540lbs/hr), without considering the wind). Yes my per hour fuel burn would have been little lower, but as I would had flown for 20 more minutes, I would have burned that "saved" fuel right away.

But on the other hand, when I start flying around switzerland (e.g. LSZH-LSZB), I'll keep the altitudes rather low (maybe around 6000ft) as these are very short flights, and this way my passengers can see much more of the countryside.