amollen

Trouble getting to documented cruise speed

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Yesterday I flew Lisbon-Dakar, a good old classic route that KLM used to fly with the DC-6, 1500 miles, well within the range.

I had trouble getting to the cruise speed I was expecting according to the manual. As a result, the trip took 8 hours instead of 6, I used a lot more fuel and I was running on fumes landing at Dakar. I have noticed this issue before, but last night was the first time I really logged what was happening.

Here's the details: I flew most of the trip using the 1100 hp table on page 291. Cruising at 14,000 feet, 2190 RPM and 142 BMEP. The table says I should be getting 189 DIAS and 240 TAS even at the heaviest load with these settings, but I was actually getting barely over 200 kts TAS (that was ground speed as per the DME but there was almost no headwind) and about 160-165 indicated. Fuel flow was about right, slightly over 500 lbs/hr. I briefly experimented with other power settings and altitudes but all with similar results.

Other details: cowl flaps closed in the minus (counterclockwise) position, low blower, auto-lean, all normal settings for cruise at that altitude. Temperature was pretty close to standard conditions and whatever little wind there was blew almost exactly at a right angle of the flight path.

Some possible factors I thought of: the plane was pretty heavy, I took off at 4,000 lbs below MTOW but according to the table, I should still get close to 240 kts at the heavy weight.

Should I be cruising initially at a much lower altitude when heavy like that? I read some posts on this forum that the Six likes to fly low, others that say it shouldn't matter much. I'll try flying low on my next leg (Dakar-Recife, which is a couple of hundred miles longer with a lot of water in between so I can't mess up there!).

I noticed a number of posts discussing the fuel capacity of the Six, the 10-tank vs. the 8-tank configuration but none of those mentioned anything about not getting the rated speed out of the bird. Weird.

I thought I found what was wrong when I read in the footnotes of page 291 that the TAS is in mph, not in knots. That would partially explain the difference, but then some posters on this forum remarked that page 291 is KNOTS after all. Does anyone know for sure? Note that both DIAS and TAS were low, by a similar margin.

Anything else I should be paying attention to? I'm a veteran simmer and a licensed private pilot, so I'd like to think I know how to plan a flight :-). I'm also spoiled by PMDG making sims that perform by the book. What's weird is that I wasn't able to find any posts with a similar problem though of course I may have overlooked some. Any suggestions?

Thanks fellow radial engine lovers 🙂

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Hi, let's start with the footnote in the manual: Page 291 is definately using knots. The original handbook that I have is using miles/h and the values for the highest weight braket at 14000 ft are DIAS 218 and TAS 277. Converting the miles/hr to knots in the PMDG manual was done because the airspeed indicator in the aircraft is in knots. Unfortunately the footnote has not been corrected accordingly. My calculator converts 218 miles/hr to 189 knots and 277 miles/hr to 240 knots and that is corresponding with the chart.

Now why did you get only 160-165? Firstly, the cowling flaps position in cruise with minimal drag is not fully closed (-4°) but slightly open (0°). We have simulated drag from cowling flaps position. On top you were heavy and by that at the edge of performance and by being slow you were close to the bottom of the power/drag curve. The additional pitch required to stay level slowed you down a bit further.

Lastly, manuals of that time were always quite optimistic and you know the sim is not perfect either in simulating the athmosphere.

During the development of the aircraft, I have tested the aircraft from 4000 to 16000 ft with low blower and from 17000 to 21000 ft with high blower at standard weight choosen 90.000 lb and the deviation on IAS in knots is within 0  to -2%.

I hope this helps to relate the sim experiance to the manual and next time keep the cowling flaps at 0°.

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Posted (edited)

Alexander,

Thanks for your suggestions, I have tried two more flights at much lower altitudes (6,000 and 8,000 feet), lighter aircraft and using cowl flaps at zero instead of -2. I still do not get better than about 200 knots TAS (170 indicated) when using the 1100 HP table.

The link below shows a screenshot. Vital data are: altitude 8,000, BMEP 155, RPM 2000, weight about 91,000 lbs (from Shift-4), cowl flaps 0, mixture autolean, temperature slightly above standard, autopilot on including altitude hold, everything else normal. Just to make sure I serviced my engines and props using Shift-6 before the flight.

IAS is 170 kts instead of the 205 kts from the table on page 291. Fuel flow right on the money at 500 lbs/hr.

I noticed I'm a bit nose-high so I figured I'd accelerate to get on the right side of the power/drag curve, but when I reduced power I fell right back to the 170 IAS you see in the screenshot.

Since I seem to be the only one with these problem, I am considering an uninstall-reinstall of the DC-6 after this flight.

BTW I forgot to mention, using FSX Acceleration and Windows 10.

https://www.dropbox.com/s/2kcynp26bodk9ot/Capture.JPG?dl=0

 

Edited by amollen

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Well, a re-install didn't solve my problem but something else did: the old "starting FSX with the default aircraft and then loading the DC-6" trick.

When I start with the AirCreation Trike and then load the DC-6, I get reasonable speeds, still a few knots lower than published but that is certainly within limits!

So I can fly Shannon-Gander without running out of fuel 🙂

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On 3/9/2019 at 1:47 PM, metzgergva said:

Firstly, the cowling flaps position in cruise with minimal drag is not fully closed (-4°) but slightly open (0°). 

@metzgergva just curious, if 0 degrees is the minimal drag position, what is the use for the negative cowl flap positions? When would one use them?

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likely securing an engine after a shutdown, fire, or severe damage... I also shut the cowl flaps when I secure the airplane after landing.. keeps mother nature outside of the nacelles 👍

 

Robert Toten

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On 4/9/2019 at 1:48 PM, amollen said:

@metzgergva just curious, if 0 degrees is the minimal drag position, what is the use for the negative cowl flap positions? When would one use them?

Wenn it is very cold outside you can increase a bit the CHT, but it generates a bit more drag. But when it is cold that is less of a concern. 

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