Delta Virtual Airlines Water Cooler - DC-8 speed settings in aircraft cfg

Delta Virtual Airlines Water Cooler | PC Support | DC-8 speed settings in aircraft cfg

DVA7689
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Posted onPost created on October 14 2013 21:57 ET by Jon Bullock

Just loaded in Windows 7 and FSX along with the DVA DC-8-71. Used to fly the DC-8-61 with Windows XP and FSX with no problems. With this new '71 I get the overspeed warning when I hit 250 kts indicated at anything over FL350. At lower altitudes the overspeed warning comes on at progressively higher indicated airspeeds. Is there a setting in the aircraft.cfg or some other file that controls at what indicated speed or mach number the overspeed warning will come on? I have FSX set to show indicated airspeed.

Jon Bullock (DVA7689)

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Posted onPost created on October 14 2013 23:07 ET by Sid Dudley

250 kits indicated at or above FL350 is probably over speed since your true airspeed would be significantly higher (if memory serves me correctly....about 425 kts true airspeed).

Sid Dudley

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Posted onPost created on October 15 2013 12:44 ET by Kenneth Dickerson

Remember the basic physics of altitude is the higher you go the less denser the air. The air volume that goes through the pitot tube at FL300 is about 1/3 of what it would be at sea level.

Try this experiment, go to your Weather and set to calm and clear, then go to the MAP set your altitude at 30,000 at 220 mph and set your A/P to hold this. Then allow everything to stabalize, then look at you true airspeed/ ground speed / indicated airspeed. If your weather settings is correct and the altimeter is 29.92 then your true and ground speed should be close and your indicated should be about 100 knots less or more.

Then set your altimeter for 15000 MSL keep the same airspeed and see what the difference is.

Finally go to 5000 msl and see what your intruments show.

All this is due to air density, that is why jets and airliners like the higher altitudes!

Kenneth Dickerson

First Officer, A330-300

Posted onPost created on October 15 2013 17:57 ET by Jon Bullock

Yeaaah I don't know... The top speed for the DC-8 is 330 knots indicated. The top corrected max true airspeed is 505 kts. At any altitude you should be able to hit the max indicated air speed without overspeeding the aircraft. The indicated airspeed, if I remember this correctly, is what the aircraft thinks it's doing through the pitot tube of the airspeed indicator, without correcting for density altitude. Once density (altitude barometric pressure and temperature) are calculated in you'll get much higher true airspeed at the upper Flight Levels where the air is thinner... and colder. I routinely flew the DC-8-61 at over 500

What is interesting though, is that the max mach number I can attain is 0.74M at FL350 before the overspeed kicks in. That is way off since the posted max mach number for the DC-8 is .088M. So something is haywire in the aircraft.cfg file. Yet when I looked at it, it showed 340kts and 0.88M as the max speeds.

Jon Bullock (DVA7689)

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Posted onPost created on October 16 2013 08:25 ET by Adam Hulse

Jon, to further explain the above points, here are a few good links. Also, you can google why indicated airspeed decreases as altitude increases, and you will find a few more.
http://www.studentpilot.com/interact/forum/showthread.php?37621-IAS-Decrease-with-altitude
http://www.luizmonteiro.com/Learning_Pitot_Sim.aspx

Adam Hulse

Captain, B777-200

Posted onPost created on October 16 2013 11:29 ET by Jon Bullock

Thanks for the links but they just verify that indicated airspeed is not the same as true airspeed based on altitude. I believe we're in agreement on that point. I'm talking about vastly different performance figures for two identical airframes with different engines is all. The DC-8-71 should fly at roughly the same numbers as the DC-8-61 just with better fuel economy due to the CFM56 engines. The '71 is only able to achieve 0.75M before the overspeed warning comes on at the same altitude as the '61 achieves 0.88M when it's overspeed warning comes on.

Thanks for responding to my post guys. I'll keep looking...

Jon Bullock (DVA7689)

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Posted onPost created on October 16 2013 13:01 ET by Luke Kolin

I looked in the aircraft.cfg and .AIR file last night for the -71; I'll keep digging.

Cheers!

Luke Kolin

Senior Captain, MD-11

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