Climbing too steeply !

[musiconair]

Hi everyone,

I don't know if this is a recognised problem but for some reason when I engage the VNAV on the Project Magenta MCP it starts to climb very steeply as much as 5600 per min, is there a setting I can adjust in the CDU ?

Thanks
Andi :)

[JWS]

Andi,

Had a similar problem yesterday (but in the opposite direction) when engaging the second AP on an autoland. Unchecked the Beta autopilot and thinghs went better.

What you have to bear in mind is that the power of the engines is pretty stiff, so when you take off in a light configuration (low on pax, cargo or fuel) it is better to select a climb 1 or climb 2 and add some assumed temps or derated stuff as well (especially when you have lot's of runway to use).

JWS

[musiconair]

thanks JWS I was using a light config I will do as you suggest :)

Cheers
Andi

[JWS]

OK, like to hear the results in due time. And don't forget your Cost Index.

JWS

[musiconair]

I usually put cost at 80, but another thing, what do you normally put underneath the flap setting **.*% I never enter anything here, I thought that might have something to do with it ?

Cheers
Andi

[mmakela]

80 seems a lot. For example in Finnair they use 40 or 30 in real life.
My problem is aircraft not descending enough steep:-)

[JWS]

80 is too much, I've heard that Trannsavia uses 011.

The Centre of Gravity is a problem, yes. If I recall correctly this should be around 26 or so. Don't know exactly. Maybe a general call for help gives us some info.

JWS

[musiconair]

Thanks JWS, this is helpful stuff, I've been so used to the PMDG CDU I never came across this CG setting before so I was a little unsure what it was.

Cheers
Andi :)

[musiconair]

Hi JWS, I altered the CG and Cost index to the values you suggested and it seems to be working much better so thanks for your help on this :)

Andi

[JWS]

Maybe you can compare the figures from PMDG with the data from PM and use the same CG figure in PM (PMDG generates this automatically, PM does not). Like to know what you use.

Glad to be of help.

JWS

[musiconair]

on the subject of compairing PMDG and PM, one thing is bugging me, on the approach ref the speeds are showing quite a low number ie flaps 30 speed 114, on the PMDG, using a similar weight the speeds are around 135-145, I downloaded the PMDG aircraft config for the 737NG but still the approach speeds seem unusually low :(

Andi

[JWS]

Yeah, known issue. I usually "repaired" this with adding some knots until I have a 3 degrees pitch up situation. I find the POSKY model better in this respect.

Can you give me the CG numbers you've come up with?

JWS

[musiconair]

ok I've compared the PMDG and PM CG'S with the same weight load and int ref settings, the PMDG shows a CG of 24.5 :)

[JWS]

Great,

Now to complete things, when you enter a figure for CG (like 24.5) PM will give you a trim value (e.g. 4.5). Now go to menu page & settings (if I'm not mistaken) and you'll find on the right hand side the word TRIM. Hit the LSK and you have a trim page. Now you can push you trim button until there is a number that comes close to the figure given before (4.5). I say "close" because it never occured with me that I hit the exact number (but who cares).

Let me know how that works ut for you.
Regards,

JWS

[musiconair]

ok found the Trim page it was under "extras" and there are a list of 3:
Elevator, Rudder etc, is this the section you mean ?

[JWS]

Yep,

And guess which one you need ......:wink:

JWS

[musiconair]

Yep,

And guess which one you need ......:wink:

JWS

cool, cheers for all this, it's been a real help :)

Best regards

Andi

[choffmann]

Hi Andi, hi JWS

Here´s some answers for the CG issues:

The stabilizer trim naturally has to be set correctly for take off. If it´s too low, you will sit on the runway until the bitter end, if it´s too high, you´ll have a tail strike, which is extremely costly.
To set the stab trim, you need to know your center of gravity (CG). The CG is the virtual point of the aircraft, on which you could balance it, if you could put it on your fingertip. If you look at the length of the aircraft, the CG is usually around the wing area. Therefore, the "dimension" for the CG is correlated to the length of the wing and is expressed as %MAC. What is MAC? It means mean aerodynamic cord. Look at this picture and you know what MAC is:
http://i938.photobucket.com/albums/ad223/christophhoffmann/IMG.jpg

Of course, the CG depends on overall aircraft weight at a certain time, e.g. during take off. T/O CG is different from Landing CG due to fuel burn. However, the changes are relatively small.
Here is a table from the 737-NG FCOM showing the stab trim settings for various T/O weights and different Flap settings.
http://i938.photobucket.com/albums/ad223/christophhoffmann/Screenshot3.jpg

If you want to calculate the CG and stab trim, you can use the free version of TOPCAThttp://topcatsim.com software, which has some very nice features to make your flight dispatch more realistic. Here is a pic of the program:
http://i938.photobucket.com/albums/ad223/christophhoffmann/Screenshot2.jpg
http://i938.photobucket.com/albums/ad223/christophhoffmann/Screenshot1.jpg

However, there are restraints with the use of these figures: Each individual aircraft has its "own" CG due to different versions as ordered by the different companies. A KLM 737-800 will differ somewhat from an AirBerlin 737-800. Knowing this, you will have to know the performance data of the specific aircraft model that you use in FlightSimulator (9/X or whatever). Unfortunately, the software developers of e.g. POSKY do not give these data (at least I couldn´t find them anywhere). I think it´s different with PMDG. They have a manual, which contains some performance data. Your would have to use the SAME performance data in PM software (and you can actually write your individual config file!) as those of the aircraft model you use.

The cost index has impact on the performance of the aircraft, but it has nothing to do with CG: The CI varies between 0 and 500. It determines the speed. CI = 0 means, that the FMC ECON SPEED is the same as the speed for maximal range. CI = 500 means, to fly in the shortest possible time. At CI = 100, the ECON SPEED will drop about 6 knots below maximum speed. A CI of around 30 to 40 is a good compromise between fast flight and fuel savings. The formula to calculate the CI is difficult because of the many items that relay to cost: cost of the airplane operation per hour, cost for the whole (flight and cabin) crew per hour, maintenance cost, leasing cost, fuel prize and other operational issues.

Cheers
Chris

[musiconair]

Thanks Chris, this is certainly turning out to be a learning curve but an enjoyable one, learning all this makes the whole flying part more realistic :)

Cheers
Andi

[JWS]

Thanks Chris,

Cheers
Andi

Dito,

JWS

PS Chris, I thought the free ware version of TOP CAT was limited to the 747 (not 737).

[musiconair]

I got TopCat last night and it's sorted out the take off / landing speeds issue...what a great piece of software :)

Thanks again
Andi

[JWS]

Trial version of full version?

JWS

[musiconair]

Full version, I paid 18 euro's for it, although this morning I saw on Simmarket it was a little more, I just registered it from within program, it was worth paying for it just for the PM cdu to show correct figures.

Cheers
Andi

[choffmann]

I don´t know why, but my free version is running when using the 737 POSKY model.
There are some limitations for computing some figures, but I can use most of the program, and that includes the Loadsheet with the trim settings, Weight&balance setup, T/O and Landing computation of V1,Vr,V2, for the various runways available, all with downloaded current weather.
Anyhow, 18 Euros is really not [I]that[I] much, is it?
Chris

[musiconair]

When it comes to flight sim my respect for money is somewhat downsized :) I fly the PMDG 737 mixed class as I have that as my prefered aircraft, not sure what Posky relates to, perhaps you could enlighten me :) please

Andi

[choffmann]

Posky (short for Project Open Sky) is a freeware product availabe e.g. here: www.avsim.com. Their 737NG models are - in view of many - superior to the FS9 default 737 flight model, but certainly inferior to PMDGs 737. Since PMDG don´t have an open politic concerning their aircraft model, especially when it comes to offsets and programming help with hardware interfaces, many homecockpit builders use the POSKY model instead. I don´t know if PMDGs attitude will eventually bank more than 45° with the upcoming 737NGX for FS X. But even if so, there will likely be problems with hardware interfaces of the various hardware manufacturers. You can add to the list of 737NG flight models the Ariane 737, and - only lately - the IFly737. I have no experience with these products, whatsoever, but I suspect, they will have their issues, too, namely the hardware compatibility. So what probably many home cockpit builders would like to have, is an accurate 737NG aircraft model, better than the Posky, maybe as good as PMDGs, but to date, there is no such accurate and pricey product, AFAIK.

[musiconair]

thanks for the info, will look into the Posky :)

[leilson]

Just use CG between 18 and 20 , and this will give you the takeoff trim set up. Cost index between 25 and 30 .

[musiconair]

Thanks Leilson :)
Andi