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Nyxx

Trim always 2.75

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Hi,

Since going from flying nothing but the freighter to flying AA around the USA.

 

I set fuel as needed and PAX/Cargo I set random until it gives me something above 60%, but trim is always 2.75! I do have fuel on short distance and lower as my flights are not long. e.g KPHX-KLAS

Is this normal or do I have a bug. It was always 4-5 range when cargo.

 

Thanks

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I set fuel as needed and PAX/Cargo I set random until it gives me something above 60%, but trim is always 2.75! I do have fuel on short distance and lower as my flights are not long. e.g KPHX-KLAS
Is this normal or do I have a bug. It was always 4-5 range when cargo.

 

The trim is based on your T/O CG.

 

If your takeoff CG is the same each flight, your trim value will be the same each flight.

 

Since you're setting a load that's nearly exactly the same each flight, with a fuel load that's exactly the same each flight, you're going to get a trim value that's exactly the same each flight.

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Kyle,
Please read again. OK a little more detail.

I have PAX/Cargo load in the range of 60-100%, yes fuel load is always around the same. But going from 60% to in the 90% still gives the same trim.

Kyle if I was using the same loads ofc it would be the same every time and I would not be wasting my time or yours asking this question :smile: And yes I do know what trim is based on. That's why I note it down when I have loaded fuel/pax/cargo.

 

Unless you saying 60-100% in weight is nothing for the T7? I don't do light loads because of the busting 250 speed problem.

Again unless you saying fuel is the main weight thing and setting this light is the main reason for the trim always being 2.75.

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Kyle,

Please read again. OK a little more detail.

 

I have PAX/Cargo load in the range of 60-100%, yes fuel load is always around the same. But going from 60% to in the 90% still gives the same trim.

 

Kyle if I was using the same loads ofc it would be the same every time and I would not be wasting my time or yours asking this question :smile: And yes I do know what trim is based on. That's why I note it down when I have loaded fuel/pax/cargo.

 

Unless you saying 60-100% in weight is nothing for the T7? I don't do light loads because of the busting 250 speed problem.

Again unless you saying fuel is the main weight thing and setting this light is the main reason for the trim always being 2.75.

 

It's not the load that matters but the distribution of that load throughout the aircraft. TO trim is affected by the CG of the aircraft which we can control via the distribution of the load in the FMC, IE putting a certain value for passengers, a certain value for front cargo bay etc... Because the 777 only has wing and central fuel tanks the CG isn't affected greatly by fuel loads hence trim barely changes. If you are flying every flight full than the only factor affecting trim is fuel which as mentioned doesn't really have a great effect on CG hence the trim value not changing. I am quite certain the PMDG loader when using a % to load the aircraft it does so evenly (IE it doesn't fill up the front cargo bay unequally to the rear bay) so CG isn't greatly influenced there either.

 

As an example, If you were to completely load out the front cargo bay but not the rear bay, I'd hazard a guess that TO trim will be much greater than 2.75

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I have PAX/Cargo load in the range of 60-100%, yes fuel load is always around the same. But going from 60% to in the 90% still gives the same trim.

 

I read your post to say "around" and not "above."  My mistake.  As Lawrence mentioned, though, it's the distribution of the weight that affects the CG, though, and as he pointed out (and I agree), the PMDG random load function probably keeps it on a certain weight distribution.

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Great, I can understand that using Random load and PMDG making that load out balance out.

 

I will do my own loading for fun and mix it up a bit.

 

Thank you for the explanation. Now it makes sense.

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Unless you saying 60-100% in weight is nothing for the T7? I don't do light loads because of the busting 250 speed problem.
Again unless you saying fuel is the main weight thing and setting this light is the main reason for the trim always being 2.75.

 

not that 250 kt problem again :O

 

http://www.youtube.com/watch?v=5FQ9179qKUw&feature=c4-overview&list=UUkW4jYLfruaR5oQF61-DgKA

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It's not the load that matters but the distribution of that load throughout the aircraft.

 

Actually it's both! Increasing ZFW, even if the distribution remains the same, will create a greater nose down pitching moment that will require offsetting with increased nose up trim.

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Dont worry about the Trim setting.

 

:-(

 

Since the fly by wire system is all wrong (an easy fix acoording to PMDG!), this 777 autotrims :-(

And it should not!

 

With that, your 2.75 wont last long anyway.

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Actually it's both! Increasing ZFW, even if the distribution remains the same, will create a greater nose down pitching moment that will require offsetting with increased nose up trim.

 

Hi, Jordan,

 

Curious to know why that is the case.

 

Thanks,

Mike

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Hi, Jordan,

 

Curious to know why that is the case.

 

Thanks,

Mike

 

4_forces_stab.jpg

 

 

If you look at the picture above, you can see that lift and weight forces are not co-incident. Centre of lift is aft of the centre of gravity causeing a nose down pitching moment; this requires the stabiliser apply a downwards force to keep the nose up. The picture is of a 172, but the arrangement holds true for almost all aircraft.

 

If weight increases, even if the centre of gravity remains in the same location, the nose down pitching moment will increase requiring more nose up trim to compensate.

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If weight increases, even if the centre of gravity remains in the same location, the nose down pitching moment will increase requiring more nose up trim to compensate.

 

Hi Jordan,

 

Thanks very much!

 

Mike

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