All the videos I've seen on Youtube seem to show the that both EICAS N1 digital values are almost always exactly the same on both engines, including changing at exactly the same time. Presuming the display is showing measured N1 fan speed rather than commanded it means the FADECs are doing an amazing job of syncronising both engines! I can see tiny variations but only occasionally, whereas the EGT and N2 values seem to vary more as I'd expect. Having never flown a real T7, I can only base my question on my experience in Citation XLSs and Bravos, which I have flown... Is this modelled correctly in the sim or is it a little too rigid? Are any rated 777 pilots able to comment?

 

Best wishes,

 

Luke

 

 

Is this modelled correctly in the sim or is it a little too rigid?

 

Well, I'm assuming the Citation XLS and Bravos you fly have two throttle levers...

 

A lot of the guys (including myself) are flying with only one throttle lever, which actually adds a lot of that variation, believe it or not.  Here's a famous example of that (granted, from the days of mechanical input, but still, lever variation):

 

 

Additionally, the other part of it is somewhat of an issue of accumulated time and associated wear, which is difficult to simulate in Flight Sim.

 

They're definitely independent values here in the sim.  They're just not going to vary widely like engines that have been hanging on an aircraft over several years of use.

 

Thanks Kyle.

To expand a little on my thoughts... If both FADECs were commanded to produce exactly the same N1 performance at exactly the same time, by the time the command was translated into fuelling variations in the turbine, all sorts of other asymmetry factors will have come into play and influence each engine's performance in slightly different ways. In reality, the FADECs will measure the commanded N1 value from the throttle or auto throttle input and make tiny instantaneous adjustments to keep N1 at the commanded value, in a continuous feedback loop. However, the turbine components, particularly the front end fan have huge inertia and therefore it is never possible to completely and perfectly synchronise two or more engines all of the time - the nearest thing is continuous and multiple FADEC corrections to give the appearance of synchronisation, particularly when they are being continuously affected by environmental and mechanical factors which differ both internally and externally for each engine.

 

That's the tech stuff! The simple result is that you'll always observe slight variations between all values of more than one engine compared to each other and I was wondering if this element was simulated realistically. The PMDG NGX does it very well, creating the very convincing illusion that you are monitoring two completely independent pieces of engine hardware :-)

Of course, as you elude to, if the throttle levers are not commanding the same fan speed value, you'd expect to see the variation between the engines. My question is really asking about variation between the engines when the throttles are exactly (or as near as possible) the same. You could also look at it like this... For a given constant throttle position on one engine alone, you should expect to see occasional fluctuations in N values for all of the influencing factors mentioned above.

P.S. Liked your video!

Luke

 

 

My question is really asking about variation between the engines when the throttles are exactly (or as near as possible) the same. You could also look at it like this... For a given constant throttle position on one engine alone, you should expect to see occasional fluctuations in N values for all of the influencing factors mentioned above.

 

For sure.  Even with my single throttle control, you can see variations between the engine speeds.  It's not huge, but it's definitely there.  You can see some of it in the Tutorial video I posted.

 

...and thanks!

In real life the variation will be down to things like manufacturing tolerances and slight differences in wear. To model that in FS you would have to toss in a random 'error' into your engine model somewhere.

I think I read somewhere that PMDG model a random error on the navigation systems, but I'm not sure if they do it for the engines too.

 

Edit: Kyle beat me to posting, looks like PMDG did indeed model engine differences, it doesn't fail to amaze how deep down the rabbit hole goes

Actually, this is worth a watch... It certainly shows only minimum variation in the real aircraft :-)

 

http://youtu.be/5HYRITT4CsI