Hey all,

 

Just was wondering how much thrust it takes you guys to keep the 777 moving when taxiing. Before I got the aircraft I was under the impression that, like all planes, you need a bit of thrust to get the plane initially moving but once that happens (with those big GE-90 engines) the plane can taxi with idle thrust pretty much (except maybe when turning). Maybe I was wrong because, for me, once I get the plane moving I will need to keep a bit of thrust throughout the taxi (even in a straight line). If I bring the power levers down to idle the 777 will slow down to an eventual stop. Is this realistic?

 

Joel

Hey,

 

it very much depends on weight. If you're very light the 777 starts to roll in idle thrust..if it's very heavy you have to give a bit thrust to get it rolling and to keep it rolling. A friend of mine is pilot on A330 and A340, he states that this behavior is quite realistic.

 

Julian

I see. I thought this might be the case. But even on the tutorial flight, with 76,000 lbs. of fuel, I still need some thrust to keep the 777 moving. Wouldn't this be a pretty light plane?

Not a problem so long as you account for it in your fuel planning. How fast are you trying to taxi?

Not a problem so long as you account for it in your fuel planning. How fast are you trying to taxi?

 

Maybe 15-20 kts

Maybe 15-20 kts

 

Yeah. 

 

I admittedly don't fly the 777 but with such a large gross weight, you're going to need more than idle throttle to keep her moving. That many tyres = lots of drag too.

I find around 23-26% N1 is fine except when very heavy, at MTOW I need 27-32% N1. Make sure to carry a bit of power through the turns to avoid stopping with nose gear angled, this requires more breakaway thrust.

Interesting. Aren't there some airliners that like to keep moving with idle thrust with an average fuel/payload combo onboard? I could have sworn I heard this somewhere. Maybe I was mistaken and its more of the narrow bodies that do this over the heavies. I don't fly the NGX but can that normally be taxied on idle thrust once it gets going?

Interesting. Aren't there some airliners that like to keep moving with idle thrust with an average fuel/payload combo onboard? I could have sworn I heard this somewhere. Maybe I was mistaken and its more of the narrow bodies that do this over the heavies. I don't fly the NGX but can that normally be taxied on idle thrust once it gets going?

 

Even the NGX slows down, just not too quickly. Its easier to get it rolling then give it a few seconds of thrust to keep it moving than it is to find the perfect N1 to keep it rolling.

This is unfortunately a problem with the FSX ground friction model that the team cannot do anything about (I know because I flagged it during the initial development). It was modified but any further tweak would have a lot of other consequences which i'm not even sure about so this is a the best compromise we could get. Normally at light weights you wouldn't even need any thrust to start taxiing and even at heavy (max tow) you only need idle once you're moving.

You can always try the DynamicFriction Lua script included with FSUIPC, It can be tweaked to your liking as well

There is a tweak to put in the FSUIPC.ini file to get rid of the ground friction problem.

In my -200LR, at very light weight, I release parking brake and no need to add thrust to start moving.

At heavy weight, I need a little thrust to start moving then back to idle, and the aircraft is moving slowly without thrust, like in real world.

In real life, if anything, you have to worry more about riding the brakes and thereby causing excisive break wear or hot brakes.

(although the 777 is not verry susceptible to hot brakes compared to other aircraft)

 

Even at idle the real 777 will just get faster and faster untill you slow it down with one firm brake application from 30kt to 15kt or so.

(one firm application causes less brake wear on carbon brakes than two medium applications....dont ask me why, but its true)

And then it all starts over again.

 

As said above, its probably the friction model in FSX.

 

Those two ideas of adapting this with FSUIPC sound nice, but I would be afraid that will mess up other things.

So I just ignore the little difference to reality.

or you could pretend you are going uphill :-)

Just a suggestion from me: I just will not encourage on hacking the pmdg but you can use dynamic friction lua script within fsuipc. I set an taxi speed of 130kts and halfened the values for friction within the script. This gives a good idle taxi speed and especially good brake heating after touchdown as there's no fsx_friction brake anymore. Above 130kts the fs physics are default.

 

 

...That many tyres = lots of drag too.

 

...well, friction at least.  Sorry, this is the enginerd in my coming out.  Drag at 15-20 notes is virtually zero (drag increases as a square of speed).  Further, drage depends on cross sectional area (primarily), so you could stack those 3 wheels in each bogey right behind each other and the drag is very near a single tire due to the cross section the air sees being equal. 

 

The loss from wheels is due to friction between the tire and the ground, as well as the tire and it's bearing on the strut.  For tires, this can get lumped in to a phrase "rolling resistance".  This also includes the angular momentum each tire sees.  This absolutely increases with each tire.

 

Sorry, I try not to pick on words normally and you're absolutely right about the concept.  I see many aviation folks (professional and sim) talk in words of drag for anything that slows them down, even if it's a wheel on the ground at 5 kts where drag effectively is non-existent.