737 NGX taxi N1 %

[Driver170 42]

I'm wanting to find out what N1 % it takes for the NG or in the sim world the NGX to start moving those big rubber wheels. I know there is alot of factors involved, But here in the sim world with my bird fully loaded fuel and payload, it only takes 24 % N1 to start her moving then i increase the N1 to 30 untill 10kts then back to idle and adjust the N1 to hold taxi speed.

 

Is 24 % N1 about right to start moving a fully loaded 737 NG?

 

Cheers.

[downscc 1,327]

Hi Vernon. Yup, that is in the ballpark. Turbojets required quite a push on the throttles to break free because their static thrust was dismal but modern fans have more than adequate static thrust to overcome the initial rolling resistance.  By the way, those big rubber wheels have about the same rolling resistance at zero as at some other speed because rolling resistance is mostly due to the energy it takes to deform the tire and the amount or deformation is pretty constant at all taxi speeds. The initial resistance is mostly just plain ole' inertia.

[kevinh 409]

 

 

Turbojets required quite a push on the throttles to break free because their static thrust was dismal

Not sure what you mean here. The static thrust of a turbojet is actually it's maximum value. There is no ram drag when you aren't moving. All jet engines use static thrust as a measure of their thrust output. As you increase forward speed, net thrust reduces.

[77west 139]

Not sure what you mean here. The static thrust of a turbojet is actually it's maximum value. There is no ram drag when you aren't moving. All jet engines use static thrust as a measure of their thrust output. As you increase forward speed, net thrust reduces.

 

I think he meant residual thrust when at idle. I don't have specifics but say a GE90 idles at 22% N1, this would equate to perhaps 25366lbs residual thrust per engine. Even if its only 20000lbs, that's still the equivalent of a 737 at takeoff power.

 

Caveat: I don't have exact numbers and realize thrust may not be linear to N1, but it gets the point across.

[downscc 1,327]

Thanks Wes.  I struggled with the right words, and looked at static for a couple minutes trying to come up with a better word.

 

I remember as a kid in LA in the 50's we'd go to LAX to watch airplanes (it was cheap and long before TSA).  Those big loud 707's had to create quite a roar to start moving and I heard back then that they could not jump chocks.  High bypass fans move a lot of air (mass) at idle.

 

I actually got to stand behind an AF E-4B NEACP (B742) on takeoff, I was right behind the localizer antenna at Offutt AFB. Never forget that or the wonderful smell of JP4 in the morning.

[77west 139]

Thanks Wes.  I struggled with the right words, and looked at static for a couple minutes trying to come up with a better word.

 

I remember as a kid in LA in the 50's we'd go to LAX to watch airplanes (it was cheap and long before TSA).  Those big loud 707's had to create quite a roar to start moving and I heard back then that they could not jump chocks.  High bypass fans move a lot of air (mass) at idle.

 

I actually got to stand behind an AF E-4B NEACP (B742) on takeoff, I was right behind the localizer antenna at Offutt AFB. Never forget that or the wonderful smell of JP4 in the morning.

 

Yeah I think the higher the bypass ratio, the higher the relative residual thrust becomes. I just returned to NZ from Rarotonga (4HR25MIN flight) on a 738 and we were close to MTOW according to the FO and they still only had to use a small amount to get rolling from what I heard sound wise.

[SKEWR 364]

24% sounds good to me. High OAT and weights will increase the N1 needed.

Try not to get to 40% or over, as you will start blowing people (and equipment) around.

[Andrea1 6]

Hi,

residual thrust , that high by-pass ratio turbofans are ground idling at 20% N1 and 60% N2 is about 3-4% of the rated thrust, you know thrust is not a linear function with N2  and N1%but exponential one (lower the compressor speed lower the compression ratios, air massflow entering to the engine, turbine inlet temperature causing a low "enthalpic delta temperatures" to be useful to produce thrust..."

So I can't believe that a GE90 can produce 25366 lbs, also if rated at 115000 pounds 4%*115000=2100-2200 lbs not 23000 and more...in fact fuel flow at idle should be something like 900 kg/h and not 9000...

 

A software to design and study jet engine (freeware): www.gasturb.de

 

Best Regards

 

Andrea Buono

[kevinh 409]

I think he meant residual thrust when at idle. I don't have specifics but say a GE90 idles at 22% N1, this would equate to perhaps 25366lbs residual thrust per engine. Even if its only 20000lbs, that's still the equivalent of a 737 at takeoff power.

 

Caveat: I don't have exact numbers and realize thrust may not be linear to N1, but it gets the point across.

There is no way a GE90 idles at that kind of thrust. You simply can't assume thrust is takeoff thrust factored by percent N1. The relationship is very non linear.

 

Turbojet idle thrust isn't necessarily lower than turbofan thrust. There's still a lot of high velocity hot air coming out of the jet pipe.

 

I don't know why Dan introduced the idea that turbofans were the reason not much extra thrust is required to get an airliner moving. Low bypass fans like the JT8D and Spey have similar characteristics to turbojets and aircraft like the 727 and F.28 don't need much thrust increase to get moving either. There is almost no drag and rolling resistance is low, so thrust need not be high.

 

Simmers think high thrust is necessary because that's what FSX requires. That's why some think it's wrong for an airliner to start moving simply be releasing brakes or with only a small increase in N1.

[Driver170 42]

24% sounds good to me. High OAT and weights will increase the N1 needed.

Try not to get to 40% or over, as you will start blowing people (and equipment) around.

 

Ok thanks this is what i'm observing, i just thought it could be my Jetmax TQ 737 throttle that was off and just wanted to know from you guys if this was normal.

 

Good knowledgeable information here aswell

[Jack C 8]

Is 24 % N1 about right to start moving a fully loaded 737 NG?

Cheers.

24% is not enough to get a fully loaded, 156,000 lb, 700 to start rolling. 24% is idle thrust. I will usually advance the thrust up to about 35% to get the momentum going and chop it back to idle. Once you get up to a taxi speed of say 15-20 kts, to maintain it you may only need to bump the thrust up to 30% then chop the thrust back to idle. Just keep doing this advancement of the thrust and chopping of the thrust to maintain 15-20 it taxi speed.

[JoeDiamond 67]

In the -800 it's not uncommon to have to push the N1 all the way to 40% to get it rolling depending on the weight and the condition of the surface and the slope of the ramp.  Once you are moving you can pull it back but it takes a fair amount of power to get it rolling.  This is the main reason you don't see many people using single engine taxi, it's too hard to get it moving on one engine.

[Andrea1 6]

HI Folks,

to give you an idea at low N1 setting thrust is almost proportional to 2nd power of N1 . Ie with 20%N1 (0,2) thrust is about 0,2*0,2=0,04 takeoff static thrust..as rule of thumb..if you design and test a turbofan by the freeware gasturb you can check by yourself (naturally in that case, being the freeware limited to "basic turbofan", not having variable nozzles vanes simulation or bleed off valves to discharge airflow at low n1% you cannot reach 20% n1 because the minimum n2% and n1% rotor speed having a "fixed geometry" are higher (at 20% n1 engine wouldn't be "self-sustaining)

 

Best

Andrea Buono

[Andrea1 6]

Hi,

you can also perform this test:

you know in reality specific fuel consumption is HIGHER at lower N2% (I think about N2 because, as you know there is a direct relation between turbine inlet temperature, and consequently fuel flow, and N2) because the overall efficiency of the engine is lower (lower compression ratio, mass airflow etc.) but assume it as a fix one i.e. at the takeoff value i.e. about 0,35 kg per kg of thrust.

Now put you B738 in an airport at sea level isa condition( 1013 mb 15°C) and record fuel flow at ground idle (i.e. 300 kg/h) from that data you can calculate your "theoric" thrust as 300/0,35= 850 kg now repeat that exercise for various N1 setting and you've a tendence of how thrust varies vs. N1.

P.S.

Naturally in real life specific fuel consumption is higher than 0,35 in idle (i.e. 0,7-0,8)

Ciao

Andrea

[77west 139]

I looked it up and for the GE90 it is around 4000lbs per engine at idle apparently.

 

I did say in my post I didn't believe it was linear.