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.

Mark Solved

 

Well jackC my NGX only takes 24 % N1 to start moving? Has PMDG messed up here because this isn't right.

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.

The fact that 707's were noisy getting moving is just because they were more noisy full stop. I used to go to LHR to watch airliners in the early 60s, it didn't seem that a lot of power was being used to get aircraft moving. It really isn't about bypass ratio. If the 777 was powered by two 100,000 lb turbojets (imagine the noise) it would get moving just as easily. And those hypothetical turbojets would be moving just as much air as the GE90 does.

Well jackC my NGX only takes 24 % N1 to start moving? Has PMDG messed up here because this isn't right.

PMDG hasn't messed up. Remember they had access to full flight sims to measure such things and pilot feedback to confirm what they do. If you are using the FSUIPC friction mod, as I do, then the NGX needs less thrust than PMDG intended to get moving.

I've just tried the -700 without the FSUIPC friction mod at max TOGW.

 

It begins to roll at 24% N1 but it takes a long time to pick up speed. It will eventually stabilise at about 15-16 knots if left at 24%. Jack's technique would allow you to reach taxi speed more quickly. When you cut to idle the deceleration is quite rapid. It takes about 30 sec for the speed to bleed off to 15 and you need to apply power again to get back up to 20.

 

So what Jack is saying does not conflict with how PMDG set the sim up. If anything it confirms it, but the whole thing is a compromise, you can't expect perfection in every detail.

 

 

The fact that 707's were noisy getting moving is just because they were more noisy full stop.

 

I'll buy this.  Just another misinformation I picked up at an early age that never got tested. Just read up on the JT3D  and marvel that about 18,000 lbs was a lot of thrust back then.  Didn't realize that it's the same basic engine as the TF33 used in the B52 and KC135.  I've watch "water burner" KC's make a lot of smoke and noise and use all the runway many times. Never got boring.

PMDG hasn't messed up. Remember they had access to full flight sims to measure such things and pilot feedback to confirm what they do. If you are using the FSUIPC friction mod, as I do, then the NGX needs less thrust than PMDG intended to get moving.

 

Well surely at MGW it would take alot more than 24 N1 but also FSX doesn't simulate gradients like in real life

 

About that LUA friction how does it work? Because if i enable it my NGX starts rolling quite quick?

It begins to roll at 24% N1 but it takes a long time to pick up speed. It will eventually stabilise at about 15-16 knots if left at 24%. Jack's technique would allow you to reach taxi speed more quickly. When you cut to idle the deceleration is quite rapid. It takes about 30 sec for the speed to bleed off to 15 and you need to apply power again to get back up to 20.

 

What i do is brake my TQ throttles to about 30-35 N1 to move off quickly then shut power and return to idle at 15 kts all depends on taxiway conditions airport speeds or corners and then adjust power as needed

This is from a C-40B Taxi guide that I have. The C-40B is basically a -700C. They call for 40% for breakaway N1.

 

General Taxi Procedures

 

A taxi diagram MUST be readily available to BOTH crewmembers during taxi

Breakout Thrust – approx 40% N1

Personnel should be no closer than 175 feet at idle and 400 feet behind at break out thrust

Max speed – 30 knots

Recommended maximum speed for tiller use:  20 knots

Speed for turns greater than 30 degrees – should be 10 knots or less

Vib range – 15-25 knots

Taxi on one engine?

Not recommended

APU should be running

Requires higher thrust on remaining engine, which creates a greater jet blast and FOD risk

Uphill slope, soft asphalt, high gross weights, congested ramp areas and wet/slippery ramps will require both engines to be operating.

Use thrust reversers?

On slippery surfaces – momentary use of reverse idle is ok, but consider having airplane towed if you think you will need extended use of reverse idle

 

 

Well surely at MGW it would take alot more than 24 N1 but also FSX doesn't simulate gradients like in real life

About that LUA friction how does it work? Because if i enable it my NGX starts rolling quite quick?

It's quite simple. In the NGX if you apply 24% the aircraft starts to roll very slowly, picking up speed slowly because the thrust is only just above the rolling friction force. If you set 35% the accelerating force is higher, so the aircraft gets to taxi speed quicker.  In the real 737 the N1 numbers might be different but the same thing would apply. There would be an N1 at which the aircraft just starts rolling, and there will be a higher N1 where the acceleration is higher and it doesn't take an age to get up to speed.

 

The problem with using the FSUIPC mod with the NGX is that PMDG have already adjusted engine thrust at low N1 so that taxiing thrust and breakaway thrust are representative. This compensates for the higher friction. If you apply friction is reduced so now the engine thrust is too high and the aircraft will roll with very little extra thrust.  On system with the mod enabled, the fully loaded -700 won't move at idle but begins to move at 22% N1.

 

However it's still worth using the mod because the lateral tyre forces and contaminated runway effects are much improved. I use the unregistered version, but with the Lua mod and a registered FSUIPC you can adjust the effects and choose which you want to use. So you could keep the rolling friction as it is and have the modified sliding and/or braking effects. Best of both worlds.

 

 

Breakout Thrust – approx 40% N1

From what others have posted I would imagine this means the thrust to aim at when moving off. If it took 40% N1 to actually get the aircraft moving that's more like the kind of figure FSX needs, which we all know is far too high.

thrust is not a linear function with N2 and N1%but exponential one

 

 

Andrea,

 

Nice explanation. Just a question, is isn't the relationship between N2 and N1 logarithmic instead of exponential (in part due to increased inefficiency as thrust increases)? 

Logarithm is exponential using base 10. The natural log is base e, but I guess you mean a square function x².

However it's still worth using the mod because the lateral tyre forces and contaminated runway effects are much improved. I use the unregistered version, but with the Lua mod and a registered FSUIPC you can adjust the effects and choose which you want to use. So you could keep the rolling friction as it is and have the modified sliding and/or braking effects. Best of both worlds.

 

Brilliant kevin!

 

How do i set it up for the mod sliding and braking effects. But keeping the NGX friction?

Logarithm is exponential using base 10. The natural log is base e, but I guess you mean a square function x².

Logarithmic is the inverse of exponential, whatever the base used.

Brilliant kevin!

 

How do i set it up for the mod sliding and braking effects. But keeping the NGX friction?

As I said I've not got the full lua mod, so I don't know how it's done, I only know it's possible. That's the whole point of the lua adjustable version, you can tune it for various addons. You'll have to look at the lua file that comes with the mod. That includes the original FSX friction values in the comments for reference. You would need to keep the original FSX rolling friction values, and probably the original FSX braking values. Use the new sliding friction values to get the improved steering and crosswind dynamics. Some people refer to these as longitudinal and lateral values. Longitudinal for rolling and braking, lateral for sliding.

 

There are also a lot of adjustment constants for contaminated runways. You could keep the modified versions of those for all sets of data as the PMDG thrust is set up for dry runways.

Hi,

Nice explanation. Just a question, is isn't the relationship between N2 and N1 logarithmic instead of exponential (in part due to increased inefficiency as thrust increases)? 

I don't think, you know: the airmassflow and compression ratios worked out by the compressor depemds on "compressor map" function of the compressor that is different for every compressor and giving for each "corrected N2" or "corrected N1" the air massflow the compression ratio the compressor efficiency etc.The relationship between N1 and N2 is simply when at given N1 and a given N2 you can elaborate the same mass flow (in the core engine)etc. etc., you know: by modyfing fuel flow you can just modify turbine inlet temperature so you can modify just N2%, N1% adjust itself based on "residual energy" remained after that gasflow (air+fuel) has delivered the work necessary to spin N2 turbine (and teorically n2 turbine work is equal to high pressure work also if , naturally, hp turbine work is a little higher because that turbine has to drive also the accessory gearbox and a small part of air delivered by the compressors is being bleeded off for pressurization etc.) but corrected N2 vs. corrected N1 values depends on the relative "compressors maps" , bleed off valves position (or "variable (pitch) nozzle vanes into the low pressure compressor) BPR (by pass ratio) etc. etc....

BTW the only one time I tried to simulate a jet engine (but a single flow turbojet not a turbofan very complicated..having a lot of variables i.e. outer fan compressor ratio, inner fan compressor ratio, lp compressor, hp compressor...) by an excel spreadsheet I've worked out a 6 degree polynominal function(i.e. y=ax^6+bx^5+cx^4+dx^3+ex^2+fx+g) by starting by 7 values worked out by the software "gasturb" (i.e. corrected N% speed vs. corrected Airflow; corrected n% vs. compression ratio etc.)

Best

Andrea Buono

 

 

As I said I've not got the full lua mod, so I don't know how it's done, I only know it's possible. That's the whole point of the lua adjustable version, you can tune it for various addons. You'll have to look at the lua file that comes with the mod. That includes the original FSX friction values in the comments for reference. You would need to keep the original FSX rolling friction values, and probably the original FSX braking values. Use the new sliding friction values to get the improved steering and crosswind dynamics. Some people refer to these as longitudinal and lateral values. Longitudinal for rolling and braking, lateral for sliding.
 
There are also a lot of adjustment constants for contaminated runways. You could keep the modified versions of those for all sets of data as the PMDG thrust is set up for dry runways.

 

Do we actually need this MOD because i thought PMDG done all these effects no?

Do we actually need this MOD because i thought PMDG done all these effects no?

PMDG modified engine thrust and ram drag to try and overcome FSX friction problems. They didn't change the friction itself so the sliding friction problems and contaminated runways issues still exist.

 

It's up to you whether you use the mod, part of the mod or not at all. But you will feel the benefit of better steering when taxiing and maybe feel lower friction on wet or icy runways,

 

 

G550flyer, on 07 Jan 2015 - 7:33 PM, said:


Breakout Thrust – approx 40% N1

From what others have posted I would imagine this means the thrust to aim at when moving off. If it took 40% N1 to actually get the aircraft moving that's more like the kind of figure FSX needs, which we all know is far too high.

I've noticed this in flight sim for a long time, the jets move to easily from stand still. In all of the real world planes I've flown, it takes some input to get going. Even on the MD80 as a passenger, I would hear the engines rev up when initially moving then cut to idle as the jet started moving. The 80 manual calls for up to 1.2 EPR max for break away. While at St Kitts last year, I was parked near an AA 737-800. While I was doing the walk around, it started and taxied out. I did notice it had a higher pitch to get moving, but sounded like idle as he taxied by. I know the bigger jets are notorious for high breakaway thrust at high weights. The DC10 had to be moved every couple of days to prevent tire damage. Our max takeoff gross weight was 590,000 pounds. In the 10-30 we used 10% of gross weight for breakaway thrust. We only used the number 1 and 3 engines for taxi and started the number 2 only when above 500,000 pounds. So if we were 510,000 pounds, we would start all 3 in the chocks but advance to 50% on 1 and 3 to get moving. It took an average of 5 to 7 seconds before it started moving. You couldn't resist looking at the parking brake handle while waiting. The 10% technique was just enough to get moving while minimizing jet blast. Every blue moon there was a report of damage from our break away thrust when in tight parking. The number 2 is really bad because its angled down with no ground friction to slow the thrust. The tire flat spots were the main problem to get it moving and the jet would clunk along as if you had square tires. This issue was worse in cold weather. By the time you cleared the parking area and taxied a bit, the clunking of the tires would have subsided.

 

The thrust output isn't as linear as most think it is, majority of the thrust is concentrated in the upper half of the engine range. This is why it takes a little higher thrust to overcome the flat spot and friction but moves easily once moving. Of course this all depends weight, engines, etc. There's no set break away thrust, just a target. The G5/550 is over powered and will move with little power. She will accel on her own with idle power. The G3 would require power every now and then to keep speed, but the 5 will speed up. We taxi with one reverser at idle to maintain speed, two in reverse idle to slow down and both stowed to speed up. In fact, you only touch the brakes when landing on short runways or when coming to a complete stop. On taxi out we select the brakes page, hold the brakes, set break away thrust and gently release for the brake test.

 

Would be nice if there was a gauge that would resist initial movement on taxi out or when sitting for a while.