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Rolling resistance after latest update?

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Posted (edited)

I noticed that after updating last night, it takes very little throttle input (even with just two engines) to move the aircraft (on a long haul load-out with 80% capacity) ... this is definitely different than prior version.  Obviously I have NO idea which is more accurate, but where changes made in the area of rolling resistance?

Cheers, Rob.

EDIT: this was in the rain at KSFO (FB's) and rain should increase rolling resistance and reduce stopping efficiency.

Edited by Guest

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Posted (edited)

Doesn’t seem realistic for an 80% load, but I have noticed with light airplanes the resistance seemed a little high (I haven’t done the update yet though). Typically with a light load and level surface the real airplane will accelerate in idle during taxi. Once it gets to 18 knots or so you would brake back down to a slower speed as to not ride the brakes. 

Edited by Jetlinker

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Posted (edited)

If your payload is at 80%, you may still be light unless the tanks are also nearly full.  I've not noticed a change, usually about 29-31% N1 will slowly get her rolling and i keep her to 10 kt unless on a long straight stretch then I let her get up to 18-20 kt but at that speed you have to close the throttle 0.5 nm before reaching a turn to slow down to 10 kt (where most turns are made).

Taxi in a heavy 748 must be done with caution.  Simply rolling resistance heats the tires even without braking so when near a million pounds ramp weight take it easy.

Edited by downscc

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I was using the same load-out (both fuel and cargo) as before the update under same wet conditions ... this update definitely requires much less throttle (at idle it will just taxi along at 15kts).  Surface wind speed was around 19 Kts but regardless of direction I could idle along at 15Kts. 

I'm NOT disputing whether this is accurate or not, I was just wondering if something was intentionally changed in relation to rolling resistance and/or wet runway/taxiway resistance?

Cheers, Rob.

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Not that I'm aware of Rob, and it wasn't in the bug list reported as fixed items.  There was an issue with landing under very specific conditions where the simple spring strut (think C-172) model in the simulator was causing strange stuff such as no rolling resistance after landing.  Some folks that like to hold the nose up after touchdown would run into this but it has nothing to do with rolling resistance while taxi.

I just taxied a B748F from the Imperial Cargo Ramp at LAX for a 07R departure with precip and at idle (about 24% N1) the aircraft would slow.  I had to bump throttle up to 28% once in awhile to maintain 18 kts.  Don't know why we are seeing different things.  Thow in some numbers for easier comparison.

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Posted (edited)

Rob,

I‘m not sure about this but it seemed to me recently that I experienced  differences in the roll friction with the A2A P51 and T6 depending on the addon airport. I can‘t tell if this is even possible but those airplanes always weight around the same each time. However I noticed that yesterday it required around 1300 RPM in the mustang to start rolling and today, an hour ago, it was only 1100. same fuel level, similar weather/temp. However yesterday ORBX Innsbruck northern GA parking, tonight it was Aerosofts EDDK GA apron. 

Edited by Ephedrin

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18 minutes ago, Ephedrin said:

However I noticed that yesterday it required around 1300 RPM in the mustang to start rolling and today, an hour ago, it was only 1100. same fuel level, similar weather/temp. However yesterday ORBX Innsbruck northern GA parking, tonight it was Aerosofts EDDK GA apron. 

 This one sounds like more of an elevation difference perhaps?

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Posted (edited)
32 minutes ago, Jetlinker said:

 This one sounds like more of an elevation difference perhaps?

could be... 1600ft difference.. but this was on the apron... on the runway it was both the same  (1000rpm)

Edited by Ephedrin

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4 hours ago, Rob Ainscough said:

I was using the same load-out (both fuel and cargo) as before the update under same wet conditions ... this update definitely requires much less throttle (at idle it will just taxi along at 15kts).  Surface wind speed was around 19 Kts but regardless of direction I could idle along at 15Kts. 

I'm NOT disputing whether this is accurate or not, I was just wondering if something was intentionally changed in relation to rolling resistance and/or wet runway/taxiway resistance?

Cheers, Rob.

As Dan mentioned it was not on the bug list.

Can you be more specific please, 400 x -8, exact location total weight and taxi route please.

There are some guidelines for taxi:

Note: When taxiways or ramps are contaminated, three engine taxi is
prohibited.

The following weights are for guidance only:
• Less than 330,000 kgs (730,000 lbs), start at least three engines.
• Less than 300,000 kgs (660,000 lbs), start at least two symmetrical
engines.

Maximum recommended break away thrust is 40% N1
Normal taxi speed is
approximately 20 knots, adjusted for conditions. On long straight taxi routes,
speeds up to 30 knots are acceptable, however at speeds greater than 20 knots
use caution when using the nose wheel steering tiller to avoid over-controlling the
nose wheels. When approaching a turn, speed should be slowed to an appropriate
speed for conditions. On a dry surface, use approximately 10 knots for turn angles
greater than those typically required for high speed runway turnoffs.

Note:
High taxi speed combined with heavy gross weight and a long taxi
distance can result in tire sidewall overheating. At maximum weights
with taxiing distances greater than 15,000 feet (4575 m), use 20 knots
as maximum taxi speed.

 


Also there is a slight difference between the two how they taxi.

Maybe something got corrupted during update...……...

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1 hour ago, killthespam said:

High taxi speed combined with heavy gross weight and a long taxi
distance can result in tire sidewall overheating.

Can s.o. explain this? Is this due to the compression of the tires and hence the rubber getting warm from it‘s own movement? Can they become THIS hot from a simple 30kts? Sure, a car is much lighter, but after an hour on the autobahn with 150+ km/h the tires are warm but you can still touch them easily. 

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10 hours ago, Rob Ainscough said:

I noticed that after updating last night, it takes very little throttle input (even with just two engines) to move the aircraft (on a long haul load-out with 80% capacity) ... this is definitely different than prior version.  Obviously I have NO idea which is more accurate, but where changes made in the area of rolling resistance?

Cheers, Rob.

EDIT: this was in the rain at KSFO (FB's) and rain should increase rolling resistance and reduce stopping efficiency.

Good to see changes are being made to this problem. But I can’t remember seeing this on the PMDG recent update change list.

Im hoping this was indeed updated. I only had time to install the update sit in the cockpit (using VR) and enjoy the new rain effects while sitting at the gate in SFO before heading out on a RW flight away from home.

Been a few years since I flew the real 744, but Rob 80% fully loaded should only require a small amount of breakaway thrust. Most times around that weight a little thrust was need just to get her up to desired speed (10-15kts) and then idle thrust for the remainder of the taxi, depending on the amount of required turns.

Thx again to PMDG if they did get to play with the ground friction.

IM

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1 hour ago, Ephedrin said:

Can s.o. explain this? Is this due to the compression of the tires and hence the rubber getting warm from it‘s own movement? Can they become THIS hot from a simple 30kts? Sure, a car is much lighter, but after an hour on the autobahn with 150+ km/h the tires are warm but you can still touch them easily. 

Even tires are filled with Nitrogen at a long taxi distance and multiple turns due to sidewall forces they gonna get hot and even blow up (melt the fuses). It can happen quite a lot like in AMS where you taxi forever sometimes to certain rwys or high elevation airports.

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I discovered my issue, I didn't actually have a 80% load, I had 9.7% ... ooops!  Set to random and got 57% and fuel set to Medium haul and took more juice to get moving and a tad more juice to stay around 10-15Kts taxi.

I've seen hot brakes on fire causing tires to burst, but I must admit, I've never see a tire explode from just taxi unless the tire already had a problem from prior usage.  I would expect engineers to know what is needs when it comes to tire reliability even for a 975,000 lb vehicle.

But here ya go for a good read on tire speed ratings: https://www.bridgestone.com/products/speciality_tires/aircraft/products/applications/pdf/tire_applications.pdf

235 MPH

Cheers, Rob.

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I agree, you wouldn't expect tire heating to be a problem but in the B748 there are several cautions in the Boeing FCTM related to higher taxi speeds and tire sidewall flex and heating.  One has to think differently when carrying 920,000 lbs around.  It's not done lightly.

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5 hours ago, downscc said:

 It's not done lightly.

I see what you did there. 

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What a fabulous informative thread this has been. Just discovered it, and really learned a lot. Glad, now that my favourite go-to aircraft has become the days old bought -8i, I kept a long taxi at EGLL to 27R no more than 12 kts.

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