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777 Rolling Resistance on the Ground

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    777 Rolling Resistance on the Ground

    Hi, just wondering if anyone else seems to think the 777's rolling resistance seems a bit much?

    In the real aircraft, at most weights south of MTOW the plane with gradually increase its taxi speed, requiring constant braking to control speed.

    But in the sim even a light weights the taxi speed always slowly decreases.

    Thoughts?

    Peter Lawrence

    #2
    Correct, having the same issue. Hopefully they will fix this in the next update?
    Like the NGXu and 747 already have.

    Comment


      #3
      Yes, I also noticed this. At lower weights it shouldn't even be required to spool up the engines to start moving for taxi. Just releasing the parking brake should be enough to get moving.
      Omar Josef
      737 FO
      757/767 rated
      Spain

      Comment


        #4
        If you have FSUIPC Registered version, you can use the DynamicFriction LUA script to alleviate this issue.
        With this script, the plane almost taxies by itself on idle power.
        Thomas Tran

        Comment


          #5
          Originally posted by Mansamoth View Post
          If you have FSUIPC Registered version, you can use the DynamicFriction LUA script to alleviate this issue.
          With this script, the plane almost taxies by itself on idle power.
          While I would agree with you for nearly all addons, PMDG addons (and also FSL and A2A for that matter) use custom taxi models that circumvent the P3D taxi physics. Rather than messing with these addons and probably breaking them in other areas it's better to talk to PMDG about it. All above as long as the 777 is still being worked on they are quick to fix these kind of things. But it has to be brought to their attention, they have other things to do than to fly the whole day.
          i7-6700k, GTX 1080TI, 32GB DDR4 RAM @2666MHz, 4k
          Marc Ehnle

          Comment


            #6
            Yeah it does feel like it can take a bit to get moving. There also feels to be a big dead zone in the amount of thrust required to actually get and then maintain momentum. Seems like the first 8-12% of N1 does nothing and then once you break past that zone the aircraft will start moving, then not even 1%n1 more than that speed will rapidly shoot up and get to 20kt+ I’m barely more than a second or two.
            Tim Smith

            Comment


              #7
              If your not sharing weights and which 777 variant and if 200ER with which engine, it’s difficult to discuss this topic.
              George Morris

              Comment


                #8
                Originally posted by Falcon99 View Post
                If your not sharing weights and which 777 variant and if 200ER with which engine, it’s difficult to discuss this topic.
                I haven't written down actual weights but it seems to me that the lighter the planes are, the more they slow down. Heavier planes seem to have more inertia that outweighs the "roll drag". The latest flight was a 300ER with 5% payload and fuel for a flight from OOMS to OMDB and I needed a) significant thrust to get moving and b) around 28% to accelerate, idle slowed be from 25 knots to full stop within approximately 30-40 seconds. In the 200ER I flew the GE and PW models so far + the freighter and noticed similar behaviour in all of them, much heavier (trans-atlantic fuel) though.
                i7-6700k, GTX 1080TI, 32GB DDR4 RAM @2666MHz, 4k
                Marc Ehnle

                Comment


                • cmdn
                  cmdn commented
                  Editing a comment
                  With that 5% payload you should not even need to spool up the engines. Releasing the parking brake should be enough. Plane would taxi itself.

                • Ephedrin
                  Ephedrin commented
                  Editing a comment
                  That‘s the point.

                #9
                Originally posted by Ephedrin View Post

                I haven't written down actual weights but it seems to me that the lighter the planes are, the more they slow down. Heavier planes seem to have more inertia that outweighs the "roll drag". The latest flight was a 300ER with 5% payload and fuel for a flight from OOMS to OMDB and I needed a) significant thrust to get moving and b) around 28% to accelerate, idle slowed be from 25 knots to full stop within approximately 30-40 seconds. In the 200ER I flew the GE and PW models so far + the freighter and noticed similar behaviour in all of them, much heavier (trans-atlantic fuel) though.
                Interesting thought.... but not sure weight is directly related with "roll drag," which I define as the rolling resistance and it is something that stays pretty constant over a range of speeds and weights. Rolling resistance is primarily caused by the energy absorbed by the tires and they flex and bend as they roll, no small amount of energy considering their inflation of over 200 psi and how big they are... and how thick the sidewalls are. Weight is related to friction so weight becomes a factor when making turns, but not in regards to rolling resistance. There is an inertia, greater the weight the more the force required to achieve a given acceleration. This is where the idea that it takes more thrust to overcome surface friction which is patently false, it is basic f=ma. None of this is realistically simulated in the simulation platform, and this is the kind of stuff that RSR loves to speed all of those late nights pounding away at his development workstation creating a realistic rolling resistance model. It's much more than I could ever bring together.

                A lighter airplane has less kinetic energy at the same speed as a heavier airplane, so there should be a difference between the deceleration of a light and heavy aircraft due to rolling resistance. Kinetic energy is the derivative (df/dt) of force so now you have the tires absorbing energy at a given rate depending on their rotation speed, but the rotation speed of the tire and not the speed of the aircraft which is the (1/2)mv^2 kinetic energy; therefore the heavier airplane at the same speed has more kinetic energy and give that rolling resistance doesn't vary significantly it takes longer (lower deceleration) to convert that increased kinetic energy to heat due to tire flex. Nothing outweighs the "roll drag" or rolling resistance, it is always there, always absorbing energy from the aircraft's movement.
                Dan Downs KCRP
                i7-10700K 32GB 3600MHz 2080Ti

                Comment


                  #10
                  Originally posted by DDowns View Post

                  Interesting thought.... but not sure weight is directly related with "roll drag," which I define as the rolling resistance and it is something that stays pretty constant over a range of speeds and weights. Rolling resistance is primarily caused by the energy absorbed by the tires and they flex and bend as they roll, no small amount of energy considering their inflation of over 200 psi and how big they are... and how thick the sidewalls are. Weight is related to friction so weight becomes a factor when making turns, but not in regards to rolling resistance. There is an inertia, greater the weight the more the force required to achieve a given acceleration. This is where the idea that it takes more thrust to overcome surface friction which is patently false, it is basic f=ma. None of this is realistically simulated in the simulation platform, and this is the kind of stuff that RSR loves to speed all of those late nights pounding away at his development workstation creating a realistic rolling resistance model. It's much more than I could ever bring together.

                  A lighter airplane has less kinetic energy at the same speed as a heavier airplane, so there should be a difference between the deceleration of a light and heavy aircraft due to rolling resistance. Kinetic energy is the derivative (df/dt) of force so now you have the tires absorbing energy at a given rate depending on their rotation speed, but the rotation speed of the tire and not the speed of the aircraft which is the (1/2)mv^2 kinetic energy; therefore the heavier airplane at the same speed has more kinetic energy and give that rolling resistance doesn't vary significantly it takes longer (lower deceleration) to convert that increased kinetic energy to heat due to tire flex. Nothing outweighs the "roll drag" or rolling resistance, it is always there, always absorbing energy from the aircraft's movement.
                  I searched the word „outweigh“ in the dictionary, maybe I picked the wrong word. I meant that it seems like once the plane is moving you can idle the engines and the heavier it is and so the more inertia it has, the less the rolling resistance can effectively brake the plane down. While with less load the rolling resistance has a stronger effect on the sim model than it should have. The resistance, as you say, doesn‘t change much while the kinetic energy increases a lot with higher mass. Of course I don‘t know how Robert calculates the acceleration against the force of engine and rolling resistance but IF there is a mistake in the equations it won‘t behave correctly. And right now what I see is a deceleration which seems to relate to the mass of the moving aircraft while it requires about the same thrust to start rolling.

                  Beside all calculations and theory it doesn‘t take much to try it out. Assuming that we don‘t have problems with our sims you should find the same. Load the aircraft lightly and heavily and taxi. In my case with the 300ER I noticed that the lighter it is the faster it slows down.
                  i7-6700k, GTX 1080TI, 32GB DDR4 RAM @2666MHz, 4k
                  Marc Ehnle

                  Comment


                    #11
                    Originally posted by Ephedrin View Post

                    Beside all calculations and theory it doesn‘t take much to try it out. Assuming that we don‘t have problems with our sims you should find the same. Load the aircraft lightly and heavily and taxi. In my case with the 300ER I noticed that the lighter it is the faster it slows down.
                    Your command of my language is excellent.

                    Rolling resistance doesn't change as much as kinetic energy with a change in mass. The tire deflection is greater with more mass but the tires have very thick walls to carry MGW loads so the deflection, which is where the energy goes, just doesn't change significantly.

                    Lighter loads slow quicker than heavy loads because they have less kinetic energy ((1/2)mv^2), less energy to be dissipated as heat by rolling resistance. A heavy load will slow down slower (deceleration is less) because it has more energy to be dissipated by about the same about of rolling resistance. The amount of energy goes up with the square of the speed.

                    Dan Downs KCRP
                    i7-10700K 32GB 3600MHz 2080Ti

                    Comment


                      #12
                      Originally posted by DDowns View Post

                      Your command of my language is excellent.

                      Rolling resistance doesn't change as much as kinetic energy with a change in mass. The tire deflection is greater with more mass but the tires have very thick walls to carry MGW loads so the deflection, which is where the energy goes, just doesn't change significantly.

                      Lighter loads slow quicker than heavy loads because they have less kinetic energy ((1/2)mv^2), less energy to be dissipated as heat by rolling resistance. A heavy load will slow down slower (deceleration is less) because it has more energy to be dissipated by about the same about of rolling resistance. The amount of energy goes up with the square of the speed.
                      However it shouldn't slow down by itself at all should it? There is not only 200 something tons of moving mass but also 2 engines with a fan of 11 ft diameter each side. They are the reason why the mantra in the 777 is to take care of those brakes, they heat up quickly. I taxied across OOMS with 5t of payload and fuel for ~2h without using my brakes at all.
                      i7-6700k, GTX 1080TI, 32GB DDR4 RAM @2666MHz, 4k
                      Marc Ehnle

                      Comment


                        #13
                        Originally posted by Ephedrin View Post

                        However it shouldn't slow down by itself at all should it? There is not only 200 something tons of moving mass but also 2 engines with a fan of 11 ft diameter each side. They are the reason why the mantra in the 777 is to take care of those brakes, they heat up quickly. I taxied across OOMS with 5t of payload and fuel for ~2h without using my brakes at all.
                        Of course it will slow on it's own with idle thrust. That rolling resistance will cause a deceleration. I've noted that a taxi speed of 18 kts will slow to 10 kts if I reduce power to idle and coast for about 900 m (1/2 nm). This is from memory that is known to be subject to degradation.
                        Dan Downs KCRP
                        i7-10700K 32GB 3600MHz 2080Ti

                        Comment


                        • Emi
                          Emi commented
                          Editing a comment
                          Dan, keep in mind that IDLE in an airplane is not the same as IDLE in a car. Just because you are in IDLE thrust you do not have zero thrust.
                          Quite the opposite, breakaway power in a 777 can be as much as takeoffthrust in a 737... and IDLE isn't much below that!

                          @all rather try to imagine it like IDLE with a gear selected in your car. It will accelerate to a certain speed in IDLE. The same goes for most airplanes.
                          However P3D's ground model imposes so many limitations, it's just impossible to accurately model this without seeing significant sideeffects and destroying other parts of the model. PMDGs version is a very good compromise between what's possible and how the real deal behaves.

                        #14
                        Originally posted by DDowns View Post

                        Of course it will slow on it's own with idle thrust. That rolling resistance will cause a deceleration. I've noted that a taxi speed of 18 kts will slow to 10 kts if I reduce power to idle and coast for about 900 m (1/2 nm). This is from memory that is known to be subject to degradation.
                        In reality it doesn't slow down. Guys release the parking brake -> airplane starts moving and accelerate to a "terminal velocity" that does depend on weight. In platforms that have a speed limit of 10 knots, that means either stepping on the brakes continuously or shutting down one engine when inbound. I've sat in the cockpit of a 777 many times and I remember how satisfying it is to just watch it go without touching the thrust levers. On straights and outside of the main platform, the airplane will accelerate to about 25kts again without touching the thrust levers and on long taxis, brake temp is an issue. The MAX also does this and with engine anti-ice it's even worse as it increases N1 by 4%.

                        This is just one of the biggest limitations in FSX/P3D. It's always been there. I guess it's one of those things that we'll have to accept until we can finally migrate to MSFS. Most high level addons like PMDG have done a good job. Wasn't so many years ago that taxiing was absolutely horrible, done with pedals (no tiller axis) and with more lag than a twitchy old cessna.
                        Omar Josef
                        737 FO
                        757/767 rated
                        Spain

                        Comment


                        • cmdn
                          cmdn commented
                          Editing a comment
                          Then How come FSlabs has managed to solve that issue (their ground friction model is quite good) and PMDG can't? just honestly trying to understand what are these " without seeing significant sideeffects and destroying other parts of the model "
                          Last edited by cmdn; 24Mar2021, 11:04.

                        • Emi
                          Emi commented
                          Editing a comment
                          Well, load the FSL with IDLE thrust and parking brake set and take a close look outside the windows. You'll see for instance that the whole aircraft is vibrating. That's caused by the excessively high idle thrust you need to overcome that ground friction model.
                          The bigger the aircraft, the stronger the effect. Now imagine what would happen if the same was done in a 777.

                        #15
                        cmdn commented
                        Today, 10:38
                        Then How come FSlabs has managed to solve that issue (their ground friction model is quite good) and PMDG can't? just honestly trying to understand what are these " without seeing significant sideeffects and destroying other parts of the model
                        don‘t take this as an official reply please as I don‘t know how the taxi bahaviour was coded of either addon. But looking at FSL‘s files they look very different to the file steucture of PMDG. The Airbus has a groundfriction.ini or similar for example which contains many values depending on the surface. And looking further the FSL addons actively change sim files and processes, just look where they place their files, what runs when you start an FSL addon, what you agree to when you install them. I assume (!) they even hack the P3D core code. I think (!) Lefteris is much more aggressive in this regard. His behaviour in the past shows little qualms to do these things to get what he wants so I don‘t think about it any further or I‘d probably had to get rid of anything by them. It works well in the sim, indeed, but I‘m not sure whether I would want to see PMDG do the same.
                        i7-6700k, GTX 1080TI, 32GB DDR4 RAM @2666MHz, 4k
                        Marc Ehnle

                        Comment


                          #16
                          Originally posted by Ephedrin View Post

                          don‘t take this as an official reply please as I don‘t know how the taxi bahaviour was coded of either addon. But looking at FSL‘s files they look very different to the file steucture of PMDG. The Airbus has a groundfriction.ini or similar for example which contains many values depending on the surface. And looking further the FSL addons actively change sim files and processes, just look where they place their files, what runs when you start an FSL addon, what you agree to when you install them. I assume (!) they even hack the P3D core code. I think (!) Lefteris is much more aggressive in this regard. His behaviour in the past shows little qualms to do these things to get what he wants so I don‘t think about it any further or I‘d probably had to get rid of anything by them. It works well in the sim, indeed, but I‘m not sure whether I would want to see PMDG do the same.
                          thanks for letting me know. I did not know about that. So the ground friction model will remain as it is for some time if understand correctly ?

                          Christophe Mondon

                          Comment


                            #17
                            Interesting all the responses......I see there are certain restrictions in the core simulation itself, and to be honest, its not a big issue if the flight dynamics etc are as good as they are.

                            Comment


                              #18
                              Originally posted by DDowns View Post

                              Your command of my language is excellent.

                              Rolling resistance doesn't change as much as kinetic energy with a change in mass. The tire deflection is greater with more mass but the tires have very thick walls to carry MGW loads so the deflection, which is where the energy goes, just doesn't change significantly.

                              Lighter loads slow quicker than heavy loads because they have less kinetic energy ((1/2)mv^2), less energy to be dissipated as heat by rolling resistance. A heavy load will slow down slower (deceleration is less) because it has more energy to be dissipated by about the same about of rolling resistance. The amount of energy goes up with the square of the speed.
                              While I agree with all of these and math is exact science of course, my issue is...or rather I dislike the fact that 542.2 TOW takes only approx 4500ft of runway for take off....Since I can't "feel" it on simulator I timed my take off few times. 14-16 seconds? Not even that. Well I get that GE engines are really strong but that is totally unrealistic. And I am not an expert by any means...but I traveled a lot with B777 and I know that AC has same engines, but each take off uses much more runway and average take off time is about 35-45 seconds depending of load of course.

                              This is just my observation for all this ground "resistance" and mass business....
                              Last edited by Rwy23; 26Mar2021, 19:05.
                              Alexander Luzajic
                              Virtual Air Canada

                              Comment


                                #19
                                Good day ,i want to share with you what i had this evening, fly to dakar from paris de Gaulle, gate k52 and take off runway 09R. Decide to wait to start engine 1 after pushback. 40T fob cargo 17T and 279 passengers. Tow 225T.
                                need 40% n1 to move the plane, need to play with steering wheel to keep the plane straight, thirst left turn no problem, just After need turn right, impossible to turn if engine is not idle, but in that case speed become to low and the plane stop. If you keep a higher idle to maintain 7-8knots plane goes straight and nose wheel slip. So i start second engine .
                                to keep 17-18 knots i need to play with throttle 25,4 % n1 i can't maintain my speed, 25,7% the plane accelerate little strong for 0.3% n1 power...
                                Frédéric Giraud

                                Comment


                                  #20
                                  Hi everyone,

                                  I have to admit, I thought it would not be a big deal but this ground friction thingy has become quite an immersion killer. It is just very weird that a 77W with 250 tons needs 30% of N1 to move forward. With that weight the aircraft would speed up with IDLE thrust and you would need to apply brakes to make it slow down.

                                  So It has been explained to me that I was trying to create an issue that actually does not exist and that anyway P3D has some limitations ; you actually can’t change that friction model. However, I do wonder how other addon providers can bypass that ground friction limitations . I am not here to speculate about this topic anyway but I would just like very much if pmdg would dig into this issue and perhaps find a fix.

                                  thanks
                                  Christophe Mondon

                                  Comment


                                    #21
                                    Originally posted by cmdn View Post
                                    Hi everyone,

                                    I have to admit, I thought it would not be a big deal but this ground friction thingy has become quite an immersion killer. It is just very weird that a 77W with 250 tons needs 30% of N1 to move forward. With that weight the aircraft would speed up with IDLE thrust and you would need to apply brakes to make it slow down.

                                    So It has been explained to me that I was trying to create an issue that actually does not exist and that anyway P3D has some limitations ; you actually can’t change that friction model. However, I do wonder how other addon providers can bypass that ground friction limitations . I am not here to speculate about this topic anyway but I would just like very much if pmdg would dig into this issue and perhaps find a fix.

                                    thanks
                                    I absolutely agree, neither do you create an issue nor is it nitpicking.

                                    I‘ll simply name the addons that I tried.

                                    FSL Airbus with CFM: a little thrust required comparable to the 737, taxis on its own then at idle N1. With IAE: ground based cruise missile, no break away thrust required at all, brakes become hot if used carelessly, aircraft accelerates to around 35 kts.

                                    Flythemaddog Md82/83: no break away thrust required at low weight, a little at higher weight. Taxis at idle.

                                    Aerosoft A330: depending on the weight it might require a little thrust. Taxis with idle or minimum thrust

                                    almost all A2A aircraft: minimum thrust required to break away, Bonanza for example taxis at 700-800RPM.

                                    Milviz Kingair 350: taxis and accelerates effordless with the power levers in the beta range.

                                    and now: PMDG 737 and 747: depending on the weight very little break away thrust required. Almost no taxi thrust. No excessive decceleration. They work as expected.

                                    There‘s nothing magical to it. I‘m sure that PMDG can correct the behaviour. Right now the lighter the 777 is the more it slows down. That‘s certainly nothing that can‘t be corrected or is limited by the sim, to me it looks like a typo. The 747 and 737 (and DC6) don‘t behave so.
                                    i7-6700k, GTX 1080TI, 32GB DDR4 RAM @2666MHz, 4k
                                    Marc Ehnle

                                    Comment


                                      #22
                                      Originally posted by Ephedrin View Post
                                      Right now the lighter the 777 is the more it slows down. That‘s certainly nothing that can‘t be corrected or is limited by the sim, to me it looks like a typo. The 747 and 737 (and DC6) don‘t behave so.
                                      A typo??? That’s a good one Marc.
                                      Last edited by Falcon99; 04Apr2021, 23:17.
                                      George Morris

                                      Comment


                                        #23
                                        Agreed, it's been bothering me for such a long time now. Marc did a great summary of what others and PMDG's other products can do, I would expect the same level of accuracy from the 777.
                                        Cedric Schaffhausen

                                        Comment

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