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[26JAN20] So a funny thing happened when using ground steering on the way to the gate...

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    [26JAN20] So a funny thing happened when using ground steering on the way to the gate...

    Captains,

    This is an interesting event that I wanted to share with you because it might give you an idea why we value your feedback, and how it becomes a constant companion in our continuing effort to improve the products we offer. Over the previous two weekends, I have been doing quite a bit of research into ground handling and wound up stumbling into something that was unexpected in how FSX/P3D respond to the steering data that we give to the sim at the end of our steering computation process.

    It has lead to some changes that we are going to push to you on Wednesday- but for the "process geeks" in our midst I thought it might be interesting for you to follow along the journey...

    When we implemented our updated ground physics model on the 737, there was quite a bit of drama about the "feel" of steering with a tiller. (Still is. ) We spent some time in analysis and found that our computations were performing within just a few percent of the numbers predicted by flight-test data, so we attributed most of the drama to the natural tendency of users not to like change, or to hardware that tended to make the input response over-sensitive. This, combined with decades of erroneous FSX/P3D ground steering, we felt was creating a bit of an adaptation curve that the community would need to climb. We made a few changes to the implementation in order to give users some control over the inputs as we felt this would make up for differences in user hardware- and largely expected users would settle right in as they have done before in a number of other areas when we improved the realism of our simulations beyond what the base platform provides. (Also: see my lengthy post about what we found was incorrect about the FSX/P3D ground steering model...)

    Two weekends ago, I had a bit of free time and set about to bring the physics model update into the 747 product line. This was supposed to be easy, since modular code is primarily designed for portability and access by multiple projects- and because the 747 already had the 1.0 physics model included. In spite of the simplicity, I was stymied by the fact that no matter how much I tweaked the steering input algorithm on the 747, the end result just didn't look or feel right and the airplane was making turning radius results that were far more narrow than were predicted by hand computations and comparison to known. flight-test collected data. From a "feel" perspective, even a slight steering input would result in what can best be described as a "neck snapping jolt" in the direction of the steering input.

    I was puzzled- for the computational mechanics of all of this should not need tweaking beyond inputting the various figures that describe the airplane's dimensions, tire friction and rigidity values, etc.

    A "quick and easy" task went sideways- which always seems to happen when you sit down to "do something easy" in code. I spent my dev time and a whole bunch of extra time creating a couple of data output solvers to get a real time data stream of the computational results. The solvers tracked the steering input, key computational results for everything from the tire/surface interface all the way to the resulting angular velocity experienced by an imaginary particle located at the exact position of the nose wheel.

    This was compared to flight-test derived data from Boeing, and the results showed pretty clearly that even though all of the computations were correct, the 747-8 airplane was turning well inside of the curve described by flight test. To put it another way, while the results of each equation in the process provided correct results to a given input, the imaginary particle at the nosehweel/ground interface had too much Angular Velocity (ω) and as a result was moving more quickly around the curve than it should.

    I could find no computational reason to explain it- but the output data clearly showed the issue. I came back to the problem the next day with some fresh ideas for how to export data from the sim in real time to see if I could determine why the airplane was behaving the way it was. The results didn't tell me anything I didn't already know- so I took these output solvers back to the 737NGXu, routed through 10,000,000 pages of documentation to find the flight test data describing the actual performance of the 737 under various turn conditions, and low-and-behold the same problem exists in PMDG 737NGXu, albeit on a much, much smaller scale.

    To phrase it differently, while we have always known that the 737NGXu would out-perform the flight test data in a turn, it was just a slight out performance that we attributed to not having turned on "nosewheel slip" in which the nosewheel will naturally slide a bit at tight turning angles, resulting in a wider realized turning angle. However, using the same equations in the 747-8 was exhibiting ω significantly higher than predicted by computation and turning radius significantly below the predicted and demonstrated figures. The only difference between these two computations was the beam length between the center of rotation and the nose-wheel...

    And then, much like that Edison guy- the light bulb came on.

    A bit more digging around the equations and I found that our physics model is *correctly* producing the desired angular velocity for the turn as experienced by the nosewheel, but when applying that solution to the simulator, the simulator was taking those results and applying them to the center of rotation of the airplane. (Center of rotation on the 737 and 747 is directly between the MLG, but on the 747 will shift fore/aft 5' depending upon the disposition of the body gear steering).

    Now- if you have followed along this far- you probably recall from your high-school physics classes that the tangential velocity (v) of a particle rotating around a fixed point will have a higher velocity the further out from the center of rotation that it travels. (Picture standing on the blade of a helo as it begins to rotate. The further from the blade hub you are- the faster you will be going, even though the entire blade is moving at a single speed of rotation.)

    For the visual learners among us- here is a sketch that explains it better:

    AngularVelocity.jpg

    If you placed an object on top of the pencil as it rotates toward the dotted line on the right, an object placed at C will travel a greater distance than an object placed at B, and likewise an object placed at B will travel further than an object placed at A. Since the entire pencil is rotating around it's eraser at a specific rate, we know that angular velocity of point C will be higher than B or A.

    In our case as it applies to our ground physics model, imagine the center of rotation of the airplane is at the erase, and we computed the angle of rotation for the nose which is out at the tip of the pencil. Since the sim then applies this number to the bottom of the eraser, it means that the speed you see in the sim is significantly higher than what we computed because the sim attributes it to the wrong location.

    Okay. THIS we can work with! A bit of modification in the data being handed to the sim, and the results for both the 747 and 737 nose correctly match the flight test data to predict computed radius and rate of turn. I ran a series of tests with the 737-600 (being the shortest) and the 747-8 (being the longest) and the results are well inside the statistical margin of error.

    What is more- from a pure FEEL perspective, the results are dramatically improved- and initial reporting back from our beta team seems to bear out that this may indeed be the improvement I've searched for off and on to see if we couldn't identify why some folks seemed to have such a bad experience with something that really shouldn't be too subjective, since it is purely a physics based result. (Interestingly, playing around with this model a bit more, I was able to decode a bit more of the problem that FSX/P3D has with it's native ground steering, which is totally and completely backward...)

    So the end result is that when users began to complain that steering seemed over sensitive- we checked (and re-checked) every aspect of the computational process. The mechanics are well documented and not terribly difficult to compute even by hand, so I was very comfortable concluding that "the performance we compute is correct" and presuming the complaints are because the new methods feel different than the old, incorrect ground steering model in FSX/P3D.

    Turns out- the problem was being introduced AFTER we hand data back to the sim and it took a bit more imagination on my part to find a way to unmask this issue.

    So in case you guys ever wonder what we do with feedback in the background- now you know. We log just about all of it- and we are always re-evaluating our conclusions because you just never known when you might discover a problem buried beneath something that managed to escape all previous efforts at evaluation.




    Attached Files
    Robert S. Randazzo
    PMDG Simulations
    http://www.pmdg.com



    #2
    Interesting
    Last edited by VoyagerP3D; 26Jan2020, 09:36.
    Christian Mbeumo BAW1138

    Comment


      #3
      Angular velocity = velocity * radius
      or
      velocity = angular velocity / radius
      The greater the radius the greater the angular velocity
      (Angular velocity has no unit in calculations, however you can Talk about radians)
      physics first semester

      Thank you very much
      Last edited by Cocobellomann; 26Jan2020, 09:52.
      Josef Kolb

      PMDG B737-600 to -900/747-400/747-8/777-200LR/777-300ER

      Comment


        #4
        I highly appreciate this type of information being given to us , thanks a lot !

        best regards
        Manolo Ruiz Carrió

        Comment


          #5
          Again, transparency from PMDG pertaining to an oft debated subject - ie: steering.

          Refreshing to say the least, and informative.......any other devs out there reading and keeping track/log of what PMDG are doing here......observe and learn.

          I don't have any concrete data points or hard empirical data - but I dare say most of us consumers would congratulate and welcome all other devs in the sim world (and every other consumer faculty) to follow the lead/example provided here. Kudos to PMDG yet again.
          Steve Summers - i7 8770K @ 4.7Ghz - 16Gb Corsair Vengeance DDR4 3000Mhz - Geforce GTX 1070 Ti 8Gb - ACER X34 Predator IPS GSYNC 3440x1440 - ASUS ROG STRIX Z370-H - 3 x Samsung Pro SSD 512Gb - Corsair H100i GTX - Win 10 Pro 64 - P3Dv4.5 + ASP4 + ASCA + PMDG NGXu / 777 + ORBX + FlightBeam + FSDT + Fly Tampa + Pacsim. Revenge is not a valid motive, it's an emotional response. No, not vengeance. Punishment.

          Comment


            #6
            Originally posted by rsrandazzo View Post
            "process geeks" in our midst
            oi! I represent that remark. 😁

            Thanks for the read.
            Last edited by Mickel; 26Jan2020, 11:13.
            Mike Dryden

            Comment


              #7
              Thanks for the explanation and update. Apology accepted.
              Will Thomas
              KMDT - Flight Sim 1998 to Present
              Aircraft Painter
              P3D, X-Plane, FSX

              Fly Virtual (flyvirtual.net)
              USAF Veteran [Aircraft Electrician / Egress Tech]

              Comment


                #8
                Hi Robert,

                I don't think "simple" and "code" should be two words allowed in the same paragraph, let alone the same sentence

                I've found that sometimes you can stare at the same code for hours, convincing yourself that nothing is wrong but knowing that something is...you're just too bogged down in the process to step back and actually read the code rather than seeing what you think is there (in no way am I implying that's what happened here - as you say the correct information was being handed back to the sim).

                Yet another example of the tireless work that goes into the simulations and I for one wholeheartedly appreciate it. A friend or two have commented that the cost of an aircraft for the sim is high; my counter argument is that you're not only paying for the dev time that has gone into it to get it to release but also the countless hours that will be spent post release making refinements or even adding brand new features.

                Andy
                Andrew Curry BAW867

                Comment


                  #9
                  Classic case of what happens when someone with the right depth of knowledge gets curious and starts digging for answers. Nicely done. As a former programmer (and now business analyst) it's refreshing to see the right questions being asked, which almost always leads to a better solution, which seems to be the case here. Post-update I imagine not having to ride the differential brakes to swing that 737 around a tight turn

                  Yet another example of the tireless work that goes into the simulations
                  - Couldn't agree more. It's what separates PMDG from the rest of the pack.
                  Dan Morand
                  i7-7700k OC 4.8, 16GB RAM, EVGA GTX 1080 Ti SC2

                  Comment


                  • Minien
                    Minien commented
                    Editing a comment
                    Barring FSL.

                  #10
                  This was some fascinating information, and it is reminiscent of some of the rabbit holes that I went down in source code when trying to figure out why something inexplicable was happening.

                  At my previous job, a colleague gave all of the coders a yellow rubber duck for our desks, and encouraged everyone to utilize "rubber duck debugging," where whenever we got bogged down in some code that wasn't doing what it should be doing we would explain what the code should be doing line by line to the duck. This seems ridiculously silly, but it is amazing how often the proverbial light bulb would illuminate with the help of this process.
                  Tim Lincoln
                  My YouTube Channel

                  Comment


                    #11
                    Hey rob, great read! I've always had a feeling it was a bug or miss calculation in the code somewhere, and people were being inpatient letting you fix this issues and resolve it. If you saw my tiller post this past week, I have literally tried everything from inputs to FSUIPC to scratching FSUIPC topretty much using a 3rd part source software connection made for battlefield since then and it all cane down to the pretty much what you have described. It's great to hear that you have found the issues and the solution to resolve the issue! I knew it would happen!
                    Alex Kulak
                    PMDG Studier and flyer
                    Ramp Agent

                    Comment


                      #12
                      ohhhh physics....I realized i hated physics/kinematics when Dodge was doing their 5 week Dodge Hellcat giveaway. It was a nightmare trying to figure out those answers. At least you guys have the patience and narrowed it down. This was an interesting read.
                      Chris Ferguson

                      Comment


                        #13
                        Very informative, thanks Robert. 10,000,000 pages of data? Did I read that correctly? That’s seems insurmountable!
                        Randall Coultas

                        Comment


                          #14
                          Originally posted by ace2029 View Post
                          Hi Robert,

                          I don't think "simple" and "code" should be two words allowed in the same paragraph, let alone the same sentence

                          I've found that sometimes you can stare at the same code for hours, convincing yourself that nothing is wrong but knowing that something is...you're just too bogged down in the process to step back and actually read the code rather than seeing what you think is there (in no way am I implying that's what happened here - as you say the correct information was being handed back to the sim).

                          Yet another example of the tireless work that goes into the simulations and I for one wholeheartedly appreciate it. A friend or two have commented that the cost of an aircraft for the sim is high; my counter argument is that you're not only paying for the dev time that has gone into it to get it to release but also the countless hours that will be spent post release making refinements or even adding brand new features.

                          Andy
                          Andy,

                          Yeah- and it should teach me to **not** sit down with an "easy" task on a Saturday. If it turns out to be otherwise, the problem solving side of the brain kicks into gear and then...well....

                          I think falling over this particular issue was a combination of having not looked at it for a few weeks, (which is good for eliminating confirmation bias) and reapplying the solution to an airframe with significantly larger inertial and moment factors. Like all things in mechanics, such changes tend to create differences that are orders of magnitude larger and thus something that is a tiny issue on a small airplane will become a big issue on a big airplane.

                          There are still some aspects of ground handling that I am not fully content with- but like everything in simming- progress will come along incrementally.



                          Robert S. Randazzo
                          PMDG Simulations
                          http://www.pmdg.com


                          Comment


                            #15
                            Originally posted by rsrandazzo View Post
                            And then, much like that Edison guy- the light bulb came on.

                            A bit more digging around the equations and I found that our physics model is *correctly* producing the desired angular velocity for the turn as experienced by the nosewheel, but when applying that solution to the simulator, the simulator was taking those results and applying them to the center of rotation of the airplane. (Center of rotation on the 737 and 747 is directly between the MLG, but on the 747 will shift fore/aft 5' depending upon the disposition of the body gear steering).

                            Now- if you have followed along this far- you probably recall from your high-school physics classes that the tangential velocity (v) of a particle rotating around a fixed point will have a higher velocity the further out from the center of rotation that it travels. (Picture standing on the blade of a helo as it begins to rotate. The further from the blade hub you are- the faster you will be going, even though the entire blade is moving at a single speed of rotation.)..................

                            ........What is more- from a pure FEEL perspective, the results are dramatically improved- and initial reporting back from our beta team seems to bear out that this may indeed be the improvement I've searched for off and on to see if we couldn't identify why some folks seemed to have such a bad experience with something that really shouldn't be too subjective, since it is purely a physics based result. (Interestingly, playing around with this model a bit more, I was able to decode a bit more of the problem that FSX/P3D has with it's native ground steering, which is totally and completely backward...)

                            So the end result is that when users began to complain that steering seemed over sensitive- we checked (and re-checked) every aspect of the computational process. The mechanics are well documented and not terribly difficult to compute even by hand, so I was very comfortable concluding that "the performance we compute is correct" and presuming the complaints are because the new methods feel different than the old, incorrect ground steering model in FSX/P3D.

                            Turns out- the problem was being introduced AFTER we hand data back to the sim and it took a bit more imagination on my part to find a way to unmask this issue.

                            So in case you guys ever wonder what we do with feedback in the background- now you know. We log just about all of it- and we are always re-evaluating our conclusions because you just never known when you might discover a problem buried beneath something that managed to escape all previous efforts at evaluation.
                            Robert, when a problem is proving very difficult to solve it is usually worth going back to basics. DODAR'ing a problem will often focus the mind on what might have been overlooked or even missed entirely, but your excellent explanation surrounding the physics to this problem and how you resolved the issue certainly takes the biscuit in my book. I would never have thought the problem you were up against was being introduced AFTER your data was handed back to the Sim.

                            I have a feeling resolving this particular issue and getting the physics to perform correctly in each Simulator must have been even harder than you describe in your post because, as you quite rightly say, the focal point of a turn in the B744 and B747-8 will shift constantly depending on the amount of body gear steering being applied. Great stuff!
                            Last edited by Michael Codd; 26Jan2020, 20:29.
                            Michael Codd

                            Comment


                            • Swaluver88
                              Swaluver88 commented
                              Editing a comment
                              What about weight depending on where its places like all the AFK (bag cans) are placed down the left side in the front and in the aft they are all on the right and people are all spread out the plane because it wasnt full. Wouldnt that also have effect on steering

                            • Michael Codd
                              Michael Codd commented
                              Editing a comment
                              Not really. The nosewheel assembly will not have as much traction if the aircraft is very light and the surface over which the aircraft is taxying is slippery. Provided the taxy speed in a turn is kept low enough to prevent any skidding then the different weights of the aircraft shouldn't affect the radius of turn.

                            #16
                            Very informative and educational now i feel like going back to school.
                            Danny Z.Cebis

                            Comment


                              #17
                              Thanks Robert. Reminded me of double physics (or maths) at school. At least I can understand your explanation. Many thanks for that! Looking forward to all the upgrades in future.
                              Keith Giannoni

                              Comment


                                #18
                                Originally posted by KG1000 View Post
                                Reminded me of double physics (or maths) at school. At least I can understand your explanation. Many thanks for that!.
                                KG1000 - you took the words right outta my mouth.........

                                Steve Summers - i7 8770K @ 4.7Ghz - 16Gb Corsair Vengeance DDR4 3000Mhz - Geforce GTX 1070 Ti 8Gb - ACER X34 Predator IPS GSYNC 3440x1440 - ASUS ROG STRIX Z370-H - 3 x Samsung Pro SSD 512Gb - Corsair H100i GTX - Win 10 Pro 64 - P3Dv4.5 + ASP4 + ASCA + PMDG NGXu / 777 + ORBX + FlightBeam + FSDT + Fly Tampa + Pacsim. Revenge is not a valid motive, it's an emotional response. No, not vengeance. Punishment.

                                Comment


                                  #19
                                  Originally posted by Michael Codd View Post

                                  Robert, when a problem is proving very difficult to solve it is usually worth going back to basics. DODAR'ing a problem will often focus the mind on what might have been overlooked or even missed entirely, but your excellent explanation surrounding the physics to this problem and how you resolved the issue certainly takes the biscuit in my book. I would never have thought the problem you were up against was being introduced AFTER your data was handed back to the Sim.

                                  I have a feeling resolving this particular issue and getting the physics to perform correctly in each Simulator must have been even harder than you describe in your post because, as you quite rightly say, the focal point of a turn in the B744 and B747-8 will shift constantly depending on the amount of body gear steering being applied. Great stuff!
                                  Michael,

                                  Yes- it was an interesting moment when I began to suspect that the sim was monkeying with the inputs, and it proved a real bugger to prove. I eventually wound up feeding the sim data that and building a table of output to see if I could develop a pattern that proved the sim was "doing something" to the input data. Once I was able to confirm for myself that this was the case- the harder task was to discern what it was doing- in hopes that identifying the "what" would allow me to adjust the input in order to correct the output. In the end- the ah-HA moment turned out to be a hand-run equation of known input- and I applied the sim's output back to my solver to see what happened- and that was how I found that the sim was mis-applying the input to the center of rotation instead of the nosewheel...

                                  I picked the wrong month to switch back to decaf...

                                  On the body gear steering- I don't recall if you have ever had the joy of operating without body gear steering- but I am hoping to replicate that particular joy for you here in the next couple of days.

                                  Robert S. Randazzo
                                  PMDG Simulations
                                  http://www.pmdg.com


                                  Comment


                                    #20
                                    The picture of the pencil with the A, B and C plots reminds of the shape made by a Fan Blade of a High By Pass Ratio Jet Engine, and the reason that the Fan Blade is wider at the tip than at the hub, in order to produce the same amount of thrust across the whole span, illustrating a variation on Roberts angular velocity principle, I think and hope, correct me if wrong!
                                    Frank Gollner

                                    Comment


                                      #21
                                      Body gear steering... We were on a flight from FRA to SFO or better we attempted it. Well, even more correct I sat somewhere just behind the flaps. Anyway we lined up RW25R (C today) applied power, held, reduced power. Applied it again, reduced it again, then left the runway and taxied back to the Gate. We were told by the Captain to wait a bit, there was an alert in the cockpit he needs to get checked. About 20-30 min later we could push back again and take off. After reaching cruise altitude the captain then told us that the Boeing 747 had this bod gear steering that helped the big aircraft to turn on narrow airports and they had an alert showing up when advancing the throttles that the body gear wasn‘t locked in center position. He needed to check this for obvious reasons. It turned out that a sensor confirming the center position had to be replaced. He joked that a „Zweimarkfuffzig“ (2,50DM old german currency before the Euro) item delayed the most expensive aircraft the Lufthansa had in its fleet.

                                      Anyway, Robert, I understand the effect the sim‘s behaviour has on the body gear equipped aircraft. But how does this improve the handling of the 737? Have you accidentally attached some body gear to the 737 with the steering module?

                                      Cheers and thanks fir the update 😁
                                      i7-6700k, GTX 1080TI, 32GB DDR4 RAM @1600MHz, 4k

                                      Comment


                                        #22
                                        Originally posted by rsrandazzo View Post
                                        I picked the wrong month to switch back to decaf...

                                        On the body gear steering- I don't recall if you have ever had the joy of operating without body gear steering- but I am hoping to replicate that particular joy for you here in the next couple of days.
                                        Sherlock Holmes would be proud of you and I suppose you could always try drinking Yorkshire decaf tea!

                                        Only once - but fortunately it's a relatively rare occurence because there are a lot of operational implications as you will no doubt know. I know that a 180 deg turn on a 150ft wide runway was not allowed; if only because the MEL implied that the aircraft would end up completing the turn at least 20ft onto the grass if we did (or words to that effect!). LOL
                                        Michael Codd

                                        Comment


                                          #23
                                          Your comment about decaf reminds me of Lyod Bridges lines in "Airplane"!
                                          Kevin M Manley

                                          Comment


                                            #24
                                            That's great news, Robert

                                            Hats off to you for figuring out a solution. I had no idea you code!
                                            Looking forward to Wednesday, Will probable scoop up that 747 tonight so I can fill out my hanger folder with some new toys!

                                            curious if the 747 we see in FS2020 is from PMDG?

                                            Cheers,

                                            RF
                                            Ro Faulk

                                            Monitor ROG PG27U 4K 144HZ
                                            i9,9900K 5.0GHZ, 64GB RAM, SSD 2TB M.2 OS drive, Dual EVGA 2080Ti NVLinked GPU's, MSI, GODlike Z390 MB.

                                            Comment


                                              #25
                                              Thank you and all the PMDG staff for such effort and your priceless contribution to our beloved community
                                              Ivan Lewis

                                              Comment


                                                #26
                                                Thanks for the in depth explanation Robert, hope this makes the next update.
                                                Zsolt Szivak

                                                Comment


                                                  #27
                                                  Originally posted by ace2029 View Post
                                                  Hi Robert,

                                                  I don't think "simple" and "code" should be two words allowed in the same paragraph, let alone the same sentence

                                                  I've found that sometimes you can stare at the same code for hours, convincing yourself that nothing is wrong but knowing that something is...you're just too bogged down in the process to step back and actually read the code rather than seeing what you think is there (in no way am I implying that's what happened here - as you say the correct information was being handed back to the sim).



                                                  Andy
                                                  When I was a programmer/designer,sometimes I would look hours at the same code, not finding the culprit. Then, I would go to a colleague, explaining my problem. While I was talking to him,
                                                  suddenly the solution came to mind, a AHA!! moment. I said to him, thanks for the help (I was merely being polite ), he said glad to be of assistance, looking at me as if I really lost it

                                                  Good times
                                                  Wijnand Lindelauf (EHBK)

                                                  Comment


                                                  • rsrandazzo
                                                    rsrandazzo commented
                                                    Editing a comment
                                                    Wijnand- This happens here frequently. Bash your head against the desk trying to find a solution to some problem, and then there mere act of explaining your incompetence to a colleague while asking for help causes you to realize that the problem was something learned in your "Introduction to Code" class. :-p - RSR

                                                  #28
                                                  I know the feeling
                                                  Ro Faulk

                                                  Monitor ROG PG27U 4K 144HZ
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                                                    #29
                                                    So is this corrected code already pushed out to the end-user? Or is this coming in a future update?
                                                    Angelo Cosma

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                                                      #30
                                                      Originally posted by Boeing 25-01 View Post
                                                      So is this corrected code already pushed out to the end-user? Or is this coming in a future update?
                                                      On the next update.
                                                      Chris Makris (Olympic260)
                                                      PMDG Technical Support
                                                      http://www.pmdg.com

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