off-topic-ish: a maglev train - scaling mass and aerodynamic drag vs. friction drag

Discussion in 'Hydrodynamics and Aerodynamics' started by nine6, Mar 17, 2015.

  1. nine6
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    nine6 New Member

    off-topic-ish......ok, probably just plain off-topic but you guys are smart

    These questions are a bit outside of the normal ken of this particular forum. My apologies for that but I am having a hard time finding a place to ask them. I have asked them in forums that a little bit more specific to the subject matter but find myself in the middle of backbiting political debates and conflicting ideologies rather than getting the answers that I am looking for.

    So....the question is actually about a train and not a boat. However, the same maths should still generally apply. It is still a vehicle moving thru a medium. In one case, air, in the other case, water and thankfully it doesn't have to fly.

    I am in the process of designing (and I'll use the word “designing” loosely here as I have no engineering background) a widebody maglev intercontinental train. Its big, its fast, it goes all the way around the world. I'm in the middle of an argument with someone in another forum about the projected mass of an individual railcar. My version is three times the width of a standard railcar at 31 feet. The person that I am arguing with seems to think that the given volume of that railcar and consequently the given mass should be 27 times the mass of the original 10 foot wide railcar. (3 to the power of 3). However, it is my belief that the railcar should be about 6 times the mass of the original railcar. Since it has three times the width and the length and height are the same, then the volume should be three times the original volume. However, the mass should be twice the mass of the original to hold up the excess cross-section.......and 3x2=6. My thoughts, 6 times and not 27 times. He is also of the belief that it would take a pylon 27 times the mass would be needed to hold up the additional 27 times the mass of the railcar whereas I am of the belief that the pylon would only need to be about three times the mass of the original as concrete is virtually incompressible. (The pylon holds up the track.)

    The next question would probably be harder for you to answer but you are welcome to add input. Its a question of aerodynamic drag versus friction drag. The friction drag being generated by a standard high speed rail over steel rails versus maglev floating on air. The aerodynamic drag of both the front end profile and the trailing surface drag. The person that I am arguing with seems to think that at 200mph, aerodynamic drag would account for 98% of the energy consumption of the vehicle and that the friction drag on the steel rail would only account for 2%. This would make maglev only 2% more efficient than standard high speed rail. My thoughts are that the faster the train moves the more heat that goes into the wheels causing them to expand and to create greater friction. My thoughts are that a maglev train would be about 30-40% more efficient at 200mph. My next question that would be the percentage of forward cross section drag as opposed to surface drag. Its a question of making the train either longer or making the train wider and taller in order to conserve more energy.

    Those are all of the questions that I have. You are either welcome to add input or to tell me take it someplace else. Either way, I wish all of the best of days.

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    FAST FRED Senior Member

    I for one do not believe the big advantage of a mag lev is due to the lessened rolling resistance of a steel wheel with roller bearings.

    On trucks and cars the rubber tires deform and eat HP, but there would be little hysterisis loss with steel.
    The commonality of some aircraft , skinny tube , fat tube ,show the induced drag from creating lift for the weight changes fuel burn far more than fat or skinny.

    Mag levs have a far smaller footprint , on the ground , perhaps easier to keep rated for 200-400 mph?
  3. snowbirder

    snowbirder Previous Member

    Air resistance is the main drag, though.
  4. daiquiri
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    daiquiri Engineering and Design

    I am unable to answer the question regarding the mass of the train. I even don't have idea about the weight of the ordinary trains...

    But when it comes to resistance of MagLev trains, I agree with Snowbirder and with that other guy mentioned by Nine6, who claims that the aero drag will be the paramount one at high speeds. Just like they are the biggest part of the drag of vehicles running on the highway, although rubber tires of cars have enormously bigger resistance than the steel wheels on rails.

    Nine6, did you perform a review of the current state of art of the Maglev and high-speed train technology? You can find a lots of info in internet, for example:
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    FAST FRED Senior Member

    I would think any difference in efficiency would be due to the drive method.

    A std train uses a big diesel , generating electric to turn the wheels , and carries its diesel with it.

    The mag lev is hooked to a Nuke or Coal power plant or 3 and does not need to create its power or carry its own fuel.

  6. jehardiman
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    jehardiman Senior Member

    Go to Hoerner, Fluid Dynamic Drag, Chapter 12, Section 2. All the basic work is there.

    FWIW, wider is not better from an economic point of view. The added structural weight offsets the volume gain resulting in a less efficient structure (payload/gross weight). This is why aircraft and ship hulls mostly fit into very narrow bounds of B/D.
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