Page 2 of 2 FirstFirst 1 2
Results 26 to 35 of 35
  1. #26
    Join Date
    Mar 2017
    Location
    SF/OV
    Posts
    52

    since we got the airplane on the treadmill figured out...helicopter in a train?

    ^ Brain fart on the whole constant angular acceleration bit. But don't get yer panties too wadded up now.

    I get what your saying, but I disagree that it will move forward relative to the train. Maybe that's not what you meant in your original post, but that's how it read.

    I agree that it will tilt forward, giving it some acceleration in the direction the train moves, but certainly not at the same rate. If the net force acting on the air is the back wall of the train, and the net force acting on the helicopter is air pressure, and air is a compressible gas, than there is no way the pressure of the air will instantly impart the same net force on the helicopter that the train imparts on the air.

    I'm not saying that it will stand still or move backwards relative to an observer outside the train. Obviously it would begin accelerating in the direction the train accelerates. It just will not accelerate at the same rate, and would appear to an observer inside the train to move towards the back wall.

    I'm with auvgeek on this one.

  2. #27
    Join Date
    Sep 2010
    Location
    Den/Baltimore
    Posts
    3,885
    Quote Originally Posted by DJSapp View Post
    To move in a circle means you are undergoing an acceleration, as your direction is constantly changing. The Earth is spinning on it's axis, and orbiting the sun, and orbiting the center of the milky way. Keep that in mind as we move to this.

    We are all accelerating with respect to a fixed point in the universe. That said, you can change reference points to suit your situation. This feels natural as the motion of the planet and stars are too large to perceive.
    Yes, but when we write the standard second-order (nonlinear, coupled) differential equation governing the dynamics of robotics and spacecraft, the matrix of Coriolis and centripetal effects is often known so it's easy to account of those forces and pretend you're in an inertial reference frame. They're also usually small compared to the forces from the inertia/mass matrix and drag matrix—I mean, the earth is rotating at 15 deg/hr. (Interestingly, for most systems, the coriolis matrix is analytically skew-symmetric and can be constructed from the inertia matrix.) We can't really just add the effects from the train's acceleration the same way to pretend the train is an inertial reference frame because we're not talking about constant acceleration in this problem and those forces are much harder to model.

    When the train accelerates, the air moves like the water within the train car, pooling toward the back and creating a surface of air that is not perpendicular to force of gravity. This creates an uneven reaction force on the heli's lift on the air and the force of gravity and causes it to pitch forward. This uneven set of forces creates motion in the direction of the accelerating train.
    We've established it will very likely pitch forward and accelerate. The question is why do you think the pressure differential will cause the heli to accelerate faster than the train is accelerating? Especially taking into account the viscosity issue I raised before and the inertia of the heli's body.
    "Alpine rock and steep, deep powder are what I seek, and I will always find solace there." - Bean Bowers

    meager stoke

  3. #28
    Join Date
    Jan 2007
    Posts
    4,504
    Is this the thread where I get to use a^2 + b^2 = c^2 for the first time in my life?

  4. #29
    Join Date
    Feb 2008
    Location
    Summit North
    Posts
    4,325
    Quote Originally Posted by DJSapp View Post
    (yes, the earth is moving)
    I suppose the next thing you are going to tell us, is that the earth is round. Heretic.
    Alpental Indiginous

  5. #30
    DJSapp's Avatar
    DJSapp is online now (╯□)╯︵ ┻━┻
    Join Date
    Oct 2003
    Location
    All over NCal
    Posts
    5,686
    Quote Originally Posted by auvgeek View Post
    We've established it will very likely pitch forward and accelerate. The question is why do you think the pressure differential will cause the heli to accelerate faster than the train is accelerating? Especially taking into account the viscosity issue I raised before and the inertia of the heli's body.
    You may have hit on something. As the train accelerates, the air will compress. The heli may initially move backwards before it moves forward and downward. This would be akin to riding a wave in the ocean. Given the slow acceleration rates of trains (0.4 - 1.3 m/s/s), the backward motion would likely not be perceived as the acceleration of gravity is much larger.
    Fat fuck bubbas are not erosion.

  6. #31
    Join Date
    May 2011
    Location
    the vails
    Posts
    1,277
    Quote Originally Posted by huckbucket View Post
    Is this the thread where I get to use a^2 + b^2 = c^2 for the first time in my life?
    Really? I use that shit all the time in real life. Maybe it's just my curiosity, though.

    Good points on the heli pitching forward. I had incorrectly assumed that it would stay level. Given a constant rotor speed and no change to inputs, it is also possible that the heli hits the floor due to the loss of lift from the tilt.

  7. #32
    Join Date
    Dec 2003
    Location
    Easy D
    Posts
    2,666
    Has anyone here actually flown an rc helicopter? The only correct answer is 'the helicopter will hit something'

  8. #33
    Join Date
    Nov 2006
    Location
    colorady
    Posts
    901
    Helicopters run on magic. All those fast moving parts are just there to keep you away and hide the secret. That said, the helicopter hits a wall and immediately ends up in a ball of flames. Trust me. I used to fly them.


    Sent from my iPhone using Tapatalk

  9. #34
    Join Date
    Mar 2008
    Location
    the ham
    Posts
    3,784
    Quote Originally Posted by ULLRismyco-pilot View Post
    Helicopters run on magic. All those fast moving parts are just there to keep you away and hide the secret. That said, the helicopter hits a wall and immediately ends up in a ball of flames. Trust me. I used to fly them.
    A lot of the guys I work with used to fly helis. When asked why they don't still, the answer is universally, "because I want to stay alive."

  10. #35
    Join Date
    Apr 2006
    Location
    SF & the Ho
    Posts
    3,101
    QFT. This^

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •