1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Net momentum for quadrotor

  1. Feb 9, 2016 #1
    While watching lecture at Coursera, i tumbled over this fourmula

    [tex]\mathbf{M}=\sum_{i=1}^{4}(\mathbf{F}_i\times \mathbf{r}_i+\mathbf{M}_i)[/tex]
    where F is uplift force from a propeller, and M is drag moment for a propeller.

    But why they add drag moment(torque) like that? Maybe it will differ for central point.

  2. jcsd
  3. Feb 9, 2016 #2


    User Avatar
    Science Advisor

    Try it out by computing the moments of the blade drag forces around different reference points.
  4. Feb 9, 2016 #3
    Thanks, it is true for two points on the picture, but can you name the rule? I want to read it whole to understand it better. Thank you.
  5. Feb 21, 2016 #4
    Could somebody write what are these [itex]M_i[/itex]? Because i still don't understand how it works, i think every moment must be calculated about some point.

    I have found this formula
    \dot{\mathbf{H}}_O = \sum(\mathbf{r}_i \times m_i\dot{\mathbf{v}}_i) = \sum(\mathbf{r}_i \times \mathbf{F}_i + \mathbf{M}_i)
    but i don't understand from where [itex]\mathbf{M}_i[/itex] came, because i think that [itex]m_i\dot{\mathbf{v}}_i = \mathbf{F}_i[/itex]
    Last edited: Feb 21, 2016
  6. Feb 21, 2016 #5


    User Avatar
    Science Advisor

    Didn't you explain it yourself:
    It's the total moment of all aerodynamic forces on the blades, which are in the plane of the propeller disc.
  7. Feb 21, 2016 #6
    Well, yes, but here i want to know from where [itex]M[/itex] came in this general formula [itex](1).[/itex] It is unrelated for quadrotor, just to understand underlying theory.
    Last edited: Feb 21, 2016
  8. Feb 21, 2016 #7


    User Avatar
    Science Advisor

    In general it's just some external moment.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Net momentum for quadrotor
  1. The Net (Replies: 1)

  2. Net force? (Replies: 1)

  3. Net torque (Replies: 4)