• Welcome to engineeringclicks.com
  • Buoyancy Problems

    Discussion in 'Calculations' started by aykhan90, Feb 16, 2014.

    1. aykhan90

      aykhan90 New Member

      Joined:
      Feb 2014
      Posts:
      1
      Likes Received:
      0
      Hi

      Recently I have been studying for the PE exam for Mechanical Engineers. I have been out of touch of studying for some time so I am having difficulty with some problems. These problems are not necessary difficult but I can't seem to understand how to go about solving them.

      Problem 1)

      A blimp contains 10000 lbm (4500 kg) of hydrogen (specific gas constant=766.5 ft-lbf/lbm-R (4124 J/kg.K) at 56 F (13 C) and 30.2 in HG (770 mm HG). What is its lift if the hydrogen and air are in thermal and pressure equilibrium?

      I am not looking for the answer just some guidance on the method I should undertake to solve this.

      Thanks
       
    2.  
    3. Virgule

      Virgule Active Member

      Joined:
      Nov 2011
      Posts:
      31
      Likes Received:
      0
      Your blimp displaces the surrounding air. Every body experiences a certain lift since it displaces air and that air tries to regain its place. All body also have a weight dragging them down. Not all bodies though will experience a net lift. Only bodies displacing an amount of surrounding air heavier than their actual weight will experience that lift.

      The force pulling down on the blimp (F1) is :

      F1 = W = m x g = rho_blimp x g x V

      where
      W : total blimp weight in N
      m : total blimp mass in kg
      rho_blimp : density of the blimp's content (helium, ...) in kg/m3
      g : gravitational constant in m/s2
      V : volume displaced by blimp in m3

      If you want to use F1 = rho_blimp x g x V, you're assuming the blimp's weight is equal to the gas content of it. In other words, that the mass of the frame, materials, ... are negligible.
      You should use F1 = m x g for more accurate force.

      Now the force (or lift) resulting from the displacement of the surrounding air, F2 is:

      F2 = rho_air x g x V

      where
      rho_air : density of surrounding air in kg/m3
      g : ...
      V : ...

      The net lift will be F2-F1.
       
      Last edited: Feb 24, 2014

    Share This Page

    By using this website you agree to our Cookies usage. We and our partners operate globally and use cookies, including for analytics, personalisation, ads and Newsletters