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Aerodynamic Forces

  1. Mar 16, 2007 #1
    I need some help from you aerodynamic masters. I am doing a small project for my controls lab course. We get to choose anything we want, and so I decided to make a pitch rate loop for an autopilot. The setup is very basic, I am going to carve a wing out of Styrofoam and attach a servo to a control surface. The wing will have a rod going through the COM that is attached w roller bearings to a stand. So the wing is fixed to the stand and can pivot clockwise/counter clockwise around the COM. The idea is to get a house fan to provide airflow and have Simulink move the servo so that it always keeps the wing level. The angle will be measured either with an encoder or a potentiometer. Once we get that working we can then suspend masses at various points along the airfoil to change the location of the CG. This will cause the wing to pivot forward or aft around the shaft passing through the original COM. Then the control loop can correct this error out by command up or down elevator until the wing remains level once again.

    We need to show some simple calculations that this idea is feasible. Since the wing is rigidly attached to the stand, the airflow only has to be sufficient to allow for rotations.

    So, what equations would be helpful here? Also, if you know how the control loops should look like, that would be nice to see as well. I am not a fluids person.......so my guess would be that the force generated on the elevator would be proportional to the velocity, area and density. I want to make a simple, standard airfoil (Clark Y maybe?). The wing will be small, about 9" x 12" max.

    Thanks for your ideas.
    Last edited: Mar 17, 2007
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  3. Mar 17, 2007 #2


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  4. Mar 17, 2007 #3
    Thanks, now where can I find a table listing the moment coefficients for differnt airfoils?

    I know theres a blue blook with an airfoil on the cover that has lots of information.
  5. Mar 19, 2007 #4
    Blue book with the airfoil on the cover...

    Theory of Wing Sections: Including a Summary of Airfoil Data
    by Ira H. Abbott, A. E. von Doenhoff

    Also, don't get above the stall angle, or otherwise your controller may not be able to stabilize the motion.
    Last edited: Mar 19, 2007
  6. Mar 19, 2007 #5
    Yep, thats exactly the blue book with the airfoil on the cover I was thinking of, thanks.

    As for the stall angle, dont worry. We want to find out when it does go uncontrollable as well. So it will be interesting to see when this happens.

    Would you know where in the book I should be looking?
    Last edited: Mar 19, 2007
  7. Mar 19, 2007 #6
    It will definitely be interesting, but the controllability will depend both on your controller design and on the stall characteristics of your airfoil (and by that I mean the hysteresis loop at large angle of attack (AOA).

    Appendix IV has plots of the lift, drag, and pitch moment with the AOA. Clark Y is not in the book.

    Cheers // Rope.
  8. Mar 20, 2007 #7


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    "Uncontrollable" will probably by the default operating mode, for your first few controller designs :wink:

    Your wing doesn't have to be stalled to be unstable, depending where the centre of pressure is relative to the centre of rotation.

    And yeah I know "unstable" doesn't necessarily mean uncontrollable!
  9. Mar 30, 2007 #8
    I want to calculate the force exerted on the elevator due to the airflow.

    I semi-derived this equation using dimensional analysis and trying to remember stuff from fluids.

    [tex]F= \rho A V^2[/tex]

    Obviously, p = density of air, A= profile area, V= velocity.

    I dont know if I should have a coefficient infront because the air flowing over the wing will speed up/slow down depending if the elevator is going up or down relative to the wing. In other words, the velocity would increase from the free stream as it curves over the wing and impacts the elevator.

    I guess for now its good enough to assume its 100%.

    Looks awful close to KE*A though its missing a 1/2.
    Last edited: Mar 30, 2007
  10. Mar 31, 2007 #9
    I'm not sure what your geometry is (a sketch wouldn't hurt). With a wing fixed to the stand, you'll only have pitch moments to control, in which case,
    [tex]M=1/2 * \rho * V^2 * A * chord * Cm [/tex]
    If you're trying to control just the flap, that's another story.
  11. Mar 31, 2007 #10
    Ok, I have a question about this equation. This seems to be the 'natural' moment of the wing itself.

    Im talking about sticking a servo attached to a hinged elevator at the back.

    What is the scope of the equation you provided?

    Setup: Picture in your mind an airfoil. A ridgid rod goes through the CG. There is a elevator on the back of the wing that can go up or down. It's that simple.

    Last edited: Mar 31, 2007
  12. Apr 1, 2007 #11
    The equation was that of the aerodynamic moment (it was rather a definition for the moment coefficient, not an equation...).

    Given your sketch though, that's not what you need. Assuming that you want to dimension the servo, you'd probably need the pressure distribution on the flap itself when deflected (not only the magnitude of the force, but its pressure center as well). Section 8.8 and corresponding table in the "blue book" of Abbot and Doenhoff may be of some help, while some basic info is provided in section 8.37 of Milne-Thomson's "Theroretical Aerodynamics".
  13. Apr 14, 2007 #12
    In this book, do they publish the MOI for a symmetrical airfoil?
  14. Apr 23, 2007 #13
    Sorry for the delay... what's MOI? If it's the moment of inertia, then the answer is no (you could nevertheless approximate it as a beam (2D) or a plate (3D).
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