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Planar vibrating airfoilcould this idea work?

  1. May 16, 2010 #1
    I have a concept for an airfoil that might be capable of providing vertical take-off and hovering (as well as forward or backward flight), but I need feedback to see if this would be feasible from an engineering / aeronautical standpoint.

    Bernoulli's Principle is classically demonstrated by by blowing air past the top of a drinking straw while the bottom is immersed in a glass of water. The resulting drop in air pressure within the straw causes the water to rise up in the straw.

    Now imagine what would happen if air were blown across the top of the straw, alternating rapidly from the left and the right. The water would still rise in the straw as it does not matter from which direction the air is being blown...Correct? Then also, it should not matter whether the air itself is being blown, or the top end of the straw is being moved back and forth rapidly in relation the the air. It is all relative right?

    So, now imagine an airfoil mounted so it can move rapidly back and forth causing the air to move over it's curved surface so as to generate lift. A system of solenoids could possibly push and pull such an air foil mounted to a frame, at say a distance of 1/2" back and forth. energizing the solenoids at the correct frequency could then approximate the speed of a conventional airfoil moving through the air at the correct speed to impart lift (in any direction or orientation) so that vertical take-off, hovering and forward / backward flight could be acheived...What are your thoughts???
     
  2. jcsd
  3. May 17, 2010 #2

    berkeman

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    Staff: Mentor

    Pretty creative. Kind of a backward-forward wing movement. I think the main problem will be in the relative velocity of the wing wrt the still air. What is the typicaly helicopter blade velocity wrt the still air that provides lift? How fast would the oscillatory movement need to be to get close to that linear blade velocity?
     
  4. May 17, 2010 #3
    I can tell you for instance that the stall velocity of a single engine airplane is approximately 90 fps. If 1/2" back and forth movement were used, I calculate that the frequency of solenoid energizing would have to be 90ft x 12"/ft divided by 1/2" for a frequency of 2,169 Hz. This would approximate a relative velocity of 90 ft/sec to the air.
     
  5. May 17, 2010 #4
    On your downstroke, you're basically trying to create lift with a backwards airfoil - and that isn't going to work. To me, this idea is like a flapping wing vehicle - without the important components of pronation and supination. These are the important wing reversal strokes during the wing-beat kinematics of insects. It gives you a large peak force due to wake capture and unsteady aerodynamics at stroke reversal. Because you do not have that in your kinematics, your performance will be worse than an actual flapping wing configuration. You should look for papers by Dr. Micheal Dickinson at Caltech for more on flapping wing flight. I have the references on my website.
     
  6. May 17, 2010 #5
    It does not make sense to talk about stall in terms of airspeed, as stall is airspeed independent. Stall is a function of the wing angle of attack.
     
  7. May 17, 2010 #6
    Hmm. Sorry to burst your bubble, but it wouldn't generate any lift to speak of. A wing requires about 4 cord lengths of travel before the bound vortex is developed to perhaps 90% of it's long term value. The bound vortex is required for lift.

    However, refer to bird and insect flight modes that get around this problem.
     
    Last edited: May 17, 2010
  8. May 17, 2010 #7
    Cyrus: So the Bernoulli analogy is flawed? Isn't the motion of air to air foil all relative? I am not advocating a "flapping airfoil" rather, one the moves forward and backward relative to the air....
     
  9. May 17, 2010 #8
    Actually, that's exactly how they create lift, a leading edge vortex (among other things).
     
  10. May 17, 2010 #9
    Cyrus: Instead of thinking in terms of conventional airfoils, please explain how the "straw principle" analogy that I originally cited, would not apply to the airfoil I described...
     
  11. May 17, 2010 #10
    My wording was a bit ambiguous. By "flapping" it can mean up/down (like a bird), or fore/aft (like an insect). My use of the word flapping here was insect like, which is what you're trying to do.

    But, back to airfoils - think about it. When you're airfoil is on its reverse stroke, its flying 'backwards'. Airfoils don't like to do that.

    *Note: In reality insects can have a culmination of up/down/fore/aft kinematics.
     
  12. May 17, 2010 #11
    The problem with your analogy is that it is inadequate to explain how airfoils generate lift. They do so by having a pressure gradient along the chord. That pressure gradient is due to the flow over the airfoil. When you start moving in the reverse direction, the pressure gradient gets screwy and you won't produce a useful amount of lift.
     
  13. May 17, 2010 #12
    Cyrus: So why then does water rise in the straw due to pressure drop no matter from what direction the air is blown across the top of the straw??
     
  14. May 17, 2010 #13
    Hu? A vortex still needs to be established. It doesn't happen all at once as soon as a lifting surface is in motion. The rate of development is dependent upon the viscosity of the fluid. Some birds use a flap-slap motion to initiate a vortex on each wing obtaining fairly instant lift.
     
  15. May 17, 2010 #14
    See the references on my http://aerospaceindustrynews.webs.com/apps/blog/show/3261925-cta-mast" [Broken]. It has been well established that there is a leading edge vortex in flapping flight.
     
    Last edited by a moderator: May 4, 2017
  16. May 17, 2010 #15
    Cyrus: So, you are saying that a vortex is created within a straw when you blow across the top of it?
     
  17. May 17, 2010 #16
    When you blow over a straw, you are increasing the velocity of the air (going over the opening). This is an increase in dynamic pressure. As a result, the static pressure goes down inside the tube of the straw, and the water rises. However, this is not how an airfoil generates lift. An airfoil is generating lift by accelerating the air that passes over it due to its shape. You might get a better primer on airfoils http://www.allstar.fiu.edu/aero/flight31.htm" [Broken].
     
    Last edited by a moderator: May 4, 2017
  18. May 17, 2010 #17
    No, not at all.
     
  19. May 17, 2010 #18
    Cyrus: So, if I take a flat sheet of paper and lay it o the table, and blow a jet of air across the top of it, it rises up. There is no leading edge geometry here, yet the pressure drop according to Bernoulli's Principle still applies...Correct?
     
  20. May 17, 2010 #19
    Yes. In the case of your paper, you are supplying a jet of air. By doing so, the static pressure of that 'streamline' of air must be lower (due to Bernoulli). The paper then feels the low pressure of air flowing over top of it, and rises accordingly. In an airfoil, it does not work the same way because you are not supplying a jet of air. The airfoil is moving through the air and reducing the pressure* due to it's shape.

    Well, keep in mind that Bernoulli's equation is valid along a streamline for steady flow and we assume viscous effects are negligible. None of this is true if you have a flapping airfoil, so it is incorrect to try and apply this equation to your oscillating airfoil.

    *In the first 1/3, the airfoil reduces the pressure. But on the last 2/3rds it increases the pressure back to static at the trailing edge.
     
  21. May 17, 2010 #20
    Cyrus: Slightly changing the subject, what if I told you there is a way to electrostatically move the air across an airfoil (in one direction) by sequentially electrifying adjacent segments on the surface of an airfoil to approximate the airflow as in a conventionally used wing. And what if I told you that the laminar flow of such electrostaically moving air molecules could be made to be in a layer so thin that the energy required to move them in this fashion would be orders of magnitude less than what a prop or turbine or jet engine currently uses to move an aircraft through the air. Would that indeed be a technology worth while?
     
    Last edited: May 17, 2010
  22. May 17, 2010 #21
    Interesting. I don't know, I'm skeptical if you could get such a thing to produce sufficient velocity to make any useful amount of lift. Are you saying this as a 'hypothetical' or have you actually made and tried this? It should be easy enough to test, all you need is a section of an airfoil attached to a bathroom scale. If it makes thrust, you'll know how much.
     
  23. May 17, 2010 #22
    Of course there is. However, your response and website are completely nonsequitar to vortex generation. I suppose your focus is divided.
     
    Last edited by a moderator: May 4, 2017
  24. May 17, 2010 #23
    Cyrus: The sequential charging and discharging of adjacent strip electrode elements spanning the surface of an air foil is only limited by the controlling circuit. That being said, you could cause air molecules to be moved over the surface at whatever speed you chose. Then, by using a frequency of the appropriate value, any portion of the craft outfitted with such electrodes could become a lifting / pushing surface. You could for instance use this concept to provide enough lift and "push" to move an aircraft in any direction. A car outfitted with such an airfoil oriented as a front and rear bumper with sufficient surface area, could literally both move the car forward and also provide the braking action...all with the same principle and nothing to wear out except the generator or battery system providing the power for the electrification of the electrode segments on the airfoil....
     
  25. May 17, 2010 #24
    I think I misread your post originally, my apologies. I'm not sure what about my response is not satisfying to you, because we are both saying the same thing (at least, I think we are).
     
    Last edited: May 17, 2010
  26. May 18, 2010 #25
    We're both saying a simple to-and-fro motion won't work. You seem to be saying the trajectory and orientation of the lifting surface need to be properly done. I'm saying that it really doesn't matter.

    Have you seen flow fields of nonviscous fluids over an airfoil shape? Begin with a two dimensional irrotational flow field around a circle. Conformally transform the circle to an airfoil shape. The field is still irrotational. Without the rotational component, there is no redirection of fluid velocity and no lift. This is the state of affairs for a perfect inviscous fluid.

    Now take a viscous fluid in uniform motion, such as an airsteam first beginning to pass over an airfoil, it's rotation is everywhere zero. It's the stickiness of the air against foil surfaces and into the fluid volume that's required to change the angular momentum of the fluid in the region of the foil required to obtain lift. Once this is established, it will persist around the foil (hopefully, or stall results upon vortex shedding).

    So it takes some time and friction between fluid and foil to get this process developed. If you move your foil back and forth, to get lift you have to establish first left handed then righthanded vorticity with each stroke.

    This will not happen over a couple cord lengths or less, and the reversal of the stroke will first have to act to reverse the previous circulation. There's all this viscous action going on, half the time reversing what's already there.

    I'd imagine that half the time, the foil could be being forced downward instead of up when it's out of phase. I dunno. I'm just guessing about this one. It only just occured to me.
     
    Last edited: May 18, 2010
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