Planar vibrating airfoil..could this idea work?

1. Frank Kecskes

13
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???

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?

3. Frank Kecskes

13
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.

4. Cyrus

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.

5. Cyrus

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.

6. Phrak

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
7. Frank Kecskes

13
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....

8. Cyrus

Actually, that's exactly how they create lift, a leading edge vortex (among other things).

9. Frank Kecskes

13
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...

10. Cyrus

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.

11. Cyrus

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.

12. Frank Kecskes

13
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??

13. Phrak

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.

14. Cyrus

See the references on my website. It has been well established that there is a leading edge vortex in flapping flight.

15. Frank Kecskes

13
Cyrus: So, you are saying that a vortex is created within a straw when you blow across the top of it?

16. Cyrus

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 here.

17. Cyrus

No, not at all.

18. Frank Kecskes

13
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?

19. Cyrus

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.

20. Frank Kecskes

13
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
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