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-   -   Planar vibrating airfoil..could this idea work? (http://www.physicsforums.com/showthread.php?t=403729)

Frank Kecskes May16-10 11:48 PM

Planar vibrating airfoil..could this idea work?
 
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???

berkeman May17-10 12:08 AM

Re: Planar vibrating airfoil..could this idea work?
 
Quote:

Quote by Frank Kecskes (Post 2720456)
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???

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?

Frank Kecskes May17-10 12:46 AM

Re: Planar vibrating airfoil..could this idea work?
 
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.

Cyrus May17-10 12:59 AM

Re: Planar vibrating airfoil..could this idea work?
 
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.

Cyrus May17-10 01:00 AM

Re: Planar vibrating airfoil..could this idea work?
 
Quote:

Quote by Frank Kecskes (Post 2720504)
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.

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.

Phrak May17-10 01:15 AM

Re: Planar vibrating airfoil..could this idea work?
 
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.

Frank Kecskes May17-10 01:20 AM

Re: Planar vibrating airfoil..could this idea work?
 
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....

Cyrus May17-10 01:30 AM

Re: Planar vibrating airfoil..could this idea work?
 
Quote:

Quote by Phrak (Post 2720519)
However, refer to bird and insect flight modes that get around this problem.

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

Frank Kecskes May17-10 01:41 AM

Re: Planar vibrating airfoil..could this idea work?
 
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...

Cyrus May17-10 01:47 AM

Re: Planar vibrating airfoil..could this idea work?
 
Quote:

Quote by Frank Kecskes (Post 2720523)
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....

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.

Cyrus May17-10 01:49 AM

Re: Planar vibrating airfoil..could this idea work?
 
Quote:

Quote by Frank Kecskes (Post 2720531)
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...

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.

Frank Kecskes May17-10 01:50 AM

Re: Planar vibrating airfoil..could this idea work?
 
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??

Phrak May17-10 02:03 AM

Re: Planar vibrating airfoil..could this idea work?
 
Quote:

Quote by Cyrus (Post 2720530)
Actually, that's exactly how they create lift, a leading edge vortex (among other things).

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.

Cyrus May17-10 02:05 AM

Re: Planar vibrating airfoil..could this idea work?
 
Quote:

Quote by Phrak (Post 2720552)
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.

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

Frank Kecskes May17-10 02:06 AM

Re: Planar vibrating airfoil..could this idea work?
 
Cyrus: So, you are saying that a vortex is created within a straw when you blow across the top of it?

Cyrus May17-10 02:13 AM

Re: Planar vibrating airfoil..could this idea work?
 
Quote:

Quote by Frank Kecskes (Post 2720537)
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??

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.

Cyrus May17-10 02:13 AM

Re: Planar vibrating airfoil..could this idea work?
 
Quote:

Quote by Frank Kecskes (Post 2720554)
Cyrus: So, you are saying that a vortex is created within a straw when you blow across the top of it?

No, not at all.

Frank Kecskes May17-10 02:19 AM

Re: Planar vibrating airfoil..could this idea work?
 
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?


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