Lifting force and flow separation

   Lift can be obtained even if peeled off on the upper surface of the wing

Mechanism of lift generation "Principle 1"

     "When an object moves in the air, air is compressed on the front surface of the object and a high pressure area is generated, and air is expanded on the rear surface of the object and generates a low pressure area."

    When replaced this with a wing,

     "When a wing with an angle of attack moves through the air, air is compressed on the front surface (lower surface) of the wing and creates a high pressure area, and air is expanded on the rear surface (upper surface) and creates a low pressure area. "

Mechanism of lift generation "Principle 2"

     "Air flows at high speed from a high pressure region to a low pressure region around an object, and exerts a force on the object when the flow is bent. "

     If you replace this with wings,

     "Air flows at high speed from the high pressure area around the wing to the low pressure area, exert a force on the wing when the flow is bent. "

(The centrifugal force of the bending air exerts a force that pulls the wing straight to the surface.) 

     For details, see "Mechanism of Lift Generation" on the homepage.

     According to "my opinion", even if the angle of attack becomes large and the air flow separates on the upper surface of the wing, lift should be obtained without problems according to "Principle 1". 

     The lift should not be zero while the flow is bent, so I tried the following experiment.

     First, when examining "stall", it is generally explained that 

     "at an angle of attack of 10 to 15 degrees, the flow on the upper surface of the wing separates and lift is lost. This phenomenon is called stall."

     So, using the model R that I built and the measuring device, I measured the lift and drag by changing the angle of attack by 2° from 0° to 30°. 　

     This is the graph below. (Re=7.9×10^4)

     Model R is a general blade cross-sectional shape.　 　

     1) In the graph, the data does not seem to lose lift even at an angle of attack of 30°, where the flow on the upper surface of the wing seems to be definitely separated. 　

     Actually I checked that at the angle of attack of 15°, the flow separated and became turbulent in the rear half of the wing model R.

     In the video, the turbulent  flow separated at approximately the midpoint on the upper surface of the wing and the laminar flow on the lower surface of the wing are compared.

     The results of this experiment saids that even if the flow is separated on the upper surface of the wing, the lift is not completely lost on the wing due to "Principle 1", and even if the air flows backward along the upper surface of the wing, Even if it is a turbulent flow, it shows that lift is generated due to the effect of "Principle 2".

・Although I wrote "separate" and "separate", this word has the image that air no longer exerts any force on the wing, but it is not.

     "Separation" means that the laminar flow becomes uncontrollable and becomes turbulent.

・Even if it becomes a turbulent flow, centrifugal force according to “Principle 2” is generated (air is expanded and air pressure is lowered) and lift is generated in the range where the flow wraps around the upper surface of the wing due to the Coanda effect. 

      If the angle of attack is further increased and the surface flows backward from the rear end of the wing, the lift generated in "Principle 2" will rapidly diminish.

     Even in that state, lift is being generated according to "Principle 1", which is the case when the kite goes up, that is, when the angle of attack is large, around 70°.

2) This graph also shows that the pressure difference between the upper and lower surfaces of the wing cannot be explained by Bernoulli's theorem. 

     Because, as Mr. Bernoulli says, "Bernoulli's theorem"  can be applied only in laminar flow, not turbulent flow.

3) Below is my guess, 

・Propulsion is insufficient near the angle of attack of 10° to 15° where the drag force increases rapidly, and the flight speed becomes the upper limit.

・The flow on the upper surface of the wing begins to separate just near the angle of attack. 

     From these two phenomena, it may be interpreted that the flow is separated and the lift is lost and the plane stalls. 

     Because, If the propulsive force is large enough, even if the flow separates, lift can be obtained as in this experiment, and the plane does not crash. 

     Actually, a jet fighter with great propulsion will give an example. 

     There is an acrobatics that uses high rebel maneuvers to bring the fighter up close to the ground in a vertical position, but in this position the separation is out of the question, In the middle of this process (angle of attack 90°), the flow should definitely separate.

     Furthermore, isn't  [Graph X]  like the above figure misleading? 

     This [graph X] is the data obtained from my previous experiment, where the vertical axis is the lift-drag ratio and the horizontal axis is the angle of attack. 

     In this graph, there is a peak near 14°, and then it falls to the right. However, this does not indicate a reduction of lift. 

     As shown in that graph [Lift & Drag up to 30 degrees of attack] above, even if the angle of attack is 30°, the lift will increase as it rises to the right.

     Looking at the [graph X], the interpretation that lift is no longer generated at around 14° and the plane stalls, is completely wrong. In my experiment, a plate-like shape like a wing will give a graph like this.

     In addition, such a graph is usually displayed as "lift coefficient" on the vertical axis.

4) I think the kite's lift is exactly the same as when the wing's angle of attack increased. 

     At the back of the kite, the kite will rise vigorously even at an angle where the flow is definitely disturbed. "Principle 1" is working well.

・There is an explanation that says about only the lower surface (front surface) of the kite.  He says that the kite is getting lift by "collision drag".

     I understand that the collision drag is used as a ”wind pressure” in the world of architectural engineering.

     But then why, in the case of an airplane wing, why not use the concept of collision drag by explaining with the pressure difference between the upper and lower surfaces?

     If you say that "the kite and the wing with a large angle of attack have different lift generation mechanisms," it is necessary to explain the condition of the boundary, but no such explanation is found.

     In general, when explaining changes in an event, you are kind and kind enough to show me the limits (with explanations). If you say that it will change to a different phenomenon from the middle, the reader will not understand it unless seeing the situation at the boundary.

     In my opinion, the kite and wing lift mechanisms are the same, only the angles of attack are different. And airplanes other than fighters have a limited propulsion power. If the angle of attack is more than a certain angle (around 15°), it will stall, but the kite has sufficient propulsion power (＝thread). That's the only difference.

     To put it in an extreme way, an airplane with propulsion to support its own weight, such as a fighter, will not crash due to a stall.

     In other words, "stall" indicates the limit of propulsive force, and does not mean that the flow on the upper surface of the wing separates and lift is lost. And it has nothing to do with [Graph X ].

     [Graph X] is determined by the cross-sectional shape of the wing, and "stall" is the limit at which the velocity does not rise anymore, and is determined by the magnitude of the propulsive force.

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