Understanding Flight

The characteristics of fluids to have zero velocity at the surface of an object explain why one is not able to hose dust off a car. Flight is a relatively simple and widely studied phenomenon. As surprising as it may sound, though, it is often misunderstood. For example, most descriptions of the physics of lift fixate on the shape of the wing (airfoil) as the key factor in understanding lift. The wings in these descriptions have a bulge on the top so that the air must travel farther over the top than under the wing. Yet we all know that wings fly quite well upside down, where the shape of the wing is inverted. The shape of the wing has little to do with how lift is generated, and any description that relies on the shape of the wing is misleading at best. It should be noted that the shape of the wing does has everything to with the efficiency of the wing at cruise speeds and with stall characteristics.

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The four forces. When an airplane is in equilibrium in straight and level cruising flight, the forces acting fore and aft (thrust and drag) are equal, as are those acting at 90 degrees to the flight path (lift and weight, or its components).

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The airfoil

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Lift acts perpendicular to the relative wind and wingspan

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Airflow past the airfoil

The four forces. When an airplane is in equilibrium in straight and level cruising flight, the forces acting fore and aft (thrust and drag) are equal, as are those acting at 90 degrees to the flight path (lift and weight, or its components).

The emphasis on wing shape in many explanations of lift is based on the principle of equal transit times. This assertion mistakenly states the air going around a wing must take the same length of time, whether going over or under, to get to the trailing edge. The argument goes that since the air goes farther over the top of the wing, it must go faster, and with Bernoulli’s principle, we have lift. Knowing that equal transit times is not defensible, the statement is often softened to say the air going over the top must go farther, it must go faster. Again, however, this is just a variation on the idea of equal transit times. Equal transit times holds only for a wing without lift. The air going over the wing arrives at the trailing edge before the air going under the wing. In fact, the greater the lift, the greater is the difference in arrival times at the trailing edge.

Another erroneous argument that leads one to believe that the shape of the wing is responsible for the generation of lift is the argument that the wing is a half venture. As the airflow constricts, it speeds up, much like putting your thumb on the end of a garden hose. Using Bernoulli’s principle, the pressure (perpendicular to the flow) in the constriction decreases. This clever device is used to create low pressure to draw fuel into automobile carburettors. The argument for a wing goes like this. Remove the top half of a venture and you have the wing. The problem, as any physics student can tell you, is that there can be no net lift if the air enters horizontally and leaves horizontally, how can there be a vertical force? A jet engine and a propeller produce thrust by blowing air back. A wing produces lift by diverting air down. A jet engine, a propeller, a helicopter’s rotor, and a wing all work by the same physics. Air is accelerated in the direction opposite the desired force.

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The stall

 

 

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Relative lift increases with angle of attack until the stall angle is reached

 

Lift as a reaction force: That is, wings develop lift by diverting air down. Since we know that a propeller produces thrust by blowing air back and that a helicopter develops lift by blowing air down, the concept of a wing diverting air down to produce lift should not be difficult to accept. After all, propellers and rotors are simply rotating wings. One should be careful not to form the mental image of the air striking the bottom of the wing and being deflected down to produce lift. This is a fairly common misconception that also was held by Sir Isaac Newton himself. Since Newton was not familiar with the details of airflow over a wing, he thought that the air was diverted down by its impact with the bottom of a bird’s wings. It is true that there can be some lift owing to the diversion of air by the bottom of the wing, but most of the lift is due to the action over the top of the wing. The low pressure that is formed accelerates the air down.

 Newton’s Three Laws: The most powerful tools for understanding flight are Newton’s three laws of motion. They are simple to understand and universal in application:

First Law: A body at rest will remain at rest, and a body in motion will continue in straight-line motion unless acted on by an external applied force. In the context of flight, this means that if a mass or blob of air is initially motionless and starts to move, there has been some force acting on it. Likewise, if a flow of air bends, such as over a wing, there also must be force acting on it. In the context of a continuum such as air, the force expresses itself as a difference of pressure.

Third Law: For every action there is an equal and opposite reaction. When one sits in a chair, you put a force on the chair, and the chair puts an equal and opposite force on you. The force you put on the chair is action, whereas the force the chair puts on you is the reaction. That is the chair is reacting to the force you are putting on it. Another example is seen in the case of a bending flow of air over a wing. The bending of the air requires a force from Newton’s first law. By Newton’s third law, the air must be putting an equal and opposite force on whatever is bending it, in this case the wing. When the air bends down, there must be a downward force on it, and there must be an equal upward force on the wing by Newton’s third law. The bending of the air is the action, whereas the lift on the wing is the reaction.

Second Law: The most common form of the second law which students are taught in early physics courses, is F=ma. [or force equals mass times acceleration]. The alternate form of Newton’s second law for a rocket can be stated: the force or thrust of a rocket is equal to the amount of gas expelled per time times the velocity of the gas. Newton second law tells how much thrust is produced by the engine of a rocket. As the air approaches the wing it splits and re-forms behind the wing going in the initial direction. This wing has no lift. There is no net action on the air, and thus there is no lift, or reaction on the wing. If the wing has no net effect on the air, the air cannot have any net effect on the wing. The air splits around the wing and leaves the wing at a slight downward angle. This downward-travelling air is the downwash and is the action that creates lift as its reaction. Thus there is a force acting on the air and a reaction force acting on the wing. There is lift. If one were to sum up how a wing generates lift in one sentence, it would be that the wing produces lift by diverting air down.

Courtesy: Understanding Flight by David F. Anderson & Scott Eberhardt, McGraw Hill Companies, New York, 2010

Newton’s Laws Of Motion

First Law: a body at rest tends to remain at rest, and a body in motion tends to remain moving at the same speed and in the same direction.

Second Law: when a body is acted upon by a constant force, its resulting acceleration is inversely proportional to the mass of the body and is directly proportional to the applied force.

Third Law: whenever one body exerts a force on another, the second body always exerts on the first, a force that is equal in magnitude but opposite in direction.

Bernoulli’s Principle: the pressure of a moving fluid (liquid or gas) varies with its speed of motion. An increase in the speed of movement or flow would cause a decrease in the fluid’s pressure.

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Pitch attitude (climb angle relative to flight path) and angle of attack

 

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Airfoil Design

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Wingtip vortices

 

As the airstream strikes the relatively flat lower surface of the wing when inclined at a small angle to its direction of motion, the air is forced to rebound downward and therefore causes an upward reaction in positive lift, while at the same time airstream striking the upper curved section of the leading edge of the wing is deflected upward. In other words, a wing shaped to cause an action on the air, and forcing it downward, will provide an equal reaction from the air, forcing the wing upward. If a wing is constructed in such form that it will cause a lift force greater than the weight of the airplane, the airplane will fly. The balance of the lift needed to support the airplane comes from the flow of air above the wing. The fact that most lift is the result of the airflow’s downwash from above the wing, must be thoroughly understood.

Momentum is the resistance a moving body offers to have its direction or amount of motion changed. When a body is forced to move in a circular path, it offers resistance in the direction away from the centre of the curved path. This is the centrifugal force. Therefore, the air pressure on the upper surface of the airfoil is distributed so that the pressure is much greater on the leading edge than the surrounding atmospheric pressure, causing strong resistance to forward motion; but the air pressure is less than surrounding pressure over a large portion of the top surface. As seen in the application of Bernoulli’s theorem to a venture, the speedup of air on top of an airfoil produces a drop-in pressure. This lowered pressure is a component of the total lift. It is a mistake, however, to assume that the pressure difference between the upper and lower surface of a wing alone accounts for the total lift force produced. One must bear in mind that, associated with the lowered pressure is downwash; a downward backward flow from the top surface of the wing.

Relative to the dynamic action of the air as it strikes the lower surface of the wing, the reaction of this downward backward flow results in an upward force on the wing. This same reaction applies to the flow of air over the top of the airfoil as well as to the bottom, and Newton’s third law is again in picture.

Because of the way air flows underneath the wing, a positive pressure results, particularly at high angles of attack. But there is another aspect to this airflow which must be considered. At a point close to the leading edge, the airflow is virtually stopped (stagnation point) and then gradually increases speed. At some point near the trailing edge, it has again reached a velocity equal to that on the upper surface. In conformance with Bernoulli’s principles, where the airflow was slowed beneath the wing, a positive upward pressure was created against the wing; i.e., as the fluid speed decreases, the pressure must increase. This increases the pressure differential between the upper and lower surface of the airfoil, and therefore increases total lift over that which would have resulted had there been no increase of pressure at the lower surface. Both Bernoulli’s principle and Newton’s laws are in operation whenever lift is being generated by an airfoil.

The FAA’s believes in: Newton’s Laws Of Motion

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