How Do Wings Work? Holger Babinsky in Physics Education , Vol. David Bloor. University of Chicago Press, Understanding Aerodynamics: Arguing from the Real Physics. Doug McLean. Wiley, You Will Never Understand Lift. Peter Garrison in Flying ; June 4, Culick; July Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue.
See Subscription Options. In Brief On a strictly mathematical level, engineers know how to design planes that will stay aloft. But equations don't explain why aerodynamic lift occurs. There are two competing theories that illuminate the forces and factors of lift. Both are incomplete explanations. Aerodynamicists have recently tried to close the gaps in understanding.
Still, no consensus exists. Climate Change. Fossil Fuels. Support science journalism. Knowledge awaits. See Subscription Options Already a subscriber? Create Account See Subscription Options. Continue reading with a Scientific American subscription. Subscribe Now You may cancel at any time. In the simulation, colored smoke is introduced periodically. One can see that the air that goes over the top of the wing gets to the trailing edge considerably before the air that goes under the wing.
In fact, close inspection shows that the air going under the wing is slowed down from the "free-stream" velocity of the air. So much for the principle of equal transit times. Fig 2 Simulation of the airflow over a wing in a wind tunnel, with colored "smoke" to show the acceleration and deceleration of the air.
The popular explanation also implies that inverted flight is impossible. It certainly does not address acrobatic airplanes, with symmetric wings the top and bottom surfaces are the same shape , or how a wing adjusts for the great changes in load such as when pulling out of a dive or in a steep turn.
So, why has the popular explanation prevailed for so long? One answer is that the Bernoulli principle is easy to understand. There is nothing wrong with the Bernoulli principle, or with the statement that the air goes faster over the top of the wing.
But, as the above discussion suggests, our understanding is not complete with this explanation. The problem is that we are missing a vital piece when we apply Bernoulli's principle. We can calculate the pressures around the wing if we know the speed of the air over and under the wing, but how do we determine the speed? Another fundamental shortcoming of the popular explanation is that it ignores the work that is done. Lift requires power which is work per time.
As will be seen later, an understanding of power is key to the understanding of many of the interesting phenomena of lift. So, how does a wing generate lift?
To begin to understand lift we must return to high school physics and review Newton's first and third laws. We will introduce Newton's second law a little later. Newton's first law states a body at rest will remain at rest, and a body in motion will continue in straight-line motion unless subjected to an external applied force.
That means, if one sees a bend in the flow of air, or if air originally at rest is accelerated into motion, there is a force acting on it. Newton's third law states that for every action there is an equal and opposite reaction.
As an example, an object sitting on a table exerts a force on the table its weight and the table puts an equal and opposite force on the object to hold it up. In order to generate lift a wing must do something to the air.
What the wing does to the air is the action while lift is the reaction. Let's compare two figures used to show streams of air streamlines over a wing. In figure 3 the air comes straight at the wing, bends around it, and then leaves straight behind the wing. We have all seen similar pictures, even in flight manuals.
But, the air leaves the wing exactly as it appeared ahead of the wing. There is no net action on the air so there can be no lift! Figure 4 shows the streamlines, as they should be drawn. The air passes over the wing and is bent down. The bending of the air is the action. The reaction is the lift on the wing. Fig 3 Common depiction of airflow over a wing. This wing has no lift.
Fig 4 True airflow over a wing with lift, showing upwash and downwash. As Newton's laws suggest, the wing must change something of the air to get lift. Changes in the air's momentum will result in forces on the wing. To generate lift a wing must divert air down, lots of air. The lift of a wing is equal to the change in momentum of the air it diverts down. Momentum is the product of mass and velocity. The lift of a wing is proportional to the amount of air diverted down times the downward velocity of that air.
Its that simple. This downward velocity behind the wing is called "downwash". Figure 5 shows how the downwash appears to the pilot or in a wind tunnel. The figure also shows how the downwash appears to an observer on the ground watching the wing go by.
To the pilot the air is coming off the wing at roughly the angle of attack. To the observer on the ground, if he or she could see the air, it would be coming off the wing almost vertically.
The greater the angle of attack, the greater the vertical velocity. Likewise, for the same angle of attack, the greater the speed of the wing the greater the vertical velocity. Both the increase in the speed and the increase of the angle of attack increase the length of the vertical arrow. It is this vertical velocity that gives the wing lift. Fig 5 How downwash appears to a pilot and to an observer on the ground.
As stated, an observer on the ground would see the air going almost straight down behind the plane. This can be demonstrated by observing the tight column of air behind a propeller, a household fan, or under the rotors of a helicopter, all of which are rotating wings. If the air were coming off the blades at an angle the air would produce a cone rather than a tight column. If a plane were to fly over a very large scale, the scale would register the weight of the plane.
If we estimate the average vertical component of the downwash of a Cessna traveling at knots to be about 9 knots, then to generate the needed 2, lbs of lift the wing pumps a whopping 2. In fact, as will be discussed later, this estimate may be as much as a factor of two too low. The amount of air pumped down for a Boeing to create lift for its roughly , pounds takeoff weight is incredible indeed.
Pumping, or diverting, so much air down is a strong argument against lift being just a surface effect as implied by the popular explanation. In fact, in order to pump 2. Air weighs about 2 pounds per cubic yard at sea level. Figure 6 illustrates the effect of the air being diverted down from a wing. A huge hole is punched through the fog by the downwash from the airplane that has just flown over it. Fig 6 Downwash and wing vortices in the fog.
So how does a thin wing divert so much air? When the air is bent around the top of the wing, it pulls on the air above it accelerating that air down, otherwise there would be voids in the air left above the wing. Air is pulled from above to prevent voids. This pulling causes the pressure to become lower above the wing.
It is the acceleration of the air above the wing in the downward direction that gives lift. Why the wing bends the air with enough force to generate lift will be discussed in the next section. As seen in figure 4, a complication in the picture of a wing is the effect of "upwash" at the leading edge of the wing. As the wing moves along, air is not only diverted down at the rear of the wing, but air is pulled up at the leading edge.
This upwash actually contributes to negative lift and more air must be diverted down to compensate for it. This will be discussed later when we consider ground effect. Normally, one looks at the air flowing over the wing in the frame of reference of the wing.
In other words, to the pilot the air is moving and the wing is standing still. We have already stated that an observer on the ground would see the air coming off the wing almost vertically. But what is the air doing above and below the wing? Figure 7 shows an instantaneous snapshot of how air molecules are moving as a wing passes by.
The symmetric airfoil in our experiment generates plenty of lift and its upper surface is the same length as the lower surface. Think of a paper airplane. This part of the theory probably got started because early airfoils were curved and shaped with a longer distance along the top.
Such airfoils do produce a lot of lift and flow turning, but it is the turning that's important, not the distance. There are modern, low-drag airfoils which produce lift on which the bottom surface is actually longer than the top. This theory also does not explain how airplanes can fly upside-down which happens often at air shows and in air-to-air combat. The longer surface is then on the bottom! You can download your own copy of the program to run off-line by clicking on this button: You can further investigate all the factors affecting lift by using the FoilSim III Java Applet.
Beginner's Guide Home Page. The component of total drag resulting from the generation of lift is termed induced drag. Lift is a function of aerofoil design shape , airspeed and angle of attack :.
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