Airfoil Design

When looking at a typical airfoil, such as a wing, from the side, several design characteristics become obvious. You can see that there is a difference in the curvatures (or camber) of the upper and lower surfaces of the wing. The camber of the upper surface is more pronounced than lower surface, which is usually somewhat flat.

The chord line is a reference line drawn from the center of the leading edge straight through the wing to the trailing edge. The distance from this chord line to the upper and lower surfaces of the wing shows the amount of upper and lower camber at any point. Another reference line, drawn from the leading edge to the trailing edge, is the mean camber line. This mean line is equidistant at all points from the upper and lower surfaces.

Different airfoils have different flight characteristics. The weight, speed, and purpose of each aircraft dictate the shape of its airfoil. The most efficient airfoil for producing the greatest lift is one that has a concave, or “scooped out” lower surface

On the other hand, an airfoil that is perfectly streamlined and offers little wind resistance sometimes does not have enough lifting power to take the airplane off the ground.

If the wing profile were in the shape of a teardrop, the speed and the pressure changes of the air passing over the top and bottom would be the same on both sides. But if the teardrop shaped wing were cut in half lengthwise, a form resembling the familiar wing section would result

The pressure difference between the upper and lower surface of a wing alone, does not account for the total lift force produced.

The downward and backward flow of air from the top surface of a wing creates a downwash. This downwash meets the flow from the bottom of the wing at the trailing edge. Applying Newton’s third law, the reaction of this downward backward flow results in an upward forward force on the wing.

Lift is also generated by pressure conditions underneath the airfoil. Because of the manner in which air flows underneath the airfoil, a positive or high pressure results.

The angle of attack of a wing is the angle between the chord line and the flow of air against the leading edge of the wing

The average of the pressure variation for any given angle of attack is referred to as the center of pressure, noted as CP. All aerodynamic force acts through the Center of Pressure. At high angles of attack, the Center of Pressure moves forward, while at low angles of attack the Center of Pressure moves aft.

This Center of Pressure travel is very important, since an airplane’s aerodynamic balance and controllability are governed by changes in the Center of Pressure.

The production of lift is much more complex than a simple differential pressure between upper and lower wing surfaces. In fact, many airfoils do not have an upper surface longer than the bottom. These are called symmetrical airfoils. Symmetrical airfoils are seen in high-speed aircraft having symmetrical wings, or on symmetrical rotor blades for many helicopters whose upper and lower surfaces are identical. With symmetrical airfoils, the relationship of the airfoil with the oncoming air stream, or angle of attack, is all that is different.

As a wing moves through air, it is inclined upward against the airflow, producing a different flow caused by the wing’s relationship to the oncoming air. Think of a hand being placed outside the car window at a high speed. If the hand is inclined in one direction or another, the hand will move upward or downward. This is caused by deflection, which in turn causes the air to turn about the object within the air stream. As a result of this change, the velocity about the object changes in both magnitude and direction, in turn resulting in a measurable velocity force and direction.

So far we have only focused on the air flow across the upper and lower surfaces of an airfoil. While most of the lift is produced by these two dimensions, a third dimension, the tip of the wing also has an aerodynamic effect. The high-pressure area on the bottom of an airfoil pushes around the tip to the low-pressure area on the top. This action creates a rotating flow called a wing tip vortex. The vortex flows behind the airfoil creating a downwash that extends back to the trailing edge of the airfoil. This downwash results in an overall reduction in lift for the affected portion of the wing.

Airplane manufacturers have developed different methods to counteract this action. Winglets can be added to the tip of an airfoil to reduce this flow. The winglets act as a dam preventing the vortex from forming. Winglets can be on the top or bottom of the airfoil. Another method of countering the flow is to taper the airfoil tip, reducing the pressure differential and smoothing the airflow around the tip.

Adapted from the Pilot's Handbook of Aeronautical Knowledge.

[tlotsmatlots]

I've been looking all over Youtube and this is the only video I got that explains everything my instructor says in an equally relatable way.. God Bless

[Observer]

This video only repeats very common misconceptions and invents a new one, as follows.
.
3:13 The low pressure above the wing allows higher pressure ahead of the wing to accelerate it to the higher speed. It is Newton. The force of pressure accelerates the mass of air.
The movement of the wing creates pressure gradients and as Euler reported to us in the mid 1700s, Pressure gradients accelerate fluids.
Bernoulli never said speed causes a pressure reduction. He only noted that the (static) pressure and velocity have an inverse relationship with no indication of cause and effect. This is a major, common misunderstanding.
Euler determined that pressure gradients accelerate fluids toward the lower pressure. That is the opposite of the common misconception.
...
3:29 The pressure difference top to bottom accounts for ALL THE LIFT. This 'a little of this and a little of that" is bad science. The same pressures that have the net upward lift force on the wing are the very same pressures that cause all accelerations around the wing including ahead, above, below, behind and around the tips.
The higher pressure under the wing pushes up on the wing AND down on the air below to contribute to the down-wash.
The lower pressure above the wing allows atmospheric pressure to push air above the wing downward, to also contribute to the down-wash.
..
NOTE: the whole upward force on the wing is equal to the force required to accelerate all the air in the down-wash. THAT is what satisfies Newton's Third Law. The upward 'push" (force) on the wing equals the downward 'push' (force) on the air accelerated downward.
The downward moving air does not push up on the wing. That air was pushed down by the pressures around the wing.
The down-wash also does not push the wing forward. You've invented a new a fallacy. The net force fore-aft is rearward and is called drag and is not caused by the down-wash as explained above.
When viewed from the still air frame of reference, the down-wash is moving slightly forward.
....
4:27 Bernoulli's Principle is again misused. The flow is not slowed under the wing which is NOT the cause of the higher pressure. . That view is valid only in the stationary wing, wind tunnel reference frame only.

In fact, REAL data from a REAL wing generating REAL lift flying through otherwise still air shows the lower air being moved forward in the direction of flight. This acceleration from still air is CAUSED BY the higher pressure at the lower surface. The increase in pressure at the surface is because the wing is pushing the air out of its way. This push increases the pressure, not the Bernoulli Principle Fallacy.
....
The pressure increase below the wing in combination with the decrease above the wing provides the ENTIRE lift force.
...
If by this magic, the down-wash did just happen to add to the lift, that "addition" would be exhibited as an increased pressure difference on the win...but it DOES NOT.
Bernoulli's Principle causes nothing in the generation of lift because the "speed-causes-pressures" is a fallacy.
It is all explained by Newton, BUT NOT with this ":reaction force" fallacy.
Newton's Third Law "reaction force" is not an originating force, but is a direct result of the originating force which, in this case, is from the pressures around the wing as described above.
....
At 7:11 Please listen to your words: “The high pressure area on the bottom of an airfoil pushes around the tip to the low pressure area on the top.”. This shows that you understand that pressure causes movement, but you fail to apply it on all the other parts of this and, therefore shows the problems with this explanation. The correct story uses this Pressure accelerated air in ALL aspects around the wing, because that I the real science.
The tip vortex does not ‘create the down wash”. You already explained the down-wash without reference to the tip vortex. This is a flaw in this explanation. The tip vortex is CONSISTENT with the wing generated down-wash moving against the still air outside the wing span. Masses of air sliding along each other create vorticity (vortices) along the boundary of the two masses.
…
At 7:47 Winglets DO NOT and CANNOT prevent the tip vortex from forming. They can only reduce the severity and the negative effect because they act to effectively lengthen the wingspan; longer wings have less severe vortices . See Doug McLean’s You Tube video on misconceptions for a full explanation.
…
Though well-meaning, the 'Pilot's Handbook of Aeronautical Knowledge.' has bad information in it as do so many other publications and on-line resources such as YouTube.

[adastra]

At 3:08 the statement is made "the air moving over the upper surface is forced to move faster than the air along the lower surface". Why is that ? What is forcing it to move faster ?

[david_porthouse]

The existence of the wingtip vortex is a topological requirement and the only way to prevent it from forming is to reduce the angle of attack to the point where the wing is not generating any lift. Winglets are able to regulate the wingtip vortex, but not to eliminate it.

The vortex system of a wing is a ring vortex which passes through the boundary layers around the wing, along one trailing wingtip vortex, through the starting vortex left behind on the runway, and then back through the other wingtip vortex. Vorticity associated with the wing generates lift by the Magnus effect if you want a short explanation, or by the Kutta-Joukowski circulation theorem if you want more detail.

[ifirekirby]

This was EXTREMELY helpful. Thank you.

[bradmcclure]

I am trying to determine the surface area and camber of a wing to lift a specific weight

[BiggMo]

6:52 doesn’t the airflow detach from the upper half of the symmetrical air foil in a high speed application. And the detachment effectively creating a lower pressure?

[blaster-zyxx]

I have an issue with your illustration at 4:34. Looking at the small blue arrows, you purpetuate the myth that the air being split at the leading edge matches up again at the trailing edge. This is false. The air on the top of the wing goes faster and beats the air under the wing at the trailing edge.

[sabercruiser.]

THANK YOU THE MOST HELPFUL LESSON EVER

[gerardmoran]

Great job breaking down and organizing all of the components of lift.

[riazhassan]

Why is it always shown with a blunt, rounded leading edge? Wouldn’t a sharp one reduce the frontal drag?

[keesvandenbroek]

Stick to the Bernoulli theorem and forget the Newtons bit, then this is an adequate explanation of ift

[thaiiexpat]

I am investigating airfoils to use for a sailboat Wingsail. Most wing sales use a symetrical foil for the front section of the wing. My question is, if possible, am I better off finding a way to designing a wing foil that matches a standard airplane wing foil? More efficient? the Trick is to be able to adjust the leading edge to adjust for left vs. right side attack. Your thoughts?

[ShonMardani]

I discovered that Parasite drag is produced by VERTICAL surfaces and Induced drag is created by HORIZONTAL surfaces, for example when the Flaps are at 0 degrees they produce Max Induced drag and Min Parasite drag, at 90 degrees Min Induced and Max Parasite drag and at 45 degrees in the middle of both.
Please let me know if you think it is a valid observation. Thanks

[henryhorak]

I fear that this video may have fallen for some common misconceptions in the aerodynamic community. For one, the figures depicting the low and high pressure arrows imply a suction on the top of the airfoil. This is not the case, it is simply less pressure on top pushing downwards while the higher pressure have much higher magnitudes and push upwards from the bottom. There is another part in the video where you explain the center of pressure. I believe you define it incorrectly, but correctly explain that topic. I think what you meant to say was coefficient of pressure, C_p at 5:30. Center of pressure varies and is a location. Coefficient of pressure is the average pressure variation.

[michaelnorris]

Ok I got a question. Where does a glider get its thrust?

[alsh]

Not another incorrect video on this topic.Will Liebhaber - Please do some research before posting. And no, I'm not gonna write an essay here explaining it to you, do your own study. Please remove this video.

[TheMatej]

This is not an "Airfoil Design" video, but rather an "airfoil description" video.

[georgechristoforou]

I was enjoying this lecture until the point the video ended. Is there a part 2 to continue this truncated lecture?

[islandfds]

I think you've got it backwards - the high pressure causes the airflow to slow down, not the other way around.