-Basic Aerodynamics-

LESSON OBJECTIVES

1.- Get a deeper understanding of the four forces of flight

2.- Understand the relationship between the atmosphere, the aircraft and the forces acting on it

3.- Discover new concepts about wings and aerofoils

-What is Aerodynamics?-

In this section, we will talk about the origins of aerodynamics, the air and its components and the atmosphere:

- Aerodynamics -> It is the study of the movement ("dynamics") of gases, and the interaction between any moving object around the atmosphere causing airflow around the body

*--> In the beginning, a movement between a movement of a body in water was studied ("hydrodynamics"), so it is not a surprise that there are concepts in aviation that have their origin in their naval counterpart. For example:

-- Rudder

-- Knots

-- Nautical Mile

- Air -> It is a mixture of gases primarily nitrogen with a concentration of 78%, oxygen with a concentration of 21% and other gases representing 1%. It is considered a fluid because it is a substance that may be pressured to flow or change its shape

- Atmosphere -> It is the blanket of gases that surrounds the Earth. It s hold near the surface of the planet by the Earths gravitational attraction. It has variable conditions such as temperature, pressure and humidity that change from one day to another.

*--> For that reason, in order to study the atmosphere, a standard value of conditions was set as a base. This is called the International Standard Atmosphere (ISA). And the standard conditions at SL (Sea Level) are:

-- Temperature -> 15ºC

-- Pressure -> 1013 hPa

-- Air Density -> 1225 gm/m3

-- Temperature Lapse Rate -> -2ºC/1000ft

-- Pressure Lapse Rate -> -1hpa/30ft

-The Four Forces of Flight (Detailed Explanation)-

All of us already know from the Long Briefings Section that the forces that work together in order to make flight possible are lift, weight, thrust and drag. In this section, we will explain in more detail all of them, so you can have a better understanding of how they work and what factors affect each one of them

Captura.JPG

-Understanding Lift-

Lift is the force that directly opposes the weight of an aircraft and holds it in the air. It acts through the Center of Pressure (CP) and its direction is perpendicular to the relative airflow of the aircraft.

- Lift = 1/2dV² x CL x S

- d = air density -> It is the mass per unit of volume of Earth's atmosphere. It is measured in kg/m3 and is determined by the temperature, altitude, and humidity of the area

- -> It is the speed at which the aircraft travels through the air. Notice that it is speed squared, do a small increase in speed will have a high impact on the lift produced.

- 1/2dV² -> All this section of the formula represents IAS (Indicated Airspeed), while V² represents TAS (True Airspeed). As you can see in order to have the same IAS, TAS increase when air density decrease and vice versa

- CL -> Lift Coefficient -> It is a measure of the difference in pressure created above and below the wing. Its value depends on two things: wing shape (invariable) and angle of attack (variable). Higher AoA -> Higher Lift

- S -> This is the area in m² of your wing from wingtip to wingtip

From a practical point of view, the factors that we can modify as pilots are two:

- CL -> Changing the AoA of the wings, you increase or decrease the coefficient of lift. Also extending the flaps increase the curvature of the wing (wing shape) and increase the CL

- -> Using power and attitude in a determined way to increase the speed and produce more lift

This is a brief resume of everything that we have learned in the Long Briefings Section, but there are some things that you don't know about lift

-For example, although most of the lift is produced by the wings, the truth is that every part of the aircraft produces lift, even the fuselage of the aircraft create some quantity of lift

- At the bottom of this section, there are two videos in which you can see this phenomenon in both cars and airplanes

-Understanding Weight-

We already know what weight is, or maybe not. Many people confuse the concept of weight with the concept of the mass, and it is important to know the difference between them because in some situations in flight, even if your mass remains the same your weight can change. 

But first, let´s see some definitions:

Mass -> In its simple definition, mass is simply the quantity of matter that a body has and it is measured in kilograms (Kg)

- Weight -> It is the force that a mass exerts on the surface of the Earth due to the Earth´s gravity attracting that mass to the center of the Earth. It is measured in Newtons (N)

If you try to remember your first grade physics lesson, probably one of the formulas tht would come to your mind is:

- Force (N) = Mass (kg) x Acceleration (m/s²) -> (F = m x a)

That means

- Weight (N) = Mass(kg) x Gravity (9.8 m/s²)

This is the reason why your weight in other planets change, because although your mass remains the same, the force attracting you to the center of that planet is different:

Did you get the idea? If the acceleration that pulls you down changes, your weight changes accordingly. This changes in acceleration also occur in flight during certain maneuvers, for example in the turns

 

Probably you have heard about the concept of Gs (1gs, 2 gs, 3 gs ...etc), but what does it mean exactly?.

- G-force is a measure of acceleration. 1G (9.8 m/s²) is the acceleration we feel due to the force of gravity.

If we experiment 2 Gs, it means that the acceleration that is affecting our mass is 2 times the gravity value:

- Mass -> 80 kg

- Weight (2 Gs) -> 80 x 2 x 9.8 = 1568 N

- In this example we can say that we would have the sensation that our body mass is 160 kg in 1G conditions.

Same effect occurs in the aircraft when we are turning. When we turn, the lift has to counteract not only the weight but also the centrifugal force that try to take us out of the turn.

- The combination of the weight and the centrifugal force creates a new force called load factor, or Gs that acts as an apparent weight of the aircraft

Weight Difference (Earth vs Moon).PNG
S&L vs Turn Aircraft.PNG

-Understanding Thrust-

Thrust is the force created by an aircraft's propeller or jet engine to move the aircraft forward. The systems used to create thrust are known as propulsions systems

But how thrust is produced, two basic laws of physics explain how propeller and jet engines create a force to move us forward:

- 3rd Law of Newton -> " Every action has a reaction of equal magnitude and opposite direction". Propeller and jets work in the same principle but using different mechanisms, both of them work by pushing air backwards causing an opposite reaction that causes the aircraft to move forward

- 2nd Law of Newton -> "The acceleration of an object is proportional to the force and inversely proportional to its mass". This translates into a simple formula:

-- Force = Mass x Acceleration

-- Thrust = Mass of Air x Acceleration by the propeller/jet

-Understanding Drag-

It is a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid, in this case, air. Drag affects all the bodies travelling through the air, but in the case of aircraft it has some special differences because two types of drag exist:

- Parasite Drag

- Induced Drag

As we did with lift, we can also define a formula for drag:

- D = 1/2dV² x CD x A

As you can see is very similar to the lift formula but with two main differences:

- CD -> Drag Coefficient is a number that engineers use to model all the complex variables (airfoil shape, angle of attack, air viscosity ... etc) on aircraft drag

- A -> Crossectional Area represents the frontal area of the aircraft exposed to the ram air

Drag can be divided into types:

- Total Drag = Parasite Drag + Induced Drag

Parasite Drag

-It is the drag produced by any object for the simple reason to be exposed to the airflow. It is proportional to the squared of the speed. There are three different types of parasite drag

-- Form or Pressure Drag -> Caused by the shape of the object exposed. Bodies with a larger frontal section will have more form drag than thinner bodies

-- Skin Friction Drag -> Caused by the viscosity of the air that creates friction when the air molecules are in contact with the aircraft skin

-- Interference Drag -> Caused by the airflow flowing from different directions, mixing between them and creating turbulence

Parasite Drag =  From Drag +  Skin Friction Drag + Interference Drag

 

Induced Drag

It is the inevitable consequence of lift and it is produced by the passage of airfoil through the air. It is inversely proportional to the aircraft's speed

-- Wingtip Vortices -> Wingtip vortices are circular patterns of rotating air left behind a wing as it generates lift. They are created when the high-presure air underneath the wing tries to flow towards the low-pressure air at the top. These vortices generate turbulence that creates drag

-- Downwash --> It is caused by the part of the air deflected downward which is not vertical to the flight pat but slightly rearward from it. As the angle of attack increases, so does drag, at a critical point, theAoA becomes so high that the airflow is broken over the upper surface of the wing and lift is lost while drag increases

Total Drag

As we have seen parasite drag increase with increasing speed and induced drag increase with decreasing speed. Putting it all together in a graph we get this:

As we can see there is a specific airspeed at which total drag is minimum this is called minimum drag speed (Vmd). This speed also corresponds to :

- Max L/D ratio -> This speed will give you the highest lift with the lowest drag produced. As you have the lowest drag to resist forward motion, this speed also represents the maximum range and best gliding speed

- Max. Range Speed -> If the Vmd is 60 kts, any increase or decrease in that speed will lead to an increase in drag, reducing the maximum range

- Best Glide Speed -> If the Vmd is 60 kts, any increase or decrease in that speed will lead to an increase in drag, so to counteract it you will have to lower the nose more, reducing the gliding distance

f0480-02.jpg
Captura.JPG
Drag1_0.jpg
Downwash Representation.PNG
Wingtip Vortices.PNG
Form Drag.PNG

Airfoil Definition And Parts

An airfoil is a body shaped to produce an aerodynamic reaction (lift) for a small penalty (drag)

If we take a wing and we cut in half and we looked at the section we would get something similar to this:

Captura.PNG

-Airfoil Parts-

An aerofoil has some geometrical definitions that we will discuss:

- Chord Line

- Mean Camber Line

- Camber

- Mean Aerodynamic Chord

- Thickness

- 1.- Chord Line -> It is the longest straight line from the leading edge to the trailing edge and divides the airfoil in two: the upper and the lower surface

- 2.- Mean Camber Line -> It is the line drawn by joining the points that lie halfway between the upper and the lower surface of the airfoil

- 3.- Camber -> It is the max distance between the chord line and the mean camber line and is a measure of the curvature of the airfoil. If equal to 0, the airfoil is symmetrical if its value is different from 0, then it is an asymmetrical airfoil or cambered airfoil

- 4.- Thickness -> It is the max. distance between the upper and the lower surface

Airfoil Characteristics.JPG