Get a deeper understanding of the four forces of flight
Understand the relationship between the atmosphere, the aircraft and the forces acting on it
Discover new concepts about wings and aerofoils
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:
-- 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 is held near the surface of the planet by the Earth gravitational attraction. It has variable conditions such as temperature, pressure and humidity that change from one day to another.
*--> For that reason, 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
-Aerodynamics Origin and Concept-
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
- V² -> 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). If you climb with a constant IAS, TAS increases because air density decreases 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).
- 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
- V² -> Using power and attitude in a determined way to increase the speed and produce more lift
Unfortunately, both methods have a limit. Not only the aircraft has a speed limit called Vne (Never Exceed Speed) that if exceeded could cause structural damage. But the wings also have a limit on the maximum angle of attack that can be achieved to produce lift
- 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. Here, there are two videos in which you can see this phenomenon in both cars and aeroplanes
At low AoA the airflow moves nice and easy over the top surface of the airflow (laminar airflow). But as the AoA increases it becomes more difficult for the air to follow the curvature of the wing and start becoming turbulent (turbulent airflow)
In the end, the AoA becomes so high that the air cannot flow over the upper surface, the lift created reduces dramatically and the drag increases exponentially due to the turbulent air.
This phenomenon is called aerodynamic stall and for most of the airfoils, the maximum AoA that can be achieved (critical AoA) is 16º
Many people confuse the concept of weight with the concept of mass, 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.
- 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 centre of the Earth. It is measured in Newtons (N)
If you try to remember your first-grade physics lesson, probably one of the formulas that would come to your mind is:
- Force (N) = Mass (kg) x Acceleration (m/s²) -> (F = m x a)
- 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 centre of that planet is different:
If the acceleration that pulls you down changes, your weight changes accordingly. These changes in acceleration also occur in flight during certain manoeuvres, for example in 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.
The 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
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 the aircraft 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, the opposite reaction moves the aircraft 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
The 2nd law explains why the thrust decreases when the air density decreases, as density decreases, so does the air mass, less air being pushed backwards means less thrust produced. The solution would be to increase the acceleration given to that air, which would translate to increasing the propeller RPM
The propeller and jet engine working principles to create thrust are completely different even if they are based on the same two physics concepts. The jet engine is straightforward to understand, you take the air, compress, mix it with the fuel and it leaves at high speeds, moving the aircraft forward
The concept around the propeller is more complex, technically the propeller's blades work in the same way as an airfoil.
What we call thrust is in reality part of the lift created by the blades when they protrude into the air as they rotate around the propeller axis
The propeller produces lift, just like the wings, but the propeller blades are sideways, so the "lift" (part of it) pushes the aeroplane forwards, instead of pushing it upwards.
The aircraft equipped with propellers suffer from certain phenomena that the jet engines aircraft do not. This phenomenon is called the left-turning tendency, as the name implies, producing a turning motion to the left when not corrected properly.
The causes of the left turning tendency on propellers are 4:
- Spiraling Slipstream
- Gyroscopic Precession
Torque is a turning force around an axis. Do you remember the 3rd Law of Newton?
Every action has an equal and opposite reaction. If the propeller is turning to the right (from the pilots perspective) (action) then the aircraft must be turning to the left (opposite reaction)
Imagine a motorcycle standing still and the driver applies a sudden acceleration, the torque of the accelerating wheel produces an opposite reaction on the motorcycle chassis, a wheelie
Higher RPM -> Higher Torque
As the propeller blades rotate around the central axis. While one blade is going downwards, the other blade is travelling upwards.
The problem is that the direction of the relative airflow for the two blades is not the same, meaning that the downgoing blade meets the air at a higher AoA than the upgoing blade, creating an unbalance in the thrust produced which translates into a left-turning tendency.
Higher the AoA of the Aircraft -> Higher AoA difference between the blades -> Stronger P-Factor Effect
The slipstream is the column of air accelerated by the propeller, we have already discussed the effect that it has on the primary flight controls (Refer to Lesson Nº3 - Effects of Controls)
But, why does the slipstream produce a left-turning tendency?
When the air is accelerated, it is also given a spinning motion which causes it to surround the aircraft and impact the rudder on its left side, deviating the tail to the right and the nose to the left.
The air only strikes the tail at low speeds. At higher speeds, the air is deviated and does not touch the rudder
A gyroscope in its most basic definition is a wheel mounted so that it can spin rapidly about an axis. a spinning propeller taking into account this definition is, in fact, a gyroscope
The gyroscopes have two unique properties, rigidity in space and precession. Precession means that if you apply a force to a specific location of the wheel, the resultant force is felt 90 degrees in its direction of rotation
How does it apply to a propeller?
Every time that you apply a force that modifies the direction of the propeller axis (Pitch and Yaw, if you bank the propeller axis in the same position but with the wings tilted), a resultant force will apply 90º in its direction of rotation.
- Pitch Up -> Right Yaw
- Pitch Down -> Left Yaw
- Right Yaw -> Pitch Down
- Left Yaw -> Pitch Up
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
It 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
-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, if you double the speed you will get 4 times more drag. 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. Closer angles create more interference drag than open angles. When an aircraft retracts the landing gear, more drag is momentarily created due to the smaller angles between the gear and the fuselage
Parasite Drag = From Drag + Skin Friction Drag + Interference 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-pressure 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 path 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
Parasite drag increase with increasing speed and induced drag increase with decreasing speed. Putting it all together in a graph we get this:
At the bottom of the graph, there is a specific airspeed at which total drag is minimum. This is called minimum drag speed (Vmd), it 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
-Airfoil Definition and Concepts-
An airfoil is a body shaped to produce an aerodynamic reaction (lift) for a small penalty (drag)
If we take a wing and cut it in half and we looked at the section we would get something similar to the image below. This shape is specially designed to accelerate the airflow passing over the upper surface and create lift
An airfoil has some geometrical definitions that we will discuss. These geometrical characteristics define the airfoil and mark its function. Some are designed to produce a high amount of lift even at low speeds, others are designed to make the aircraft glide as far as possible, others are designed for supersonic flight...etc
- Chord Line
- Mean Camber Line
- Mean Aerodynamic Chord
- 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. The angle between this line and the relative airflow is the AoA
- 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. Technically asymmetrical wings can not create lift at 0º AoA because at that angle the difference in distance that the air has to cover would be equal for the upper and the lower surface, so no pressure difference is created, thus no lift
- 4.- Thickness -> It is the max. distance between the upper and the lower surface. Airfoils with higher thickness create a higher amount of drag, but the advantage is that they produce a high quantity of lift even at low airspeed. You can find them in aircraft with STOL (Short Take-Off and Landing) capabilities