1) What is Aerodynamics
2) What is Lift
2.1) Bernoulli’s Theory of Lift (incorrect theory)
2.2) Real Theories on Lift (correct theory)
3) What is Drag
4) What is Thrust
5) What are high Angles of Attack (AOA)
6) What is a Spin & Stall
Section 1 :: What is Aerodynamics?
Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is often used synonymously with fluid and gas dynamics, with the difference being fluid dynamics is applied to liquids, and gas dynamics to gases. Understanding the motion of air (aka “flow field“) around an object enables the calculation of forces and moments acting on the object. Typical properties calculated for a flow field include: velocity, pressure, density, and temperature. By defining a control volute around the flow field, equations for the conservation of mass, momentum, and energy can be defined and used to solve for the properties.
Section 2 :: What is Lift?
Lift is a force, which is perpendicular to the flow field direction. It is opposite of the drag force (explained later), which isparallel to the flow field. Lift is generated in accordance with the fundamental principles of physics, such as: Newton’s Laws of Motion, Bernoulli’s Principle, Conservation of Mass, and Balance of Momentum (similar to Newton’s 2nd law).
* Bernoulli’s Theory of Lift:
This theory ignores friction over surfaces, which is easier to understand, but is not completely accurate.
In this diagram, “streamline” is the air which goes over the wing (aka airfoil from a side-view). As air flows over the top surface of the airfoil, which has a longer distance than the lower surface, it has a lower pressure, thus a faster velocity or speed. Thus, airflow on the lower surface has a higher pressure, and slower velocity.
Because airflow over the top surface is faster, it produces greater force than the bottom, thus creating our “Lift” force perpendicular to the airflow.
(NOTE: Circles = time intervals of airflow over the airfoil. Notice how the times are all the same.)
* Real Theories on Lift:
The above theory in a way is correct, but it does not explain correctly how airflow over the wing produces lift. No single theory can explain how a wing produces lift, it is a combination of the four main theories mentioned above, and many other more complex theories and formulas.
(No formulas will be given here either, so if you wish to know those, please GOOGLE or read up on aviation books on lift and aerodynamics.)
If you look at the above image, you will noticed various colored lines. These lines show where on the airfoil air is flowing over it, and at what times (ms). Unlike Bernoulli’s simplified version, you will notice air does not meet at the end of the airfoil at the same time.
Above is a similar images (with various Angles of Attack), but it also shows the various pressure bubbles formed over the airfoil. When the Angle of Attack is at 0-degrees, you’ll notice the pressure bubbles are roughly equal; with the top blue bubble being larger and further back. When the Angle of Attack reaches 10-degrees, the top blue pressure is larger and further back than the smaller red bubble.
It is actually the size and location of these pressure bubbles which causes lift on the wing, rather than the top airfoil having a longer travel distance than the bottom airfoil. The Angle of Attack and shape of the airfoil does affect airflow speed over the airfoil, which leads to various shapes and designs.
If the Angle of Attack is too high, and the airfoil is not designed properly, the pressure bubble on the top wing will not form correctly, thus produce almost zero lift force.
Section 3 :: What is Drag?
There are various types of drag, and the most common type of drag everyone talks about is Lift Induced Drag. This type of drag is caused by the lift force and angle of the wing. The greater the wing angle is with respect to the airflow and airplane direction, the greater the induced drag. This type of drag is greatest at low airspeeds. Since there is little force or thrustfrom the engines to push the aircraft forward (which provides greater airflow), higher Angles of Attack of the wing are required to produce sufficient lift. This higher Angle of Attack is what causes greater induced drag.
A second type of drag force is Parasitic Drag (aka Profile Drag). This form of drag is caused by a combination of three other drag forces: Form drag, Skin friction, and Interference drag.
Skin friction is basically friction between the air and wing or airplane surfaces (smoother surface = less skin friction).
Form drag is drag caused by the shape of the wing airfoil and/or airplane design. Smoother and more flowing designs result in lower form drag.
Interference drag is caused by odd shapes on the body of the aircraft, such as engines on a commercial airliner.
The last type of drag is called Wave Drag. This type of drag only takes affect when an airplanes is within the transonicairspeed range. This is a specific range of speeds (Mach 0.8 to 1.2) which is close to the speed of sound. This form of drag is caused by minor shock-waves formed by breaking the sound-barrier on various parts of the plane due to its shape. The greatest resistance is at exactly Mach 1.0, and is least at Mach 0.8 and 1.2.
Speeds under Mach 0.8 are considered Subsonic, speeds over Mach 1.2 are Supersonic, and speeds over Mach 5.0 are considered Hypersonic.
Section 4 :: What is Thrust?
Thrust is a force, which is described quantitatively by Newton’s 2nd and 3rd Laws of Physics. When an engine accelerates an airplane in one direction, the acceleration will cause a proportional but opposite force on the body.
If there is insufficient thrust from the engines to force air over the wings and other bodies of the plane to cause lift, the plane will not fly.
In the case of a flat winged plane (like foamy planes), if there is sufficient thrust, a plane will fly. This also applies to paper airplanes.
Section 5 :: What are High Angles of Attack (AOA)?
Angle of Attack basically means the angle of the wing or airfoil with respect to the direction of the airflow and airplane. If the wing is not designed properly, very low or high Angles of Attack could cause bad airflow over the wings (not forming good pressure bubbles), thus insufficient lift on the wings. When this happens, a stall occurs on the wing and its control surfaces.
Section 6 :: What is a Spin & Stall?
Stalling occurs when insufficient airflow or pressure over the wings and control surfaces causes the airplane to lose control. This can be caused by insufficient thrust from the engines, poorly designed wing airfoils, bad control surface locations, or simply the aircraft reaching its maximum Angle of Attack. This last reason is very important for aircraft pilots to know, so they can avoid such stalling conditions.
A common and deadly type of stall is called a “Flat Spin”. When this occurs, the plane has almost no airflow over the wing or tail control surfaces. Since there is no air on the surfaces to cause lift to turn the plane’s direction, it keeps on spinning and falls towards the Earth. Sometimes, because the plane is falling and spinning so fast, the stress on the hull actually rips the plane apart before it hits the ground.
(Diagram of where wing stalls START from…)
— If you find any errors, please let me know, and I will fix them.