Aerodynamic forces- lift

Have you ever wondered how an aeroplane can fly? Well, the answer is right here. Even if you didn't ask :) There are two forces when we refer to a special branch of physics, aerodynamics. These two forces act in both an upwards direction and a downwards direction whilst they both operate from right angles. This idea can seem complex, this is why these fundamental "two" forces are split and referenced to as 4 forces. Simply for the purposes of simplification. These 4 forces are known as lift, weight, drag and thrust. I will write about the other 3 later, however for all intensive purposes this post will focus primarily on lift.

The idea of lift can seem highly mysterious. However, not to threat. Whilst the idea is indeed highly complicated, we can simplify it for the purposes of understanding. To begin to understand lift, we MUST consider Bernoulli's law. This was a law determined by Swiss physicist, Daniel Bernoulli in the 1700s. The law simply means that the further the distance air particles have to travel the faster they will travel and hence the lighter they will be (meaning the pressure is less). The shape of an aerofoil (any shape that can be used to generate lift) applies this law well.

Here is an aerofoil:

 



As you can observe is the above diagram, the top of the aerofoil is slightly curved. This, in turn creates more room for the air particles to travel. As we already know (stated above) The more distance a given set of air particles must cover in a given set of time, the lighter they will be. In flight, the reference to this principle is very true.

So we have already covered one principle to how lift is generated. Now you must note that when lift is generated there are two pressures one is still or dense pressure and the other one is "moving" pressure where the air particles are significantly lighter than in still pressure. The former pressure is known as static pressure whilst the latter is known as dynamic pressure. It is very realistic that we can have static pressure below the aerofoil and dynamic pressure above the aerofoil. These differences in pressure create a difference in velocity, and in turn we have a net force acting at a 90 degree angle (lift and drag). The drag will be explained in another post, whilst the majority of the attention of this force is lift.

Here is lift in act:

 

Although the above diagram is an example of approach so the AOA is lower than other times in flight. It is still a relevant image that displays the effect of lift. I will explain AOA on another post :-)

When it comes to the concept of calculations for flight, lift must be calculated. The equation for this is: L=Cl1/2(v squared)pS
In case you didn't understand what the letters mean:

L= Lift
Cl= lift coefficient
1/2= constant value for this equation
V (squared)= velocity squared, I don't have a keyboard to write squared numbers sorry :(
p= density of surrounding air
S= surface of the wing area.

L is the total lift generated through correctly operating the equation. The lift coefficient varies dependant on flight whilst the 1/2 is a constant values that is ALWAYS applied to the lift equation. V squared is the velocity at which the aircraft is traveling, this, of course, changes numerous times in the duration of a flight. p refers to the air density, this is dependant in meteorological conditions, however alters depending on altitude in addition (e.g. 30,000ft would have lighter air density than 10,000ft). And the final part of the equation is S. This basically is the surface area of the wing. This varies dependant on aircraft (E.g A380 wing is bigger than A330 wing) and it also can vary depending on the stage of flight. For example, On take off as more lift needs to be generated the flaps are extended in order to produce more lift. This differs when you reach a higher altitude however as the air density is lighter and you are already airborne so having the flaps extended creates excess drag. This is when the flaps can be detracted.

Thank you for reading! Also, note none of the images are mine. References can be provided.