LIFT EQUATION
§ Have you ever wondered what 'lift' is, and how can it be
calculated? If so, then read this article to learn all about it's importance in
the design of aircraft.
§ In this article we'll look at the basics of lift, the factors
that influence it and how effects the take-off of commercial aircraft. It's a
crucial equation for the safe flight of aircraft, and it can be used to
calculate a range of important variables from the force required, through to
the surface area of the wing.
§ We'll go over the individual component parts of the equation
and how they all fit together, to help give you a much better overall
understanding.
It has been observed that most
aircraft accidents occur during the take-off or landing phase of the flight.
Lift is a major part of takeoff. Before getting technical and diving right into
the, let’s understand what lift is in simple terms. This will make it easier to
grasp the concepts involved in the equation itself.
So, what is lift? Basically, lift is
a force that causes an aircraft to fly. It either equals or exceeds
gravitational force to create a tendency to rise into the air. The force that
pushes up under the wings of an aeroplane, given the right circumstances and
conditions, is lift.
What
is the Lift Equation?
The lift equation is a mathematical
representation which can be used to properly measure an aircraft’s lifting
capabilities. The lift generated by an aircraft depends on a number of factors
that are listed below:
§ Velocity between the air and the object
§ Compressibility and viscosity of air
§ The surface area of the wing of the aircraft
§ Shape of the body under consideration
§ And finally, the angle of attack (e. the body’s inclination
relative to the flow)
Who
uses the lift equation?
The lift equation is required to
calculate the lift Coefficient used by aerodynamicists,
to design all of the complex dependencies of inclination, some flow conditions,
and shape of lift.
Definition
of the lift Equation
In reality, the calculation of
precise lift values on an aircraft’s wing is very complex, involving factors
and parameters such as body shape and air viscosity. Therefore, a simplified
model is often used, with the equation:
L=1/2 ρv2 Sref CL
where:
§ L denotes
lift force.
§ V defines
the velocity of aircraft expressed in m/s.
§ ρ is
air density, affected by altitude.
§ Sref is the reference area or the wing area of an aircraft
measured in square metres.
§ CL is the coefficient of lift, depending on the angle of attack
and the type of aerofoil.
In the lift equation, v is also
known as the true airspeed. This is defined as the real, measured speed that
the aircraft attains in flight.
Similarly, ρ is air density, so the
value of this variable depends on the height at which you want to find the lift
and if it changes, altitude is influenced too.
We know that the lift formula is
dependent on its component properties. So, if you change any one of these
variables then the amount of lift will also change. Consider the following
example, if you just change the velocity, keeping everything else constant in
the lift equation, the amount of lift force will change, and the altitude of
the aircraft will start to vary. Therefore, precise take off speeds are very
important when flying and aircraft.
The inner workings of the lift
equation – everything you need to know
Now, let’s break down the lift
equation and examine it’s constituent parts.
Dynamic energy, (1/2 V2)
Air density multiplied by true
airspeed will result in dynamic energy. Dynamic energy is caused by the
aircraft movement which disturbs the surrounding air stream.
Coefficient lift (CL)
The lift coefficient CL is influenced by air viscosity and compressibility. It
is a dimensionless value which is dependent on the air craft being examined.
Any given aircraft wing always lifts
at the same CL max (with a specific angle of attack) for that
configuration.
Angle of Attack, (AOA)
Angle of Attack also referred to as
AOA, is basically the angle at which wind moves
against an aerofoil. It is formed by a chord of the aerofoil and the
direction of the vector defining the motion. In non-technical language,
angle of attack is the difference between the direction a wing is pointing and
where it is actually heading.
Wing area, (Sref)
Wing Area, denoted by Sref, is the surface area. Changing it will result in a change in
the amount of lift experienced by the air craft.
Conclusion
To sum it all up, the lift equation
has made things very easy for aerodynamicists and engineers. If you use the modern
lift equation, and consider the lift coefficient given above, you can easily
calculate the amount of lift produced at a given velocity for a given wing
area.
In a different scenario, where wing
area is not mentioned, and where we have been given velocity, we can also
determine how large the wings need to be to be able to lift a certain weight,
using the lift equation.
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