Lift to Drag Ratio | Types of Drag in Aircraft

Lift/Drag Ratio

Before discussing about Lift to Drag ratio we must know about the lift and drag produced on an airfoil.

Lift

Lift is a force that is created by the air's dynamic pressure on the airfoil. It acts perpendicular to the flight path through the center of lift (C_L) and perpendicular to the lateral axis. Lift battles weight's downward force during level flight. Aerodynamics of Flight.

The lift may be managed by the pilot. The AOA changes every time the control yoke or stick is adjusted foreward or rearward. Lift rises as AOA rises (all other factors being equal). Lift starts to rapidly decrease when the aircraft hits its maximum AOA.

Drag

Drag is the force that prevents an aircraft from flying through the air. Drag produced on an aircraft can be divided into two categories: parasitic drag and induced drag. The first is referred to as a parasite because it does nothing to assist with flight, whereas the second, known as induced drag, arises from an airfoil gaining lift.

Lift/Drag Ratio

The lift-to-drag ratio (L/D) evaluates how much lift an airfoil or wing produces in relation to its drag. L/D ratios show the effectiveness of an airfoil. Higher L/D ratio aircraft are more effective than lower L/D ratio aircraft. For a given AOA, the ratios of the coefficients of lift (C_L) and coefficient of drag (C_D) can be determined in unaccelerated flight with the lift and drag data stable.

The lift produced by a lifting body, the dynamic pressure of the fluid flow around the body, and a reference region connected to the body are all related by the dimensionless coefficient of lift. The coefficient of drag, which measures how much drag an object experiences in a fluid environment like the air, is also dimensionless and is always connected to a certain surface area.

By dividing the C_L by the C_D, which is equivalent to dividing the lift equation by the drag equation because all variables save the coefficients cancel out, the L/D ratio may be calculated. L = Lift in pounds; D = Drag; C_L = Coefficient of Lift; = Density (expressed in slugs per cubic feet); V = Velocity (in feet per second); q = Dynamic Pressure per Square Foot (q = 1/2 𝛒V²); S = The Area of the Lifting Body (in Square Feet); and C_D=Ratio of Drag Pressure to Dynamic Pressure are the lift and drag equations.

D = (C_D.𝛒.V².S)/2

The coefficient of drag is often low at low AOA, and tiny variations in AOA typically result in relatively slight changes in the coefficient of drag. Small AOA variations result in large changes in drag when the AOA is high. The generation of lift is influenced by variations in the AOA as well as an airfoil's form.

Observe how the coefficient of lift curve (red) for this specific wing section increases to its maximum at 20° AOA before rapidly decreasing. The crucial angle of attack is thus 20° AOA. At 14° AOA, the coefficient of drag curve (orange) gradually climbs until it totally supplants the lift curve. At 6° AOA, the lift/drag ratio (green) achieves its greatest, suggesting that the most lift is obtained for the least amount of drag at this angle.

Note that one particular CL and AOA is where the maximum lift/drag ratio (L/DMAX) occurs. The total drag is at its lowest when the Aircraft is flown steadily at L/DMAX. Any AOA below or above the L/DMAX value decreases the L/D, which raises the overall drag for a given aircraft's lift. The L/DMAX is shown in Figure as the lowest point of the blue line with the caption "total drag." The L/D greatly depends on an aircraft's configuration.

Types of Drag

The following types of drag are produced when an airfoil moves through the air:

Parasite Drag

All the forces that work to slow down an aircraft's movement together constitute parasitic drag. It is the drag that is not connected to the creation of lift, as the term parasite indicates. This includes air movement caused by the aircraft, turbulence created in the airstream, or an obstruction to airflow over the aircraft and airfoil surfaces. The three types of parasite drag are skin friction, interference drag, and form drag.

Form Drag

The element of parasite drag known as "form drag" is produced by the aircraft as a result of its shape and the airflow around it. Examples include antennas, engine cowlings, and other parts' aerodynamic shapes. The air finally rejoins after passing the body when it must separate to manoeuvre around a moving aircraft and its parts. The ease and speed with which it rejoins is indicative of the resistance it erects, which calls for more energy to overcome. When constructing an aircraft, form drag is the most straightforward to eliminate. The answer is to simplify as many of the components as you can.

Interference Drag

Eddy currents, turbulence, and obstructions to smooth airflow are all caused by the intersection of airstreams, which causes interference drag. One location where strong interference drag occurs is where the wing and fuselage meet at the wing root. When air travelling around the fuselage and air over the wing collide, they combine to form a new air current that is distinct from the two original currents. Two surfaces coming together at perpendicular angles produce the maximum interference drag. To lessen this propensity, fairings are utilized. Because both of these create and generate interference drag, if a jet fighter is equipped with two identical wing tanks, the overall drag will be more than the sum of the individual tanks. Interference drag is reduced by fairings and the separation of lifting surfaces from exterior parts (like radar antennae suspended from wings).

Drag Versus Speed

Skin Friction Drag

Skin friction drag is the aerodynamic resistance caused when moving air makes contact with an aircraft's surface. No matter how seemingly smooth a surface appears to be, when examined under a microscope, it always has a rough, jagged surface. The air molecules that come into direct touch with the wing's surface are essentially still. Up until the molecules are flying at the speed of the air moving around the aircraft, each layer of molecules above the surface moves a little faster. The free-stream velocity is the name given to this speed. The boundary layer is the region between the wing and the free-stream velocity level, and it is about the size of a playing card. The molecules accelerate and reach the same speed as the molecules outside the boundary layer at the boundary layer's top. The shape of the wing, the viscosity (stickiness) of the air it is travelling in, and its compressibility all affect how quickly the molecules actually move (how much it can be compacted).

The boundary layer's border has the same physical effect on the airflow outside of it as an object's physical surface would. Any item has a "effective" shape that is typically slightly different from its physical shape thanks to the boundary layer. Additionally, the boundary layer may split off from the body, giving rise to an effective shape that differs greatly from the actual shape of the object. Lift and drag are dramatically reduced as a result of the boundary layer's altered physical form. The airfoil has stalled when this happens.

Aircraft designers use flush mount rivets and eliminate any flaws that may protrude beyond the wing surface to lessen the effect of skin friction drag. Additionally, a glossy, smooth coating facilitates the passage of air over the wing's surface. Keep the surfaces of an aeroplane clean and waxed since dirt on them hinders airflow and causes drag.

Induced Drag

Induced drag is the second fundamental kind of drag. It is a well-known physical reality that no system that operates mechanically can function at 100% efficiency. This implies that the essential work is achieved at the expense of some additional work that is wasted or dissipated in the system, regardless of the nature of the system. This loss decreases with system efficiency.

The aerodynamics of a wing or rotor create the necessary lift for level flight, but this can only be attained by incurring a specific cost. Induced drag is the name given to this punishment. Every time an airfoil generates lift, there is induced drag; in fact, the generation of lift and this form of drag are inextricably linked. As a result, if lift is produced, it is constantly there.

The energy of the free airstream is used by an airfoil (wing or rotor blade) to generate lift force. According to Bernoulli's Principle, whenever an airfoil is creating lift, its lower surface is under more pressure than its top surface. As a result, the air has a tendency to go from the region of high pressure beneath the tip up to the region of low pressure on the upper surface. There is a propensity for these pressures to equalize around the tips, which causes a lateral flow from the underside to the upper surface. The air at the tips is given a rotating velocity by this lateral flow, which produces vortices that follow the airfoil.

These vortices rotate clockwise around the right tip and counterclockwise around the left tip when the aircraft is viewed from the tail. Downwash is the term for the downward angle that the air (and vortices) take as they roll down the back of your wing. It is obvious that these vortices cause an upward air flow beyond the tip and a downwash flow behind the trailing edge of the wing when considering the rotational orientation of these vortices. The downwash required to create lift is completely different from the downwash being created here. In actuality, it causes induced drag.

The relative wind is oriented downward by downwash, so the more downwash you have, the more the relative wind is oriented downward. Lift is always perpendicular to the relative wind, which is a very strong reason why this is significant. You could say that your lift vector is more vertical and resists gravity when there is less downwash. Additionally, increased downwash causes your lift vector to point backward, creating induced drag. Additionally, the energy required for your wings to generate downwash and vortices results in drag.

The downwash component on the net airflow over the airfoil, which is a function of the vortices' size and strength, determines how much drag is created. The lift vector is bent slightly backward as a result of the downwash over the top of the airfoil at the tip; as a result, there is a rearward lift component and the lift is slightly aft of perpendicular to the relative wind. It's called induced drag.

An airfoil can be inclined to a higher AOA in order to increase the negative pressure on its top. There would be no pressure differential, which would also mean that there would be no downwash component and no induced drag if the AOA of a symmetrical airfoil were zero. In any scenario, induced drag grows proportionally with AOA. To put it another way, the greater the AOA needed to provide lift equivalent to the aircraft's weight and, thus, the greater the produced drag, the lower the airspeed. The airspeed squared has an inverse relationship with the quantity of induced drag.

On the other hand, parasite drag grows as the square of airspeed. As a result, in steady state, as airspeed falls to a level close to stalling speed, the total drag increases, primarily because of the abrupt increase in induced drag. Similar to how the parasitic drag grows sharply as the aircraft achieves its never-exceed speed (VNE), the overall drag increases quickly. When calculating an aircraft's maximum range, the minimal amount of thrust is needed to overcome drag. At a different moment, the minimum power and maximum endurance are present.