There are four forces that act on an aircraft in flight: lift, weight, thrust, and drag. Aircraft’s motion in air is dependent on the relative magnitude and direction of these forces. Fig -1 below shows the direction of these forces. Fig 1 (Benson, 2006) The weight of an airplane is always directed towards the center of the earth. The thrust is normally directed forward along the center-line of the aircraft. Lift and drag are aerodynamic forces on the airplane.
Drag acts in a direction opposite to the motion of the aircraft and hence is sometimes referred to as the aerodynamic friction, while lift force acts perpendicular to the motion. An aircraft is in a state of equilibrium when the thrust and drag are equal and opposite. It will continue to move forward at the same uniform speed. If thrust or drag becomes greater than the opposite force, the aircraft loses its state of equilibrium. If thrust is greater than drag, the aircraft will accelerate. If drag is greater than thrust, the aircraft will lose speed and eventually descend.
When lift and weight are equal and opposite, the airplane is in a state of equilibrium. If lift is greater than weight, the aircraft will climb. If weight is greater than lift, the airplane will descend. Drag is the aerodynamic force encountered as an airplane pushes through the air, which tends to slow the airplane down. Drag is generated by the contact of a solid body with a fluid, in this case due to the interaction between the plane body and air. Drag force, which is a mechanical force, is generated by every part of the airplane including the engines.
It is a vector quantity i. e. has both magnitude and direction. Drag must be overcome by thrust in order to achieve forward motion. Drag is generated by nine conditions associated with the motion of air particles over the aircraft. Although prediction of drag and wind tunnel drag measurements of models yield good results, final drag evaluation must be obtained by flight tests. Sources of Drag in aircrafts Drag can be thought of as aerodynamic friction, and one of the sources of drag is the skin friction between the molecules of the air and the solid surface of the aircraft.
Drag can also be thought of as aerodynamic resistance to the motion of the object through the fluid. This source of drag depends on the shape of the aircraft and is called form drag. As air flows around a body, the local velocity and pressure are changed. Since pressure is a measure of the momentum of the gas molecules and a change in momentum produces a force, a varying pressure distribution will produce a force on the body. This causes pressure drag. As an aircraft approaches the speed of sound, shock waves are generated along the surface.
There is a drag penalty, known as wave drag that is associated with the formation of the shock waves. The magnitude of the wave drag depends on the Mach number of the flow. Ram drag is associated with slowing down the free stream air as air is brought inside the aircraft. Jet engines and cooling inlets on the aircraft are sources of ram drag. (Benson, 2006) There is an additional drag component caused by the generation of lift, known as induced drag, is the drag due to lift. It is also called “drag due to lift” because it only occurs on finite, lifting wings.
This drag occurs because the flow near the wing tips is distorted p wise as a result of the pressure difference from the top to the bottom of the wing. Swirling vortices are formed at the wing tips, which produce a downwash of air behind the wing which is very strong near the wing tips and decreases toward the wing root. The local angle of attack of the wing is increased by the induced flow of the down wash, giving this, downstream-facing, component to the aerodynamic force acting over the entire wing. Types of Drag in aircrafts There are several types of drag: form, pressure, skin friction, parasite, induced, wave and ram.
However, form, pressure, skin friction, wave and ram drags are collectively known as parasite drag. Hence, there are only two types of drag: parasite and induced Parasite drag – Profile or parasite drag is caused by the airplane pushing the air out of the way as it moves forward. The parasite drag of a typical airplane consists primarily of the skin friction, roughness, and pressure drag of the major components. Some additional parasite drag is also due to things like fuselage upsweep, control surface gaps, base areas, and other extraneous items.
The basic parasite drag area for airfoil and body shapes can be computed from the following expression: f = k cf Swet, where the skin friction coefficient, cf , which is based on the exposed wetted area includes the effects of roughness, and the form factor, k, accounts for the effects of both super-velocities and pressure drag. Swet is the total wetted area of the body or surface. Computation of the overall parasite drag requires that we compute the drag area of each of the major components (fuselage, wing, nacelles and pylons, and tail surfaces) and then evaluate the additional parasite drag components described above.
Hence it is written as: CDp = S ki cfi Sweti / Sref + CDupsweep + CDgap+ CDnac_base + CDmisc, where the first term includes skin friction, and pressure drag at zero lift of the major components. cfi is the average skin friction coefficient for a rough plate with transition at flight Reynolds number. Equivalent roughness is determined from flight test data. (http://adg. stanford. edu/aa241/drag/parasitedrag. html) Induced drag – Induced drag is the part of the force produced by the wing that is parallel to the relative wind, i. e. the lift.
As it is a consequence of the vortices it is sometimes called vortex drag. Induced drag is least at minimum AOA and is greatest at the maximum AOA i. e. angle of attack. Induced drag = (k ? CL? / A) ? Q ? S where A is the wing aspect ratio. (Preston, R) The magnitude of induced drag depends on the amount of lift being generated by the wing and on the wing geometry Long, thin (chord wise) wings have low induced drag; short wings with a large chord have high induced drag. An airplane must fight its way through both kinds of drag in order to maintain steady flight.
. Total drag is a sum of Parasite and Induced drag. Total Drag = Parasite drag + Induced drag However, the total drag of an aircraft is not simply the sum of the drag of its components. When the components are combined into a complete aircraft, one component can affect the air flowing around and over the airplane, and hence, the drag of one component can affect the drag associated with another component. These effects are called interference effects, and the change in the sum of the component drags is called interference drag. Thus, (Drag)1+2 = (Drag)1 + (Drag)2 + (Drag)interference (Johnston, D)
Generally, interference drag will add to the component drags but in a few cases, for example, adding tip tanks to a wing, total drag will be less than the sum of the two component drags because of the reduction of induced drag. Total drag and its variation with altitude The equation for total drag is: D = CD x S x ? rV2 (Preston, R) where, CD is the coefficient of drag. It must be subdivided into two parts, the Cdi (Coefficient of induced drag) and CDp (Coefficient of parasite drag. ). Therefore it can be written as: D = (Cdi + Cdp) x S x ? rV2 (Preston, R)
The airplane’s total drag determines the amount of thrust required at a given airspeed. Thrust must equal drag in steady flight. Lift and drag vary directly with the density of the air. As air density increases, lift and drag increase and as air density decreases, lift and drag decrease. Thus, both lift and drag will decrease at higher altitudes. Fig 1 shows the total drag curve which represents drag against velocity of the object. The fuel-flow versus velocity graph for an air graph is derived from this graph, and generally looks as shown in Fig 2
Fig – 1 (Preston, R) Fig – 2 (Preston, R) From the above drag it is seen that the total drag is minimum at a certain velocity. This occurs when the parasitic drag is equal to the induced drag. Below this speed induced drag dominates, and above this speed parasite drag dominates. Design engineers are interested in minimizing the total drag. Unfortunately many factors may conflict. For example, longer wing p reduces induced drag, but the larger frontal area usually means a higher coefficient of parasite drag. Conversely, a high wing loading (i. e.
a small wing) with a small aspect ratio produces the lowest possible parasite drag but unfortunately is the produces for a lot of induced drag. In recent time it is seen that jet airliners have longer wings, to reduce induced drag, and then fly at higher altitudes to reduce the parasite drag. This causes no improvement in aerodynamic efficiency, but the higher altitudes do result in more efficient engine operation. (Preston, R) Angle of Attack (AOA), is the angle between the wing and the relative wind. Everything else being costant, an increase in AOA results in an increase in lift.
This increase continues until the stall AOA is reached then the trend reverses itself and an increase in AOA results in decreased lift. The pilot uses the elevators to change the angle of attack until the wings produce the lift necessary for the desired maneuver. Besides AOA other factors also contribute to the production of lift, like relative wind velocity and air density i. e. temperature and altitude. Changing the size or shape of the wing (lowering the flaps) will also change the production of lift. Airspeed is absolutely necessary to produce lift.
If there is no airflow past the wing, no air can be diverted downward. At low airspeed, the wing must fly at a high AOA to divert enough air downward to produce adequate lift. As airspeed increases, the wing can fly at lower AOAs to produce the needed lift. This is why airplanes flying relatively slow must be nose high (like an airliner just before landing or just as it takes off) but at high airspeeds fly with the fuselage fairly level. The key is that the wings don’t have to divert fast moving air down nearly as much as they do to slow moving air.
Air density also contributes to the wing’s ability to produce lift. This is manifested primarily in an increase in altitude, which decreases air density. As the density decreases, the wing must push a greater volume of air downward by flying faster or push it down harder by increasing the angle of attack. This is why aircraft that fly very high must either go very fast e. g. Mach 3, or must have a very large wing for its weight. This is why the large passenger airplanes cruise at higher altitude to reduce drag, and hence save on the furl costs.
(“Aircraft for Amateurs”, 1999) Small sized aircrafts have lower than normal Reynolds number. The drag coefficient attributable to skin friction is hence higher for the small aircraft. For this reason, the maximum lift-drag ratios characteristic of business jet aircraft tend to be lower than those of the large transports. Hence, the smaller flights can fly at relatively lower altitudes. References Books John A. Roberson & Clayton T. Crowe, 1997, Engineering fluid Mechanics, 6th ed. , John Weily & Sons Inc. , ISBN 0-471-14735-4.
Clement Klienstreuer, 1997, Engineering Fluid Dynamics, Cambridge University Press, ISBN 0-521-49670-5 Websites “Aircraft for Amateurs”, 11th Jan. 1999 http://www. fas. org/man/dod-101/sys/ac/intro. htm Benson, T. , “The Beginner’s guide to Aeronautics”. , 14th March 2006 http://www. grc. nasa. gov/WWW/K-12/////airplane/ Johnston, D. , “Drag”, http://www. centennialofflight. gov/essay/Theories_of_Flight/drag/TH4. htm “Parasitic Drag”, http://adg. stanford. edu/aa241/drag/parasitedrag. html Preston, R. , “Total Drag” and “Flight Controls”, http://selair. selkirk. bc. ca/aerodynamics1/