Basic Aerodynamics& Theory of Flight

ByAhmad Ahsan

Winglets• Winglets are vertical extensions of wingtips that improve an

aircraft's fuel efficiency and cruising range. • Designed as small airfoils, winglets reduce the aerodynamic drag

associated with vortices. By reducing wingtip drag:a)fuel consumption goes down b)range is extended

• For transport-size aircraft with winglets efficiency during cruise is improved to between 6% and 9%.

• Research by NASA in 1979-80 on Boeing 707 jetliner recorded an increased fuel mileage rate of 6.5%.Source:NASA Dryden Flight Research Center

Winglets• Winglets increase an aircraft's operating

efficiency by reducing induced drag at the tips of the wings.

• Winglets produce a forward thrust inside the circulation field of the vortices and reduce their strength.

• Weaker vortices mean less drag at the wingtips and lift is restored.

• Improved wing efficiency translates to more payload, reduced fuel consumption, and a longer cruising range that can allow an air carrier to expand routes and destinations.

• To produce as much forward thrust as possible, the winglet's airfoil is designed with the same attention as the airfoil of the wings themselves. 

Forward thrust vector

Winglets• Reduce drag and improve aircraft performance• Can be installed on various aircraft easily• Can be installed after production. Do not need extensive technical

support• Small change to aspect ratio and aircraft weight, means negligible

effect on aircraft handling

Raked Wingtips• Raked wingtips can be

described as horizontal wingtip extensions which have a higher degree of sweep as compared to the rest of the wing.

• They work on the same principle as winglets.

• In testing by Boeing and NASA, raked wingtips have been shown to reduce drag by as much as 5.5%, as opposed to improvements of 3.5% to 4.5% from conventional winglets.

• Airliners to use raked wingtips: Boeing 767-400ER,

• Boeing 777(-200LR,300ER, Freighter)

777-300ER 6.5-foot (1.98-meter) raked wing tips are highly tapered wing extensions used to improve an airplane's performance. The raked wing tips reduce takeoff field length and increase fuel efficiency and climb performance. Each wing tip weighs 105 pounds (48 kilograms) and are installed using 12 bolts.

Trailing Vortices & Downwash• Trailing vortices are by-product of wing lift, their influence changes the air

flow pattern. • They alter the flow direction and speed in the vicinity of the wing and tail

surfaces. • The air behind the wing is drawn downwards, and this is called downwash.• Downwash also influences the approaching air, the flow over the wing, and

causes the air to be deflected downwards as it flows past the wing.• Due to downwash, the angle of attack relative to the local airstream is

reduced. This means that less lift will be generated at certain angles. • It also produces “trailing vortex drag”• Trailing vortices also produce a large upwash outboard of the wing tips. The

upward momentum change thus produced cancels out the downward momentum change of the downwash.

Trailing Vortices & Downwash• The trailing vortices are not just a mildly interesting by-product of wing lift.• Their influence on the flow extends well beyond their central core,

modifying the whole flow pattern. • In particular, they alter the flow direction and speed in the vicinity of the

wing and tail surfaces. • The trailing vortices thus have a strong influence on the lift, drag and

handling properties of the aircraft.• Downwash, is apparent not only behind the wing, but also influences the

approaching air, and the flow over the wing itself. It causes the air to be deflected downwards as it flows past the wing.

• The angle of attack relative to the modified local airstream direction, is reduced. This reduction in effective angle of attack means that less lift will be generated at a given AoA.

Downwash

Increase AoA,increase drag

Trailing Vortices & Downwash• A strong starting vortex is formed and left behind just above the runwaywhen the aircraft rotates at take-off. More starting vorticity is produced andleft behind whenever the aircraft produces an increase in wing circulation. • An additional starting vortex is thus formed, when an aircraft starts to pull

out of a dive.• The counterpart of starting vorticity is stopping vorticity, which rotatesin the opposite sense, and is shed every time the circulation is reduced, as onlanding.• In level flight, the amount of circulation required reduces as the speed

increases, so stopping vorticity is shed when an aircraft accelerates in level flight.

• Strong starting and stopping vortices can be generated during violentmanoeuvres, and may significantly affect the handling.

Downwash

Trailing Vortices & Downwash• Trailing vortices are by-product of wing lift, their influence changes the air

flow pattern. • They alter the flow direction and speed in the vicinity of the wing and tail

surfaces. • The air behind the wing is drawn downwards, and this is called downwash.• Downwash also influences the approaching air, the flow over the wing, and

causes the air to be deflected downwards as it flows past the wing.• Due to downwash, the angle of attack relative to the local airstream is

reduced. This means that less lift will be generated at certain angles. • It also produces “trailing vortex drag”• Trailing vortices also produce a large upwash outboard of the wing tips. The

upward momentum change thus produced cancels out the downward momentum change of the downwash.

Trailing Vortices & Downwash• Vortices in the wake of heavy aircraft pose a serious threat to following

aircraft.• The danger is particularly severe during landings and take-offs for two

reasons.• First, the extension of the flaps of the leading aircraft may create trailing

vortices that are even stronger than the wing tip vortices. • Second, the proximity of the following aircraft to the ground means that

even the smallest disturbance to it can be disastrous. • Thus sufficient separation between planes must be maintained to allow

time for the dispersal of trailing vortices.• Minimum 2-3 minutes of waiting behind a taking off aircraft• Wingtip depend on weight, AoA, speed)and configuration. • They are greatest at high weight, high AoA, slow speed.

Trailing Vortices & Downwash

Drag

Drag• “The net aerodynamic force parallel to the relative wind, usually the sum

of two components: induced drag and parasite drag.”• “Drag is the force that resists movement of an aircraft through the air.”• Drag is the aerodynamic force that opposes an aircraft's motion through

the air. Drag is generated by every part of the airplane (even the engines)• Drag is a mechanical force. It is generated by the interaction and contact of

a solid body with a fluid (liquid or gas).• Drag is a force and is therefore a vector quantity having both a magnitude

and a direction. Drag acts in a direction that is opposite to the motion of the aircraft. Lift acts perpendicular to the motion. There are many factors that affect the magnitude of the drag.

Factors that Affect Drag• Geometry has a large effect on the amount of drag generated by an object.

Drag depends linearly on the size of the object moving through the air. • The cross-sectional shape of an object determines the form drag created by

the pressure variation around the object. • The three dimensional planform shape affects the induced drag of a lifting

wing. • The amount of drag also depends on the surface roughness of the object; a

smooth, waxed surface produces less drag than a roughened surface. This effect is called skin friction and is usually included in the measured drag coefficient of the object.

Factors that Affect Drag• Drag is associated with the movement of the aircraft through the air, and

varies with the square of the velocity of the air. • If the object moves through the air at speeds near the speed of sound,

shock waves are formed on the object which create an additional drag component called wave drag.

• The motion of the object through the air also causes boundary layers to form on the object. A boundary layer is a region of very low speed flow near the surface which contributes to the skin friction.

• Drag depends directly on the mass of the airflow going past the aircraft. The drag also depends on viscosity and compressibility of air.

• We can gather all of this information through the Drag Equation.

Drag• There are two basic types: parasite drag and induced drag. • The first is called parasite because it in no way functions to aid flight, while

the second, induced drag, is a result of an airfoil developing lift.• Parasite drag is comprised of all the forces that work to slow an aircraft’s

movement. As the term parasite implies, it is the drag that is not associated with the production of lift. This includes the displacement of the air by the aircraft, turbulence generated in the airstream, or a hindrance of air moving over the surface of the aircraft and airfoil. There are three types of parasite drag: form drag, interference drag, and skin friction.

Drag• The second basic type of drag is induced drag. It is an established physical

fact that no system that does work in the mechanical sense can be 100 percent efficient. This means that whatever the nature of the system, the required work is obtained at the expense of certain additional work that is dissipated or lost in the system. The more efficient the system, the smaller this loss.

• In level flight the aerodynamic properties of a wing or rotor produce a required lift, but this can be obtained only at the expense of a certain penalty. The name given to this penalty is induced drag. Induced drag is inherent whenever an airfoil is producing lift and, in fact, this type of drag is inseparable from the production of lift. Consequently, it is always present if lift is produced.

Lowest drag, max gliding dist. Decreasewith weight

Boundary Layer• Prandtl found that the effects of viscosity were only important and

apparent in a very thin layer adjacent to the surface. • He called this the boundary layer. For an aircraft wing in cruising flight, it is,

at most, only a few centimetres thick.• For the purposes of calculations, however, it is necessary to define this

area.

Assignment• 1 page analysis of research paper.• List of dangers of wingtip vortices (at least 3)• What is boundary layer?• Summary of Glen Research Center NASA article on Boundary Layer.• Describe types of drag.• Email: ahmed@vpia.org• Time: 23/1/2013

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