Drag Flashcards
(16 cards)
Drag acts parallel to and in the ____________ as the relative airflow (in the ____________ to the flight path)
Drag acts parallel to and in the same direction as the relative airflow (in the opposite direction to the flight path)
PARASITE DRAG - __________
INDUCED DRAG - ______________.
Parasite drag is further sub-divided into:
__________ Drag
____________________ Drag, and
_____________________________ Drag
NOTE: Skin Friction and Form Drag are together known as _______________–.
PARASITE DRAG - independent of lift generation, and
INDUCED DRAG - the result of lift generation.
Parasite drag is further sub-divided into:
Skin Friction Drag
Form (Pressure) Drag, and
Interference Drag
NOTE: Skin Friction and Form Drag are together known as PROFILE DRAG.
The layer of air extending from the surface to the point where __________ is known as the boundary layer. In flight, the nature of the boundary layer will determine the _________, the ______, the value of ______, and to some extent the high speed characteristics of an aircraft.
The layer of air extending from the surface to the point where no viscous effect is detectable is known as the boundary layer. In flight, the nature of the boundary layer will determine the maximum lift coefficient, the stalling characteristics, the value of form drag, and to some extent the high speed characteristics of an aircraft.
The increased rate of change in velocity at the surface in the turbulent flow will give _____ skin friction than the laminar flow. A turbulent boundary layer also has a ________ of kinetic energy than a
laminar layer.
The increased rate of change in velocity at the surface in the turbulent flow will give more skin friction than the laminar flow. A turbulent boundary layer also has a higher level of kinetic energy than a
laminar layer.
Forward movement of the transition point will increase skin friction because there will be a
greater area of turbulent flow. The position of the transition point is dependent upon:
—Surface condition - The thin laminar layer is extremely sensitive to surface irregularities. Any roughness on the skin of a leading portion of an aircraft will cause transition to turbulence at that point and the thickening, turbulent boundary layer will spread out fanwise down-stream causing a marked increase in skin friction drag.
—Adverse pressure gradient (Figure 6.3) - A laminar layer cannot exist when pressure is rising in the direction of flow. On a curved surface, such as an aerofoil, the transition point is usually at, or near to the point of maximum thickness. Because of the adverse pressure gradient existing on a curved surface the transition point will be further forward than if the surface was flat.
FORM (PRESSURE) DRAG: Results from ____________ ___________.Because of separation there will be a ____ pressure at the trailing edge than the leading edge. An aerodynamic force will act in the direction of the lower pressure - form drag. Separation will occur when the boundary layer _________________________
FORM (PRESSURE) DRAG: Results from the pressure at the leading edge of a body being greater than the pressure at the trailing edge.Because of separation there will be a lower pressure at the trailing edge than the leading edge. An aerodynamic force will act in the direction of the lower pressure - form drag.Separation will occur when the boundary layer does not have sufficient kinetic energy in the
presence of a given adverse pressure gradient.
Loss of kinetic energy in the boundary layer can be caused by various factors
–As angle of attack increases, the transition point moves closer to the leading edge and the adverse pressure gradient becomes stronger. This causes the separation point to move forward. Eventually, boundary layer separation will occur so close to the leading edge that there will be insufficient wing area to provide the required lift force, C L will decrease and stall occurs.
–When a shock wave forms on the upper surface, the increase of static pressure through the shock wave will create an extreme adverse pressure gradient. If the shock wave is sufficiently strong, separation will occur immediately behind the shock wave.
Laminar and Turbulent Separation: Separation has been shown to be caused by the airflow meeting an adverse pressure gradient, but it is found that a turbulent boundary layer is _______ to separation than a laminar one when meeting the same pressure gradient. In this respect the turbulent boundary layer is preferable to the laminar one, but from the point of view of _____ the laminar flow is preferable.
Laminar and Turbulent Separation: Separation has been shown to be caused by the airflow meeting an adverse pressure gradient, but it is found that a turbulent boundary layer is more resistant to separation than a laminar one when meeting the same pressure gradient. In this respect the turbulent boundary layer is preferable to the laminar one, but from the point of view of drag the laminar flow is preferable.
Streamlining: Each part of an aircraft will be subject to form (pressure) drag. To reduce form drag it is necessary to ___________________. Streamlining increases the ratio between the _________________, reducing the curvature of the surfaces and thus the _______________. Fineness ratio is the measure of streamlining. It has been found that the ideal fineness ratio is ________.
Streamlining: Each part of an aircraft will be subject to form (pressure) drag. To reduce form drag it is necessary to delay separation to a point as close to the trailing edge as possible. Streamlining increases the ratio between the length and depth of a body, reducing the curvature of the surfaces and thus the adverse pressure gradient. Fineness ratio is the measure of streamlining. It has been found that the ideal fineness ratio is 3:1.
Profile Drag: __________________________. It can be considered that these drags result from the ______________ of the aircraft presented to the relative airflow.
Profile Drag: The combination of skin friction and form drag is known as profile drag. It can be considered that these drags result from the “profile” (or cross-sectional area) of the aircraft presented to the relative airflow.
INTERFERENCE DRAG: When considering a complete aircraft, parasite drag ___________. Additional drag results from ____________. ________________ is necessary to minimise interference drag.
INTERFERENCE DRAG: When considering a complete aircraft, parasite drag will be greater than the sum of the parts. Additional drag results from boundary layer ‘interference’ at wing/ fuselage, wing/engine nacelle and other such junctions. Filleting is necessary to minimise interference drag.
FACTORS AFFECTING PARASITE DRAG
If IAS is doubled the Parasite Drag will be _________ - if IAS is halved the Parasite Drag will be _______ of its previous value.
FACTORS AFFECTING PARASITE DRAG
– Indicated Air Speed
Parasite Drag varies directly with the square of the Indicated Air Speed (IAS).
– Configuration
Parasite Drag varies directly in proportion to the frontal area presented to the airflow; this is known as ‘Parasite Area’. If flaps are deployed, the undercarriage lowered, speed brakes selected or roll control spoilers operated, ‘Parasite Area’ is increased and Parasite drag will increase.
–Airframe Contamination
Contamination by ice, frost, snow, mud or slush will increase the Parasite Drag Coefficient, and
in the case of severe airframe icing, the area.
The lower the IAS, the _____ the angle of attack - the _______ the vortices.
The stronger the vortices - the greater the induced drag.
The lower the IAS, the higher the angle of attack - the stronger the vortices.
The stronger the vortices - the greater the induced drag.
An aircraft is viewed from the rear. An __________ vortex will be induced at the right wing-tip and a ___________ vortex at the left wing-tip. At higher angles of attack (Lower IAS) the _____ chordwise vector will increase the resultant spanwise flow, making the vortices stronger.
An aircraft is viewed from the rear. An anti-clockwise vortex will be induced at the right wing-tip and a clock-wise vortex at the left wing-tip, Figure 6.6. At higher angles of attack (Lower IAS) the decreased chordwise vector will increase the resultant spanwise flow, making the vortices stronger.
Induced Downwash: Wingtip vortices create certain vertical velocity components in the airflow in the vicinity of the wing, both in front of and behind it. These vertical velocities ______ upwash and downwash which ________ the effective angle of attack. The stronger the vortices, the ______ ______________.
Induced Downwash: Wingtip vortices create certain vertical velocity components in the airflow in the vicinity of the wing, both in front of and behind it. These vertical velocities strengthen upwash and downwash which reduces the effective angle of attack. The stronger the vortices, the greater the reduction in effective angle of attack.
To replace the lift lost by the increased upwash and downwash the wing must be flown at a
higher angle of attack, than would otherwise be necessary. This increases drag. This extra drag
is called ______________
To replace the lift lost by the increased upwash and downwash the wing must be flown at a
higher angle of attack, than would otherwise be necessary. This increases drag. This extra drag
is called Induced drag
