MODULE 6: DRAG MEASUREMENTS Flashcards
4 forces of flight:
- Lift
- Weight
- Drag
- Thrust
As an aircraft moves/travels through air, every part of it that is exposed to the airflow produces a different type of resistance to the forward motion which constitutes the Total Drag.
Drag
Forms of drag:
- Parasite drag
- Induced drag
This is comprised of all the forces that work to slow an aircraft’s movement which includes resistance of air moving over the aircraft’s surface.
Parasite drag
This is an inevitable consequence of lift and is produced by the passage of an airfoil through the air.
Induced drag
Factors that affect the type of flow in the boundary layer:
- Shape
- Texture
- Flow quality
- Pressure gradient
- Reynolds number
- Heating of the fluid
This type of drag is the aerodynamic resistance due to the contact of moving air with the surface of an object, and in this particular case that object would be an aircraft. Every surface, no matter how apparently smooth, has a rough, ragged surface when observed under a microscope.
Skin friction drag
A _________ cannot exist when pressure is rising in the direction of the flow.
Laminar flow
On a curved surface, such as an airfoil, the transition point is usually __________.
At or near to the point of the maximum thickness
Because of the adverse pressure gradient existing on a curved surface the transition point will be ________ than if the surface was flat.
Further forward
This is generated due to the aircraft’s shape and the airflow around it. When the air has to separate to move around a moving aircraft and its components, it eventually rejoins after passing the body. How quickly and smoothly it rejoins is representative of the resistance it creates, which creates additional force to overcome.
Form drag
To reduce form drag, it is necessary to ________.
Delay the separation as close to the trailing edge as possible (Streamlining)
________ increases the ratio between the length and depth of the body, reducing the curvature of the surfaces and thus the adverse pressure gradient.
Streamlining
________ is the measure of streamlining.
Fitness ratio
An example of this would be the intersection created by the connection of the wing and the fuselage at the wing root.
Interference drag
The most interference drag is observed when_______. _______ are used to reduce this tendency.
Two surfaces meet at perpendicular angles
Factors affecting parasite drag:
- Indicated air speed
- Configuration
- Airframe contamination
Formula of parasite drag:
Dp = qSCdp
Parasite drag varies with the ________.
Square of the IAS
What is the relationship of the variation of parasite drag to the parasite area?
Directly proportional
Contamination by these will increase the parasite drag coefficient.
By ice, frost, snow, mud, or slush
________ is an inherent component, a rather undesirable by-product of lift.
Induced drag
_______ modify upwash and downwash in the vicinity of the wing which produces a rearward component to the lift vector known as the “________”.
Wing tip vortices; Induced drag
Wingtip vortices induce downwash, which tilts the lift vector ________, creating induced drag.
Backward
Wing tip vortices create certain ________ in the airflow in the vicinity of the wing, both in front of and behind it. These strengthen upwash and downwash which reduces the _______.
Vertical velocity components; Effective angle of attack
What is the relationship between the strength of the vortices and the effective angle of attack?
Inverse relationship
_______ must be equal to weight in flight so induced drag will depend on the weight of the aircraft.
Lift
Factors affecting induced drag:
- Size of the lift force
- Speed of the aircraft
- AR of the wing
Induced drag will be _______ at higher aircraft weights.
Greater
Certain maneuvers require the lift force to be greater than the aircraft weight. This relationship of lift to weight ratio is also known as the _______.
Load factor
Formula of coefficient of induced drag:
Cdi = CL^2/piARe
What is the relationship of the load factor to the induced drag?
Directly proportional
What is the relationship between the induced drag and the speed of the aircraft? How does increased speed mean less induced drag?
Inverse relationship; less downwash = less lift inclination = less induced drag
What is the relationship between the AR of the wing and the induced drag? How does a higher AR decrease the induced drag?
Inverse relationship; higher AR = longer span = less wing vortices effect; less overall change in downwash = smaller rearward lift tilt = less induced drag
Factors to consider if one decides to continue the increase of AR:
- Excessive wing bending moments
- Reduced rate of roll
- Reduced ground clearance in roll during takeoff and landing.
How to reduce excessive wing bending moments?
By carrying fuel in the wings and mounting the engines in pods beneath the wing
Reduced rate of roll is caused by the down-going wing experiencing an _______.
Increased angle of attack
These are small vertical aerofoils which form part of the wing tip, shaped and angled to the induced flow, and generate a small forward force.
Winglets
This is placed at the wing tip to restrict the tip vortices and have a similar effect to an increased aspect ratio, but without the extra bending loads.
Wing-end plates
This will have a similar beneficial effect to an end plate, which is to reduce induced drag.
Tip tanks
These partly block the air flowing from the bottom to the top surface of the wing, reducing the strength of the tip vortices.
Winglets
This can affect the strength of the tip vortices, and designs such as turned down or turned up wing tips have been used to reduce induced drag.
Wing-tip shape
The _______ of an aircraft in flight is the sum of its induced drag and parasite drag.
Total drag
Formula of total drag:
D = qsCd
The _______ is a number that aerodynamicists use to model all of the complex dependencies of shape, inclination, and flow conditions on aircraft drag.
Drag coefficient
Formula of total drag coefficient:
Cd = Cdo + Cdi
What is the effect of the surface roughness to the turbulent flow and the laminar flow?
Surface roughness increases the skin friction drag. The surface from the turbulent flow is rougher than that of the laminar, hence, it gives a greater skin friction drag than that of the laminar.
