Drag Flashcards
Total drag
Is the sum of all the components of aerodynamic force which act parallel and opposite to the direction of flight
What makes up total drag
Induced drag and parasite drag
What makes up parasite drag
Profile drag and interference drag
What makes up profile drag
Skin friction and form drag
Parasite drag
Is the drag force not directly associated with lift production, all elements of parasite drag arise because air is a viscous medium
Induced drag
Is the drag associated with the production of lift, it arises from the downwash induced by the wingtip + trailing edge vortices which for a given amount of lift being produced tilts the total reaction force further backwards through the induced downwash angle, this increases the length of the drag vector
Skin friction drag
Is the result of shear stress between successive layers of air within the boundary layer. Skin friction drag is lower with a laminar boundary layer compared to a turbulent one
Skin friction drag - shear stress
The force required to separate air particles at one level from those at the next and move them along at a faster rate
Factors effecting skin friction drag
- Speed
- Shape
- Size
- Surface area
- Surface condition
- Angle of attack
How does speed effect skin friction drag
Skin friction drag increases in proportion to IAS^2
(because as speed increases the rate of change of velocity across the flow in the boundary layer increases, which increases the shear stress)
How does shape effect skin friction drag
Laminar flow boundary layers are sensitive to adverse pressure gradients which occur when the flow is toward an area of higher static pressure.
If an aerofoil is at a low AoA the lowest static pressure is located about the point of max thickness. Laminar boundary flow can usually be maintained from the leading edge to this point (i.e across the area with a favourable pressure gradient).
As airflow progresses beyond this point of lowest pressure toward the trailing edge the pressure becomes increasingly adverse, and the boundary layer responds by transitioning to turbulent flow.
Hence long slender aerodynamic shapes have the point of max thickness located well back to encourage laminar boundary flow
How does surface condition effect skin friction drag
Laminar flow is sensitive to surface condition irregularities and roughness, so smooth surfaces are needed to keep flow laminar and in turbulent flow areas to prevent the boundary layer from getting too thick
How does size effect skin friction drag
If the size is increased, then there is an increase in drag that is out of proportion to the increase in size (scale effect), therefore small aerodynamic shapes decrease drag
How does surface area effect skin friction drag
Increased SA exposed to the airflow = increased drag
How does Angle of attack effect skin friction drag
Increase in AoA = increase in drag
due to the transition point moving forward and a greater proportion of the surface becomes covered with a turbulent boundary layer
Form drag
Whenever a solid body moves through air the pressure on the forward facing surfaces will be higher than on the rearward facing surfaces. The component of the force generated by the pressure difference which is parallel to the airstream is form drag
What is important in determining how form drag is generated?
The separation point
When does the separation occur
When the lower flow in the boundary layer slows to stop and begins to reverse, the flow as a whole is no longer able to conform to the shape of the body and separation occurs
Separated flow
Turbulent flow in the wake is referred to as separated flow
Separation point
The point where the streamline flow around an aerodynamic surface breaks down to form a turbulent wake
What are the main factors effecting form drag
- shape
- size
- AoA
- airspeed
How does shape effect form drag
Streamlining is aimed at reducing the effect adverse pressure gradients by making the curvature of the surfaces more gradual (particularly towards the rear), this delays separation which in turn reduces the pressure difference between the forward and rearward surfaces
How does size effect form drag
Form drag is proportional to the size of the surface on which the front and rear pressure differential acts, so small cross-sectional shapes when viewed from the forward aspect keep form drag to a min
How does AoA effect form drag
As AoA increases at a constant speed form drag also increases because:
- the seperation point moves forward
- a greater frontal area is presented to the oncoming airflow
How does airspeed effect form drag
Form drag is a function of the dynamic pressure acting on an a/c and therefore increases with IAS^2
What can make a shape more streamlined
Retracting undercarriage or adding fairings
What effect does surface irregularities have on streamlining shapes
Anything that interrupts the smooth streamlined flow such as ice / damage will speed up separation and increase form drag
Fineness ratio
The effectiveness of streamlining a body of a given cross-sectional area is determined by the fineness ratio, which is the ratio between the length of a body and its depth (length / depth), the least amount of form drag is achieved with a fineness ratio between 3 - 4 and with max depth placed 1/3 of the distance back from the leading edge
Interference drag
Is caused by mixing or interference of converging airflows at the junctions of various surfaces (such as the wing / fuselage junction). Whenever the airflows from various surfaces of the a/c meet a wake is formed behind the a/c, this additional turbulence that forms in the wake causes a greater pressure difference between the front and rear surfaces of the a/c, incerasing drag.
How can interference drag be reduced
By filtering and blending shapes. A fairing is a part of the a/c skin that’s added to encourage smoother blending of different airflows, reducing eddying and the resultant drag
What effects Induced drag
- Aspect ratio
- Shape
- AoA
- Airspeed
- Weight
How does AR effect induced drag
High AR = smaller vortices = decreased induced drag
How does shape effect induced drag
Elliptical planform shape produces the smallest vortices and therefore the lowest induced drag
How does AoA effect induced drag
Increased AoA = stronger vortices = increased induced drag (therefore induced drag is at a max at stalling angle)
How does airspeed effect induced drag
Induced drag is inversely proportional to IAS^2 so induced drag is at a highest at low airspeeds
How does weight effect induced drag
Increasing weight means that a higher AoA must be used to produce a given amount of lift at any given speed so induced drag increases in proportion to W^2
How can induced drag be decreased
- Washout combined with a taper
- Wing fences (to straighten and control spanwise flow)
- Wingtip modification (such as drooping, winglets, wingtip tanks)
What is washout
A reduction of the angle of incidence (the angle between the chord of the wings or tailplane with respect to the fore and aft axis of the airframe) and therefore the geometric AoA of the wings toward the wingtip
CD
The coeffiecnt of total drag
CD = CD (parasite) + CD (induced)
CD curve features
- Min CD will occur when the wing is about 0° AoA
- As AoA increases past the min CD, CD also increases due to induced drag and increments of parasite drag
- Beyond stalling the increase in CD becomes more rapid mainly due to the effect of separation and increase in form drag
Drag formula
Drag = CD x 1/2pv^2 x S
3 functions of the drag formula
- CD (coefficient of drag)
- Dynamic energy
- S (total frontal area)
Drag curve
Produced by plotting drag against IAS. It shows that drag is high at low speeds due to induced drag and it is also high at high speeds due to parasite drag. Minimum drag is experienced at an intermediate airspeed where induced drag and parasite drag are = (this is also the speed at which the ratio of lift to drag is at a max)
What does the L/D ratio determine
The aerodynamic performance and efficiency of the a/c is determined by the L/D ratio at different AoA
L/D curve
Obtained by plotting CL/CD against AoA. Key features:
- Peak of the curve indicates the AoA for max L/D ratio (most efficient angle)
- In straight and level flight here is only one IAS associated with the AoA for max L/D
- At airspeeds higher then the peak (lower AoA) the L/D ratio falls off rapidly
- At airspeeds lower than the peak (higher AoA) the L/D ratio also reduces but not as rapidly until stalling AoA is reached where there is a marked reduction in
How can you obtain the L/D ratio at different AoA
By comparing CL against CD (as both the lift and drag formula’s 1/2pv^2 and S cancel each other out )
