Drag Flashcards
what happens in laminar airflow?
- streamlines travel undisturbed in parallel paths or the pressure of a solid object.
- streamlines are deflected from the parallel paths and swirl in all directions described as turbulent airflow
what are the two types of airflow?
laminar and turbulent.
at the leading edge of the surface how big is the boundary layer?
1-2mm in depth but increases in thickness as it travels further back along the surface.
in turbulent airflow air, what happens?
air is disturbed and travels in all directions.
what is the turbulent boundary layer?
as laminar airflow moves along the surface it loses energy making it more susceptible to any disturbance, eventually becoming turbulent (turbulent boundary layer)
the point where the boundary layer changes from laminar to turbulent is called what?
the transition point.
in the turbulent boundary layer, speed increases are higher closer to what than in the laminar boundary layer?
closer to the surface, the turbulent airflow produces much more friction drag than the laminar airflow.
what is the lift coefficient formula?
Cl1/2pV^2S
where Cl = lift coefficient
p = density of air
V = velocity or dynamic pressure of air
S = frontal cross-sectional area
What is the seperation point?
A point where adverse pressure gradient becomes too strong and airflow can no longer conform to the aerofoil shape.
What is the transition point?
The point where the boundary layer changes from laminar to turbulent.
what is the simplified lift formula?
lift is proportional to the angle of attack and airspeed so Lift = AoA x A/S
what are the factors affecting lift? (6)
- shape of the wing
- angle of attack
- surface condition of the wing
- density of air
- airspeed
- wing area
what are the factors affecting the coefficient of lift?
flap stages and wing contamination.
what is in a typical CL against AoA graph?
Cl will increase as AoA increases up until what is know as the critical AoA which is 16 degrees.
If AoA is increased beyond this point, airflow over the aerofoil won’t be able to remain laminar and will become turbulent and break away from the aerofoil causing stall seen on the graph where the line drops off.
how does wing contamination affect flight?
aerofoils are designed to be streamlined so keeping airflow laminar reduces drag.
damage or contaminants on the wing have the opposite effect and reduce a wing’s ability to develop lift, forcing airflow to become turbulent, creating drag.
what is total drag?
all the drag being produced by an aircraft and can be split down into the sum of two types of drag - parasite and induced drag
what is parasite drag (zero-lift drag)?
the drag that doesn’t contribute to lift. proportional to TAS squared, so as airspeed increases so does parasite drag.
what is induced drag?
the penalty for producing lift. inversely proportional to TAS squared so as TAS decreases it increases
how does downwash affect relative airflow?
it tilts the air flowing over the wing downwards so the wing experiences airflow coming from a different angle which is know as “sees”
meaning that the Angle of Attack that the wing sees is reduced reducing lift.
what is downwash?
change in direction of air deflected by the aerodynamic action of an airfoil or wing in motion, as part of the process of producing lift.
what causes downwash?
air wanting to flow from the higher to the lower pressure so the higher pressure flow around the wingtips towards the area above the wing.
what are the factors affecting induced drag? (5)
- Aspect Ratio
- Angle of Attack
- Wing planform shape
- airspeed
- weight
what is aspect ratio?
the comparison (ratio) of wingspan to wing chord
how does aspect ratio affect induced drag?
the higher the aspect ratio the less the induced drag because the air has less time to bend sideways as the higher aspect wing has a shorter chord. meaning that the angle at which the higher pressure air meets the lower pressure air is less.
meeting of two air streams at an angle produce vortices known as what?
trailing edge and wingtip vortices which both contribute to induced drag.
how does the angle of attack contribute to induced drag?
the higher the AoA, the more lift is being developed so the pressure difference between the upper and lower surfaces is greater.
what is profile drag?
drag broken drown into skin friction drag and form drag.
what is skin friction drag?
drag caused by layers of air that are affected by the aircraft and slowed down. the rougher the surface the more skin friction drag.
what are the factors affecting skin friction drag? (6)
speed - higher speeds leads to more friction
surface condition - keep wings clean
size - double-wing size = double drag
surface area - more surface area more skin drag
shape - point of max thickness
AoA - increase bring s transition point forward which increases drag.
what is form drag?
form drag is drag affected by the shape of an object
what causes form drag?
caused by the pressure differential between the front and rear surface when an object is subject to an airflow.
air can’t conform to the shape of an object and so breaks away making turbulent eddies making drag.
how is form drag reduced?
by streamlining an object making them into smooth shapes.
what are the factors affecting form drag? (4)
surface condition - how clean
size - smaller cross-section the lower the form drag
AoA - increase in AoA, increase in form drag
Speed - increase in speed, increase in form drag
what is interference drag?
drag caused by the mixing and interference of converging airflows at various junctions of the aircraft surfaces.
how is interference drag reduced?
by the blending of one surface into another by reducing the angles at which different layers of air meet.
what is the best angle for the Lift to Drag Ratio?
4 degrees
what does the typical Lift to Drag graph look like?
Max L/D ratio is at 4 degrees any any operation at a higher or lower AoA than this results in a drop in the L/D ratio.
And once the wing stalls (16 degrees) the L/D ratio drops off more steeply indicating the wing is producing less lift and more drag.
what does the drag curve look like?
minimum drag occurs where the curves for parasite and induced drag cross and that is at some intermediate speed between the stalling speed and maximum speed for flight.
this speed corresponded with the most efficient operational AoA of 4 degrees.
