Drag Flashcards
Profile Drag
Combination of Skin Drag and Form Drag
Skin Drag
Surface roughness of the surface of aerofoil skin
Traps and slows air molecules in boundary layer
Direct result from friction in boundary layer
Form Drag
Pressure differences
Static pressure to rear is lower so creates force rearward
Air speed
Frontal Area
Shape/Streamline
Surface
Boundary Layer
All skin friction occurs within
Molecules at surface at a stand still
Layers above slowed down until outside boundary lower
Greater viscous air = greater shearing force
Laminar/Turbulent flow within
Laminar Boundary Layer
Few mm deep
Shallow velocity gradient
Not much friction/less heat/less energy loss
Turbulent Boundary Layer
Thick measured in cm - more mass = more turbulent = more skin friction than laminar
Flow in all directions
High energy exchange between layers
Steep velocity gradient
Greater K resist separation compared to laminar
Transition Point
Point of change from laminar to turbulent flow
Laminar flow when viscous force more dominant
Turbulent flow when high inertial forces dominant
Factors determining transition point
Speed:
- High speed = greater laminar flow - transition point further back
- Low speed - Greater turbulent flow - transition point closer to LE
Surface Roughness:
- More friction reduce flow of energy laminar to turbulent earlier
Adverse Pressure Gradient
High static pressure moves from lower surface to upper surface of wing
Causes separation at boundary layer
Reduction in surface area therefore drop in lift force
VMD Means
Minimum drag speed best lift/drag ratio
Induced = parasite
Thrust equal drag to maintain stable constant speed in lvl flight
Speed Stability
Natural tendency to return to original speed
Drop in say speed would result in a reduction drag force so natural acceleration back up to the set speed
Speeds greater than VMD
Speed Unstable
Back of drag curve - natural tendency to diverge
Speed increases drag increases
Danger of stalling
Speed Netural
No tendency to return or diverge
Located around VMD
Effect of Alt on Drag Curve
IAS no difference (density not taken into account)
TAS whole drag curve moves right as altitude increase
Effect of weight on the drag curve
Increase in weight moves drag curve up and right
Higher induced drag for mass (induced drag curve moves up and right)
Faster VMD/stall speed increases
Effect on configuration on drag curve (flaps/landing gear etc)
Increase parasite drag
Induced drag constant
Total drag curve moves up and left
Slower VMD
Parasite drag + CL
No variation as CL increases parasite drag is the same
Induced drag with CL
Coefficient of induced drag varies with the square of coefficient of lift
CDI = CL2/Aspect ratio
Drag Polar Curve
Total Drag (Induced + Parasite) against coefficient of list
Cl/Dmax occurs at Vmd
More efficient wing steeper cl for cd
Flaps move up and to the right (increase parasite and increase in clmax)
Factors impacting induced drag
Speed - Slower more (Double speed quarter of drag)
Mass - Heavier more drag (More Cl/More induced AOA)
Manoeuvre - More cl more di
Aspect ratio - High aspect less vortices/less AOA so less induced drag
CDI = Cl2/AR
Interference Drag
Occurs in 3D flow
Around structural joints (Wings and fuselage)
Use of filets to smooth joints and reduce
FILLET OF FISH
Value of induced drag in S+L varies with
1/V2
Effects of fitting tip tanks on induced drag and parasite drag
Parasite drag will increase as surface area has increased more friction
Induced drag will decrease as tanks prevent flow around the tip of wing to decrease the vortices
How do CDI vary with speed
If speed drops by a half (1/4) therefore CDI will be 1/16
