Drag and the Boundary Layer Flashcards
State the Drag Equation
D = CD x ½pV2S
What does VIMP stand for?
Is this the setting for Range or Endurance?
VIMP = Velocity Indicated Minimum Power
Max Endurance
What does VIMD stand for?
Is this the setting for Range or Endurance?
VIMD = Velocity Indicated Minimum Drag
Best Range
What are the two components of Total Drag?
- Zero-Lift Drag (Profile Drag); and
- Lift Dependant Drag (Induced)
What are the three components of Profile Drag?
- Skin Friction Drag;
- Form Drag; and
- Interference Drag.
What is Skin Friction Drag?
Drag within the boundary layer due to viscosity
What is form drag?
Drag due to shape, wake turbulence behind an object.
What is Interference Drag?
Drag at intersections (e.g. wing root and fuselage)
What is the equation for the coefficient of drag CD?
CD = CD0 + CDi where, CDi = CL2 / (πAR)
CD0 = Zero Lift Drag
CDi = Induced Drag
Zero Lift Drag:
a) Increases with airspeed;
b) decreases with airspeed
a) Increases with airspeed
Define the Boundary layer
The layer of air extending from the surface of the wing to the point where no dragging effect is discernable.
The airflow over the upper surface of an aerofoil experiences a:
a) Proverse Pressure Gradient;
b) Adverse Pressure gradient;
c) Proverse PG then Adverse PG; or
d) Adverse PG then Proverse PG.
c) Proverse PG then Adverse PG
The airflow over the lower surface of an aerofoil experiences a:
a) Proverse Pressure Gradient;
b) Adverse Pressure gradient;
c) Proverse PG then Adverse PG; or
d) Adverse PG then Proverse PG.
a) Proverse Pressure Gradient
It’s just a much lower magnitude than the upper surface.
Which is Thicker?
a) Laminar flow; or
b) Turbulent Flow
b) Turbulent Flow
Which Boundary Layer has the highest velocity next to the surface?
a) Laminar Flow;
b) Turbulent Flow
b) Turbulent Flow
Which has the most gradual Velocity Change?
a) Laminar Flow; or
b) Turbulent flow
a) Laminar Flow
The tubulent boundary layer flow has a sharp velocity change.
Which Boundary Layer has the lowest skin friction?
a) Laminar Flow;
b) Turbulent Flow
a) Laminar Flow
Which Boundary Layer is better able to go against an Adverse Pressure Gradient and remain attached to the wing?
a) Laminar Flow;
b) Turbulent Flow
b) A Turbulent boundary layer
How is the skin friction determined?
HINT: 5 factors used to determine the skin friction
- Total surface area (wetted area);
- Coefficient of viscosity of the medium (density of the air);
- Type of boundary layer (laminar or turbulent);
- Location of transition point within the BL; and
- Surface roughness (e.g gloss paint for reduced skin friction).
What are the 6 factors affecting Form Drag?
- Object shape (Streamlining)
- Cross sectional area ( thickness to chord ratio)
- Speed (Form drag is proportional to the square of the speed)
- Type of boundary layer (turbulent BL better)
- AoA (form drag increases with AoA)
- BL controls (delay separation)
What are two ways of reducing interference drag?
- Following the ‘area rule’ (Concorde)
- ‘Waist-ing’ of fuselage (F-5)
How does AR affect drag?
An increase in AR (glider) will decrease induced drag. A decrease in AR (e.g Starfighter) will increase induced drag)
CDi = CL2 / πAR
Describe how the three components of Drag change with an increase in AoA up to stall. I.e. Which has the most effect etc
See Image
Downwash adds to which type of Drag?
a) Skin Friction
b) Form Drag
c) Interference Drag
d) Induced Drag
d) Induced Drag
What is the ideal wing planform for reducing Induced Drag?
An elliptical wing planform (Spitfire)
What are the factors affecting Induced Drag?
- CL2 (as it increases so does ID);
- 1/V2 (As speed increases, ID decreases);
- Weight2 or LF2 or Lift2 (as lift increases [changes in V or CL] ID increases);
- 1/AR (as AR increases, ID decreases)
- 1/S
Why is Induced Drag highest at takeoff?
- Weight is max;
- Speed is lowest;
- Flap deflection is least;
- AoA is highest;
- Pressure envelopes are strongest;
- Wing tip vorticies are strongest; and
- Downwash is maximum
What is ground effect?
The presence of ground or water within approx 1 wingspan curtails downwash by not allowing the flow to deflect downwards. It is most appreciable at takeoff and at the point of landing.
When in Ground Effect, is the thrust required greater, the same, or lesser?
The thrust required in ground effect is less than if you were not in ground effect.
What are 4 methods of Boundary Layer Control (BLC)?
- Slats;
- Blowing;
- Suction;
- Vortex Generators
