Drag

Question 1

What is drag and total drag

Answer 1

Drag is the component of total reaction acting inline with the RAF. The resistance of a body to movement through a fluid

Total drag is made of two components:

• zero lift drag (parasite) proportional to V^2
• lift dependant drag (induced drag)

Total drag = zero lift + lift dependent drag

Question 2

What is the drag tree

Answer 2

Total Drag = induced and parasite drag

Parasite = profile and interference drag

profile = skin friction and form drag

Question 3

What is viscosity?

Answer 3

How thick a fluid is.

Question 4

What is the boundary layer?

Answer 4

When air moves past a solid body, its viscosity causes particles next to the surface to adhere to the surface of the body, and the airflow in the immediate vicinity to be slowed down.

This layer of retarded flow, which is sandwiched between the local free stream flow and the surface of the body is known as the boundary layer.

Question 5

Describe laminar boundary layer flow, turbulent layer flow and transition point.

Answer 5

Laminar - increase in velocity is uniform and low drag. The layer is only about 2mm in depth

Turbulent - higher rate of change in velocity, more surface friction = more drag. Eddying causes air with more kinetic energy to mix with lower levels. The layer about 20mm in depth.

Transition point - the point at which the flow changes from laminar to turbulent.

Question 6

What is adverse pressure gradient?

Answer 6

When airflow is moving from an area of low pressure to an area of high pressure.

Question 7

What happens with the boundary layer with AoA?

Answer 7

As AoA increases separation and transition points move forward with an increase of AoA.

Question 8

Explain skin-friction drag

Answer 8

The shear stress between successive layers of air as it accelerates from stationary to free stream flow.

(Shear stress is the force required to seperate air particles at one level from those at the next).

To keep it to minimum, laminar airflow must be kept for as long as possible.

Question 9

Explain factors affecting skin friction drag (5S 1A)

Answer 9

Speed - higher speed = more shear stress
Surface condition - keep clean
Size - doubling the size will double the drag
Surface area - more surface area more drag
Shape - lowest static pressure is located at point of maximum thickness, thus laminar flow can be maintained up to this point. Passing it, pressure gradient becomes adverse and turbulent flow will occur.
AoA - Increased AoA, brings transition point forward

Question 10

Explain form drag

Answer 10

Drag caused by the pressure differential between the forward and rear facing surfaces of object in an airflow.

Basically caused by air not conforming to the shape of an object.

Question 11

Factors affecting form drag

Answer 11

Surface condition - keep clean or wont conform
Size - smaller cross section, lower drag
AoA - increased AoA, can’t conform as well
Speed - increasing speed increases form drag.

Question 12

Describe the effect of streamlining in reducing form drag.

Answer 12

Elongates the forward and rear surfaces which reduces pressure differential, delays separation, reduces turbulent wake.

Question 13

Describe interference drag and the measures for reducing it.

Answer 13

Caused by the mixing of converging airflows at the junctions of different aircraft surfaces

Reduced via filleting and blending of shapes which reduces the angle at which airflows meet.

Question 14

Explain the origin of induced drag

Answer 14

The penalty for producing lift, created whenever lift is produced, caused by the downwash created by the trailing edge and wingtip vortices.

Question 15

Factors affecting induced drag

Answer 15

Aspect Ratio - Increasing AR decreases induced drag
Wing planform shape - shape of the wing viewed from above
AoA - drag increase with increase in AoA
Airspeed - higher speeds, induced drag is lower
Weight - more weight requires a higher AoA for a given airspeed

Question 16

What to do to reduce induced drag

Answer 16

Straighten span wise flow and reduce pressure gradient between the upper and lower surfaces

Question 17

Ways of reducing induced drag (6) (aspect ratio, washout, taper, pressure gradient, spanwise flow)

Answer 17

Increase aspect ratio so that the wingtip vortices are a smaller percentage of the wing as it gives less time for the spanwise flow to affect the chordwise flow so less air spills over the wingtip.

Washout - reduction in angle of incidence towards the wingtip, reducing AoA at the wingtips reducing pressure differential at wingtips.

Taper:

• in chord: reduction in chord towards the wingtip, reducing AR at wingtip.
• in depth: reduction towards the wingtips so less lift is produced by the wingtip.

Pressure gradient : reducing pressure gradient will reduce size and intensity of the tip vortices. E.g winglets

Spanwise flow - straightening the spanwise flow so the severity of the vortices will reduce.

Question 18

State the meaning of the term coefficient of drag (CD); and describe the main features of a typical curve of CD versus α.

Answer 18

Total drag is a combination of parasite and induced drag so coefficient drag is therefore these two added.

In the curve, minimum Cd occurs at 0º AoA and the rate of increase becomes marked at AoA above 8º and it greatly increases after the critical angle due to the increase in turbulent flow as airflow breaks down.

Question 19

What is the drag formula and what does it consist of

Answer 19

Drag = Cd1/2pV^2S

Where 
S = total frontal area
Cd = coefficient of drag
V = velocity
P = rho/density

Question 20

Explain from a typical graph the most efficient angle of attack, the zero lift position, and the stalling angle.

Answer 20

4º is most efficient

16º is stall angle

Zero lift is 4º