Parasite and Induced Drag

Question 1

what are the three components or parasite drag?

Answer 1

form drag, skin friction and interference drag

Question 2

what is the transition point?

Answer 2

in a laminar boundary layer the airflow is smooth and there is no vertical movement to mix the layers but as this layer flows further back over an aerofoil, it is likely to be disturbed and become turbulent at a point known as the transition point

the major factor is that, aft of the point of max curvature the airflow encounters an adverse pressure gradient

Question 3

where over the wing is skin friction highest?

Answer 3

turbulent flow

Surface friction drag depends on the rate at which the speed of the air changes just near the surface – the surface velocity gradient. The surface velocity gradient is sharper in the turbulent flow, so this layer has more skin friction drag

Question 4

what produces more lift, laminar or turbulent flow and why?

Answer 4

laminar flow

The air molecules are now moving in all directions, at a greater speed than the laminar flow, so this thicker layer has more kinetic energy than the laminar flow. But the speed of movement aft over the aerofoil decreases, which means that this turbulent flow produces less lift than a laminar flow

Question 5

what casues the separated flow?

Answer 5

as the turbulent layer moves further aft over the wing, the adverse pressure gradient, the falling free stream velocity and the surface friction all combine to further slow the air in the boundary layer. Eventually the air in the layer near the surface stops moving and even reverses direction. The boundary layer now breaks down and separates from the surface, leaving a wake of random and disturbed flow called separated flow

Question 6

what is an advantage of turbulent flow, even though it procuces drag and less lift?

Answer 6

energises the boundary layer, delaying forward movement of the separation point

Question 7

what is the distribution of spanwise flow over a conventional straight wing?

what does this cause?

Answer 7

root to tip below, tip to root on top

causes wing tip vortices

Question 8

what is interference drag?

Where is it commonly found?

Answer 8

created where the two airflows collide, because energy will be required to make one or both of the flows change direction. The two flows may combine to form a vortex – smaller than the wingtip vortex but still a source of drag

commonly found at the wing root

Question 9

what is the drag formula?

Answer 9

drag = CD x 1/2rhoVsquared x S

Question 10

difference between drag and lift in level flight?

Answer 10

drag will change with changes of speed, unlike lift which does not change with speed if the pilot maintains level flight

Question 11

how does parasite drag increase with speed?

Answer 11

CDP is a constant in the drag equation and so parasite drag will increase directly with the square of the speed (Vsquared). Therefore, if we double the speed, the parasite drag will increase by a factor of 4

Question 12

what are the two components of induced drag?

Answer 12

wingtip vortices and the direction in which the force of the total reaction is applied (total reaction titls back as alpha is increased)

Question 13

what is the effect of upwash and downwash on induced drag?

Answer 13

The upwash in front of the wing and the downwash behind it cause the actual airflow over the wing to change direction and effectively reduce the angle of attack – effective airflow

The effective alpha between the effective flow and the chord line is now smaller and we will not get the amount of lift we expect. To achieve the amount of lift we require, we will need to increase alpha to compensate for the loss of lift, caused by the downwash

But this has involved leaning the total reaction even further, increasing the size of the induced drag component

Question 14

what is the effect of wing tip vortices on downwash?

Answer 14

Wingtip vortices increase downwash behind the trailing edge, which further reduces the local effective alpha, and further decreases lift. When we have large wingtip vortices creating a lot of downwash, maintaining the require value of lift will require considerable increases in alpha, giving us high levels of induced drag. It is therefore highly desirable to minimise the size of wingtip vortices

Question 15

wha is the angle called between affective airflow and the undisturbed flow?

Answer 15

induced angle, or induced angle of attack

Question 16

effect of increased downwash on iduced drag?

Answer 16

will reduce the effective alpha and increase the induced alpha, which will lead to increased levels of induced drag

Question 17

what happens to downwash when in ground effect?

Answer 17

In ground effect the runway suppresses the downwash because it has nowhere to go

When we enter ground effect on landing, both downwash and the induced alpha decrease, while the effective alpha increases

Lift will increase and induced drag decreases because the size of the wingtip vortices is reduced

This combination gives the aircraft a tendency to float, which may affect the landing distance required

Question 18

what effect does ground effect have on critical alpha?

Answer 18

Entering ground effect will lower our critical alpha (which is of course measured against undisturbed airflow) because the increase in effective alpha puts us closer to the stall

Question 19

how does Cdi vary with CL?

Answer 19

induced drag varies with the square of the increase in lift

Question 20

CL(sqrd) =

Answer 20

Cdi x aspect ratio