Aerodynamic Forces (Drag)

Question 1

What components make up total drag?

Explain the two main forms briefly

Answer 1

Total drag = Induced & Parasite drag
Parasite drag= interference & profile drag
Profile drag= form & skin friction drag
Induced drag is due to the production of lift by the rearwards tilt of the L vector due to downwash and parasite drag is “carried” by the a/c as it is related to the movement of the a/c through the fluid.

Question 2

How is parasite drag proportional to TAS?

What components make up the coefficient of parasite drag?

Answer 2

PD p TAS^2
Related to:
-nature of the fluid
-geometry of the a/c body
-surface quality of the a/c body
-changes in airspeed... and therefore Re
-INDIRECTLY by AoA

Question 3

What is skin friction drag?

Answer 3

Is due to viscous friction from shear forces (from a viscous fluid) on surfaces within the boundary layer.
As the speed of a particle is 0 on the surface due to its viscosity and it increases in y direction, it must overcome shear forces which is SFD.

Question 4

What is the skin drag coefficient affected by? Explain further where needed.
How is it calculated?

Answer 4

SFD is related to the flow regime (and so Re), the properties of the air and the quality of the surface. It is calculated through equations.
As increase in airspeed decreases Re, this will reduce SFD, but SFD is p TAS^2 so this overcomes this and will increase with increase in airspeed.
AoA INDIRECTLY affects SFD (AoA change regime which change SFD)

Question 5

What can help reduce SFD? Why?

Answer 5

Smooth clean surfaces. Unsmooth & contaminated surfaces can cause early T BL, which then draws Ek from the mainstream to increase particles speed within BL, and SFD in T BL is always higher because of the increased Ek… remember that equation.
Reduce surface area of wing
Max thickness rearwards
Reduce speed

Question 6

What is form drag? How does it create drag?

How is it calculated?

Answer 6

Caused when flow separates from a streamline creating fore/aft pressure differences.
As the flow separates vorticies form detach from the surface, forming a vortex wake. Wake produces a local low px zone and therefore creates this fore/aft px difference which causes resistance to movement.
Is calculated through experiments.

Question 7

What affects the vortex wake intensity?
What affects form drag?
How can we reduce form drag?

Answer 7

Vortex: shape of object and Re. Inc Re inc vortex intensity and this draws Ek from the mainstream, so it is weaker when vortexes are present.
FD: TAS^2 p FD
Inc speed will inc Re which inc wakes
INDIRECTLY by AoA (inc AoA… early BL sep… inc FD)
To reduce want to streamline and reduce speed

Question 8

What causes interference drag?

How is it calculated?

Answer 8

Is caused at the junctions of different a/c components, where the different air flows and wakes of those components interact and intrude on each other. So these airstreams are unable to follow their original path and can become compressed, which can cause local pressure disturbances, turbulence and flow separation and vorticies may be created, which all cause drag.
Calculated through experiments

Question 9

How do we get constructively interfering wakes?

How do we get noise from wakes?

Answer 9

As 2 wakes interfere they can be reflected/deflected where they intersect and if it joins back with the original wake than it may constructively/destructively interfere and create violent oscillations which draw Ek from the mainstream and therefore produce drag.
Wakes detach from the a/c body and if the propagate back onto the a/c body and reflect off this created noise.

Question 10

What is the coefficient or interference drag related to?

How can we reduce interference drag?

Answer 10

Related to shape of a/c body, Re, airspeed (TAS^2) and type of manoeuvre.
Should have a smooth pressure distribution at joints to avoid wakes interfering and constructively interfering. This is done by fairings or the smooth gradual blend of a/c components.

Question 11

Explain the formation of wingtip and trailing edge vorticies

Answer 11

Air over a finite aerofoil has a spanwise component toward the wing root and under the aerofoil it is towards the wing tip. These 2 airstreams will meet at the trailing edge and due to viscosity, interact in a way to reduce their difference in movement… by creating a rotational motion which is the trailing edge vortices.
This spanwise flow is greatest at the wingtips.
Px on the upper surface of the aerofoil is greater than the freestream and px below the aerofoil is less than the freestream. The difference in pressure at the wingtip drives the air into a rotational motion (provide a torque) or wingtip vorticies

Question 12

Explain how these vortices create induced drag.

What effect does this have for AoA?

Answer 12

The reduction in pressure of the vortex changes the direction of flow around the wing and induce downwash. This downwash tilts the ERAF rearwards which tilts the L vector rearwards (which slightly reduces lift). The vertical component of the “true” lift is what overcomes the weight of the a/c and is less than bathe original amount. The extra horizontal component of the new lift is induced drag.
The reduction in the amount of lift means AoA must increase to produce more but this just increases induced drag further.

Question 13

What are the equations for Cdw/Cid?
What is the k and e value for a symmetrical aerofoil?
What about for a high speed range/cambered aerofoils?
How to calculate induced drag?

Answer 13

C=k x Cl^2/(Pi x AR) = Cl^2/(eAR x Pi)
S: 1
High speed range/cambered aerofoils k>1 e<1
ID= Cid x 0.5pv^2S

Question 14

Draw Cl, CD, l/d and total drag curves

Answer 14

Refer to notes

Note parasite drag p TAS^2 & induced drag p 1/TAS^2

Question 15

When is an aerofoil thin, general, thick?

What specifically is important when talking about thickness?

Answer 15

The thickness to chord ratio

T/c<7 thin T/c approx 10 general T/c>15 thick

Question 16

Describe a thick vs thin aerofoil.

Compare their stalling angles

Answer 16

Thin aerofoils have a reduced pressure differential between the upper and lower surfaces of an aerofoil, so have reduced downwash… vortices and induced drag, they also have reduced Cl as a result. (Could use CTOL which states less DW= less circulation = less Cl). They have a reduced surface area per span x chord so have less skin friction drag. Better for high speed a/c.
Thick aerofoils are the opposite. They are able to produce a higher Cl, especially at high AoA. (Refer to graph in notes).

Stalling is due to BL separation which requires an adverse pressure gradient. This condition occurs later on in a thick aerofoil if at the same AoA as a think aerofoil so the stall occurs at a higher AoA.

Question 17

Describe the flow of a high and low aspect ratio wing.

Compare stall angles

Answer 17

Over a low AR, flow resembles 3D. Over a high AR, flow resembles 2D.
In low AR there will be increased TE/WT vortices due to increased spanwise flow so DW increases and so does Cid.
High AR are the opposite and are better at producing higher Cl at equal AoA while minimising drag… but require increased structural strength.
Low AR wing have a higher stall angle as Eraf decreases so Cl dec and CD inc.

Question 18

Describe how camber has an effect on the wings.

Compare stall angles

Answer 18

Cambered AF produce more lift at the same AoA (Cl>0) assuming all else remains the same while symmetrical AF Cl=0.
High camber wings have greater circulation/pressure differential so have higher Cl. Using L= Cl x 0.5pv^2S, a higher Cl means a lower V for a given L.
Increasing camber increases DW so has effect on Eraf and inc Cid.
The stall AoA decreases however, especially if the max camber is close to the LE as it brings forward the adverse px gradient.

Question 19

How does elliptical and tapered wings affect ID?

Answer 19

Elliptical wings have no wingtip so little ID due to reduced Wt vortices.
Tapered wings have a reduced sized wingtip, which increases the aspect ratio at the wing tip and decreases surface area, which helps reduced WT vortices.

Question 20

Describe the design of a laminar flow AF and its effects (Stall & drag)

Answer 20

Has a thin leading ledge to reduce rate of px and velocity change over an aerofoil, increasing time spent in laminar flow.
Rear max camber causes adverse px gradient to be further toward the TE, delaying BL separation.
They therefore have low SFD, low FD and a high stalling AoA.

Question 21

What method are there to delay BL separation? Explain how they work

Answer 21

Suction: near TE mini hole suck in lazy particles in the BL. Requires structural change, increased weight and power.
Blow: -can work through opposite of suction through bleed air
-MEMS micro electrical mechanical system, a piezo electric sensor detect change in pressure and generates an electrical signal that causes a diaphragm to expand and blow air out of a cavity to remove the lazy particles.
Vortex generators: produce micro vortices that cause the transfer of Ek from mainstream into BL which makes it harder for flow to separate therefore stall.
LE slot/slat: slat extends and increases surface area to inc Cl and creates a slot which up air under the wing passes through reenergising (inc Ek) the BL flow so delays BL separation and stall. -useful T/O with low IAS/high AoA.

Question 22

What is the purpose of wingtip modification and how do they work?
What types are there?

Answer 22

Work by changing the pressure distribution around the wingtip which causes smaller vortices and therefore reduces DW and induced drag.

• plain wing
• winglet
• wing fence
• midwifed wingtip
• wingtip tanks
• elliptical

Question 23

What is washout? How does it work?

Does it have any effect on stalling?

Answer 23

Is a reduction in the angle of incidence at the wingtip. This reduces the intensity of vortices at the wingtip which reduce induced drag due to decreased downwash, because of a reduced pressure differential at the wingtip.
Can also delay adverse pressure gradient at the wingtip and cause it to occur first at the wing root so reduces chances of tip stalls.

Question 24

Explain the effects on entering and exiting ground effect.

Answer 24

Entering: ground interferes with vortices forming so DW is reduced and therefore so is induced drag. This increases L as the L vector tilts forward (EAoA inc) and the a/c may float. There is a nose pitch down moment as less load on the tailplane (more noticeable on low tailplane a/c as it enters ground effect first) and there will be a false low reading of airspeed.
Exit: increasing DW means more ID and reduced lift. There will be a nose pitch up (as less load on TP as it leaves GE) and a false high reading of airspeed.

Question 25

Summarise what can help reduce ID

Answer 25

Washout
Tapered wings
AR
Ground effect
Elliptical wing
Modified wingtip
(Also less camber and thickness)

Question 26

What drags are not directly affected by AoA?

Answer 26

Form
Skin friction
Interference