Ch 11 - Aircraft Drag

Question 1

Total Drag

Answer 1

Total Drag = Parasite Drag + Induced Drag

Parasite drag = Form Drag + Skin Friction Drag + Interference Drag

Profile Drag = Form Drag + Skin Friction

Question 2

Form Drag

Answer 2

Impacted by Speed, Frontal Area, Shape and streamlining, Surface (smooth/dimpled)

To decrease form drag:
- Increase the distance aft of the point of max. thickness

Question 3

Skin Friction Drag

Answer 3

Impacted by; Speed, surface area, boundary layer type and surface roughness

Question 4

Interference Drag

Answer 4

All to do with how smooth the transition from air to flow around the object is.

Think of Fish

• Fillets and fairings all aimed to reduce interference drag

Question 5

Parasite Drag

Answer 5

Proportional to:

• Airspeed squared
• Surface area of the aircraft
• Coefficient of parasite drag

Parasite drag is a factor of CAS/IAS and dynamic Pressure

Question 6

Coefficient of Parasite Drag

Answer 6

Determined by;

• Form drag
• Skin Friction drag
• Interference drag

For a given config, Cdp does not vary with normal AoAs

Question 7

Induced Drag

Answer 7

Cdi is inversely proportional to IAS squared

Weight makes the induced drag curve move up and to the left
- The heavier you are, the more lift you will need which means more induced drag

Cdi is inversely proportional to aspect ratio. So as aspect ratio increases, Cdi decreases as there is less downwash and less vortices

Question 8

Reducing Tip Stalling Tendency

Answer 8

Vortilon - Creates vortex (barrier) to stop span wise flow

Wing Fence - Physical barrier to stop spanwise flow

Chine - Engine notch, at slow speeds, reduces root to tip flow on upper surface of wing

Notch - Create vortex (barrier) to help reduce spanwise flow

More commonly though:

• Washout at the tip
• Decreased camber at the tip

Question 9

Cross Section on Stalling Angle

Answer 9

Sharpening the leading edge of the wing root will encourage the root to stall first

Some light aircraft have stall strips (Toblerone )

Question 10

Wing Loading

Answer 10

=Aircraft Weight / Wing Area

High wing loading; increases the adverse pressure gradient, the flow energy is lost more quickly which encourages separation and an earlier stall onset

Question 11

Aileron Reversal

Answer 11

When approaching the stall, you try to roll one way and in trying to increase the lift on the wing that you are trying to lift, cause it to stall which means that you actually roll the other way

Question 12

Effect of mass on stalling speed

Answer 12

Increased mass = increased required lift (faster or increased AoA)

Which means the hat the stalling speed is Higher for Heavier aircraft

Question 13

Flap Configuration on Stalling Speed

Answer 13

Flaps and slats will increase CLMAX which means the that flaps and slats decrease the stalling speed.
Can fly slower

VS1 = Stall speed at a specified configuration 
VS0 = Stall speed in landing configuration

Question 14

Landing Gear on Stall Speed

Answer 14

Landing gear will create a pitch down moment which will increase the stall speed (normal orientation)

Question 15

Propeller Thrust on Stalling Speed

Answer 15

Power-off Stall = Higher Stall Speed

Power-on Stall = Lower stall speed as you are gaining ‘free lift’

Question 16

Jet Thrust on Stall Speed

Answer 16

Power-off Stall = Higher stall speed

Power-on Stall = Lower stall speed (pitch up moment created which is good for a stall)

Question 17

CofG Positions on Stall Speed

Answer 17

CG Forward = Higher Stall Speed

Question 18

Swept Wing on Stall Speed

Answer 18

Reduced CLMAX compared to a rectangular wing (much lower)

Which means a higher stalling angle of attack but a less defined stalling angle of attack

Question 19

Altitude on Stall Speed

Answer 19

With altitude, the aerofoil loses energy at the leading edge which gives you less warning to when you are about to stall.

At high alt. stalling speed increases due to compressibility
At other alt. Ranges, the stalling speed is consistent

Question 20

Contamination on Stalling Speed

Answer 20

Decreased stalling AoA
Decreased CLMAX
Increased stalling speed

Question 21

Accelerated Stall

Answer 21

A manoeuvre that requires additional lift.

The wing has the same CLMAX, speed therefore has to increase

Question 22

Stalling in the Turn

Answer 22

Load Factor = 1/cosThi

VSNEW = 3 step approach

Question 23

Stall Speed in a Pitching Manoeuvre

Answer 23

Higher load factor which means the same as an accelerated stall
Stalling speed increases

To recover, reduce AoA