1. CGS Ground School Principles Of Flight Drag © Crown Copyright 2012
No Part of this presentation may be reproduced without the permission of the issuing authority.
The views expressed in this presentation do not necessarily reflect the views or policy of the MOD.

2. Total reaction
In flight, an aircraft produces an aerodynamic force called the total reaction.
Drag is the component of the total reaction acting parallel to the relative airflow.
RAF
Total reaction
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Drag

3. Drag Drag is normally considered under two main headings.
1. Zero Lift Drag.
2. Lift Dependent Drag.
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4. Zero lift drag can be sub-divided into:
When an aircraft is flying at an angle of attack which gives zero lift, the resultant of the forces acting parallel and opposite to the direction of flight is called Zero Lift Drag.
Zero lift drag can be sub-divided into:
1. Surface Friction Drag.
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2. Form Drag.
3. Interference Drag.

5. Surface friction drag
In flight, due to the viscosity of the air, a thin layer of air adheres to the aircraft's surfaces.
This thin layer of air is called the Boundary Layer
and is the cause of Surface Friction Drag.
The boundary layer consists of two types of airflow.
1. Laminar Flow.
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2. Turbulent Flow.

6. The boundary layer
Laminar flow, is an orderly flow of air, where successive layers of air slide over each other with little change of direction.
At the transition point the laminar flow becomes turbulent.
Turbulent Flow consists of many small vortices and eddies in which there is no set direction.
Transition point
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7. The boundary layer
When this happens the boundary layer thickens and the amount of surface friction drag increases.
If the turbulent flow becomes very turbulent then it will break away from the wing.
The point at which this occurs is called the separation point.
Separation point
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Transition point

8. The boundary layer
Since turbulent flow causes more drag than laminar flow, surface friction drag can be reduced by keeping the airflow laminar for as long as possible.
This is achieved by keeping the wings smooth,
clean and polished.
Any minor imperfections, such as scratches, dead flies, water or ice on the surface will move the transition point forward and therefore increase Surface Friction Drag.
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9. Form drag Form Drag is produced by an aircraft as it forces
the air that it meets out of its way.
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High form drag
Low form drag
Form drag can be minimised by Streamlining.

10. Form drag
The effectiveness of streamlining an object can be seen using the following example:
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If a flat, circular plate is placed in a wind tunnel and it is found that 20 men are required to prevent the plate from moving:

11. Form drag
The effectiveness of streamlining an object can be seen using the following example:
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Then streamlining the same plate, by making it into a ball of the same diameter, will only require 10 men to prevent it from moving (drag reduced by 50 %).

12. Form drag
The effectiveness of streamlining an object can be seen using the following example:
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Further streamlining the same plate will require only 1 man to prevent it from moving (drag reduced by
95 %).

13. Interference drag
Interference Drag is caused by the air flowing around junctions such as wing to fuselage, tailplane to fin etc.
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It can be reduced by fairings or fillets at these junctions.

14. 2. Increments of Zero lift drag.
Lift dependent drag
When an aircraft produces lift it also produces extra drag, called lift dependent drag.
This drag consists of:
1. Vortex (Induced) drag.
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2. Increments of Zero lift drag.

15. Vortex drag
Vortex Drag is caused by the difference in pressure above and below the wing.
Air from the high pressure area beneath the wings spills around the wing tips, to the low pressure area above the wings.
This creates large wing tip vortices.
Low pressure
Low pressure
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High pressure
High pressure

16. Vortex drag
The flow around the wing tips will cause air underneath the wings to start flowing towards the wing tips,
and the air above the wings to start flowing towards the fuselage.
When the two airflows meet at the wing trailing edge they are travelling in different directions, which causes the formation of trailing edge vortices.
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17. There are four factors which affect vortex drag:
1. Wing Shape.
2. Aspect Ratio.
3. Speed.
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4. Weight.

18. Vortex drag - wing shape
Vortex drag is greatest where the vortices are largest, that is at the wing tips.
An elliptical wing (such as the Spitfire) reduces the size of the wing tip vortices.
In most modern gliders a combination of wing taper and washout produces a similar effect, but only at one angle of attack.
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Both of these techniques therefore reduce vortex drag.

19. Vortex drag – aspect ratio
A wing's aspect ratio is the ratio of its span to mean chord.
A high aspect ratio wing is long and thin.
A low aspect ratio wing is short and wide.
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20. Vortex drag – aspect ratio
The pressure differential on a high aspect ratio wing is lower than a low aspect ratio wing.
Additionally the airflow is affected by the spanwise flow for a shorter time on a high aspect ratio wing due to the smaller chord length.
Both of these effects cause a high aspect ratio wing to have less vortex drag than a low aspect ratio wing.
Low pressure
Low pressure
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High pressure
High pressure

21. Vortex drag – speed
An increase in speed produces a smaller vortex because the air flowing under and over the wing has less time to be influenced by spanwise flow.
In fact Lift Dependent Drag is inversely proportional to the square of the speed.
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22. Vortex drag – weight In level flight: Lift = Weight.
Any increase in weight therefore requires an increase in lift.
If the speed is not increased, then the angle of attack must be increased to generate the extra lift.
This will produce a greater pressure differential between the upper and lower wing surfaces, and therefore stronger vortices.
An increase in weight therefore increases vortex drag.
Lift
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Weight

23. Increments of zero lift drag
Form drag
When an aircraft is pitched up to increase the angle of attack in order to produce more lift, a larger frontal area is presented to the airflow, therefore increasing form drag.
Surface friction drag
If the angle of attack is increased to produce more lift, the transition point in the boundary layer moves further forwards. As such the boundary layer is thicker over a larger part of the wing therefore increasing surface friction drag.
Interference drag
As lift is produced the air is deflected
spanwise. This spanwise flow increases
the interference drag where it interacts
with the airflow around the fuselage.
Transition point
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24. Total drag
The Total Drag of an aircraft is the sum of the Zero Lift Drag and Lift Dependent Drag of all its parts.
Total drag
Zero lift drag
Lift dependent drag
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Surface friction drag
Form drag
Interference drag
Increments of
surface friction drag, form drag and interference drag
Vortex drag

25. Total drag
Total drag is the sum of both zero lift drag and lift dependent drag.
As speed increases from stalling speed, Total Drag reduces due to the rapid reduction in lift dependent drag.
This continues until the minimum drag speed is reached.
As speed increases above the minimum drag speed Total drag increases due to the rapid increase in zero lift drag.
Drag varies with airspeed:
Zero lift drag varies as the square of the airspeed.
Lift dependent drag varies inversely as the square of the airspeed.
It can therefore be clearly seen that minimum drag occurs when zero lift drag and lift dependent drag are equal, and not at the lowest possible airspeed.
Stalling speed
Total drag
Drag
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Zero lift drag
Lift dependent drag
Speed for minimum drag
IAS

26. Drag equation
When an airflow is brought to rest in a tube facing into the airflow, the pressure exerted (drag) is given by the equation:
Drag = 1/2 ρV2S
Where:
ρ = Air density.
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V = Velocity.
S = Surface area.

27. Drag equation Drag = CD x (½ρV²S)
When a flat plate is placed in an airflow the air is not brought to rest, but flows around the edges causing turbulence to the rear of the plate.
This turbulence reduces the pressure behind the plate, creating a "suction".
Because of this suction the drag equation is altered to
Drag = 1.28 x (½ρV²S).
The figure 1.28 is a correction to the theoretical drag equation and is called the coefficient of drag (CD).
Each body will have its own coefficient of drag (CD) such as 1.28 for a flat plate.
Therefore the drag equation for any given body is:
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Drag = CD x (½ρV²S)

28. Drag equation
The CD for any given body will alter as the attitude of the body is changed.
The CD of an aerofoil therefore varies as the angle of attack changes.
Stalling angle
0.32
Normal angles
of flight
0.28
0.24
0.20
Drag Coefficient
0.16
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0.12
0.08
0.04
-4° 0° 4° 8°
12°
16°
20°
Angle of attack

29. THE END
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