Lecture

Leading Cadet Training

Principles of Flight

3Drag

Every part of the aircraft over which air flowsproduces drag.

This drag directly opposes thrust.

The more drag there is,the more thrust is needed to overcome it.

DRAGTHRUST

Drag

Anyone on a bicycle knows the faster you go, the more air resistance you encounter.

The force which hinders your progress

is called ‘DRAG’.

Drag

Drag will be least when parallel to the airflow.

What causes Drag ?

Drag is formed by the shape of an object.

When holding a flat plate in an airflow: Drag will be greatest when the plate is at 90°

The object is to reduce this Form Drag.

Drag is formed by the shape of an object.

When an object passes through the air

it is accompanied by a wake.

Wake is the turbulent airflow

immediately behind the aircraft.

Wake produces drag –

the bigger the wake,

the bigger the drag.

Drag

Surface Friction Drag Is caused by the ‘rough’ skinof an object.

Interference DragTwo surfaces meeting at an angle, causes airflow drag.

This is minimized by fairing and filleting to inducesmooth mixing of air past the affected surfaces.

Types of Drag

Drag

Lift Dependent DragIs drag caused by the lift associated with the changein the angle of attack.

Most of the drag caused at take-off is lift-dependent.

Wingtip VorticesAre tubes of circulating air behinda wing as it generates lift.

A wingtip vortex trails from the tipof each wing.

Relatively High Pressure

Low PressureLow Pressure

VORTEX

Types of Drag

Drag

Zero Lift Drag

Streamlined Shape5lbs drag

Form Drag(The Shape) Flat Plate

100lbs drag

Smooth - 4lbs drag

Surface Friction Drag(Smooth/Rough)

Rough - 6lbs drag

Interference Drag(Components)

5lbs drag

11lbs drag

Drag

Wingtip Vortices

LOW SPEED HIGH SPEED

The effects of Speed on Wingtip Vortices.

LARGEVORTEX

SMALLVORTEX

Drag

Wingtip Vortices

HIGH ASPECT LOW ASPECT

The effects of Aspect Ratio on Wingtip Vortices.

LARGEVORTEX

SMALLVORTEX

CHORD CHORD

Drag

How can Drag be minimised ?

By not building in drag at the design stage.

By eliminating as many protruding edges as possible….

By ‘streamlining’ the design.

Drag

How can Drag be minimised ?

Drag

By designing the shape to have a fineness ratioof between 3:1 and 4:1

The fineness ratio is the ratio of the length of a body to its maximum width;

"short and fat" shapes have low ratios, while "long and skinny" have high ratios.

DragDrag and Airspeed

The amount of drag varies with the square of the airspeed -

Twice the airspeed = 4 times the drag.

3 times the airspeed = 9 times the drag.

AIRSPEED DRAG

Drag and Thrust

THRUST DRAG

In un-accelerated straight and level flight:

The aircraft will maintain speed.

Thrust = Drag

If thrust is greater than drag

the aircraft will speed up.

Thrust > Drag

If drag is greater than thrust

the aircraft will slow down.

Drag > Thrust

Drag

An Aircraft in Balance

In straight and level flight, at constant speed, two pairs of forces act on the aircraft.

The thrust opposes the drag and is equal to it,

The lift equally opposes the weight.

WEIGHT

LIFT

DRAGTHRUST

Summary

Check of UnderstandingWhich part(s) of an aircraft produce drag?

Only those parts producing thrust

Only those parts producing lift

Every part that air flows over

The fuselage but not the wings

Check of UnderstandingDrag directly opposes what?

Thrust

Pressure

Lift

Weight

Check of UnderstandingWhich of these objects of equal width,

will produce the least drag?

Airflow

D

A

C

B

Check of UnderstandingHow can drag best be minimised?

By decreasing the airspeed

By increasing the airspeed

By reducing the weight

By streamlining the design

Check of UnderstandingWhat type of drag is formed

when two surfaces meet at a sharp angle?

Interference Drag

Surface Friction Drag

Wingtip Vortex Drag

Lift Dependant Drag

Check of UnderstandingWhen an object passes through the air,

what is it accompanied by?

Wake

Wind

Thrust

Lift

Check of UnderstandingWhat type of drag is dependant upon

the rate of change of velocity through the boundary layer?

Wing Vortex Drag

Interference Drag

Surface Friction Drag

Lift Dependant Drag

Check of UnderstandingWhat is Lift Dependent Drag?

Drag caused by the wingtip vortex

Drag caused by the shape of the object

Drag caused by lift associated withthe change in the angle of attack

Drag caused by the ‘rough’ skin of an object

Check of UnderstandingA streamlined shape with an airspeed of 100 kts

has a drag of 200 n.If the airspeed is increaded to 300 kts

what will the drag be?

3600 n

400 n

1800 n

800 n

Principles of Flight

End of Presentation

Leading Cadet Training