Thrust and Drag

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Principles of Flight Thrust and Drag

Principles

of Flight

Thrust and Drag


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Principles of Flight So Far

You already know that:

Every action has an equal and oppositereaction

Every object has weight To generate lift, you need airflow over the

wings.

Lift varies with airspeed, angle of attack,

wing shape and air density For straight and level flight, lift and weight

must be equal


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Aim of the Lesson

By the end of the lesson, you should havelearnt:

What thrust and drag are, and how they

are produced How to minimise drag

How drag varies with the airspeed

How the levels of thrust and drag affect an

aircrafts flight What is necessary for an aircraft to be in

balance


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An Aircraft In Flight

Besides lift and weight, there are twoother forces that act on an aircraft in flight

These are thrust and drag


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Thrust

The airflow over the wings that is neededto generate lift is caused by thrusting theaircraft forwards through the air.

Thrust is therefore the forward forceacting on an aircraft

It is produced by the engines by throwingair backwards

Either a propeller can do this, or air canbe expelled from the rear (in the case of a

jet engine)


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Thrust

In both cases, throwing air backwardsthrusts the aircraft forces

This is because every action has an equal

and opposite reaction The size of the thrust depends on the

amount of engine power selected by thepilot


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Drag

When youre on a bicycle, the faster yougo the more resistance you encounter

The force which hinders your progress is

called drag The same can be said for aircraft.

Every part of the aircraft over which airflows produces drag which resists forward

motion Thus the more drag there is, the more

thrust is needed to over come it


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Drag

However to get more thrust you need abigger engine, more fuel and more weight,and therefore more expense

The aircrafts designer wants to make theaircraft fly at the best possible speed forthe available thrust

Therefore he will try and reduce drag

This makes the aircraft more efficient andeconomical


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Causes of Drag

The shape of the aircraft causes a greatdeal of drag

When any object moves through the air it

is accompanied by a wake of complicatededdies and vortices

In flight, engine power which should beused for forward power is wasted inmaking these vortices


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Drag

Here is an extremeexample: a flatplate in an airflow

The drag isgreatest at 90 tothe airflow

The drag is leastwhen it is parallelto the airflow


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Minimising Drag

Drag can be minimised by:

This means smoothing all the parts of theaircraft which are in the airflow, thus makingthe air flow as smoothly as possible.

It also means minimising the number ofprotruding external parts.


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How much drag would this object produce?

Minimising Drag

A flat circular plate

Answer = Maximum Drag

i i l f li h h d


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Minimising Drag

A ball of the same diameter as the plate

Answer = 50% Drag (less!)

P i i l f Fli h Th d D


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Minimising Drag

A streamlined shape (same diameter)

Answer = 5% Drag (a lot less!)

P i i l f Fli ht Th t d D


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Minimising Drag

An effective way to streamline shapes is tohave a fineness ratio (length compared tobreadth) of 4(or 3):1

length four units

widthone unit

P i i l f Fli ht Th t d D


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Variation of Drag with Airspeed

The amount of drag is proportional to thesquare of the airspeed

That is, when an aircraft is going at twice

the airspeed there is four times as muchdrag.

So, when an aircraft is going at threetimes the airspeed, how many timesmore drag will there be?

Answer = 9 times as much drag

P i i le of Fli ht Th t d D


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Thrust and Drag

in Straight and Level Flight

You have already learnt that for straight andlevel flight, lift must equal weight.

Now we must learn that for the aircraft totravel at a constant speed, the thrust must beequal to the drag.



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Thrust and Drag

in Straight and Level Flight

When thrust = drag, the aircraft travels aconstant speed.

thrust drag

When thrust is greater than drag, the aircraftaccelerates.

When drag is greater than thrust, the aircraft

slows down.



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The Aircraft in Balance

thrust drag

lift

weight

When thrust=drag and lift=weight, the aircraftflies straight and level at a constantspeed.

To summarise:

The aircraft is in balance.



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Questions



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Documents

Thrust and Drag