[ GRAPHS ] Jets and Propellers

1. Total Drag (Required Thrust)

The total drag of an aircraft consists of both induced and parasite drag.

- Parasite Drag

Any kind of drag, not related to lift generation
(Can be further subdivided into pressure (form) drag, skin friction drag and interference drag)

Parasite drag increases proportional to the speed squared (the faster the aircraft flies, the more parasite drag will be created)



- Induced Drag

Is drag related to lift generation. From the moment the aircraft generates lift, it generates induced drag.

At low airspeeds we need to generate a higher coefficient of lift to keep flying, this is done by increasing the angle of attack. The lift vector will bend more backwards and induced drag increases. The faster the aircraft flies, the lower the induced drag becomes.



- Total Drag

If we now add both together we get the Total Drag that an aircraft experiences while flying. To maintain straight and unaccelerated flight, the aircraft will need a thrust force that is equal to the total drag.

The total drag curve can therefore also be called the "thrust required" curve!

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2. The effect of changing variables on drag

With the graphs above as a visual tool we can now see what will happen in the following scenarios;

- Increase in mass

When mass increases, the aircraft needs more lift for a certain speed, this is achieved by increasing the angle of attack which thus increases the induced drag curve.

When the induced drag curve (green) moves upwards, the total drag curve (red) will move up and to the right.

- Change in configuration

When the aircraft changes its configuration (flaps, gear, etc) the parasite drag will increase which moves the parasite drag curve (orange) upwards. The result on the total drag curve (red) will be that it moves up and to the left.

- Change in altitude

When altitude increases, air density decreases. For a given IAS that means that the TAS must increase (to compensate for this reduction). All curves will shift to the right when related to TAS. In relation to IAS, there is no change. (When climbing with a constant IAS for example, drag will remain constant).

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3. Thrust and Power

Thrust is the actual FORCE which is delivered by the aircraft's engines
Power is the FORCE x SPEED or "THRUST x TAS"

- Jet aircraft

The maximum thrust output of a jet engine is constant for any TAS (the efficiency of the engine is independent of speed).



- Propeller aircraft

The maximum thrust output of a propeller engine however is dependent on TAS. This is because the efficiency of a propeller decreases with increasing airspeed.

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4. Thrust and Power graph for a JET

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5. Thrust and Power graph for a PROPELLER