FORCES DURING TAKE OFF

During the performance subject of the private Pilot and the Airline Transport Pilot License, we study what helps us during the manouver of take off. For understanding it with more detail I suggest to review this post (Aircraft take off forces), as we are going to use the concepts explained on it.

How does this forces vary during take off?

We consider an static take off which consist on releasing the brakes once we have got the maximum power. In the table just below we can see a summary of how the forces are changing during the take off run

Factors affecting the take off run

All factors affecting the take of run are those which affects the acceleration (review the third newton law).

Weight of the aircraft (W): As heavier the aircraft is, more speed we need for taking off ( L=1/2 d v ̂2 s Cl). Also the acceleration factor decreases. Resulting in an increased runway .

An increased of 10% of the weight leads on an increased of 5% of the speed.

Runway Slope: The runway slope makes that one component of weight sum up the thrust or rest it, depends on if it is downhill ( It is better for the acceleration factor , so we need less runway) or uphill (we need more runway)

Density Altitude (DA): ( review of : density altitude factors ) as the density altitude increases the acceleration factor decreases, because the engine works worse. So we will need more runway.

There is a less important factor, the take off speed is based to the air speed (TAS, true air speed) which depends on the density of the air . as lower it is the DA the greater the TAS is. So a certain IAS speed ( the speed we have on the anemometer) the TAS is greater.

Wind: As we saw above the aircrafts allways takes off at certain TAS ( speed based on the air). So as greater the head wind is the lower the ground speed is ( GS, Speed related to the ground), so when we have have head wind (HW) we use less runway lenght . Whith Tail wind (TW) we use more runway lenght.

note : Tail wind is a limitation, however cross wind is certified ( so it is a demostrated speed).

For performance calculations we consider the 50% HW and 150% of TW

Flap Setting : The flap setting affects the Take off speed , due to the lift depends on the wing surface , but unfortunately also the drag . So depending on the other factors such us the runway lenght , the obstacles during the climb phase ….. we have to choose one flap setting or other.

Contamination of the runway : That part affects the friction force. It is not the same taking off from a new runway , that taking off from old and wet runway. Of course on the second situation you need more runway lenght. We know the friction factor because ATC tells us.

In summary, the conditions that leads to an increase of the runway lenght are : HIGH EXTERIOR TEMPERATURE, HIGH AERODROME ELEVATION, LOW ALTIMETER ADJUSTMENT, MAXIMUM TAKE OFF WEIGHT AND TAIL WIND.