The take-off can be split into three distinct parts: ¾ The take-off roll
¾ The transition to the climb
¾ The steady climb to a “screen height”
Screen Height The height of an imaginary screen which the aeroplane would just clear when taking off or landing in an unbanked attitude with landing gear extended.
This chapter looks at the general principles of the climb and the factors that need to be taken into account.
Aerodynamic Forces
The forces that affect the aircraft during the take-off are the same as those discussed in Chapter 2.
Thrust The engine thrust depends upon the type of engine being used: Jet Engine The net thrust attained is the gross thrust minus the momentum drag. As the aeroplane accelerates the momentum drag increases and the thrust will reduce. As the speed increases the momentum drag is countered by “ram effect” as the aeroplane accelerates. For a jet engined aeroplane during take-off:
Lift
Thrust
Drag
¾ There is an initial decrease in net thrust ¾ As the speed increases the net thrust increases
¾ The overall effect is that there is a decrease in thrust during take- off
Propeller For a fixed pitch propeller, the angle of attack decreases with forward speed and the thrust decreases. For a variable pitch propeller:
¾ Maximum rpm is selected which sets the propeller in fully fine pitch. The angle of attack will decrease with the initial increase in forward speed causing an initial thrust decrease
¾ As the speed increases the load on the propeller reduces and the constant speed unit will operate. The propeller pitch is increased to hold maximum rpm which reduces the rate at which the thrust decreases
Drag The drag that affects the aeroplane on take-off is a combination of: ¾ Aerodynamic drag which is dependent on:
¾ IAS
¾ Configuration
¾ The drag will increase for increased flap setting ¾ For a given flap setting the drag increases by IAS2 ¾ Angle of attack
¾ Wheel friction (drag) which is dependent on: ¾ The load on the wheel
¾ Initially the weight of the aeroplane ¾ The drag reducing as lift is produced ¾ The runway surface resistance
Once the aeroplane is rotated then the induced drag will increase due to the
increased angle of attack. Where an aeroplane is rotated earlier than VR then the
Variables That Affect the Take-Off
The following affect an aeroplane on take-off. Specific figures will be specified when the JAR Performance Class aeroplanes are discussed.
Weight Acceleration is calculated by the equation: Force = Mass x Acceleration
The greater the mass of an aeroplane the slower the acceleration and thus the longer the take-off run. The thrust of the aeroplane (force) is little changed so the greater mass must be offset by a decrease in the acceleration.
A secondary effect of increased weight is one of increasing the friction on the wheels which will also suppress the acceleration.
Increased weight:
¾
Requires an increase in the lift required to balance the forces during the take-off run. To increase the lift increased airspeed is required which will increase the take-off run.¾
The increase in weight means an increase in the aeroplane stallingspeed. For safety the aeroplane will use a higher lift-off speed which again increases the take-off run
Wind The affect of either a headwind or a tailwind can be quite marked on the take off distance.
Example A 30 knot headwind means that the aeroplanes relative speed is already 30 knots and the aeroplane has 30 knots less to achieve the lift off airspeed. Obviously this will decrease the take-off run
A 30 knot tailwind means that the aeroplane has to accelerate to 30 knots before the relative airspeed is zero. To achieve the lift off speed will require a longer take off run
The headwind and tailwind also have an affect on the climb gradient: ¾ A headwind increases the climb gradient
¾ A tailwind decreases the climb gradient
When calculating take-off distance the following is used to allow for variations in the wind:
¾ 150% of a tailwind is assumed
When the wind is across the runway both the lateral and directional control are affected. This crosswind leads to the publication of crosswind limitations for aeroplanes.
Runway Slope The slope of a runway affects the take-off distance:
Downslope The downslope will assist in the acceleration process and thus decrease the take-off distance
Upslope The upslope will counter the accelerating force and will cause an increase in the take-off distance
Density Density is affected by: ¾ Pressure
¾ Temperature ¾ Humidity
Density will decrease when there is an increase in the temperature and humidity and a decrease in the pressure (increase in altitude). Remember that pressure falls and temperature falls with an increase in altitude. This means that the density is decreased by the decrease in pressure but increased by the fall in temperature. Pressure is the dominant force so it is fair to say that with an increase in altitude the density of the air is reduced.
Any decrease in density will obviously decrease the power output of an engine. Any lowering of the density will increase the take-off run. When aeroplanes operate at high altitude airfields and high temperatures then limits on take-off weight may apply. The WAT limits are normally produced in the aeroplane manual to allow a pilot easy calculation of the limitations.
When the aeroplane lifts off the screen height has to be achieved which means that the aeroplane must have a minimum climb gradient. If the density reduces the power output then this will reduce the climb gradient. This may result in the take-off weight being reduced.
TAS The take-off speeds are determined by reference to:
¾ AUW
¾ Configuration ¾ Density
High temperature increases the TAS which also increases the groundspeed. Take-off calculations are with respect to EAS/CAS and to achieve these speeds with the
increased TAS means that the take-off run is increased. The initial climb is affected in a similar way; the lower density lowers the climb gradient.
Groundspeed When low density situations occur the take-off run is increased, this means that a higher groundspeed is achieved which could mean that the aeroplane reaches the tyre limiting speed before lift off. Under these circumstances the TOW may be limited.
Flap Setting The flap setting will affect the take-off run. The lower the flap angle the longer the take-off run required. The flap setting will also affect the climb gradient