Take-Off The take off part of a flight is the distance from the brake release point to the point at which the aircraft reaches a defined height over the surface (35ft). During the take off roll, lift is created on the wings to overcome the aircraft weight. This is done by forward acceleration of the aircraft produced by greater thrust force then drag.
4 Forces Acting On Aircraft During Take Off
The take off distance required depends on the interaction of forces: The thrust varies during take off, in general it decreases as aircraft speeds up. The total drag of the aircraft during take off results from aerodynamic drag and wheel drag. As the aircraft speeds up the aerodynamic drag will increase. The wheel drag depends on the load and the runway surface resistance. But as the aircraft speeds up the lift force increases, which reduces the load on the wheels and therefore reduces the wheel drag (eventually to zero). The lift force increases, as aircraft speeds up. The aircraft weight remains constant.
Take-Off Performance The more powerful the engine and the lighter the aircraft, the quickly the aircraft will accelerate and the shorter the take-off distance will be. Take-off Distance is the total length of the take-off run (or take-off roll) and the initial climb distance to 35ft. Take-off should not be attempted if the take-off distance available is less than the take-off distance required.
Actually, the factors that affect lift, weight, thrust and drag forces are the factors that affect aircraft performance during take off. Factors Affecting Take-off Performance 1. Aircraft’s Weight 2. Air Density 3. Wind 4. Runway Conditions 5. Aircraft Configurations a) Flap Setting b)Airframe Contamination
1. Aircraft Weight Weight limitations (MTOW) are set to ensure adequate margins of strength and performance during take-off. When loading the aircraft care must be taken not to exceed the limits given. The greater the weight, the greater the lift force is required to overcome the weight, therefore greater speed is necessary for take off (lift formula).
2. AIR DENSITY ↑ Air Density = ↓ Distance The greater the air density, the shorter the take-off distance required. Air density is mass of air per volume of air. Air density is influenced by temperature, humidity, airfield elevation & atmospheric pressure Lower the temperature, higher the air density. Lower the humidity, higher the air density. Lower the airfield elevation, higher the atmospheric pressure, thus higher the air density. 10
2. AIR DENSITY As air density is reduced (for example, with increasing altitude), take-off distance begins to increase quickly. In aircraft performance the term Density Altitude is used. Density Altitude is the International Standard Atmosphere (ISA) altitude with the same density as the existing pressure and temperature.
3. WIND The distance required for take off depends on the ground speed. While the lift and the drag during take off depend on air speed. ↑ Headwind = ↓ Distance 12
3. WIND Headwind is wind which is blowing in the opposite direction of the aircraft movement. Tailwind is wind which is blowing in the parallel direction of the aircraft movement. A headwind reduces the ground speed at a required take off air speed and reduces the take off distance. A tailwind increases the ground speed thus increases the take off distance. Crosswind (wind from left or right of the aircraft) component has no effect on the take off distance.
4. RUNWAY CONDITIONS Runway slope affects the rate of acceleration ↑ Slope = ↑ Take-Off Roll Runway surface conditions affect the wheel drag Standing water Snow Slush ↑ Friction = ↑ Take-Off Roll 14
4. RUNWAY CONDITIONS Any runway slope (a surface which one side is at a higher level than another) will affect take-off performance. A down slope allows the aircraft the aircraft to accelerate more quickly thus reducing the take off distance required. An up slope reduces the accelerating force thus increasing the take off distance. The runway surface condition has effect on the wheel drag. If the runway is contaminated by snow, slush or standing water, the wheel drag will be greater. Thus the accelerating force decreases and the take off distance required increases.
5. Aircraft Configurations 16 a) Flap Setting ↑ Flaps = ↓ Take-off Roll
a) Flap Setting Flap setting has an affect on the aerodynamic drag. Most aircraft use 10 to 15 degree flaps on take- off. Take off distance will decrease with increase of flap angle initially, but increasing the flap angle increases the drag, and so reduces the climb gradient for a given aircraft mass. If there are obstacles to be considered in the take off flight path, the flap setting that gives the shortest take off distance may not give the required climb gradient for obstacle clearance.
a) Flap Setting Flaps can give the aircraft extra Lift. The use of flaps during take-off is to reduce take-off roll distance. While during landing is to reduce landing speed and landing roll distance.
b)Airframe Contamination In addition if the airframe is contaminated by frost, ice or snow during take off the aircraft performance will be reduced, and the take off distance will be increased.
identify the maximum weight at take off from the performance chart in a given set of conditions.
Determine maximum weight at take off from the performance chart This action is performed before take off in order to confirm that the actual weight is below the maximum permissible take off weight at particular conditions. The determination of Maximum Weight are done by considering available runway length, temperature (air density), wind direction, runway slope, flap setting, and airfield elevation.
22 Determine the MAXIMUM TAKE-OFF WEIGHT (MTOW) Given the conditions determine the maximum permissible take off weight: - Elevation 4000 feet - Temperature 32 degC - RWY 3000m dry slope 1% up - Wind 20kt head component - Flap setting 15
Steps to Read the Chart Step 1: From the bottom of the chart, vertical line is drawn at a corresponding runway length (in this example 3000m) until the reference line for the runway slope adjustments. Step 2: Then the line follows the closest trend line for the runway slope until it reaches the corresponding value (in this case 1% up). Step 3: From that position vertical is drawn to the intersection with the wind reference line. Then the line follows the closest trend line for wind adjustment until it reaches the value for the wind (20 kt headwind).
Steps to Read the Chart Step 4: Further on, same principle is used. A vertical line is drawn from that position to the flap reference line. Then the line follows the trend lines until the corresponding value (15 degrees). Step 5: From that position on the chart the vertical line is drawn until the intersection with the second line taking into account airfield elevation and temperature, which starts from the bottom left at the corresponding air temperature (32 C).
Steps to Read the Chart Step 5: The line is drawn until the intersection with the elevation line (4000ft). Then the line continues horizontally to the intersection with the reference line and from there it follows the closest trend line to the intersection with the first correction line. The intersection is the maximum permissible weight at take off ( kg).
28 Given the conditions determine the maximum permissible take off weight: - Elevation 4000 feet - Temperature 32 degC - RWY 2500m dry slope 2% down - Wind 10kt tail component - Flap setting kg Determine the MAXIMUM TAKE-OFF WEIGHT (MTOW)