Feb 27, 2019

Passenger Plane In FlightWhat makes an aeroplane climb? During flight there are four main forces at play on an aircraft, lift, weight, thrust, and drag. These forces constantly impact the movement and orientation of the aircraft in the air, but are particularly important when an aircraft climbs or descends. To understand the forces at play when an aircraft climbs and descends, first lets briefly define the four main forces acting on that aircraft when it moves through the air.

  • Weight. This is the force pulling the aircraft in a downward direction towards the earth.
  • Lift. This is the opposing force needed to overcome the burden of weight, in simple terms lift is generated by the combination of the shape of the aircraft’s wings and the forward motion of the aircraft.
  • Drag. This is resistance the aircraft is faced with when moving through the air.
  • Thrust. This is the propulsion required to overcome drag, put simply this is created by the engine and its propulsion systems (propeller etc.).

When an aircraft is in normal level flight the thrust force must equal drag and weight must be equal to lift – placing the aircraft in a state of equilibrium. This does not however mean the four forces are equal simply that the opposing forces are equal to the effects of each other (effectively cancelling each other out). According to the US Federal Aviation Administration (FAA) this can be defined as:

  • The sum of all upward components of forces (not only lift) equals the sum of all downward components of forces (not only weight) and;
  • The sum of all forward components of forces (not only thrust) equals the sum of all backward components of forces (not only drag).

Forces In A Climb

View Of Wing & TurbineWhen the aircraft begins climbing or descending it is necessary for the ratio of these forces to be temporarily altered as more/less thrust is required. Climbing requires an increase in the thrust to offset the increased drag associated with the increased angle of the aircraft (as the aircraft inclines upward a portion of the weight force acts in the same direction as the drag).

The thrust required for a climb will equal drag plus a percentage of the weight – this is dependent on the angle of climb. For example; a climb with an angle of 10° requires thrust equal to drag plus 17 percent of the weight. So it follows that theoretically if you were able to climb straight up, this would require thrust to equal all of the weight and drag.

If thrust is not increased this will result in a loss of airspeed, the amount of thrust depends on the angle of climb (how steep you are climbing). Note that aircraft are able to sustain a climb due to excess thrust. When the excess thrust is gone, the aircraft is no longer able to climb. At this point, the aircraft has reached its limits.

4 Things That Effect Climb Performance

  1. Weight. The greater the weight, the lesser the rate of climb and potential angle of climb.
  2. Flaps. Increase lift but also drag, any increase in drag will reduce the rate and angle of climb.
  3. Power. The more power (engine capacity) available, the better the climb performance.
  4. Wind. This can affect the climb angle and the distance traveled (over ground) required to reach the desired altitude.

Forces In A Descent

Aeroplane Above CityThe forces that act on an aircraft also alter when descending, in order to descend at the same speed as you were in level flight the thrust must decrease. Descent is possible without dipping the nose of the aircraft, effectively ‘sinking’ as long as the equilibrium between the thrust and weight, which are causing a forward motion are balanced by the lift and drag (both acting in a rearward direction).

Dropping the nose of the aircraft creates an angle of descent (AoD), resulting in a more rapid descent. The weight force increases as the angle of descent increases resulting in a shift in weight, which means adjustments must be made to the lift and drag ratio.

The rate and angle of descent will depend on the required altitude at the end of the descent, and the distance you have to complete the manoeuvre.

4 Things That Effect Descent Performance

  1. Weight. A change in weight will affect the rate of descent.
  2. Flaps. The increased drag produced by the flap will increase the descent angle thereby increasing the rate of descent.
  3. Power. The amount of power applied controls the rate of descent (RoD), reducing the descent angle and increasing the distance traveled over the ground.
  4. Wind. Affects the descent angle and the distance traveled over ground from a given altitude.

For more detailed information of how lift, weight, drag and thrust affect an aircraft during climb and descent it is best to speak to a professional flight training institute.

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