- The following SKYbrary Articles:
Gain an understanding of:
- Centre of Gravity and influence on aircraft performance
Gravity is the downward force acting on all bodies vertically towards the centre of the Earth. The name given to the gravitational force is WEIGHT and for the principles of flight it is the total weight of the loaded aircraft. The magnitude of the weight force is determined by the aircraft’s mass. The greater the aircraft’s mass, the greater the weight force.
The weight force may be considered to act as a single force through the Centre of Gravity (CG). The CG is the point of balance and its position depends upon the mass and position of all the individual parts of the aircraft and the load that it is carrying. If the aircraft was suspended by a rope attached to its centre of gravity, the aircraft would balance.
Centre of Gravity
The Balance point (Centre of Gravity - CG) is very important during flight because of its effect on the stability and performance of the aircraft. It must remain within carefully defined limits at all stages of flight.
The CG will move if the distribution of the load changes, e.g. by passengers moving about or by transferring fuel from one tank to another. The CG may move as the weight changes by fuel burning off or by parachutists leaping out. The all-up weight always decreases as the flight progresses.
The designers of an aircraft have determined the maximum weight / maximum take-off mass (MTOM) or weight (MTOW), based on the amount of lift the wings or rotors can provide under the operating conditions for which the aircraft is designed. The structural strength of the aircraft also limits the maximum weight the aircraft can safely carry. The ideal location of the center of gravity (CG) is very carefully determined by the designers, and the maximum deviation allowed from this specific location calculated.
A useful means of describing the load that the wings carry in straight and level flight (when the lift from the wings supports the weight of the aeroplane) is the Wing Loading factor, which is simply the weight supported per unit area of wing.
Wing Loading = Weight of the Aircraft / Wing Area
The pilot in command of the aircraft has the responsibility on every flight to know the maximum allowable gross weight of the aircraft and its CG limits. This allows the pilot to determine, on the preflight inspection, that the aircraft is loaded in such a way that the CG is within the allowable limits.
Weight and balance is one of the most important factors affecting safety of flight. An overweight aircraft, or one whose center of gravity is outside the allowable limits, is not airworthy.
Weight and Balance Control
It is very important to have in mind that any excessive weight reduces the efficiency of an aircraft and the safety margin available if an emergency condition should arise.
When the weight of an aircraft is increased, the wings or rotors must produce additional lift and the structure must support not only the additional static loads, but also the dynamic loads imposed by flight manoeuvres. For example, the wings of a 3000 kg aeroplane must support 3000 kgs in level flight, but when the aeroplane is turned smoothly and sharply using a bank angle of 60°, the dynamic load requires the wings to support twice this, or 6000 kgs. Severe uncoordinated manoeuvres, or flight into turbulence, can impose dynamic loads on the structure great enough to cause failure.
Effects of Weight
Most modern aircraft are so designed that if all seats are occupied, all baggage allowed by the baggage compartment structure is carried, and all of the fuel tanks are full, the aircraft will be grossly overloaded. This type of design gives the pilot a great deal of latitude in loading the aircraft for a particular flight. If maximum range is required, occupants or baggage must be left behind, or if the maximum load must be carried, the range, dictated by the amount of fuel on board, must be reduced.
Effect of Weight on Stability and Controllability
Overloading effects stability. An aircraft that is stable and controllable when loaded normally may have very different flight characteristics when overloaded. Although the distribution of weight has the most direct effect on this, an increase in the aircraft’s gross weight may be expected to have an adverse effect on stability, regardless of location of the CG. The stability of many certificated aircraft is completely unsatisfactory if the gross weight is exceeded.
Effect of Load Distribution
The effect of the position of the CG on the load imposed on an aircraft’s wing in flight is significant to climb and cruising performance. An aircraft with forward loading is “heavier” and consequently, slower than the same aircraft with the CG further aft.
With forward loading, “nose-up” trim is required in most aircraft to maintain level cruising flight. Nose-up trim involves setting the tail surfaces to produce a greater down load on the aft portion of the fuselage, which adds to the wing loading and the total lift required from the wing if altitude is to be maintained. This requires a higher AOA (Angle of Attack) of the wing, which results in more drag and, in turn, produces a higher stalling speed.
The recovery from a stall in any aircraft becomes progressively more difficult as its CG moves aft. This is particularly important in spin recovery, as there is a point in rearward loading of any aircraft at which a “flat” spin develops. A flat spin is one in which centrifugal force, acting through a CG located well to the rear, pulls the tail of the aircraft out away from the axis of the spin, making it impossible to get the nose down and recover.
An aircraft loaded to the rear limit of its permissible CG range handles differently in turns and stall maneuvers and has different landing characteristics than when it is loaded near the forward limit.
The forward CG limit is determined by a number of considerations. As a safety measure, it is required that the trimming device, whether tab or adjustable stabilizer, be capable of holding the aircraft in a normal glide with the power off.
A tailwheel-type aircraft loaded excessively nose-heavy is difficult to taxi, particularly in high winds. It can be nosed over easily by use of the brakes, and it is difficult to land without bouncing since it tends to pitch down on the wheels as it is slowed down and flared for landing. Steering difficulties on the ground may occur in nosewheel-type aircraft, particularly during the landing roll and takeoff.
To summarize the effects of load distribution:
- The CG position influences the lift and AOA of the wing, the amount and direction of force on the tail, and the degree of deflection of the stabilizer needed to supply the proper tail force for equilibrium. The latter is very important because of its relationship to elevator control force.
- The aircraft stalls at a higher speed with a forward CG location. This is because the stalling AOA is reached at a higher speed due to increased wing loading.
- Higher elevator control forces normally exist with a forward CG location due to the increased stabilizer deflection required to balance the aircraft.
- The aircraft cruises faster with an aft CG location because of reduced drag. The drag is reduced because a smaller AOA and less downward deflection of the stabilizer are required to support the aircraft and overcome the nose-down pitching tendency.
- The aircraft becomes less stable as the CG is moved rearward. This is because when the CG is moved rearward it causes an increase in the AOA. Therefore, the wing contribution to the aircraft’s stability is now decreased, while the tail contribution is still stabilizing. When the point is reached that the wing and tail contributions balance, then neutral stability exists. Any CG movement further aft results in an unstable aircraft.
- A forward CG location increases the need for greater back elevator pressure. The elevator may no longer be able to oppose any increase in nose-down pitching. Adequate elevator control is needed to control the aircraft throughout the airspeed range down to the stall.
The pilot should always be aware of the consequences of overloading. An overloaded aircraft may not be able to leave the ground, or if it does become airborne, it may exhibit unexpected and unusually poor flight characteristics. If not properly loaded, the initial indication of poor performance usually takes place during takeoff.
Excessive weight reduces the flight performance in almost every respect.
The most important performance deficiencies of an overloaded aircraft are:
- Higher takeoff speed
- Longer takeoff run
- Reduced rate and angle of climb
- Lower maximum altitude
- Shorter range
- Reduced cruising speed
- Reduced maneuverability
- Higher stalling speed
- Higher approach and landing speed
- Longer landing roll
- Excessive weight on the nose wheel or tail wheel
The maximum allowable gross weight for an aircraft is determined by design considerations. The maximum gross weight may also be limited by the departure or arrival airport’s runway length. One important preflight consideration is the distribution of the load in the aircraft.
Loading an aircraft so the gross weight is less than the maximum allowable is not enough. This weight must be distributed to keep the CG within the limits specified.
If the CG is too far forward, passengers can be moved to rear seats or baggage can be shifted from a forward baggage compartment to a rear compartment. If the CG is too far aft, passenger weight or baggage can be shifted forward. The fuel load should be balanced laterally.
Weight and balance of a helicopter is far more critical than for an aeroplane. A helicopter may be properly loaded for takeoff, but near the end of a long flight when the fuel tanks are almost empty, the CG may have shifted enough for the helicopter to be out of balance laterally or longitudinally. Before making any long flight, the CG with the fuel available for landing must be checked to ensure it will be within the allowable range.
Stability and Balance Control
Balance control refers to the location of the CG of an aircraft. This is of primary importance to aircraft stability, which determines safety in flight.
The CG is the point at which the total weight of the aircraft is assumed to be concentrated, and the CG must be located within specific limits for safe flight.
Both lateral and longitudinal balance are important, but the prime concern is longitudinal balance; that is, the location of the CG along the longitudinal or lengthwise axis.
As long as the CG is maintained within the allowable limits for its weight, the airplane will have adequate longitudinal stability and control. If the CG is too far aft, it will be too near the center of lift and the airplane will be unstable, and difficult to recover from a stall. If the unstable airplane should ever enter a spin, the spin could become flat and recovery would be difficult or impossible.
A serious problem, caused by the CG being too far forward, is the lack of sufficient elevator authority. At slow takeoff speeds, the elevator might not produce enough nose up force to rotate and on landing there may not be enough elevator force to flare the airplane. Both takeoff and landing runs will be lengthened if the CG is too far forward.
The lateral balance can be upset by uneven fuel loading or burnoff. The position of the lateral CG is not normally computed for an airplane, but the pilot must be aware of the adverse effects that will result from a laterally unbalanced condition.
Helicopters are affected by lateral imbalance more than airplanes. If a helicopter is loaded with heavy occupants and fuel on the same side, it could be enough out of balance to make it unsafe to fly.
Q1: The position of the aircraft's centre of gravity depends on
- the aircraft's total mass
- the distribution of the aircraft's mass and load
- the aircraft's surface area
- the angle of attack
Q2: The position of the aircraft's centre of gravity
- must always be the same
- is limited to a specific range and the limits must not be exceeded
- has no effect on the aircraft's controllability
- is fixed during the flight
Q3: Which of the items listed below are performance deficiencies of an overloaded aircraft:
- Higher takeoff speed
- Longer take-off roll
- Reduced cruising speed
- Reduced manoeuvrability
- Higher stalling speed
- Higher approach and landing speed