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Encyclopedia > Center of gravity (aircraft)

The center-of-gravity (CG) is the point at which an aircraft would balance if it were possible to suspend it at that point. It is the mass center of the aircraft, or the theoretical point at which the entire weight of the aircraft is assumed to be concentrated. [1] Its distance from the reference datum is determined by dividing the total moment by the total weight of the aircraft.[2] The center-of-gravity is an important point on an aircraft, which significantly affects the stability of the aircraft. To ensure the aircraft is safe to fly, it is critical that the center-of-gravity fall within specified limits.

Contents

Terms

  • Ballast
Removable or permanently installed weight in an aircraft used to bring the center of gravity into the allowable range.
  • CG Limits
The specified longitudinal (forward and aft) or lateral (left and right) points within which the CG must be located during flight. These limits are indicated in the aircraft operator manuals.
  • CG Range
The distance between the forward and aft (or left and right) CG limits indicated in the aircraft operator manuals.
  • Weight and Balance
The aircraft is said to be in weight and balance when the gross weight of the aircraft is under the max gross weight, and the center of gravity is within limits and will remain in limits for the duration of the flight.
  • Reference Datum
Balance is determined by the location of the CG, which is usually described as a given number of inches from the reference datum.
  • Arm
The horizontal distance from the datum to any component of the helicopter or to any object located within the helicopter is called the arm. Another term that can be used interchangeably with arm is station.
  • Moment
If the weight of an object is multiplied by its arm, the result is known as its moment. You may think of moment as a force that results from an object’s weight acting at a distance. Moment is also referred to as the tendency of an object to rotate or pivot about a point. The farther an object is from a pivotal point, the greater its force.
  • Center-of-gravity Computation
By totaling the weights and moments of all components and objects carried, you can determine the point where a loaded aircraft would balance. This point is known as the center-of-gravity.

Helicopters

Weight and balance of a helicopter is far more critical than for an airplane. 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.[2] For helicopters with a single main rotor, the CG is usually close to the main rotor mast. Improper balance of a helicopter’s load can result in serious control problems. In addition to making a helicopter difficult to control, an out-of-balance loading condition also decreases maneuverability since cyclic control is less effective in the direction opposite to the CG location.


Ideally, the pilot tries to perfectly balance a helicopter so that the fuselage remains horizontal in hovering flight, with no cyclic pitch control needed except for wind correction. Since the fuselage acts as a pendulum suspended from the rotor, changing the center of gravity changes the angle at which the aircraft hangs from the rotor. When the center of gravity is directly under the rotor mast, the helicopter hangs horizontal; if the CG is too far forward of the mast, the helicopter hangs with its nose tilted down; if the CG is too far aft of the mast, the nose tilts up.


CG Forward of Forward Limit

A forward CG may occur when a heavy pilot and passenger take off without baggage or proper ballast located aft of the rotor mast. This situation becomes worse if the fuel tanks are located aft of the rotor mast because as fuel burns the weight located aft of the rotor mast becomes less.


This condition is recognizable when coming to a hover following a vertical takeoff. The helicopter will have a nose-low attitude, and the pilot will need excessive rearward displacement of the cyclic control to maintain a hover in a no-wind condition. In this condition, the pilot could rapidly run out of rearward cyclic control as the helicopter consumes fuel. The pilot may also find it impossible to decelerate sufficiently to bring the helicopter to a stop. In the event of engine failure and the resulting autorotation, the pilot may not have enough cyclic control to flare properly for the landing.


A forward CG will not be as obvious when hovering into a strong wind, since less rearward cyclic displacement is required than when hovering with no wind. When determining whether a critical balance condition exists, it is essential to consider the wind velocity and its relation to the rearward displacement of the cyclic control.


CG Aft of Aft Limit

Without proper ballast in the cockpit, exceeding the aft CG may occur when:

  • A lightweight pilot takes off solo with a full load of fuel located aft of the rotor mast.
  • A lightweight pilot takes off with maximum baggage allowed in a baggage compartment located aft of the rotor mast.
  • A lightweight pilot takes off with a combination of baggage and substantial fuel where both are aft of the rotor mast.

An aft CG condition can be recognized by the pilot when coming to a hover following a vertical takeoff. The helicopter will have a tail-low attitude, and the pilot will need excessive forward displacement of cyclic control to maintain a hover in a no-wind condition. If there is a wind, the pilot needs even greater forward cyclic. If flight is continued in this condition, the pilot may find it impossible to fly in the upper allowable airspeed range due to inadequate forward cyclic authority to maintain a nose-low attitude. In addition, with an extreme aft CG, gusty or rough air could accelerate the helicopter to a speed faster than that produced with full forward cyclic control. In this case, dissymmetry of lift and blade flapping could cause the rotor disc to tilt aft. With full forward cyclic control already applied, you might not be able to lower the rotor disc, resulting in possible loss of control, or the rotor blades striking the tailboom.


Lateral balance

For most helicopters, it is usually not necessary to determine the lateral CG for normal flight instruction and passenger flights. This is because helicopter cabins are relatively narrow and most optional equipment is located near the center line. However, some helicopter manuals specify the seat from which you must conduct solo flight. In addition, if there is an unusual situation, such as a heavy pilot and a full load of fuel on one side of the helicopter, which could affect the lateral CG, its position should be checked against the CG envelope. If carrying external loads in a position that requires large lateral cyclic control displacement to maintain level flight, fore and aft cyclic effectiveness could be dramatically limited.


Weight and Balance Calculations

When determining whether an aircraft is properly loaded, the pilot must answer two questions:

  1. Is the gross weight less than or equal to the maximum allowable gross weight?
  2. Is the center of gravity within the allowable CG range, and will it stay within the allowable range as fuel is burned off?

To answer the first question, just add the weight of the items comprising the useful load (pilot, passengers, fuel, oil, if applicable, cargo, and baggage) to the basic empty weight of the aircraft. Check that the total weight does not exceed the maximum allowable gross weight.


To answer the second question, the pilot needs to use CG or moment information from loading charts, tables, or graphs in the manual. Then calculate the loaded moment and/or loaded CG and verify that it falls within the allowable CG range, also shown in the manual.


The location of the reference datums is established by the manufacturer and is defined in the aircraft flight manual. The horizontal reference datum is an imaginary vertical plane or point, arbitrarily fixed somewhere along the longitudinal axis of the aircraft, from which all horizontal distances are measured for weight and balance purposes. There is no fixed rule for its location. For helicopters, it may be located at the rotor mast, the nose of the helicopter, or even at a point in space ahead of the helicopter. While the horizontal reference datum can be anywhere the manufacturer chooses, most small training helicopters have the horizontal reference datum 100 inches forward of the main rotor shaft centerline. This is to keep all the computed values positive. The lateral reference datum, is usually located at the center of the helicopter.[3]


References

  1. ^ Aircraft Flying Handbook. Federal Aviation Administration (2004).
  2. ^ a b Aircraft Weight and Balance Handbook. Federal Aviation Administration (1999).
  3. ^ Rotorcraft Flying Handbook. Federal Aviation Administration (2000).

See Also

List of aviation topics 0–9 $100 hamburger A Accidents and incidents in aviation ADF Aerobatics Aerodynamics Aeronautical Information Manual (AIM) Aeronautics Aileron Airband Aircraft Aircraft engine controls Aircraft Maintenance Technician (AMT) Aircraft Maintenance Engineer (AME) Airfield Airfield traffic zone (ATZ, and MATZ) Airfoil Airline Transport Pilot License Air navigation Airport Airship Airspace classes...


 
 

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