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ALC-103: Helicopter - Weight & Balance, Performance
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Weight and Balance - Terms


It is vital to comply with weight and balance limits established for helicopters. Operating above the maximum weight limitation compromises the structural integrity of the helicopter and adversely affects performance. Balance is also critical because on some fully loaded helicopters, center of gravity deviations as small as three inches can dramatically change a helicopter’s handling characteristics. Taking off in a helicopter that is not within the weight and balance limitations is extremely unsafe.


 
The following terms are used when discussing and determining weight and balance:
 
 


When determining if a helicopter is within the weight limits, it is crucial to consider the weight of the basic helicopter, crew, passengers, cargo, and fuel. Although the effective weight (load factor) varies during maneuvering flight, this chapter primarily considers the weight of the loaded helicopter while at rest. The following terms are used when computing a helicopter’s weight.
 
  • Basic Empty Weight - The starting point for weight computations is the basic empty weight, which is the weight of the standard helicopter, optional equipment, unusable fuel, and full operating fluids including full engine oil. Some helicopters might use the term “licensed empty weight,” which is nearly the same as basic empty weight, except that it does not include full engine oil, only non-drainable oil. If you fly a helicopter that lists a licensed empty weight, be sure to add the weight of the oil to your computations.

  • Useful Load - The difference between the gross weight and the basic empty weight is referred to as useful load. It includes the flight crew, usable fuel, drainable oil, if applicable, and payload.

  • Payload - The weight of the passengers, cargo, and baggage.

  • Gross Weight - Total of the basic empty weight and useful load.

  • Maximum Gross Weight - Maximum allowable weight of the helicopter. Most helicopters have an internal maximum gross weight, which refers to the weight within the helicopter structure and an external maximum gross weight, which refers to the weight of the helicopter with an external load.

  • Weight Limitations - Weight limitations are necessary to guarantee the structural integrity of the helicopter, as well as enabling accurate determination of helicopter performance. Operating above a maximum weight could result in structural deformation or failure during flight if excessive load factors, strong wind gusts, or turbulence are encountered. Operating below a minimum weight could adversely affect the handling characteristics of the helicopter, including inability to achieve the desirable RPM during autorotation. During single-pilot operations in some helicopters, a large amount of forward cyclic may be required in order to maintain a hover. Adding ballast to the helicopter will allow the cyclic to be closer to center, which provides greater range of control motion in every direction. Additional weight also improves autorotation characteristics as the autorotation descent can be established sooner.

Although a helicopter is certificated for a specified maximum gross weight, it is not safe to take off with this load under all conditions. Anything that adversely affects takeoff, climb, hovering, and landing performance may require off-loading of fuel, passengers, or baggage to some weight less than the published maximum. Factors which can affect performance include high altitude, high temperature, and high humidity conditions, which result in a high density altitude.
  • Empty Weight - A helicopter’s weight and balance records contain essential data, including a complete list of all installed optional equipment. Use these records to determine the weight and balance condition of the empty helicopter. When a helicopter is delivered from the factory, the basic empty weight, empty weight center of gravity (CG), and useful load are recorded on a weight and balance data sheet included in the approved Rotorcraft Flight Manual. The basic empty weight can vary even in the same model of helicopter because of differences in installed equipment. If the owner or operator of a helicopter has equipment removed, replaced, or additional equipment installed, these changes must be reflected in the weight and balance records. In addition, major repairs or alterations must be recorded by a certified mechanic. When the revised weight and moment are recorded on a new form, the old record is marked with the word “superseded” and dated with the effective date of the new record. This makes it easy to determine which weight and balance form is the latest version. The latest weight and balance data must be used for computing all loading problems.

 


Helicopter performance is not only affected by gross weight, but also by the position of that weight. It is essential to load the aircraft within the allowable range specified in the rotorcraft flight manual’s weight and balance limitations. 
 


The center of gravity is defined as the theoretical point where all of the aircraft’s weight is considered to be concentrated. If a helicopter was suspended by a cable attached to the center-of-gravity point, it would balance like a teeter-totter. 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. The allowable range in which the CG may fall is called the “CG range.” The exact CG location and range are specified in the rotorcraft flight manual for each helicopter.
 
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. The ideal CG location would perfectly balance the 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. See figure 1-1.

 fig1-1.png

Figure 1-1: The locaton of the center of gravity strongly influences how the helicopter handles.
 


 

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 can be recognized when coming to a hover following a vertical takeoff. The helicopter will have a nose-low attitude, and excessive rearward displacement of the cyclic control is required to maintain a hover in a no-wind condition. Flight in this condition should not be attempted, since it is possible to rapidly run out of available rearward cyclic control travel as fuel is consumed. It may also be impossible to decelerate sufficiently to bring the helicopter to a stop, or flair sufficiently at the end of an autorotation. A forward CG will not be excessively noticeable 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.


 


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 and / or maximum allowable baggage 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.

The aft CG condition is recognizable when coming to a hover following a vertical takeoff. The helicopter will have a tail-low attitude, and excessive forward displacement of cyclic control is required to maintain a hover in a no-wind condition. If there is a wind, even greater forward cyclic will be required. If flight is continued in this condition, it 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, it might not be possible to lower the rotor disc, resulting in possible loss of control, or a rotor blade strike of the tail-boom.
 
 


 

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.

 


 Fig1-2.jpg
Balance is determined by the location of the CG, which is usually described as a given number of inches from the reference datum. The horizontal reference datum is an imaginary vertical plane or point, arbitrarily fixed somewhere along the longitudinal axis of the helicopter, from which all horizontal distances are measured for weight and balance purposes. There is no fixed rule for its location. It may be located at the rotor mast, the nose of the helicopter, or even at a point in space ahead of the helicopter. See figure 1-2.

The lateral reference datum is usually located at the center of the helicopter. The location of the reference datum is established by the manufacturer and is defined in the rotorcraft flight manual. See figure 1-3

 Fig1-3.jpg

 


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. If the component or object is located to the rear of the datum, it is measured as a positive number and usually is referred to as inches aft of the datum. Conversely, if the component or object is located forward of the datum, it is indicated as a negative number and is usually referred to as inches forward of the datum.

 


 

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.

 


 

By totaling the weights and moments of all components and objects carried, the point where a loaded helicopter will “balance” can be computed. This point is known as the center of gravity.