Chapter 6 Weight And Balance

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Chapter 6Weight and BalanceIntroductionIt is vital to comply with weight and balance limitsestablished for helicopters. Operating above the maximumweight limitation compromises the structural integrity ofthe helicopter and adversely affects performance. Balanceis also critical because, on some fully loaded helicopters,center of gravity (CG) deviations as small as three inches candramatically change a helicopter’s handling characteristics.Operating a helicopter that is not within the weight andbalance limitations is unsafe. Refer to FAA-H-8083-1 (asrevised), Aircraft Weight and Balance Handbook, for moredetailed information.6-1

WeightWhen determining if a helicopter is within the weight limits,consider the weight of the basic helicopter, crew, passengers,cargo, and fuel. Although the effective weight (load factor)varies during maneuvering flight, this chapter primarilyaddresses the weight of the loaded helicopter while at rest.It is critical to understand that the maximum allowable weightmay change during the flight. When operations include out ofground effect (OGE) hovers and confined areas, planning mustbe done to ensure that the helicopter is capable of lifting theweight during all phases of flight. The weight may be acceptableduring the early morning hours, but as the density altitudeincreases during the day, the maximum allowable weight mayhave to be reduced to keep the helicopter within its capability.The following terms are used when computing a helicopter’sweight: Basic Empty Weight Maximum Gross Weight Weight LimitationsBasic Empty WeightThe starting point for weight computations is the basic emptyweight. This is the weight of the standard helicopter, optionalequipment, unusable fuel, and all operating fluids includingengine and transmission oil, and hydraulic fluid for thoseaircraft so equipped. Some helicopters might use the term“licensed empty weight,” which is nearly the same as basicempty weight, except that it does not include full engine andtransmission oil, just undrainable oil. If flying a helicopterthat lists a licensed empty weight, be sure to add the weightof the oil to the computations.Maximum Gross WeightThe maximum weight of the helicopter is referred to itsmaximum gross weight. Most helicopters have an internalmaximum gross weight, which refers to the weight within thehelicopter structure and an external maximum gross weight,which refers to the weight of the helicopter with an externalload. The external maximum weight may vary dependingon where it is attached to the helicopter. Some large cargohelicopters may have several attachment points for sling loador winch operations. These helicopters can carry a tremendousamount of weight when the attachment point is directly underthe CG of the aircraft.Weight LimitationsWeight limits are necessary to guarantee the structuralintegrity of the helicopter, enable pilots to predict helicopterperformance and insure aircraft controllability. Althoughaircraft manufacturers build in safety factors, a pilot should6-2never intentionally exceed the load limits for which ahelicopter is certificated.Operating below a minimum weight could adversely affectthe handling characteristics of the helicopter. During singlepilot operations in some helicopters, a pilot needs to use alarge amount of forward cyclic to maintain a hover. By addingballast to the helicopter, the neutral cyclic position can beshifted toward the center of its range, thus giving a greaterrange of control outward from neutral in every direction.When operating at or below the minimum weight of thehelicopter, additional weight also improves autorotationalcharacteristics since the autorotational descent can beestablished sooner. In addition, operating below minimumweight could prevent achieving the desirable rotor revolutionsper minute (rpm) during autorotations.Operating above a maximum weight could result instructural deformation or failure during flight if encounteringexcessive load factors, strong wind gusts, or turbulence.Weight and maneuvering limitations also are factors inestablishing fatigue life of components. Overweight, meaningoverstressed, parts fail sooner than anticipated. Therefore,premature failure is a major consideration in determinationof fatigue life and life cycles of parts.Although a helicopter is certificated for a specified maximumgross weight, it is not safe to take off with this load undersome conditions. Anything that adversely affects takeoff,climb, hovering, and landing performance may requireoff-loading of fuel, passengers, or baggage to some weightless than the published maximum. Factors that can affectperformance include high altitude, high temperature, and highhumidity conditions, which result in a high-density altitude.In-depth performance planning is critical when operating inthese conditions.BalanceHelicopter performance is not only affected by gross weight,but also by the position of that weight. It is essential to load theaircraft within the allowable CG range specified in the rotorcraftflight manual’s (RFM) weight and balance limitations. Loadingoutside approved limits can result in insufficient control travelfor safe operation.Center of GravityThe pilot should ensure that the helicopter is properly balancedand within its center of gravity limitations, so that minimalcyclic input is required during hovering flight, except forany wind corrections. Since the fuselage acts as a pendulumsuspended from the rotor, changing the CG changes the angleat which the aircraft hangs from the rotor. When the CG isdirectly under the rotor mast, the helicopter hangs horizontally;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. [Figure 6-1]CG Forward of Forward LimitA forward CG may occur when a heavy pilot and passengertake off without baggage or proper ballast located aft of therotor mast. This situation becomes worse if the fuel tanksare located aft of the rotor mast because as fuel burns theCG continues to shift forward.This condition is easily recognized when coming to a hoverfollowing a vertical takeoff. The helicopter has a nose-lowattitude, and excessive rearward displacement of the cycliccontrol is needed to maintain a hover in a no-wind condition.Do not continue flight in this condition, since a pilot couldrapidly lose rearward cyclic control as fuel is consumed. Apilot may also find it impossible to decelerate sufficiently tobring the helicopter to a stop. In the event of engine failureand the resulting autorotation, there may not be enough cycliccontrol to flare properly for the landing.A forward CG is not as obvious when hovering into a strongwind, since less rearward cyclic displacement is required thanwhen hovering with no wind. When determining whether acritical balance condition exists, it is essential to consider thewind velocity and its relation to the rearward displacementof the cyclic control.CG Aft of Aft LimitWithout proper ballast in the cockpit, exceeding the aft CGmay occur when: A lightweight pilot takes off solo with a full load offuel located aft of the rotor mast. A lightweight pilot takes off with maximum baggageallowed in a baggage compartment located aft of therotor mast. A lightweight pilot takes off with a combination ofbaggage and substantial fuel where both are aft of therotor mast.CG Directly Under The Rotor MastCGA pilot can recognize the aft CG condition when comingto a hover following a vertical takeoff. The helicopter willhave a tail-low attitude and will need excessive forwarddisplacement of cyclic control to maintain a hover in a nowind condition. When facing upwind, even greater forwardcyclic is needed.If flight is continued in this condition, it may be impossibleto fly in the upper allowable airspeed range due to inadequateforward cyclic authority to maintain a nose-low attitude. Inaddition, with an extreme aft CG, gusty or rough air couldaccelerate the helicopter to a speed faster than that producedwith full forward cyclic control. In this case, dissymmetry oflift and blade flapping could cause the rotor disk to tilt aft.With full forward cyclic control already applied, a pilot mightnot be able to lower the rotor disk, resulting in possible lossof control, or the rotor blades striking the tailboom.Lateral BalanceFor smaller helicopters, it is generally unnecessary todetermine the lateral CG for normal flight instruction andpassenger flights. This is because helicopter cabins arerelatively narrow and most optional equipment is locatednear the centerline. However, some helicopter manualsspecify the seat from which a pilot must conduct solo flight.In addition, if there is an unusual situation that could affectthe lateral CG, such as a heavy pilot and a full load of fuelon one side of the helicopter, its position should be checkedagainst the CG envelope. If carrying external loads in aposition that requires large lateral cyclic control displacementto maintain level flight, fore and aft cyclic effectiveness couldbe limited dramatically. Manufacturers generally accountfor known lateral CG displacements by locating externalattachment points opposite the lateral imbalance. Examplesare placement of hoist systems attached to the side, and wingstores commonly used on military aircraft for external fuelpods or armament systems.Forward CGCGAft CGCGFigure 6-1. The location of the CG strongly influences how the helicopter handles.6-3

Weight and Balance CalculationsWhen determining whether a helicopter is properly loaded,two questions must be answered:1.Is the gross weight less than or equal to the maximumallowable gross weight?2.Is the CG within the allowable CG range, and willit stay within the allowable range throughout theduration of flight including all loading configurationsthat may be encountered?To answer the first question, just add the weight of the itemscomprising the useful load (pilot, passengers, fuel, oil [ifapplicable] cargo, and baggage) to the basic empty weight ofthe helicopter. Ensure that the total weight does not exceedthe maximum allowable gross weight.purposes. There is no fixed rule for its location. It may belocated at the rotor mast, the nose of the helicopter, or evenat a point in space ahead of the helicopter. [Figure 6-3]The lateral reference datum is usually located at the centerof the helicopter. The location of the reference datum isestablished by the manufacturer and is defined in the RFM.[Figure 6-4]Chapter SummaryThis chapter discusses the importance of computing theweight and balance of the helicopter. The chapter alsodiscusses the common terms and meanings associate withweight and balance.To answer the second question, use CG or momentinformation from loading charts, tables, or graphs in theRFM. It is important to note that any weight and balancecomputation is only as accurate as the information provided.Therefore, ask passengers what they weigh and add a fewpounds to account for the additional weight of clothing,especially during the winter months. Baggage should beweighed on a scale, if practical. If a scale is not available,compute personal loading values according to each individualestimate. Figure 6-2 indicates the standard weights forspecific operating fluids. These values are used whencomputing a helicopter’s balance.Reference DatumBalance is determined by the location of the CG, whichis usually described as a given number of inches from thereference datum. The horizontal reference datum is animaginary vertical plane or point, arbitrarily fixed somewherealong the longitudinal axis of the helicopter, from which allhorizontal distances are measured for weight and balanceAviation Gasoline (AVGAS). . . . . . . . . . . . . . . . . . . . . 6 lb/galJet Fuel (JP-4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 lb/galJet Fuel (JP-5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.8 lb/galReciprocating Engine Oil . . . . . . . . . . . . . . . . . . . . . 7.5 lb/gal*Turbine Engine Oil. . . . . . . . . . Varies between 6 and 8 lb/gal*Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.35 lb/gal* Oil weight is given in pounds per gallon while oil capacity isusually given in quarts; therefore, convert the amount of oil togallons before calculating its weight. Remember, four quartsequal one gallon.Figure 6-2. When making weight and balance computations, alwaysuse actual weights if they are available, especially if the helicopteris loaded near the weight and balance limits.6-4 Horizontaldatum Figure 6-3. While the horizontal reference datum can be anywherethe manufacturer chooses, some manufacturers choose the datumline at or ahead of the most forward structural point on thehelicopter, in which case all moments are positive. This aids insimplifying calculations. Other manufacturers choose the datumline at some point in the middle of the helicopter, in which casemoments produced by weight in front of the datum are negative andmoments produced by weight aft of the datum are positive.

Front view Lateral datumTop view Figure 6-4. The lateral reference datum is located longitudinallythrough the center of the helicopter; therefore, there are positiveand negative values.6-5

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increases during the day, the maximum allowable weight may have to be reduced to keep the helicopter within its capability. The following terms are used when computing a helicopter’s weight: Basic Empty Weight Maximum Gross Weight Weight Limitations Basic Empty Weight The