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Introduction
Compliance with the weight and balance limits of any aircraft is critical to flight safety. Operating above the maximum weight limitation compromis the structural integrity of an aircraft and adverly affects its performance. Operation with the center of gravity (CG) outside the approved limits results in control difficulty.
Weight Control
As discusd in Chapter 4, Aerodynamics of Flight, weight is the force with which gravity attracts a body toward the center of the Earth. It is a product of the mass of a body and the acceleration acting on the body. Weight is a major factor in aircraft construction and operation, and demands respect from all pilots.
The force of gravity continuously attempts to pull an aircraft down toward Earth. The force of lift is the only force that counteracts weight and sustains an aircraft in flight. The amount of lift produced by an airfoil is limited by the airfoil design, angle of attack (AOA), airspeed, and air density. To assure that the
lift generated is sufficient to counteract weight, loading an aircraft beyond the manufacturer’s recommended weight must be avoided. If the weight is greater than the lift generated, the aircraft may be incapable of flight.
Effects of Weight
Any item aboard the aircraft that increas the total weight is undesirable for performance. Manufacturers attempt to make an aircraft as light as possible without sacrificing strength or safety.
Weight and Balance
Chapter 9
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Lateral or longitudinal unbalance
Lateral unbalance will cau wing heaviness.
Empty
Full
Excess baggage
Longitudinal unbalance will cau either no or tail heaviness.
Figure 9-1. Lateral and longitudinal unbalance.
The pilot should always be aware of the conquences 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. For example, 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小学暑假班英语
• Shorter range
• Reduced cruising speed • Reduced maneuverability
• Higher stalling speeddisturb
• Higher approach and landing speed • Longer landing roll
• Excessive weight on the no wheel or tail wheel The pilot must be knowledgeable about the effect of weight on the performance of the particular aircraft being flown. Preflight planning should include a check of performance charts to determine if the aircraft’s weight may contribute to hazardous flight operations. Excessive weight in itlf reduces the safety margins available to the pilot, and becomes even more hazardous when other performance-reducing factors are combined with excess weight. The pilot must also consider the conquences of an overweight aircraft if an emergency condition aris. If an engine fails on takeoff or airframe ice forms at low altitude, it is usually too late to reduce an aircraft’s weight to keep it in the air.
Weight Changes
The operating weight of an aircraft can be changed by simply altering the fuel load. Gasoline has considerable weight—6 pounds per gallon. Thirty gallons of fuel may weigh more than one pasnger. If a pilot lowers airplane weight by reducing fuel, the resulting decrea in the range of the airplane must be taken into consideration during flight planning. During flight, fuel burn is normally the only weight change that takes place. As fuel is ud, an aircraft becomes lighter and performance is improved.
Changes of fixed equipment have a major effect upon the weight of an aircraft. The installation of extra radios or instruments, as well as repairs or modifications may also affect the weight of an aircraft.Balance, Stability, and Center of Gravity
Balance refers to the location of the CG of an aircraft, and is important to stability and safety in flight. The CG is a point at which the aircraft would balance if it were suspended at that point.
The primary concern in balancing an aircraft is the fore and
aft location of the CG along the longitudinal axis. The CG
is not necessarily a fixed point; its location depends on the
distribution of weight in the aircraft. As variable load items are
shifted or expended, there is a resultant shift in CG location.
The distance between the forward and back limits for the position of the center for gravity or CG range is certified for
an aircraft by the manufacturer. The pilot should realize that
if the CG is displaced too far forward on the longitudinal axis, a no-heavy condition will result. Converly, if the CG is displaced too far aft on the longitudinal axis, a tail
heavy condition results. It is possible that the pilot could not
control the aircraft if the CG location produced an unstable condition. [Figure 9-1]
Location of the CG with reference to the lateral axis is also important. For each item of weight existing to the left of the fulage centerline, there is an equal weight existing at a corresponding location on the right. This may be upt by unbalanced lateral loading. The position of the lateral CG is not computed in all aircraft, but the pilot must be aware that adver effects ari as a result of a laterally unbalanced condition. In an airplane, lateral unbalance occurs if the fuel load is mismanaged by supplying the engine(s) unevenly from tanks on one side of the airplane. The pilot can compensate for the resulting wing-heavy condition by adjusting the
trim or by holding a constant control pressure. This action places the aircraft controls in an out-of-streamline condition, increas drag, and results in decread operating efficiency. Since lateral balance is addresd when needed in the aircraft flight manual (AFM) and longitudinal balance is more critical, further reference to balance in this handbook means longitudinal location of the CG. A
single pilot operating a small rotorcraft, may require additional weight to keep the aircraft laterally balanced.
Flying an aircraft that is out of balance can produce incread pilot fatigue with obvious effects on the safety and efficiency of flight. The pilot’s natural correction for longitudinal unbalance is a change of trim to remove the excessive control pressure. Excessive trim, however, has the effect of reducing not only aerodynamic efficiency but also primary control travel distance in the direction the trim is applied. Effects of Adver Balance
Adver balance conditions affect flight characteristics in much the same manner as tho mentioned for an excess weight condition. It is vital to comply with weight and balance limits established for all aircraft, especially rotorcraft. Operating above the maximum weight limitation compromis the structural integrity of the rotorcraft and adverly affects performance. Balance is also critical becau on some fully loaded rotorcraft, CG deviations as small as three inches can dramatically change handling characteristics. Stability and control are also affected by improper balance.
Stability
Loading in a no-heavy condition caus problems in controlling and raising the no, especially during takeoff and landing. Loading in a tail heavy condition has a rious effect upon longitudinal stability, and reduces the capability to recover from stalls and spins. Tail heavy loading also produces very light control forces, another undesirable characteristic. This makes it easy for the pilot to inadvertently overstress an aircraft.
It is important to reevaluate the balance in a rotorcraft whenever loading changes. In most aircraft, off-loading a pasnger is unlikely to adverly affect the CG, but off-loading a pasnger from a rotorcraft can create an unsafe flight condition. An out-of-balance loading condition also decreas maneuverability since cyclic control is less effective in the direction opposite to the CG location. Limits for the location of the CG are established by the manufacturer. The are the fore and aft limits beyond which the CG should not be located for flight. The limits are published for each aircraft in the Type Certificate Data Sheet (TCDS), or aircraft specification and the AFM or pilot’s operating handbook (POH). If the CG is not within the allowable limits after loading, it will be necessary to relocate some items before flight is attempted.哈韩
The forward CG limit is often established at a location that is determined by the landing characteristics of an aircraft. During landing, one of the most critical phas of flight, exceeding the f
orward CG limit may result in excessive loads on the nowheel, a tendency to no over on tailwheel type airplanes, decread performance, higher stalling speeds, and higher control forces.
Control
In extreme cas, a CG location that is beyond the forward limit may result in no heaviness, making it difficult or impossible to flare for landing. Manufacturers purpoly place the forward CG limit as far rearward as possible to aid pilots in avoiding damage when landing. In addition to decread static and dynamic longitudinal stability, other undesirable effects caud by a CG location aft of the allowable range may include extreme control difficulty, violent stall characteristics, and very light control forces which make it easy to overstress an aircraft inadvertently.
A restricted forward CG limit is also specified to assure that sufficient elevator/control deflection is available at minimum airspeed. When structural limitations do not limit the forward CG position, it is located at the position where full-up elevator/control deflection is required to obtain a high AOA for landing.
The aft CG limit is the most rearward position at which the CG can be located for the most critical maneuver or operation. As the CG moves aft, a less stable condition occurs, which decreas the ab
ility of the aircraft to right itlf after maneuvering or turbulence.
For some aircraft, both fore and aft CG limits may be specified to vary as gross weight changes. They may also be changed for certain operations, such as acrobatic flight, retraction of the landing gear, or the installation of special loads and devices that change the flight characteristics. The actual location of the CG can be altered by many variable factors and is usually controlled by the pilot. Placement of baggage and cargo items determines the CG location. The assignment of ats to pasngers can also be ud as a means of obtaining a favorable balance. If an aircraft is tail heavy, it is only logical to place heavy pasngers in forward ats. Fuel burn can also affect the CG bad on the location of the fuel tanks. For example, most small aircraft carry fuel in the
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wings very near the CG and burning off fuel has little effect on the loaded CG. On rotorcraft, the fuel tanks are often located behind the CG and fuel consumption from a tank aft of the rotor mast caus the loaded CG to move forward.
A rotorcraft in this condition has a no-low attitude when coming to a hover following a vertical takeoff. Excessive rearward displacement of the cyclic control is needed to maintain a hover in a no-
wind condition. Flight should not be continued since rearward cyclic control fades as fuel is consumed. Deceleration to a stop may also be impossible. In the event of engine failure and autorotation, there may not be enough cyclic control to flare properly for a landing. Management of Weight and Balance Control
Title 14 of the Code of Federal Regulations (14 CFR) ction 23.23 requires establishment of the ranges of weights and CGs within which an aircraft may be operated safely. The manufacturer provides this information, which is included in the approved AFM, TCDS, or aircraft specifications. While there are no specified requirements for a pilot operating under 14 CFR part 91 to conduct weight and balance calculations prior to each flight, 14 CFR ction 91.9 requires the pilot in command (PIC) to comply with the operating limits in the approved AFM. The limits include the weight and balance of the aircraft. To enable pilots to make weight and balance computations, charts and graphs are provided in the approved AFM.
Weight and balance control should be a matter of concern to all pilots. The pilot controls loading and fuel management (the two variable factors that can change both total weight and CG location) of a particular aircraft. The aircraft owner or operator should make certain that up-to-date information is available for pilot u, and should ensure that appropriate entries are made in the records when repa
irs or modifications have been accomplished. The removal or addition of equipment results in changes to the CG.
Weight changes must be accounted for and the proper notations made in weight and balance records. The equipment list must be updated, if appropriate. Without such information, the pilot has no foundation upon which to ba the necessary calculations and decisions.number的缩写
Standard parts with negligible weight or the addition of minor items of equipment such as nuts, bolts, washers, rivets, and similar standard parts of negligible weight on fixed-wing aircraft do not require a weight and balance check. Rotorcraft are, in general, more critical with respect to control with changes in the CG position. The following criteria for negligible weight change is outlined in Advisory Circular (AC) 43.13-1 (as revid), Methods Techniques and Practices—Aircraft Inspection and Repair:
• One pound or less for an aircraft who weight empty is less than 5,000 pounds;
• Two pounds or less for aircraft with an empty weight of more than 5,000 pounds to 50,000 pounds;
• Five pounds or less for aircraft with an empty weight of more than 50,000 pounds.
Negligible CG change is any change of less than 0.05 percent Mean Aerodynamic Chord (MAC) for fixed-wing aircraft, 0.2 percent of the maximum allowable CG range for rotorcraft. Exceeding the limits would require a weight and balance check.
Before any flight, the pilot should determine the weight and balance condition of the aircraft. Simple and orderly procedures bad on sound principles have been devid by the manufacturer for the determination of loading conditions. The pilot us the procedures and exercis good judgment when determining weight and balance. In many modern aircraft, it is not possible to fill all ats, baggage compartments, and fuel tanks, and still remain within the approved weight and balance limits. If the maximum pasnger load is carried, the pilot must often reduce the fuel load or reduce the amount of baggage.
14 CFR part 125 requires aircraft with 20 or more ats or weighing 6,000 pounds or more to be weighed every 36 calendar months. Multi-engine aircraft operated under a 14 CFR part 135 are also required to be weighed every 36 months. Aircraft operated under 14 CFR part 135 are exempt from the 36 month requirement if operated under a weight and balance system approved in the operations specifications of the certificate holder. AC 43.13-1, Acceptable Methods, Techniques and Practices—Aircraft Inspection and Repair also requires that the aircraft mechanic must ensure the w
天使与魔鬼的对话eight and balance data in the aircraft records is current and accurate after a 100-hour or annual inspection.
Terms and Definitions
The pilot should be familiar with terms ud in working problems related to weight and balance. The following list of terms and their definitions is standardized, and knowledge of the terms aids the pilot to better understand weight and balance calculations of any aircraft. Terms defined by the General Aviation Manufacturers Association (GAMA) as industry standard are marked in the titles with GAMA.
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• Arm (moment arm)—the horizontal distance in inches from the reference datum line to the CG of an item. The algebraic sign is plus (+) if measured aft of the datum, and minus (–) if measured forward of the datum.• Basic empty weight (GAMA)—the standard empty weight plus the weight of optional and special equipment that have been installed.
• Center of gravity (CG)—the point about which an aircraft would balance if it were possible to suspe
nd 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. It may be expresd in inches from the reference datum, or in percent of MAC. The CG is a three-dimensional point with longitudinal, lateral, and vertical positioning in the aircraft.
• CG limits—the specified forward and aft points within which the CG must be located during flight.
The limits are indicated on pertinent aircraft specifications.
• CG range—the distance between the forward and aft CG limits indicated on pertinent aircraft specifications.• Datum (reference datum)—an imaginary vertical plane or line from which all measurements of arm are taken.
The datum is established by the manufacturer. Once the datum has been lected, all moment arms and the location of CG range are measured from this point.• Delta—a Greek letter expresd by the symbol ❒ to indicate a change of values. As an example, ❒CG indicates a change (or movement) of the CG.
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• Floor load limit—the maximum weight the floor can sustain per square inch/foot as provided by the manufacturer.
• Fuel load—the expendable part of the load of the aircraft. It includes only usable fuel, not fuel required to fill the lines or that which remains trapped in the tank sumps.
• Licend empty weight—the empty weight that consists of the airframe, engine(s), unusable fuel, and undrainable oil plus standard and optional equipment as specified in the equipment list. Some manufacturers ud this term prior to GAMA standardization.
• Maximum landing weight—the greatest weight that an aircraft normally is allowed to have at landing.• Maximum ramp weight—the total weight of a loaded aircraft, and includes all fuel. It is greater than the takeoff weight due to the fuel that will be burned during the taxi and runup operations. Ramp weight may also be referred to as taxi weight.• Maximum takeoff weight—the maximum allowable weight for takeoff.怎么清理毛孔
• Maximum weight—the maximum authorized weight of the aircraft and all of its equipment as specified in the TCDS for the aircraft.pha是什么意思
国庆节英文怎么说• Maximum zero fuel weight (GAMA)—the maximum weight, exclusive of usable fuel.
• Mean aerodynamic chord (MAC)—the average distance from the leading edge to the trailing edge of the wing.
• Moment—the product of the weight of an item multiplied by its arm. Moments are expresd in pound-inches (in-lb). Total moment is the weight of the airplane multiplied by the distance between the datum and the CG.
• Moment index (or index)—a moment divided by a constant such as 100, 1,000, or 10,000. The purpo of using a moment index is to simplify weight and balance computations of aircraft where heavy items and long arms result in large, unmanageable numbers.
• Payload (GAMA)—the weight of occupants, cargo, and baggage.
• Standard empty weight (GAMA)—aircraft weight that consists of the airframe, engines, and all items of operating equipment that have fixed locations and are permanently installed in the aircraft, including fixed ballast, hydraulic fluid, unusable fuel, and full engine oil.
• Standard weights—established weights for numerous items involved in weight and balance computations.迈克尔福克斯
The weights should not be ud if actual weights are available. Some of the standard weights are:
Gasoline ............................................... 6 lb/US gal Jet A, Jet A-1 .................................... 6.8 lb/US g
al Jet B ...................................................6.5 lb/US gal Oil ......................................................7.5 lb/US gal Water .....................................................8.35 lb/US gal • Station—a location in the aircraft that is identified by
a number designating its distance in inches from the
datum. The datum is, therefore, identified as station zero. An item located at station +50 would have an arm of 50 inches.
• Uful load—the weight of the pilot, copilot, pasngers, baggage, usable fuel, and drainable oil. It is the basic empty weight subtracted from the maximum allowable gross weight. This term applies to general aviation (GA) aircraft only.
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