All helicopters are designed for certain limit loads and balance conditions. The pilot is responsible for making sure that the weight and balance limitations are met before takeoff. Any pilot who takes off in a helicopter that is not within the designed load and balance condition is not only violating FAA regulations but is inviting disaster.
Three kinds of weight must be considered in the loading of every helicopter. These are empty weight, useful load, and gross weight.
Empty weight - The weight of the helicopter including the structure, the powerplant, all fixed equipment, all fixed ballast, unusable fuel, oil, total quantity of engine coolant, and total quantity of hydraulic fluid.
Useful load (payload) - The weight of the pilot, passengers, baggage (including removable ballast and usable fuel).
Gross Weight - The empty weight plus the useful load.
Maximum gross weight - The maximum weight for which the helicopter is certificated for flight.
Although a helicopter is certificated for a specified maximum gross weight, it will not be safe to take off with this load under all conditions. Conditions that affect takeoff, climb, hovering, and landing performance may require the "off loading" of fuel, passengers, or baggage to some weight less than maximum allowable. Such conditions would include high altitudes, high temperatures, and high humidity, the combination of which determines the density altitude at any given place. Additional factors to consider are takeoff and landing surfaces, takeoff and landing distances available, and the presence of obstacles.
Because of the various adverse conditions that may exist, many times the helicopter pilot must decide the needs of the type mission to be flown and load the helicopter accordingly. For example, if all seats are occupied and maximum baggage is carried, gross weight limitations may require that less than maximum fuel be carried. On the other hand, if the pilot is interested in range, a full fuel load but fewer passengers and less baggage may be decided upon.
Not only must the pilot consider the gross weight of the helicopter, but also must determine that the load is arranged to fall within the allowable center-of-gravity range specified in the helicopter weight and balance limitations contained in the helicopter flight manual. The center of gravity, often referred to as the "CG," is the point where the helicopter is in balance - the point at which all the weight of the system is considered to be concentrated. If the helicopter were suspended by a string attached to the center-of-gravity point, the helicopter fuselage would remain parallel to the surface much as a perfectly balanced teeter-totter (seesaw). The allowable range in which the CG may fall is referred to as the CG range. The exact location and length of this range is specified for each helicopter, but it usually extends a short distance fore and aft of the main rotor mast. For most types of helicopters, the location of the CG must be kept within much narrower limits than that in airplanes - less than 3 inches in some cases.
The ideal condition is to have the helicopter in such perfect balance that the fuselage will remain horizontal in hovering flight, with no cyclic pitch control necessary except that which may be made necessary by wind. The fuselage acts as a pendulum suspended from the rotor. Any change in the center of gravity changes the angle at which it hangs from this point of support. If the center of gravity is located directly under the rotor mast, the helicopter hangs horizontal; if the center of gravity is too far aft of the mast, the helicopter hangs with the nose tilted up; and if the center of gravity is too far forward of the mast, the nose tilts down (fig. 45). Out of balance loading of the helicopter makes control more difficult and decreases maneuverability since cyclic stick effectiveness is restricted in the direction opposite to CG location. Because helicopters are relatively narrow and high sideward speeds will not be attained, lateral balance presents no problems in normal flight instruction and passenger flights, although some light helicopter manuals specify the seat from which solo flight will be made. However, if external loads are carried in such a position that a large lateral displacement of the cyclic stick is required to maintain level flight, fore and aft cyclic stick effectiveness will be very limited.
CG forward of forward limit
This condition may arise in a two-place helicopter when a heavy pilot and passenger take off without baggage or proper ballast located aft of the rotor mast. The condition will become worse as the flight progresses due to fuel consumption, if the main fuel tank is also located aft of the rotor mast.
The pilot will recognize this condition after coming to a hover following a vertical takeoff. The helicopter will have a nose-low attitude and an excessive rearward displacement of the cyclic stick will be required to maintain a hover in a no-wind condition, if hovering flight can be maintained at all (fig. 45). Flight under this condition should not be continued since the possibility of running out of rearward cyclic control will increase rapidly as fuel is consumed, and the pilot may find it impossible to decelerate sufficiently to bring the helicopter to a stop. Also, in case of engine failure and the resulting autorotation, sufficient cyclic control may not be available to flare properly for the landing.
Hovering into a strong wind will make this condition less easily recognizable since less rearward displacement of the cyclic control will be required than when hovering in a no-wind condition. Therefore, in determining if a critical balance condition in this situation exists, the pilot should consider the wind velocity and its relation to the rearward displacement of the cyclic stick.
CG aft of aft limit
Without proper ballast in the cockpit, this condition may arise when (1) a lightweight pilot takes off solo with a full load of fuel located aft of the rotor mast, (2) a lightweight pilot takes off with maximum baggage allowed in a baggage compartment located aft of the rotor mast, or (3) a lightweight pilot takes off with a combination of baggage and substantial fuel where both are aft of the rotor mast.
The pilot will recognize this condition after coming to a hover following a vertical takeoff. The helicopter will have a tail-low attitude and an excessive forward displacement of the cyclic stick will be required to maintain a hover in a no-wind condition, if a hover can be maintained at all. If there is a wind, an even greater forward displacement will be required.
If flight is continued in this condition, the pilot may find it impossible to fly in the upper allowable airspeed range due to insufficient forward cyclic displacement to maintain a nose-low attitude. This particular condition may become quite dangerous if gusty or rough air accelerates the helicopter to a higher airspeed than forward cyclic control will allow. The nose will start to rise and full forward cyclic stick may not be sufficient to hold it down or to lower it once it rises.
Figure 45 - Effect of center of gravity on cyclic stick position and helicopter attitude during hovering flight.
Weight and balance information
When a helicopter is delivered from the factory, the empty weight, empty weight CG, and useful load for each particular helicopter are noted on a weight and balance data sheet included in the helicopter flight manual. These quantities will vary for different helicopters of a given series depending upon changes or variations in the fixed equipment included in each helicopter when delivered.
If, after delivery, additional fixed equipment is added, or if some is removed, or a major repair or alteration is made which may affect the empty weight, empty weight CG, or useful load, the weight and balance data must be revised to reflect these new values. All weight and balance changes will be entered in the appropriate aircraft record. This generally will be the aircraft logbook. The latest weight and balance data should be used in computing all loading problems.
Sample weight and balance problems
In loading a helicopter for flight the problem is two-fold:
* Is the gross weight within the maximum allowable gross weight?
* Does the helicopter meet balance requirements, i.e., is the CG within the allowable CG range?
To answer the first question, merely add the weight of the items comprising the useful load (pilot, passengers, fuel, and baggage) to the empty weight of the helicopter. (Obtain the latest empty weight information from the appropriate aircraft record.) Then check the total weight obtained to see that it does not exceed maximum allowable gross weight.
To answer the second question, use the loading chart or loading table in the helicopter flight manual for the particular helicopter that is being flown.
Sample problem 1 - Determine if the gross weight and center of gravity are within allowable limits under the following loading conditions for a helicopter based on the loading chart in figure 46.
Empty weight .............................. 1,040
Pilot .................................... 135
Passenger ................................ 200
Baggage ................................... 25
Fuel (30 gallons) ........................ 180
SOLUTION: To use the loading chart for the helicopter in this example, the items comprising the useful load must be added to the empty weight in a certain order. The maximum allowable gross weight is 1,600 pounds.
Empty helicopter weight ................... 1,040
Seat load (pilot - 135 pounds;
passenger - 200 pounds) ....... 335
Subtotal ............................... 1,375 (point A)
Baggage compartment load .................. 25
Subtotal .............................. 1,400 (point B)
Fuel load ................................ 180
Total weight ........................ 1,580 (point C)
The total weight of the helicopter does not exceed the maximum allowable gross weight. By following the sequence of steps (locating points A, B, and C) on the loading chart, it is found that the CG is within allowable limits (fig. 46).
Sample problem 2 - For this example, assume that the pilot in sample problem 1 discharges the passenger after using only 20 pounds of fuel.
Empty helicopter weight ................... 1,040
Seat load (pilot - 135 pounds) ............ 135
Subtotal ............................... 1,175 (point D)
Baggage compartment load .................. 25
Subtotal ............................... 1,200 (point E)
Fuel load ................................. 160
Total weight ............................ 1,360 (point F)
Although the total weight of the helicopter is well below the maximum allowable gross weight, the CG falls outside of the aft allowable limit as defined by the loading chart (fig. 46).
This example illustrates the importance of reevaluating the balance problem in a helicopter whenever a change is made in the loading. In most airplanes, the discharging of a passenger would have little effect on the CG. Because of this fact, the airplane pilot may not be aware of the critical loading problems that must constantly be kept in mind as a helicopter pilot. If the pilot in this example takes off again after discharging the passenger, there will be insufficient forward cyclic stick control to hover in a strong wind; it may be impossible to fly in the upper airspeed range due to insufficient forward cyclic control to maintain a nose-low attitude; and a dangerous situation will exist if gusty or rough air accelerates the helicopter to a higher airspeed than forward cyclic control will allow.
Figure 46 - Loading chart illustrating the solution to sample problems 1 and 2.
Just the opposite situation from that illustrated by sample problems 1 and 2 could exist if a very heavy pilot proceeded solo on a flight until a substantial amount of fuel was used, then stopped to pick up a very heavy passenger and proceeded on the flight without refueling. During the solo portion of the flight, the helicopter CG would be within allowable limits. During the second portion of the flight with the passenger, the CG would be forward of allowable limits. If this pilot did take off with the passenger in this example, insufficient aft cyclic control would be available to slow the helicopter to a hover and, in case of an autorotative landing, insufficient aft cyclic control would be available to flare for a landing.
Determining the CG without the use of a loading chart or table
An alternate method of computing center of gravity is to use the arm-weight-moment computation method. To use this method, we must know the empty weight and empty weight CG (obtained from the appropriate aircraft record) and the weight and distance from the datum line of each portion of the useful load - pilot and passengers, usable fuel, and baggage. The following formulas will be used:
Weight x arm = Moment
Moment + moment = Total Moment
------------- = CG
Some manufacturers choose the datum line at or ahead of the most forward structural point on the helicopter, in which case all moments are positive. Other manufacturers choose the datum line at some point in the middle of the helicopter in which case moments produced by weight ahead of the datum line are negative, and moments produced by weight aft of the datum line are positive. A sample problem will be shown for each type.
If external loads are carried, this is the method by which the center of gravity will normally be computed.
Sample Problem 3 - For this example, the datum line is chosen ahead of the most forward structural point on the helicopter (fig. 47).
Weight Arm Moment
Item (pounds) (inches) (inch-pounds)
Empty weight ................ 1,004 x 101.0 = 101,404
Fuel (25 gallons) ........... 150 x 107.0 = 16,050
Seat load ................... 330 x 83.9 = 27,687
TOTALS .................... 1,484 145,141
Total moment 145,141
CG = ------------ = ------- = 97.8 inches aft of datum line
Total weight 1,484
Since the approved center-of-gravity limits are station 95 (95 inches from datum line) and station 100, and the maximum allowable gross weight is 1,600 pounds, the helicopter meets the weight and balance requirements for flight.
Figure 47 - Datum line forward of helicopter.
Figures 48 and 49 show how the charts in the helicopter flight manual would normally be used to compute the CG in this helicopter. To compute the CG from these charts, the first step is to determine the total weight of the helicopter (in pounds) and the total moment (in thousands of inch-pounds).
1. The empty weight and empty weight CG are obtained from the latest weight and balance information in the helicopter flight manual. In this case 1,004 pounds and 101.4.
2. The total moments for the fuel load and seat load are found from the loading chart (fig. 48). Locate the diagonal line indicating fuel quantities and stations. Draw a horizontal line from the 25-gallon mark to the left side of the chart where the moment is read to be 16.
3. Locate the point representing the seat load along the bottom of the chart. From this point, draw a vertical line until it intersects the diagonal line marked "Pilot and Passenger." From this point of intersection, draw a horizontal line to the left side of the chart where the moment is read as 27.7. Tabulating these results and totaling, we have the following:
Item Weight Moment (thousands
(pounds) of inch-pounds)
Empty weight ................... 1,004 101.4
Fuel (25 gallons) .............. 150 16.0
Seat load ...................... 330 27.7
TOTALS ...................... 1,484 145.1
The second step is to locate the point on the center-of-gravity chart (fig. 49) represented by the total weight and total moment just computed. This point falls within the center-of-gravity envelope; therefore, the helicopter is loaded within center-of-gravity and weight limits.
Sample Problem 4 - In this example, the datum line is chosen at a point in the middle of the helicopter (fig. 50).
Item Weight Arm (inch-pound)
(pounds) (inches) Positive Negative
Empty weight ......... 1,820 + 6 10,920 ......
Fuel (41 gallons) .... 246 + 2 492 ......
Seat load ............ 330 - 31 ...... 10,230
----- ------ ------
TOTALS ............ 2,396 11,412 10,230
Total moment = 11,412 - 10,230 = 1,182
Total moment 1,182
CG = ------------ = ----- = +0.5 inches aft of datum line
Total weight 2,396
Figure 48 - Loading chart.
Figure 49 - Center-of-gravity chart.
Since the approved center-of-gravity limits are 3 inches forward of station 0, to 4 inches aft of station 0, and the maximum allowable gross weight is 2,850 pounds, the helicopter meets the weight and balance requirements for flight.
Because the total positive moment exceeds the total negative moment, the center of gravity is aft of the datum line. Had the negative moment exceeded the positive moment, the CG would be forward of the datum line.
Figure 50 - Datum line near rotor mast.