Helicopter Flight Information
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Helicopter Flight Information |
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WEIGHT & BALANCE and FUEL BURN
The factors concerning weight and balance are: the datum, weight, arm, moment, and CG (center of gravity). As fuel is burned, the CG (center of gravity) moves forward, so fuel burn is quite relative to weight & balance data. Furthermore landing weight is significant in many weight & balance calculations. Although most modern weight and balance data are calculated using a graph, many pilots prefer to use the manual calculation method. Datum The datum is an imaginary line from which all components on an aircraft are measured. The datum can be a number of inches in front of the aircraft, or it could be the most forward point of the aircraft, or even some point in the middle of the aircraft. All components located aft of the datum are positive (+), and all components located forward of the datum are negative (–). For lateral weight & balance, the datum is the centerline of the aircraft. All components left of center are negative (–), and all components right of center are positive (+). The datum is also known, or referred to as, “station 0.” Weight Weight is the weight of the aircraft or the weight of any item such as fuel, oil, passengers, baggage or cargo that will be placed within the aircraft. The weight of articles should be accurate, not just guessed. 100LL fuel weighs 6 lbs per gallon, and oil weighs 7.5 lbs per gallon. Arm The arm is the distance from the datum that any component is located. The arm is also the CG of that specific item. The fuel is a given distance from datum as is the oil, the passengers, etc. The weight of any relative object is multiplied by the arm, to find the moment. Moment The moment is the result of the weight of an object multiplied by the arm. The total moment is then divided by the total weight to find the CG. CG (Center of Gravity) The CG is ultimately the balance point of the aircraft. It must be located within specifications for the controllability of the aircraft. If the CG is located aft, the controls will be limited in forward movements, and likewise if the CG is forward, the controls will be centered aft. In helicopters, many times lateral CG is critical for lateral control centering. In that case, the CG should be as near center as possible. Calculating the CG It is helpful to have a worksheet for calculating weight and balance (see figures 7a and 7b). After locating the weight and balance data for the aircraft you are going to fly, fill in the worksheet with the necessary information. Example: Problem data: Pilot 175# Passenger 180# Oil 8 qts. Fuel 41 gal. Actual weight and balance data from an aircraft:
WEIGHT AND BALANCE DATA BELL 47 G-2 - N6746D - S/N 2250 Maximum gross weight for this helicopter is 2450 lbs. All calculations based on weights recorded per Saliaka Aviation on Oct. 11, 1992, and calculated per Maverick Helicopters, with battery installed in aft location. ITEM WEIGHT ARM MOMENT BASIC EMPTY WEIGHT Less oil and usable fuel 1,676.90 +7.05 11,832.00 Calculated most forward CG Basic empty weight 1,676.90 +7.05 +11,832.00 Pilot and 2 Passengers (170 # ea.) 510.00 - 30.00 -15,300.00 Oil (8 qts @ 7.5 lbs gal.) 15.00 +26.50 +397.50 Fuel (5.0 Gal. 20 min. reserve) 30.00 +4.95 +148.50 Totals 2,231.90 -1.31 2,992.00 CALCULATED MOST AFT CG Basic empty weight 1,676.90 +7.05 +11,832.00 Pilot 170.00 -30.00 -5,100.00 Oil (8 qts. @ 7.5 lbs gal.) 15.00 +26.50 +397.50 Fuel (41 gal.) 246.00 +4.95 +1,217.70 Totals 2,107.90 +3.96* +8,347.20 *Note: Although the calculated most aft CG is within limitations, a ballast must be placed in the cabin in single pilot operations of sufficient weight to move the CG forward to center the cyclic, and to prevent a tail strike on approach. The CG limitations are –3.0 to +4.0. Station 0 is approximately 2 inches forward of the mast centerline. Using this problem data, calculate the weight and balance of the aircraft and determine if it is within CG (see figures 7a and 7b). Filled in a form, it should look like this: ITEM WEIGHT ARM MOMENT N6746D 1,676.90 +7.03 11.832.00 Pilot 175 -30.0 -5,250.00 Passenger 180 -30.0 -5,400.00 Oil 15 +26.50 +397.50 Fuel 246 +4.95 +1,217.70 Totals 2,292.9 +2,797.2 CG = +1.22 Notice in this problem, there are both negative and positive arms. This is due to the fact that the datum (station 0) is just forward of the mast centerline. In an aircraft where there is a lateral arm specified, for example in this case if the empty aircraft had a lateral CG of .02 with a range of +0.2 to –0.2 specified, the lateral moment and CG are found the same way as the longitudinal moment and CG. So with a pilot (right seat position) lateral arm of +12.5, and a passenger lateral arm of –13.3 specified, when you calculate the lateral CG it would look like this: Item Weight/Arm/Moment Aircraft 1676.9 x .02 = 33.538 Pilot 175 x +12.5 = +2187.5 Passenger 180 x –13.3 = –2394.0 Totals 2031.9 –172.96 Total lateral moment (–172.96) divided by total weight (2031.9) = CG –0.08 Moving the CG (weight shift) If you calculated the CG and found it to be out of range, you could move baggage from one point to another, to move the CG within allowable limits. The formula for recalculating the CG is very simple: Weight (of object) x distance moved ÷ gross weight = change in CG. If in the above example the passenger were moved 2 inches forward, we could put the formula to work: 180 x 2 = 360 ÷ 2,292.9 = 0.15. If the weight change is forward, then the change is subtracted from the previous CG. The previous CG of 1.22 – 0.15 (Change in CG) = a new CG of +1.07. If the weight change were aft, the change in the CG would be added to the previous CG. Weight Change This formula is effective for calculating the new CG after a fuel burn. It is still a simple formula, and first you must know the CG before the fuel burn. Using the above example, the CG was found to be +1.22. The CG of the fuel is (fuel arm) +4.95, calculate the distance between the two, +4.95 – +1.22 = +3.73. Next calculate the new gross weight using a fuel burn of 125 lbs. Subtract the weight of the fuel burned (125 lbs) from the previous gross weight (2,292.9) = 2,167.9 (new gross weight). Having consumed 125 lbs of fuel, the formula is: 125 (fuel burned) x 3.73 (difference of CG) ÷ 2167.9 (new total weight) = +0.21 (the change in CG). The weight shift is forward so the change in CG is subtracted from the old CG. The new CG is +1.22 – 0.21 = A new CG of + 1.01 Fuel burn Information on the fuel burn characteristics of an aircraft are not found in the operator’s handbook. This information must come either from personal experience, or from someone else who is familiar with the aircraft. The fuel burn can vary significantly between different aircraft with similar engines, but it will remain close on the same make and model of aircraft using the same engines. The best way to determine fuel burn accurately is to land and refuel with a large safety margin during the first leg, and then you can make accurate calculations for the aircraft after that. Never rely on fuel level indicators, always make a visual check of the fuel level regardless of how difficult it might be. An effective way to do this is to purchase a piece of doweling from your local hardware store and keep it with you for all flights regardless of the aircraft you will be flying. Become familiar with what level of fuel is equivalent to how many gallons or pounds of fuel in a given aircraft. Even those plastic calibrated indicators available from various sources are only an estimation of the actual fuel level. One good method is to calculate the fuel burn per minute of any aircraft flown, and then use the following formula for fuel burns enroute. For this example, using a burn rate of 15 gallons per hour. (Gallons per hour burned) 15 x 6 (weight of fuel per gallon) = 90 (pounds per hour) ÷ 60 (minutes) = 1.5 (pounds per minute). Then for the minutes flown calculate the fuel burned, 90 (minutes) x 1.5 (pounds per minute) = 135 (pounds of fuel burned) or 22.5 gallons. Although the above example may seem complicated, it is very effective when it is necessary to calculate a new weight and balance for every departure, when you do not have to take on fuel. It is just as easy to calculate only gallons per hour burn rates when that is your only concern for flight planning. The formula is: Time (in minutes) ÷ 60 x GPH = fuel burned. For time remaining subtract the gallons burned from the initial quantity and divide gallons remaining by rate of burn (GPH). 15 (gallons remaining) ÷ 5.5 (gallons per hour burn rate) = 2.7 (hours remaining). For hours and minutes remaining, multiply .7 x 60 = 42 (2 hours 42 minutes remaining). ☺
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