Weight & Balance (Aeronautical Engineering) PDF

Weight & Balance Chapter 8, Section B  Overview  Importance of Weight  Importance of Balance  Weight and Balance Terms  Principles of Weight and Balance  Determining total weight and CG  Weight-Shift Formula  Effects of Forward and Aft CG...

Weight & Balance Chapter 8, Section B  Overview  Importance of Weight  Importance of Balance  Weight and Balance Terms  Principles of Weight and Balance  Determining total weight and CG  Weight-Shift Formula  Effects of Forward and Aft CG  PHAK Ch. 10 Importance of Weight  Increased weight reduces all aspects of aircraft performance ◦ Higher takeoff speed ◦ Longer takeoff run ◦ Reduced rate and angle of climb ◦ Lower maximum altitude ◦ Shorter range ◦ Reduced cruising speed ◦ Reduced maneuverability ◦ Higher stalling speed ◦ Higher approach and landing speed ◦ Longer landing roll ◦ Excessive weight on the nose wheel or tail wheel  PHAK 10-3 Importance of Balance  Balance also affects performance ◦ Forward Center of Gravity  Higher load on nosewheel  Increased longitudinal stability  Decreased performance  Higher stalling speed  Higher control forces ◦ Aft Center of Gravity  Increased performance  Reduced longitudinal stability (less stable at all speeds)  Decreased ability to right aircraft after turbulence or recover from stalls  PHAK 10-4 Weight and Balance Terms  Reference Datum ◦ Imaginary vertical plane from which all horizontal distances are measured  Basic Empty Weight ◦ Weight of the airplane including optional (installed) equipment, unusable fuel, full oil, and full hydraulic fluid ◦ The weight may change due to modification or changing equipment  Changes must be documented and measured by mechanic  Unusable Fuel ◦ Small amount of fuel in the tanks which cannot safely be used in flight or drained on the ground ◦ Standard weight of 100LL (AvGas) is 6lbs per gallon  PHAK 10-5 Weight and Balance Terms  Maximum Ramp Weight ◦ Maximum weight the aircraft can support for taxiing  Takeoff Weight ◦ Maximum weight with which the aircraft can takeoff  Maximum Landing Weight  Maximum Takeoff Weight  Useful Load ◦ Weight of the flight crew, usable fuel, passengers, baggage, and cargo ◦ Ramp weight or Takeoff Weight – Basic Empty Weight  PHAK 10-5 Weight and Balance Terms  Payload ◦ Weight of passengers, baggage, and cargo  Usable Fuel ◦ Gasoline weighs 6 lbs. per gallon  Station ◦ Location in aircraft identified by inches from the reference datum ◦ Reference datum is therefore station 0  PHAK 10-6 Principles of Weight and Balance  Arm ◦ Distance from reference datum  Moment ◦ Tendency of a weight to cause rotation at the fulcrum ◦ Expressed in pound-inches  Weight x Arm = Moment  Calculating the Position of the CG ◦ Expressed in inches from reference datum ◦ Total moment divided by total weight  PHAK 10-7 Determining total weight and CG  Computation  Table  Graph  PHAK 10-7 Computational Method  Add up all weights ◦ Aircraft Basic Empty Weight and moment are provided ◦ Remember fuel weighs 6 lbs per gal. ◦ Arms are provided  All this information is located in the Pilot Operating Handbook (POH)  Calculate moments  Add up all moments  Divide total moment by total weight to determine CG location  Weight Arm Moment Airplane 1,642.6 - 62,623.8 Front Seats 400 37 14,800 Fuel 25 gal (150 48 7,200 lb) Rear Seats 150 73 10,950 Baggage 1 20 95 1,800 Baggage 2 10 123 1,230 Max Ramp Weight: Ramp 2,372.6 - 98,603.8 2,558 lb Weight Taxi -8 48 -384 Takeoff 2,364.6 - 98,219.8 Flight -24.8 gal (- 48 -7142.4 Max T/O & Lnd: 148.8 lb) 2,550 lb Landing 2,215.8 - 91,077.4 Calculating CG Position  Divide total moment by total weight Weight Moment CG (in aft of datum) Ramp 2372.6 98,603.8 41.56 T/O 2364.6 98,219.8 41.54 Landing 2215.8 91,077.4 41.10  PHAK 10-7 Graph Method  Utilize graph to calculate moments  Sum weights and sum moments  Remember to add airplane weight and moment Graph Method  Plot CG on graph using total weight and total moment  PHAK 10-9 Table Method  Utilize the given chart to determine moments  Remember, unless otherwise specified, oil is included in basic empty weight  PHAK 10-10 Weight Shift Formula Effects of Forward & Aft CG  Forward CG ◦ The airplane is “effectively heavier”  Decreased performance  Higher stalling speed  Better Stall Characteristics  Aft CG ◦ The airplane is “effectively lighter”  Improved Performance  Lower Stall Speed  Worse Stall Characteristics  But how? Balancing Tail-Down Force So to summarize  With a forward CG: ◦ Rotational moment created by the CG and Center of Lift is greater ◦ This requires increased balancing tail down force ◦ This requires a higher AoA to generate more lift ◦ This increase in AoA increases lift AND drag (specifically induced drag) ◦ This increase in drag results in a loss in aircraft performance  Since you are flying at a higher AoA with increased wing loading, stall speed increases as you are flying “closer to your critical AoA”  Elevator effectiveness increases as there is a greater arm between the elevator and the CG making stall recoveries easier So to summarize  With an aft CG: ◦ Rotational moment created by the CG and Center of Lift is less ◦ This requires decreased balancing tail down force ◦ This requires a lower AoA to generate less lift ◦ This decrease in AoA decreases lift AND drag (specifically induced drag) ◦ This decrease in drag results in an increase in aircraft performance  Since you are flying at a lower AoA with decreased wing loading, stall speed decreases as you are flying “farther from your critical AoA”  Elevator effectiveness decreases as there is a decreased arm between the elevator and the CG making stall recoveries harder Forward and Aft CG  So which one is better?  It depends… ◦ If you’re looking for better performance: aft CG ◦ If you’re looking for improved stability: forward CG ◦ If you’re worried about stall speeds: aft CG ◦ If you’re looking for more effective flight controls: forward CG What happens when you operate outside of the “envelope”?  Overweight ◦ Aerosucre Boeing 727: https://www.youtube.com/watch?v=EnfQJfsY2Po  High Density Altitude ◦ Stinson 108-3: https://www.youtube.com/watch?v=OVM3RRd1vf0  Aft CG (graphic content) ◦ National Airlines Flight 102: https://www.youtube.com/watch?v=5fpxm0D46iQ  There’s what the book says and then there’s the real world. Operating at the edge of the envelope can be deadly. (Personal Minimums are Important) ◦ https://www.youtube.com/watch?v=ZmEVwyMRYIY Questions?