Pilot training

Ground Lesson Weight and Balance

Flight-with-JC 2020. 5. 28. 13:48

 

Objective:

 

Teach the student the importance of weight and balance of an airplane and how it affects the handling and stability of the aircraft.

 

Elements 

  • Weight and balance definitions

  • Effects of weight and balance on performance 

  • Methods of weight and balance control 

  • Determination of total weight and center of gravity  and the changes that occur when adding, remove, or shifting weight 

 

Equipment

 

Aircraft, POH 

 

Instructor Actions 

 

Weight and balance definitions 

  • Reference datum: An imaginary vertical plane from which all horizontal distances are measured for balance. Reference datum may be anywhere in the plane as the manufacturer chooses.

  • Arm: The horizontal distance from the reference datum to the C.G. of an item. Arms ahead of datum are negative and behind are positive. 

  • Station: A location in the aircraft that is identified by a number designating its distance in inches from the datum. An item located at station +50 would have an arm of fifty inches. 

  • Standard empty weight: Includes weight of airframe, engines, fixed items, unusable fuel, full operating fluids, and oil. 

  • Basic empty weight: Standard empty weight plus any optional equipment installed.

  • Center of gravity: Balance point of an aircraft. CG=m/w. The CG is a three-dimensional point with longitudinal, lateral, and vertical positioning in the aircraft. 

  • C.G. Limits: The extreme forward and aft center of gravity location within which the airplane must be operated at a given weight,

  • Maximum landing weight: Max weight approved for the landing touchdown. 

  • Maximum ramp weight: Max weight approved for ground maneuvers. 

  • Maximum takeoff weight: Max weight approved for start or takeoff run.

  • Maximum zero fuel: max weight exclusive of useable fuel. 

  • Moment: A force that causes an object to rotate.

  • Moment Index: Moment divided by a reduction number such as 100 or 1000 to make the moment value a small number.  

  • Payload: Weight of occupants, cargo, and baggage. 

  • Standard weights

    • Gasoline: 6 lb/US gal 

    • Jet A: 6.8 lb/US gal 

    • Oil: 7.5 lb/gal 

    • Water: 8.35lb/gal

  • Unusable: Fuel remaining in the airplane’s fuel system after a runout test has been completed according to the FARs

  • Useable fuel: Fuel available for flight planning

  • Useful load: the difference between the takeoff weight and basic empty weight

 

Effects of weight and balance on performance 

  • Planes that are too heavy produce load factors that can cause structural damage

  • Metal fatigue can occur over time if you continuously overload the plane 

  • Reduce flight performance 

    • Higher takeoff speed

    • Longer takeoff run 

    • Reduced climb rate and angle 

    • Shorter range 

    • Reduced cruising speed 

    • Reduced maneuverability 

    • Higher stall speed 

    • Faster landing approach used 

    • Longer landing distance required 

    • Less margin for error

    • Fatigue 

 

Effects of balance and stability on aircraft 

  • Forward CG

    • More tail downforce required 

    • Total lift from wings is increased

    • Wings fly at higher AOA which increases drag Nose heavy 

    • More stable due to the elevator having a longer arm and requires less deflection. 

    • Lower cruising speed 

    • Higher stall speed 

    • Harder to hold the nose up at slower speeds (landing, takeoff)

  • Aft CG

    • Less tail downforce 

    • Less lift required by wings 

    • Less drag 

    • Higher cruise speed 

    • Lower stall speed 

    • Not as stable 

    • Harder stall recovery 

Methods of weight and balance

  • Computation method 

    • List the weight for the aircraft, occupants, fuel, and baggage. 

    • Enter the moment for each item. Weight times arm equals the moment 

    • Find the total weight and total moment 

    • To determine the CG, divide the total moment by the total weight 

 

  • Graph Method 

    • To determine the moment, find the weight and draw straight across until it intercepts the item for which the moment is to be calculated. 

    • Draw a line straight down to determine the moment. 

    • Once this has been done for each item, total the weight and moments and draga line for both weight and moment on the CG envelope graph.

    • If the lines intersect within the envelope, the plane is loaded within limits

 

Table method 

  • Applies the same principles as the previous methods.

 

Weight shifting 

  1. CG=(Moment x change in moment) / (Weight x change in weight)

    1. Any increase in weight causes a (+) moment change.

    2. Any shift of weight rearwards will cause a (+) in the moment. 

    3. A weight shift only changes the moment. 

    4. Example 

    5.  

How much weight could be added at Station 120 without exceeding the aft CG limit?

 
      1.  

 

90.5  =  

9,500(90.0) + 120.0(X)  
 

9,500 + X

9,500(90.5) + 90.5(X)  =  

9,500(90.0) + 120.0(X)

859,750 + 90.5X  =  

855,000 + 120X

4,750  =  

29.5X

161.0  =  

X

 

3. 

What is the location of the CG if 90 pounds are removed from Station 140?

Total weight

6,230 lb

CG location

Station 79

 

= New CG

=New CG



4. How much weight could be added at Station 120 without exceeding the aft CG limit?

Total weight

9,500 lb

CG location

Station 90.0

Aft CG limit

Station 90.5

 



90.5  =  

9,500(90.0) + 120.0(X)
 

9,500 + X

9,500(90.5) + 90.5(X)  =  

9,500(90.0) + 120.0(X)

859,750 + 90.5X  =  

855,000 + 120X

4,750  =  

29.5X

161.0  =  

X

 

Weight and balance homework 

 

  1. Determine our plane’s weight and balance when you have a front seat passenger weighing 200 Lbs, baggage weighing 76 Lbs, and full fuel, 

  2. Determine the change of CG if your passenger wants to sit in the back. 

  3. Which CG from the previous question would be better for stall recovery?