Weight and Balance Flashcards
Introduction
● Although it may be a bit of an oversimplification, when an aircraft is designed, it is done very much like as a teeter-totter.
● All of the weights of the aircraft must fall within a set of limits that will balance this teeter totter.
● We must also take care that this balance is maintained during the flight.
Adding Weight Means?
● Remember that loading an aircraft to the point that it is over gross invalidates the COA.
○ Being too heavy also contravenes the CARs.
● That said, it isn’t just about the rules. Putting more weight in an aircraft will result in:
● Higher Stalling Speed
● Higher Take-off Speed
● Reduced Rate and Angle of Climb
● Reduced Range
● Reduced Cruising Speed
● Longer Landing Roll and Take Off Run
● Higher fuel consumption
Effect of Balance Forward Center of Gravity (Nose Heavy)
● A downward force will be required on the horizontal tail surfaces to counteract the nose heaviness.
● This results in:
○ A higher stalling speed.
○ Difficulty raising the nose during take-off and landing.
○ Decreased cruise performance.
○ A reduction in range (more drag needs more thrust which burns more fuel).
○ Increased stability about the lateral axis (nose pitches down in stall leading to recovery).
Effect of Balance Aft Center of Gravity (Tail Heavy)
● In this case, an upwards force will required on the horizontal tail surfaces to counteract the tail heaviness
● This results in:
○ A lower stalling speed
○ Increased cruise performance
○ Increased range
○ Less stable about the lateral axis…
○ So, it may be difficult or impossible to recover from a stall or spin
Definitions
● Datum line: An imaginary line from which all measurements are taken.
● Arm: Distance from the datum line to the weight.
● Basic Empty Weight: Standard empty weight of aircraft plus all equipment installed.
● Moment: Weight * Arm
● Center of Gravity: Point through which an aircraft would balance if it was suspended at that point.
● Max Zero Fuel Weight: The maximum weight exclusive of fuel.
● Operational empty weight: the basic empty weight including removable equipment, flight crew members and crew members (including baggage), oil, unusable fuel and emergency equipment.
● Max Payload: Max Take-off weight - (OEW + Fuel) Alternatively it can be found as MZFW - OEW
Calculating Weight and Balance
● Determine the weights and arms of all the mass placed within the aircraft.
● Multiply weights by the arms for all the items to calculate moments.
● Add all the weights and the moments of all mass together.
● Divide the total moment by the total weight of the aircraft to give an overall arm.
● This figure is your Center of Gravity (C of G)
● Some AFMs/POHs will include loading graphs which can be used instead of calculations
Sample Weight and Balance Calculation
● First we need the basic empty weight and arm
● Weight of Front Passengers x Arm=Moment (note moment is divided by 100 for ease of calculation)
● Subtotal the weights and moment for each item
● Divide the moment by weight x 100 to get the C of G for Zero Fuel Weight
● Add the Weight and Moment of Fuel
● Subtract Fuel used in taxi and run-up
● Moment/Take of weight = Take off C of G
● Subtract Fuel expected to be used in flight
● Moment/Landing Weight= Landing C of G
Weight and Balance Chart
We plot the landing Weight and C of G on the Chart to ensure it is within limits. Weight 3514 C of G is 113.6
We plot the take off Weight and C of G (or moment) on the Chart to ensure it is within limits. Weight 3754 C of G is 113.8
Categories
● Normal: Maximum gross weight is allowed but certain maneuvers are prohibited
● Utility: Requirements for weight and C of G are within a tighter range.
○ Maneuvers such as spins, lazy eights are allowed.
● Aerobatic: Full aerobatics are permitted
Practice Question 1
● C of G is 1.5 inches aft of the limits (distance cofg will need to move back to limits). Gross weight is 2300 lbs. Weight that can be moved is 150 lbs.
● How far do we have to move the weight?
23 in forward
Practice Question 2
● The aircraft’s gross weight is 3380 lbs with a C of G of 110.8.
We want to add 200 lbs at station 167.
○ What is the new C of G?
w = 200
W = total gross 3380+200
d = change in cg
D = distance btw station and old cg 167 - 110.8
3.14” backwards. new c of g is +110.8 =113.94
when calculating change in cg (d), add answer onto cg
Practice Question 3
● Gross weight is 3900 lbs and the C of G is 118.5. The aft C of G limit is 117.5 We can move the weight from station 167(baggage compartment) to station 142(a vacant bench seat).
● How many pounds do we need to move to bring the C of G within limits?
d=118.5-117.5 = 1” (We need to move the C of G forward by 1”)
D=167”-142” = 25” (The distance between the stations is 25”)
We will need to move 156 lbs from station 167 to station 142.
MAC
● In some aircraft the C of G limits will be expressed as percentage of MAC instead of inches from the datum.
● MAC (Mean aerodynamic chord): the average distance from the leading edge to the trailing edge of the wing.
● LEMAC: Leading edge of MAC
● Example: Instead of saying C of G limits are between 140.1 and 165.2, using MAC we can say C of G limits are 15% to 30% of MAC
- MAC = temac - lemac
● C of G as % of MAC = C of G - lemac/ MAC
Practice question 4
● Given Aircraft C of G that is 144” aft of the datum
● Lemac is 120”
● MAC is 95”
● What is C of G as a percentage of MAC
144-120/95 = 25.56% of the MAC
(If we take 25.26% of the MAC of 95”, we get 24”, 24” + 120 = 144” which is our C of G)
