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§ 18

Centre of gravity as percentage of MAC

Where is the centre of gravity as a percentage of MAC?

Answer: On larger aircraft the centre of gravity limit is often given as a percent of mean aerodynamic chord (MAC) not in cm from datum. You must know the leading edge of MAC (LEMAC) measured from datum and MAC's length. Both are in the aircraft's weight and balance manual. The calculator converts CG from datum to %MAC so you can compare against the limits.

Formula

%MAC = (CG − LEMAC) ÷ MAC × 100

CG and LEMAC are measured from the same datum and in the same unit

Omvendt: CG = LEMAC + %MAC ÷ 100 × MAC

Symbols and units in the formula
SymbolMeaningUnit
cgCentre of gravity (CG) from datumcm
lemacLeading edge of MAC (LEMAC) from datumcm
macMAC's lengthcm
=Centre of gravity in % MAC% MAC
=Distance from LEMAC to CGcm

Validity and source

Source
FAA-H-8083-1 (Aircraft Weight and Balance Handbook) %MAC

Does not apply when

  • LEMAC and MAC are aircraft-specific and are in the weight and balance manual. Always compare with the aircraft's approved CG limits for the current weight.

Worked example

The calculator's own default values inserted. The figures are calculated with exactly the formula the tool uses.

Substituted

  • Centre of gravity (CG) from datum1,250 cm
  • Leading edge of MAC (LEMAC) from datum1,180 cm
  • MAC's length230 cm

Giver

  • Centre of gravity in % MAC30.4 % MAC
  • Distance from LEMAC to CG70.0 cm

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Sådan bruger du den

På turboprop- og jetfly er tyngdepunktets grænser og trimindstillingen ofte angivet i procent af MAC i stedet for i cm fra datum. Efter en vejning eller en ændring af kabinen skal du kunne omregne. Beregneren omregner CG fra datum til % MAC ud fra LEMAC og MAC fra vægt- og balancemanualen.

Step by step

  1. Find CG fra datum i vægt- og balanceberegningen.
  2. Find LEMAC (forkanten af MAC) og MAC's længde i vægt- og balancemanualen, i samme enhed.
  3. Aflæs CG i % MAC.
  4. Sammenlign med grænserne for den aktuelle vægt.

Typical errors

  • At bruge forskellige datum.
  • At blande tommer og cm.
  • At glemme at % MAC kan være negativ.

Examples from everyday life

Turboprop efter vejning

Efter en vejning af en turboprop ligger tomvægtens CG 1.250 cm fra datum. Manualen giver LEMAC 1.180 cm og MAC 230 cm.

Centre of gravity (CG) from datum
1,250 cm
Leading edge of MAC (LEMAC) from datum
1,180 cm
MAC's length
230 cm

It means: CG ligger på 30.4 % MAC, 70 cm bag LEMAC.

Last flyttet frem

Med passagerer og last placeret efter loadsheetet flytter CG frem til 1.215 cm fra datum.

Centre of gravity (CG) from datum
1,215 cm
Leading edge of MAC (LEMAC) from datum
1,180 cm
MAC's length
230 cm

It means: CG ligger nu på 15.2 % MAC, kun 35 cm bag LEMAC. Tjek den forreste grænse.

Rule of thumb

1 % MAC er en hundrededel af MAC's længde. Ved en MAC på 230 cm er det 2,3 cm.

How to check the answer

CG = LEMAC giver 0 %.