Weight and Balance

[JUSTNUZZA]

I have had a new prop installed on my aircraft and need to re calculate the rear C.G. The maintenance company has recalculated the empty weight moment and MAC %. I am now having trouble calculating the figures in the POH that the manufacturer has worked out. Some help would be appreciated.

 

Original figures are as follows.

 

C.G RANGE 24-35 %

 

Empty weight 348kg. CG 25.5 % MAC. Moment 112695.

 

Rear C.G Figures are:

 

Pilot: 80kg ARM 0.6 Moment 48

 

Passenger: 80kg ARM 0.6 Moment 48

 

Bagage: 14kg ARM 2.0 Moment 28

 

Fuel 22kg ARM .20 Moment 4.4

 

WEIGHT TOTAL 196kg. MOMENT 128.4.

 

TAKE OFF WEIGHT 544kg. CG =0.443M. 35% MAC.

 

If I use the formula CG=Total Moment/Total weight [in] x 100/MAC [%] I can not get it right using the formula and see in the original calculation how it equals 35%.

 

New Figures are as follows.

 

Empty weight 343.6kg. C.G 27.4 % MAC. Moment 119251.

 

 

[turboplanner]

Justnuzza, any chance of scanning and posting a readable page(s) of the POH.

 

The fundamentals of the formula couldn't be simpler but some manufacturers turn it into water torture.

 

If I can see how the list it I can probably find the error.

 

 

[djpacro]

Need to know what the MAC is, where the datum is and leading edge of the MAC wrt that datum.

 

 

[JUSTNUZZA]

OTE="djpacro, post: 424963, member: 369"]Need to know what the MAC is, where the datum is and leading edge of the MAC wrt that datum.

 

Sorry can not scan documents at the moment. The MAC is 1267mm the calculation I did in the above example was empty moment 112695 plus loading moment 128.4=112823.4/take off weight 544kg= 207.395 x 100/1267= 16.36 % not 35 % ?

 

 

[Dafydd Llewellyn]

It's most unusual for a Flight Manual to talk of the centre of gravity in terms of % MAC (mean aerodynamic chord); we use that form for flight test work, because it tells an aerodynamicist quite a bit. In a flight manual, it should be expressed as a distance from a specified datum. The MAC may be given as a dimension in the flight manual. If not, it can be calculated from the geometry of the planform. The datum is required to be specified in the flight manual.

 

 

[djpacro]

OK, I see that the leading edge of the MAC is the datum.

 

With the original data, the total moment of the fully loaded aeroplane is 241097.

 

I can email a simple spreadsheet to you if you like.

 

Note that as the fuel is forward of that aft CG - as you burn fuel the CG moves aft so you need to look at the case with zero fuel.

 

 

[planesmaker]

JN,

 

Ok we also need to know the wing chord in mm. It appears the datum is the LE of wing? Please explain just what you need to know. Tom

 

 

[planesmaker]

I see the problem. The moment of empty aircraft is worked out using mm (347.06mm) and you are using metres in working out moment for loading. Convert these to mm and problem solved.

 

PS 1% MAC = 12.67 mm aft of datum

 

35% MAC = 12.67 x 35 = 443.45 mm

 

Tom

 

 

[JUSTNUZZA]

Thanks guys. Planesmaker you are right, the moment just needed to be converted from metres to millimetres and i can now get the calculations to work out.