W&B, second pair of eyes please
 

Annual coming up want to make sure my calcs are right.

old empty weight --------------- arm -----------moment
1440 ---------- 13.77 ------ 19822.8

Removing (old tryes)

25.2lbs -2.5

Adding new tyres (bigger ones)

52.0lbs -2.5

Want to make sure your answer jives with mine, Go for it

Is '-2.5' 2.5 forward, or aft of the Datum?


MJ:ok:

Forward, ironically this is the shyte I failed my CFI checkride on

I await with baited breathe !!!!!

Weight added = 26.8 ..................New weight = 1466.8


+26.8 x -2.5 = -67 .................. New moment = 19755.8


19755.8/1466.8 = 13.46864.............New CofG = 13.47 AOD

Hope that's what you got. Just checking my figures now. :)


MJ:ok:

Yours is right, its back to the books for me, Looks like that DPE was on to something.:ok:

I had 1466.8 / 13.01 / 19095.1, and i have idea how i got there :confused:

What type of aircraft do you have that has it's wheels in front of the Datum?


MJ:ok:

Maule MX-7-180a

https://www.google.co.uk/search?q=n3...8%3B1024%3B678

My figures agree with Mach Jump's calculation and the new CG position passes the TLAR test. It has moved forward, because you have added weight forward of the datum. :ok:

This is why I dislike datums that are not at the nose of the aircraft. It's easy to make a mistake due to sign errors. When preparing to teach W&B at a glider ground school this year, I read that in light helicopters, the datum is often 100" forward of the mast, just to avoid sign problems.

A lot of gliders have the datum at the LE of the wing root, which gives negative arms for most, if not all of the cockpit loads.

Ah. Think we met at Breighton the other week.

Not thinking of going 'water skiing' I hope! :=

;)


MJ:ok:

Funny you mention it, I just got back this afternoon from the Lake District passed by Ullswater, in the Nissan off course :ok:

I expect that most (all?) taildraggers that use the wing leading edge as a datum, have a negative arm for the main gear. Scouts and Citabrias do.

Got some more items to finish up if someone wants to have a crack at it, using MJ's finishing numbers as the starting point.


Item 1 LBS 1.34 Removed -3.4
Item 2 Ounces 6.50 Added -1
Item 3 Ounces 12.50 Added 3
Item 4 LBS 1.10 Added -3.4
Item 5 LBS 3.25 Added -5.3
Item 6 Ounces 15.50 Added -8
Item 7 LBS 0.50 Added 2.5
Item 8 Ounces 1.00 Added -1

Why not do it the easy way?
 

An Excel (or other spreadsheet) would seem the ideal way to account for these multiple changes.

Alternatively, as the aircraft is going in for its annual, why not just ask the engineers to re-weigh it and get an accurate new Basic Empty Weight and CofG (NB: I have no idea how much it costs to weigh a light aircraft).

pb84,

Been messing with the instrument panel? ;)

Since I'm terminally jet lagged and wide awake in the middle of the night (an 11 hour time zone change will do that to you), I did the calculation.

Empty CG has moved forward by 0.1" and the empty weight has gone up by 5.7 pounds. Better cut down on the Forfar Bridies. :E

I suspect the increase in "pounds invested" might be a lot more!

Mucho gracias I42

I make your initial moment wrong:

1,440*13.77 = 19,828.8

(Unless it's a typo?)

I think you are missing how the data originates ...

(a) sum weights, sum moments

(b) arm = sum moments/sum weights

The as-published arm is then rounded off as you might like.

When you do the calculation the other way around, as you have done .. invariably, there will be a round-off "error" in the moment so calculated ....

londonblue,

I thought the same thing at first, but I think it is a rounding issue:

19822.8 / 1440 = 13.7658 = 13.77 to 2 dp.

PS I see John beat me to it.

India Four Two. I see what you're saying, but I disagree with JT.

After all, how's that possible with the empty weight? You can't know your moment. You can only know your weight and how far away from the CoG the weight is (i.e. the arm). From those you can derive the moment.

When you have worked out all the individual moments, you then have to work backwards (i.e. sum moments/sum weights) to derive the arm.