W&B, second pair of eyes please

[piperboy84]

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

[Mach Jump]

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


MJ:ok:

[piperboy84]

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

[piperboy84]

I await with baited breathe !!!!!

[Mach Jump]

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:

[piperboy84]

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:

[Mach Jump]

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


MJ:ok:

[piperboy84]

Maule MX-7-180a

https://www.google.co.uk/search?q=n3110j&biw=1366&bih=705&tbm=isch&imgil=r9InrG8X4jwEVM%253A%253BFFYBdTo_nUSzWM%253Bhttps%25253 A%25252F%25252Fwww.flickr.com%25252Fphotos%25252Felaref%2525 2F14039260108%25252F&source=iu&pf=m&fir=r9InrG8X4jwEVM%253A%252CFFYBdTo_nUSzWM%252C_&usg=__9BLusKPgLggkQuNrx3rbNtyApHQ%3D&dpr=1&ved=0CDYQyjdqFQoTCL-YkICHjcYCFYWa2wod3vcASw&ei=hlV8Vb-9IIW17gbe74PYBA#imgrc=r9InrG8X4jwEVM%253A%3BHdlnbYkPE4XhPM%3 Bhttps%253A%252F%252Fc2.staticflickr.com%252F6%252F5536%252F 14039260108_405f88118a_b.jpg%3Bhttps%253A%252F%252Fwww.flick r.com%252Fphotos%252Felaref%252F14039260108%3B1024%3B678

[India Four Two]

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:

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

[Mach Jump]

Ah. Think we met at Breighton the other week.

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

;)


MJ:ok:

[piperboy84]

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

[India Four Two]

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

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.

[piperboy84]

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

[Level Attitude]

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).

[India Four Two]

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!

[piperboy84]

Mucho gracias I42

[londonblue]

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

I make your initial moment wrong:

1,440*13.77 = 19,828.8

(Unless it's a typo?)

[john_tullamarine]

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 ....

[India Four Two]

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.

[londonblue]

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.

[India Four Two]

Sorry londonblue, JT is correct.

When weighing an aircraft - like pb84's Maule for example - to calculate the empty CG location, you measure the weight on the mainwheels and the weight on the tailwheel (with the fuselage levelled as per the factory instructions).

You then calculate the mainwheel moment - mainwheel weight x mainwheel arm (from datum) and the tailwheel moment - tailwheel weight x tailwheel arm.

You sum the two weights to get the empty weight. You then sum the two moments and divide by the empty weight, to get the empty CG arm.

There are important details in this process, for example, you must do the weighing in a closed hangar to avoid any wind-induced lift affecting the scale readings and with a taildragger, the scale used for the tailwheel must be more accurate than the one used for the mainwheels, because the weight is significantly less and so any error would be a bigger percentage.

Interestingly, the location of the datum does not matter. As long as you pay attention to the signs of the arms, the empty CG location will come out correctly with any datum. For example, you could use the mainwheel axle as the datum, in which case the mainwheel moment is zero and only the tailwheel moment would be used in the calculation.

[londonblue]

Cheers India Four Two. It seems that the empty weight is also a sum of other moments.

That makes more sense...

[India Four Two]

londonblue,

An important correction to your statement. It's the empty weight moment that is the sum of other moments. The empty weight is just the sum of the scale readings.

[john_tullamarine]

Another sideline point re datum position -

(a) the position of the datum is entirely arbitrary .. ie it doesn't matter where you choose to hang the end of your tape measure as the end result will be the same (different numbers but the same significance). Usually, the OEM will choose to place the fuse station datum somewhere out toward the nose of the aircraft, solely for convenience.

(b) however, once you choose a datum, then ALL calculations relating to each other must be with reference to that datum otherwise it all turns to worms.

(c) be wary when your aircraft uses trimsheets for loading systems. It is typical for the weight control person to use the OEM datum for the weighing stuff but then choose a different datum (usually around or at the aft limit of the CG envelope) to minimise trimsheet execution errors.

I had one occasion, many years ago, where a mate (who also was a weight control engineer) redid some configuration stuff on a cabin class twin, reissued the empty weight data (good boy) but then, without thinking what he was doing .. reissued the loading system (trimsheet) entry data using the OEM datum .. which made it incompatible with my trimsheet .. fortunately the chief pilot was the next to fly and, being a mate, gave me a call to sort out what was going on ...

Weight control is straight forward and really only requires good housekeeping .. but you do need to know what you are on about ..