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#1778724
I think I read recently the LAA suggest your navel is your CoG. Assuming legs moderately forward this sounds plausible.

If really keen, take the seat out and mount it on a level surface with a transverse broom handle that would give the same height and distance to some imaginary pedals.

Then find at which point you balance, by putting the broom handle in different fore and aft locations.
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#1779037
Sooty,
You need to determine your datum point from the TADS for your particular aircraft by looking on the LAA website.
You will find everything there in the TADS regarding the CofG and Weight and Balance for any variant of your aircraft always assuming that the LAA have approved it of course.
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By Sooty25
#1779181
Shoestring Flyer wrote:Sooty,
You need to determine your datum point from the TADS for your particular aircraft by looking on the LAA website.
You will find everything there in the TADS regarding the CofG and Weight and Balance for any variant of your aircraft always assuming that the LAA have approved it of course.


Got the TADS, got the datum, got the limits, its the loading arms for crew and fuel tank that are the problem
#1779462
Sooty25,

If the aircraft is CofA, the arms should be in the certified documents that form part of the official W&B for the issue of a CofA.

If the aircraft is PTF, the arms should be in the certified documents that form part of the official W&B for the issue of a PTF.

Simples.
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By Sooty25
#1779468
@Rob L it's PtF and there are no official figures for crew and fuel tank arms from the kit manufacturer.

They've been administered by the LAA for 30 years and nearly 50 are registered. Everyone I've asked so far has come up with different figures, all of which keep it with CofG limits (just), but none of which have any explanation!
#1779515
Just a note on fuel arms. Before getting a proper weighing of my tail wheel a/c I was trying to work it out for myself, and trying for a first approximation just put the tail wheel on the scales. What I hadn't considered was that with the tail down such fuel as there was was very much to the rear of the tanks and so at a greater arm than if evenly distributed, introducing errors into the calculation.

Assuming the base of the tanks are sensibly level during horizontal flight and of a regular shape then the mid point of the tank (fore/aft) would give you the centre of mass and be the point to measure to. If the tanks taper in some way then I suppose you would need to calculate the centre of area of the shape and use that.

Of course if really keen you can remove the tanks hold them in the correct attitude, fill them with incremental fuel volumes, and with the aid of your trusty broom handle see how the CoG moves.

"Do you expect anyone to take that suggestion seriously?"

"No."
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By foxmoth
#1780567
Charles Hunt wrote:
Assuming the base of the tanks are sensibly level during horizontal flight and of a regular shape then the mid point of the tank (fore/aft) would give you the centre of mass and be the point to measure to. If the tanks taper in some way then I suppose you would need to calculate the centre of area of the shape and use that.

Of course if really keen you can remove the tanks hold them in the correct attitude, fill them with incremental fuel volumes, and with the aid of your trusty broom handle see how the CoG moves.


Why not put the tail on a trestle on top of the scale so it is in the right attitude - or put the scale on top of the trestle if easier?
#1780589
I was looking for a gross error - which has now been confirmed at 73 lb!!

Yes a trestle with the scale on top is the right way to go but working alone this didn't feel a very secure option.
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By Rob L
#1780592
Sooty25 wrote:@Rob L it's PtF and there are no official figures for crew and fuel tank arms from the kit manufacturer.

They've been administered by the LAA for 30 years and nearly 50 are registered. Everyone I've asked so far has come up with different figures, all of which keep it with CofG limits (just), but none of which have any explanation!


Sorry for the delay replying, Sooty25.
Let me understand this a little better (please forgive me):

The LAA have administered this design for 30 years, and there are "nearly 50 of these aircraft registered".

May I ask how many of the "nearly 50 registered" aircraft actually have an LAA PtF?

Of these, if there are no "official figures for crew and fuel tank arms" from the CG Datum
(My bold italic) Then how come they have an LAA PtF?

There must be some confusion here (probably by me!) because an LAA-approved design must surely have CG arms for these variables to determine the W&B during use? But perhaps the design CG position does not vary with pilot/fuel load, only the weight; hence my interest.

This link gives links to the LAA W&B schedule requirements for all aircraft on their fleet, about a quarter of the way down the page.

It's not a Cri-Cri is it?
#1781145
Is it a secret what aircraft type this is?

Most of what you need is in the LAA Docs.

If the seat is on runners you do examples for max forward CofG with the seat forward and then a max aft example with the seat fully back. If the fuel tank is forward of the CofG you do the example with max fuel in, but not exceeding total weight. If the fuel tank is aft of the CofG then you do the max aft calculation with fuel to max weight

The datum for the seat is taken midway (front to back) of the seat squab. The aircraft should be set with the horizontal datum set horizontal by a variable jack on the tailwheel that is sitting on the scales.

If the kit or tank maker has not defined the CofG centre for the tank(s) then you have to find where it is before you build the kit by experimentation, or you ask LAA what you should use, as they will have to have this recorded on the LAA data sheets for the type, otherwise the other 30 of the type that have been weighed would not have been able to complete the LAA Weighing form and therefore not have a PtF.

I would be interested to know what LAA engineering have said when you raised this with them.