Weight & Balance question.

[IBob]

My Savannah (no sweep on the wings) came with a set of standard 'arms' in metres for use when calculating W & B for a given load.
Eg for baggage area: Kg baggage X 1.32M, 1.32 being the given 'arm' for baggage.
These arms all appear to be measured from the leading edge of the wing.
I would have thought they should be measured from the centre of lift. As any weight forward of that would push the nose down and vice versa.
I'm interested as I've been thinking about the difference of weight in baggage vs weight in tail, and with the given arm for baggage, the (theoretical) difference does not seem as great as I would have expected.

Can anyone explain this (why the leading edge?)  to me.....or point me to a thread that will do that?

[djpacro]

https://www.aviation.govt.nz/assets/publications/gaps/caa-gap-weight-and-balance-october-2023-web.pdf

A bit more comprehensive from the USA: https://www.faa.gov/sites/faa.gov/files/2023-09/Weight_Balance_Handbook.pdf

CASA here still has this obsolete document as the current advice: https://www.casa.gov.au/sites/default/files/2021-09/weight_control.pdf 50 years old and some things have changed with the way CASA requires us to deal with the subject.

[facthunter]

If you have it at the front or even before it you get no negatives. which reduces the likelihood of error. The result has to fall within a stipulated range to be acceptable. Nev

[Red]

Well the centre of lift (or centre of pressure) isnt strictly fixed and moves with AoA so not a good candidate for use as a Datum

Its partly a convenience thing as long as its a fixed airframe point that wont ever be modified and isnt a ridiculous distance from the centre of mass then jobs a good un

[aro]

The datum is just a point chosen for convenience as the origin of all the other measurements. The datum doesn't have any aerodynamic significance.

 

The datum could even be e.g. a point in front of the aircraft. The advantage of that is that the arm values for e.g. engine oil or a change of propeller are positive values and it simplifies the calculations.

[facthunter]

I said the two posts back.  It's the "normal" way to do it. Nev

[aro]

24 minutes ago, facthunter said:

I said the two posts back.  It's the "normal" way to do it. Nev

Yes, but you're answer only makes sense if you already know that the datum is just an arbitrary point chosen for convenience.

 

Given that the original question was about how the datum is derived, it didn't necessarily help. I was trying to write a more complete answer.

[facthunter]

I had already given you a "LIKE"  I was wondering IF my posts are in "invisible" ink.  Nev

[turboplanner]

As previously said, a datum can be an imaginary line.

 

or If you plumb bob from, say a rafter, you can set up a vertical face to sight laser dimensions.

 

or If you make the datum the vertical line through the spinner or prop flange you can plumb bob fuel tank changes, cargo changes,       seat changes etc and get repeatable results. 

[IBob]

Thanks all for your input.....I'm learning!
I see now that there is no fixed centre of lift, and that a forward datum avoids possible errors with negative values.

What brought this up is the question of weight in the baggage area (immediately behind me)  vs weight in a tail locker.
In a steady flight situation, a much smaller weight in the tail locker will have a similar balance effect as a larger weight in the baggage area. But how much smaller?
Measuring from the LE as the POH does, the tail locker 'arm' would be approx 3.35M.
As previously stated, the baggage area 'arm' is 1.32M.
This suggests that 10Kg in the baggage area has the same w & b effect as almost 4Kg in the tail locker.
That just doesn't feel right, somehow.............(

 

[turboplanner]

14 minutes ago, IBob said:

This suggests that 10Kg in the baggage area has the same w & b effect as almost 4Kg in the tail locker.
That just doesn't feel right, somehow.............
 

Once you get the equation correct and your calcs are coming out to the exact correct figure, you'll get more confident with the result.

It's just a version of the seesaw with a heavy mass close to the pivot only needing a small mass to balance as long as the other moment it much longer.

 

What you are describing is why PPLs are supposed to do a WB calc before a cross country trip.

I can remember a Member on this site telling us he put a 15 kg tool box on the floor of his Morgan, and lost control on takeoff, luckily managing to get it down. The moment arm from around 50 mm in front of the right seat squab maybe allowed 90 kg in balance, but the moment arm of that tool box was a lont longer.

[facthunter]

Your CofG varies and Something for from it will have a bigger effect at moving it it, Extra weight AT the CofG will not alter it. Anything  ALTERED in the allowable CofG range won't take it Out of the allowable range. For ANY Particular gross wt the Moments added up must be withing 2 figures (aft and fwd) already worked out. There is ALSO a Graphical way of doing all this. I drew up one for the Citabria. That way you can put compartment limits on the chart.. It will show TO Wt . Wt for intended landing and a ZERO fuel wt and CofG and for the other fuel wts as well. Look at examples and work through each, till it makes sense. W&B is a 100%pass exam as is fuel management.  nev

[kgwilson]

I chose the tip of the spinner as the datum when I did the W&B. The advantage of this is that is a fixed point as far as you can go forward on the aircraft so it is easier to do another one somewhere else using the same point. It is a pretty common practice. My COG limits are 15% to 30% aft of the leading edge wing root cord. I have a forward baggage area and one behind the seats and a forward fuselage fuel tank. Wing tanks are within the COG limits. It is all in the phone & easy to calculate when I chuck things on the scales before heading off. If I am bringing something heavy back with me I need to know what it weighs or it gets left behind.

[facthunter]

IF you have LONG lockers have a way of securing the cargo so it cannot move in flight,  Nev

[IBob]

51 minutes ago, facthunter said:

................. There is ALSO a Graphical way of doing all this. I drew up one for the Citabria. That way you can put compartment limits on the chart.. It will show TO Wt . Wt for intended landing and a ZERO fuel wt and CofG and for the other fuel wts as well. Look at examples and work through each, till it makes sense. W&B is a 100%pass exam as is fuel management.  nev

This is how ICP present it for the Savannah, facthunter: a graph with 2 lines showing the forward and rear limits. Using the 'arms' provided and the weights (for fuel, people and baggage) you combine the results with the original aircraft w & b figures, then plot the result on the graph to see where you are on the allowable range for that all-up weight. That part is very straightforward (and it's also near impossible to load outside the limits unless you were to grossly overload the baggage, which has a limit of 20Kg.)

[facthunter]

It's the Method Airlines used and I find it the most foolproof. Of Course nothings foolproof. if planes take off on another shorter runway, or put on extra fuel for weather and it doesn't show on the Loadsheet..  Nev

[onetrack]

The Perth airport crash of VH-MME, a Douglas C-47A-20-DL, on 2nd July 1949, which killed all 18 on board, was due to inadequate W&B checking procedures which meant the rear baggage compartment was overloaded with newspapers.

The aircraft climbed steeply to only 500 feet before it stalled and spiralled straight down, resulting in almost total destruction of the aircraft - aided by an intense fire, thanks to full fuel tanks.

It was W.A.'s worst aircraft crash to that time, and it was a combination of poor company procedures, and individuals mistakes on the ground crew, that led to the crash.

 

No-one was ever blamed directly for the crash, but investigations revealed the gent responsible for W&B checking when loading the aircraft (the senior despatch officer) had inadequate knowledge of aircraft loading principles and limits.

He also believed it was not his job to weigh cargo, only the passengers and their luggage. The two ground crew loading the cargo compartments were not involved in the W&B checking.

The Dakota was heavily loaded with newspapers for the Northern towns of W.A., and newsprint is dreadfully heavy stuff.

 

1949 MacRobertson Miller Aviation DC-3 crash - Wikipedia

EN.WIKIPEDIA.ORG

 

[skippydiesel]

OzRunways has a very handy W&B calculator, that can be customised for your particular aircraft. Very easy to use, instant report on in/out of balance.

 

Word of warning; Computer W&B calculation aids, are only as good as the information put in 😈

[facthunter]

Who knows where the CofG point of their engine is. It's essential when you calculate it's moment. Same with pilot's and Pax weights   When It's all together you should Have a Basic wt and moment (index)on your loadsheet where you start from.   Nev

[Neil_S]

21 hours ago, IBob said:

In a steady flight situation, a much smaller weight in the tail locker will have a similar balance effect as a larger weight in the baggage area. But how much smaller?
Measuring from the LE as the POH does, the tail locker 'arm' would be approx 3.35M.
As previously stated, the baggage area 'arm' is 1.32M.
This suggests that 10Kg in the baggage area has the same w & b effect as almost 4Kg in the tail locker.
That just doesn't feel right, somehow.............(

 

Hi IBob,

 

I guess it comes down to the principle of moments.

I remember doing all that stuff at school with levers and weights at different lengths from the fulcrum. I guess the CG for the aircraft is the "fulcrum" here, and a smaller weight a longer distance away has the same effect as a larger weight that is closer.

Being a bit of a pedant I frequently put more fuel in the starboard outer tank on my Savvy than the port outer tank as I usually fly solo, and I worked out that even a relatively small extra amount of fuel acts as if I had a (light) passenger in the right hand seat, and therefore the plane is more evenly balanced in the roll axis.

 

Cheers,

Neil

[facthunter]

Nice understanding of Levers. Rather than fiddle with fuel levels I'd recommend a  trim tab on the back of one of the ailerons, that you can bend to get the required result  UP makes the aileron go DOWN which makes the wing come UP. Nev

[skippydiesel]

21 hours ago, Neil_S said:

Hi IBob,

Being a bit of a pedant I frequently put more fuel in the starboard outer tank on my Savvy than the port outer tank as I usually fly solo, and I worked out that even a relatively small extra amount of fuel acts as if I had a (light) passenger in the right hand seat, and therefore the plane is more evenly balanced in the roll axis.

 

Cheers,

Neil

Hi Neil,

 

I have wing tanks & like you mostly fly solo. For a "Nav" I fill my tanks to the brim, use some of my right tank first, followed by my left. I repeat this as often as fuel is required, aiming to use about 10-15L per change.😈