DFC (and the VERY great majority of pilots),
aahhh .... yeeeesss ... and/but no !!!
This post addresses some important things which are very poorly understood amongst the aviating fraternity. Hopefully it will generate some follow-on posts to make sure that those watching the thread end up understanding a bit more about these concerns.
Beware of being too cavalier and relaxed with plots as there are a few little traps lurking in the background for young players who are insufficiently cautious due to technical ignorance (and I use the latter term in a strict definition sense .. not at all wanting to be impolite here).
First up, I apologise to the purists for whom "mass" is preferable to "weight" .. and they are correct ... but I'm a "weight" dinosaur ... so please make allowances for me ...
Now, there are two main forms in which we might plot loading data
(a) weight vs CG
(b) weight vs moment
Both of these are shown in the typical POH. Such manuals nowadays usually follow the ICAO ops manual format which, subsequently, was adopted by GAMA. This all dates back some years ago ...
Now, if one does the normal longhand calculation sets and plots the RESULTS of the calculations on either format, there is no problem .. one ends up with a few points representing, eg,
(a) empty (or operating .. whatever particular starting configuration definition you might like to use) weight,
(b) zero fuel weight,
(c) landing weight (for as many fuel states as you might find convenient, useful, or otherwise interesting) and
(d) takeoff (or ramp) weight.
The problems arise when people start drawing lines between these points without understanding what the lines might represent and whether such lines have any relationship with real world loading cases.
We need to look at two sets of graphing problems .. and I need to make the point that the scales and datum positions chosen can vary the visual appearance quite dramatically ... but that is another concern altogether ...
First, simple loading cases where we might PRESUME that the loading arm remains (reasonably) constant (and this is the simple case to which I think DFC refers).
Looking at the two plot cases ..
(a) if the plot is drawn on a weight vs CG chart, then one MUST NOT join points with a straight line and presume that the line represents the variation of CG with weight. The actual plot is a complicated curve. Test it .. do a series of calculations and plot the incrementing (weight, CG) points ... not a straight line ....
(b) if the plot is drawn on a weight vs moment (or IU) chart, then the straight line DOES represent the actual CG variation.
Second, consider the notion of constant arm for particular loading cases .. and this is where the main problems can arise
(a) approximately correct for loads such as personnel sitting in seats and we would not normally worry about any minor residual errors
(b) a problem with baggage and freight compartments as the Manual-quoted arm will be for a notional centroidal loading. This is rarely achieved in practice so there will near always be a residual error to consider
(c) fuel loads .. and here is the BIG problem.
(i) as an aside, consider that the "standard" fuel SG values often quoted in pilot training and other documents are not so standard. Fuel density varies with a bunch of parameters .. mainly oil field origin, refining, and fuel temperature. If you want to know the value today at your fuel bowser while you are gassing up the bird, you need either to take a sample and measure the SG in an hydrometer (a simple float gadget to measure liquid density) or check with the fuel farm people who do just that for each incoming load of bulk fuel. Now I can do either as I have half a dozen calibrated hydrometers .. but most pilots can only get the information by ringing the fuel farm office folk .. and they will give you the data without any worries ...
(ii) if we are looking at a small prismatic (eg a nice box shape) tank then the presumption of constant arm usually is OK
(iii) most aircraft tanks are not quite prismatic and there will be a variation of resultant CG as the tank fuel level varies. Be wary that, sometimes, documents will quote a CG for the full fuel case ... which may not be accurate for a part load case. Mostly, the POH will give information if this is a problem.
(iv) many tanks are wildly non-prismatic, eg, swept wing jet fuel tanks .... to use a single arm figure is nonsensical. (Having said that, sometimes we might do just that for particular reasons and take out the error in envelope adjustments on a trim sheet .. but let's not go there at the moment ... )
(v) in the case of multi-tank installations the overall fuel line will comprise several separate lines representing the various tanks. Often a multi-tank installation will have a combination of prismatic and non-prismatic tanks and the overall fuel line can be quite strange-looking
(vi) and, one also needs to be aware of the POH-mandated fuel usage sequence as that will constrain how the fuel is managed and how the loading case for fuel varies.
Another thing one needs to consider is the shape of the CG envelope.
(a) for very small lighties, the envelope might be a simple single forward and aft limit.
(b) for larger lighties, it might include an upper forward (and sometimes aft) sloping limit where the forward limit moves aft at higher weights and the aft limit moves forward.
The problem here is that the weight-CG variation typically is linear (straight line) - although there are aircraft around where (usually aft) limits are complex curves. When plotted on a weight-moment chart, such a straight line becomes a fairly simple curve. Often the curve is drawn as a straight line and, depending on the datum and scaling chosen by the drafter, this difference may be obvious to a lesser or greater extent. However, for the normal lightie chart, using a straight line will be conservative (as it will cut off a bit of the available envelope) so we are not concerned from a safety point of view. We might, however, have a loading (read "dollar") penalty ...
(c) for large aircraft, the envelopes can be quite complicated and there may be some additional problem areas. Two which come to mind involve
(i) sloping limits which go in the reverse sense to what one sees in lighties .. ie the limit becomes less restrictive with increasing weight. In this case, using a straight line on the moment chart is non-conservative and ought not to be done.
(ii) aircraft which check CG for the zero fuel case and stab setting for the takeoff case.
I can recall a number of aircraft where inappropriate use of straight lines can either suggest you are inside (when you are outside) or outside (when you are inside) the envelope ..... as I suggested at the start in respect of drawing straight lines on charts .... not necessarily as simple as people might think.
On a related subject .. years ago I was commissioned to run up a trim sheet for a quite complicated aircraft .. and that was an interesting job as I had to figure out some way of actually doing the task.
However, the point of the tale is that the operator, on the very first local asy training exercise after introducing the new loading system .. rang me up to query how come their standard training ballast load put them outside the envelope ? .. they weren't at all impressed when, on checking it out, I suggested that, for the past however many years, they had been operating outside the envelope on asy sessions ... this was a simple consequence of an inappropriate and ill-conditioned envelope datum/scale in the superseded system.
The plot said it was OK .. but the longhand calculation clearly showed it was not ... The chap who had developed the earlier system was a very experienced consulting engineer for whom I had a high regard .. he just got caught out by a simple design consideration error.
Beware of traps out there, good folk ..
Cup of coffee now finished so I shall stop. There probably are a few other things I should mention but enough is enough for now.
Last edited by john_tullamarine; 19th Feb 2005 at 00:57.