Weight & Balance..............?

Perhaps I can help you?

The problem lies around the fact that a lot of people own American aircraft, which 'quite rightly' supply a "POH" (Pilots Operating Handbook), I think the reason for this is that our American cousins have had a law about "Freedom of Information" for many more years than us chaps here in good old Blighty!

So everything you buy in America, has an "idiots Guide" to accompany it.....draw your own conclusions?

The POH has a plethora of intricate detail and all the calculations that you require to build a G of G graph, like the one you see in this programs' instructions.

My aircraft, a Grob 109B Motorglider has a "Flight Manual", which is Europe’s equivalent of the POH, but it only has half the information the POH has! So you have to dig deep into the wording to extract what you are looking for.

In order to work out the loading envelope for my aircraft, the loading chart only has 4 points; instead of 5 (the Cessna one has 5, which is the one in the program instructions).

Another piece of information is also needed to work out your numbers and apply them, my aircraft's data shows that the distances from the datum point are:

Empty Weight 0.360m, Crew 0.083m, Baggage 0.72m and finally fuel 1.0m (If like the Cessna you have 2 sets of seats then more distances will be shown).

Finally you will need the latest weighing chart from your engineer, or whoever does your Servicing/ARC etc, by the way an aircraft must be re-weighed every 8 years and or if significant changes are made to the "Weight and Balance".

OK, so now lets get down to detail: My last weighing shows that my aircraft is: 673.47Kg with an empty C of G at 363.34mm (if your aircraft is in inches/Lbs, change all the values to mm and Kg's its much easier to work with).

The forward C of G limit shown on my last weighing is 271mm and the aft limit is 427mm.

Now lets look at the weights and the "distance" i.e. 'Moment' these are stable and constant and set by the manufacturers, so we can use them in our example, there are 4 things listed in the moment scale: Empty weight, Crew (there are 2, side by side seats in my A/C so they are both at the same moment), Baggage and fuel, you may have more seats, so you will have to add weights and moments to your aircraft editor.

Let’s put some numbers with this: Empty weight 673.47 x 0.36 = 244.69m kg (Momentum).

The next calculation is for the pilot's: Pilot 1 (me) 83kg x 0.083 = 6.89m kg

Baggage (max), if carried: 20kg x 0.72 = 14.4m kg

Fuel (Avgas weighs 0.7 kg per Litre, 100 litres x 0.7) 70 x 1.0 = 70m kg

Now if we add all those together: weights=846.47kg. Momentum: 335.98m kg.

We now divide the momentum/weight = 0.3969m or 397mm, as we can see from the Fwd and Aft limits it is in between them, so we are safe!
However, if I add another pilot weighing the same as me the calculation will look like this: 929.47kg/342.87m kg = 369mm still in the C of G, however the chart will show "red", because I have exceeded the MTOW (Maximum Take Off Weight), which in my case is 850kg!

So because the moment arm for my fuel (1.0) is equal (what you add/take away from the weight calculation you also add/take away from the m kg calculation), all I need to do is reduce the fuel weight i.e. 929.47kg - 850kg = 79.47 kg or 113.52 Litres, BUT my aircraft only carries 100 Litres of fuel, so I can't fly with 2 pilots of similar weight at all, unfortunately there isn't any way around this, my pilot 2 or passenger needs to be skinny! Or I need to lose weight myself! (This is why; sometimes you can be in the correct C of G, but still get a red chart because you are overweight).

Lets now get rid of the baggage: numbers look like this 909.47kg/328.47m kg = 361.16mm still in C of G but red lined again because of MTOW, so now I need to reduce the amount of fuel to bring the weight down to 850 i.e. 59.47kg = 84.95 Litres, so now I can fly but only with 15.04 Litres of fuel on board, the numbers are: 850.00kg/268.99m kg = 316.45mm back within the C of G limits.

Hope I haven't lost you at this stage, complicated I know, but bear with me?

So how do we draw our "polygon" to make this happen automatically, my graph only has 4 points, with weight on the vertical left scale and momentum at the bottom horizontal scale, it also has a pre-determined start at 700kg weight (that’s because a new Grob 109B only weighs 630kg, which is below the minimum C of G), so a minimum pilot of 70kg must be added to bring the weight and balance "Within Limits", next the first point at the bottom of the left hand side states that:

Momentum at the forward limit C of G is: 190 @ 700kg the highest forward limit is 230 @ 850kg; so now we have: 190,700, 230,850, now do you see where we are going?

So the whole picture, using all four points on the graph will look like this: 190,700, 230,850, 363,850, 299,700, these are the points that are input into the weight & balance page in Skydemon; (you need to open the "aircraft library" page and choose your aircraft and then “edit” to get to the weight and balance), you can see that it only has four points, but it is a graphic showing you if, when you set your pilot's/passengers’/fuel/baggage, if you are within your C of G limits.

When entering your numbers, the format is so: 000,000, space 000,000, space etc.

I do hope this helps you and others in this forum?

Dave

Hi dave

That was an interesting point you made Andy. It also applies to the Rockwell Commander 112A, that has similar characteristics to the Arrow. One can easily go outside the C of G if not careful. Perhaps more easily than the Arrow. I do eye-ball things sometimes with a total of 3 persons on board (not 4).

By the way, I came across this little brief story, albeit about a model RC. However, it illustrates the importance of C of G. Fortunately it was about a RC aeroplane!

"Hi, I have a Grob 109 by ESM, or KMP depending if your in the U.S or elsewhere. I'v had it for 5 months but never flown it. Why, because i destroyed it on the first flight. Balance was way off. Having the battery way forward (out of the battery compartment) and it still wouldn’t balance anywhere near the instructions so i decided it must be a misprint and off i went. They weren’t a misprint! She tracked out nice for a bit and then was spinning towards disaster in a few seconds."


Very Newbytheway

DurhamTeaseAirport

sorry Andy, it's what and how to put the info into sky demon thats causing me problems. but thanks for the advice.

help mogas

Sorry it worries you Andy.

Dave
Very very informative , simply and thoroughly explained, well done and thank you

new user of SD

Iain