Mass and balance calculations

[Airlinepilot1687]

Hi all I was just wondering if anyone had any helpful tips or websites to look at to help with mass/balance calculations and also the c of g. I dont find this section at all easy and could just do with some more info.

Many thanks :confused:

[Jan Olieslagers]

As an extreme measure, you could consider the five-day training as mentioned at

http://www.luchtzak.be/forums/viewtopic.php?f=7&t=45293 (http://www.pprune.org//http://www.luchtzak.be/forums/viewtopic.php?f=7&t=45293)

[24Carrot]

Or look at a POH. The C172 POH has diagrams, a worked example, the works.

[patowalker]

w&b (http://www.lightaircraftassociation.co.uk/engineering/Weight%20and%20balance/w&b_main.html)

[Genghis the Engineer]

I'd suggest just do a lot of weight and balance calculations, using the material that should be available at your flying club.

It really is one of those things that just comes together with practice.

G

[Gertrude the Wombat]

The calculations are straightforward ... but some people, myself included, find it difficult to do calculations when we're just following formulae by rote and don't actually understand what is going on.

I remembered enough O level mechanics to know what concepts such as "centre of gravity" and "moment" were, but the idea of calculating moments with respect to an aribtrary reference point (eg the firewall) rather than actually finding out where the CofG is was a rather different way of looking at it. Plus of course we only bother with front-to-back balance calculations for light aircraft, I never saw it written down anywhere "of course the CofG is located somewhere in three dimensional space, there are monents about it in all directions, but we don't worry about how far off the centre line the CofG is because you can't load a light aircraft in such a way as to cause problems here, and we don't worry about how high up the CofG is for similar reasons".

So what some people might find helpful is not a magic procedure - "stick these numbers in these formulae, plot the result on the graph, and you get the answer, honest, trust me on this" - but an explanation of some O level mechanics that they might have missed out on in their schooldays so that they have some chance of actually understanding what is going on.

[stiknruda]

An alternative option:

I learned all the W&B formulae and passed all of the CPL exams. I felt I understood it , too!

H O W E V E R - what really cemented it into my little brain was a few years ago watching an aeroplane going through a new weight and balance. (I'd seen it done a few times but never really took an active interest...)

Seeing the a/c jacked, levelled and weighed - understanding where the datum was, watching all the measurements being done and the primary school maths required............... chalk lines on the hangar floor, tape measures and multiplying and dividing big numbers!!......

well it all became so obvious!

I would recommend anyone to go and see this happening as it might just all "click" as it did for me!

Stik

[BackPacker]

chalk lines on the hangar floor,

To be honest, I don't get that part.

When designing an aircraft, the manufacturer decides on an arbitrary datum for all W&B calculations. It really doesn't matter what that datum is, as long as it is unambiguously defined, easy to find, and as long as everybody uses it afterwards, from designers to pilots.

From the plans you can measure the distance from the wheels to the datum, if they're not simply listed in the POH. Combine that with the measured weight on the wheels and you have the individual moment. Add up all weights on the wheels, add up all the moments, divide and you have the actual CofG of the aircraft. No need for chalk lines on the floor, tape measures and whatnot.

At least, that's what my high school physics, and looking at a few actual W&B sheets, tells me. You can work it all out from the individual weights on the wheels and the distance from the wheels to the datum.

So why the chalk lines on the floor?

(I have seen the situation that you describe though. But that was during the construction of a Europa, to fix the exact location of the main wheels before bonding in the legs.)

[patowalker]

It's to make sure the wheels are where they are supposed to be.

[BackPacker]

'been googling a bit based on your comment. Yeah, if it is to cater for individual variations due to manufacturing tolerances, damage, rigging and whatnot, and to get the most accuracy, I can imagine that you don't take the POH values for the arm of the wheels, but measure them. Fair enough. Thanks.

[stiknruda]

It's also to do with the datum (100" fwd of the lower wing leading edge) and the fact that with the tail on the ground and the tail raised the datum shifts!

[Airlinepilot1687]

Thanks guys these really help. Its just one of those things i will need to practice more. Once i get it hopefully it should stick. I find doing more practical work alot easier. Thanks again. :O