Datums

[bmb7jiw]

Hi,
You probably think this is a very basic question but.........

Im currently studying the Thom books for my PPL and Ive just been reading the mass/balance chapter. Can someone please explain exactly what a datum is?
I understand the balance theory (I think) but in Thom I havent seen it explain what a datum is exactly, and what the difference is between the datum and the pivot point.


Thanks a lot




James

[BEagle]

It's the theory of moments. Moments have to be taken about a reference point; each element (pilot, fuel, bags) can be thought of as mass acting through the point of application. The datum is just a convenient point for the manufacturer to measure the moment arm of each element from. Then the centre of mass is at a distance from the datum equal to the sum of the moments divided by the total mass.

I've had students who can't understand this - it seems that basic 'mechanics' isn't taught in any depth at school these days......

[Saab Dastard]

James,

I wrote some notes on this a while back - follow the link:

http://www.pprune.org/forums/showthr...ht+AND+balance

HTH

SD

[bmb7jiw]

Thanks a lot
I understand the theory. I had just never heard datum being used before. I assumed that the pivot point was the only thing used when calculating moments!

Thanks again

James

[GRP]

Sounds to me like you are still confused. The pivot point and the datum point are one and the same thing. You may be confusing the pivot point with the actual point where the c of g lies but there is no single place where this happens. There is a point where the c of g lies when the aircraft is in its empty state which 'could' be used as the datum or pivot point but it would make for a more complex calculation than you need. Typically the pilot, front seat pax and fuel would probably be in front of that point and everything else you load would be behind and your calculation would involve some positive elements and some negative ones. In this case the C of G limits would also need to be quoted as a negative forward limit and a positive rearward limit - measured from that empty c of g position.

So... to make things easier for us C of G is calculated by picking "any" point and saying that the c of g must be between x and y inches from that point. The point chosen is completely irrelevant in the calculation but it helps if it can be guaranteed that it is always in front of any object that you might place in the aircraft so that you do not have to start working with positive and negative moments. The point chosen could be 10 feet in front of the propellor or 500 feet behind it and the result would be the same. The distance that you calculate is always measured from that point, as are the points that your c of g must be within to remain in balance.

Datum points I have come across always seem to be given as a set number of inches ahead of where the wing meets the fuselage and although I've not measured, this probably coincide with a point at the most forward part of the aircraft (the tip of the spinner or the nose maybe). This would ensure that any weight you add anywhere on the aircraft (whether it be a heavier prop or an item of baggage) is behind the datum point so all moments calculated would still be positive.

I may well have confisued you further here.... hope not!

[Tinstaafl]

I wonder if you're confusing 'datum' with 'Centre of Gravity (CG)'?

A datum is nothing more than reference point from which everything else is measured eg elevations based on mean sea level, flight levels on 1023Hpa or....CG location.

The CG is located somewhere in the a/c, but where, and where do you measure the 'where' from? Ditto everything that can be placed in or on the a/c. A starting point has to be defined somewhere - and that's the datum. It could be the furthermost forward or rearward point on the a/c or some convenient point in between.

If a mid-point is chosen then you start having to use +ve & -ve combinations in your calculations (all things to one side of this mid-point will be +ve, all things to the other side must therefore be -ve).

If you choose to use the CG position when the a/c is empty then you would have to define the loaded CG in relation to where the empty CG is ie the Empty CG position is your datum. You would still have to deal with a sometimes forward CG, sometimes aft CG - leading to lots of +ves & -ves in your calculations.

It's much easier to ensure that whatever measuring point is used (datum) that it lies somewhere where all items that can be placed in the a/c will always be to one side of this position. That means that you can avoid mixing +ve/-ves and simplifies things a bit.

Note that everything, including the empty CG gets measured from this point. You haven't changed the physical position of the CG & CG boundaries, just where you choose to measure the location from. No different from describing your house position from the mid-point of your street or from one end.

Cessna commonly uses the front face of the firewall as the datum on their piston singles. Piper likes to use xx inches forward of the wing leading edge at a certain span position. Can't remember what Beech like to do. Notice that all those points in space are fixed relativel to the aircraft? That means that no matter how the a/c is loaded or what modifations get done to it, the measurement of the CG and CG limits remains constant. Much easier to deal with than if you chose, say, the tip of the spinner but then had optional spinner sizes.

Hope this helps.

[bmb7jiw]

Ok!!

I think I have it now. Thanks a lot for those replies.
In Thom it never mentioned exactly what a Datum was, so thats what threw me.
Im not starting my training till January, so I dont have an Instructor to ask yet.


Thanks again


James

[Genghis the Engineer]

The datum can be almost anywhere, I've seen particular bolts, wing leading edge, mainwheel axle, nosewheel axle, mainspar rivet line, point in space in front of the nose, etc. etc. Once only when working on an aeroplane (a BAe Hawk T1) I went and asked the groundcrew where the CG datum is, and they pointed out a small metal plate under the belly with a line engraved saying "this is the CG datum" - very helpful indeed, that sort of behaviour should be encouraged.

There's a French aeroplane where they decided to use the prop hub, then kept fitting different engines and causing confusion. When the type was approved for the UK thankfully some sensible soul decided to relocate the datum to the mainwheel axle centreline which made like much easier.

(Then again there's another French aeroplane which did use the mainwheel axle, then the UK version got a different maingear design, so the datum is now 40mm forward of the maingear axle centreline - ho hum).

G


N.B. Why do pilots never notice that the plural of "datum" is "data", and the plural of "minimum" is "minima" ? Always troubled me that.

[Bottle Fatigue]

Likewise with agenda and agendum. It always annoys me. I think I must be getting old.....

[Keef]

To be blamed on the demise of Latin as a compulsory subject in British schools.

The one that sends me up the wall is "media" - plural of "medium", but far too often treated as a singular in its own right.

[Timothy]

Less and less people understand grammer these day's.

W

[DubTrub]

...and peeples speling aint to good eether