Mass & Balance-Performance questions

[N1 Limit]

Hi There!

I'm undertook to do a bit of studies to refresh my knowledge while seated at home looking for a job,i elected to use OXFORD Books which i've got some.So i'm now busy on the Flight Planing and performance and my question concerns the DATUM.While Datum is determined by manufacturers i'm having some troubles when it comes to resolving questions dealing with Datum.On many examples they've drawned in the book the Datum to illustrate them.I myself don't exactly know where to draw the datum to start with when solving some questions.I'll give u an example and u help me sort it out

An aircraft has a MAC of 82 inches.The leading edge of the MAC is 103 inches aft of the datum.If the CG position is 14.7% MAC,what is the CG distance from the datum?

Any help is appreciated

[Tigermagicjohn II]

If all the datums are positives, draw the datum in the front.
Good idea also to draw the LeMac, MAC, and whatever else information they provide.

If you draw the datum at the nose, then it's linar scale, 0 at the nose and whatever measurement given.
All you need to figure out is where the MAC starts and ends, normally they give LeMac or C of G position, with this you should be able to calculate the different positions.

Example LeMac is 10m, MAC is 1m, C of G is 25% of MAC, your C of G would be at 10.25m.
Some questions however put the datum in the centre, then you need to use Plus and Minus signs, in front of datum Minus, behind datum Plus.

Hope this helps.

[Alex Whittingham]

I wouldn't draw this example. You are told the MAC is 82 inches long, the CG is 14.7% of that distance back from the leading edge of the MAC, i.e. 14.7 x 82 / 100 = 12 inches back from the leading edge. The leading edge itself is 103 inches aft of the datum so the CG should be 103 + 12 = 115 inches aft.

[N1 Limit]

Thanx very much guys for ur help,i appreciate this

[Keith.Williams.]

Too many people view their groundschool studies as if they were a race, with prizes being awarded for finishing in the minimum time. To win this race many students devote their entire efforts to simply memorising the answers to questions, without ever attempting to understand any of the subject matter. I'm not suggesting that you did so N1, but you probably know a lot of people who did.

Revising (some of) the ATPL theory subjects is certainly a good way of using the time while waiting for the employment situation to improve. To maximise the benefit gained, it should be viewed as a second chance to get to actually understand the material.

As Alex has said you do not need to draw anything to solve the question posed in this thread. But that does not mean nothing will be gained from doing so. They fact that you are asking where to draw the datum suggests that you will probably gain something from making a sketch.

A good starting point would be to ask what the information given in the question actually means.

If the C of G is located at 14.7% MAC, this means that it is 14.7% of the way back from the MAC leading edge to the MAC trailing edge

In this question the MAC is 82 inches from leading dege to trailing edge.

So the C of G is 14.7% of 82 inches aft of the leading edge.

As Alex has said, that is 14.7 x 82 / 100 = 12 inches aft of the MAC leading edge.

The question also states that the MAC leading edge is 103 inches aft of the datum.

So the total distance from the datum to the C of G is 103 + 12 = 115 inches.

If all of this is not obvious, it will help to draw a sketch.

As Tigermagicjohn has said, all of the distances given in the question are aft of datum, so we may as well draw the datum at the nose of the aircraft.

The MAC leading edge will be 103 inches aft of the datum.

And the C of G will be 12 inches aft of the MAC leading edge.

The sketch will show quite clearly that the C of G is 115 inches aft of the datum.

[N1 Limit]

Hey Keith Williams

What u just mentionned is indeed true but thanx to braking it down for and that's the way i intend going so that my knowledge can improve in all aspect of flying.Thanx a bunch again

[fly_antonov]

Is this a joke or something?
April' s fool posted a day early?