MAC Trim
 

On a load sheet we are given the mac trim for takeoff. This is entered into the fmgc and set on the stab. Typical figures are in the region of 20-30 for my aircraft. Can anyone tell me what this value actually is and how to make sense of it.
Thanks

The MAC is the Medium Aerodynamic Chord. You give the position of the CG in % of the MAC. Depending of its position you will need a different angle for the THS (or the trim tab in other aircrafts not the A320). You can find the required THS angles (1UP, 0, 2DOWN...) required for each CG between the thrust levers and the trim wheel (this is in the A320, but I suppose will be the same in other acfts.)

Is this what you're asking?

MAC
 

So if I had a trim of 25% MAC with a wing of 100cm chord then that refers to 25cm with respect to the leading or trailing edge? How does this relate to the C of G? What exactly can I practically do with this piece of information. I'm just trying to bolt it down to the day to day world. MAC 25% doesn't mean much in isolation.
Thanks

You don't have a trim of 25% MAC. You have the CG at the 25% of the MAC, which gives you a specific angle for the THS. And yes, if your CG is at the 25% of MAC, if MAC measures 100cm. you will have the CG 25cm. behind the LEMAC.

In little acfts (the ones I fly:) ) we don't work with MAC. For us the CG position is measured from the datum (generally at the nose). But in big acfts the CG is measured by %'s of MAC. By aerodynamics you can only have the CG in the MAC, but also if must be within limits (which are also measured in %'s of the MAC).

To sum up, you should know the position of the CG (in % of MAC) at T/O to assure is within limits and also to know what is the required angle of the THS.

Do you understand?

PS: What plane do you fly?

MAC
 

lo siento!
This is an expression of C of G.
Thanks
(soon to fly an airbus)

The "M" in "MAC" stands for "Mean" - in the sense of "average" - not "Medium".

The MAC is actually a theoretical concept; it doesn't really exist on the aircraft, it's a way of averaging (hence the "mean" part) the overall wing shape to a single notional wing chord. Consequently, it doesn't really have a "leading edge" or "trailing edge" - it's just a reference concept.

Just for aerodynamic pedantry...

You're right! Sorry for my Spanglish :O , we say "Cuerda Media Aerodinámica", so I confused the word.

So is it wrong if for example I say the LEMAC is xxx inches from the datum?

Not really, since what you're saying in shorthand is really that the zero point of the mean aerodynamic chord (which does indeed tend to get called the "leading edge") is at a certain position. What's important is that its a geometric reference point, not a "real" leading edge.

As I said, my point was to some extent being pedantic. Its the engineer in me ;)

Oh, and if I were conversing in anything but my native tongue (English, of course) I'd appear to be a mentally challenged 8 year old, so no apology needed.

Ok. Thank you for the explanation! :ok:

Quite useful to join the model aircraft ranks to sort out MAC!! If you build a swept wing model, the MAC is needed as reference for a standard C.G at about 25% MAC.
How's it done? - simple ... draw a scale plan view of one wing; reverse the view and attach it to the first one such that the trailing edges coincide; draw lines joining the four 'corners'; where these lines cross is the MAC point on each wing. On one (or both) wing plan(s) draw a chord line at this (these) point(s). Mark a point at, say, 25% of that chord line on each wing and join them. For easy handling CofG purposes, the aircraft should balance longitudinally when suspended at that point. Simple, innit?:ok:
Then all you have to do is learn to fly radio!!!:eek:

Quite useful to join the model aircraft ranks to sort out MAC
 

Cornish beware.
What model makers refer to as MAC is NOT MAC it is the mean geometric chord. This does not take into acount changes in profile or twist and is very very crude. Mac is the chord of an imaginary straight wing with the same area and AERODYNAMIC FORCE VECTORS as the aeroplanes swept wing.

FE Hoppy
Point well made and taken:ok:
However, as with most things to do with aerodynamics, unless one is working as, or training as a TP or TE/Obs, most average 'Joes' want some sort of 'simple' visualisation of the more abstruse areas of these functions.:confused:
I'm reasonably sure that the difference between MAC and MDC is not going to overly exercise most new RHS occupants but a reasonably simple mental picture is usually 'close enough for Government work'.;)

Even on a fairly simple swept wing there's quite a difference between the mean aerodynamic and geometric chords.

If memory serves me correctly, the values for the BAe Hawk were 1.777m for geometric and about 2.1-2.2m for aerodynamic - interestingly, perhaps, it was the gemoetric chord that was used EVEN for aerodynamic purposes - it is, after all, just an arbitrary reference length.

I'm with you Cornish, but It adds time in the classroom when I have to demonstrate the difference between the two to disbelieving pilots. I would rather we all used the same definitions for the same terms.

........ and in physical terms, i.e. INCHES, say, for a typical 747, that difference would amount to ... ?
Just slightly OT, for 47-400 operators, a little question - if you entered the wrong MAC value into the FMC, would there be anything to advise you of the fact, prior to take-off?
Worth looking in the book for this one:E

Some thoughts ..

(a) aerodynamicists like measuring CG in %MAC for engineering reasons as handling qualities are linked conveniently to %MAC for design and flight test work. Apart from TP/FTE types, who are interested in the same aspects, the piloting fraternity doesn't need to worry too much about the subtleties.

(b) the Flight Manual and loading data generally are prepared by the OEM's aerodynamics section

(c) guess what .. the aerodynamicists take the easy way out and prepare the AFM/loading manual data using %MAC rather than refiguring it in pilot friendly CG arm values.

(d) for the operator, %MAC is just a different, more complicated, way of saying the CG is so many units aft of whatever datum you choose.

The two are related by a simple equation

%MAC = ((CG measured from datum - LEMAC referenced to datum)/length MAC)*100

The original post identified the reasons for having the %MAC overlaid on the trimsheet. Depending on the aircraft, an actual CG near either the forward or aft limit and the stab set for the opposite end of the envelope can result in some interesting times for the pilot on the day ... so it augurs well to get it right at the sums stage ..

Is one better than the other ? ... with my occasional aerodynamicist/flight test hat on, %MAC is better .. but, with a routine pilot hat on .. give me inches or mm aft of the datum every time, please ..