A320 and aerodynamic center

[basics]

I wonder what is the location of the aerodynamic center in A320, considering it is a subsonic aircraft..
most of the books on the aerodynamics says that AC of a subsonic aircraft is about quarter-chord. so it be for A320 also ... but the aft CG limit in flight of the A320 is 45%. for positive longitudinal stability CG should always be ahead of the AC. it means that aerodynamic center in A320 lies beyond 45% chord... could that be true?

thanks in advance

[bigfatchris]

Well, without going through all the manuals, 30% is the rough trim position for take off. I have been lazy by assuming that is %MAC. If I am corrected I would be grateful, if not I'll go on blissfully unaware.

[hvogt]

I'm not the technical type, but from what I have read, only the aerodynamic centre of the whole aeroplane must be behind the centre of gravity to ensure positive longitudinal stability. However, modern airliners are designed in a way which allows the aerodynamic centre of the wings to be in front of the centre of gravity.

[basics]

I'm not the technical type, but from what I have read, only the aerodynamic centre of the whole aeroplane must be behind the centre of gravity to ensure positive longitudinal stability. However, modern airliners are designed in a way which allows the aerodynamic centre of the wings to be in front of the centre of gravity.

could you please refer the title the book you read it from? I could explore more...

[hvogt]

I read this in a German university skript I downloaded some years ago. I'm afraid the title won't help you, unless you speak German. Sorry.

[cwatters]

Just try throwing a dart or arrow backwards (eg flights/MAC ahead of the CoG) see how it's flight path diverges until it stops and falls the right way around!

Longitudinal static stability - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Longitudinal_static_stability)

Neutral point

A mathematical analysis of the longitudinal static stability of a complete aircraft (including horizontal stabilizer) yields the position of center of gravity at which stability is neutral. This position is called the neutral point. (The larger the area of the horizontal stabilizer, and the greater the moment arm of the horizontal stabilizer about the aerodynamic center, the further aft is the neutral point.)

The static center of gravity margin (c.g. margin) or static margin is the distance between the center of gravity (or mass) and the neutral point. It is usually quoted as a percentage of the Mean Aerodynamic Chord. The center of gravity must lie ahead of the neutral point for positive stability (positive static margin). If the center of gravity is behind the neutral point, the aircraft is longitudinally unstable (the static margin is negative), and active inputs to the control surfaces are required to maintain stable flight.

[rudderrudderrat]

Hi Basics,
it means that aerodynamic center in A320 lies beyond 45% chord... could that be true?
From FCOM DSC -20 Principle Dimensions:
"CG limits are given in percentage of the reference chord length aft of the leading edge.
The reference chord length is 4.193 m (13.76 ft). It is 16.31 m (53.51 ft) aft of the aircraft nose.
The CG must always be within these limits, regardless of fuel load."

The main wheels are 17.7 m behind the nose, and the C of G is always forward of the main wheels.
The reference chord leading edge is therefore close to where the wing intersects the fuselage.
The swept wing has the centre of pressure further aft, so the C of G is always forward of the Centre of Pressure.

[cwatters]

I'm not the technical type, but from what I have read, only the aerodynamic centre of the whole aeroplane must be behind the centre of gravity to ensure positive longitudinal stability. However, modern airliners are designed in a way which allows the aerodynamic centre of the wings to be in front of the centre of gravity.

I believe that's possible due to the combination of sweep, washout and a wing section that varies from root to tip. All tricks used on tailless aircraft to manipulate the pitching moment..

Basic Design of Flying Wing Models (http://www.mh-aerotools.de/airfoils/flywing1.htm)

[Microburst2002]

It is the airplane's aerodynamic center that matters.

The 25% of MAC applies to the wing, only. If you include al the other elements (fuselage, nacelles...) then it is somewhere else. Then you have to put the CG ahead of it.

Normally, airplanes would be unstable if they had no tailplane (AC ahead of CG), but the great effect of the tailplane (also called stabilizer) makes the airplane estable, as a whole. Otherwise the airplane would be inefficient (large negative lift at the tailplane).

I think that in the case of the 320 all these things about stabiilty are complicated if we consider normal law because we cannot even talk about stick free or stick fixed... but I have the idea that 320s are less stable than similar conventional flight control airplanes, which makes them more fuel efficient. Normal control law augments their stability by means of the computers.

[FE Hoppy]

These chaps have a reasonable understanding:chapt9 (http://history.nasa.gov/SP-367/chapt9.htm#f132)

Another thing to note is that supercritical airfoils are quite aft loaded meaning their and therefore the aeroplanes AC can be quite far aft of the simple 25% figure we learned as kids.