View Full Version : AFCS really makes my heid hurt
21st Feb 2002, 03:11
The autopilot controls the aircraft about which point...CG or AC?
I reckon it's the CG, but my brain's too full of circling minima and touch down marker separation distances to work it out properly. help
21st Feb 2002, 04:11
Not exactly sure what you're trying to get at here, Lumpy, but...
The autopilot responds to the difference between what you asking of it and what the aircraft is doing now. Airplane sensors provide the autopilot computers with data relating to what the airplane is doing now (e.g. gyro's, IRS's, air data sensors, compasses, etc).
Attitude sensors, such as gyros and IRS's or whatever are usually located in an Equipment Centre in the forward part of the airplane. In this case, I guess you could say that the autopilot thinks this is where the airplane is... at least in terms of pitch, roll and yaw (and controls the airplane from this point). However, when it comes to things like altitude, it depends on where the static ports are on the aircraft and what baro settings the pilot's instruments are set at. If the pilot has set these incorrectly, the autopilot will think you are in a different place to what you actually are... and send you to the wrong altitude.
Any clues here?
[ 21 February 2002: Message edited by: QAVION ]</p>
21st Feb 2002, 06:27
The autopilot controls the aircraft in exactly the same way as a human pilot.
Let's consider pitch control, as it is the most significant in respect to the question. The Centre of Gravity (CG) is a physical characteristic of the aircraft and the weight of the aircraft acts down from this point. The weight is opposed by the lift acting upwards at the Aerodynamic Chord (AC) which shifts in flight according to the speed and configuration of the aircraft. When the CG and AC are not aligned, the aircraft will pitch up or down as the case may be unless opposed by a control force. Lets say the AC moves aft of the CG; the aircraft will pitch nose-down and nose-up control must be applied to counter it. This is initially done by moving the column aft to deflect the elevator up, then the resulting column force is trimmed out using the trim system and the aircraft remains steady "hands-off" Both the CG and the AC remain in the same positions as before the control input, but the aircraft is now balanced about the AC by a nose-down force acting through the CG and an equal but opposite nose-up trim force acting upon the tail surface. Just like a set of beam scales.
So both the autopilot and the human pilot control the aircraft about the AC. They do this by generating control forces to balance the difference between the weight acting at the CG and the lift acting at the AC. Both human and automatic pilots detect the changes through a change in aircraft attitude and apply the same control inputs to correct the attitude. If a standing control input results, both human and automatic pilots trim the control force back to neutral using the trim system.
It is of course possible to control the aircraft by moving the CG, this usually being accomplished by moving fuel about. (Premium passengers in transport aircraft are resistant to being moved down the back whereas coach class passengers are much less resistant to being moved forward, thus nose down trim changes are more easily achieved than nose up) Such methods are a long term means of trim and do not affect the explanation. The CG is relatively fixed and only changes slowly, AC changes occur more rapidly.. . . .**********************************. .Through difficulties to the cinema
21st Feb 2002, 14:40
Thanks to both.
22nd Feb 2002, 08:07
In actual fact the AC moves as the pressure distribution changes with angle of attack. The point about which the aircraft pitches (the axis of rotation) is neither the CG nor the AC, but changes, and would have to be determined by adding the moments of the forces involved.
I'm not surprised that the JAA exams are incorrect on the technicalities <img src="rolleyes.gif" border="0">
(I'm studying these myself at the moment, in preparation for a move to the U.K.)
22nd Feb 2002, 08:55
Help me out here. Is this new aerodynamic feature the "AC" (Aerodynamic Chord) something I have missed? Are we not talking CofP (Center of Pressure) here. AC or more exactly MAC (Mean AeroDynamic Chord - [in anything with other than totally straight wings]) is a line position on the longditudinal axis that is quoted as a % point (usually) to provide an indication of trim position - surely....
[ 22 February 2002: Message edited by: MasterGreen ]</p>
The question asks ABOUT which does the autopilot control not what does it change to effect the control.
An autopilot (full authority) controls about 3 axis, vertical (yaw), longitudinal (roll) and lateral (pitch). The AC is a function of pitch only so is not the right answer, and, as has been stated moves about so would be rather difficult to use as a reference point. The three axis converge at a point which would equate (roughly) to the a/c C of G (which also moves but to a much lesser degree) so I think that is the answer they are looking for.
As an aside, Qavion, it does not matter where the static port is located, the pressure at the sensing device (ADC or altimeter) is what is measured. If the static port were on the top of the fin (say 60 feet agl) and the ADC in the E&E bay (say 10 ft agl) then for a given local pressure the altimeter would read 10 ft. :)
[ 22 February 2002: Message edited by: mono ]</p>
22nd Feb 2002, 15:47
The Aerodynamic Centre (AC) is the theoretical point on the chord whereby any change in angle of attack and subsequently the Coefficient of lift (CL) does not alter the pitching moment about the wing centre of gravity (CG).. .An aircraft is controlled by applying a moment (pitching, rolling or yawing) about it's (the aircraft's) CG.
23rd Feb 2002, 15:42
"As an aside, Qavion, it does not matter where the static port is located,..."
Good point, Mono. I realise the error of my ways :)
Of course, technology will ever put an endless spin on things... The Boeing 777, I am told, measures pressure at the probe itself, and transmits the altitude signal electrically to the ADC's (or should I say ADIRU's). Unless some sort of correction is used in this case, I'd say that the altitude should read the same as the probe height.
Yup your right there, the scarebus also uses Air Data Modules (ADM's) located very close to the pitot and static probes (less plumbing, less hastle and less weight) which send raw atmospheric data to the ADIRU's for processing so as these are the sensing device (the ADM's) that is the measured pressure.
M. . <img src="eek.gif" border="0">
[ 23 February 2002: Message edited by: mono ]</p>
lumpy The autopilot controls the aircraft about which point...CG or AC?
I remember there was a time when I knew this stuff cold. It just doen't seem like it was that long ago. Anyway, the following link is very illustrative of this topic:<a href="http://188.8.131.52/aerodynamics1/Stability/Page7.html" target="_blank">?</a>
To be excruciatingly precise, you can add a force to a point on a rigid body (which will add a moment unless the point is the CoG) but if you apply a moment you apply it to the entire body.
Cheers,. . /ft