bobsaget123
I wrote a long tome about autopilots then clicked the wrong thing and lost it all! So here goes again...
Several books could be written about this but I will address 3 areas: 1) flight path prediction, 2) power / Nr awareness, and 3) flight envelope protection / Go-around (Flyaway) mode.
1) Most autopilots, including the 332L, L2 and S92, work by detecting a deviation, and a rate of deviation, from the required flight path. So for example when engaged on ALT mode, the datum altitude is compared with the actual altitude to get an altitude error. As that error starts to increase from zero, a correction is made according to the magnitude of the error and the rate of change of that error. This means that there has to have been an error in the first place before the autopilot does anything. In other words, the autopilot waits for an error and then fixes it.
The 225 uses accelerometer data to predict the flight path and correct it before an error has developed. In the case of ALT mode it would resolve the triaxial accelerometer data (lateral, longitudinal and normal) into the earth's plane using the pitch roll and yaw data and monitor the normal (relative to the earth) acceleration. In level flight this should of course be 1g, so if it sees the g falling to 0.99 then it knows that a descent will ensue. So before the descent has even started, it can make a correction to restore 1.0g.
This flight path prediction is why, for example, a 225 engaged on ALT and IAS in straight flight can be banked to 45 degrees by the pilot, holding it for 360 deg then rolling out and, with no pilot input to fore/aft cyclic or collective, throughout the whole manoeuvre the altitude will change by no more than 10'.
This is why it can level off at a preselected altitude having climbed/descended at 2000'/min with no more than 10' overshoot, more normally zero overshoot.
And the principle also applies to lateral navigation. Using gps ground track and wind data it of course knows what heading to fly to intercept and track the localiser (it doesn't just set the heading to match the localiser course and then adjust heading in response to a deviation), but its also monitoring the accelerometers to predict the flight path, so when a lateral gust occurs it knows that its flight path will start to deviate but doesn't have to wait for an error in the flight path to develop before it makes a correction. This is why it can hold the localiser and glidepath exactly in the centre, down to DA and below, when there is a 70kt crosswind blowing over the hills at Dyce.
This is why you can come down the ILS at 165kts / 80% torque and at 500' dial up 30kts IAS. As the torque goes from 80% to 20% in a couple of seconds to deccelerate, perhaps the glidepath might move a smigin but it would be no more that 5% of the scale.
And because its fixing acceleration deviations as they occur, its a very smooth experience for pilots and pax.
225 pilots relish really crap days because they love to fly down the ILS and have it auto-level off, slow it down to 30kts IAS and trickle up the runway exactly on the centreline, beep it down to 30' or so all on the automatics. If you can't see the ground then..... Not that we ever bust the minima, but we get a cosy feeling knowing that if despite the TAFS it all turned to rats# we are fine.
The 92 will of course fly a coupled approach but only on nice smooth days.
2) Unlike the 92 autopilot, the 225 is power and rotor rpm aware. Power is a limited commodity and the 225 will give priority to vertical mode when IAS is above Vy, and to IAS mode when below Vy. This means for instance that you can select 175 kts IAS and 5000' (or whatever) and the 225 will fly along at 5000', at max continuous power minus about 2% (allowance for turbulence etc), going as fast as it can (less than 175!) without fuss.
This means for instance that when going around from an ILS on one engine, it will climb at 1000'/min, washing off airspeed to achieve the climb rate. But as the IAS reaches Vy, priority will change to maintaining Vy and now it will sacrifice climb rate. So the OEI go-around procedure is to press go-around and watch it all happen.
The 225 is also rotor rpm aware so when an engine fails, of course there is no need to touch the controls as the collective is automatically lowered (if necessary) to give 96% Nr (the optimum Nr OEI). If the daft pilot pulls the collective up too far then lets go, it just adjusts itself to give 96% Nr again. If upper modes are engaged it will raise the collective as required to give no less than 96% Nr.
Because its power-aware if (on twin engine) you exceed MCP or takeoff power (according to airspeed), as well as an audio cue, if you release the collective it will automatically gently reduce to take you out of the transient range.
The cleverness of the S92 is limited to a statement in the RFM that tells you not to demand excessive power from the autopilot, otherwise the collective will rise without limit even though the rotor rpm slows down at the AEO FADEC stop.
3) There are various clever features - here is one: You have ALT engaged on cyclic, then you reduce power to slow down, but too much. The nose is raised to maintain altitude at the expense of IAS and in some helis this could result in excessive loss of speed so that either the mode drops out or worse, you enter a tail slide! Not in the 225 - at around 65kts it flashes a warning and then automatically engages IAS on the cyclic, moving ALT to the collective and increasing power to stabilise things.
The GA (Go-around) mode is really clever, eg you are in an OGE hover, when an engine fails. Of course there is no need to touch the controls - just press GA (under your thumb on the collective) and the aircraft will transition away by selecting 10 degrees nose down, adjusting collective to maintain 96%, then as the airspeed is around Vtoss-10kts, starting to raise the nose so that it gets to the climbing attitude just as Vtoss is reached.
We use GA mode on every takeoff. Preselect the desired IAS and altitude on the ground, preset the nominated Vtoss (if its greater than the actual Vtoss) then (for an airfield takeoff) press GA at decision point (V1) and let go. GA mode will set 1000'/min on vertical speed and raise the collective to MCP, climbing at whatever IAS it can muster. Should an engine fail, of course there is no need to touch the controls as it will reset the IAS datum to Vtoss (or current airspeed if that's higher) and climb away OEI. You do have to press the FADEC stop buttons to take the power down from 30 second power to 2 minute power, and again to continuous power. Its a hard life!
Now I am going to post this quickly before I lose it again!
HC
Last edited by HeliComparator; 11th November 2007 at 20:26.