autothrottle 'hunting'

[RAFAIR100]

Can any of the .cfg [autopilot] entries be tweaked to minimise autothrottle (and, hence, rpm) hunting?     On some aircraft the autothrottle appears to be over sensitive.

[Staiduk]

Dunno - I rarely use it myself, but give me a couple of days and I'll check it out. First though; can you be a bit more specific about what you mean by 'hunting'? I'm assuming you mean it tends to overspeed/underspeed rather than going straight to the desired speed, right? And can you give me some idea of when this happens - i.e. during cruise/approach/which aircraft you notice this on, etc?

Thanx!

[-Crossfire-]

I've had this happen to me too... not on the default aircraft, but on some of the freeware airliners.  It usually only happens on approach for me, even in calm weather - the autothrottle will not maintain the desired speed, it will let the speed drop a little bit below desired, then come back up a bit higher than the set speed.

[Nessim]

Don

[JBaymore]

On descent you likely need to manually tweak the throttles and sometimes even use the airbrakes/spoilers.  Also deploying the correct flaps degrees settings and gear extension at the correct times is important in maintaining appropriate speeds.

If you sit at a window seat with a view of the wings on RW commercial flights.... you'll likely see the pilot using the spoilers to control descent speed.

Best,

......................john

[microlight]

You can also get this effect if the plane (especially a jetliner) is over-powered. The Posky 757 was prone to do this until I judiciously adjusted the engine thrust via the [jet_engine] command in the aircraft.cfg file.

Didn't take off like a rocket then, either!

[RAFAIR100]

Thanks all for your replies.     The latest aircraft on which I have had this problem are the Victor B2 and the Vulcan B2.      I did some of the autothrottle tests on both these aircraft and, early on, 'hunting' was a real problem.    You would set a speed and then, to maintain that speed, the throttles would slam from fully open to fully shut.    I can't remember now what was the speed increase or decrease from datum which activated the autothrottle response, but I do remember that when we slackened that value off, the 'hunting' was much reduced.  To come back to the sim world, this 'hunting' manifests itself most noticeably at relatively low speeds - say below about 220K.     I just wondered if there was some parameter I could 'tweak' to minimise this effect.

[Staiduk]

OK; good enough for government work - that's the info I need. I'll D/L both those aircraft and check it out; though I'm pretty much certain what the answer is going to be (Thrust/drag imbalance). I'll get back to you later.

[Nessim]

[quote]Thanks all for your replies.

[Staiduk]

Well, I was able to download the appropriate aircraft and I can certainly see the effect - while it seems to have little effect on the A/C's flight performance those engines spooling up and down like that is a little irritating, innit?

I haven't been able to fix it though - the fix could either be in the autopilot or in the engines themselves and unfortunately today I don't really have enough time to really explore.

Rafair, if you want to fiddle with it, the appropriate lines in the .air file (I'm assuming you have AirEd, if not it's here on SimV) are line 1199, sublines 85-91. Those are the 'Speed Hold' lines - and tinkering with them should produc e the effect you want. Be warned - making this kind of adjustment is likely to include a wide variety of parameters such as throttle rate, drag, engine performance/inertia, etc. Challenging, but a good intro into the world of .air file adjusting.

This doesn't help much, but hopefully it points you inthe right direction.

[RAFAIR100]

Thank you Staiduk.     I'll try it out on a copy of the aircraft.     Incidentally, I'm intrigued.    I haven't heard 'good enough for government work' since I exchanged with the USAF.

[C]

If you sit at a window seat with a view of the wings on RW commercial flights.... you'll likely see the pilot using the spoilers to control descent speed.

Only if he's planned it badly or been held high by ATC!

My only other suggestion is that you stabilise at or near the desired speed before engaging the autothrottles, as it minimises the power change needed by the automatic system, and hence is less likely to constantly overcorrect in a sort of "throttle PIO".

[JBaymore]

Only if he's planned it badly or been held high by ATC!

I'd say that I've seen it on about 100% of my many international flights in 747s and about 90% of my domestic flights.

best,

...............john

[C]

Only if he's planned it badly or been held high by ATC!

I'd say that I've seen it on about 100% of my many international flights in 747s and about 90% of my domestic flights.

best,

...............john

Going into busier airports it's far more likely to be fair. We try and avoid it as it's unpleasant/uncomfortable for passengers more than anything else; noisy and (slightly) bumpy.

Mind you, not being particularly knowledgeable on modern type, I suppose some have them operated by the autopilot as it requires to maintain speed and RoD.

[JBaymore]

Going into busier airports it's far more likely to be fair.

I'm usually going into / out of places like KJFK, O'Hare, Narita, and so on.... so that counts as "busy" I guess.  

best,

.................john