Hunting throttles

[qratrev]

Wonder if anyone has come across this problem. On several downloaded a/c f.s 2004 when selecting autopilot in cruise, the throttles 'hunt'. i.e move slowly backwards and forwards. Seen it on 'live' aircraft but wonder if its a cfg problem in the program. Would appreciate any input.

[old_wombat]

Yes, a modereately common problem. Fixes for it have been seen here several times. THere is a change to an aircraft.cfg parameter. Looking for it now.

 

Here we are.

 

[TurbineEngineData]

fuel_flow_gain=xxxxx

[napamule2]

Well, that is not Auto Pilot related. Nice try but no cigar. As far as 'hunting' throttle it is due to modelers not using the latest parameters for the autopilot values. What is in the SDK is not always the 'latest'. But to be fair, they also use 'old' values in the AIR file, so we might have to make changes to those section in AIR file. In ac cfg in (autopilot) section look at this line: 'autothrottle_max_rpm=90' . If it's '90' then change it to '110.000' and do SAVE. Then go check auto throttle when in AP. And report back with results if you will. We might have to tweak the AIR file too, that's why. Thanks.

Chuck B

[qratrev]

Well, that is not Auto Pilot related. Nice try but no cigar. As far as 'hunting' throttle it is due to modelers not using the latest parameters for the autopilot values. What is in the SDK is not always the 'latest'. But to be fair, they also use 'old' values in the AIR file, so we might have to make changes to those section in AIR file. In ac cfg in (autopilot) section look at this line: 'autothrottle_max_rpm=90' . If it's '90' then change it to '110.000' and do SAVE. Then go check auto throttle when in AP. And report back with results if you will. We might have to tweak the AIR file too, that's why. Thanks.

Chuck B

 

Thanks gentlemen will dust the spanners off and give the old girl a tweak ! many thanks

[qratrev]

Thanks for that old wombat.

[RCChopper]

Wonder if anyone has come across this problem. On several downloaded a/c f.s 2004 when selecting autopilot in cruise, the throttles 'hunt'. i.e move slowly backwards and forwards. Seen it on 'live' aircraft but wonder if its a cfg problem in the program. Would appreciate any input.

 

I have had a few planes do this myself. The fix is quite easy. It is amazing to me how many things always get posted about "solutions" when this hunting issue comes up. All you need to do is look in the autopilot section of the aircraft.cfg. Find the line "max_throttle_rate" and then look at the value after it. 0.100000 is most common. You will likely have a different value in the plane giving you problems. Try changing this to 0.100000 or even something close until it is to your liking. From the SDK: This value sets the maximum rate at which the autothrottle will move the throttle position. In the example, the maximum rate is set to 10% of the total throttle range per second.

 

In other words, this simply sets the speed of the throttle response, therefore eliminating (or greatly reducing) the throttle hunting.

[old_wombat]

Uhh, the fix that I posted, was given to me by someone on this forum, I tried it and it worked. On more than one aircraft. I wouldn't have posted it if it hadn't been successful.

[napamule2]

Well that works for jets and (maybe) turbo-props, but what about piston engine aircraft? And why is it 'increase -or- decrease' the value. Give us an example of what the ('wrong') value was (and on what aircraft & modeler) and what you changed the value to (increased or decreased).

 

It don't sound right that it could be both higher or lower. And higher or lower than WHAT (ie: '=xxx'). I suspect that the AIR file would have to be 'right' for any change in CFG to make that much difference. But I could be wrong. I realize this problem involves several parameters, both in the cfg and the air file, and as such makes it a headache to determine exactly WHAT needs to be modified. There is no 'book' on this so can't look it up. It's got to be trial & error. And there is the values in different 'tables' and of course the hex data types. I don't do hex. Or 'tables' (hehe).

 

So, don't get me wrong. I am stumped as I tried 'messing' it up (by changing several parameters in cfg and air both) on default and 3rd party ac but could not do it so did not 'find' the definitive answer. I wll try again tomorrow (when my brain is fresh). This is what makes tweaking FDEs fun and simming exciting. So be cool. But keep digging.

Chuck B

[old_wombat]

Chuck, a fair enough comment and question. I have seen this problem only on jets, specifically jet fighters off the carrier.

 

On the Abacus Flight Deck 4 F-18 Hornet, I have this line:

fuel_flow_gain=0.1// 0.012

So, current value, 0.1, previous value 0.012

 

On the Iris A-7 Corsair II, I have:

fuel_flow_gain = 0.006 // 0.0025

 

As to higher/lower, I was told to try both directions, starting from the value provided.

 

I also did the same thing on the Iris F14-A Tomcat, but I don't have any previous values. This sticks in my mind because I was tut-tutted for doing this, because I was told that in real life that aircraft doesn't have an autopilot, therefore the problem "shouldn't" exist. Reality, huh? Who needs it? LOL.

[napamule2]

Thanks for the feedback. I learned something today. Now to experiment with what is in the air file. Specifically 'fs2k2' engine sections and what role it plays. News at 11.

 

I'm with you on making changes. It's YOUR Sim. And we all know that the F-14 has all kinds of electronics so why not AP. That is not such a big 'stretch'. On the other hand a AP on a DC-3 is..well, sacrilidge? (I put one in anyway but don't tell anyone).

Chuck B

[tiger1962]

On the other hand a AP on a DC-3 is..well, sacrilidge? (I put one in anyway but don't tell anyone)

 

Well, don't you tell anyone that I've installed Jim Robinson's DH89a Autopilot (dh89a_ap.zip) into all my FS2004 default historic aircraft - except the Wright Flyer - it looks the part and works a treat too, thanks Jim!

[old_wombat]

Chuck, I was playing the piano last night and experimenting with some harmonies when the explanation revealed itself to me. This answers both of your objections.

 

What we have here is an oscillation. In the most general sense, we get an oscillation in a "system" (such as a vibrating spring, or a simulated jet engine, or an electronic circuit) when the energy travels cyclically through the system at a speed that matches one of the resonant frequencies of the system. Are we happy with that as a concept? So, one way to stop the oscillation, then, is to change the "speed" of energy travel so it no longer matches the resonant frequency. The simplest analogy here is that when you have your TV tuned to channel X, the tuner part will resonate at X's frequency and thus pick up that channel, and won't resonate at the frequency of channel Y and thus won't pick it up.

 

Changing the fuel_flow_gain is analagous to changing the "strength" or "stiffness" of a vibrating spring and thus changes the frequency of energy travel. And here is the important point. To stop the oscillation, you only need to get it off resonance. That is, whether you make is go higher or lower is not important. Thus, either lowering or raising that parameter will move the frequeny off resonance and it will no longer oscillate. Your objection #1.

 

A system like an FS9 simulated jet engine is "complex" in the sense that there are many parameters that affect it. Specifically, there may be more than one parameter that when changed (up OR down) will change the frequency and thus stop the oscillation. Your objection #2.

 

This came to me in less than three bars of Mendellson.