Updated IRIS F-14A flies a lot better, but still has a severe Autothrottle problem.

[old_wombat]

Someone here (jgf???) suggested to get another F14 after all the problems with the IRIS one. In looking for one, I found an update to the IRIS aircraft.cfg file which has improved things a LOT. So I now have an F-14 I can land, yay! But the autothrottle still doesn't work properly. At moderately high speeds (200 - 300kn) it takes a long long time to get to the commanded speed.

Now I can live with that, but I mention it in case it is relevant to the main problem, which I can't live with. Below about 200kn, and getting worse the slower it flies, the autothrottle causes tremendous but slow engine oscillations. Of the order of 15 knots, which at carrier landing speeds is disastrous. I set say 112 knots, I get (seemingly) sinusoidal variations between 115 and 130 knots. Makes it very difficult to hit the arrestor wires.

Thinking it might be a low-RPM problem, I try the landing with the airbrake/spoiler deployed. N1 is now obviously a lot higher, but the oscillations are still there, albeit slightly not quite as bad.

Many of the other fast carrier jets, both from similar and different stables, have similar problems. Some have it it worse, some not as bad. A handful do not have this problem at all, right down to and below full-flaps stalling speed. I tested many and found huge variations.

This is definitely an AT problem. With the AT off, N1 varies with F1-F4 keys, as it should, but it reaches a particular value and stays there.


Where to from here? Comparing the many aircraft.cfg parameters that are different between the both good and bad planes gives such great variations that it's way beyond my analytical skills to figure out what to change and in what direction.

Can anyone make some concrete suggestions as to what to look at? Is it worth looking at any particular aircraft.cfg parameters, either prima face on their own, or comparing them from the good vs bad example? Or should I reluctantly and after all this water under the bridge abandon this plane after all?

Steve from Mudgee.

[pfflyers]

One thing that might help if you haven't done it already is go to the [autopilot] section in the config and change this line:

autothrottle_max_rpm=90.000

to this:

autothrottle_max_rpm=110.000

Makes the AT a little more responsive.

Ultimately I think it is probably unrealistic to expect most fighter jets to land using autothrottle. The turbine lag time causes the AT to "hunt" for the selected speed, especially in a dirty, nose high, low-speed flight condition. Real world Navy pilots use throttle to control their glideslope/rate of descent. I wonder if the real F-14 even had an autothrottle.

[old_wombat]

Will give this config line a go later this evening.

Your comments about the real F-14 and possible total lack of A/T are noted, and accepted. Having said that, this is just a computer game, and landing the virtual F-14 on the virtual carrier is so difficult for me that I am not skilled enough to do it without A/T, and so I accept that that is a compromise/assistance that would not exist in the real word.

However, the problem goes deeper than just landing, although that is all that concerns me. The "hunt" happens to several similar planes, and is really an issue of low-speed flying rather than landing per se, which is only an example [of low speed flying]. The main task of a real F-14A in the 1980s would have been the terribly boring one of flying a BARCAP for several hours a couple of hundred miles from the carrier, heavily fueled and armed, and probably <would> have been flying totally on AP and/or AT, and certainly at low speed and a constant altitude.

Steve from Mudgee.

[old_wombat]

** SUCCESS ** !!!


Changing autothrottle_max_rpm to 110 or even 140 (%) did nothing. But nothing ventured nothing gained, I looked around that same area and found max_throttle_rate=0.100 . A little experimentation of both increasing and decreasing this value very quickly produced the optimum rate of max_throttle_rate=0.008, about 12X lower. I can now autoland the F14, empty, within 1/2 knot of its designated full-flaps stall speed, airbrakes on, mind you with a pitch-up of nearly 12 degrees. At this speed the delta-V of the oscillation is about one knot, barely noticeable.

With greater weights the landing speed of course increases. At maximum gross landing weight and landing speed of about 122 knots and no airbrake the oscillation is less than a third of a knot, almost undetectable.

I also discovered that increasing max_pitch_acceleration from 1.0 to 3.0 increases the elevator response without compromising stability, but that max_pitch_velocity needs to be left strictly alone.

These parameters have successfully flowed through to other jet fighters that had had the same problem. The EA6B, which I had dismissed as uncontrollable under auto, can now be auto-landed, with some difficulty. Under AP control, it doesn't want to pitch up as the speed decreases, but it will land. And the Abacus Flight Deck Hornet, which I suspect is here greatly over-powered compared to the real thing, can now also be auto-landed reasonably smoothly.

So many thanks to everyone who has helped me with this project. I have no idea how I am going to return the favour, as I must, eventually.


Steve from Mudgee