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On the Various FM Issues


CptSmiley
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Hi all,

 

I thought I'd provide some info on the FM issues being discussed and what we are doing about them. First off there are a handful of issues here and there that are being worked. However there are other issues being noted where the data and behavior is correct. Since I am not a fan of opinion or feeling and prefer quantitative results lets discuss military power behavior:

 

tpsjJIG.png

 

As you can see there is clearly work to be done across the flight envelope in regards to military power level flight acceleration. I have been working on a fully automated test case tool that stimulates the FM code to generate the above outputs along with FM adjustments to much more rapidly make corrections and adjustments. The nice thing is, it utilizes the same EFM API function calls that all EFM aircraft use, so it will become an extremely valuable tool across all airframes.

 

We are doing this for every data plot that we have and are capable of digitizing and automating to make further improvements.

 

In summary, here are the issues we are currently looking to tackle:

- Improving level flight military power behavior (in general too high for most of envelope)

- Improving adverse yaw behavior (not sufficiently modeled)

- Improving dynamic stall behavior (not sufficiently modeled) - along with spin behavior

- Improving accelerated stall behavior (not sufficiently modeled)

- Investigating transonic control modeling

- Increasing drag at high angles of attack

- Improving turn rate (slightly too low currently across flight envelope)

 

Finally, an issue I'm seeing being brought up multiple times is ground behavior when moving from 9000 RPM through 10000 RM with too quick thrust build up when advancing throttle. Per our steady state engine charts each engine outputs around 800 kg of thrust at 9000 RPM this exponentially spikes to around 1400 kg of thrust at 10000 RPM. I am still investigating this and will create an automated engine test like the mil power test above to ensure the code is generating thrust for a given RPM as expected.

 

The last issue I'm seeing is roll rate and inertia behavior. From all indications the inertia data is correctly applied but will generate an automated test that analyzes effective roll moment for a given aileron deflection to ensure it is behaving correctly.

 

My objective is to provide a summary of these test results once their are generated and any corrections applied so you can all take a look at it.

 

Thanks all!


Edited by CptSmiley

"Witness mere F-14s taking off from adjacent flight decks, gracefully canting left and right, afterburners flaming, and there’s something that sweeps you away—or at least it does me. And no amount of knowledge of the potential abuses of carrier task forces can affect the depth of that feeling. It simply speaks to another part of me. It doesn’t want recriminations or politics. It just wants to fly.”

― Carl Sagan

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Nice !

 

 

I did notice that taxiing needs a lot of care because of that spike in power output when going past 9000RPM, making careful throttle control a must. Hopefully those tweaks will make it easier.

 

 

On the topic of spins, is it really impossible to make the Famer spin ? I've tried level flight + rudder, high attitude + rudder, botched turn reversal (which works in the hornet), but I only got to this state where it gently stalls, and yaw inputs bring the nose down, going into an alternating stall/recovery situation with the nose down and pro-spin controls applied.

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+1

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Nice !

 

 

I did notice that taxiing needs a lot of care because of that spike in power output when going past 9000RPM, making careful throttle control a must. Hopefully those tweaks will make it easier.

 

 

On the topic of spins, is it really impossible to make the Famer spin ? I've tried level flight + rudder, high attitude + rudder, botched turn reversal (which works in the hornet), but I only got to this state where it gently stalls, and yaw inputs bring the nose down, going into an alternating stall/recovery situation with the nose down and pro-spin controls applied.

 

Sorry, I meant to add along with adverse yaw and dynamic stall behavior, spin work will be involved in that as well.

"Witness mere F-14s taking off from adjacent flight decks, gracefully canting left and right, afterburners flaming, and there’s something that sweeps you away—or at least it does me. And no amount of knowledge of the potential abuses of carrier task forces can affect the depth of that feeling. It simply speaks to another part of me. It doesn’t want recriminations or politics. It just wants to fly.”

― Carl Sagan

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We have closed all other FM threads in this forum. If you want to discuss this issue, this is the thread for it.

Any new thread on the FM will be deleted.

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

"The three most dangerous things in the world are a programmer with a soldering iron, a hardware type with a program patch and a user with an idea."

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Hi CaptSmiley,

 

Thanks for the heads up, I have a couple of observations.

 

The roll rate at subsonic speed seems really high but what I believe needs to be looked into are inertia and damping. At sea level both seem almost non existent as the a/c hits maximum roll rate almost immediately upon stick deflection but then when the stick is brought back to neutral the roll ceases almost instantly (high damping and no inertia). That behavior is typical for a FBW a/c like the F-16 for instance where the computer actually applies slight reverse roll command in order to match zero roll with neutral stick. At high alt the damping is decreasing as it should and it takes a bit of an opposite command to stop the roll but low in the weeds the MiG-19 has barely any competition. Hornets and Vipers beware! :)

 

Drag in level flight - seems really low. At 12k m the a/c easily maintains Mach 1.01 at mil power with no stores and it takes at least 20 sec to slow down to Mach 0.9 after cutting engines to idle. Similar at sea level cutting engines to idle at 1000km/h IAS the a/c needs good 24-25 sec to slow down to 900km/h IAS in level flight. I've got the feeling that it's mostly due to low drag rather than high idle power.

 

I would be interesting to read your thoughts on this.

"See, to me that's a stupid instrument. It tells what your angle of attack is. If you don't know you shouldn't be flying." - Chuck Yeager, from the back seat of F-15D at age 89.

=RvE=

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Hi CaptSmiley,

 

Thanks for the heads up, I have a couple of observations.

 

The roll rate at subsonic speed seems really high but what I believe needs to be looked into are inertia and damping. At sea level both seem almost non existent as the a/c hits maximum roll rate almost immediately upon stick deflection but then when the stick is brought back to neutral the roll ceases almost instantly (high damping and no inertia). That behavior is typical for a FBW a/c like the F-16 for instance where the computer actually applies slight reverse roll command in order to match zero roll with neutral stick. At high alt the damping is decreasing as it should and it takes a bit of an opposite command to stop the roll but low in the weeds the MiG-19 has barely any competition. Hornets and Vipers beware! :)

 

Drag in level flight - seems really low. At 12k m the a/c easily maintains Mach 1.01 at mil power with no stores and it takes at least 20 sec to slow down to Mach 0.9 after cutting engines to idle. Similar at sea level cutting engines to idle at 1000km/h IAS the a/c needs good 24-25 sec to slow down to 900km/h IAS in level flight. I've got the feeling that it's mostly due to low drag rather than high idle power.

 

I would be interesting to read your thoughts on this.

 

I was going to write a post that said exactly this, it feels like the aircraft is fly by wire but also at the same time has no drag.


Edited by scampaboy
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Hi all,

 

 

 

tpsjJIG.png

 

 

Thanks for all the hard work, i've got many hours of enjoyment out of your modules. What do the three axes represent in that plot, and what are the units? I'm not familiar with FM data analysis.

 

Thanks

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Thanks for all the hard work, i've got many hours of enjoyment out of your modules. What do the three axes represent in that plot, and what are the units? I'm not familiar with FM data analysis.

 

Thanks

 

 

Judging from the legend and what people have reported, my gut feeling is that it represents level flight acceleration as a function of Mach number for different RPMs at a given aircraft weight. Given what people are saying about the jet, my guess is the dashed lines is what the FM "predicts", and the continuous ones what is actually happening - that is, the jet accelerates too well across the flight envelope (as confirmed by the points CptSmiley pointed out).

 

 

Note that this is only speculation on my part and I do agree with you - plots without axis labels are a sin!

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question: Mig-19 can't do it spin maneuver?

I tried spin maneuver and plane never fall in spin.

 

Here pilot Mig-19 describe how ecape from spin:

https://hushkit.net/2017/10/30/flying-and-fighting-in-the-mig-19-in-conversation-with-wg-cdr-irfan-masum-part-1/

 

citate:

“The MiG 19 was notorious for getting into spins without much warning due to it’s ‘adverse yaw’ attribute. And my most frightening episode also relates to this aspect.

 

Is something completely wrong with Mig-19 FM?

 

I' am in real glider pilot, so I know how enter in spin .. of course jet planes are differ but, this behaviour Mi-19P looks weird ..

 

I know personally Mig-15 pilots and for example Mig-15 can do it spin only on one side (in direction of rotating engine) and Belsimtek do it correct on Mig-15.

 

Count that Mig-19 have similar configuration .. so Mig-19 is two engined plane, engines have contra-rotating turbines if I' am correct, but when I OFF one engine, plane must go in spin in direction on this engine which is active.


Edited by Magot
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Thanks for all the hard work, i've got many hours of enjoyment out of your modules. What do the three axes represent in that plot, and what are the units? I'm not familiar with FM data analysis.

 

Thanks

 

Sorry about that, the tool is still in work and forgot to add axis legends. X-axis is mach, and Y-axis is acceleration represented in G's

"Witness mere F-14s taking off from adjacent flight decks, gracefully canting left and right, afterburners flaming, and there’s something that sweeps you away—or at least it does me. And no amount of knowledge of the potential abuses of carrier task forces can affect the depth of that feeling. It simply speaks to another part of me. It doesn’t want recriminations or politics. It just wants to fly.”

― Carl Sagan

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Looking forwards to the FM improvements. It is really rewarding to fly a plane and learn it's 'happy place', when to push - and when to stop pushing or it will bite back. BUt I feel like MiG-19 is not quite there yet - feels a bit like beast that has been tamed. Glad to see all the dangerous stuff, quirks and other real plane issues is looked into to make the plane really 'come alive'.

A-10C Warthog | AJS-37 Viggen | F-5E Tiger II | Mig-15bis | MiG-19P Farmer

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Hi all thanks for the feedback, it looks like many comments are asking about stuff mentioned in the title post. I would STRONGLY recommend you please take just a moment to read the title post.

 

A little progress report, 42 total performance test cases have been developed within the automated tool. I should be getting to performance tuning by the end of this week once I get a full picture from the automated tests.

 

I'm particularly excited about this tool as it allowed me to do currently 42 "test flights" in less than 30 seconds of trimming routines what would probably take days of manual test flying and processing of the data. This allows a quick change of FM behavior and can instantly see the effect across the full flight envelope.

 

Once the performance test cases are complete I'll post a current state report before making tuning so you all can see a before/after.

"Witness mere F-14s taking off from adjacent flight decks, gracefully canting left and right, afterburners flaming, and there’s something that sweeps you away—or at least it does me. And no amount of knowledge of the potential abuses of carrier task forces can affect the depth of that feeling. It simply speaks to another part of me. It doesn’t want recriminations or politics. It just wants to fly.”

― Carl Sagan

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That's great news. Automation is always a welcome addition to development I bet. I am sure the end-results will speak for themselves.

 

Q: Will this tool also be used to go back and evaluate/refine the M200C and Harrier FMs?

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A little bit of a progress report on the steady state engine thrust tuning, here is the current release. Not horrible, but certainly not correct:

 

AeFka5n.png

 

Spending the evening tuning now that I have the test cases automated and here are the results so far:

 

VZIuKqR.png

 

More to come!

"Witness mere F-14s taking off from adjacent flight decks, gracefully canting left and right, afterburners flaming, and there’s something that sweeps you away—or at least it does me. And no amount of knowledge of the potential abuses of carrier task forces can affect the depth of that feeling. It simply speaks to another part of me. It doesn’t want recriminations or politics. It just wants to fly.”

― Carl Sagan

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