[NO BUG]take off auto flaps - f18 - pitch up? - Page 4 - ED Forums
 


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Old 08-14-2019, 12:20 AM   #31
Wizard_03
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How do you know the real jet doesn’t violently pitch up on TO with AUTO? How do you know demo teams don’t have too apply forward stick and or neutralize trim as a result of that condition? Or that the pitch up is a desired behavior for their purposes? It sounded like that pilot used that setting specifically for a vertical zoom right after TO.

It’s part of the procedure for a reason, it throws a caution when the switch is in the wrong position for a reason. It’s not a mechanical flap switch, it’s an FCS mode switch, being in the wrong FCS mode at the wrong time is going to have consequences, that’s why we have procedures. So again how do you know it won’t pitch up in that condition?
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Old 08-14-2019, 12:58 AM   #32
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Because I literally just asked the guy who flies the Swiss display jet and he said he pulls +2G after takeoff to the vertical. I’ll ask him some questions tomorrow and clarify what the DCS Hornet does and ask if that’s anything like what the real jet does.

I’m also trying to get in contact with a Blue Angel who can tell me exactly how the Blue Angel FCS is different from the stock jet, and how the stock jet would behave in the auto takeoff situation.

Now don’t misunderstand me. I don’t think this is an important bug, it’s not something that needs to be fixed with any priority, because it effects a relatively small amount of players. But if we are trying to make the most realistic commercial simulation of an F-18 possible then it should behave realistically even when people do unusual things. And to say “it’s not a bug until proven” is just the kind of poke I need to set me on the route to proving it, if only to cure my own curiosity.

I cannot find any evidence to support the assertion that the real jet does this, this includes talking to real Hornet pilots and combing through the Natops manual. IF the real jet does this then fine, but I don’t believe that it does. I will get to the bottom of it one way or another.

Also guys. Ever heard of Occam’s razor? What is more likely, a jet that cost billions and has an FCS system specifically designed to reduce pilot workload has this one critical failure mode... which isn’t noted anywhere in the manual for said aircraft.

Or

A flight sim doesn’t model an unusual configuration quite right...
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Old 08-14-2019, 01:37 AM   #33
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I know but he also was only at +3 for trim not +12 and 2gs is awful low for a straight to vertical climb immediately after TO, unless the FCS was “helping” him, and he’s super light. Also I know for a fact hornet demo teams use bob weights for tight formation flying, which literally makes the stick harder to pull.

And I wouldn’t call it critical, it would only kill you if you didn’t take any positive action to correct it. I’m not saying it’s incorrect, just that if your outside guidelines, you can and should expect abnormal behavior. People other then you have posted about this before, and we found out they had configured the jet way outside of proper guidelines, and were blaming ED for their woes

We can’t call it a bug unless we know it should behave differently then what we’re seeing, that being said it’s gonna be hard to find real evidence of that because no professional would use those settings in actual operation, unless he/she had a specific reason such as the one you posted. Otherwise if they did have an accident they’d be at fault for not following guidelines. But yeah ask around someone’s gotta know.
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Old 08-14-2019, 02:00 AM   #34
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Quote:
Originally Posted by Wizard_03 View Post
I know but he also was only at +3 for trim not +12 and 2gs is awful low for a straight to vertical climb immediately after TO, unless the FCS was “helping” him, and he’s super light. Also I know for a fact hornet demo teams use bob weights for tight formation flying, which literally makes the stick harder to pull.

And I wouldn’t call it critical, it would only kill you if you didn’t take any positive action to correct it. I’m not saying it’s incorrect, just that if your outside guidelines, you can and should expect abnormal behavior. People other then you have posted about this before, and we found out they had configured the jet way outside of proper guidelines, and were blaming ED for their woes

We can’t call it a bug unless we know it should behave differently then what we’re seeing, that being said it’s gonna be hard to find real evidence of that because no professional would use those settings in actual operation, unless he/she had a specific reason such as the one you posted. Otherwise if they did have an accident they’d be at fault for not following guidelines. But yeah ask around someone’s gotta know.
The only Hornet team that uses “bob weights” is the Blue Angels and that’s actually a literal spring on the the stick which forces the stick forward. The other solo Hornet displays don’t use such a system.

He’s doing a 2G pull because of the gear retraction G limit and yes he is very light because he’s just got airborne for a solo display.
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Old 08-14-2019, 02:12 AM   #35
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Quote:
Originally Posted by Deano87 View Post
Sierra, I can’t find anything in the -000 about this violent pitch up. Can you direct me to the appropriate chapter/page where It explains that if you attempt a takeoff in AUTO the aircraft will uncontrollably pitch up and require a cycle of the flaps to reset the FCS. Such a serious handling fault would no doubt be documented in the Natops manual, where all other handling oddities (including unusual configuration operations) are listed.
You’re absolutely correct. I can’t direct me to the appropriate chapter/page where It explains that if you attempt a takeoff in AUTO the aircraft will uncontrollably pitch up and require a cycle of the flaps to reset the FCS. I ALSO can’t direct you to the chapter/page where it says landing with the gear handle UP...Alters the landing distance and requires more power to taxi to parking. Does that mean the ability to land with the dear UP is a bug?

Now that being said...I can direct you to step 13.b of the before Taxi checklist that directs

“FLAP switch - HALF”

Not “FLAP switch - As Desired”
Not “FLAP switch - AUTO”
Not “FLAP switch - FULL”

“FLAP switch -HALF”

This is the proper procedure. End of discussion. Everyone “yeah but-ing” is doing it wrong.

That is the procedural end to the argument. That being said as far as the uncontrolale pitch up is concerned...

It can be surmised that since FLAPS significantly increase available lift...Not setting the correct flaps would result in insufficient lift. It can be assumed Insufficient lift at the end of the cat shot would cause the flight control computer ito compensate for the missing lift By increasing the AOA...I.E.A violent pitchup.

But that’s probably not in the book either.
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Old 08-14-2019, 02:52 AM   #36
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This is the proper procedure. End of discussion. Everyone “yeah but-ing” is doing it wrong.
Including the people that do it in the real jet? I’ll be sure to let them know you don’t approve.

Quote:
It can be surmised that since FLAPS significantly increase available lift...Not setting the correct flaps would result in insufficient lift. It can be assumed Insufficient lift at the end of the cat shot would cause the flight control computer ito compensate for the missing lift By increasing the AOA...I.E.A violent pitchup.

But that’s probably not in the book either.
Nice idea, Except that’s not the way the F-18 FCS works. With the flaps in AUTO the FCS tries to attain 1G of load on the jet. So yes it would pitch up to account for the less lift, but then once it reached 1G it would stop pitching up, instead of doing what it does in the sim which is continue pitching up. It’ll happily fly around pulling 5G when you let go of the stick if you don’t slow down and cycle the flaps at which point it goes back to flying normally. Also it does it even when taking off from a normal runway.

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Originally Posted by Wizard_03 View Post
but he also was only at +3 for trim not +12
You know what Wizard, I’ll give you that, that’s a good point. That is a big difference in trim. I wonder if the initial trim setting on the ground influences the G figure that the FCS in AUTO is trying to attain, I’m not sure it should work like that but maybe it does in the sim. Maybe +3 is correct for 1G after the gear is up? I’ll experiment with this tomorrow.
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Old 08-14-2019, 03:26 AM   #37
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Did a few tests (clean, 50% fuel) and while there is a very noticable pitch up, it's easy to counteract/control.
Since you are not a passenger and hence you should always actively control your aircraft, this would not be a real problem.

The main problem comes from an apparently wrong trim simulation if you take off with the flaps in auto!

Resetting the trim before take off is somewhat sursprising.

1. flaps half > decrease trim from +12° to 0° = 6sec
2. flaps auto > decrease trim from +12° to 0° = 120sec, that's 0.1°/sec!

3. Half flaps (+12°), mil thrust, full aft stick at 110 (to confirm 24° stab deflection), neutral stick at 20° pitch attitude and the stab deflection will be around +3°
The pitch attitude will continue to increase, the climb speed will initially increase to ~190kts but will decrease to 110kts with the highest pitch attitude being a tad over 50°. So you have the dreaded 'pitch up' with half flaps as well.

4. auto flaps (+12°), mil thrust, full aft stick at 110 (to confirm 24° stab deflection), neutral stick at 20° pitch attitude and the stab deflection will be around 9° with a much higher pitch up which will quite a bit of forward stick to counteract.

4a. with auto flaps and the trim reset to 0°, the stabs will 'reset' to ~5° after take off rotation with a less aggressive pitch up.

The main problem is that the stab trim still runs at 0.1°/sec which means it's completely ineffective since the FCS is much faster, hence the impression that trim doesn't have any effect.

But the trim is actually not moving at all.

Last edited by bbrz; 08-14-2019 at 03:28 AM.
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Old 08-14-2019, 03:59 AM   #38
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4a. with auto flaps and the trim reset to 0°, the stabs will 'reset' to ~5° after take off rotation with a less aggressive pitch up.
Really interesting stuff bbrz! Thanks for that. With the above point, what G would the jet try and attain once you let it accelerate? And could you trim from the ~5° stab position back to 1G normal? Or wasn’t that possible?
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Old 08-14-2019, 04:16 AM   #39
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Once you have established the +20deg pitch attitude the plane will initially continue to pitch up with approx 1.6-1.7G.

If you take off with auto flaps and +12deg stab trim, the stabilators will very quickly reset to the same +5deg after reaching the+ 20deg pitch attitude, like in the 0deg trim case and the result will be the same 1.6-1.7G pitch up.

You can trim to 0deg, but this will take around 1min!
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Old 08-14-2019, 04:29 AM   #40
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It maintains that g-loading up to 500 knots? Or does it scale with speed?
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