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B777-300 Unable to stall (UNSTALLABLE)

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B777-300 Unable to stall (UNSTALLABLE)

Postby milolouis » Sun Sep 03, 2017 4:27 pm

Does anyone know why with the default B777-300 You can turn of both engines, deploy spoilers and lower the landing gear and you will get only 600fpm descent rate.
I really like flightgear but this concerns me as to the flight dynamics of all the models.
I'm not exaggerating either seems quite happy to glide at nose up 15 degrees and 160 knots IAS
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Re: B777-300 Unable to stall (UNSTALLABLE)

Postby it0uchpods » Sun Sep 03, 2017 4:30 pm

Umm. Each aircraft has a different flight model, and they all work differently. Just because one isn't accurate, doesn't mean that all are inaccurate.

I've found that most "YaSim" aircraft have very odd or completely broken stall characteristics. (Like the 777)

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Re: B777-300 Unable to stall (UNSTALLABLE)

Postby Thorsten » Sun Sep 03, 2017 5:14 pm

I really like flightgear but this concerns me as to the flight dynamics of all the models.


In theory, there ought to be a rating for the FDM telling you how accurate it is - so you only get to complain that the flight dynamics is unrealistic if the rating is sufficiently high, otherwise you've been told before and decided to still use the plane.

Oh, and YaSim doesn't stall properly. It just doesn't. YaSim aircraft may be nice inside the nominal envelope, but for anything outside of it, only JSBSim cuts it (still means the data has to be good, if you feed JSBSim with garbage you get garbage back - but there are some hi-fidelity planes around which have proper stall characteristics).
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Re: B777-300 Unable to stall (UNSTALLABLE)

Postby Alant » Mon Sep 04, 2017 12:01 am

But who wants to stall a 777?

The only folk really interested d«in this are the manufacturer and the licensing authorities.

Anybody else flying this type of aircraft should be protected from stall etc by the aircraft designers.

That is unless you are a top test pilot, ignore all the warnings, turn off the safety systems and ignore the warnings..

Or am I taking to an experienced test pilot or accident investigator?

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Re: B777-300 Unable to stall (UNSTALLABLE)

Postby it0uchpods » Mon Sep 04, 2017 12:26 am

That's not even stalling tho, that is just unrealistic behavior in general.

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Re: B777-300 Unable to stall (UNSTALLABLE)

Postby Alant » Mon Sep 04, 2017 12:39 am

Does Flightgear´s simulation include the the stall protection systems?

Next does the current simulation have aero data in, or near, the stall region? If not where will this data come from?

I repeat, any pilot flying a 777 in real life will never be exposed to a stall. Therefore why simulate it?

Perhaps it is of interest to a game player for his 5 minutes attention span, but unless someone has a serious use for something like this I do not see the purpose.

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Re: B777-300 Unable to stall (UNSTALLABLE)

Postby Alant » Mon Sep 04, 2017 1:11 am

The 777 uses Yasim. If you look athttp://www.buckarooshangar.com/flightgear/yasimtut_stall.html you may find clues as to make the stall behaviour appear better.

For a game player this approach may be adequate.

Otherwise you can use JSBSim, and try to find the data required. This is probably impossible unless you have god contacts within the aero dept at Seattle, It is very unlikely that you will find such data in the public domain.

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Re: B777-300 Unable to stall (UNSTALLABLE)

Postby GinGin » Mon Sep 04, 2017 7:09 am

@Alant : In my opinion, It's interesting to simulate stall capability because we can test it, learn procedure to avoid it and react to it if its happen in game by mistake in flying, or low speed buffet near thunderstorms etc.

Stall protections systems on 777 would be hard to reproduce on a sim because it's mainly dealing with adding some weight on the yoke when approaching flight enveloppe limits.
For example in real, if you are approaching stall with high pitch, it's gonna be very hard to pull more on the yoke as it would bring the aircraft even closer to a pre stall situation .
Basically with Boeing, aircraft will tell you that it's not good to continue in this direction by adding weight on the controls, but will not stop you by overriding controls to go outside the flight envelop ( need to be a really stupid pilot though)
Philosophy is different from airbus where there is much more automatic stall protection and aircraft will not allow you in normal laws to go in certain flight envelop by limiting angle of attack.



Of course real pilots don't do that on line, but do plenty of upset recoveries situations two times a year in the simulator.
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Re: B777-300 Unable to stall (UNSTALLABLE)

Postby Alant » Mon Sep 04, 2017 6:30 pm

Back to the subject.

It will be difficult to do much useful with a PC based simulator of the 777 stall systems. To get the aircraft to stall it is first necessary to disable the auto-throttle and disable both PFC´s Primary Flight Controls). These are al meant to be engaged permanently. After that the pilot must hold the stick back (or apply pitch trim) with the throttles closed. He will get a warning in the form of a stick shaker before stall. I guess you could send a rattling noise through the speakers to simulate the stick shaker. If he ignores that the stick pusher will operate and the pilot will have to fight quite hard to overcome that. You could add a nose down signal to the elevator or pitch trim to simulate that.
There is plenty of discussion going on here and in the developers email forum about the (lack of) Yasim stall simulation.

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Re: B777-300 Unable to stall (UNSTALLABLE)

Postby Alant » Mon Sep 04, 2017 6:34 pm

PS The pilot will also have to ignore warnings on his main display screen.

To do what you need will really require a full size training system with powered artificial feel system, not a laptop with a toy joystick or keypad.

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Re: B777-300 Unable to stall (UNSTALLABLE)

Postby GinGin » Mon Sep 04, 2017 6:52 pm

@Alant: Yep, that why I said it's not possible to reproduce stall warning devices on a 777 in a PC sim. Or just like you said stick shaker sound, Airspeed Low aural alert and "Buffet Alert" on FMC.

Stall can happen even with AP/AT engaged.
An example, strong windshear caused by Cb's ( can cause a decrease in 40/50 kts of IAS) or Jetstream when climbing in thick air, with tight margin between stick shaker speed and MMO.
Or AP engaged during final approach and AT disconected with Idle thrust, it's gonna end up in stall if nothing is done ( Real accidents happened like that, hard to believe )
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Re: B777-300 Unable to stall (UNSTALLABLE)

Postby Alant » Mon Sep 04, 2017 7:23 pm

I have experienced this,just once.
I was part of a team building F100 and F50 systems simulators for Focker. I already knew one of the test pilots and a technician who worked on their research handling simulator (the connection was that we all were owners of antique Riley motor cars). They had it set up fr demonstrating wind shear on the approach and asked me to have a go. Despite being warned that this would happen they caught me out. No doubt the shear was turned up to full for my benefit ;-) The aircraft just dropped out of the sky at about 500ft on the descent.

Back to your problem. As others are having stall simulation problems with Yasim, I would recommend that you set up a JSBSim FDM. With JSBSim you can directly tune Cl CD,Cm and all other aerodynamic parameters to match the best available data. The problem of finding good aero data remains. Perhaps some papers have been published, otherwise start with Aeromatic which will give a good start point for your FSBSim model.

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Re: B777-300 Unable to stall (UNSTALLABLE)

Postby bugman » Tue Sep 05, 2017 2:00 pm

As requested, I have split off a number of posts to:

Regards,
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Re: B777-300 Unable to stall (UNSTALLABLE)

Postby legoboyvdlp » Wed Sep 06, 2017 12:59 pm

milolouis wrote in Sun Sep 03, 2017 4:27 pm:Does anyone know why with the default B777-300 You can turn of both engines, deploy spoilers and lower the landing gear and you will get only 600fpm descent rate.


This is the issue.
When you turn off the engines in the air, they still provide a massive amount of thrust on the 777-300, which is incorrect.
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Re: B777-300 Unable to stall (UNSTALLABLE)

Postby it0uchpods » Wed Sep 06, 2017 4:56 pm

That is a Yasim thing... Jets can start, but can't shut down.

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