Setting ZFW CG?

[Jeroen Hoppenbrouwers]

Catastrophic trouble is not possible because the master CG can never exceed a [catastrophic] limit.

Aha, this is the detail that I overlooked. Got it!

Does the slider have limits based on available test flight data (tables in flight manual) that may slightly exceed recommendations/restrictions but are still flyable, though not necessarily safe?

Or does the slider stop short of sitations that Boeing did not even try because they were deemed unsafe?

I know that sims don't usually include edge cases with untested "what if" assumptions. I presume there simply is no reliable actual data on a CG too far forward to rotate, or too far aft to keep the bird from sitting on  its tail after airspeed bleeds off after touchdown. Let alone uncontrollable situations in flight.

Boeing definitely has the capability to do silly:



https://www.metabunk.org/debunked-chemtrail-plane-interior-ballast-barrels.t661/

PS. Rumours that this is a Worldflight aircraft are only slightly exaggerated.


Hoppie

[Hardy Heinlin]

Does the slider have limits based on available test flight data (tables in flight manual) that may slightly exceed recommendations/restrictions but are still flyable, though not necessarily safe?

Yes.

... sitting on its tail after airspeed bleeds off after touchdown.

This is possible in PSX when the CG is far aft and the body gear is up. (Actually, at low airspeeds, when the elevator is ineffective, the on-tail-sitting is not a function of airspeed but of deceleration and acceleration.)


|-|ardy

[Hardy Heinlin]

CG slider system is now redesigned in PSX 10.14:

http://aerowinx.com/board/index.php?topic=4191.0


|-|ardy

[Will]

So Hardy, it looks like you put constraints on the master CG slider that vary with weight. Is your limit the same as Boeing's limit of a given weight? I ask because previously in your conversation with Hoppie (above), it seemed like the instructor page would let you select CG values that were outside of official legal limit.

This is another way of asking whether setting the slider at maximum forward CG or maximum aft CG would ever bring us outside of Bpeing's envelope.

[Hardy Heinlin]

The CG range in PSX is the same CG range used in Boeing's operation manuals and in the FMC.

My model will not go beyond these known limits as it would otherwise be very complicated to design (both re user interface and re aerodynamics) and would also be irrelevant for flight training since an uncontrollable flight would end deadly anyway.

Like in previous versions, the CG does not vary with fuel weight. It varies with fuel distribution. If you set an abnormal distribution, e.g. AUX tank, center tank and main tanks 2+3 full, and all others empty, the CG will be abnormally far forward.

The user sets the CG. When the "CG varies by fuel distribution" checkbox is selected, the fuel distribution will modulate the user's CG setting. The CG slider position indicates the sum which is used in the aerodynamics model. That is, when the CG slider is at the limit, the actual CG is really at that limit and not beyond it.

It behaves like a transistor amplifier getting a DC modulated by an AC: The DC is set by the user, and the AC is the modulation by the fuel distribution. As long as the sum of the two voltages exceed the transistor's operating limit, the sum is clipped. The clipping will vanish when the sum is reduced to a normal value.


Cheers,

|-|ardy

[skelsey]

Thank you Hardy!

[Phil Bunch]

I hope my comments/questions, re the 747 firefighting tanker are not too far off-topic for this CG thread:

----------------
First, a couple of excerpts from the company's web site FAQs:

http://globalsupertanker.com/b747-400-supertanker/question-answer/

Q: "You can't fly wide body jets safety in low level environment."

A: "In a typical fire drop scenario, the SuperTanker is approximately 200' above the tallest obstacle in the immediate drop zone.  With the GE CF6-80 engines escape from difficult or dangerous situations is quite easy, in fact, the tanking system empties in 6 seconds and the climb out rate is more than 6000 fpm!
Low, slow drops and steep terrain are no problem."

-------------------------------------------------------
Now for my questions and comments:

Flying a 747 firefighting aircraft at a precise very low height over complex terrain while simultaneously dumping a large water/chemical load continues to confuse me.  I can't see how this could possibly work -- it seems to me that the aircraft would either (1) increase its altitude very rapidly as its gross weight rapidly decreased and (2) become very hard to fly precisely and smoothly at very low height due to shifting the CG and (?) loss of trim.  Is the CG approximately stable during this liquid cargo dumping task?  Would trim settings be stable?

The 747's water tanks hold 19,200 gallons, which I estimate would weigh about 160,231 pounds (using a web calculator and 8.34 lbs per gallon). That sounds like a *lot* of weight to lose in 6 seconds while flying manually 200 feet over rapidly varying terrain!

A cropduster aircraft does this type of thing all the time, of course, but it is designed for this task and its pilot are highly trained for this high-risk task.

Yet one can watch videos of their 747 dropping firefighting liquid onto a fire at very low height with a rapidly varying terrain profile without obvious imprecision or instability.  How do they do this?   Is it as simple as emptying the water tanks in a smooth fashion while manually and visually flying? 

How does the pilot keep the plane from suddenly gaining height as the load is dumped within 6 seconds (see below)?   Since the water is dumped in 6 seconds, I assume the pilot would have to make many complex control adjustments (yoke, throttle, trim) in the same time frame.

Firefighting at low height with a 747 is so different from flying the carefully established stable approach that is always used for a normal airport landing...  Perhaps I should think of the 747-400 as an oversized fighter plane, especially after the liquid cargo has been dumped?

-------
Of course, we expect Hardy to add a firefighting option to PSX, now that he's finished the variable CG feature! <insert friendly grins here!>



Hoppie added some typography for clarity

[cagarini]

Some intersting read on the Firefighter theme...

http://oatao.univ-toulouse.fr/11196/1/Legendre_11196.pdf

https://www.wired.com/story/southern-california-thomas-wildfire-boeing-747-global-supertanker/

[skelsey]

How does the pilot keep the plane from suddenly gaining height as the load is dumped within 6 seconds (see below)?   Since the water is dumped in 6 seconds, I assume the pilot would have to make many complex control adjustments (yoke, throttle, trim) in the same time frame.

Absolutely (and don't forget that the hot, rapidly rising air above a fire is going to be far from stable in the first place).

I remember watching a video of a C130 (I think) airdropping something heavy out the back and the control inputs required to counter the trim change as several tonnes of Humvee or whatever it was slid along the cargo deck and out the ramp were quite something. Annoyingly I can't find it now, but these guys are definitely doing some aggressive flying: https://youtu.be/2w6N3LQ5uR8

[G-CIVA]

https://youtu.be/T6pBbIl23U8

https://youtu.be/GmmR3jB9j-Q

https://youtu.be/Fox_xomGgFg