What happens when an aircraft hits something that is not inclined to move?

[chillzero]

OK. A reminder to re-read the Membership Agreement, people. Do not discuss perceived slights and insults in the threads - if you feel the MA has been breached, report it. If you have some objection to using the report function, don't discuss it in the thread.

Please get this thread back on the rails, and up the civility levels. Do not attack each other - discuss the arguments being brought.

Replying to this modbox in thread will be off topic  Posted By: chillzero

 

[BenBurch]

Terral?

Please answer my question?

 

[Furcifer]

Apollo: You guestimate s=20m and I guestimate v(final)=250m/s. I've got video evidence that proves my case easier than yours. No case closed here

I haven't read the tests you cited in regards to this triangular load profile. Perhaps to could explain if the unique exterior of the WTC was considered? While I can see this "load profile" may be triangular for a more concentrated mass striking a denser barrier, I don't necessarily believe this extends to the WTC. I think the exterior columns would have shred the plane resulting in a different load profile. Would the load profile of a bullet pentrating a semi-rigid body be the same as the load profile of shot with the same mass and velocity?

 

[fezzic]

Fezzic:

Thanks for the nice table!
<snip>
Omika et al. calculate the impact load time history for WCT 1 & 2. In both cases it is approximately triangular and, for WTC 2, peaks after about 0.1 seconds at a value of about 375 MN. Without velocity reduction this means the aircraft has moved about 25 meters into the building. But, 3bodyproblem, please recognize that there WAS velocity reduction - this is why I used v^2 - u^2 = 2as. If we assume that the aircraft was brought to rest after a distance of s, we have u = 0, and v^2 = 2as. I guestimated that s ~ 20 meters and did my calculation, which from Fezzics table leads to a resisting force of 194 MN. (Or 97 MN if we consider the average acceleration as 1/2 the peak)

Mendis et al. calculate an impact load-time history for a Boeing 767-300ER travelling at 140 m/s striking a "typical tall building". The load profile is triangular and peaks at 320 MN.

Riera's formalism leads to a triangular load profile and for A Boeing 707-320 travelling at 103 m/s the peak resistive foce is estimated to be 88 MN. And please note this is a low speed 707!

T. Sugano et al. measured, (Yes, measured!), the impact force vs. time profile for a F-4 Phantom jet weighing 19 tonnes and travelling at 215 m/s striking a concrete block mounted on air bearings. The peak impact load was found to be about 180 MN.

Now when we look at NIST's treatment of this problem, well, we have a problem!

First of all NIST assumes that the aircraft travelled 60 meters inside the building . NIST also show an acceleration profile for the aircraft (Figure 2-45). This STARTS at about 60 g's, (equivalent to about 75 MN), and drops off in a linear fashion to zero in about 0.6 seconds.

THIS CANNOT BE CORRECT!

As we have seen, the load profile is approximately TRIANGULAR, starting from zero, increasing to a maximum and then falling back to zero.

Also NIST's calculation assumes the resistive force starts when the aircraft strikes the perimeter wall. This is strictly true, but remember Wierzbicki has shown that the perimeter wall offered very little resistance. If this was not true we would have seen the aircraft smashed to pieces OUTSIDE of the tower. Since this did not happen, and using Wierzbicki's estimate of 3 % of the initial kinetic energy expended to penetrate the exterior wall, we can safely assume that the resistive force only begins when the aircraft strikes the CORE!

Therefore I am sorry to say NIST really messed up on this calculation. Its load profile is non-physical and its stopping distance is too long.....

CASE CLOSED
<snip>

I don't pretend to really understand it all though I will say that ok I see where you are coming from. It seems to be in the assumptions that the analyst made.

I ask what is the practical result in general?

I can see where, for the purposes of accuracy and precision, it might make a difference in the quality of analysis, but what I would like to know is what practical difference does the greatly increased resistive force (excuse me if I characterize it wrong) make?

 

[jaydeehess]

I don't pretend to really understand it all though I will say that ok I see where you are coming from. It seems to be in the assumptions that the analyst made.

I ask what is the practical result in general?

I can see where, for the purposes of accuracy and precision, it might make a difference in the quality of analysis, but what I would like to know is what practical difference does the greatly increased resistive force (excuse me if I characterize it wrong) make?

It has to do with the amount of force imparted to the core columns by momentum transfer. If the perimeter resisted a great deal of the available momentum transfer then less is available to affect the core, and if the perimeter took a very large percentage then one would expect to see a lot of material flying off the sides at impact.
The Sandia f-4 test illustrates this as the concrete block was sufficiently strong to resist the transfer of momentum. The concrete block does move indicating that some momentum was transfered to the block but the force of that impact was insufficient to penetrate the block. That results in the plane debris exiting the impact zone at right angles to the original line of travel.

If one ran a series of tests on walls of decreasing strength and of less continuous design (more/larger window openings) the amount of aircraft debris exiting the impact zone at right angles to the original line of travel would decrease until a point is reached at which the wall is breached quickly enough for all of the aircraft debris to travel through the wall.

 

[jaydeehess]

Throw a basketball at a plywood wall and it bounces back. The wall is more than sufficiently able to resist the force of impact, the ball flexes and compresses , storing the momentum transfer, over the time of contact, through the deformation of the ball and compression of the air inside. Once velocity reachs zero the compression of the air and return to spherical shape causes that stored energy to be released, propelling the ball in the opposite direction. (minus energy that goes into sound and heat)
If the ball were made of damp clay there would be no way to store that momentum transfer over time of impact. Instead it would simply deform and flatten out.

But if what is impacting the wall, breaches it and uses only a small percentage of its momentum to do so, then it will still have a lot of momentum in the original line of travel and continue through the breach, even if that object also breaks up during the impact.

 

[rwguinn]

It has to do with the amount of force imparted to the core columns by momentum transfer. If the perimeter resisted a great deal of the available momentum transfer then less is available to affect the core, and if the perimeter took a very large percentage then one would expect to see a lot of material flying off the sides at impact.
The Sandia f-4 test illustrates this as the concrete block was sufficiently strong to resist the transfer of momentum. The concrete block does move indicating that some momentum was transfered to the block but the force of that impact was insufficient to penetrate the block. That results in the plane debris exiting the impact zone at right angles to the original line of travel.

If one ran a series of tests on walls of decreasing strength and of less continuous design (more/larger window openings) the amount of aircraft debris exiting the impact zone at right angles to the original line of travel would decrease until a point is reached at which the wall is breached quickly enough for all of the aircraft debris to travel through the wall.

Actually, the figures and data Apollo20 shoved at us was to used to determine the test set-up for shedding insulation on the beams. It had little or nothing to do with the overall action.
If the rise time is significantly shorter than the decay time on a force or acceleration input, a Step function is a "Reasonable approximation". When the force (or acceleration) at T=0 is any value other than zero, and the force (or acceleration) at T<0 is zero, that is a step function. That is what is depicted in the graphs.
Aplool20 has no working knowledge of engineering, and makes assumptions based on his own specialty.
Engineering is real-world and has to deal with all its unknowns. We have to make assumptions, based on reality.
Chemstry is exact. Do the same thing twice, get the same results
In reality, two very similar events (as it would seem to the uninitiated) can, and often do, have results that are dissimilar in the extreme. The analysis and detailed examination of these events, and the assumptions that cause them to make sense from a physical standpoint is what engineers do.

 

[Apollo20]

jaydeehess, 3bodyproblem and fezzic:

Thanks for your helpful posts!

While it is true that the TAIL of the aircraft was still moving at 250 m/s 0.2 seconds into its impact on WTC 2, this is not really helpful in discussing the impact. Figure 9-28 of NCSTAR 1-2B shows why. This is a plot of the aircraft momentum vs. time after impact based, as NIST point out, on ALL of the aircraft structures and fuel. It shows that about 75 % of the initial momentum of the aircraft had been lost 0.2 seconds into the impact. This is equivalent to a 75 % reduction in velocity or a loss of about 94 % of the aircraft's impact KE.

Now look at Figures 9-31 and 9-33 of NCSTAR 1-2B which show NIST's estimate of the impact damage to the core of WTC 2 and you will see that it really was quite MINIMAL. Only five columns were severed and four columns were heavily damaged over a floor height of about 5 floors. This is only about 20 % of the core columns damaged on only 5 % of the building's floors!

But we know that the entire mass of WTC 2 was displaced a distance of about 0.6 meters at the roofline and oscillated for several minutes as a consequence of the aircraft impact. This shows that the building behaved as a vertical cantilever and absorbed ELASTIC energy. Thus, to a certain degree, the core of WTC 2 did exhibit at least some of the behavior seen in the crash of the F4 Phantom jet into a concrete block. Interestingly, in Sugano's discussion of the impact of the F4 on a concrete block we read on page 377:

"The concrete target experienced a slight rocking motion during impact".

 

[Apollo20]

RWGUINN:

I hate to tell you this, but CHEMISTRY is "real world" too, and is certainly NOT exact because of statistical fluctuations in the motions of atoms and molecules!

 

[Max Photon]

Science, with Max Photon

RWGUINN:

I hate to tell you this, but CHEMISTRY is "real world" too, and is certainly NOT exact because of statistical fluctuations in the motions of atoms and molecules!

Personally, I fail to see the difference between chemistry and physics.

And engineering is just applied physics.



Conclusion: engineers are chemists.

 

[Apollo20]

MaX:

Answer me this:

If someone who works on the design and construction of buildings is called an engineer,

Why isn't a guy who works on the design and construction of engines called a buildineer?


2.

 

[Max Photon]

Life's toughies

MaX:

Answer me this:

If someone who works on the design and construction of buildings is called an engineer,

Why isn't a guy who works on the design and construction of engines called a buildineer?

Dude I'm still trying to figure out why we

drive on parkways

and park in driveways.


* * *

 

[jaydeehess]

jaydeehess, 3bodyproblem and fezzic:

Thanks for your helpful posts!

While it is true that the TAIL of the aircraft was still moving at 250 m/s 0.2 seconds into its impact on WTC 2, this is not really helpful in discussing the impact. Figure 9-28 of NCSTAR 1-2B shows why. This is a plot of the aircraft momentum vs. time after impact based, as NIST point out, on ALL of the aircraft structures and fuel. It shows that about 75 % of the initial momentum of the aircraft had been lost 0.2 seconds into the impact. This is equivalent to a 75 % reduction in velocity or a loss of about 94 % of the aircraft's impact KE.

A point of view that does have its limitations. As the aircraft passed beyond the outer perimeter it was no longer a single object. So NIST's showing that the momentum of the aircraft as a whole was at 75% of its original value can be quite correct and some parts of the aircraft having reached a velocity of zero while others still are moving at several hundred MPH. The engine core that exited WTC 2 (IIRC) still had about 20% of its original velocity after having passed through the entire building.

So the NIST numbers are good for determining the amount of momentum that the building absorbed but assigning specific damages gets less sure the more specific you try to be with this data.

Now look at Figures 9-31 and 9-33 of NCSTAR 1-2B which show NIST's estimate of the impact damage to the core of WTC 2 and you will see that it really was quite MINIMAL. Only five columns were severed and four columns were heavily damaged over a floor height of about 5 floors. This is only about 20 % of the core columns damaged on only 5 % of the building's floors!

Which was determined partly from empirical evidence of photos and videos and mostly by the FEA analysis that was an attempt to model the debris behaviour and the structure reaction to the debris.

But we know that the entire mass of WTC 2 was displaced a distance of about 0.6 meters at the roofline and oscillated for several minutes as a consequence of the aircraft impact. This shows that the building behaved as a vertical cantilever and absorbed ELASTIC energy.

Which goes back to the momentum absorbed(transfered to) the building as a whole, by the aircraft as a whole.

Thus, to a certain degree, the core of WTC 2 did exhibit at least some of the behavior seen in the crash of the F4 Phantom jet into a concrete block. Interestingly, in Sugano's discussion of the impact of the F4 on a concrete block we read on page 377:

"The concrete target experienced a slight rocking motion during impact".

Indeed, that concrete block would have outweighed the Phantom by many times, and was strong enough not to shatter. The momentum of the aircraft became the momentum of its debris, heat energy, sound energy and the momentum of the block. It is pretty basic physics but if the block had a mass of 100 times that of the Phantom and the impact transfered 50% of the Phantom's momentum to the block then (ignoring the considerable friction resisting the block's movement) the block would then be moving at 0.005 the original velocity of the Phantom. 0.005 times 500 MPH is 2.5 MPH. If the block was only 10 times more massive then it would be moving at 25 MPH. BUT, as I said this would require that one ignore the friction of the block on whatever surface or hold-down mechanism it had.

On the macro level this illustrates that a large aircraft will transfer a lot of its momentum to a much larger structure. Odd that such a simple concept escapes some CT's.

 

[jaydeehess]

Personally, I fail to see the difference between chemistry and physics.

And engineering is just applied physics.



Conclusion: engineers are chemists.

Depends on what level of detail you are at. Simple chemistry that has , for eg. hydrogen combining with oxygen to produce water and give off heat, is not neccessarily concerned with the subatomic physics that occurs during this process.

Going up a level towards the marco situation, an engineer needs to know that steel, for eg. loses its strength as it is heated and at what rate. He does not neccessarily need to know what chemical reactions might be occuring between the gases surrounding the steel and the steel nor does he need to know the details of the changing atomic structure of the steel.

The physicist is not concerned with calculating the size and type of steel needed to take the stresses it must withstand in a building if all he is doing is investigating the atomic structure of steel while it heats.

If you wish you can also include biologists since the processes that occur within living organisms also are driven by atomic level changes.

Therefore all science and technology is physics.

 

[Max Photon]

The Art of Model Max Photon

* * *

By the way Apollo,


Did you ponder what would produce NIST's acceleration profile?

(I have my ideas.)


Remember, NIST does not lie.

That is a HUGE modeling constraint.

Use it.


Max Photon


Author of:

The Art of Modeling Teleologically-Driven Far-From-Equilibrium Dynamical Systems Cloaked by Paul-Trap-type Dynamic Stabilization

In a Tree NIST is Stuck, I See

Dien Bien Phu II


* * *

 

[jaydeehess]

Dude I'm still trying to figure out why we

drive on parkways

and park in driveways.


* * *

because you are ignoring the second syllable. A parkway is the path you take to get to the place you park ('this way to a parking spot'), a driveway is the path you take to get to the place where you drive ('this way to a driving area').

 

[Max Photon]

Remember plebs, all science and techology is physics. So without a physics degree...

[snip]

Therefore all science and technology is physics.

Whew. Glad I'm a Berkeley physicist.


(Hear that, Oprah?)

* * *

 

[uruk]

Whew. Glad I'm a Berkeley physicist.


(Hear that, Oprah?)

* * *

Don't you have a BS in geophysics and an MBA in marketing?
http://maxphoton.com/2007/06/02/who-the-hell-are-max-photon-and-paul-bouvet.aspx

Just asking.

 

[Max Photon]

* * *

Correct.


And thanks for linking to my site.

Management gets grumpy if site owners link to their own site.

(And we don't want the web to be full of links now do we?)



ETA: I just reread it. Thanks again uruk. I forgot what a great post that is.

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[BenBurch]

Max,

I judge people by their work, not their credentials. And on that basis, the click you are about to hear is you being put on ignore...

-Ben