Allancw's World

[Allan F]

I like to compare it to a Japanese China Airlines Flight 006 Boeing 747, which plunged from cruise altitude to 3500 feet before being recovered. It had much of it's tail control surfaces ripped off, due to near-supersonic or supersonic airflow.

I don't know if that is the way to calculate the dynamic pressure, but would (airpressure) x (speed)^2 be a reliable indicator to the forces experienced by the planes?

[gwiz]

Strictly, dynamic pressure is half the air density times the speed squared.  A common alternative way of expressing this is "equivalent airspeed", which is the speed at sea level that would give the same dynamic pressure as the higher actual speed in the reduced density air at altitude.

[Noldi400]

Sometimes conspiracy theorists bring up when a B-25 bomber hit the Empire State Building without causing significant structural damage. Not only was a B-25 bomber significantly slower than a 767, not only was it significantly smaller, but, correct me if I am wrong, but the Empire State Building was rather massively over-engineered, being, I believe, the first skyscraper of that kind of height.

I'm not sure if you would call it over-engineered, but the construction was of a totally different type. The building's "skeleton" was a grid of steel columns spaced about 6 meters apart, cross connected by girders at each floor.  Of course, this made for an extremely strong structure, but the downside was that any office (or whatever) space had, obviously, columns every six meters in any direction... 

The height of a building constructed with this technique was limited by the fact that the taller the building, the closer together the supporting columns on the lower floors had to be (because the were holding up more weight). Not an engineer, o'course, but I suspect it would take a hellacious impact to inflict serious structural damage, and even then, the lower floors would probably hold together.

http://mcadams.posc.mu.edu/vietnam.htm

He was not planning to pull out.

Look up NSAM #263  (National Security Action Memorandum), issued 11 October 1963. JFK ordered the quiet withdrawal of 1000 military personnel by the end of 1963, and approved the recommendation of Robert McNamara and Gen. Maxwell Taylor that "A program be established to train Vietnamese so that essential functions now performed by U.S. military personnel can be carried out by Vietnamese by the end of 1965. It should be possible to withdraw the bulk of U.S. personnel by that time."

I agree with you that JFK very much wanted South Viet Nam to remain non-communist.  I personally am skeptical that the McNamara-Taylor recommendation would have been successful, but even RFK (in the interview you referenced) stopped short of saying that we would have sent combat troops in. He even states "Yes, because I, everybody including General MacArthur felt that land conflict between our troops, white troops and Asian, would only lead to, end in disaster." 

The question is certainly open to debate, but we'll never know what JFK would actually have done. Just IMHO, though, I don't believe he would have ended up sending hundreds of thousands of troops there, losing something on the order of 60,000 US lives in the process.

[Allan F]

Strictly, dynamic pressure is half the air density times the speed squared.  A common alternative way of expressing this is "equivalent airspeed", which is the speed at sea level that would give the same dynamic pressure as the higher actual speed in the reduced density air at altitude.

AKA KIAS?

Not bad, really - was only missing the 1/2 factor from my not-relevant-educated guess.

[gwiz]

AKA KIAS?

Close.

Indicated airspeed is what the airspeed instrument is showing.  To convert to equivalent airspeed you need to take account of any errors in the instrument and, if you're going fast enough, you also need a compressibility correction.

[raven]

I'm not sure if you would call it over-engineered, but the construction was of a totally different type. The building's "skeleton" was a grid of steel columns spaced about 6 meters apart, cross connected by girders at each floor.  Of course, this made for an extremely strong structure, but the downside was that any office (or whatever) space had, obviously, columns every six meters in any direction... 

The height of a building constructed with this technique was limited by the fact that the taller the building, the closer together the supporting columns on the lower floors had to be (because the were holding up more weight). Not an engineer, o'course, but I suspect it would take a hellacious impact to inflict serious structural damage, and even then, the lower floors would probably hold together.

Interesting and thank you. How does this compare with how WTC was constructed?

[Allan F]

WTC 1 and 2 was in essence two hollow tubes, one inside the other. The floors were connected to the inner and outer structure, keeping them evenly spaced and rigid. This ensured wide open floors, with no colums breaking up the space. When the collapse started, the points where the floors were connected to the inner and outer tube, failed quite rapidly, causing a cascading collapse.

Edit: During the collapse, the connection between the outer and inner tube was broken, allowing the outer tube to break up and fall independently. Looking at the videos of the collapse, it is obvious that the falling pieces ejected from the collapse, fall much faster than the actual collapse, rendering the truthers claim of "free fall" null and void. Also, it is clear that parts of the inner tube - the core - stays standing for several seconds after the collapse has ended, also countering the claim that the core was destroyed from the bottom using explosives.

[ka9q]

Both planes were still well below Mach 1, so what is it that damages an aircraft from flying so fast at low altitudes? Is it the denser air?

[Sus_pilot]

Both planes were still well below Mach 1, so what is it that damages an aircraft from flying so fast at low altitudes? Is it the denser air?

Dynamic pressure on the airframe and the load for which the airframe is designed.  A Cessna 172 starts getting bent over 160 KIAS, which isn't all that fast...

[Allan F]

How fast is the first pass in this video?

[ka9q]

Dynamic pressure on the airframe and the load for which the airframe is designed. 

Got it. Same thing that stresses rockets near max-Q, except that it doesn't last long as they ascend rapidly into thinner air.

Oh btw, to put those kinetic energy figures into perspective, a "ton tnt", as used with nuclear weapon yields, is defined as a million kilocalories ("food calories"). That's  4.184 GJ, very roughly the kinetic energy of the 767s that hit the WTC.

Added: it's also very roughly the energy in the food consumed by the passengers in a few days.

The fuel carried considerably more energy (as it has to, since it provided all of the plane's kinetic energy in the first place) but the kinetic energy was released almost instantly.

[raven]

WTC 1 and 2 was in essence two hollow tubes, one inside the other. The floors were connected to the inner and outer structure, keeping them evenly spaced and rigid. This ensured wide open floors, with no colums breaking up the space. When the collapse started, the points where the floors were connected to the inner and outer tube, failed quite rapidly, causing a cascading collapse.

Again, I thank you.

[Andromeda]

WTC 1 and 2 was in essence two hollow tubes, one inside the other. The floors were connected to the inner and outer structure, keeping them evenly spaced and rigid. This ensured wide open floors, with no colums breaking up the space. When the collapse started, the points where the floors were connected to the inner and outer tube, failed quite rapidly, causing a cascading collapse.

Edit: During the collapse, the connection between the outer and inner tube was broken, allowing the outer tube to break up and fall independently. Looking at the videos of the collapse, it is obvious that the falling pieces ejected from the collapse, fall much faster than the actual collapse, rendering the truthers claim of "free fall" null and void. Also, it is clear that parts of the inner tube - the core - stays standing for several seconds after the collapse has ended, also countering the claim that the core was destroyed from the bottom using explosives.

Yes, one of Heiwa's claims is, I believe, that dropping a brick onto a pile of pizza boxes is an appropriate model for what happened...

[ka9q]

Strictly, dynamic pressure is half the air density times the speed squared.

I don't know much about aeronautics, so let me work out the physical meaning of these units. This looks a lot like the classic kinetic energy equation, 1/2 * m * v², with air density substituted for mass to give units of energy per volume. So dynamic pressure is just the kinetic energy density of the air in joules (J) per cubic meter.

The basic units of J/m³ are the same as the pascal (Pa), the SI unit of pressure. And multiplying that pressure by an area (like the effective cross section of an airplane fuselage) gives the dynamic force, which in SI units would be newtons (N).

Did I get all that right?

[Allan F]

Pizza boxes? I hate to resort to catch-phrases, but he's not even wrong.