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Open Thread - Sunday | 141 comments (141 topical, editorial, 0 hidden)
Gravity - reposted (none / 0)
Let's see... Gravity in fact determines the forces that the aircraft engines can generate, and the lift from aurodynamic effects. As long as the structural integrity of the aircraft is not an issu (i.e., at all times before impact) one can assume that there are no sustained accelerations larger than gravity in order of magnitude. Otherwise passenger flights would be so uncomfortable so as to be impractical. Normal lift basically balances gravity, etc.

So, we have the acceleration of gravity (9.8 m/s^2) and the speed of the plane (230 m/s from 762 ft/s). From this I can construct a characteristic length (v^2/g = 5.4 Km) and a characteristic time scale (v/g = 23 s).

Flight 77 was a 757 with a length of 50m, a wingspan of 40m and a tail height of 14m. The Pentagon is 24m tall and each of the outer walls is 280m long. A typical runway will be typically narrower than the pentagon's walls (under 100m) but at least a couple of kilometres long (the relevant dimension to compare with the height of the pentagon).

Terminal velocity is irrelevant: a plane is not spherical, to recall the famous joke about the mathematician and the cow.

This is all before impact, but there was some discussion of the target approach last night. On impact, gravity indeed seems irrelevant again, except that structures such as planes and buildings are, again, designed so that internal stresses at rest balance gravity. So gravity again can give a useful idea of orders of magnitude.

The great Richard Feynman was once giving a lecture about the forces of nature and he says "gravity is incredibly weak", at which point one of the loudspeakers in the lecture hall fell from the wall. Feynman said "weak, but not negligible".

P.S. If you don't know where this comment is coming from, don't ask, you really don't want to know. But this is an open thread, anyway.

Those whom the Gods wish to destroy They first make mad. — Euripides

by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 04:34:50 PM EST
Re: Gravity - reposted (none / 0)
where's my response.

gravity has nothing to do with aircraft engines.

my response had a very good explanation of that.  where is it?

by manon (m@gmail.com) on Sun Sep 17th, 2006 at 04:36:33 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
sorry, no - gravity does not determine the forces that an aircraft engine can generate

there are two ways to address this:

 the lift you can theoretically get from an aircraft wing

 the thrust or torque that an aircraft engine can generate

gravity plays a small part in the lift that you can generate, as a downwards force, but the Bernoulli equation for incompressible fluids (of which air is with a Mach number under .6) is the determining equation here - the lift generated is the result of the pressure differential across a surface (and gravity is taken into account in that equation)

gravity plays no part in the equation of the thrust or the torque that an engine can generate - this is limited by the efficiency of the compressor, which is dictated by its mechanical design and the physical properties of the fluid (air is considered to be a fluid) being used

by manon (m@gmail.com) on Sun Sep 17th, 2006 at 04:38:08 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Why don't you repost it yourself? Copy and paste...
sorry, no - gravity does not determine the forces that an aircraft engine can generate

there are two ways to address this:

   1.  the lift you can theoretically get from an aircraft wing

   2.  the thrust or torque that an aircraft engine can generate

gravity plays a small part in the lift that you can generate, as a downwards force, but the Bernoulli equation for incompressible fluids (of which air is with a Mach number under .6) is the determining equation here - the lift generated is the result of the pressure differential across a surface (and gravity is taken into account in that equation)

gravity plays no part in the equation of the thrust or the torque that an engine can generate - this is limited by the efficiency of the compressor, which is dictated by its mechanical design and the physical properties of the fluid (air is considered to be a fluid) being used

How many g of thrust can a passenger aircraft produce, and how many g of lift? I submit anything higher that 10g would be overkill and would be interested in knowing why, from an engineering point of view, one would want to have the ability to sustain more than 10g as that would kill the occupants anyway.

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 04:41:43 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
gravity limits the payload that you can carry of course, but not the lift - so you have to design for a certain lift for an aircraft taking into account the weight  -  get it?

military aircraft have engines that can develop thrust equivalent to about 6 g's of acceleration but they have afterburners

a commercial plane would only have about 1 g

here's another brain twisting principle - when you want your plane to land, what you effectively do is lower the lift to drag ratio so that the plane lowers in altitude and in speed - you do this by changing the aerodynamic properties of the wing ->  angle of attack, and with the ailerons, the drag of the wing

by manon (m@gmail.com) on Sun Sep 17th, 2006 at 04:51:31 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Sorry, I should have said "the ratio of lift to payload" and "the ratio of thrust to payload" instead of "lift" and "thrust". Other than that, there's nothing brain-twisting about it. I could even calculate the lift from a wing profile if my life depended on it, but not in my head.

Ah, ok, so the ratio of thrust to payload of a commercial aircraft is about 1g. That's what I meant (suuuuuure...... ;-p  )

Those whom the Gods wish to destroy They first make mad. — Euripides

by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 04:56:37 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
when one is landing a plane, there is such a thing as a ground effect that in effect prevents one from landing.  it's easier to think of it as the incompressible cushion of air under the wings, but it's really due to the conservation of angular velocity of the air surrounding the wings

(i'd have to launch into the sink and source theory of aerodynamics to explain the mathematical model to you but we have no time here for such things)

by manon (m@gmail.com) on Sun Sep 17th, 2006 at 04:57:37 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
No, you don't have to explain the relationship between circulation around the wing profile and lift. What makes you think just because I'm a physicst I don't know fluid dynamics?

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 05:00:11 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Though I have to admit that might be a reasonable assumption...

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 05:01:08 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
no, that is not what I was trying to explain.  

I didn't want to get into a discussion of vertocity, not circulation (two different things) around a wing.

by manon (m@gmail.com) on Sun Sep 17th, 2006 at 05:03:42 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Ok, you've said enough. I'll put my thinking cap on...

Now back to the question: from an engineering design point of view, what would be the point of designing a manned flying machine capable of sustaining accelerations significantly larger than 1g?

Those whom the Gods wish to destroy They first make mad. — Euripides

by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 05:06:55 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
I don't know?  space flight?
by manon (m@gmail.com) on Sun Sep 17th, 2006 at 05:08:44 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
You'd kill the crew.

And we're talking commercial aircraft.

Those whom the Gods wish to destroy They first make mad. — Euripides

by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 05:09:33 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
i thought you said unmanned

how about commercial missiles ;-)

I once worked beside the aircraft simulator for the F18 and the person who was using it had to have special pumps placed on the legs and arms to help with blood flow otherwise they would pass out.  and that was at 6G's

by manon (m@gmail.com) on Sun Sep 17th, 2006 at 05:15:02 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Oh, yes, combat pilots (and I presume astronauts during liftoff) have special uniforms designed to keep all their blood from ending up in their legs.

Anyway, my general point is this: gravity actually being pervasive, impossible to shield, and being one of the defining characteristics of the enviroment in which humans have evolved, means that 1g is a good first guess as to the typical accelerations involved in any machine meant to interact with a human without killing it. So without knowing anything else about airplanes other than the fact that they are designed to be comfortable for humans inside them, I can guess that the accelerations involved are all of the order of 1g.

Those whom the Gods wish to destroy They first make mad. — Euripides

by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 05:22:13 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Do you mean vertocity or vorticity?

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 05:08:28 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
vorticity
by manon (m@gmail.com) on Sun Sep 17th, 2006 at 05:09:34 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
it's easier to think of it as the incompressible cushion of air under the wings, but it's really due to the conservation of angular velocity of the air surrounding the wings

Actually, the "cushion of air" model would imply that the effect depends on the altitude, while the angular momentum model would imply that the effect is independent of altitude. Which one is it?

Those whom the Gods wish to destroy They first make mad. — Euripides

by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 05:04:25 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
from wikipedia:

The physics which describe ground effect are still very much under debate.

It appears that one reason for the improvement in efficiency is the "cushion" of air formed beneath the wing, but the inability of the air below the wing to behave as "free air" also seems to affect the way in which the turbulence around the end of the wing (and to a lesser extent off the back of the wing) is able to develop into downward curls in the first place. With less turbulence generated behind and around the wing to impede the airflow, it is able to flow faster over the top surface and slower underneath and the wing therefore becomes more efficient and produces better lift.

Within one wingspan of the ground the ground effect begins to change the behaviour of the airflow round the end of the wings, and at closer distances (around one-tenth of the wingspan) it begins to affect the turbulence generated off the back of the wing as well.

by manon (m@gmail.com) on Sun Sep 17th, 2006 at 05:12:55 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Within one wingspan of the ground the ground effect begins to change the behaviour of the airflow round the end of the wings, and at closer distances (around one-tenth of the wingspan) it begins to affect the turbulence generated off the back of the wing as well.
I have no intuition of the following situation [2D fluid dynamics]: what happens when you take a circle and move it parallel to an infinite line at high reynolds number? Do you get an attractive or repulsive force?

Now knowing about the ground effect for aircraft I begin to have an intuition: you get a repulsive force.

But I'd havee to check that because the reference to the role played by the wingtip means that this is not a strictly 2D effect (unlike a simple wind lift calculation).

Those whom the Gods wish to destroy They first make mad. — Euripides

by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 05:18:06 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
the wing tip causes a vortex behind each wing - maybe you've seen pictures of these behind the Concorde, for example
by manon (m@gmail.com) on Sun Sep 17th, 2006 at 05:28:56 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Ah, so this is a full 3d effect as the axis of the wingtip vorticity is [almost] orthogonal to the wing and the lift...



Those whom the Gods wish to destroy They first make mad. — Euripides

by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 05:38:21 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
yup
by manon (m@gmail.com) on Sun Sep 17th, 2006 at 05:40:28 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
You cheated, I was thinking about lift and thrust in 2D and you threw me a 3D curveball. ;-)

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 05:43:46 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
For an airfoil of fixed cross-section, as the wingspan approaches infinity, the flow far from the ends approaches that of a 2D model (nit-picking here involves a time-average of 3D wiggles in turbulent flow, which produces a funny sort of viscosity).

Since an infinitely long wing won't feel a ground effect when at an infinite height, the appropriate dimensionless number is chord/height, not wingspan/height.

Words and ideas I offer here may be used freely and without attribution.

by technopolitical on Sun Sep 17th, 2006 at 06:20:01 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
you're bringing back some memories of school here

what do you do for a living techno?

by manon (m@gmail.com) on Sun Sep 17th, 2006 at 06:25:41 PM EST
[ Parent ]
Re: Gravity - reposted (none / 1)
He blazes trails.

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 06:26:06 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
I mentioned ground effect to demonstrate how difficult it is to target a building and hit it directly.  Over an uneven surface  (unlike a runway) the ground effect is inconsistent and makes the task much more difficult
by manon (m@gmail.com) on Sun Sep 17th, 2006 at 05:36:23 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
The way I understand it, it is appropriate to imagine an airplane actually dragging two cones of vorticity behind it, which would indeed interact with nearby obstacles.

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 05:41:35 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
originating at the wingtips
by manon (m@gmail.com) on Sun Sep 17th, 2006 at 05:45:13 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
I have no intuition of how a tip manages the feat of turning angular momentum by 90 degrees. It's fascinating.

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 05:48:36 PM EST
[ Parent ]
Re: Gravity - reposted (4.00 / 2)
Aerodynamicists seem to get lost in their math sometimes. In plain terms:

The wing pushes down on the air underneath it (and pulls down on the air above it), hence the air behind the wing flows downward (d/dt momentum = aircraft weight). Differential speeds perpendicular to the stream direction imply vorticity. This is obvious, and rest follows.

The air passing the wing moves down, swirling relative to the air beyond the end of the wing, hence the tip vortex. "Circulation around the wing" is the same story. The air behind the wing is moving down; the air in front isn't.

Words and ideas I offer here may be used freely and without attribution.

by technopolitical on Sun Sep 17th, 2006 at 06:07:40 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Well, thank you again.

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 06:09:45 PM EST
[ Parent ]
Re: Gravity - reposted (none / 1)
When landing, a pilot approaches the ground on a more-or-less tangent trajectory and therefore dwells in the ground-effect regime.

By contrast, a dive aiming through the ground (or better, a building) will experience a significant ground effect only in the last moments of flight. The resulting brief force will have little effect on the trajectory. This is true even for a dive at a quite shallow angle.

A multistory building isn't a small target. Just align the aircraft to keep the base of the building at a constant angle as seen through the windscreen, and there you go.

Words and ideas I offer here may be used freely and without attribution.

by technopolitical on Sun Sep 17th, 2006 at 06:33:10 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
You don't think a half a degree target is small?

A full moon is about half a degree. Imagine a full moon - not at the horizon, which distorts the perceived size, but high enough not to be affected by that.

Personally I doubt if I could drive a car into something that wide in around 12 seconds at 400mph. Never mind a plane, which is infinitely less responsive.

And you can't line up on an arbitrary part of the windscreen. It has to be the right part of the windscreen, calibrated for your own personal eye level. Otherwise you'll drift up or down.

Considering that you only have a few seconds to make the calibration, and that - if you're the official hijacker - you're supposedly rather crap as a pilot, this is still stretching credibility significantly.

by ThatBritGuy (thatbritguy (at) googlemail.com) on Sun Sep 17th, 2006 at 08:15:54 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Consider landing an aircraft:

The speed is slower, but of the same order, and latter accuracy is excellent -- the nosewheel doesn't descend merely toward the runway, but toward the middle of the runway. The width of a Pentagon wall is huge by comparison, by more than enough to make up for any disadvantage of a faster speed.

As for height, the maneuverability of aircraft in this direction is relatively large, since the lift force is quicker to modulate than the side force (it doesn't require rolling the aircraft, just adjusting the flaps). I'd be surprised if the height accuracy were much worse than the lateral accuracy, though the variance in the touchdown point will be magnified by the small slope.

Also, regarding height, a problem for pilots is to hit the target altitude and position while also slowing to landing speed. Just aiming would be easier.

A hundred runs in a flight simulator would also be a big help.

Words and ideas I offer here may be used freely and without attribution.

by technopolitical on Sun Sep 17th, 2006 at 10:05:42 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Well, that's a rather disappointing response, on a purely factual basis.

The plane wasn't anywhere close to landing speed. The speed was estimated by the official report to be 400mph. Many of the eye witness accounts in the links describe it as running at 'full throttle'.

The width has never been the issue. It's the height I'm talking about.

To pull off a stunt like this you have to pull out of a dive, hit a vertical angle on a target that's so far away you can't even see it, all while travelling at 400mph - which is 400ft a second.

Airliners are simply not that vertically maneuverable. The proof is that commercial airliners don't land with that kind of vertical accuracy, even with all of the benefits of ILS glideslope and a low approach speed. A realistic landing window with full instrument support at low speed is two to three degrees.

And yet someone who was such a bad pilot that he wasn't allowed to solo, pulled a military-quality perfect spiral dive, and then hit a half degree corridor at 400mph by eye?

by ThatBritGuy (thatbritguy (at) googlemail.com) on Mon Sep 18th, 2006 at 06:32:23 AM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
The trick is in the preparation of the flight.
You choose a 'waypoint'
 which (for a 757) can be reeched on automatic pilot (inclusive steep dive), from that point you have to take over manually but heading and altitude ore calculated beforehand : so no visual corrections needed.
With some knowledge that even can be programmed in the navigation system.

The struggle of man against tyranny is the struggle of memory against forgetting.(Kundera)
by Elco B (elcob at scarlet dot be) on Mon Sep 18th, 2006 at 07:10:55 AM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
I addressed those points:

As I said, "The [landing] speed is slower, but of the same order". A factor of four or less in speed is, I suggested, compensated for by the large target size.

I discussed the height issue. Please look again.

Can't see the target? I thought it was the size of a full moon, a long 12 seconds before impact.

A realistic landing window with full instrument support at low speed is two to three degrees? Therefore...?

As I said, "a problem for pilots is to hit the target altitude and position while also slowing to landing speed". Simply hitting a target is easier.

Where is the evidence for a "perfect spiral dive", and what would this mean? I see no reason to think that internet talk of the near-impossibility of an alleged event here is anything more than the usual story-telling. Someone misrepresents an event or how it could happen, this fits a conspiracy narrative, and gets repeated endlessly.

After seeing tenacious adherence the absurd idea that steel doesn't weaken when hot, I have no respect for the quality of critical thinking in this crowd. They're obviously far from reality-based with respect to elementary observations.

For example, at a glide slope of 3 degrees, a height error equal to the height of the Pentagon wall would correspond to an error in touch-down point of well over a kilometer. This is huge compared to real-world accuracy, therefore vertical control is much better than that. And yes, pilots can land without instruments, provided that they can see.

This is another non-issue, bloated up into a great blob of nonsensical argument.

Words and ideas I offer here may be used freely and without attribution.

by technopolitical on Mon Sep 18th, 2006 at 03:18:05 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Part of the point of my original post is to give estimates of the sizes of things, since I have no intuition of how fast or small things are as I have never flown an airplane.

I have argued that "natural units" in this case are a length of 5.4 Km and a time of 23 s. Indeed, the height of the Pentagon (24 m) is 0.0044. You can interpret this as radians as 5.4 Km is a good approsimation of the radius of curvature of the plane's trajectory, and you indeed get 0.25 degrees. But you still have 23 seconds to manoeuvre the aircraft. This is not like aiming a rifle and shooting a bullet, where there is no scope for corrections once the bullet is fired.

The Twin towers were over 420m tall and 63m wide, which are the dimensions of a smallish runway and present the same (vertical) aspect on approach (unlike the Pentagon's outer wall, which presents a horizontal aspect.

Those whom the Gods wish to destroy They first make mad. — Euripides

by Migeru (migeru at eurotrib dot com) on Mon Sep 18th, 2006 at 03:59:41 AM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
I meant "about 420m tall" not "over 420m tall".

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Mon Sep 18th, 2006 at 04:07:05 AM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
A multistory building isn't a small target. Just align the aircraft to keep the base of the building at a constant angle as seen through the windscreen, and there you go.
ElcoB mentioned being unable to replicate the Pentgon crash on his flight simulator... Maybe this tip would help, and if it doesn't, it would be interesting to know why.

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Mon Sep 18th, 2006 at 04:18:37 AM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
If the wall is wider than a runway, and the acceptable height-range is greater than for landing on a runway, what's the extraordinary problem?

Words and ideas I offer here may be used freely and without attribution.
by technopolitical on Mon Sep 18th, 2006 at 02:43:20 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
I'm chipping in here on non-scientific grounds.

Perhaps the extraordinary problem here could be that the person who is thought to have flown the Pentagon plane, Hani Hanjour, was described as "having trouble controlling and landing a Cessna" by flight instructors, as reported by the New York Times.

Now I'm not saying this is actually relevant, as I myself was very good at crashing planes in Flight Simulator.

by Alex in Toulouse on Mon Sep 18th, 2006 at 03:00:42 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
It has been widely noted that a suspicious aspect of the hijackers' training is precisely that they didn't train to land.

Words and ideas I offer here may be used freely and without attribution.
by technopolitical on Mon Sep 18th, 2006 at 03:20:27 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
I would say that being incapable of controlling a Cessna would be suspicious too (were they only interested in controlling Boeings?).
by Alex in Toulouse on Mon Sep 18th, 2006 at 03:40:05 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
He replied on the breakfast that there is realy no problem after all. However, the height of the wall is 24m versus a tolerance of hundreds of metres as to where on the runway to tough down.

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Mon Sep 18th, 2006 at 03:01:12 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
tough => touch

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Mon Sep 18th, 2006 at 03:02:42 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
And at a 3 degree glide slope, a touchdown error of hundreds of meters corresponds to what vertical error? (Small.)

Words and ideas I offer here may be used freely and without attribution.
by technopolitical on Mon Sep 18th, 2006 at 03:22:08 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Yes, I was just figuring out that was what you meant. The 24m height corresponds to the entire length of a runway at a 3deg slope.

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Mon Sep 18th, 2006 at 03:25:35 PM EST
[ Parent ]
Re: Gravity - reposted (4.00 / 2)
I'd guess it would depend on how wide the surrounding area has to be to be influential. If there's laminar flow over and under the wing then it's a non-issue. If there's a huge vortex, which seems more likely, then it's going to be very significant.

I have to say that I don't see how gravity is relevant to the discussion at all. The impact angle - however achieved - was close to horizontal, which means most of the kinetic energy from the momentum vector would be dissipated into the building and not into the ground.

So the 1G question seems like a non-issue. It would be more useful to estimate local kinetic energy at impact - which is easy as a ball park calculation, but harder if you want an accurate estimate of structural deformation and heating effects during the impact.

If I had more patience than I do right now, it probably wouldn't be too hard to work out the amount of energy needed to melt and/or vaporise a chunk of fuselage alloy and compare it with the kinetic energy produced by coming to a very fast stop from an approach speed of approx 400mph.

Obviously you'll get some energy lost to mechanical deformation, so it's not likely to be a very accurate model of the crash. But a ballpart sort of figure could still be interesting.

As for 1G - the 757 is designed to survive at least 3Gs. Turbulence and other extreme conditions can create an environment in which 2G or 3G accelerations are possible.

Some more information about the approach run here.

The 757 doesn't actually have a computer capable of automating the flight path. Elsewhere there are a lot of contradictory points made about the level of flying skill required.

by ThatBritGuy (thatbritguy (at) googlemail.com) on Sun Sep 17th, 2006 at 05:48:16 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
The heat of vaporisation of aluminium is huge compared to the kinetic energy at aircraft speeds. Aluminium does, however, have a very low melting point. I've melted it (deliberately, even) on an electric-range heating element. A nice, hot fire would do the job quite well, and blackened blobs of melted aluminium mixed with blackened rubble wouldn't look much like bits of an aircraft.

Regarding piloting skills, note my comment above about ground effect (negligible in this instance) and flying while keeping the target at a constant view angle. Your linkee makes a good point about forces during a turn, but overestimates the difficulty of flying in a straight line aimed at something. No computers needed.

Regarding any notion that there wasn't a 757 impact, where did the missing 757 go, then? The 4th dimension, along with all the mysteriously lost socks?
--------

Technopolitical's law: For every unusual disaster in the news, there will be at least one expert who can't understand it and proposes a different, conspiratorial cause.

Words and ideas I offer here may be used freely and without attribution.

by technopolitical on Sun Sep 17th, 2006 at 07:01:10 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
I disagree on a few points:

  •  there is no evidence to suggest a dive that i've seen

  •  you must have a very powerful appliance there to burn aluminium

  •  i've studied melting points of metals but vaporisation is not something you see everyday, so not taught in school

  •  even when you vaporise turbine blades, there is a residue that is left downstream
by manon (m@gmail.com) on Sun Sep 17th, 2006 at 07:56:57 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Your linkee makes a good point about forces during a turn, but overestimates the difficulty of flying in a straight line aimed at something. No computers needed.

I think you're hugely underestimating the degree of skill involved. I covered the vertical angle aspect yesterday - the impact corridor required for an accurate hit on the facade of a not very tall building is tiny. Especially after a fierce spiral dive.

Horizontally of course there's no problem. But that's not the issue. It would take a fantastically talented pilot to pull off a turn like that. And - much as I hate to quote a conspiracy site - there seem to be reasonable grounds for believing that the 757 pilot wasn't particularly gifted.

I'm mostly happy with the bulk of the 757 story. I'm not quite so happy about how it came to be flown quite so precisely by someone who - by all accounts - could barely fly at all, into a section of the building that was, very conveniently, closed for renovation at the time.

Some interesting eye witness comments here.

by ThatBritGuy (thatbritguy (at) googlemail.com) on Sun Sep 17th, 2006 at 08:07:07 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
If the last part of the trajectory is straight, and the problem is like hitting a patch of runway (a very precise height!), why should this be an exceptionally difficult problem?

It was done, after all, unless the plane disappeared by magic, etc.

Words and ideas I offer here may be used freely and without attribution.

by technopolitical on Mon Sep 18th, 2006 at 02:40:56 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
I doubt pure aluminium was used.  Aircraft structural elements are some alloy of Aluminium stemming from its low tensile strength.  Thus, melting and combustion temperatures will depend on the alloy.

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre
by ATinNM on Sun Sep 17th, 2006 at 10:46:09 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
The melting point should be just a bit lower. I can't find any indication that the stuff burns in ordinary fires.

Words and ideas I offer here may be used freely and without attribution.
by technopolitical on Mon Sep 18th, 2006 at 02:38:39 PM EST
[ Parent ]
Re: Gravity - reposted (none / 1)
gravity has nothing to do with aircraft engines.

I think that Migeru's point could be summarised as follows:

Gravity has everything to do with the thrust of aircraft engines, the lift of their wings, and so on, because gravity is what sets their requirements. This is a matter of engineering purpose. Physics is merely a constraint on how (and whether) the purpose can be achieved, and with what trade-offs, but for passenger jets, engineering purpose limits thrust and lift to be adequate to produce accelerations of the order of 1 g, not a lot more or a lot less.

Words and ideas I offer here may be used freely and without attribution.

by technopolitical on Sun Sep 17th, 2006 at 05:50:00 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Thank you.

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 05:54:47 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
yes, that's it
by manon (m@gmail.com) on Sun Sep 17th, 2006 at 06:06:04 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
But surely vaporisation - or not - of a plane is more linked to the kind of fire you get going, and the temperatures you reach, no?

In the long run, we're all dead. John Maynard Keynes
by Jerome a Paris (etg@eurotrib.com) on Sun Sep 17th, 2006 at 05:11:29 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
DoDo has ridiculed 9/11 conspiracy theories based on the temperature at which kerosene burns.

Those whom the Gods wish to destroy They first make mad. — Euripides
by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 05:13:55 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
And DoDo does so quite properly. Since I just concocted it, I'll repeat, with a small improvement in wording --

Technopolitical's law: For every unusual disaster in the news, there will be at least one expert who can't understand how it could happen and proposes a different, conspiratorial cause.

Words and ideas I offer here may be used freely and without attribution.

by technopolitical on Sun Sep 17th, 2006 at 07:06:12 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
ok.  you can burn turbine blades in an aircraft engine if you increase the fuel flow too much.  the turbine blades are  designed to withstand the temperature at which the flame exists, but by increasing the amount of flame, you increase the temperature but this assumes that you are burning, not exploding

in a fuel like kerosene, which doesn't allow for flame speed to increase dramatically, which I think is called the combustibility of the fuel, you can only get limited explosive characteristics compared to something like say, ammonia and diesel fuel mixed together (which is what was used in the Oklahoma bombing) or in a true explosive like nitroglycerine

so the metal didn't burn that quickly, and kerosene doesn't really explode that quickly either  (and even if it did, there would still be huge pieces of wreckage)

so then look at mechanical forces - did the impact cause the plane to virtually disappear - not from what we know of other plane crashes where you get wreckage and entire engines still intact (most of the time)  - the engines are extremely heavy

so maybe both together - kerosene and impact - well no, because planes that have crashed (for example a 757 that crashed in Greece with full fuel tanks at high speed into the side of a mountain) still had huge pieces of wreckage.

were the conditions that much different in the two crashes?  not in any way  that anyone can identify

by manon (m@gmail.com) on Sun Sep 17th, 2006 at 05:27:13 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
combustion is a field of mechanical engineering by the way, because of the need to calculate flame speed and other characteristics for internal combustion engine
by manon (m@gmail.com) on Sun Sep 17th, 2006 at 05:46:26 PM EST
[ Parent ]
Thanks for that reply (none / 0)
I remember reading the discussions about the fire at WTC, and someone explaining why it was not surprising that the steel pillars were weakened by the fires - there was some mention of other things burning in the WTC in addition to the kerosene itself.

I'd need to dig it up, but i can't replace it, and google brings up tons of conspiracy theory sites!

In the long run, we're all dead. John Maynard Keynes

by Jerome a Paris (etg@eurotrib.com) on Sun Sep 17th, 2006 at 05:50:46 PM EST
[ Parent ]
Re: Thanks for that reply (none / 0)
My summary: Think of fire, steel, hammers, and pictures of blacksmiths at work. Fire, obviously, softens steel. Common knowledge suffices here.

Words and ideas I offer here may be used freely and without attribution.
by technopolitical on Sun Sep 17th, 2006 at 07:12:17 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
try googling pictures of Helios airplane crash August 15 2005 to see pictures of the same type of plane crashing into a mountain at full speed
by manon (m@gmail.com) on Sun Sep 17th, 2006 at 06:04:19 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Well, apart from the tail, you don't see anything left of the plane. Nothing that wouldn't fit in a suitcase anyway... ;-)

And you did not have lots of burning material like inside the Pentagon...

In the long run, we're all dead. John Maynard Keynes

by Jerome a Paris (etg@eurotrib.com) on Sun Sep 17th, 2006 at 06:30:31 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
The main source of heat would have been burning jet fuel in both cases, but inside the Pentagon (or rubble pile) the heat would be better confined and hence better able to melt things. Fuel splashed over the ground would be relatively ineffective.

Words and ideas I offer here may be used freely and without attribution.
by technopolitical on Sun Sep 17th, 2006 at 07:26:00 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
but the vaporisation is supposed to have occurred in the second after impact
by manon (m@gmail.com) on Sun Sep 17th, 2006 at 07:50:23 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
The kinetic energy of impact at aircraft speed is insufficient (by a large factor) to vaporise the fuel:

At 300 m/s, 1/2 m V^2 = 45 J/gm, which isn't enough to bring kerosene near it's vaporisation temperature, much less to supply the additional heat necessary to actually vaporise it.

I can believe that a typical impact might convert much of a fuel load into an aerosol, but I can't see how this could remove more than a fraction of the fuel from the building. Slamming a volume of liquid in through the walls will leave a lot of fuel deep inside.

Words and ideas I offer here may be used freely and without attribution.

by technopolitical on Sun Sep 17th, 2006 at 08:39:44 PM EST
[ Parent ]
Re: Gravity - reposted (none / 1)
Bizzarely That was the air crash that coincided with my last trip to Greece. (and the hotel I stayed in the next day appears to have burned down in this summers forest fires).

The time before I was in Greece was September 2001, so flying back from that on september 12th was an experience.

If they have any brains, they'll never let me back into Greece again

Any idiot can face a crisis - it's day to day living that wears you out.

by ceebs (ceebs (at) eurotrib (dot) com) on Sun Sep 17th, 2006 at 07:41:06 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
"Bizzarely That was the air crash that coincided with my last trip to Greece. (and the hotel I stayed in the next day appears to have burned down in this summers forest fires)."

Ho! What are the odds of that? Something suspicious here. Put up a web site about it, and look for more odd events that can be linked.

Words and ideas I offer here may be used freely and without attribution.

by technopolitical on Mon Sep 18th, 2006 at 03:25:40 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
you can burn turbine blades in an aircraft engine

I don't know of a high-temperature turbine-blade material that burns at a temperature lower than its melting point. The issue is weakening and breaking under stress at high temperatures, or eroding due to oxidation, or (perhaps, if thermal gradients are very high) melting.

by increasing the amount of flame, you increase the temperature but this assumes that you are burning, not exploding

I don't understand this. How can quantity of flame (an extensive property) be related to temperature (an intensive property)? What is the relevance to temperature of burning vs. exploding ?

in a fuel like kerosene, which doesn't allow for flame speed to increase dramatically, which I think is called the combustibility of the fuel, you can only get limited explosive characteristics compared to something like say, ammonia and diesel fuel mixed together (which is what was used in the Oklahoma bombing) or in a true explosive like nitroglycerine

"Combustibility" -- This refers to how easy it is to burn something, not to flame speed (which has meaning only in a uniform, premixed material).

"ammonia and diesel fuel mixed together (which is what was used in the Oklahoma bombing)" -- That would be ammonium nitrate and a fuel oil (ANFO). Ammonia, of course, cannot serve as an oxidiser.

"a true explosive like nitroglycerine" -- Ammonium nitrate and a fuel oil are a true explosive. In fact, ANFO is a high explosive (that is, detonating via shock-wave compression, rather than by heat at a rapid burn-front). It differs from nitroglycerin in being insensitive to heat and ordinary mechanical impacts.

the metal didn't burn that quickly

I don't know of instances of any substantial amount of aircraft aluminium burning at all (other than in the wing of the Challenger during reentry -- a special case!).

kerosene doesn't really explode that quickly either

I don't know of instances of kerosene exploding in an impact (aside from vapour-air mixtures inside a fuel tank, or perhaps mist-clouds created on impact). For an explosion to occur, rather than rapid burning, the fuel and oxidiser must be premixed.

did the impact cause the plane to virtually disappear - not from what we know of other plane crashes where you get wreckage and entire engines still intact (most of the time)  - the engines are extremely heavy

In most crashes, the engines strike soft ground at a small angle, not a wall straight-on. I wouldn't be surprised if engines often survive intact, but not this time.

planes that have crashed (for example a 757 that crashed in Greece with full fuel tanks at high speed into the side of a mountain) still had huge pieces of wreckage.

Kerosene burning in open air wouldn't greatly heat wreckage, and much of which would anyway be scattered away from the fuel. For example, a quick Google image search <greece crash> shows that the tail (of the 737) is sitting in unburned vegetation.

In contrast, an enclosed space (or rubble pile) is more like a furnace, and better able to confine wreckage to the burning zone.

were the conditions that much different in the two crashes?  not in any way  that anyone can identify

The differences seem both identifiable and substantial.

 

Words and ideas I offer here may be used freely and without attribution.

by technopolitical on Sun Sep 17th, 2006 at 08:27:07 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
I'd like to get all the perpetrators into a vole hole and transport them to Vahalla, at this point.

Anyone know the coordinates.

Let's do the Kennedy assination, Pearl Harbor, Yalta, Chariots of the Gods, bremuda triangle, crop rings, auger cow pies, Rorsach test tea leaves, whatever.

Jesus. Enough with the freaking airplanes!

"When the abyss stares at me, it wets its pants." Brian Hopkins

by EricC on Sun Sep 17th, 2006 at 05:29:42 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
Enough with the freaking airplanes!

Sorry, dude, this is an open thread and I'm enjoying the opportunity to pick the brain of a fellow geek in her area of expertise.

Those whom the Gods wish to destroy They first make mad. — Euripides

by Migeru (migeru at eurotrib dot com) on Sun Sep 17th, 2006 at 05:35:39 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
why thank you
by manon (m@gmail.com) on Sun Sep 17th, 2006 at 05:41:07 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
GRRR!!(a ferocious little dog)

"When the abyss stares at me, it wets its pants." Brian Hopkins
by EricC on Sun Sep 17th, 2006 at 06:17:31 PM EST
[ Parent ]
Re: Gravity - reposted (none / 0)
I just want to know who the hell is behind the conspiracy to have this subject come up in every freaking open thread!  

Those who can make you believe absurdities can make you commit atrocities. -Voltaire
by p------- on Sun Sep 17th, 2006 at 11:28:37 PM EST
[ Parent ]
Re: Gravity - reposted (4.00 / 4)
Me.

Because I asked the participants to come over here and discuss instead of hi-jacking IdiotSavant's diary.

BTW, I think it's a very good and fascinating discussion.

by afew (afew(a in a circle)eurotrib_dot_com) on Mon Sep 18th, 2006 at 03:21:16 AM EST
[ Parent ]
Re: Gravity - reposted (4.00 / 2)
Yes, once the mud-slinging part was over, it was interesting and I learned a lot, not about conspiracies but about airplanes and connected stuff.
by Fran (fran at eurotrib dot com) on Mon Sep 18th, 2006 at 04:31:36 AM EST
[ Parent ]

Open Thread - Sunday | 141 comments (141 topical, 0 editorial, 0 hidden)
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