Atlas Air 767 down/Texas
Join Date: Jul 2013
Location: Norway
Age: 57
Posts: 140
Likes: 0
Received 0 Likes
on
0 Posts
I think we in reality agrees, but we might have a slightly different meaning regarding acceleration.
According to Einstein a sattelite in orbit is free falling around the earth in its orbit, experiencing zero G. Standing still on the face of the earth we are experiencing 1G.
What you was describing is what most people would count as an acceleration, the (vertical) speed starts at zero, then the speed increases as the object is free falling towards the earth. And thus the object must be experiencing a G of some magnitude since it is falling faster and faster. But this acceleration is only perceived and is dependent on the observer standing still and experiencing 1G. An observer free falling together with the object would see no acceleration of the object and would therefore conclude the object was not under any acceleration and thus experiencing zero G.
Join Date: Jun 2001
Location: Rockytop, Tennessee, USA
Posts: 5,898
Likes: 0
Received 1 Like
on
1 Post
Join Date: Aug 2006
Location: USA
Posts: 142
Likes: 0
Received 0 Likes
on
0 Posts
Let us all wait for the NTSB to do the work.
I understand some of the pprune group needs to vent all the ideas from the comfort of there arm chairs. But, all this discussion is not helping answer the question of "what really happened." Let us all wait for the professional to produce the result.
We lost three fellow airman, may they rest in peace.
We lost three fellow airman, may they rest in peace.
Join Date: Jun 1999
Location: world
Posts: 3,424
Likes: 0
Received 0 Likes
on
0 Posts
mustanfsally, there's nothing wrong with venting ideas as long as it comes from those sufficiently qualified to make comment (though admittedly that's not always the case). That's what PPRuNe is all about didn't you know? If it upsets you, I guess it's best just to ignore this thread.
Join Date: Jul 2013
Location: Norway
Age: 57
Posts: 140
Likes: 0
Received 0 Likes
on
0 Posts
Well, they both did. You have Newtonian gravity, and you have Einsteinian gravity. In most cases the equations yeld practically identical results, however, when the values are extreme, as in an object in close orbit around another extremely dense object, like a white dwarf, a neutron star or a black hole, the equations yeld completely different answers.
Here on earth we need to resort to Einsteinian gravity and his theory of relativity in order to get the gps system to work, among other things. If we didnt apply Einsteins equations to the gps system we would see the position of a non moving gps receiver on the surface of the earth drift a couple km each hour. This is due to the fact that the time, and hence the clocks, on the gps satelites is running ever so slightly faster than the time and clocks on earth.
Here on earth we need to resort to Einsteinian gravity and his theory of relativity in order to get the gps system to work, among other things. If we didnt apply Einsteins equations to the gps system we would see the position of a non moving gps receiver on the surface of the earth drift a couple km each hour. This is due to the fact that the time, and hence the clocks, on the gps satelites is running ever so slightly faster than the time and clocks on earth.
Join Date: Mar 2014
Location: WA STATE
Age: 78
Posts: 0
Likes: 0
Received 0 Likes
on
0 Posts
Join Date: Jul 2013
Location: Norway
Age: 57
Posts: 140
Likes: 0
Received 0 Likes
on
0 Posts
Join Date: Feb 2009
Location: Seattle
Posts: 379
Likes: 0
Received 0 Likes
on
0 Posts
Response of the stabilizer to a failure of the jack screw or its nut and braking system would be greatly influenced by the position of the attached elevators. The horizontal tail normally carries as downward load (less so at aft CG, more so at forward CG). With the stabilizer hinge line forward of its center of pressure, the load on the jack screw is usually in the direction associated with rotating the stabilizer in the leading edge up (i.e., airplane nose down) direction. If the linkage controlling stabilizer position were to let loose I would expect the stabilizer to rotate in that direction causing the airplane to pitch nose down. In response (either by way of the autopilot or the crew) the elevator would be commanded in the airplane nose up direction (elevator surface trailing edge up) that would put further moment on the stabilizer to drive it in the airplane nose down direction.
We also need to remember that the lift generated by the tail is only a small portion of the total lift of the airplane. For the overall airplane to enter an essentially ballistic free-fall the wing angle of attack would have to decrease to that for zero lift. A horizontal tail mechanism failure that allows the stabilizer to float will result in nose down pitching moment that would tend to decrease wing AOA, but it is not correct to assume that a floating stabilizer leads to the wing experiencing AOA corresponding to zero lift. It is very possible that such a failure would result in sufficient nose down pitching moment to drive the wing to a significantly negative AOA to cause negative g.
We also need to remember that the lift generated by the tail is only a small portion of the total lift of the airplane. For the overall airplane to enter an essentially ballistic free-fall the wing angle of attack would have to decrease to that for zero lift. A horizontal tail mechanism failure that allows the stabilizer to float will result in nose down pitching moment that would tend to decrease wing AOA, but it is not correct to assume that a floating stabilizer leads to the wing experiencing AOA corresponding to zero lift. It is very possible that such a failure would result in sufficient nose down pitching moment to drive the wing to a significantly negative AOA to cause negative g.
Unless I’m recalling my rusty theory incorrect G-forces are accelerating forces and indicate a rate of change rather then velocity. Same as a VSI shows rate of change.
Standing in an elevator I feel acceleration and deceleration forces.
If the vehicle that I’m in reaches a constant speed and I’m at the same speed I won’t feel ( measure) anything.
If the speed of the vehicle changes and I’m firmly attached to this vehicle I will sense (measure) a change.
Starting at altitude I can have an average rate of descent which is based on altitude change/time but it can be continuously accelerating or deceleration after initial acceleration.
This is where Vg diagrams come in and the definitions of the various speeds such as Va.
Intact airframes do not free fall in the atmosphere
Just a honest question.
What do you calculate the average vertical G to be in order to start at zero vertical speed at 6000 feet and then reach the ground in 18sec?
Drag, thrust and lift work in three dimensions. Only gravity works in a single dimension.
Etkin's Dynamics of Atmospheric Flight devotes 75 pages of serious math to General Equations of Unsteady Motion
Etkin's Dynamics of Atmospheric Flight devotes 75 pages of serious math to General Equations of Unsteady Motion
Salute!
@ the wait-and-see folks ----- we should be fairly confident that our discussion of the MCAS on the 610 accident was worthwhikle and may have prevented another accident, ya think? And all that good info was before much of a formal report was released from the relevant safety agency. As tdracer , et al have stated, an obvious system or hardware failure that might exist, or be possible on other aircraft of this type would likely be announced early on by the NTSB.
If I were flying this type today I would be glommed onto this thread like flies on sierra.
Gums sends...
.
@ the wait-and-see folks ----- we should be fairly confident that our discussion of the MCAS on the 610 accident was worthwhikle and may have prevented another accident, ya think? And all that good info was before much of a formal report was released from the relevant safety agency. As tdracer , et al have stated, an obvious system or hardware failure that might exist, or be possible on other aircraft of this type would likely be announced early on by the NTSB.
If I were flying this type today I would be glommed onto this thread like flies on sierra.
Gums sends...
.
6000' in 18 seconds gives a = 37 ft/s² if my maths is correct.
Join Date: Jul 2013
Location: Norway
Age: 57
Posts: 140
Likes: 0
Received 0 Likes
on
0 Posts
Response of the stabilizer to a failure of the jack screw or its nut and braking system would be greatly influenced by the position of the attached elevators. The horizontal tail normally carries as downward load (less so at aft CG, more so at forward CG). With the stabilizer hinge line forward of its center of pressure, the load on the jack screw is usually in the direction associated with rotating the stabilizer in the leading edge up (i.e., airplane nose down) direction. If the linkage controlling stabilizer position were to let loose I would expect the stabilizer to rotate in that direction causing the airplane to pitch nose down. In response (either by way of the autopilot or the crew) the elevator would be commanded in the airplane nose up direction (elevator surface trailing edge up) that would put further moment on the stabilizer to drive it in the airplane nose down direction.
We also need to remember that the lift generated by the tail is only a small portion of the total lift of the airplane. For the overall airplane to enter an essentially ballistic free-fall the wing angle of attack would have to decrease to that for zero lift. A horizontal tail mechanism failure that allows the stabilizer to float will result in nose down pitching moment that would tend to decrease wing AOA, but it is not correct to assume that a floating stabilizer leads to the wing experiencing AOA corresponding to zero lift. It is very possible that such a failure would result in sufficient nose down pitching moment to drive the wing to a significantly negative AOA to cause negative g.
We also need to remember that the lift generated by the tail is only a small portion of the total lift of the airplane. For the overall airplane to enter an essentially ballistic free-fall the wing angle of attack would have to decrease to that for zero lift. A horizontal tail mechanism failure that allows the stabilizer to float will result in nose down pitching moment that would tend to decrease wing AOA, but it is not correct to assume that a floating stabilizer leads to the wing experiencing AOA corresponding to zero lift. It is very possible that such a failure would result in sufficient nose down pitching moment to drive the wing to a significantly negative AOA to cause negative g.
So, I would speculate that a free floating HT where the hinge point is forward of the center of lift line will cause the aircraft to quickly reach approximately zero G and thereafter increase vertical speed at approximately 10m/s2 (1G) which would take an object from 6000 feet to the ground in 20 sec.
Join Date: Jul 2013
Location: Norway
Age: 57
Posts: 140
Likes: 0
Received 0 Likes
on
0 Posts
6000 feet in 20 sec gives zero G, free fall or approximately 10m/s2, 6000 feet in 18 sec gives your slightly larger value which is a slightly negative G.
My idea with the "average" value is the same as yours, namely what value of constant G or acceleration would cause the object to travel the required vertical distance in the required time.
6000 feet in 20 sec gives zero G, free fall or approximately 10m/s2, 6000 feet in 18 sec gives your slightly larger value which is a slightly negative G.
6000 feet in 20 sec gives zero G, free fall or approximately 10m/s2, 6000 feet in 18 sec gives your slightly larger value which is a slightly negative G.
A "zero G" descent (or indeed a descent at any constant vertical acceleration value) follows a parabolic profile (ask any trainee astronaut). The actual descent profile, as far as it's possible to ascertain from FR24, was at an almost constant flightpath angle.