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USAF MC-12 and National Cargo 747-400F crash in Afghanistan

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posted on May, 9 2013 @ 08:46 AM
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Originally posted by C0bzz
The cargo doesn't care about the pitch angle on takeoff. The acceleration the cargo "feels" longitudinally should be directly proportional to excess power. The excess power will be maximized at the beginning of the takeoff role, not during climb. Therefore, it should not matter what the pitch angle was during takeoff, as it is irrelevant.


Pitch angle is HIGHLY relevant to cargo shifting. Cargo can hold steady during level acceleration, but shift dramatically due to a high pitch angle. It's like when you're standing on a bus that starts moving. You're saying that when that bus starts up a hill, you're only going to get pushed backwards when the bus starts moving, and not when you go up a hill. It's the same concept.

There is some shifting that will occur on acceleration, but nowhere near as much as will occur on climb out.




posted on May, 9 2013 @ 09:34 AM
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No, that's not what I am saying at all.

Assuming no frictional resistance, if a bus is accelerating at 2.5 m/s on level ground due to the torque of the engine, and then the bus pitches up due to a hill by 15 degrees, then assuming constant torque, the bus will no longer be accelerating, because the force due to engine torque and the force due to gravity down the slope will be equal. The longitudinal reaction force in the bus (i.e. the force in the direction from the back of the bus to the front of the bus) will be the same throughout this entire exercise, due to constant engine torque. Passengers will be pushed back into there seats at about 0.255 g throughout this entire exercise.

Assuming constant torque:
Hill > 15 degrees = Deceleration
Hill < 15 degrees = Acceleration

The longitudinal reaction force will remain the same throughout, this is entirely due to engine torque!

The aircraft is essentially identical to this. At the beginning of takeoff roll, the thrust to drag ratio is maximized because there is no air resistance, and the engines are producing maximum thrust. As the drag increases, the longitudinal acceleration will decrease somewhat. When the aircraft rotates, the power the engines are developing stops going to accelerating the aircraft, but goes to making the aircraft gain height.

There will also initially be an increase in reaction force from the floor, as the aircraft takes off and its velocity changes from horizontal to diagonal, and then later a decrease in reaction force from the floor as it is climbing (due to the being pitched up) but not accelerating. Due to a decrease in friction against the floor, an increase in force the straps or bracing has to accommodate has likely increased.

However, the longitudinal reaction force (i.e. what the cargos feel longitudinally) will go from being from aircraft acceleration, to fighting gravity, but will be fairly similar in magnitude throughout. The angle of attack would be non-zero after rotation, this is the only other reason I can think of which would increase the force on the straps as it would increase the longitudinal force by a relatively small amount.

Point is, longitudinal reaction force is mostly dependent on the thrust-to-drag and thrust-to-mass of the aircraft.
edit on 9/5/13 by C0bzz because: (no reason given)



posted on May, 9 2013 @ 10:32 AM
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I found flight test data:

machdiamonds.com...
source: www.pprune.org...

Looks like longitudinal acceleration is maximized during liftoff.



posted on May, 9 2013 @ 11:08 AM
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reply to post by C0bzz
 


It's at its highest during climb, just after liftoff. It rises slightly at rotation, but goes up to just over 0.3Gs after liftoff. So there's a definite increase after takeoff. At the 14 degree mark it's about 0.35, when it was less than 0.2 at rotation. So the test data shows that pitch angle IS relevant. Once they pass through 16 degrees, and lower the nose back to about 14 degrees, it levels off to about the same as on the runway, but there is a definite increase on climb out.



posted on Jun, 3 2013 @ 12:25 PM
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I've received word that several chains securing the MRAPs on the 747 were found broken, and at least one vehicle shifted during take off, possibly more than one.






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