Forces on a pallet
 

It sounds like there's a bit of confusion about the forces acting on a pallet during acceleration, rotation and climb.

There are two frames of reference to view it in - relative to the direction of gravity, and relative to the deck of the aircraft. It doesn't much matter which.

Accelerating the pallet to stay with the aircraft is done through the attachments. Combined with gravity there are forces due to the acceleration of the aircraft. These can increase or reduce the demands on the attachments as compared with standing parked on the ramp - in extremis, flying a parabolic vomit comet trajectory would mean the pallet had no forces through the attachments. The mix of locks, straps, and friction all have to provide these forces.

If the cargo's not to move, effective gravity (incorporating weight and acceleration of the aircraft) and the reaction forces through the attachments, which you could choose to resolve parallel and perpendicular to the deck, always balance. If there's no acceleration, then the total force through the attachments is just 1g from gravity, although the direction it points changes based on orientation of the deck to horizontal.

On initial acceleration for take off, the attachments deal with g from gravity and about 0.3g forwards - leading to a total of 1.044g on the attachments, acting 16.7 degrees forwards of the vertical. This forward component reduces as drag builds going down the runway with the same thrust.

On rotation, sharply increasing drag should reduce the acceleration perhaps even pulling back on the load, as the gravity vector also rotates backwards.

Acceleration during the climb pulls gently forwards to keep the load with the deck, while the gravity vector remains laid back. A constant speed climb means you just get a total of 1g of gravity acting backwards from vertical: loading the attachments more lightly perpendicular to the deck, but requiring a forward along-the-deck component of force from the attachments.

The total forces to accommodate, without turbulence or sharp turns, aren't much bigger than 1.1g - however, the direction shifts more, which could affect the relative burden on straps and locks.

That's from a physicist's perspective: maybe that helps, maybe it doesn't.

With the small amount of information available to ponder, that would remain a possible cause for the NTSB to consider, and possibly find clues for. Or not. Other sources for clues to that might lie in the realm of interviews or "humint" of varying kinds.

If anyone is looking for background on National Airlines cargo, the Airforwarder assoc magazine for Spring of 2013 has a pre-crash story about them. Eerily it shows one of their 747's with MRAPS parked in front of it.

All right, something I'm expert in. (I would definitely set the parking brake and put it in gear, regardless of strapping..)

Acceleration is vectorial. The acceleration of gravity is a fixed acceleration perpendicular to the ground. It adds to the other accelerations, from pitch/roll/yaw/thrust/drag, vectorially.

Good point, I agree it could be a factor, but of course anything goes at this stage.

The Taliban did claim responsibility, but then of course they would, regardless, if they considered that there was any chance of enhancing their position amongst their followers.

I wonder if the crew had to remove their shoes before they re-boarded ?
( sorry - bad taste, but you get the point )

Normal pitch is 12.5 degrees for flap 20 and 15 degrees for flap 10.

A few degrees higher if they had excess takeoff performance and chose to use it. ie max thrust.

FWIW, my vote is “surely yes”, and for Machdiamond being right.

Question:
What would be the tension on the tiedowns resisting rearwards motion of the cargo if the aircraft pulled up to 90 degrees and the engine thrust was zero?

Answer:
The tension in the tiedowns would be zero, because then the aircraft and it’s load would both be accelerating downwards (decelerating) at the same rate. Like in an orbiting spacecraft. And in NASA's "vomit comet".


On the other hand, if the aircraft were stationary on the ground it is true that the tiedown tension would increase with pitch-up angle until at 90º the tension would equal the total weight of the load.

But a flying aircraft is not stationary. It behaves exactly as Machdiamond says in Post#545.

FTR, In the last second

116 kts IAS
V/S -12800 ft/min (126 kts)
+35 deg AOA
-28 deg pitch
-63 deg flight path at 142 kts
532 ft (AGL at 90 deg roll)
.


EDIT: JF: Why do you assume I will learn about something that I already know.

alph2z

Thanks for the teaser.

Now can you give us the pitch change between Vr and V2 ?

alph2z

Total crap.
Show your working and justify the accuracy. You have not even showed your error +-
plus IAS is what is shown in the cockpit, which at very high angles of attack may not be even remotely accurate.

I call this plain wrong. If this was true there would be no need for brakes to be locked on the catering trolleys in a passenger cabin, and people would levitate from their seats. As I understand it, all objects in an aeroplane are travelling at the same speed, but because of the slight nose-up flying attitude, gravity still 'pulls' items to the rear of the aircraft. Test it yourself when you walk in an aircraft in flight. Gravity certainly does exert force on any object in an aeroplane (and I hope I don't need to rely on your expertise in this field)

Quote: So if he pulled up to 90 degrees (heaven forbid) the tiedowns resisting rearwards motion experience the same load?

Surely not?
FWIW, my vote is “surely yes”, and for Machdiamond being right. Quote

That is true, BUT...... think, what happened to the 1G that was pulling the load onto the deck?..its dissapeared, it is no longer in the equation. and thus a large force that was helping keep the load planted is now gone.
You have to look at the whole situation not just forces in isolation

Oh Dear, ..... get a house brick, glue a piece of draught-excluder along two long edges on the large,flat side, place on a tea-tray, rubber strips downwards, "strap" down with parcel-tape (cellotape) then tilt the tray.

Lower -edge tends to dig in to tray, upper edge tends to rise from tray, whole thing tries to slide down the tray.

OK, the dynamic situation is a " bit " more complex, but anyone who has ever had a loose item in a motor-vehicle will have experienced the forces occuring in a change of direction.

Perhaps looking at a few flat-bed lorries might help. Drivers apparently fear long tubes and sheet-materials (especially steel sheet)
They understand that if that lot shifts, it's difficult to stop it and being guillotined like a sliced loaf is a real possibility (tube would punch you full of holes!)...Normally they use "Sylvesters"- a schain -version of a ratchet-strap with a lever about 50 Cm long. Yes, there's a pre-load but i'd estimate that each chain would have a breaking-strain ~100 tons.

As long as they're tight, there's no problem, soon as you have movement, you massively increase the loading....

that's the problem with transporting vehicles, Tyre-compliance ( chassis bouncing) suspension compliance - body bouncing. Very difficult to draw the line between too slack and too tight.

Loading is not a totally unskilled job!

Alph2z, the reason John Farley asked if you understood stalls is that you clearly don't know sh1t from Shinola. If (as you postulate) the alpha was +35 degrees I'd be amazed if the ASI was indicating anything of any value, so your '116kts IAS' is as meaningless as most of the drivel on this thread..
Back to MS FlightSim for you methinks

:ugh:
Ok, here is actual flight test data for a jet taking off:
http://machdiamonds.com/takeoff.jpg
Horizontal axis is runway distance. The three vertical dash lines are rotate, liftoff and obstacle.
AX is longitudinal acceleration measured by an accelerometer on board the aircraft. It starts as 0.2 g and then drag picks up with airspeed.
Peak longitudinal acceleration takes place at liftoff in this case, but this is not always the case. It also depends on where you measure it in the aircraft when there is a pitch rate. Near the tail it will be higher (centrifugal effect around the aircraft cg).

While established in the climb at 14 degrees pitch, the longitudinal acceleration is EXACTLY THE SAME as it was on the runway. :eek:

Now if you do not believe this flight test data and want to argue about it as well, I don't know what else I can do.

Thud105 - on a cold rainy day in the UK you brought a big smile to my face.

I love it when people try and criticise John Farley (who I had the great pleasure of meeting a few years ago).

Been Reading this with great interest, not as a Pilot but as an Ex Movements Controller in the RAF.
Now i have been out for the best part of nearly 30 Years but i"m pretty sure we used to Restrain Equipment, including Wheeled Equipment at 4G Forward and 1.5G Rearward and Lateral , like i say, hard to remember.
Thus - 4G Multiplied by 25000 lbs = 100000 lbs,
Divide the 100000 by the 10000 lb (Restraint factor of a given Chain) and that Equals 10. Now, Single Chain Tie Downs are always in Pairs believe it or not so as such you would have 5 times 10000 lb Chains one side and 5 the other for forward restraint and for the Rear, personally, i would put 6 Chains, (3 aside).

Someone mentioned 100 Ton Chains or something, basically, the Chain is only as Strong as its Weakest Link, for instance, if a 10000 lb Chain was used on a 5000 lb tie down point on a Herc Ramp then that is what the Chains Restraint Factor is.

Someone also mentioned Restraint Equipment is Tested to 1.5 times it"s Restraint Factor, True, so as such, a 10000 lbs Chain should not Break until around 15000 lbs.

The Wooden Dunnage under the Load may have been used for Load Spreading.
The Aircraft Pallet that The Vehicle was Sited on has a Floor Loading Intensity, (FLI), much the same as Aircraft Floors, and at 25000 lbs, that particular Vehicle may have Exceeded it and as such, they may have Spread the Weight elsewhere on the Pallet.

As for Load Movement in Transit, i have never not seen the Loadie Check his Load on Arrival at another Station, they always check it, also in Flight as well, now this is from my Experience.
Can"t Comment on the Crash itself as i have already mentioned, i not a Pilot, except to say how Tragic it was.

I leave it to you, as an exercise, to prove me wrong; otherwise your statement is not pertinent to this forum.

How can you read JF's mind ? Prove my data wrong. I'm a pilot, engineer, did not want to go back to the engineering dept of a major airline (i.e. no high-speed semiconductor physics). IAS is theoretical and does not include high AOA effects.

Does he know who JF is?

We feel the force needed to resist gravity, not gravity itself
 

Followup to david1300 - I have years of experience working in this field and know full well of what I speak here.

All objects with mass are acted upon by gravity such that they would experience a uniform acceleration in the absence of other forces. The concept of weight comes from the force needed to resist the acceleration that gravity would impart if left unchecked. In order for a mass to remain in an unaccelerated state relative to an earth based reference frame a force must be applied to that mass to oppose gravity. Remove that force and the mass will accelerate toward the earth at approximately 9.8 meters per second^2.

Force is measured between objects. In the case of an airplane and cargo, between the body of the aircraft and its contents. Because both the airplane and the cargo are subject to the same gravity field, the force between the two does not represent gravity. There cannot be any force between the airplane and its cargo without some external force acting on the airplane (and here gravity is not a force). On the ground at rest, the external force comes from the gear. In the air the external force comes from the engines and aerodynamic forces. In the air, the forces felt within an airplane have nothing to do with pitch or roll attitude. They are entirely dependent on thrust and the combination of angle of attack, sideslip angle, control surface positions, dynamic pressure (i.e., airspeed), and Mach number.

As the data provided by Machdiamond shows, the greatest X-axis acceleration occurs when thrust is high and during the rotation when the total lift is greater than 1g and along the stability axis such that some of it is experienced along the body X-axis. X-axis acceleration during a climb is roughly the same as during takeoff roll. It should also be noted that for transport aircraft with a thrust to weight ratio of 0.3 or less deceleration during landing rollout can result in an even higher X-axis acceleration in the opposite direction due to reverse thrust and braking.

This whole topic illustrates the criticality of pilots having a reliable visual reference for pitch and roll attitude as there is no means to determine those important airplane state parameters from the accelerations that the crew is experiencing.