6.3.4. Forces breakdown
In order to compute the number of ropes or straps needed to restrain a load, the force applied to each tie-down point in any direction must be carefully studied. For that purpose, it is necessary to consider a reference axis system (X, Y, Z) in the Forward, Left and Up directions.
In the above example, two straps are attached to each tie-down point, generating two forces: the force F1 is generated by a strap restraining the load in the forward direction, whereas the force F2 is generated by a strap restraining the load in the up direction.
LOADING LOADING
A. LOADING GENERALITIES
. Forward strap: F1 force breakdown In X direction
In Y direction
F1X = F1.cos(α).cos(β)
F1Y = F1.cos(α).sin(β)
In Z direction
As each strap shall make a maximum angle of 30 degrees with the direction of restraint, α and β shall be less than 30o. Consequently, for a FORWARD (or AFT) strap, the minimum restraint force in each direction can be expressed as follows:
F1x = 75% F1 F1y = 43% F1 F1z = 50% F1
A. LOADING GENERALITIES
Note : the recommended 30° between the rope or the strap with the main direction of restraint enables a high restrained force in the needed direction, indeed by increasing this angle, one decreases the weight that can be restrained by the item. Nevertheless some operators might choose even when applying the recommended 30° to perform the computation with an angle of 45° in order to take into account a margin in case one of the ropes in not correctly tied down.
In this case the above force breakdown is F1x = 50% F1, F1y = 50% F1, F1z = 70% F1
. Up strap: F2 force breakdown
F2X =0 , F2Y = F2.sin(δ) , F2Z = F2.cos(δ)
As each strap shall make a maximum angle of 30 degrees with the direction of restraint, δ shall be less than 30o. Consequently, for an UP strap, the minimum restraint force in each direction can be expressed as follows:
F2x = 0
F2y = 50% F2
F2z = 86% F2
Note : the recommended 30° between the rope or the strap with the main direction of restraint enables a high restrained force in the needed direction, indeed by increasing this angle, one decreases the weight that can be restrained by the item. Nevertheless some operators might choose even when applying the recommended 30° to perform the computation with an angle of 45° in order to take into account a margin in case one of the ropes in not correctly tied down.
In this case the above force breakdown is F2x = 0, F2y = 70% F2, F2z = 70% F2
Note2: When the force is applied in the forward, left or up directions, it is considered as positive. It is counted negative in the aft and right directions.
LOADING LOADING
A. LOADING GENERALITIES
6.3.5. Maximum load
Let‘s assume, as an example, a sudden deceleration of the aircraft. Without restraining system, the load would slip forward. To prevent it from moving in the forward direction, a strap is tied-down to a single stud on each side of the load, as shown below.
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