Collision of steel post with aircraft wing
Analytical Service Pty Ltd
TECHNICAL NOTE 102

TECHNICAL NOTE 102

This Note is quite similar to TN101, except for having a broader scope and title. In the previous Note a single event of cutting the post was presented. The approach taken here was to gradually increase the wall thickness of the post until the wing is completely cut during the collision.

In the previous work the point of impact was 10.8 m from the plane of symmetry of the fuselage. Now, that point is assumed farther away, at 13.36 m, where a smaller section of the wing resists impact.

The post models have 300 mm outer diameter and wall thickness of 4,8,12 and 15 mm in the four impact cases presented. The post is 12 m tall and is impacted at mid-height. According to the official reports, the fall of the aircraft was caused by a birch-tree cutting through the wing. The shear strength of the "suspect birch" is quoted below and compared with that of the post. Kinematic conditions at impact differ from those in TN101 in that the wing has its chord inclined by 15⁰ to the horizontal (rather than being horizontal) and that it also has the vertical velocity component. (Fig.1)

Fig. 1 (left) Wing position and velocity components when impacting the post. (For the sake of ease of computer work, the wing chord was taken as horizontal and the post was inclined.)

Probably the most interesting is impact against the weakest post considered, with 4 mm thick wall, as it is similar in strength to the suspect birch. As shown in Figs. 4.1-4.4 the nose becomes damaged and the front longeron has its web bent, but is otherwise intact. The front ends of two ribs behind are also partially damaged. The piece of the post between the top and bottom skin is caught by the wing and carried away. The resultant reaction force at the base of post is presented in Fig. 4.5. It is the geometrical sum of three perpendicular components. The largest peak among the three is reached by a vertical force, associated with the wing pulling the post along its axis.

Fig. 4.1 Wing cutting 4 mm post. View from below.

Fig. 4.2 Wing cutting 4 mm post. View from below.

Fig. 4.3 Wing cutting 4 mm post. View from below.

Fig. 4.4 Wing cutting 4 mm post. The post shown by itself, before failure on the left and after, on the right. The badly deformed piece flies off with the wing.

Fig. 4.5 Wing cutting 4 mm post. The reaction force at the base of the post.

The damage inflicted on the wing by the 8 mm thick post is illustrated in Figs. 8.1-8.3. Not only the nose segment but also a segment of the front longeron is destroyed. The breaking of the post happens at t =15 ms, but the damaging effect of the upper part of the post lasts longer.

Fig. 8.1 Wing cutting 8 mm thick post

Fig. 8.2 Damage to the wing shown in greater detail. A single-bay segment of the front longeron is destroyed. View from top, tip segment on the right. (Careful reading makes it possible to distinguish fragments of stringers as very thin lines.)

Fig. 8.3 The same damage illustrated when skin is removed.

With the longeron broken, there is a question of eventual breaking off of the wing, in view of the same aerodynamic forces still being applied. To clarify this, the simulation was carried up to t = 300 ms, longer than in the other cases and long past the end of damage inflicted by the post. The damage at the end of simulation, illustrated in Fig.8.4, is not much different from that in the preceding figures. There is no visible tendency for the wing to break off. In Fig. 8.5 we see the record of vertical velocity at the tip, the point which undergoes the largest movement. In a steady state it should be 5 m/s, which is the vertical component of the motion. Here we have oscillations superposed on that 5 mm/s. (Note that no damping was used in our model, so the vibrations do not die down as fast as they should.) The stress history of vibrations exhibits a stronger decay. Still, the stress pulsations, which persist, could conceivably destroy the wing after a longer while due to metal fatigue. But this takes, typically, hours or minutes at best. For the few seconds remaining to the end of flight no such danger is real.

Fig. 8.4 Damage after prolonged time, seen from below.

Fig. 8.5 History of vertical speed at the tip of the wing.

Fig. 8.6 History of effective stress of an element near the break cavity.

Fig. 8.7 The resultant reaction force at the base of the 8mm post. Upon the impact, the post begins to vibrate, which shows itself in the magnitude of the base reaction..

With the 12mm post, the damage is more extensive, but the center longeron remains largely intact, Fig.12.1-12.3. Finally, a 15 mm post completely cuts through the wing, Fig.15.1-15.3. (Fig.15.3 is mainly shown to demonstrate that the cut-off part does not continue the steady- state flight like the rest of the wing, but that it begins rotation in space, i.e. it enters a random flight mode.)

Fig. 12.1 The effect of impact against 12mm post..

Fig. 12.2 The damaged area shown in a greater detail.

Fig. 12.3 The same area shown without skin

Fig. 12.4 The resultant reaction force at the base of the 12mm thick post.

Fig. 15.1 The thickest, 15 mm post cutting the wing

Fig. 15.2 The damaged wing as seen from the trailing edge.

Fig. 15.3 The three components after the event: The wing, the cut-off tip and the bent and nearly broken post.

Fig. 15.4 The resultant reaction force at the base of the 15mm thick post.

The change of forward velocity of the craft as well as that of the yaw were nearly imperceptible as a result of the described events. The lift and the drag forces were applied to the wing model during the considered time span.

COMPARISON OF THE SHEAR STRENGTHS

Steel pole (use 8mm as the example):
D = 300 mm, h = 8 mm, therefore section area A₁ = 7339 mm²
Construction steel, F_y = 350 MPa, F_u = 430 MPa, e = 16%.
Shear strength, F_su = 0.6 F_u = 258 MPa
Section strength, single shear: P₁ = A₁F_su = 7339x258 = 1893.4 kN

The mass density of the pole is increased above that of steel, so that its mass per unit length is the same as for suspect birch. (Specific mass of the latter was assumed as 700 kg/m³.)

h is wall thickness, d_av is mean diameter, A is section area, m is mass per unit height of the pole, ρ' is the mass density of the model pole to give the same mass per unit length as that of the suspect birch.

Suspect birch: D = 400 mm, therefore A₂ = 125,660 mm²
Shear strength : F_su = 5 MPa
Section strength, single shear: P₂ = A₂ F_su = 125,660 x 5 = 628.3 kN
(Upper values were used here, but not the absolute maxima. It would be unreasonable for these circumstances to do otherwise.)

Strength ratio for the 15mm post:
P₁/P₂ = 3465/628.3 = 5.51

When comparing steel with timber note that for the latter the elongation prior to rupture is 5% at the most. Keeping in mind the importance of ductility under fast impact conditions we can say that using much larger maximum elongation makes the object much stronger. We have increased the mass per unit length of the post to match that of the tree, but we have not degraded the elongation of the post to make it more similar to the tree. This treatment was very favorable for the survival of the tree. With the lower elongation assumed, the post material would need a much greater static shearing strength (say P₁/P₂ = 7 or 7.5) to cut off the wing.

FURTHER DETAILS

The material properties of the aluminum alloys used were
2024-T3:F_y =293MPa,Fu =448 MPa and e=16%
7075-T6: F_y = 493 MPa, F_u = 545 MPa and e = 9% (Stringers only)

which are the averages of published data and which are similar to the original Russian alloys involved.

Structural properties near impact point: Main skin: 4 mm,
Longeron walls: Front: 2 mm, Center: 4 mm, Rear: 2 mm

Nose skin: 2 mm and Rib: 2 mm. The stringers are thick-wall I-beams which, in this region, have the section of 329 mm². As there are 3 longerons, 6 of the stringers are used as caps.

Instead of the steady flight lift pressure corresponding to 1g the design lift, 4x as large was applied. This was to make the effect of aerodynamic loads more visible.