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Old 10-22-2021, 09:51 AM
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JSF-TC
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Spent the morning stripping the wreckage and looking for clues, especially in regards to some form of structural failure.

All control surfaces were still attached and linkages intact and able to move freely. Servos felt smooth, but have not been powered to check if they still work.The rear fuselage bulkhead that holds the elevator servo had fractured in multiple places and allowed the elevator servo to come loose, but I suspect that was a result of the impact.

Both flaperons and the left inner flap are undamaged. The tailplane has one minor crack in the upper fairing, but is otherwise undamaged.

The left wing was virtually undamaged, with just the loss of the last 1" on the trailing edge at the wing tip. No structural damage was observed. After loosening the retaining screws, it slid off the wing joiners easily at the crash site so that we could get it in the car.

The right wing showed significant damage, and it had separated along with a section of the fuselage, which was easily removed at the wing joiners. The upper and lower skins were cracked in multiple places. On initial inspection it was not possible to tell if the damage was caused in-flight or by the impact.

Further checking of the right wing showed that the upper skin was loose and could easily be removed. As I peeled back the skin it became apparent that it was not bonded to the internal structure. The Hysol on the structure showed very little contact with the skin.











De-skinning the left wing showed that it was firmly attached and in contact in all areas, although the tolerances could be tightened up to reduce the glue thickness.


I feel confident that this lack of upper skin bonding to the internal structure resulted in a loss of torsional rigidity of the right wing. With the 3deg washout, there will be significant torsion acting on the outer wing and this is likely to have caused the right outer wing to fail, probably in a nose down direction, leading to the snap/ spin.

I'm currently not sure if I made an error in generating the internal structure in CAD, or if the built-up wing plugs caused a variation in wing thickness. Probably a combination of both, but it allowed at least a 1.5mm gap between the top of the structure and the upper skin across most of the structure, both ribs and spars. I had bonded the structure to the lower skin first and confirmed it was seated correctly, but it is really hard to check the closed structure once the top skin is down.

No one to blame apart from myself.......

Paul



Last edited by JSF-TC; 10-22-2021 at 09:56 AM.