Hi. I have finally got my Airbike certified, and had it on my second flight yesterday. I was doing my third touch and go, when suddenly the tail wheel broke off. As you can see from the picture, I found that there has been a crack in the weld, probably for a long time, and yesterday the remaining material (the grey area) was not able to take the forces anymore. The Airbike has flown about 6 hours after I restored it, but I was not able to detect the crack when I cleaned and inspected the part before painting. The Airbike is built in 1996, has about 255 flight hours and 500 landings. Thought I should give you a heads up, so that you guys can check yours. My tail wheel is attached to the frame via a rod, while the drawings that I have show a leaf spring. If any of you got a picture or a pdf, of my version, it would be very much appreciated.
Make it a hard landing/rough ground inspection item. Easy way to spot trouble, clean sand and paint with an epoxy paint (from model airplane hobby shop). If the paint flakes off you will know the part has been flexing and will soon crack.
Peter Strand The broken part in the first picture,is it a free bolt or welded in threaded rod? If the thread portion was located in this area then the crack can be explained. Doesnt look like a weak design.Seems the bolt or rod was brittle or became brittle with time.A proper choice of material looks important here.A material which does not harden or brittle with use.
It seems to me that it is a welded in bolt. I has the hole for the cotter pin. It also seems that the head of the bolt has been filed round, and then it has been welded around the head. The area that cracked is hidden when the tail wheel is installed, so painting in that area will not show. I will have to remove the cotter pin and nut, and pull the bolt out, before I can inspect it. By the way, how hard is a hard landing
After a bit of reading I found that onlya mild steel bolt can be welded or a grade 4.6. Alloy and high carbon bolts shouldnt be welded.
Therefore welding AN bolts which are hightensile alloy steel seems to be a bad practice even in a tailwheel assembly.
It seems like a re design is called for in this area to avoid welding the bolt. I came up with this idea. I am sure there can be other variations.
If a grade 4.6 bolt could be made to work then the design wouldnt need changing.This grade can be welded.
Thanks for the heads up and new design, mullacharjak. I don't think I will re design the thing now. Maybe I can save it, if I am careful and it weld it short distances at the time, and let it cool down in between. I was not aware of the problem with welding high tensile steel, so thank you for the information.
How do you know for certain what grade bolt that is? How do you know from a picture this is from EMbrittlement?
The head marking would show what bolt it is. The part has been sheared off almost horizontally. A sign of brittleness. The failure has been initiated simultaneaously from two sides in line with the axle on opposite sides of rod and then circling circumferentially to meet in the middle.Pre existing isolated crack seems unlikely.Case hardening of rod due to welding can make rod brittle at any point. A crack can initiate at a brittle point.
I don't see how you could weld an aircraft grade bolt. They are heat treated bolts. If they are re-heated by welding they would lose most of their strength. The heated area would fatigue much faster. Further as shown you are developing a lot of leverage in landing on a very small area. I would say a re-design is in order.
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