Screw in blade type anchor testing
Hi All,
I finished my propeller install for the last time the other day, but I had one last task for the engine hoist before I return it to its owner. That task was validating the pull resistance of the screw in anchors I purchased.
Bill R. mentions an avweb anchor analysis in his post. Avweb actually have two videos of anchor testing, however neither test reviewed the blade-type screw in anchor (only the corkscrew anchor).
https://www.youtube.com/watch?v=UIuYK_eEQ9c
The anchors I purchased were sold as trampoline anchor set of 4 for just under AUD$30
http://www.lifespankids.com.au/ANCH...MI2Y_I2rSM2gIV2CMrCh0s8gKLEAQYAyABEgJz8_D_BwE
The anchors appear to be hardened and heat treated steel (the heat treatment marks are still evident on the eye, and approximately 12" in length.
Since I had 4 anchors, I though I'd sacrifice one and do a pull style test like the Avweb lads, setting the test up per the photo below.
The ground was slightly moist sandy red soil, as I was trying to emulate the conditions that the avweb team utilized. The anchor was easily screwed in using a 5/8" OD pipe of 12" in length.
On the first pull, the metal buckle fractured at 170lbs. I then tied the straps and tried again. On the second pull, the ground in a radius of about a foot around the anchor began to lift in a dome shape. As I reached maximum load, the anchor began pulling a core of material out of the middle, and the load on the digital scale began dropping. The maximum load achieved was 305lbs, almost identical to what Avweb achieved with all the other dedicated tie down kits.
At maximum load there was no opening of the eyelet and upon removal, the cutting edge and rod in general appeared to be in excellent condition.
If I had an aircraft of larger wing area or very light weight or was expecting a severe storm, I think I'd be using two of them per wing, with the rope tied to one, up over the aircraft attach point, then back down and tied off at the second, but with approximately 1.5-2ft between the two anchors, as it seems to lift the entire ground around it as I mentioned previously.
I weighed the anchors, and they came to 230g each, including the tie down straps. If you assume you need 3 of these, plus a 12" long 5/8" OD pipe (200g) plus a carry bag, the total is coming to around 900g, or 2.2lbs. To put this into perspective, by the time I added the hammer and bag to the "Claw" tie down set, it was significantly more than the claimed 8 lbs, for almost exactly the same tested load in sandy soil. Oh, and they retail price of the claw here in Australia is $225, almost 8 times the price of the trampoline anchors.
Anyway, I just thought this might be of use to people out there looking to get a set of tie downs anchors.
Tom.