rvbuilder2002 said:
e
Even to aero engineers, things don't always look right ...thats why they test.
The push rod as designed has been tested. I don't know what the failure load was but it seems to me I heard that it was more than three times the load expected from tha flap that is used on the two seat RV's. Except for the RV-9(A) which has a much larger flap. Which was the reason the heavier pushrod material was used.
Yeah...and I wonder why some RVs out there have 1000s of hours on them with no flap pushrod failure... Wonder what the factory demo planes have on 'em... hm... I'm guessing the STOCK design!
I think there are at least 2 problems at work here, both of which lie in the builder's hands:
1. Drilling and tapping the pushrod properly. First of all, did the builder drill a proper clearance hole before tapping? Did the builder drill that STRAIGHT? If the hole wasn't drilled straight, it could leave a thin area in one spot on the wall of the pushrod -- essentially a stress riser. Wonder where it's gonna break if it does...
2. Rigging the flaps. If you allow the pushrod to be in TENSION when the flaps are retracted, i.e. the flaps are tugged up to the point where they're forced tightly against the fuselage by the PULLING, not by the FIT...well, that probably contributes to some degree.
I know several builders at Chino who opted to replace the aluminum with steel (of the same size) -- some from the get-go, and others at some point along the way...after having flown and, as Scott points out, tested.
I also know one RV-8 builder with whom I fly weekly...he did have one aluminum pushrod fail at ~250 hours (iirc). It was a non-event that he, as a terrific pilot, handled calmly. I wasn't able to see the part after the failure to get a sense of whether #1 or #2 above played into it. Wish I had been able to see it. He replaced it with steel tubing, and of course he preached doing the same to all of the local RVators...understandably.
My RV-7 with 1400+ hours still has the bone stock aluminum tubes, which I drilled & tapped in my garage. I *KNOW* I did not drill 'em perfectly straight, but I am not losing sleep over it.
I'm not trying to convince people NOT to use the larger hex stock pushrods. I actually think it's a great idea for builders who aren't confident that they can do a "good enough" job with the stockers. Scott, your point about testing is valid to me, but you gotta consider "the worst builder" out there. A factory-built pushrod may have been tested, but was "the worst built" pushrod tested?
I think I'm just seeing this argument from both angles, and at this point I'm just babbling...