john smith wrote:

Okay... so what is the theory as to why it is cracking there?
Is it a result of the spar flexing?
Stay with the two-blade... less weight, more efficient at cruise.

Ah...should have mentioned that. My A&P, as well as the guys JimB
pinged, say that it's oil canning. Some speculate that it's due to the
stress from walking on the wing-walk area (and claim that this is more
prevalent on the right wing). Everyone advised a good look at the spar
area for any signs of anything wrong. Fortunately, between the gear
well and inboard wing inspection panels, you can get a good look at the
problem areas.

While at OSH, I plan to check each Arrow I come across for this fix.
Sort of like my brother crawling around RVs seeing how folks put certain
parts together :-)

Yeah, the 2-blade is our lowest cost option. Based on the Hartzell AD,
I'd like to ditch them and go with McCaulley...but the 40% discount from
Hartzell makes them the least expensive. Right at 5.8 AMUs including
shipping old prop back/new prop to us. The McCaulley option from a Bay
Area prop shop is 7 AMUs.