Re: Pietenpol-List: 1/4 x 1/4 rib material; rib design & construction
Posted: Fri Mar 03, 2006 8:28 pm
Original Posted By: Gary Gower
As Leon Stefan said, the capstrips on Pietenpol ribs tend to sag between clusters with age. My Pietenpol is over 35 years old and the 1/4" x 1/2" inch spruce upper capstrips have been sagging for perhaps the past fifteen years. Not much, but enough to make a visible irregularity in the once-smooth curvature. What this does to the airplane's performance is difficult to measure; in fact, it seems to fly as well as it ever did. (But it sure doesn't look very good!)The rib truss is essentially a Warren type which has rather long unsupported capstrip lengths. Over time, fabric tension tends to straighten them. I regret not installing extra vertical members (see tip rib construction in plans) when I recovered my airplane twenty years ago. Since then, I repaired the wings from another Pietenpol which exhibited upper capstrip sagging, and installed 1/4" x 1/2" vertical members, using epoxy adhesive and a single 3/8" brass nail to hold one end in position until the epoxy cured. The other end fits in the gusseted cluster and is held in place by a dab of epoxy. No additional gussets are necessary. [If you are incorporating this modification when building a new set of ribs, take care to position the vertical members so they will clear the drag/antidrag bracing!]This reduces the unsupported length of the capstrip by about one half. Ergo, no more sagging problems.In my humble opinion, one should stay with the 1/4" x 1/2" rib material and install these extra vertical members. The small amount of extra weight is well-justified in providing long-term durability. Using 1/4" x 1/4" spruce isn't worth the weight savings--and would in any case make the addition of the extra vertical members essential to prevent sagging.Rather than skimping on the rib structure to save weight, significant weight savings could be achieved by using a formed .016", or .020", 2024 T3 leading edge instead of the very "sturdy" wooden leading edge. If this approach isn't acceptable, then remove excess wood from this part by routing. A vee-shaped aluminum channel for the trailing edge could save some weight.It really isn't a good idea to try to save a few ounces in the wing ribs--which are perhaps the most fragile parts of a fabric-covered light airplane.Graham Hansen (Pietenpol CF-AUN)________________________________________________________________________________Date: Fri, 3 Mar 2006 22:04:31 -0800 (PST)
As Leon Stefan said, the capstrips on Pietenpol ribs tend to sag between clusters with age. My Pietenpol is over 35 years old and the 1/4" x 1/2" inch spruce upper capstrips have been sagging for perhaps the past fifteen years. Not much, but enough to make a visible irregularity in the once-smooth curvature. What this does to the airplane's performance is difficult to measure; in fact, it seems to fly as well as it ever did. (But it sure doesn't look very good!)The rib truss is essentially a Warren type which has rather long unsupported capstrip lengths. Over time, fabric tension tends to straighten them. I regret not installing extra vertical members (see tip rib construction in plans) when I recovered my airplane twenty years ago. Since then, I repaired the wings from another Pietenpol which exhibited upper capstrip sagging, and installed 1/4" x 1/2" vertical members, using epoxy adhesive and a single 3/8" brass nail to hold one end in position until the epoxy cured. The other end fits in the gusseted cluster and is held in place by a dab of epoxy. No additional gussets are necessary. [If you are incorporating this modification when building a new set of ribs, take care to position the vertical members so they will clear the drag/antidrag bracing!]This reduces the unsupported length of the capstrip by about one half. Ergo, no more sagging problems.In my humble opinion, one should stay with the 1/4" x 1/2" rib material and install these extra vertical members. The small amount of extra weight is well-justified in providing long-term durability. Using 1/4" x 1/4" spruce isn't worth the weight savings--and would in any case make the addition of the extra vertical members essential to prevent sagging.Rather than skimping on the rib structure to save weight, significant weight savings could be achieved by using a formed .016", or .020", 2024 T3 leading edge instead of the very "sturdy" wooden leading edge. If this approach isn't acceptable, then remove excess wood from this part by routing. A vee-shaped aluminum channel for the trailing edge could save some weight.It really isn't a good idea to try to save a few ounces in the wing ribs--which are perhaps the most fragile parts of a fabric-covered light airplane.Graham Hansen (Pietenpol CF-AUN)________________________________________________________________________________Date: Fri, 3 Mar 2006 22:04:31 -0800 (PST)