To stitch or not to stitch

[Jim Heffelfinger]

Again my yahoo group has become bored and brought up one of the 3 deadly topics for experimental aircraft - Rib Stitching. Sometimes it gets asked quite innocently, other times its truly a troll - just to get the group all wound up - again. We know where it will lead.
So, not to troll here but to ask the question honestly.
Parameters - 85 VNE, wing loading under 4 #. Gross about 600#. Wood ribs >>>> To stitch or not to stitch.
Where is the demarcation, if there is one, to definitely stitch.
Is there one "glue" better than another if not stitching. AC43.13 has not been updated in the is area for a LONG time. Is it outdated?
Just because we have always done it this way - is it still REALLY valid?
Since much of the country is frozen to their door knobs this might be a way of warming things up.
OH - this is about facts. Not personalities. Keep it informative or I will pull the thread.

[Aaron Novak]

Under factory controlled condtions I would think adhesive bonding would be more at home. Stitching is a nice mechanical means of attacing the fabric that has strengths you can rely on. Adhesives have strength you "think" you can count on (provided its application was perfect). History on stitching says it WILL work, the history on gluing says it CAN work. Or as I like to think of it, the more proven methods I can use, the less I have to worry. Time spent stitching vs. having to get the fabric just right so it shrinks up.....maybe the same? I am curious, what are the true advantages to gluing from the people that chose to do it?

[WLIU]

So if you rib stitch, you first glue the fabric down to the rib. And before you rib stitch, you apply an additional tape over the fabric to protect the fabric from the stress of the stitch around the rib. All Correct?

I think that what I understand is that using only adhesive to attach the fabric to the ribs reduces the parts count as well as labor hours.

So to go with gluing the fabric to the ribs as the primary attachment, you need to A) see if any other aircraft has a history of this method, or B) come up with a plan for a method, adhesive to use, and how it will be tested to an appropriate safety factor.

Now if the airplane has a wing loading of 4 lbs/sq ft, we still need to know how many ribs are in the wing, and we have to factor in that the 4 is an average calculation, but the loading is higher near the wing root due to prop wash. So we need to calculate how much load the attachment to each rib must sustain and figure out an appropriate safety factor. After all, we do not build our wings for just 1 G flight in smooth air.

Certificated airplanes use different rib stitch spacing on the inboard vs the outboard ribs due to propwash. My Pitts S-2A uses 1 inch spacing inboard and 3 or 4 ribs outboard transitions to 2" spacing.

So if you come up with some numbers, build some test ribs, and do some sort of load testing, you might get the answer you are looking for.

So if this has not been done on an existing airplane yet, would you rather rib stitch towards a finished airplane or make test articles and break them to see what the answer is? The former might get you a flying airplane sooner, but the latter might help all of your builder friends for the future.

Best of luck,

Wes
N78PS

[Matt Gonitzke]

Here's one data point, which may or may not be useful, but nonetheless... My Schempp-Hirth SH1 glider, which has wing loading of about 4.9 lb/sq. ft. and a 155 mph VNE, has fabric on the top of the wing, aft ~20% of the chord, and the entire structure of the all-flying V-tail is fabric-covered, and there is no rib stiching anywhere on it. The original fabric was doped cotton, and now it's Superflite glued and finished with the Stewart Systems process.

[1600vw]

Went to Oshkosh 2010 and I saw a few airplanes that had no stitching.

When doing the repair on my wing we had to rib stitch for my airplane has this. I found it not hard to do and really did not take that long for what piece of mind you get from it.

But I have also owned a couple airplanes that had no rib stitching. One was just like the airplane I fly today. This bird had no rib stitching. All are airplanes with speeds in the 50-80 range.

Tony

[WLIU]

If I may ask, what is the wing chord and the rib spacing on the Schempp-Hirth SH1?

Thanks!

Wes
N78PS

[Sam Buchanan]

I have never seen hard numbers as to where the line of demarcation should be in regard to non-rib stitch or rib stitch. However, scores of sub-85mph VNE aircraft have safely flown for the past few decades without rib stitching. I have built and owned three such aircraft; Fisher Super Koala, TEAM MiniMax, and Legal Eagle XL. All these aircraft have wood wings with similar rib spacing (~14").

The MiniMax allowed me to observe the top surface of the wing during flight and I never saw fabric deformation that was cause for concern. The Koala and Eagle allow me to observe the bottom surface and while there is some buffeting behind the prop arc, the slipstream presses the fabric against the ribs, not away from them.

I did rib stitch the vertical stab on the Eagle after video showed considerable buffeting of the fabric due to propwash.

All these aircraft were flown in a conservative manner per the designer's intentions. The low wing loading, low VNE, small performance envelope, and extensive field history lead me to believe the omission of rib stitching is a safe construction technique and provides weight savings on these very light aircraft.

[Matt Gonitzke]

Wes,

Wing rib spacing is 230mm according to the 3-view I have. Root chord is 1200mm. Only the aft foot or so of the wing has fabric, and only on the top surface. The tails are about 6' in span and have a root chord of about 2' or so, with a plywood D-tube leading edge. Rib spacing on the tails is slightly less than the wings, but my 3-view doesn't have that dimensioned, and the glider is currently too far away for me to measure. Hope that helps.

[Jim Hann]

So if you rib stitch, you first glue the fabric down to the rib. And before you rib stitch, you apply an additional tape over the fabric to protect the fabric from the stress of the stitch around the rib. All Correct?

Wes,

I'm in the process of recovering the tail and wings of my PA-22/20. I'm using Stewarts and approved fabric (of course.) Following their STC for my certificated airplane you do not glue the fabric to each rib, you glue it on the perimeter. My flaps and ailerons I did a one piece wrap so there is no glue on the trailing edge. Then do an initial shrink, apply reinforcements, finish shrinking and then stitch.

The biggest single problem I see with gluing is that you need to make sure you don't introduce undesirable stresses into the structure. If you glue to each rib before any shrinking takes place, you'll have a mess in my opinion. The alternate would be to apply glue, let it dry, apply the fabric, shrink it, then glue down through it like the Stewart process.

Obviously (well to me at least) is that the tapes are the bulk of the added weight. Will there be tapes over the ribs to reinforce the covering so you don't have tears? Is this all being done with the 104 fabric? I don't mind doing the stitching, and it sure secures the fabric well leaving no question about its stability.

Good luck,
Jim