Stringers in 170B wings.

[cfiatzph]

Anybody have any idea why the last section of wing on the outboard of the last rib is missing two stringers? Why did'nt Cessna just run them the whole way? Kinda strangers I thought somebody had a bad repair on the wing because the rib has the holes for the stringers. My 56 172 is the same way also. Does'nt really matter just curious. Also how does the wings get its twisting on the ends?


Thanks

[GAHorn]

Those are called "breakaway" wingtips and are simpler, easier to repair as they are frequently the area damaged in minor accidents/hangar rash.

[cessna170bdriver]

Anybody have any idea why the last section of wing on the outboard of the last rib is missing two stringers? Why did'nt Cessna just run them the whole way? Kinda strangers I thought somebody had a bad repair on the wing because the rib has the holes for the stringers. My 56 172 is the same way also. Does'nt really matter just curious. Also how does the wings get its twisting on the ends?


Thanks

Just a guess, but since the bending moment on a cantilevered wing section increases as you go inboard (each spanwise point in the structure has to carry the total load generated outboard of that point), the engineers figured there wasn't enough bending moment in the last bay to warrant the stiffening that would be supplied by the stringers.

I don't have a copy of the IPC to see exactly where the stringers are "missing" but it could be that the same shape rib is used in several spanwise locations, and they didn't bother to design a rib without the stringer cutouts.

The twist is implemented by mounting the internal structure (spars & ribs) in a jig to set specific angles on each rib. When the skin is riveted on, it holds each part in its proper relative orientation.

Miles

[Metal Master]

Having just recently repaired both wings on my 170A I discovered some interesting things. I have a few later model Cessna wings laying around. The last rib out board in the early Cessna 170A, B, 180 and 182 is symmetrical. The later Cessna’s that did away with the (easily replaceable repairable) ? Reference (Section with just the two stringers in it) (the last bay) has an out board rib that is Not symmetrical. It has the same airfoil shape as the ribs inboard. This change also occurred at the same time that Cessna drooped the leading edges on the wings from the factory.
The wing twist in Cessna 100 series airplanes is all in the wing from the wing strut out board. The wing from the fuselage to the strut has no twist. The twist can easily be seen by standing at the wing tip and looking inboard towards the cabin. You will see the twist starting at the skin lap at the strut and continues to the second to the last rib out board. The twist continues to the last rib on all airplanes but in the earlier airplanes (Easily recognized by having a section of skin between the last and second to last rib) the twist appears to be accentuated by the last rib being symmetrical in the earlier airplanes. The angle of twist is measured at the wing tip rib at the tooling holes in the rib forward and aft. They are #10 drill size holes. The angle is referenced to a line drawn through the centerline of the attachment holes at the wing root (At the cabin) to a line drawn through the centerline of the two holes at the tip rib.

If you stand behind the wing of a Cessna 100 with an all-metal wing (that has twist) and look at the trailing edge of the aileron you will note that the trailing edge curves up towards the tip. This is built into the aileron to compensate for the twist in the wing.

Wing twist issues and measurements are discussed elsewhere in these forums and in the Service Related Articles Manual Cessna 170 sold by the association.

I have noticed by looking at airplanes at the airport (Every time I see one) that looking at the outboard leading edges of the last bay, that it appears to sort of swoop up at the tip. This is mostly noticeable at the leading edge outboard. In careful measurements taken by taping a string to the forward spar inboard and taking measurements at the tip rib between the spar and the string as it lies on the wing. I have found that in fact the outboard rib is actually centered between the upper and lower spar caps and that the wing actually tapers at the forward spar in the last bay. These measurements I have taken on several wings takes into consideration that there are four layers of skin at the point where the inboard leading edge, outboard leading edge and the center and outboard mid bay skins overlap. This layering of the skins constitutes .040 of an inch in thickness at one surface and .080 of an inch in total thickness of the wing localized the overlap. In this I have found that the rib lies .10, That’s a tenth of an inch below a line drawn along the top of forward spar and a .10 above the same line draw along the bottom of the bottom spar.

I am sure this is all very difficult to follow without drawings.
And sort of for the curious at heart.

[GAHorn]

If you stand behind the wing of a Cessna 100 with an all-metal wing (that has twist) and look at the trailing edge of the aileron you will note that the trailing edge curves up towards the tip. This is built into the aileron to compensate for the twist in the wing.

Or said another way (as in WD Thompsons book), the wingtips were made symetrical in order to simplify production and in order that wingtip fairings may be identical/interchangeable left and right, therefore the aileron trailing edge must curve upward in an effort to close the gap. (In actual fact, the ailerons in a correctly rigged 170 will not actually meet the tip fairing's trailing edge. It will be about 1/2" below it.