The International Cessna^® 170 Association

I was thinking a couple of rockets...the one on the right filled w/ballast such as St. Pauli Girl. The one on the left filled with Torpex or RDX.

wingnut wrote:... wing twist (washout). Cessna specs allow plus/minus 1/2 degree (meaning you could have a total of one degree difference between left and right wings)

Del (or other sheetmetal gurus) as I get ready to rivet my wing back together I've got a couple of questions. I'm curious where you found this 1/2 degree spec. What I found was on page 18-7 of the 69-76 C-172 manual which allows up to .10" tolerance. Over the span of 24" I believe that is about a 1/4 degree per wing. Another question I had concerns access/inspection holes in the wing flap bay skin. A previous mechanic cut out two holes on the outboard end of this skin. He did a very crude job without any doublers behind the holes. I'm wondering if I have to replace the entire skin or whether to simply try and add doublers to replicate the holes on the inboard end of the skin. One A&P I talked to said that this is not a structural skin; in his opinion having the two holes there would be a minor alteration. I see in this photo of a wing for sale on the internet that someone has added several inspection holes.Here's the illustration from the C-170 manual of what the skin should look like:It'll be a challenge riveting the wing back together without the additional holes.

You wrote:

"One A&P I talked to said that this is not a structural skin;"

I'll get back to you later on the wing washout info, after I get caught up this morning. But, this skin is VERY STRUCTURAL. Imagine the load on the flap tracks as you deploy flaps. This is one of two skins that form a "BOX" stucture for the flap track assemblies.

Gary,
In response to your question about where I got the info regarding washout being 3 degrees, with a tolerance of plus/minus .5 degrees, I cannot find where I obtained this information. I do know that I have seen it in print, and confirmed verbally with tech support many years ago.

You are absolutely correct when you state that .1 inches at a 24 inch span equals approx .25 degrees. So, going with this data in the 100 series service manual, or the 172 manual your using, it would appear that the correct tolerance is 3 degrees plus/minus .25. I can’t argue with the service manual. The particular figure for measuring washout in the service manual, that you referenced in your recent post, seems to be instruction for checking/inspecting a wing suspected of having been damaged, and contains words such as “approximately” and “avoid canned areas”. The .1 inch at a 24 inch span is the amount that the eccentrics could adjust for, if both wings were within this spec.

An interesting note, if you look in the 1969-1976 172 Service Manual, paragraph 18-7 indicates 3 degrees washout, and references Figure 18-2 for checking washout. You’ll notice beginning with 1973 year models, they use a different (shorter) length bolt for the sta 208 forward spar, which has to mean they increased the washout approx .35 degrees, and they still allow the .1” lift off to center bubble. Yet, with the obvious change noted in figure 18-2, paragraph 18-7 does not indicate anything other than 3 degrees for all.

As for tolerances, I may be wrong, but I don’t cull a wing that is within .5 degrees of 3 degrees washout, so long as both wings are within a tolerance range with each other so that the eccentrics can be adjusted to fly right. And, my wing fixture is set-up to build them perfect. The word tolerance seems to imply that there is some “degree” of imperfection allowed, and when you build a wing there is no reason perfect washout cannot be achieved.

If it were indeed true that the tolerance is .25 degrees, there are a lot of wings out there flying that are out of spec. I have measured the twist on new aircraft that are not within this washout specification.

When I build a pair of wings for one aircraft, I build them perfect, without regard to what the tolerance may be. If I’m building one wing for an aircraft that has one undamaged wing, then I build the damaged wing as close to the same washout as the undamaged wing, staying within a 2.5-3.5 degree washout. If I were rigging a plane, I would rather have both wings at 2.5, than one at 2.75 and the other at 3.25.

Del,
Thanks for the replies. Although it's not the answer I was hoping for, I'd agree with you that the flap well skin is structural. It's interesting that Cessna changed the location of the inspection holes on that skin in later models. Here's a page from a later C-172 service manual that gives directions for adding inspection holes. If I don't replace the entire flap bay skin I may try and use this as acceptable data for the field approval of the undocumented inspection holes in that skin.

wingnut wrote:An interesting note, if you look in the 1969-1976 172 Service Manual, paragraph 18-7 indicates 3 degrees washout, and references Figure 18-2 for checking washout. You’ll notice beginning with 1973 year models, they use a different (shorter) length bolt for the sta 208 forward spar, which has to mean they increased the washout approx .35 degrees, and they still allow the .1” lift off to center bubble. Yet, with the obvious change noted in figure 18-2, paragraph 18-7 does not indicate anything other than 3 degrees for all.

In 1973 Cessna (C-172M) changed to the "camber-lift" wing with the drooped leading edge. My guess is that if one were to measure the twist from the chord leading edge to trailing edge it would still be 3 degrees although the twist in the spar between the pre-1973 wing and the 1973 and following wings would be slightly different.