Wing Rigging thoughts 8)

[Bruce Fenstermacher]

Have recently been adjusting the rigging of the wings on my airplane. The the eccentrics weren't centered and so I adjusted them to center. In doing so I happened to place the thickest part of the eccentric towards the cabin. In doing so I noticed the aileron cables go tighter which makes sense because the rear of the wing is being pushed out. I looked in the 100 Series 62 and prior to see if there was a proper direction for the eccentrics when in neutral. There doesn't seem do be or at least I couldn't find it in the manual.

So question #1 does anyone know if there is a proper position for the eccentrics when in the neutral position?

Ok now since the forum has been a little slow and free of controversy I'll ask the next few questions.

As I stated moving the eccentrics fat side from up or down to either side actually sweeps the wing forward or back.

The first question to ponder. If you swept one wing forward and the other back would you feel the difference in the rigging? And a followup to that is would you just correct with rudder realigning the wings perpendicular to the route of flight?

But here is the tough question. Does anybody think either Joe or George have their wings swept back to help with stability when they fly near the mach numbers?8)

[GAHorn]

Well,...all that is kinda funny, Bruce.... But ...
I think you are going to stop laughing when you discover the eccentrics are not supposed to sweep the wings forward OR back. They are supposed to only move the trailing edge UP or DOWN. It makes no difference whether or not the thick portion of the eccentric is UP/DOWN/INBOARD/OUTBOARD ..... The wing itself never sweeps forward or back. (And the neutral position is whenever the wing is in it's mid-range of travel.)
Sorry. (But Joe's wings actually are swept back slightly, ever since he took it to Mach 1.25)

[Bruce Fenstermacher]

OK George explain to me how the eccentric bushing would only move the wing trailing edge up and down yet not lead or lag (helicopter terms) the wing when the thicker part is toward or away from the cabin or in other words neutral travel up and down with relation to the incidence of the wing

I think if you reviewed a picture and thought about it you would agree with me that it does swept the wing forward and back.

[jon s blocker]

It does in fact move the wing forward or back depending on the position. I have been working with mine to adjust the wings after a rebuild and you can see and measure a difference in the distance between the trailing edge of the wing and the side of the fuselage. Although the movement there is small, multiply it by the almost 14 ft of wing, it does move it.
Jon S Blocker

[GAHorn]

Correctomundo, Bruce...... When imagining "sweep" I guess I was imagining some huge amount rather than the relatively infinitesmally small amount actually incurred. The original neutral position was with the thickest portion inboard. Since maximum up or down would be incurred with that thickest portion either up or down...the movement of the wingtip forward/aft would be very small indeed. Only if the bushings were subsequently further adjusted outboard would the "sweep" then be doubled that intended.

[Bruce Fenstermacher]

Well George I don't doubt you know some how that the thick portion of the eccentric bushing should always travel towards the cabin. Do you remember your source because I couldn't find it.

I'm of course having fun thinking of this and you gave me another thought. That is the wing may be straight with the thick part of the eccentric towards the cabin and that any adjustment would sweep the wing with the maximum sweep with the fat part of the bushing adjusted away from the cabin. In other word the wing would never be swept forward.

Just for fun I electrronically drew a theoretical wing with the spars 3ft apart and the span of 10 ft. If you adjusted the one spar at the root closer to the cabin by 1/8th inch then the other it looks like the wing would be swept 1/2 inch at the wing tip.

[Jr.CubBuilder]

Bruce, can you offer any opinion on how much difference in flight moving one of those eccentrics a 1/4 or 1/2 of it's travel makes. There is a trim tab that got put on my right aileron at some point, it seems to small to make any difference so I've been flattening it out, but it does now seem like I have a slight tendance to turn to the left. I know my fat but is in the left seat and nobody is in the right so I'm not sure what to make of it yet, but ultimately would like to get that tab off there.

[GAHorn]

I'm out of town, so I can't send you a copy, but the source was the Cessna commercial aircraft conformity specification sheet. I have a portion of a poor copy of that multi-page document that came along with my airplane's paperwork when I bought it. I've subsequently tried to get a copy of the entire document but these days Cessna refuses to part with proprietary blueprints. I believe I've also seen the comment in rigging instructions, but don't recall if they were genuine Cessna or if they were a type club's procedure. (CPA perhaps?)
Tom Hall has sent me his own personally-developed rigging instructions and as soon as I can "massage" them a bit they'll become part of our service manual.
Did you check cable tensions prior/subsequent to your change? How much difference did it make?

[jon s blocker]

I can't answer for Bruce, but I can tell you that moving the eccentrics up or down 1/4 to 1/2 of their travel will make a difference. Remember if your rigging is out far enough that moving one won't correct the problem, you have the other side to move in the opposite direction and double the results. Evidentaly Cessna didn't want you to put a trim tab on the aileron, because the next step they recommended was to grab the trailing edge of the aileron and gently bend it to get the desired results. I think if the rigging is that far out, I would be looking for the root of the problem. Good luck, Jon

[GAHorn]

There should be no trim tabs on a 170 aileron. I once saw an aftermarket electrically-driven trim tab, installed with a 337 on a 170, but since then I've been convinced the 337 was bogus. No block 3 approval was signed off, and that particular trim tab mfr. does not have approval for 170's or 172's. (I've seen it on several home-builts, but that's a color of a different horse.)
Last convention in Tehachapi there was a 170 with a trim tab on EACH aileron....both bent so as to exactly counteract one another....both bent the same direction.

[Bruce Fenstermacher]

My aircraft is a special aircraft in that there are so many little things that could be effecting the trim but besides an STC'd electric trim tab being installed it also had full up and full down adjusted with the eccentric bushings.

Yes George there is an electric aileron trim tab STC'd for a 17O and I have one. And the paper work is now correct for it but that is another story for Petit Jean maybe. The trim tab at full deflection in either direction does very little for at least my airplane in its current trim.

As for the cables getting tighter yes they did. I hadn't measured them before I started but felt that they were on the loose side of the tolerance if not more.

After the wing adjustment I felt the cables had tightened but thought that they were OK and not having a cable tensioner handy I didn't measure them. Wish I had now.

3 weeks and maybe 2 flight hours later I decided to finish my re-rigging. I adjusted the aileron stops to center by loosening on turnbuckle then took up the slack with another in this way Iwouldn't disturb the tension. When I checked the tension with a gauge it was more than double what it should have been.

So either my cable tension was correct at the start and by just changing the eccentric bushings from up and down to towards the cabin doubled the tension or worse if my cable tension was loose as I suspect.

So yes if you adjust your wing eccentric bushings you should definitely check you cable tensions and if you only move one side you may have to correct the aileron stop position as well.

[zero.one.victor]

Seems to me that random tinkering with an airplane that's out of rig is only likely to correct that out of rig condition by sheer luck. IMHO, the best thing to do is start right at square one & re-rig everything, as per the owner's manual &/or the Cessna series 100 manual. A friend just had his 182 re-rigged by an A&P who had attended a course put on by the Cessna Owner's (or Pilot's?) Assn & was evidently "certified" at rigging Cessna's. You might check into this. But I would wait to see if completely re-rigging as per manual does the trick.

Eric

[N170BP]

Do I hear an ehco?

Do I hear an echo?

[Bruce Fenstermacher]

Eric I totally agree that when rigging an aircraft you should start from the factory setup. I'm doing just that just not all at one sitting.

What I was referring to when I say there are so many little things that would cause my aircraft to be out of trim I'm referring to wings, ailerons and flaps that have been repaired and probably would not fit in the factory jigs any more. I believe that if I really want to trim my aircraft to be as straight as it was when new I'd have to remove the wings, ailerons and flaps and put them in a jig and straighten them. It's something you really have to look at closely. I find something new every time I look.

[GAHorn]

Hey, Bruce...I wasn't referring to your airplane at all. (I didn't recall you having a trim tab.) The airplane I had seen so modified (w/electric trim) was in Pagosa Sprgs, CO back in '87, sitting in the same hangar as Larry Bartlett's 195. (Taming the Taildragger)
Larry is who pointed out the discrepancy to me.