Wing Washout and flight characteristics
Ever since I purchased my 108-3, I have had to have the control yoke turned slightly to the right and a touch of left rudder to get the airplane to fly straight. If I put the yoke level (which places the ailerons level with the flaps), the airplane wants to turn left. For some reason, my airplane has a trim tab on both ailerons which seems odd and playing with the trim tabs has not fixed this issue.
Since I was due to have my annual anyway, I decided to have the washout checked. I got the fairings off at the base of the V Strut and counted threads to see if they were different. The left strut shows 8 threads while the right strut shows 7 threads. After looking at the general service manual in the wing installation section, it shows that you put the eyebolt against the bottom of the strut and then unscrew it five complete turns (which presumably would have five five threads showing). Page 34 of the General Service Manual then states, “The rear lift strut is then the proper length to warp the wing panel the required amount for best aerodynamic efficiency.”
So I am asking for some personal experience from those that have done this. Does five complete turns of the eyebolt give you five threads or more than that? How many threads are showing on your airplanes? Would the one thread difference on my airplane make a difference in the turning characteristics as described? Please help as I would like this resolved.
Karl A VogelheimS108-3 N596C
Hi Karl; There is a detailed explanation for setting washout either in Larry Westin's Stinson page or in the technical resources data on this
website. First, I would check the washout degrees before changing anything, I suspect the manual stating 5 threads is only a starting point.
Has your plane ever been damaged? If so, forget the 5 threads. All Stinsons left the factory with aileron trim tabs, they have been removed from my -1 sometime in the past.
i would start by checking the angle of the flaps first there is a hard to find bolt under the belcrank in the wing that adjusts the flap angle.
Then tape a straight edge under the control wheels, adjust the flaps to the flaps. make sure the cable tension is 25 lbs. After that tackle
the washout. I should have mentioned this first, is your turn and bank installed correctly in the panel if it is at a angle you will fight yourself.
My-1 banked slowly to the right with the ball centered, i could not adjust the right aileron, I found the right turnbuckle behind the rear
seat was the long part number which ran out of threads. I installed the correct PN turnbuckle, adjusted the flaps, and ailerons flies
hands off now and gained 5 knots, 5 knot is not a true statement. I have a Airtex headliner the zipper is in the middle of the cabin top
the idler cable turnbuckle is on the right side, now I have very nice slit in the headliner. I hope this longwinded email helps.
Good luck Jim
One thing I think gets overlooked is that the ailerons are neutral not when they are faired with the flaps, but 1/8" below the flap. Similarly, the elevator neutral is not faired, and varies by serial number. Page 42 if you're interested.
Karl, I'm sure you've seen this in the General Service Manual, but on the dash 3 the aileron bell crank has an alignment hole into which you can insert a pin--this fixes the neutral position of the ailerons. The flaps of course are held up by the springs and the up position is easily adjusted by a stop bolt in the wing, with the 1/8" aileron droop mentioned above by resto108. As far as I know, at least for the dash 3's, there were fixed tabs on both ailerons. Before you change the wing incidence, you should probably bend these so that they fair with the bottom surface of the aileron. It would be wise before bending these tabs to take some thin slat boards and clamp to both side of the aileron trailing edge on top and bottom; and do the same on the trim tab before you bend them; otherwise you could distort the trailing edge of the aileron and the trim tab. As it sits in the hangar, with my airplane more or less laterally level, has the ball out to the left just a smidgeon. Put a bubble level across the top of your door frames and compare it with your ball. It goes without saying that since the dash 3 has an adjustable rudder tab, make sure it's neutral before fiddling with the aforementioned adjustments. Keep us posted!
Karl, When ever I rig a plane I like to start at factory zero, verify my T&B is right then test fly. Always make sure the ball is center when noting any roll. I have found most rig problems are self (mechanic) inflicted. Cable tensions get adjusted with no concern to rig. Tabs are bent trying to compensate for a miss rig. You always end up with a slow wonky flying plane. My last Stinson rig the flaps were not matched, with that fixed she flew perfectly. All with the factory settings. Follow the manual, it’s faster in the long run.
I am going to try and tackle this project tomorrow with my A&P. He is bringing along an electronic angle measurement device so we can compare one side to the other. Can anyone tell me what the degree difference of the washed out (outer panels) of the wing is supposed to be compared to the wing root?(example, the washed out panels should be 2.5 degrees different than the inner panels. This is a made up number as I don’t know what the difference is supposed to be). Thank you if you can help.
Karl A VogelheimS108-3 N596C
I agree that they should be as close as possible if not identical. However, what I don't know is what that number should be in an ideal world (brand new wings or as close to that as possible on a brand new fuselage as it came from the factory). Other than the five full rotations as specified in the General Service manual, does anyone know what that translates to in degrees? It would help me get this right if someone knows. I will let you all know what happens when we get done later today.
Karl A VogelheimS108-3 N596C
Well, we made the wings the same with seven threads on each side. I took it for a test flight today and I can’t really say if it was any better. At the beginning of the flight I would have told you that there wasn’t any difference. By the end of the flight, I think there might have been a slight improvement. However, if there is an improvement, it is difficult to determine which probably means there isn’t any. Still a nice flying airplane though.
Karl A VogelheimS108-3 N596C
Karl and everyone else:
When I recovered my wings several years ago, I found one wing that the factory riveted ribs had been installed not quite perpendicular to the spars. So, I concluded that not all factory built wings are perfect.
To get it to fly hands off, I started with the manual recommended number of turns, flew it, and then changed one of the adjustments a half turn different, flew it and after 2 or 3 trials and changes it flew perfectly hands off and still does. The differences in number of threads to get it to fly straight is probably due to the difference in the wings. Both wings are original for this plane as far as I know, but I have only owned it since 1968 so something could have been changed in the preceeding 22 years of its life. I have checked washout with an electronic level but do not remember the numbers. As I recall, it was close to the recommended amount.
I do have the rudder bungee trim system and I can easily trim the rudder for straight and level flight and even with a little turbulence it will maintain level and heading for more than 5 minutes hands and feet off.
Larry Wheelock, A&P/IAStinson 108 N584LW 180 LycTexas in Winter; Indiana in Summer
Larry, I've never been through this rigging exercise, but your method makes the most sense and conforms with the recommended procedure in the General Service Manual. I wouldn't not be concerned with the actual measurement (in degrees) of the washout, as long as the end result of a straight flying airplane was achieved. I do have a question for you though. Since yaw affects roll, and a change in lateral rigging would affect the ball, does this necessarily become an iterative process between adjusting the wings and adjusting the rudder trim until a centered ball (or as close as you can get it) and level wings are achieved?
Another thought, we have to recognize that the airplane is not perfectly symmetrical with an offset vertical fin; and with turbulent and swirling air from the propeller blast it doesn't fly in perfectly calm, smooth air. It's interesting that Stinson provided for bendable aileron tabs on the ailerons which can be bent up to 26 degrees. The airframes were rolling out of the factory at a pretty rapid pace, and I imagine it was all test pilot Al Schramm could do to put each airplane through its paces. I can imagine if he encountered an airplane with a rolling tendency he just tweaked those tabs a bit and moved on to the next airplane. On a side note, I encountered several airplanes through the years while flying as a corporate pilot which would not fly straight. One was a late 70's vintage King Air 200; and the most recent was a 2013 Gulfstream G-280. You could trim those airplanes until the cows came home, but they would not fly straight.