Trying to catch the ball
I bought my 108-1 last year, and am still getting used to it. My other plane is an N3N, and I've flown Cesnas, Pipers & helicopters with a few thousand hours. This is the most ball/rudder sensitive aircraft I've flown by far. Is this normal? It has such a small rudder & big tail it doesn't seem logical. Pitch and roll are stable as I would expect, but this airplane is very yaw sensitive. It takes tiny movements of the rudder pedals to send the ball outside the cockpit. You have to keep your feet extremely still, and it still isn't happy. It goes in either direction, as long as it's not in the middle (not just to the left or right). Is there a place to find the rigging instructions? All I can find is that you lengthen the eye on the aft strut by 5 turns after even with the front to get the proper "wing twist". I measured the washout & I get 3 degrees Port side and 2 degrees Starboard. You can see the washout sitting in the cockpit & both sides look about the same by eyeball (but 1 degree different). Any inputs on flying qualities and possible solutions if this is not normal would be appreciated.
I am new to my 108-2 as well. I have noticed what you describe, and am getting more used to it. I just have to fly it all the time unlike my C-140 where i can take my feet off the pedals in cruise.
Stinson 108s are "rudder" airplanes and the sensitivity is a good thing and desireable provided the plane is rigged for straight and level flying in non turbulent air. The 108-2 and later has rudder trim bungees and makes the rudder much stiffer.
I have found many 108s and 108-1s not properly rigged. (others,too)There is a bendable trim tab on the rudder which may need to be adjusted. Then there is the strut lengthing/shortening which must be done with care and at the same time maintaining the correct wingtip washout/washin. Another thing to check is that the rudder pedal return springs in the cockpit are in place and are the correct springs. Some have been replaced with screen door springs, which may or may not be correct. New ones are available from Univair. They should exert equal tension on both left and right pedals.
Another thing I have noticed is that over the years and who knows how many re-coverings, the original aileron trim tabs on both ailerons are missing.
Again, these are bendable tabs and if missing, one of the lowest cost parts available from Univair as they apparently have a barrel full of originals.
A Stinson 108, properly rigged, will fly hands and feet off for long periods even with light turbulance. The plane will correct itself if left alone.
Another source of yaw problems is improper tailwheel steering springs. These depend on which tailwheel is used, but I have often seen wrong springs here too, but no matter which tail wheel system you have, these springs should not be in tension on either side when the rudder is straight ahead.
Since you have a 108-2 with rudder trim bungees, you should not have to fly it all the time. If you don't get this corrected, next winter when I am back here in deep south Texas, fly it down here, and I will first take you for a ride in my Stinson so you can see how a Stinson is supposed to fly, then I will help you figure out and correct why you "have to fly it all the time". That is NOT a Stinson characteristic!
Thank you. I'll look at all of those things. After jacking the tail, I noticed the tail wheel main support spring is loose side to side at the lower rear point of the fuselage. Something isn't right.
That is a common ear point. Actually there is a bushing within a bushing and often the weldemount gets hogged out. But, it is doubtful that this is your problem with yaw stability, but it is not correct either. The Stinson General Service Manual from Univair is absolutely a must to have. It is also available on the Yahoo website I like real pages to look through at the airplane. I am still learning after 50 years of ownership and maintenance and flying my Stinson all over the USA.
Correction: common wear point not ear point
To add to Larry's comments, a U-bolt secures the spring to the fuselage structure. I had one of these break one time. Also, the forward end of the spring fits snugly into the fuselage structure and is nested in a short section of rubber hose to cushion it. If your plane has spent a lot of time tied down outside, you could have some rusted tubes back there. All things to check. Should be easy to see what the cause of the wiggle is.
Dennis Crenshaw. N6102M.
Got it all apart. Rubber bushing thingie on the front of the leaf spring into the fuselage is missing. Steel tube on fuselage is wallowed out a bit. Springs look like they may be Scott brand. Tailwheel assy. is Scott brand. You have to pull each spring back about 3/4" to get them to hook up. They're pretty tight. Got all the parts ordered from Unavair. Stinson springs, some of those chain link things, bushings, etc. Should be able to get it much better. Hopefully it will help the sensitive yaw a bit. I still need to dig into the front end springs & check that out next. Thank you for the help.
The cushion P/N 108-5311000-6 is simply a piece of Mil 6000 hose of the correct diameter.
What tail wheel do you have? Early tailwheels (original) may have been Scott, but probably the small version. Most have been retrofitted to the Scott Model 3200 (large) which takes a different size bolt than the original. The steering springs listed in the Stnson parts manual, P/N 2632 are not correct for the Scott 3200 tailwheel (AKA Alaska BushMaster 3200). For a 3200 tailwheel you need springs Scott P/N 3239. Hope this is what you got. Some Univair folks have some knowledge of what is correct, some do not.
The Scott P/N 3239 kit comes with 2 springs, 4 chain clips and some lengths of chain. They should be installed with zero to slight snugness equal on both sides. Tight springs will wear out and damage the rudder hinge and rudder. The correct springs are about 4 1/8 to 4 /14 overall length, the coil portion shouild be about 2 1/2 in length and about 1 1/4" outside diameter. They are heavy springs and provide very positive steering on the ground with a properly functioning Scott 3200 tail wheel.
Yaw sensitivity is one of the indearing qualities of a properly rigged and flown Stinson 108. Most people are used to airplanes that require full time attention and a Stinson never does if properly set up. I have taken long (thousands of miles) cross country trips and one can relax, take off your shoes, keep feet flat on the floor and occasionally nudge the rudder with your big toes on the part of the rudder pedal assemblies that projects through the floor
That was before I added the rudder trim, bungee system in 1978 for future larger engine installation, which I did in 2012 with the Lycoming O-360, but while FAA required to have the trim system, Stinsons don't really need it. Once the rudder trim is set, it is rarely needed again.
A Stinson that requires full time attention to the rudder is set up Wrong, Wrong, Wrong!
I tried to add a photo of the correct tailwheel steering spring installation but this system would not allow it even though less than 300KB.
I will see if webmaster can help.
here's the image
The rudder cable tension springs are also messed up. Figured something wasn't right after I removed the tail wheel and the natural position of the rudder was almost full left. They look like the correct springs, but they're stretched out a bit and one is about 1/4" longer than the other. New ones on order. The rudder is centered with the pedals even, so at least that looks proper. If this doesn't fix the problem, it darn sure oughta help a lot.
In view of all the problems you are finding with your Stinson, you need to make sure that AD-50-17-02 was actually complied with and not just a pen and ink compliance. The AD calls for a pair of larger pulleys under the floor for the rudder cables along with pulley brackets and guards. When I installed the kit in my Stinson probably soon after I bought it, the larger pulleys did slightly affect the geometry at the pedal attach of the cable and had to modify adjustment of the turnbuckles which also affected the rudder cable return springs although new ones of the correct part number are still the ones to use.
I installed new rudder cables too at that time which was nearly 50 years ago.
Again, a proper pre-purchase inspection by a Stinson/Franklin knowledgable person can be worth a lot more than it costs. One person I did a prepurchase for still bought the Stinson but for thousands less than he originally was going to pay for it. I found that the fabric was well below minimum, though others had a year previously signed off the fabric. Also found such things as automotive type hoses connecting the engine to the fuel system and for oil filter connections. Lots of stuff gets done and while it may or may not work and be safe, it can make a dog out of a sweet flying aircraft type.
Scott, I can't add any more than Larry has other than to reiterate that after owning three Stinson's over the years, I know of no other airplane that flies nicer than it does. A Bonanza comes close. I taught at an A&P school where we had a stripped down Stinson that we would use for teaching assembly and rigging. Also I have experimented with the rigging on my Stinson and found only very subtle effects from large amounts of adjustment change. Most people adjust wash in/out to compensate for wing heaviness. This is absolutely wrong. After adjusting the struts per the manual, the airplane should be flow and stalled. It should be noted which wing drops. That wing should have it's rear strut lengthened one half turn of the strut fitting and flown again. Never adjust the other wing. Any wing heaviness during cruise, should be adjusted out with the aileron tab as Larry mentioned.
Of course, the rudder system should be examined to make sure it has the correct parts, springs and such. The manual has a procedure for adjusting the trim. That should be done. Then after that's correct, the pedal turnbuckles are used to center the rudder. With the rudder trim centered, go fly the airplane. If the ball is not centered, go back and land, and adjust the rudder tab slightly and go back and fly it again. (Make sure you have the gas tanks equally filled or full.) You should be able to bring the ball to the center with feet on the floor.
As Larry said, you should be able to fly with your feet flat on the floor and your arms folded, making only very minor course corrections with the rudder trim.
Take Larry up on flying his Stinson, or come to Mississippi, and I'll help you with it.
Thank you all for the advise and help. I flew the Stinson today after replacing the worn out/uneven rudder return springs, the tail too tight wheel springs & installing the missing tail wheel spring piece of hose where it goes into the fuselage. All of those combined fixed the problem. It goes where its pointed as it should. I am going to recover the plane in the next year or two, since it has Razorback that is REALLY tight (think snare drum). Plus I want to give everything a good inspection & service. This is a huge improvement. Thank you again for the help.