C-170A Tailwheel Behavior

[N5561C]

A CFI that I have been flying with has evaluated the tailwheel of our aicraft as very "loose".

We put the aircraft on jacks to see how much pressure it would take to break the tailwheel out of over-center. It did not take much pressure at all (tested by hand).

Does anyone have any experience or knowledge how the tailwheel over-center is supposed to "act". Can these over-center components wear out?

Thank you - Richard DeWalt
N5561C 1950 C-170A

[n3437d]

First, I am not an AI nor AP and am no "George" but... there are series of washers (spacers) and locking (?) mechanism that over time and hard landing can result in breakage of one or more of these components. Check Service bulletin for breakdown of parts. Typical rotation should be around 45 degrees left or right of longitudnal axis before it "breaks." I am guessing that the main purpose for this design is to help prevent ground looping during take off landings, taxiing and more important during x-winds while on the ground help minimize weather vaning effect.

I was haveing a heck of a time taxiing on the ground using rudder alone, always had to tap breaks left or right. Discovered during annual that one of these "spacers" (?) had failed. Once replaced it was a "world of difference" tracking true on the ground with ruddr alone. Also much better x-wind handeling.

"my story and sticking to it" unitl corrected by powers to be smarter than I

[GAHorn]

First, I am not an AI nor AP and am no "George" ....

Well, so far you've got a great reputation then!

The Scott 3200 has an internal, flat, angular spring (PN 3222) which works in conjunction with a "pawl" (PN 3219) and some detents to provide steering which, when excessive force is applied, will allow the tailwheel to "unlock" or "break" free to swivel. The spring can be broken or worn, as can the pawl and/or the detents, and then the swivel will occur too easily.
The spring is backed up by two spacers, PN's 3258 and 3258-1, which if missing will also fail to provide a positive detent. (With the tailwheel in the air it doesn't take a lot of effort to move it into full swivel, but on the ground with aircraft weight, it takes more force.)
There are also some thrust washers and some compression (coil) springs which provide a sort of "friction-clutch" to help prevent excessively loose steering action. While some illustrations show 5 such springs, in fact onl 3 are used for the 170 application. (If your CFI has been flying a heavier airplane using all 5 compresion springs, that may help explain his view.)
Just because your tailwheel is being evaluated by a "tailwheel CFI", unless he's been flying several 170's which he can use to make comparisons, ... He may be prone to accuse the Scott 3200 on a 170 as being "loose" when in fact, it is just fine. 170's do indeed require a bit of braking action during ground manuevers. Simple rudder pressure alone is rarely sufficient, in my experience.
With the aircraft resting on the ground and the tailwheel "in trail" as if it has been rolling forward several feet... will require a healthy shove on the fuselage just forward of the horizontal stabilizer before the tailwheel will completely unlock and swivel. (Don't push on the vertical stab. There's a bracket in there that's easily broken at the fwd portion of the dorsal fin.)
The Aircraft Spruce catalog lists the parts, as does Sacramento Sky Ranch but unfortunately the illustration leaves a lot to be desired, if you're looking for an illustration which will lend an understanding of this tailwheel.
http://www.sacskyranch.com/acatalog/scott_tailwheel.pdf

[mbram]

I would start by adjusting the King pin and checking for loose spring
attachment bolts. If your not getting correct tension on the shimmy
dampner or the pawl is not engaging at center. tear the assembly
down and check for broken parts.
Spruce has a break down of the 3200 tail wheel.

Avox web site
http://www.scottaviation.com/
Brochure 5126


http://cessna170.org/phpBB2/viewtopic.php?p=2734#2734

Service News Letter (Date: 10-24-50)

"We have had some reports on Scott pneumatic tailwheels shimmying. The Scott 8" pneumatic tailwheel is designed with a friction type shimmy dampener. This friction is regulated by 3 small compression springs inside the upper casting. If the main king pin nut becomes too loose the shimmy dampener effect will be lost. When the shimmy occurs, it is recommended that the large nut be tightened in increments in 1/6 of a turn until the tailwheel starts to bind when rotated by hand. Then back off 1/16 of a turn for correct tension on the shimmy dampener. The king pin nut is on the underneath side of the lower housing directly above the tire. This procedure will definitely assure that compression springs in the dampener are acting against the friction dampener disc. We recommend a tire pressure aof 30 pounds (Note: Scott recommends 34 lbs.-George) on both the 140 and 170 for the 8" pneumatic tailwheel. When connecting the springs and chains on this assembly, it is recommended that the strength on the springs be from 1/8 to 1/4". (Note: I believe they are saying to place 1/8" - 1/4" amount of stretch upon those steering springs. Remember that Cessna makes all landing gear adjustments at 2,000 lbs operating weight. Only Duane Wallace knows where the CG is at that 2,000# wt. Shocked -George) The springs used with the 3200 assembly have a high tension rate and if stretched to any extent on assembly the mechanism may not release easily. No slack should be left in the chain and spring hook-up when both sides are connected. <end>

[GAHorn]

Another common problem found with tailwheels is "shimmy" due to improper "caster". See:
http://cessna170.org/phpBB2/viewtopic.p ... =tailwheel

[N5561C]

Thank you for the data.

RP DeWalt