Big Tires and Expensive Lessons

[Scooter7779h]

Well, I have to weigh in on this. I have been flying my -12 for 32 years, and it has had about every type of gear on it. 8:00, 8:50, Airstreaks, 26" Goodyears, 29" ABW, 31"ABW, Dodge Board skis, Landes 2500 skis, Schnider Wheel Skis, AWB 2500 wheel skis with Big Foots, and trim has never run out of range or been that big of pitch change between different tires. Most forward CG was on the AWB's with min fuel and no passengers/baggage. Still no problem landing with full nose up trim or anything that would make you want to land it at something higher than 40mph. I have the standard length mount although motor mount beefed by FA Dodge. Always 0320 engines with 82" Borer props. Mike Skup of Mikes MCS rebuilt my -12 in 96, but it flew the same way before we rebuilt as after.

My call is something with your trim is whacked, or the relationship of the tail plane to the wings is out of rig. I am no mechanic, but I would get someone good at measuring to see what is really going on with the rigging and frame / wing / tail surfaces relationships. A PA-12 is a sweet handling easy to fly plane, not one that normally goes radical on you for a minor tire change.

 

[Seaworthy]

Thanks for that Scooter. Problem is that there isn't anyone around for 2000 miles that has that kind of knowledge. I guess I learn to live with it.

 

[mike mcs repair]

Thanks for that Scooter. Problem is that there isn't anyone around for 2000 miles that has that kind of knowledge. I guess I learn to live with it.

got a good digital level, and know how to calibrate it? we can walk you through what measurements to take....

its quite simple.....

hardest part is lifting it up it up in level flight position....

 

[WileyHunter]

I guess I learn to live with it.

PLEASE - Don't do that!!! It's just not worth the risks IMHO. There's gotta be someone that can help ya out.

 

[Coyote Ugly]

My 2 Cents... Mike is absolutely right, it's something to do with the spring or bungie on the top elevator cable. I tried taking that spring off once, and my cub flew just like that. You still have all the same control movements, it just changes the "feel" big time. Not sure why changing tires would make it worse tho... :-?

 

[Gordon Misch]

Hey Tom, As somebody suggested before, the cross tube sleeve that attaches to the aluminum yoke with two bolts, might be upside down? That would give full nose down trim (full stabilizer front-up), but it might well severely limit nose up trim (stabilizer down).

Notice in the photos, the ears on that black cross tube are up. Are you sure they're in the same position on your plane, as well as the yoke being pointed the same direction?

I'm with Mike and the others on this one, it sure sounds like something is significantly out of rig, and as he says, it's easy enough to check. Not to be critical, but if the mechanic left the oil drain nut loose, what else might not be quite right?

No digital level? I (and others) can help you do the calculations with a carpenter's level. Really, it's not that hard, and I'm not even an A&P! If you don't like math, email me the necessary data (involving length of level, and distance of one end of the level from the surface) and I'll do the calculations for you if you want - - gordon(dot)misch(at)gmail.com.

 

[T.J.]

delete

 

[StewartB]

TJ and Mike,

Please educate me. In my -12 I can fly hands-off at any speed by adjusting the trim. Fast, slow, loaded, light, I always have trim authority to completely relieve elevator input pressure on my part. How would an elevator bungee play into that? It's been stated here that the "feel" would be heavy without the bungee. In my example there is no feel.

Stewart

 

[mike mcs repair]

TJ and Mike,

Please educate me. In my -12 I can fly hands-off at any speed by adjusting the trim. Fast, slow, loaded, light, I always have trim authority to completely relieve elevator input pressure on my part. How would an elevator bungee play into that? It's been stated here that the "feel" would be heavy without the bungee. In my example there is no feel.

Stewart

as your stab front edge move down almost to hitting top longeron the bungee cable and spring come under tension helping pull the stick back some,

Its also a way the get a little more nose up, but only at the extremist end of the trim adjustment.....

when stab is in middle area the bungee cable is loose with elevators neutral

even your 180/185 has this (do all have that cable, or just some I think???)

 

[StewartB]

I think you're right about the 180 spring, at least in later models. Then there's the rudder centering spring, too.

So in Tom's case, lacking a knowledgeable mechanic, how can he check or correct this bungee tension? Is there a prescribed value or is this a "feel" thing for the mechanic?

SB

 

[mike mcs repair]

I think you're right about the 180 spring, at least in later models. Then there's the rudder centering spring, too.

So in Tom's case, lacking a knowledgeable mechanic, how can he check or correct this bungee tension? Is there a prescribed value or is this a "feel" thing for the mechanic?

SB

FIRST... he needs to check the angles of the plane, tail, wing, firewall, might not be a bungie problem,

but that said, you may be able to compensate for other problems with its length....

remember the original cable length was set for how the plane was configured as it left the factory, empty CG wise, which is MUCH different now

And I am sure it was based on trial and test flights, then all set at one length.....

I wounder what a minimum everything(fuel/pilot) W&B looks like on his, is it still inside the forward CG envelope range?

 

[StewartB]

Windy and I have posted W&B data on our planes on the thread I linked earlier. I had hoped more -12 owners would do the same to establish some empirical baseline for modified planes, but most guys turn into Secret Squirrel when asked about airplane weights. I never understood that.

SB

 

[Scouter]

Seaworthy;

Reid Campbell, AKA SuperCrow on this site can figure it out. He lives in Millinocket Maine. If you actually have something out of rig, he knows cubs and would be a help. There is also a PA12 on the field with the same mods you have. He might chime in, if not and you want to contact him let me know I have his cell. You are welcome to stay with me if you come up.

Jim

 

[kevin]

I think you're right about the 180 spring, at least in later models. Then there's the rudder centering spring, too.

So in Tom's case, lacking a knowledgeable mechanic, how can he check or correct this bungee tension? Is there a prescribed value or is this a "feel" thing for the mechanic?

SB

FIRST... he needs to check the angles of the plane, tail, wing, firewall, might not be a bungie problem,

but that said, you may be able to compensate for other problems with its length....

remember the original cable length was set for how the plane was configured as it left the factory, empty CG wise, which is MUCH different now

And I am sure it was based on trial and test flights, then all set at one length.....

Tom,

When I put my bungee cable and spring on, I used a spring and cable off an -18. I got all my parts from Univair. I did this because I used a new yoke, carry thru tube and jackscrew for an -18 and will be installing -18 tailfeathers. The yoke, carry thru tube and screw might be the same as a -12 but I wanted to play it safe. Since those pictures were taken, I took the feathers off. I can put them back on and hook up the cable and spring and take a picture if you want me to. Just let me know.

One thing I did notice is that the bungee spring I put on(PA-18 spring) is quite a bit heavier than the spring that came off(assuming it was the proper spring).

Kevin

 

[Paul Heinrich]

I think the -18 bungee cable and spring is longer than a -12. Anyone know the exact length for the -12 overall with spring and everything?

 

[kevin]

I think I still have my old spring. I can look for it and measure it.

 

[mike mcs repair]

I think you're right about the 180 spring, at least in later models. Then there's the rudder centering spring, too.

So in Tom's case, lacking a knowledgeable mechanic, how can he check or correct this bungee tension? Is there a prescribed value or is this a "feel" thing for the mechanic?

SB

actually now that i think of it the 180/185 bungie works opposite, give more nose down....

 

[mike mcs repair]

I think you're right about the 180 spring, at least in later models. Then there's the rudder centering spring, too.

So in Tom's case, lacking a knowledgeable mechanic, how can he check or correct this bungee tension? Is there a prescribed value or is this a "feel" thing for the mechanic?

SB

actually now that i think of it the 180/185 bungie works opposite, give more nose down....

 

[Paul Heinrich]

If you think the closest qualified Cub mechanic is 2000 miles away I suggest you actually look for one in your area, and not just one who will paper whip an airplane through inspection. They are there, you just have to pay them what they deserve for their superior skill and knowledge when you find them.

Sadly, too many pilots are too cheap to do things correctly; and unsavory, unethical mechanics are a dime a dozen.

 

[T.J.]

delete

 

[Bugs66]

On the 26" Goodyear subject... I just got back flying. I couldn't tell any difference in the air. Landings were SWEEEEET!

 

[Crash]

26" Goodyears are NOT "big tires". They're actually only two inches larger then 8:50 x 6's and should have little to no effect on your trim.

Even going up to 31" Bushwheels seemed to have little effect (trim) in my experience on my PA-18 (O-320 / O-360) or PA-14 (O-320).

It's not your tires.

Crash

 

[Seaworthy]

Pictures of PA-12 tailspring and cable compliments of Kevin Gazarian
Kevin is now rebuilding a 12.















OLD SPRING AND CABLE---the old spring is significantly shorter

 

[Paul Heinrich]

So why did he put in a longer spring? What was the overall orginal length? and what is the new length with the longer spring?

 

[kevin]

So why did he put in a longer spring? What was the overall orginal length? and what is the new length with the longer spring?

I put in a longer spring because I am planning on using an O 320, PA-18 feathers and wanted to duplicate the PA-18 tail section. The back half of the plane will be -18. The lengths are shown in the pictures with the tape measures.

 

[StewartB]

The longer spring may contribute to what Seaworthy describes, right? Longer spring - less up elevator at full nose-up trim?

Stewart

 

[kevin]

I didn't think it would since the PA-18 has this spring/cable and does not have these tendencies. If it does, it won't be a problem for me to change it since I haven't covered yet. I am curious as to if Seaworthy has the smaller/lighter spring...

 

[cubdriver2]

Kevin, if the measurement in the photos is correct or close it looks like the cable on the shorter spring is also 1.5" shorter then the one on the new longer spring ?

Glenn

 

[Seaworthy]

I'll ask this again: Does anyone have in their possession or know where to obtain an illustrated parts breakdown for an original PA-18 and PA-12 with nomenclature? Like military aircraft have? Do they exist?

I truly appreciate all the advise from everyone---really, however, when folks ask me about XYZ and I should do thus and such it would be analogous of me to ask someone to take a feeler guage and determine the clearance of the swash plate from the mast on a UH-1N or set the voltage regulator on an OV-10, or track the blades of a helicopter using the old Q tip. If you haven't done it---you don't know. I have yet to find a book, or mainteance manual, or an IPB for an 18 or 12. Knowledge is personal, hearsay, and closely guarded. Why aren't there any "How To's out there or Piper Cubs for Dummy's? Like Pete said----"Seek out a good 18 mechanic and be prepared to pay $100.00 /hr for expert advise". What is the criteria for "expert advise." One guy says his 12 has a heavy nose, a second says his 12 is tail heavy a third says the other two have no idea what they are talking about. Who's right? I have no idea.

 

[StewartB]

Tom,

I understand your frustration but I doubt a parts manual for a 108hp stock 1946 PA-12 would do you much good. You've got a big motor, big prop, modified accessories, flaps, and Supercub tail feathers, right? Every mod has an impact. The interrelationship between the mods is the responsibility of the mechanic. That may frustrate you now, but it's a beautiful thing. It provides some freedom to personalize our planes. You just need to find a guy who can make the necessary adjustments to what otherwise sounds like a nice airplane.

Keep the faith.

Stewart