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Trim, Weight & Balance Question

P

P51DACE

Registered User
Covington La.
I have a Kenmore Air Harbor O-320, Sensenich, PA-12.
Here's the issue: I run out of nose down trim at high cruise power/speeds. I recently put new cylinders on and during the break in period was running near wide open etc for the specified time.
IAS is between 130- 140 at these settings and I have full nose down trim and am still pushing forward on the stick, fairly heavily, to maintain level flight.
I checked the trim jack screw and it is bottoming out, so I am getting full travel. I also noticed in the logbook that in 1998 a mechanic moved the battery from "sta. 25 to sta. 60 as per the TC".
The battery is currently in the traditional baggaqge area location. I wonder if someone had moved it forward to help with this situation? Is relocating the battery legal?

Any thoughts?
 
Is the jack screw yoke pointing down.
100_6379.webp
 

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Which way is your trim yoke installed? Flat side on top on on the bottom? I think this picture will illustrate the relationship of yoke to nose down trim pretty well.

PA-12 trim 006.webp

To the battery relocation question, review this thread.
http://www.supercub.org/forum/showthread.php?42691-Which-Battery&highlight=AC43.13-2B

It seems most guys with Kenmore -12s prefer to have some weight in back in order to balance the plane at approach speeds so your mechanic's moving the battery back isn't unprecedented. Or maybe the documentation was incorrect and he took the easiest path to correct it by moving the battery back to stock. Your situation is unusual but you've got the fastest -12 I've ever heard of. How's the trim at normal operating speeds? Add some Cub gear and big tires. That'll slow you down!
 

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Last edited:
That is a lot faster than my 180 hp Super Decathlon.

Also check to make sure the upper longeron has about a 15 degree down turn at the trim opening. Not sure that should be the case on the 12, but a lot of 2-place Cubs get that angle removed when the aft end is repaired.
 
Also, probably should check the CG by weighing. This sounds way out of whack to me. I had the Kenmore setup on my -12 and it was nose-heavy. Battery was in the baggage area. I now have the short-mount setup from Crosswinds, light starter and alternator, and it's still a bit nose-heavy.
 
No it is pointing up, the opposite of your picture.
sierra bravo said:
Which way is your trim yoke installed? Flat side on top on on the bottom? I think this picture will illustrate the relationship of yoke to nose down trim pretty well.

View attachment 7405

To the battery relocation question, review this thread.
http://www.supercub.org/forum/showthread.php?42691-Which-Battery&highlight=AC43.13-2B

It seems most guys with Kenmore -12s prefer to have some weight in back in order to balance the plane at approach speeds so your mechanic's moving the battery back isn't unprecedented. Or maybe the documentation was incorrect and he took the easiest path to correct it by moving the battery back to stock. Your situation is unusual but you've got the fastest -12 I've ever heard of. How's the trim at normal operating speeds? Add some Cub gear and big tires. That'll slow you down!
Click to expand...
 
Often discussed in the past on this site. A common issue in modified -12s. Generally agreed upon that the flat side should be on top. In my case had the yoke been installed point up my nose down trim would have been limited. You can see in my picture (as well as in Steve's but not as clearly) that the nose down stabilizer travel is limited by the airframe structure, which coincides with the yoke's max up travel on the screw. Had I installed it pointed up the yoke would have topped out on the screw with the stab tube short of that position (lower, more towards neutral trim).

While what I said above is true, I can't imagine using full forward trim in my plane at cruise. My plane is very neutral at most speeds/weights until I slow it down.
 
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sierra bravo said:
Often discussed in the past on this site. A common issue in modified -12s. Generally agreed upon that the flat side should be on top. In my case had the yoke been installed point up my nose down trim would have been limited. You can see in my picture (as well as in Steve's but not as clearly) that the nose down stabilizer travel is limited by the airframe structure, which coincides with the yoke's max up travel on the screw. Had I installed it pointed up the yoke would have topped out on the screw with the stab tube short of that position (lower, more towards neutral trim).

While what I said above is true, I can't imagine using full forward trim in my plane at cruise. My plane is very neutral at most speeds/weights until I slow it down.
Click to expand...

Does turning it over involved cutting fabric? Or can I get to everything via the side inspection holes?
I can do fabric work but obviously want to be as least invasive as possible.
 
This is exactly the main reason I installed the Atlee Dodge clean out pan just forward of the tailwheel, so I can easily get to this stuff without having to cut the fabric. If I had to cut the fabric to get to it, I would cut it on the bottom side and go ahead and install the clean out pan while you've got it open. Makes it easy to get back in there if you ever need to. I think the clean out pan was only about $175 if I remember correctly, and fairly easy to install. Had to TIG five 1" tabs to the frame to fasten the pan to. I think all Cubs should have this mod so access is easy to the jackscrew, etc. JMHO.
 
Cut a hole just above the jackscrew. It can be removed fairly easy. Make an aluminum patch to cover the hole after you rotate it.
 
If you decide to proceed with the flip I'd sure appreciate some pictures of the procedure and a pirep about the resulting change. We often discuss the "before" conditions and never close the threads with "after" reports.

Best wishes for satisfactory results.

SB
 
Yes, and if you proceed with cutting the hole in the top like Steve suggested I would appreciate seeing how that turned out as well. I never heard of it being done like that, sounds like a really good way of doing it if you don't have the pan. My question to Steve is, how big a hole would one cut, and exactly how would you apply the aluminum patch? One piece of aluminum on the inside and one on the outside with the edges of the fabric sandwiched in between the two pieces of aluminum? Or just one piece of aluminum glued to the outside? Just curious, as I have never run across that type of situation before. And what would you glue the aluminum patch to the fabric with? Enquiring minds wanna know.
 
The fin/fuselage transition former is a sheet metal piece with a hole in the middle for the jack screw to rise up out of. On the original, there is a small sheet metal cover over that hole in the transition former, attached with two PK screws. Bottom line - Piper already arranged for removing the jack screw without significantly tearing up the fabric. After the work, the hole could be patched with a small piece of fabric, or perhaps the existing fabric glued down and an aluminum cover screwed on from the outside. If I were to recover from scratch again, that's how I'd set it up from the beginning - an external aluminum cover like Steve suggested rather than the internal one provided by Piper.

By the way, it's not possible to remove the jack screw from the bottom, because it is shouldered in the lower bearing - it must come out upward to remove the yoke.
 
Thanks for that info, Geezer. I wasn't aware you couldn't get the jackscrew out from the bottom as I had never had to do it before. When I put my Dakota Trim system in, it was right before I recovered the fuselage, so I did it with fabric off. I do remember seeing that fin to fuselage transition former with the hole now that you mention it. I just didn't realize at the time that that was specifically what the hole was for. I learn something new every day, that is why I am on this site, so I, like a few others, can learn from guys like you, Steve, Sierra Bravo, and all the others. Thanks again to ALL you guys for helping me understand these things.
 
sierra bravo said:
If you decide to proceed with the flip I'd sure appreciate some pictures of the procedure and a pirep about the resulting change. We often discuss the "before" conditions and never close the threads with "after" reports.

Best wishes for satisfactory results.

SB
Click to expand...


Okay I did it, it solved the problem. At 138 IAS I still have some nose down trim remaining as opposed to before when I had none and had to really push the stick forward for level flight. It took about 5 hours minus the fabric work which I'll do later. Steve was right on with his instructions except for the "fairly easy" part. I'd categorize it as "not toooo bad".

Here's the skinny and a couple tricks. No pictures but if you need to reverse the jack screw horn as I did you'll know what I'm talking about. The jack screw cannot be removed out the bottom, only the top. I'll skip the obvious steps in this adventure.

Remove the two AN-5 bolts attaching the horn to the horizontal stab.

Regarding the two trim cables and the pulley. We do not want to mix up the trim cables by letting them fall off the pulley. Furthermore we do not need to remove the tensioner spring. Take a tie wrap and run it through one of the pulley holes then around the two trim cables and sinch it down tight. Do the same on the opposite side of the pulley thus securing the trim cables,in their proper position, to the pulley.

Now take another tie wrap, run it through one of the pulley holes, then around the horizontal stab. Do the same on the opposite side of the pulley. Not too tight. Leave some slack. We are doing this so when the pulley is removed from the jack screw it will not go flying forward nor will the cables get twisted and create a real headache.

From the bottom fuselage inspection holes: If you only have one hole I would highly recommend cutting another. You will be working your arm up through one and looking through the other. Remove the cotter key and AN6 nut from the bottom of the jack screw. Now go back top side and gently push the pulley down off the jack screw.Be careful and keep an eye out for the woodruff key located in the shoulder of the jackscrew.

Reach in and start turning the jack screw by hand so that it rotates upward. We are unscrewing the jack screw out of the horn. Mine was very tight. Old hardened grease I suppose. I could only turn it by hand for about half the required travel. I then got two thin AN6 nuts and jammed them against each other on the threaded end (bottom) of the jack screw. Very easy after this trick. Look for the upper end of the jack screw as it protrudes agains the fuselage fabric. Cut a hole about 1" square. The jack screw will then come all the way out.

Obviously I cleaned both the jack screw and the horn before reassembling.Don't forget the woodruff key!

Hope this helps.
 
Bravo, and thanks for the detailed report. Your speed is something to behold. I just flew home, a tad heavier than usual but not heavy by any stretch, and at 2500 I couldn't break 85!

Enjoy it.

SB
 
P51DACE, Thankyou very much for the detailed report. I am printing out and filing your report in with my maintenance reference info in case I or any of my local friends ever need it in the future. It is detailed information like this that is worth it's weight in gold when you need it, (especially for guys like me who know nothing about absolutely anything, according to my wife). Again, I have learned something new today.

And another thing. I am very familiar with St. Tammany Parish and Slidell. I do pass through there occasionally. Maybe one day in the not too distant future I can drop in and say Hey as you are not too far from me.
 
DesperadoPilot said:
P51DACE, Thankyou very much for the detailed report. I am printing out and filing your report in with my maintenance reference info in case I or any of my local friends ever need it in the future. It is detailed information like this that is worth it's weight in gold when you need it, (especially for guys like me who know nothing about absolutely anything, according to my wife). Again, I have learned something new today.

And another thing. I am very familiar with St. Tammany Parish and Slidell. I do pass through there occasionally. Maybe one day in the not too distant future I can drop in and say Hey as you are not too far from me.
Click to expand...

Drop by the Bogalusa Airport (BXA). It's north of Covington and Slidell. This is where we keep our planes.My Dad has been the airport manager and FBO for thirty years. Always something going on. Cheapest AVGAS around also. Be glad to visit with you.
 
sierra bravo said:
Bravo, and thanks for the detailed report. Your speed is something to behold. I just flew home, a tad heavier than usual but not heavy by any stretch, and at 2500 I couldn't break 85!

Enjoy it.

SB
Click to expand...

Looks like you've gotta working airplane. Mine is a city slicker.
 
David---

Glad you got it sorted out; I too had the same problem and flipping the trim yoke fixed it.

I have to ask about your IAS--- VNE is 138 and it's not common that a plane that can maintain that speed in level flight (maybe a Mooney or Bonanza??). Have you checked your IAS against a GPS? Having said that, it stands to reason that you'd run out of nose down trim at such a high speed in a plane with a low speed airfoil designed to cruise at 100. Most planes struggle even to make it up into the yellow arc (110 in this case).

I am absolutely not questioning what you're seeing, mostly just expressing extreme jealously! You're up past Pacer speeds even with a flatter prop than they use. My plane has Supercub gear (= draggier with external bungees) and 8.50 tires--- if I firewall it and wait....and then wait longer...and stay below 2000'...it'll make 128. It's like a Ford Pinto on the freeway. An easy cruise is 100 in it with an O-320 and Sensenich.

Andrew
 
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