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PA-12 Replica Elevator Problems

H

Heater

Registered User
Hi Folks,

I recently purchased a PA 12 Replica Kit. The frame was done by Javeron and the kit, I believe from Back Country Super Cubs. Here's the problem. The plane won't stall. There's not enough back stick/up elevator to get the plane to stall. My understanding is that this airplane needs 27 degrees up elevator. This one has only about 21 degrees. The problem is that the horn is at the stop, there's no where left for it to go. Has anyone encountered this? I've talked to Javeron and the builder and they claim that they know of no fix. No point in having a STOL airplane that won't STOL.. Anyone have any ideas for a fix or anyone have this problem?? Any ideas would be appreciated. Thanks.
 
Do you have a good, recent empty CG number for the plane? Modified 12's have the habit of getting nose heavy.
Can you post a picture of the front of the H.stab with the trim set in its normal single occupant cruise position? That tells a lot by itself. I see a lot of modded 12's with the front of the stab all the way down, or nearly so.
 
If it needs 6 more degrees of up elevator than you're getting now, how much down elevator is it getting?
 
Look at the control horn. Is it perpendicular to the surface of the elevators? If not, the elevators may be upside down. Also as Perry said, you may be loaded too far forward and need ballast of some type.

Also check that the leading edge of the stabilizer touches the top longeron when in the full nose up position.
 
As others said, you are nose heavy. Throw a nicely stocked tool bag way back in fuselage. Or a bucket of gravel.( you can fix up you remote strip one bucket at a time)
 
Take a look at the stop. If the link on the lower horn is too long, you won't get full up elevator.

P_20170118_102022_p_1.jpg P_20170118_102041_1_p_1.jpg

The elevator stop on a PA-12 should be 1/2" closer to the tailpost than on a supercub. The link shown in the photos is the one called out in the -12 parts book. When used with the stop shown (-18 ) it stops up travel too soon. You can shorten the link or move the stop.

Note that on the second picture, my elevator cable is slack so I could rotate the link out of the way.
Wayne
 

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PA 12 Replica~Thanks

PerryB said:
Do you have a good, recent empty CG number for the plane? Modified 12's have the habit of getting nose heavy.
Can you post a picture of the front of the H.stab with the trim set in its normal single occupant cruise position? That tells a lot by itself. I see a lot of modded 12's with the front of the stab all the way down, or nearly so.
Click to expand...

Thanks very much Perry. I'll get a picture for you. This entire airplane is all new to me and I'm trying to dial it in. Honestly, I don't know where the trim set would be in a normal single occupant cruise position. CG is good but obviously with an 036 on the front it's going to be nose heavy. I'm leary and so is my I.A. of putting a weight in the back of the tail. This could cause a serious problem in a stall scenario.

I'm trying to dial the airplane in so that it flies like it should without bastardizing it too much. Does this make sense? I'll experiment with the trim and try and get you a picture or information on that. Thanks so much for your help. I'm hoping I can get this airplane to be just what we're looking for. It's nose heavy as hell right now, won't stall and not much fun to fly. One thing is for sure Perry, there is something not manufactured or built right back at the up elevator stop. With full stick back, I'm on the stop and 5-6 degrees shy of 27 degrees up elevator. Is it your understanding that 27 degrees is proper for up elevator? Thanks again Perry. I'll get back to you if you don't mind.

Best regards,
Scott
P.S. I'm Scott Sackett. (940) 367-1992
 
27* up, yes. Overall range of elevator is 59*. TCDS A780 lists the trim, ailerons, rudder, and elevator deflection. And CG. What is your empty CG?
 
CG

stewartb said:
27* up, yes. Overall range of elevator is 59*. TCDS A780 lists the trim, ailerons, rudder, and elevator deflection. And CG. What is your empty CG?
Click to expand...

Hi Stewart,

Empty Weight CG is 13.7

Most forward CG is 14.0

Most aft CG is 23.5

I'm Scott Sackett. Certainly appreciate your thoughts.

Best regards,
Scott
 
Forward CG is not your problem. At least not if you're calculating it like a certificated version.

Specifications Pertinent to All Models
Datum- Leading edge of wing
Leveling Means- Plumb bob from machine screw at door frame channels near upper rear corner of door tohole in plate near rear seat.
C. G. Range (+9.0) to (+18.6)
Empty Weight C.G. Range (+9.5) to (+13.3)
Click to expand...

https://cubclub.org/tc/Piper/A780.PA-12.pdf
 
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stewartb said:
Forward CG is not your problem. At least not if you're calculating it like a certificated version.



https://cubclub.org/tc/Piper/A780.PA-12.pdf
Click to expand...

Thanks Stewart. Now, here's the thing. When one buys an airplane like this, in all actuality, who knows how everything was calculated. Thank you for the spec sheet. So, the A-780 sheet gives you CG values based on the LYC 0-235-C Mine has a LYC 0360 hanging on the front. Do you think the difference in weight between these two engines is significant and throwing everything out of wack?? Thanks Stewart.
 
I'd expect your CG to be forward of what it is. Thus the comment about if it's calculated like a stock Cub. If it's based on a -12, the CG needs to be in spec. Adding a big engine isn't anything new. Lots of guys have big engines on -12s and they perform fine even if a little nose heavy when empty. Easy solution, don't fly it empty. :) Seriously, you have a couple of head scratching issues. The CG probably needs to be verified using the leading edge as datum and the elevator travel needs to be corrected. Find a Cub mechanic and get it tuned up. It should be a great plane.
 
Super Cruiser fix

stewartb said:
I'd expect your CG to be forward of what it is. Thus the comment about if it's calculated like a stock Cub. If it's based on a -12, the CG needs to be in spec. Adding a big engine isn't anything new. Lots of guys have big engines on -12s and they perform fine even if a little nose heavy when empty. Easy solution, don't fly it empty. :) Seriously, you have a couple of head scratching issues. The CG probably needs to be verified using the leading edge as datum and the elevator travel needs to be corrected. Find a Cub mechanic and get it tuned up. It should be a great plane.
Click to expand...

Thanks again Stewart. I'm going to certainly do another CG based on your information. Fixing the elevator travel is going to be a challenge but I think we have an idea of how to do it. I was pretty sure we'd have to go through a "dial in" process when I bought the plane. BTW, this is the first time this airplane (100 total hours) has ever flown on wheels. Always floats before so the former owner didn't notice the lack of up elevator problem as much as I do fly off a grass strip. Thanks again, I'll keep you posted as I move along.
 
Heater said:
CG is good but obviously with an 036 on the front it's going to be nose heavy. I'm leary and so is my I.A. of putting a weight in the back of the tail. This could cause a serious problem in a stall scenario.
Click to expand...
Do not be concerned about placing ballast in the tail as long as it is securely fastened in place. Your IA should know this, as it is part of the training to be an A&P.

If your empty CG is correct at 13.7" your plane's empty CG is 0.4" aft of the published empty CG range. This is indicating that your plane is not nose heavy. Since you say that it is flying nose heavy, it would be wise for you to reweigh the plane for confirmation of the numbers. For your information it is unusual for the FAA to publish an empty weight cg range. There aren't many planes where this is done.
 
Thanks very much. We're going to go through the process of re-weighing the plane to see where everything really is. My I.A. and I are both aware of using a ballast if necessary. My feeling is that, ballast or not, this airplane is waaaay nose heavy. With me in the front seat at 230#, my wife in the back seat at 150#, I have to trim the airplane nearly fully stab up to get it to fly straight and level. Simply not right. Again, I'm still lacking 5 to 6 degrees of up elevator because of an improper build.
 
Re: the empty weight CG range, the -12 doesn't have a CG envelope when operating on wheels or skis.

When empty weight C.G. falls within this range, computation of critical fore and aftC. G. positions is unnecessary. Range is not valid for non-standard arrangements.
Click to expand...
 
Just for the fun of it, measure the distance from the leading edge of the wing (datum) to the propeller. If it is the same as a PA-12, it will be 62". If it is more than 62", your nose heaviness is in the engine mount being too long. Granted the 0-360 is a heavier engine than the 0-235. But it is a place to start.

This information is found in the Type Certificate for the PA-12, A-780.
 
You definitely need to weigh it. Over time the continuity chain gets broken and the numbers start getting wonky.
When I rebuilt mine, I went to considerable effort to get my CG back while keeping my long (Kenmore) mount/O-320.
I have a Niagara cooler, B&C alt and Skytek starter. Also about 5 lb in the tail at about 180-ish inches. My empty CG is 12.85 if memory serves, too lazy to dig the books out right now. In the attached pic, I'm up front at 210 and my son is/was about 150. Trim is in the middle. With no passenger I crank in 1.5 turns up (stab down) and the L.E. drops about 3/8". I wouldn't be surprised if your weighed CG ends up in the 11" range.

As to limited up travel, I'd never considered the possibility of inverted elevators as Skywagon suggested, but I'd be looking there first.
 

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PerryB said:
.

As to limited up travel, I'd never considered the possibility of inverted elevators as Skywagon suggested, but I'd be looking there first.
Click to expand...

on a Cessna that maters, I can't think of a real difference on a -18 tail. I assume you have -18 tail feathers? At least?
 
I have a bare 18 elevator sitting right behind me in the office. I see no difference right side up to upside down in relation to the control horn.
 
One thing that has been mentioned, but not emphasized yet, is that the aluminum bracket that connects the vertical stabilizer to the jack screw (I don't recall the proper name for it) must be in the correct orientation such that the leading edge of the stabilizer can touch, or nearly touch, the top fuselage longeron with full nose-up trim applied. If it won't, you're not getting full nose-up trim and possibly that bracket is incorrectly installed.

Also - don't worry about any individual location of weight affecting stall characteristics. It is only the sum of weight-moments that affects the CG location, and only that location that can adversely affect stall, airframe geometery being equal.

Also, full elevator travel per the TCDS is essential.
 
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Hey Gordon, I think they call that yoke shaped thing a yoke. I can't imagine a guy who drives mules missing THAT! Oh well, yokes on you....
 
Perry, weather's been keeping you inside WAY too long. Do your taxes or something. That Ford should give you plenty of deductions - - -
 
I already did my taxes, now I'm broke so all I can afford to do is hang out here and harass you. And yes, the Ford actually did recently give me a couple thousand in "deductions". I was considering trading in for a fresher horse until we (mostly she) ran the numbers. I was informed I'd be repairing rather than replacing it.
 
Depreciation plus repairs - how can that be bad for a farmer with adequate water? And see what you did with the thread-drift? Jeeze. :Ginnocent:
 
mike mcs repair said:
on a Cessna that maters, I can't think of a real difference on a -18 tail. I assume you have -18 tail feathers? At least?
Click to expand...
Good point. For heater's benefit, -18 tail feathers have a balancing section ahead of the hinge line, -12 elevators do not.
skukum12 said:
I have a bare 18 elevator sitting right behind me in the office. I see no difference right side up to upside down in relation to the control horn.
Click to expand...
The elevator horn should be perpendicular to the elevator surface. On my Backcountry Cub there was a small difference. Enough so that I had to try them on both sides to get the best position before covering since I wanted the drain grommets on the bottom. That is the reason why I mentioned this.

The up stop as shown in the first picture of post #6 is also a good place to look. Perhaps should be the first place to look. Cut some of it off for more travel.

Another thing to consider is that with a pilot of 250 pounds there could be a restriction in stick travel. Rig the cables to stick accordingly.

Still with all of the above accomplished, the nose heaviness situation is not solved. Only the ability to increase elevator authority. Proper CG is important for proper and good flight performance.
 
Heater said:
My understanding is that this airplane needs 27 degrees up elevator. This one has only about 21 degrees. The problem is that the horn is at the stop, there's no where left for it to go.
Click to expand...
Heater, Another question. What did you use for a reference from which you measured your angles? Piper had a reference line on their drawings from which dimensions were taken. This line is perpendicular to the firewall. If you are using a digital level you ought to zero it on the firewall, then check your elevator travel. Is it still 21 degrees?

Unless you know for certain that your measurements are correct, all of the advise we give you could be in error.
 
Turbine Cubs PA12, Can't 3 Point

Hello,

Has anyone heard anything about Steve Pierce. I've tried to contact him for several months to no avail. Thanks in advance.

Scott
 
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