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Inadequate elevator fo J-3 with flaps: looking for thoughts about this issue

reliableflyer

Registered User
Seldovia, Alaska
I have recently sold my cub which I have had for more than 40 years and purchased a J-3 C65 with a
C-90. I have an 80 X 34 Catto on it. It has been converted to PA-11 configuration including wing tanks and removal of the nose tank as required by the STC and is soloed from the front seat. This was done many years ago. It has a pacer balanced tail also with vg's under the tail. The wing is metal with 18 flaps, vg's and Stewart tips and of course wing tanks for the pa11 conversion. It also has the CubCrafter header less fuel system.

The empty CG weight falls in the center of the range specified in the type certificate. With the wings off, the aircraft balances on the lifting fitting at the front spar location. Trim range is about 28 or 30 turns stop to stop. The v on the jack screw yolk is pointing down and the horizontal cross tube touches the longeron on the bottom and the yolk touches the frame brace on the top. I have Full trim range.

Measuring the the movement of the horizontal stabilizer I have within hundredths of an inch on both up and down position on the horizontals as compared to a new Carbon Cub which is being built here. The carbon cub has approximately 28 degrees up deflection with full up elevator and I have 33 degrees full up elevator..

Problem is that while I have full trim range and all is well with two notches of flaps, full flaps require full forward trim and forward stick pressure during a three point landing. It is the slowest landing cub I have ever flown but there is no reserve for additional elevator. Seems to make little difrence even when I have weight aft.

I am am speculating that the full flaps move the center of lift aft and I simply run out is elevator power on a very slow full stall landing. I am looking for a set of pa18 horizontals and ailerons and hope that they will be able to provide the additional elevator power required. If any one knows of a set available please advise.

I have never ever flown the zero thrust mod and read the recent discussion with interest and wondered of that would have any bearing on this issue. This cub was built to existing configuration in the 90's and I don't remember when the zero thrust angle became popular and I don't know if this cub has it or not.

if anybody has had any experience with overly powerful flaps or lack of elevator effectiveness or other thoughts on this issue I would be interested in hearing your thoughts.

stu
 
I have a similar issue with my stock 150HP original tailwheel Pacer.. with full flaps I have an awful lot of forward trim. For that reason, I've never loaded up the back seat.
 
...If anybody has had any experience with overly powerful flaps or lack of elevator effectiveness or other thoughts on this issue I would be interested in hearing your thoughts.

stu
stu, I have seen this, Many years ago (1983) a friend had a J-3 modified to a -11. It had a no electrics O-320, Hendricks tips. Center LE cuff and flaps. I was there when the FAA was certifying the flaps on the wood spar wings. While I'm not certain of the reason, the FAA engineer in Anchorage told my friend to remove (1/2"?) from the lower longerons 32" ahead of the tail post in order to increase the down effectiveness of the elevators. I made the "repair" and have a copy of the 337 in my file. The 1/2" is from memory as this was a "repair :lol:".

This was a very light weight and super performing "J-3" which was based at Birchwood at the time. I do not recall which tail feathers were on it, though I suspect that they were PA-18.
 
Skywagon has the correct final fix as usual. But the description is still a bit unclear to me. Full flaps should make the tail lighter Just to clarify. When you do full forward (nose down trim) is the front of the horizontal stabilator full up or full down? Level the aircraft and see how many degrees positive and negative you have with stabilator travel that will help figure out if you need to add or remove from the lower longerons . Tails bend over time and it is not uncommon for one to need cutting due to rough use or pair repair when new tail section added. It won't take much to have a big change on the tail.
DENNY

Edit: Full flaps on the ground makes the tail lighter. On final full flaps requires more forward stick pressure. I haven't even started drinking yet!!
DENNY
 
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You might look at your elevator rigging as well. I've heard of the primary stop (back in the tail section) for nose down elevator being welded in the wrong place from the piper factory or from a repair somewhere down the line. This could limit your travel to less than the specified amount, appearing as a lack of authority.

I think Skywagon's got the right answer in your case for the fix, but rigging might also be contributing.

My plane also requires lots of nose down trim for full flap flight.

Sent from my LG-D850 using Tapatalk
 
Somebody welded in a Zerk fitting on one of our Cubs, interfering with up elevator travel. Most Super Cubs seem to require forward trim when flaps are extended - always seemed to me that those with weak bungees were easier to fly without always wheeling on the trim handle.

I flew a J-3 last year with balanced elevators. It required trim for each different power setting and airspeed, quite unlike a normal J-3. I did not like the way it handled.
 
stu, I have seen this, Many years ago (1983) a friend had a J-3 modified to a -11. It had a no electrics O-320, Hendricks tips. Center LE cuff and flaps. I was there when the FAA was certifying the flaps on the wood spar wings. While I'm not certain of the reason, the FAA engineer in Anchorage told my friend to remove (1/2"?) from the lower longerons 32" ahead of the tail post in order to increase the down effectiveness of the elevators. I made the "repair" and have a copy of the 337 in my file. The 1/2" is from memory as this was a "repair :lol:".

This was a very light weight and super performing "J-3" which was based at Birchwood at the time. I do not recall which tail feathers were on it, though I suspect that they were PA-18.


I appreciate your input. years ago when we could still get field approvals for significant mods we changed the angle of incidence on the wing. To keep the tail in the same relationship to the wing we shortened the lower longerons. The fuselage of course is just along for the ride and the wing and tail angles are what is significant.

What at I have done is checked the angle of the tail in relationship to the wing and did the same with a new carbon cub currently being built from a kit. The horizontal on mine matched that ot the freshly built carbon cub to within a few hundredths on the stabilizer and had more up trim on the elevator. I started to look for a tail section being bent upward and thereby lengthening the lower longeron but decided to measure the angle relative to the wing instead.

I appreciate your thoughts and will consider this. While not a cheap or easy option I think it is a good one.
 
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