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Exp cub out of nose down trim....Ideas please?

View attachment 47282

Not sure if this explains or not
The straight lines would be your digital level
It was set to zero at the fw vertically to start.
The horizontal ref is 90 to the fw for reference.
The other 3 lines are the wing and stab measurements.
I am assuming that is the way you measured Adam to come up with numbers like that?
Your up stabilizer has a value of 2.5 which is greater than a wing value of 1.0


yes that is how I measured. I’m not sure what the problem is. Thanks for the drawing it is more clear and something I can wrap my head around. I posters w&b thinking maybe that had something to do with it.
 
I was just trying to answer a question asked of me with a reply that makes sense. As far as wt and bal--- I know nothing lol.
 
I think it’s time to assume that the dimensions of the fuselage may not conform to the factory drawings.....

Is the nose too short? Is the tail too long?
 
So you can't get more trim with a new jackscrew assembly from Javron or Dakota? I run out of nose down trim even before I get to 2000lbs. PA-18 experimental. AOI changed to 1.84 I believe and tail was adjusted to proper angles but I'm still running out of nose down trim often.

No doubt 2300 lbs aft cg will cause issues. But less than 2000 lbs should not be a problem unless he is outside the aft limits.
This is why I thought it might be angles or possibly too short of bungee assist cable
 
The stab deflection angles appear to be workable. We have to figure out WHY the tail has to be producing an up thrust even at forward cg loading. Pilot only loading should have a net elevator up deflection and leading edge down slightly ....at full aft cg loading, the stab should be neutral, not leading edge up, elevator down.

The elevator should be exactly in trail of the stab, at all trim settings...
 
The elevator should be exactly in trail of the stab, at all trim settings...

I am not sure if you meant quite the way you wrote it or I misinterpreted. Pretty sure my elevator does not trail my stab in all trim settings. But then I try not to look at those shaking tail surfaces much:lol:
 
The bungee cord is supposed to keep it exactly in trail, but in truth, it doesn’t seem to matter much. My bungee cord was way too long (how the heck did that happen?) when I got it, but when I replaced it with the correct part number, it didn’t change the trim at all....but, it would probably make more of a change if it was too short, and the elevator was deflected up. When it’s just hanging down, the slipstream tends to lift it into trail.
 
The bungee cord is supposed to keep it exactly in trail, but in truth, it doesn’t seem to matter much. My bungee cord was way too long (how the heck did that happen?) when I got it, but when I replaced it with the correct part number, it didn’t change the trim at all....but, it would probably make more of a change if it was too short, and the elevator was deflected up. When it’s just hanging down, the slipstream tends to lift it into trail.

Not in my experience in any of the Super Cubs I have been around.
 
Not in trail, trim nose up and it manipulates the elevator up, not in trail with the stab.
 
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Now my head is starting to hurt again...when you trim nose down, it raises the leading edge of the stab, which loosens the bungee cord, which deflects the elevator down, in order to keep it at the same level as the stab...(rather than lowers, lets gravity take the elevator down...)

The elevator mostly stays in trail near neutral trim, at the extremes, it moves more than the stab, but it matters more when the elevator is pulled up.
 
Now my head is starting to hurt again...when you trim nose down, it raises the leading edge of the stab, which loosens the bungee cord, which deflects the elevator down, in order to keep it at the same level as the stab...(rather than lowers, lets gravity take the elevator down...)

That is all correct. Loose bungee is a cruise setting elevator in trail. If the bungee is too short the elevator will not be in trail but deflected up and then you can run out of nose down trim. Elevator is not in trail with the bungee system in a nose up trim setting.

I need a good disclaimer today. 8:lol:
Everything I have posted to this morning, could be wrong.
 
Now my head is starting to hurt again...when you trim nose down, it raises the leading edge of the stab, which loosens the bungee cord, which deflects the elevator down, in order to keep it at the same level as the stab...(rather than lowers, lets gravity take the elevator down...)

The elevator mostly stays in trail near neutral trim, at the extremes, it moves more than the stab, but it matters more when the elevator is pulled up.

Sorry, nose up trim, making the leading edge of the stab go down, pulling the elevator up.
 
Read the third sentence in post #73. I had a friend with the same problem after rebuild. The culprit was the bungie cable was too short. There were kits that were sold locally that had cables that were too short.

Maybe take a side view shot of the elevator position with the trim in both extremes and post it. Your wing incidence as has been said is not the problem.

Jerry
 
Small drift here - What happens to flight characteristics if no elevator bungee is installed? The bungee on my plane is weak or too long; the elevators sag, which results in the horiz stab leading edge downward (nose up trim). But it seems to fly ok? It seems I've read in threads long ago about eliminating the elevator bungee, but I'm not certain what was said.
 
Adam, is this 555 ZW ? If so I thought it flew great in the fall of 16 when I borrowed it. I would venture to guess your answer is in the straight edge on the lower longhorns. They should be straight and bent up that much would explain the lack of nose down trim. When I flew it it was close to perfect in the way it flew and performed.
Dave
 
Small drift here - What happens to flight characteristics if no elevator bungee is installed? The bungee on my plane is weak or too long; the elevators sag, which results in the horiz stab leading edge downward (nose up trim). But it seems to fly ok? It seems I've read in threads long ago about eliminating the elevator bungee, but I'm not certain what was said.
I believe that the primary purpose of the bungee spring(s) is to provide resistance feedback in the elevator control. Secondarily it assists the stabilizer in pitch trim. There are specific control pressure limits to meet the certification requirements. If there was no resistance when you move the stick, it would be easy to over control the airplane (PIO) pilot induced oscillation.
 
My weak bungee spring doesn't result in PIOs, but it bugs me that the stab rides so low. (CG is well within limits - don't recall the number offhand).
 
As I trim nose up my stick sneaks back. Trim nose down and the stick eases forward. I tried a different yoke that had a little more nose up bias and the bungee position left the spring limp. I never noticed any control pressure difference.
 
mark I guess I thought based on the numbers that the stab angle nose down trim would be a greater number that the bottom of wing. What about longerons coming up and 3/4” gap at tail post?

Adam,
The butt rib and fw can be difficult to measure and by the time you get to the tail they can be way off. That is why I suggested earlier to make it simple and just measure the relationship between the wing and nose down trim. The longeron should be straight. It is bent upwards which will lose your nose down authority. You said some work was already done in the tail but not what was done. This simple measurement could show you if there is a problem. Just measure off the bottom of the cleaned spar caps under the wng root fairing and and the stabilizer in the up position. Not trying to make a bunch of work for you but it might be a telling measurement.
I used the words could and might in this post because you just never know. You asked so that's all I have. If this doesn't help I am back to lurking lol. Good luck
 
Side question. How do lower longerons get bowed? Ive see tail sections get twisted and single longerons bent by impact but both longerons bowed equally is new to me. And wouldn't bowed lower longerons improve nose down trim? I don't see how that alone would hurt it.
 
Around here most all fabric planes have bowed longerons since the one shop who covers planes has never learned how not to over tighten the fabric.
 
Distortion from welding.

I have looked the the planes this guy does, the tubes are good when starting cover but bowed in more than an inch. It is work I would not pay for nor refer anyone. But it is the only game in town.
 
I remember the J3 fuselage drawing shows bowing the tubes out so much. I assumed it was for the fabric to pull.
 
Side question. How do lower longerons get bowed? Ive see tail sections get twisted and single longerons bent by impact but both longerons bowed equally is new to me. And wouldn't bowed lower longerons improve nose down trim? I don't see how that alone would hurt it.
Stewart
I don't know about bowing but those lower longerons look stretched bent upward. If we use Adam's .75 inch number bent upward in the tail that could translate to more than 1.5 degrees less nose down trim. That is quite a bit. I hope Adam meaures again between the wing and tail. Curious as to how full of **** my figuring is.:lol:
 
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