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

[S2D]

Just take a sawhorse and come along out to your tie down spot and do a little head scratching. Im sure you will come up with some interesting ideas.






Disclaimer. ( I'm just joking)

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[reliableflyer]

I’m no mechanic and no expert. There are many on this site that do this sort of repair for a living and are better equipt than I to give advice.

if You need more down trim you will need to lower the aft end of the horizontals. That can be done by grinding out the tail post tube and bending the upper longerons downward. Then weld in a new tail post tube. Currently 145$ from univair. While not the proper fix you could instead cut off the bracket that has the bushing for the aft horizontal cross link tube and cut some off the bottom and reweld it on.

How much to change shouldn't be a guess but calculated. My cub drawings didn’t show the tail angle in relationship to the wing. If you can’t find that angle on the drawings you might get it off a good cub. All this is assuming your experimental is built according to Supercub specs.

If you are trying to land this cub short and fly it slow on landing its an accident looking for a place to happen. Good luck with this.

 

[mike mcs repair]

I’m no mechanic and no expert. There are many on this site that do this sort of repair for a living and are better equipt than I to give advice.

if You need more down trim you will need to lower the aft end of the horizontals. That can be done by grinding out the tail post tube and bending the upper longerons downward. Then weld in a new tail post tube. Currently 145$ from univair. While not the proper fix you could instead cut off the bracket that has the bushing for the aft horizontal cross link tube and cut some off the bottom and reweld it on.

How much to change shouldn't be a guess but calculated. My cub drawings didn’t show the tail angle in relationship to the wing. If you can’t find that angle on the drawings you might get it off a good cub. All this is assuming your experimental is built according to cub Supercub specs.

If you are trying to land this cub short and fly it slow on landing its an accident looking for a place to happen. Good luck with this.

Be careful. Tail wires might not fit then. Don’t ask


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[skywagon8a]

I’m no mechanic and no expert. There are many on this site that do this sort of repair for a living and are better equipt than I to give advice.

if You need more down trim you will need to lower the aft end of the horizontals. That can be done by grinding out the tail post tube and bending the upper longerons downward. Then weld in a new tail post tube. Currently 145$ from univair. While not the proper fix you could instead cut off the bracket that has the bushing for the aft horizontal cross link tube and cut some off the bottom and reweld it on.

How much to change shouldn't be a guess but calculated. My cub drawings didn’t show the tail angle in relationship to the wing. If you can’t find that angle on the drawings you might get it off a good cub. All this is assuming your experimental is built according to cub Supercub specs.

If you are trying to land this cub short and fly it slow on landing its an accident looking for a place to happen. Good luck with this.

Shorten the bottom longerons 32" ahead of the tail post. Calculate and cut out the appropriate amount, make an outside sleeve repair. Nothing else will be effected. Done.

 

[Jonnyo]

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

Thanks for posting !

 

[fobjob]

I’m concerned that the focus of this thread seems to be how to get even more positive lift on the stab instead of finding out WHY the tail has positive lift on it at all........when it shouldn’t have ANY.

 

[DENNY]

Are you sure about that? In level flight the stabilizer should have 2° or so of a up travel that should Create a lifting force.
DENNY

 

[reliableflyer]

I’m concerned that the focus of this thread seems to be how to get even more positive lift on the stab instead of finding out WHY the tail has positive lift on it at all........when it shouldn’t have ANY.

Your missing the point. The tail is creating a downward force to counter balance the forward weight of the plane. It always creates a downward force . By trimming up or down you are increasing or decreasing the downward force Nose down trim decreases the downward force (but there is still downward force, just less of it) and nose up trim increases the downward force (more downward force) of the tail. The tail in effect is flying upside down.

In a cub type system the leading edge of the horizontal stabilizer moves up moves up or down as you change trim ( you change the amount of downward force) If you trim nose up you will see that the horizontal leading edge moves down. If you leave the trim where it is and move the elevator upward as in trying to move the nose of the aircraft upward it goes up increasing downward force.

If the tail isn't on the plane at the appropriate angle when you trim up the tail can’t develop adequate downward force or downward lift.

To fix it you either have to decrease the forward weight weight in the system or be able to increase the downward force to counter balance the forward weight as you slow down and the elevators become less effective.

What you need to do is not increase positive lift of the tail but increase the negative lift (downward force) of the tail when required.

 

[fobjob]

When I say “positive lift” I am referring to upwards force....if nobody knows the difference between positive and negative, this could explain the confusion in this thread....

 

[stewartb]

The original post was about trim in a Cub "consistently" flown at weights up to 550# over standard gross. Load your plane to that weight and to the aft limit and see how it feels to fly. The tail will be very heavy at takeoff and without added speed at landing the tail will drop and you won't have elevator authority to overcome it. Hopefully you're close to the ground, but even then it makes things uncomfortable.

When CC did the 2300# gross did the CG aft limit move at max weight?

 

[Gordon Misch]

Stewart is asking a good question. The tail geometry is engineered to be effective in balancing the maximum CG MOMENTS. At max moments it must be still lifting downward, otherwise the aircraft is unstable in pitch. Potentially dangerously so.

 

[hunt/trapak88]

This may have already been said but, I almost/do run out of down trim sometimes when loaded fairly aft. My
incidence is increased. And did nothing to the tail. I took the spar fittings up as far as possible without messing a bunch of things up. If your forearm is beefy you can cut out your nose down stop which is just a small tube. I did this and gained gobs of down elevator movement. Forearm beefy as in, once you run out of nose down trim you can do what you need to do by physically using the stick to get you home when heavy. When hauling loads of plywood in the lumber rack I could only land with one notch as the elevator would be at the stop. Pull another notch and nose is still going up with the stick to the stop. When I trimmed full nose down before removing the stop, I had about a 1/2” movement left in the stick is all. Or get a pod and keep the weight forward. But then again your saying you fly regularly over 2000lbs...which is safe to say that is consistently flying HEAVY AS *hit. No thanks