Simple Cub Rolls Left: Suggestions?

gijoecam

Rank: Plane Smasher
The background: Scratch-built a 3 channel Simple Cub for my third FT project. Maiden flight a couple days ago showed a left roll with power on. It was severe enough that even with it trimmed full right, it still took about 50% right rudder to keep it in the air. I adjusted the thrust angle down and to the right using some shims I made up. It helped a bit, but at full right trim, it still rolls left. Power-off glide is fairly level, though free glide slope isn't great.

The plane is balanced just like Josh shows in the video. It's ruining a B- pack motor and esc with an 1800 mah 3s battery.

Structurally, it's not perfect. The tail section of the fuselage is slightly out of square, so the tail is ever-so-slightly off counterclockwise. The rudder is parallel with the motor axis, but not perfect. Also the dihedral angle of the wing is slightly different from one side to the other, putting the left wing tip about 4mm higher than the left. Both wing halves appear to be even and symmetrical otherwise.

The questions:
Could the asymmetry of the wing be causing the roll? I would think that if it glides straight power-off, it's probably not a wing issue, correct?
Do I keep shimming the motor until it goes away? How much shimming is too much?
Is there anything else that could be causing this roll? Or is it just the torque it's fighting?

Thanks, gang!
 
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kdobson83

Well-known member
Mine fly's good with no shims, but it is 4 channel. You plane could have issues due to both the bad dihedral and tail issue. I can attest personally that just a little out of square in the tail can be really bad. Do some repairs and try to straighten things out. The wing should be easy. If you don't have the dihedral gauges anymore they r in the free plans. The tail will be a tad harder but if you have help, you can use a heat gun or hair dryer and try to twist it back to square. I personally added two popsicle sticks to the bottom side of my elevator to straighten mine back out after a crash. I did what FT does with the explorer tail with BBQ sticks, went from underside of elevator, about halfway to tip, to the fuselage. Straightened it right out.

Anyway, good luck. Have fun.
 

daxian

Elite member
hi gijoecam...
" Also the dihedral angle of the wing is slightly different from one side to the other, putting the left wing tip about 4'm higher than the left. Both wing halves appear to be even and symmetrical otherwise."
this will be the problem,as i found on a tinytrainer wing ...it will cause roll !!!
mine was the opposite and had right roll under power...packed it up with some foam tape and it has flown perfect since!!
hope this helps ...
 

gijoecam

Rank: Plane Smasher
Thanks, guys. Do you think I can just shim the main wing until it's level, or do I need to somehow reshape it all together? A heat gun may work to separate the glue, but it's under the packing tape, and will be difficult to correct...
 

DarkFire

Member
Could it be a torque roll issue? With the B pack on a 1800 mah 3s it could pack quite a bit of torque. Also, if the rudder is just SLIGHTLY out of alignment, it should only take 1 or 2 clicks of trim to counteract that.
 

Bricks

Master member
Since it seems to be worse with the power on I would shim the motor more maybe being scratch built your power pod holes for offset were not right on. If it flies well with power off then it is a motor problem at least IMO.
 

Hai-Lee

Old and Bold RC PILOT
A mismatched dihedral angle will cause some slight issues but the tail possibly being misaligned would compound the issue. Something to check is that the wings are at the same angle to the airflow and not twisted with respect to each other.

A simple check can be done using a scrap of FB or similar and making a profile gauge. Cut the wing profile out of one side or edge of the FB scrap so that it sits flat upon the wing upper surface leaving no gaps. Cut the opposite edge to be flat and roughly parallel to the wing profile.

Now fit the profile gauge on the wing and using a level check the angle of the gauge and by shifting the gauge from wing to wing compare the gauge angles. Ideally they should all be the same, if not your wings have warps and twists which will make stable flight very difficult to attain.

Another thing to check is that the wing saddle, (the part of the fuselage where the wing sits normally), is square and not twisted. A twisted wing saddle will apply twisting forces to the wing when fitted and will cause the wings to be skewed to the airflow and give roll forces not part of the original design. As stated above packing the saddle to level the wings can help with this issue.

The profile gauge can be made so that the tail angles can be measured as well and a scrap FB gauge can be made to check vertical tail alignment.

Have fun!
 

PsyBorg

Wake up! Time to fly!
Just to throw the obvious out and confirm things.. Does your prop spin counter clockwise or clock wise? incorrect prop rotation will according to what I have read while working on the SE5 show those exact symptoms with hard turns one direction and EVERYTHING possible to keep straight or barely turn in the other.
 

gijoecam

Rank: Plane Smasher
Partial update: I cut the glue seams out of the wing with an exact-o blade, and reset the wing dihedral. On the test flight, it was better, only requiring about 50 percent right trim to make it a little better than it was before.

On closer inspection in the daylight yesterday, it does appear the rudder may be angled ever-so-slightly to starboard. That would cause either a left roll or right turn (or both). Unfortunately that's going to be a bit tougher to correct...

Before I could try any other adjustments, I treed it on the edge of our flying field. It survived the deciduous landing mostly unscathed, but when the wind finally blew it out of the tree, it didn't survive the ground landing well.... Damaged the left wing tip, stuffed the nose a bit to the right, broke the firewall and bent the motor shaft, and smashed the power pod. I am making a new power pod, and I think I can straighten the nose and wing well enough to fly again...we'll see!
 
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gijoecam

Rank: Plane Smasher
Just to throw the obvious out and confirm things.. Does your prop spin counter clockwise or clock wise? incorrect prop rotation will according to what I have read while working on the SE5 show those exact symptoms with hard turns one direction and EVERYTHING possible to keep straight or barely turn in the other.

Not sure what you mean... If the prop was rotating the wrong direction, it would be flying backwards, no?

It's counterclockwise rotation, by the way...
 

Hai-Lee

Old and Bold RC PILOT
Not sure what you mean... If the prop was rotating the wrong direction, it would be flying backwards, no?

It's counterclockwise rotation, by the way...

I believe that he was questioning that you are using a CCW prop on a design which is designed for a CCW rotation setup.

Some users have unfortunately fitted a CW prop and changed the motor rotation to suit and then found that it was almost completely uncontrollable. Right thrust angle for CCW and a left thrust angle for a CW setup.

Have fun!
 

gijoecam

Rank: Plane Smasher
I believe that he was questioning that you are using a CCW prop on a design which is designed for a CCW rotation setup.

Some users have unfortunately fitted a CW prop and changed the motor rotation to suit and then found that it was almost completely uncontrollable. Right thrust angle for CCW and a left thrust angle for a CW setup.

Have fun!

Gotcha... It's the CCW prop that came with the B- pack. And trees, it's installed properly as well.

Another update: I Tweaked a few things this afternoon and made things a lot better. I was able to un-glue the rudder and shift it more in-line with the prop. After that, I was able to dial down the rudder trim to about 25%, but it still tended to roll left a bit. I was walking back, and also noticed the elevators were twisted slightly.... Not off-level, but actually port side trailing edge was up, starboard side was down. In other words, the tail was still twisting the plane through the air. I tweaked it some by hand, and was able to reduce the trim to just 16%.

I wouldn't have thought that would be so critical, but clearly it was. I need to shift the rudder a hair more, and flatten the elevator a touch, and I think that should mostly eliminate the issue.
 

Shakeyjake

New member
Control rod flex

After I crashed my Simple Cub several times I finally figured out that the rudder and elevator control rods were flexing so bad that in the right conditions the rudder would go slightly left even when I was full rudder right. I ended up slitting some control rod/antenna tube and sliding them over the rods and gluing them in place. I have soooooo much more rudder authority but now to trim it out to stay straight it yaws horribly. It's my opinion that it's just not a good three channel plane unless you adjust the thrust angle. It's a much better 4 channel plane.
 
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Forster

Slow, low and dirty.
I just finished my Cub and found a article about adding nose down/right thrust angle (4mm from the upper left to the lower right of the swappable pod). I'm starting there.
 
After I crashed my Simple Cub several times I finally figured out that the rudder and elevator control rods were flexing so bad that in the right conditions the rudder would go slightly left even when I was full rudder right. I ended up slitting some control rod/antenna tube and sliding them over the rods and gluing them in place. I have soooooo much more rudder authority but now to trim it out to stay straight it yaws horribly. It's my opinion that it's just not a good three channel plane unless you adjust the thrust angle. It's a much better 4 channel plane.

The Cub handles so much better if you move the rudder and elevator servos to the rear and beef up the (now much shorter) control rods with proper 2-56 rods and hardware. In fact, the only FT model (I have seven) that I haven't ditched the included small gauge control rods and replaced with proper ones is my Tiny Trainer.
 

Brett_N

Well-known member
I second the better control rods. I'm only using the small gauge on the mini's and even then I zip tie them. They're OK for aeilerons on the bigger planes, but no good for anything requiring more than about 6 inches of length.