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Trim Tab on the Elevator

A trimming stabilizer is more aerodynamically efficient (less drag at cruise speeds) and enables a greater loaded center of gravity range. The travel range of the trimming stabilizer should be determined by the ability of the elevators to overpower a failure. Balanced elevators increase the surface area giving more elevator authority with less force on the control stick.

Cub Builder's experience is a scary and possible fatal circumstance. A trim tab can have any number of different failure modes with the same result. The only possible trim tab design that I would consider is one with an aerodynamic balance which could float without being connected. Flutter is an experience which you do not want to experiment with. It can happen instantaneously without warning at the most inopportune time. Been there twice (for different causes), "got the T shirt" and I do not wish to be there again.

Do what you wish, but please be very aware of what you are building and what any failure can produce.
 
Had a Taylorcraft come in for an annual with the trim tab in the right seat because the actuator froze up. 8)
 
Coyote Ugly said:
After having flying both on Cub quite a bit, I gotta say I like the tab better. My 2 cents... It's much easier to operate and seems to work just great slow and fast, light or heavy..
Click to expand...

That's also been my experience, the broken cable incident not withstanding. It works well heavy, light, forward CG or aft CG. With the trim tab, I don't have other cables and springs (that can fail) attached to the elevator controls. The majority of light planes out there have trim tabs, so I'm thinking it's not such a terrible solution. But Wag screwed up their design with the single bowden cable used to push and pull. It was pushing with it trimmed nose down that caused the cable to bend, fatigue, and break. Replacing it with a Ray Allen servo and putting the trim control in the stick grips is a permanent (and lighter) solution.

-Cub Builder
 
Cub Builder said:
..... It was pushing with it trimmed nose down that caused the cable to bend, fatigue, and break......
-Cub Builder
Click to expand...
This is key. Consider when the tab is under the highest load. That is likely to be at the highest cruise speed. Design your push rod installation to be pulling the tab under those conditions. IF the rod is pushing the tab at the high load it must be stronger to offset the compression bending strength of the rod. I have seen the tab push rod bend under these conditions. When the mechanism was reversed to pull under the highest loads there was no more trouble.
 
I experienced the same thing as Cub Builder on my 2+2. It happened to me on take off, about 100 feet in the air. I slowed down to about 60 and the shuttering went away. I made a 360 and landed, I had no idea what just happened. I put a couple small bolts with washers between the elevator and the tab and went on my way. I had a Bowden cable and the center wire broke. A friend owns a company that makes all kinds of cables for the military and civilian use. He made me a cable you could tow a truck with. A little hard to get installed but it will be there from now on. If you use a trim tab on the elevator make sure you make it big enough, mine is a little small
 
Tim said:
I experienced the same thing as Cub Builder on my 2+2. It happened to me on take off, about 100 feet in the air. I slowed down to about 60 and the shuttering went away. I made a 360 and landed, I had no idea what just happened. I put a couple small bolts with washers between the elevator and the tab and went on my way. I had a Bowden cable and the center wire broke. A friend owns a company that makes all kinds of cables for the military and civilian use. He made me a cable you could tow a truck with. A little hard to get installed but it will be there from now on. If you use a trim tab on the elevator make sure you make it big enough,

mine is a little small
Click to expand...

Jackscrew wanabe

Glenn
 
I have often wondered why Piper did it on the vagabond other than maybe cheaper to produce. I realize it was their airplane to ease them financially and get them back in the black but if you look at an original vagabond trim tab they aren't an easy deal to make. They are not hinged at the front like the wag aero plans depict. At their widest point they are about 5 " wide and hinged at half way point on their chord line. I don't see it easier to make over a jack screw set up which they had already built thousands of and then they went right to the jack screw on the clipper. I bought a set of tipping dies for my bead roller to reproduce the vagabond trim but I haven't tried it yet. My 2 cents worth for free.
 
Cub junkie said:
I have often wondered why Piper did it on the vagabond other than maybe cheaper to produce. I realize it was their airplane to ease them financially and get them back in the black but if you look at an original vagabond trim tab they aren't an easy deal to make. They are not hinged at the front like the wag aero plans depict. At their widest point they are about 5 " wide and hinged at half way point on their chord line. I don't see it easier to make over a jack screw set up which they had already built thousands of and then they went right to the jack screw on the clipper. I bought a set of tipping dies for my bead roller to reproduce the vagabond trim but I haven't tried it yet. My 2 cents worth for free.
Click to expand...

Oragami sheet metal is cheap. Jackscrew has 2 machined expensive parts

Glenn
 
Why are you guys arguing over which is the better method of trimming?

Just man-up, eat your Wheaties and forget the trim wheel! :wink:
 
Cub Builder said:
That's also been my experience, the broken cable incident not withstanding. It works well heavy, light, forward CG or aft CG. With the trim tab, I don't have other cables and springs (that can fail) attached to the elevator controls. The majority of light planes out there have trim tabs, so I'm thinking it's not such a terrible solution. But Wag screwed up their design with the single bowden cable used to push and pull. It was pushing with it trimmed nose down that caused the cable to bend, fatigue, and break. Replacing it with a Ray Allen servo and putting the trim control in the stick grips is a permanent (and lighter) solution.

-Cub Builder
Click to expand...
I'd be interested in seeing where you mounted the servo and how it works. Thinking of doing the same.
 
We took out the push pull cable on the trim too, that was not a good solution between your legs on the floor. I have the ray allen servo now and like it much better. The only thing different is that I was cautioned by a friend to not put it on the stick (don't remember his reasoning) so we put a switch on the left next to the throttle that you can reach with your index finger. I really like that. I don't find that you have to use the trim all that much as well.
 
You guys continually manage to scare me with your haphazard approaches to trim designs. All of you appear to have no idea of the consequences of flutter or the little design features which can help to prevent it. I have discussed this extensively in other threads.

These latest designs above need a question to be answered. Which direction is the tab going to be deflected during high speed flight? Then, is that flimsy push/pull wire pulling or pushing to hold the tab in that position? IT SHOULD BE PULLING to prevent it from buckling under compression loads.

IF that tab can produce a vibration at high speed, that vibration can move into the flimsy push rod/wire causing the rod/wire to vibrate. This vibration causes minor flexing/bending which can buckle under the high load.

All sorts of things can initiate the ability of the vibrations to occur.

THINK THIS THROUGH!
 
skywagon8a said:
You guys continually manage to scare me with your haphazard approaches to trim designs. All of you appear to have no idea of the consequences of flutter or the little design features which can help to prevent it. I have discussed this extensively in other threads.
Click to expand...

Actually, the haphazard design of using a single push/pull bowden cable is in the Wag Aero plans. A couple of us have been bit by building per plans. A number of the Wag Aero Cub Clones are out there, which is why we are having a discussion about it.

skywagon8a said:
These latest designs above need a question to be answered. Which direction is the tab going to be deflected during high speed flight? Then, is that flimsy push/pull wire pulling or pushing to hold the tab in that position? IT SHOULD BE PULLING to prevent it from buckling under compression loads.

IF that tab can produce a vibration at high speed, that vibration can move into the flimsy push rod/wire causing the rod/wire to vibrate. This vibration causes minor flexing/bending which can buckle under the high load.

All sorts of things can initiate the ability of the vibrations to occur.

THINK THIS THROUGH!
Click to expand...

You are absolutely correct. You don't push with a cable. I've done this long enough that I should have known better, but figured Wag had a clue what they were doing, so built that control to plans. As I said, I should have known better. This design appears to be copied from an Aeronca... almost. If you look at the Aeroncas, they use two cables for a pull/pull design. First clue to failure was a slow speed fore and aft cycling in the stick while trimmed nose down. That was the cable flexing back and forth just prior to failure. Had I have slowed down and trimmed nose up at that point, all would have been well. Once it failed, the flutter was high speed and violent. As for the electric trim, I used a 10/32 push/pull rod, which is the heavy duty option rod used with the Ray Allen trim. The Ray Allen trim system has been well proven and is used on many, if not most, Experimentals. Failures are pretty rare in that I have never heard of one actually breaking.

I have also seen this happen on a Pegasair under the same circumstances. I was unable to convince the builder/owner to change to either a pull/pull cable design or an electric trim servo. He chose to use a larger diameter bowden cable instead, and apparently doesn't plan to fly expedited approaches any more. IMHO, not a wise choice, but it's not my decision to make.

-Cub Builder
 
The wire push pull on a certified Pitts S1 always made me wonder. The system uses a bare .071 music wire in a secured aluminum tube from the cockpit lever to the rear of the fuselage then it's encased in a bowden flex housing to the tab on the elevator. I built an EAB Pitts to the same specs and dove it to 210 with no problems. Glad I didn't know all this when I built it in 1983.
 
My Ray Allen actuator on my experimental has worked flawlessly for 1800 hrs. now. Simple and light also. Pretty sure I could take off, fly, and land using just pitch trim, that powerful and that fast responding.
 
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