Rudder Trim Tab...NOT!

[VictorBravo]

If this was posted before, please disregard.

I just wanted to share a little tidbit with the group. Since I've owned N29544, I had to hold a couple of pounds of right rudder pressure to keep it straight. I just lived with it, rather than put on a bent metal tab. I took a look at the wing rigging, I took a look to see if the fuselage was bent, I took a look to see if the ailerons were bent, etc.

I finally found what I believe is the problem. The previous restorer used two equal length rudder pedal return springs, even tough the left and right pivot points are about an inch apart in the fore-aft axis.

Sure enough the left side spring was in more tension because that pedal pivot point was further aft.

An immediate appointment was scheduled with a pair of diagonal cutters and pliers. Next flight will tell...

[n6346m]

Re: Rudder Trim Tab...NOT!

Bill,
I too had a rudder trim problem with my L-2B. Even with the return springs disconnected I needed some rudder input to keep the ball centered. I do not have any fixed trim tabs on the ailerons or rudder and I don't think any are needed on a properly rigged airplane. I cured my problem by simply removing one link of my tailwheel chain from on side and added to the other and voila a perfectly trimmed airplane. That tailwheel makes a pretty effective trim tab.

Bob

[flyguy]

Re: Rudder Trim Tab...NOT!

Now that is clever.
DC

[alwaysoar]

Re: Rudder Trim Tab...NOT!

BC12-D's came from the factory with rudder trim tabs. Way back, Forrest sent me a template for a trim tab, told me I'd find three small holes in the rudder. I did, I made one, and I screwed it on. But aileron tabs, NEVER

[M Towsley]

Re: Rudder Trim Tab...NOT!

The L-2 had a rudder tab also, though I wasn't as lucky as Ed was in finding the holes, (cuz they weren't there, figures....). I have a full-sized mockup I can send to anyone who needs it, I'll just make a photo-copy and send it to you. I think they even indicated how many degrees to make it.

[flyguy]

Re: Rudder Trim Tab...NOT!

Finally got mine where it will fly hands off at 2300 RPM. Drop down to 2150 and it takes just a hint of left rudder. I still find myself flying with the left wing up and the ball off to the right if I'm not really paying that much attention. The darn curve on the nose throws me off if I'm just flying instinct. I've been tempted to put some kind of straight line up there for reference, or maybe a little dot on the windshield straight across from the center top of the nose.
DC

[M Towsley]

Re: Rudder Trim Tab...NOT!

Hey DC,

Fly an L-2, its always in the center!

[N74DV]

Re: Rudder Trim Tab...NOT!

BC12-D's came from the factory with rudder trim tabs. Way back, Forrest sent me a template for a trim tab, told me I'd find three small holes in the rudder. I did

hmmm... where might those holes be found? is the tab pop riveted in place??

[Dick Huish]

Re: Rudder Trim Tab...NOT!

They are held on with 1/4" #4PK screws. You need the tab to locate the holes. They'll be on the left side of the rudder at the lower rib of the rudder. Dick

[M Towsley]

Re: Rudder Trim Tab...NOT!

DJ.

You can run your finger along the left side of the rudder's lower tube and find two of them. They are slightly triangulated. The center one will be slightly forward of those two on a fore and aft tube.

[flyguy]

Re: Rudder Trim Tab...NOT!

Just thinking about it. I'll bet getting the vertical stabilizer offset at just the right angle to be near to correct for cruise flight required some pretty clever designing and probably more than a little trial and error testing. All three of my Taylorcrafts required a similar left hand bend in the tab to give a little more right rudder at cruise once the wings were tweaked in. Because of the mounting of the tab on the left side of the rudder the vertical stabilizer appears to have been designed to utilize the most efficient functioning of the tab; ie, always bent to the left for a better airflow as opposed to a right bend that would have the airflow blocked by the rudder tube that it is mounted on. Airplanes in general and Taylorcrafts in particular never cease to amaze me.
Darryl

[VictorBravo]

Re: Rudder Trim Tab...NOT!

I'll bet getting the vertical stabilizer offset at just the right angle to be near to correct for cruise flight required some pretty clever designing and probably more than a little trial and error testing.

IMHO vertical fin offset is the wrong way to go. But then, as I have pointed out more than once, I am an old free-flight model airplane type of fellow

I suspect they did it at the fin because they didn't want to trust the mechanics to have it set up correctly at the engine mount every time the rubber cones were changed.

The way it's SUPPOSED to work, in my opinion, is that the torque is balanced out by having offset in the thrust line. If you can accomplish that, then the airplane can be set up straight.

A straight airplane (no warps, no fin offset, no bends) will stay straight at any speed power off, from a VNE power off dive to a power off stall.

Proper engine offset will counteract torque at any power setting, meaning that at full power there is full torque and also that power is pulling it to the right to offset that torque. At low power there's little torque, but the engine offset doesn't pull hard either.

So ideally you could increase or decrease power at any speed and have the airplane go straight.

Using fin offset (and no thrust offset), if you are in a fast cruise at low power the fin will turn the plane to the right more than the torque pulls it to the left. So you need left rudder input. At slow climb with full power, the engine is pulling left more than the slow airflow over the fin is pulling it to the right. So you need right rudder input.

Just a thought from a guy who still smells of Nitrate dope and balsa dust and rubber motor lube

[Bobdog]

Re: Rudder Trim Tab...NOT!

I'm with Bill on this one. Piper used quite a bit of engine offset on the PA-28 Cherokee. As I recall the engine points down and to the left. The fist time I noticed it, I thought I had a bent motor mount!

[high time cub]

Re: Rudder Trim Tab...NOT!

Just a thought from a guy who still smells of rubber motor lube

Ode à glycerin & green soap?