Rigging

I've looked through the archives a bit but have not found what I am looking for.

Aircraft 1st flight. Hands off it flies solid level. The ball is just under 1/2 out on the left side though. When I step on the ball I get a gradual left bank. Chase plane says ailerons both look the same, matched up with the tips. Flaps trail what looks to be fine as well. When I weighed the plane, the left main was 9 lbs heavier than the right (I don't think it is unusual though).

Is there a way to adjust tail or washout to get flying straight, or am I going to have to put a tab on the rudder?

Thanks

Brent

Brent,
Don't put a tab on the rudder. I was about to do this until somebody showed me the correct way to resolve this. build a big gust-lock looking device out of a couple of 2 by 4's, about 5 or 6 feet long. Slip it over the vertical stabilizer, and bend it in the direction desired. I understand this is what Piper did at the factory. Did it to mine, and it flies as straight as an arrow........no tab!

Thanks Ron. I like this idea a lot. Did you make the bend at the middle and top of the vertical? Or did you just grab it in the middle? I bet a little bend goes a long way.

So if I need to step on the left rudder peddle to center the ball, that means I bend the front of the vertical to the right. Do you concur?

After the bend did you need to adjust washout to get it to fly wings level. Mine does now but wanders to the left when ball is centered.

Thanks

Brent

Before doing anything else, I'd level the plane on the ground and be certain the bubble is centered then. I had the same trouble you're describing, and discovered that the turn & bank instrument was installed crooked!

That's a good idea Gordon. If it does turn out to need rigging, I would bend the vertical stabilizer, and then adjust the washout, as you say. I concur with the direction that you would apply the correction. (Had to think about it for a minute). I honestly wasn't there to watch the process when it was done to mine. Dan's at Merrill Field did it for me. At first, he didn't want to tell me what he had done, but I talked it out of him. I assume you would adjust somewhere near the top, but not so much that the fabric pooches out on one side.

Wait: It flies level, hands-off, with wings equi-distant from the horizon, and it is not turning?

Try adjusting your ball instrument.

You know, I installed that instrument so long ago that I'm not certain the plane was level when I did it. I think I would have but don't remember now. Good catch. I'll start there. 1/2 ball is quite a bit however.

Hands off the plane is level with the horizon and has no tendency to drop a wing. When I apply left rudder to center the ball the plane does drop the left wing, but only slightly.

Brent

One other thing - if the dihedral is unequal the wings would tend to fly level, but with the fuselage tipped.

Again, level the plane as accurately as possible by playing with tire pressure (roll axis should be exact, and pitch axis should be close). You'll have to decide what to define as level; a straight transverse tube, or the tops of the front spar caps at the wing root.

Stretch a tight string from wingtip to wingtip, exactly parallel to the front spar, and over the centerline of the front spar. Then measure to the top surface of the front spar cap at the wing root. The measurement should be equal on both sides.

I would check for equal dihedral, equal washout, and the instrument alignment before changing anything else.

Disclaimer: I am NOT an A&P. However I recently went through this process with my IA looking over my shoulder - - -.

This is the order I rig... (hope i haven't forgot anything..easier for me to do than to type up)

This way it's quick and keeps the chasing things to one or two adjustments usually, and you are Done...(unless its really bent, but this will average that out also...)

Basically do what the books says, with a few extra checks/averages....

FIRST! make sure.. or at least an attempt at the bottoms of flaps and ailerons being level with bottom of wing using a strait edge...... don't care if the ends don't line up on side of fuselage or tip, we only care how it will fly.( they will line up if the tip and fuselage gingerbread are in right place)

Set fin so if you were to look forward at where top of it points, it would end about 1 inch to left wing side of center skylight Chanel at front spar.

Do the string and get the dihedral height right...

Set up plane to approx level flight for now..

**Then use level on fuselage side to side..find happy medium, across: spar attach points, gear fittings, engine mount studs.. sometimes you need to look behind gear area to find a untweaked area.... just find what you will now on call level, it may have to be an average... and adjust plane to it..

**This is now where you note how much T&B bubble is off... write this down... half bubble to the left or whatever... you could set plane down and adjust this now, but I wait till I have the plane down for another step.....

Now use the level at about strut attach area on leading edge bottom..... set plane down, turn one front strut in same amount as you turn other front strut out to make level read same amount off level on each wing..

**Now to set plane up in level flight, I use 1 degree trailing edge down at root rib(which I got off a virgin piper fuselage), check both root ribs, and split the difference. there are other thoughts on where level is..... but this has always worked great....my $0.02....... note. Univair rear PA-18 spars have wing attach holes about an eighth inch to high or low...cant remember which...

Check twist with the 3/8" block and adjust with rear struts as needed...
.....
As for what to fix/chase FIRST after test flights, we know the ball is telling the truth now, so adjust fin to make it right( use top wire to move tail post some, bend fin some also)
only concern yourself with which wing is heavy when ball is centered... (by foot pressure or rigged right), for rigging we don't care what it does when the ball is not centered....

**Sometimes its nice..or scary to measure from rear strut attach bolt ends on wing to the tail spring mount bolt to see how crooked wings are on plane, bent, or trammed wrong, if you are still having trouble..... have seen 5" & 6" difference side to side on a few flying planes.... but it does make the rigging rules kinda moot making one of these behave... a little over 1" is about normal... have only seen 1 plane in almost 20 years with no difference....

Mike/Gordon

Thank you for the detailed response. Now that I'm done building I really miss the process and need a good project like this to keep me busy... and of course flying. I'll delve back into the rigging as shown above to see if I can't get her flying straight.

Thanks again.

Brent

Speeds

Make sure that you do your testing at your desired cruise speed. Because the vert fin is offset ( The one inch when looking from the rear to the top of the cabin ) your ball position will change with speed. If you change a prop and go faster or slower or just get in a hurry, the ball will change accordingly. Jerry B.

My little J-3 has a top speed close to the stall, so the ball doesn't seem to move at different cruise speeds. Come to think of it, the old electronic ball didn't move at different cruise speeds in the Airbus, either.

I could be wrong, but it seems to me the ball ought to be centered anytime the wings are level and the aircraft is on a steady heading. At any speed.

Did that Airbus have rudder trim? :lol:

I know exactly what you are talking about Jerry. Steve at Steve's A/C designed rudder trim for the Pacer for just that reason.

No, it had all kinds of computers to keep it going straight. But I think the ball worked the same as in my Cub. I have never seen an aircraft that would fly straight and level with the ball in different spots at different airspeeds.

I agree that an aircraft that will fly straight and level hands and feet off at one speed will not necessarily do so at some other speed. But once it is straight and level, the ball will be centered. Opinion.

Speed

Hi Bob. Are we not saying the same thing? At those other speeds the ball is not centered. You have a elevator trim speed. You set the trim for the speed you want to travel. There is also a trim speed for the vertical fin. That's why the big ones have cockpit adjustable trim tabs on the rudder. We just fly a little crooked when not at that speed. Fly faster it will turn right. Or if you hold the wings level the ball will just be out a bit. Look at it this way. If you have a fixed rudder that is not aligned with your direction of flight ( the vert stab ) different speeds give it different forces. :-? Jerry B.

No Jerry - we are talking past each other. Go back to the original problem - the poster, as I understand it, had an aircraft that, in straight and level flight, had the ball off center. You are talking about an aircraft that is turning with the wings level - in a skid - the cure for which is one of three things: rudder, rudder trim, or fin adjustment.

These, to me, are two totally different problems.

One poster suggested checking dihedral - I agree; it is either that or the instrument is mounted at an angle. We use the "smart level" to check such things.

Think about it - if your wings are on straight with respect to the instrument panel, and you fly with the wings level, and you are not turning (skidding), the ball has to point down. F= GmMe/r2, or something?

Another thing to check is the compression of the engine mount rubbers. Make sure they're all the same. If tightened too much (compressed) on one side verse the other, you'll have a thrustline (left to right) deflection that can cause it to fly slightly off center.

While we're on that point...If correcting the fin angle doesn't do the trick, an 1/8" shim (.0125 machine washer) under the engine mount firewall attachment points (top and bottom, one side) on the opposite side of the pull will usually give some good results.

On a lot of older airframes, the firewall is not perfectly square to the rest of the plane, especially if it's ever been crashed or stood on it's nose.

Take care.

Crash

???

Hi Bob. I am also talking about straight and level flight. Even hands off for a while. But the fin will usually win. Or as Crash says even the Motor mount. Mine is set up so the ball is centered at gross wt at cruise. Ratting around local with little weight the ball is 1/4 to 1/2 off center in straight and level non turning flight. And I'm going a lot faster. On long flights (at gross or above) I would like it to fly as straight as possible. Around here I don't much care. As long as you have engine torque trying to roll the airplane and AOI in the wings to counter the torque and a crooked fin trying to counter the uneven AOI it's just not going to be a perfect world. Sorry! Perhaps you will explain why the darn fin is on crooked in the first place or why it should fly straight at all speeds with a crooked fin? My suggestion is to load the airplane then fly it at the desired speed, then adjust the ball in flight until centered. At any other speed it will have to be centered with rudder. :-? Jerry B.

You still misunderstand me. What force is it that forces the ball off center if your wings are level, and you are proceeding in a straight line? It cannot be acceleration, since you are not accelerating, by definition. Let's say for argument that your fin and motor mounts mandate, say, a ten degree fuselage angle with wings level. Sure, that slows you down, but how does it force the ball off center?

If you say you cruise at certain speeds with one wing down, then that's a different story. I have a buddy with a Cherokee that cannot be flown with the wings level. No force on the ball from some acceleration, and wings level, the ball should be centered. Rudder and engine angle should have nothing to do with it.

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Who's really straight ?

OK try this. You can carry two levels in the cockpit for testing purposes. One to measure wings level and the other for pitch. But there is nothing to determine how straight the airplane is flying. Compass or GPS won't do it. I have flown behind Cubs with uneven elevators, cocked tailwheel, whole tail cocked to one side or twisted. You can't see it well on the ground but it sure is visible in the air. And they all thought they were flying straight. The slightest mismatch anywhere will put the ball out of center and you can be convinced you are straight and level. Have somebody follow you is the best way I know. The ball is not stupid. You have to interpret which one of a thousand things it's trying to tell you. Yes even that it's mounted crooked. Jerry B.

What an interesting thread. Another reminder of how much I don't know. (that reminder is a GoodThing)

FYI my balls are OK, but when straight/level (hands off) my bank indicator (needle) shows a slight left bank. No biggie, but is that an indicator fault or can it be one of those "thousand things" that it's trying to lie to me?

...a thank-you nod to the Gurus

My balls are OK too.

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NimpoCub,
Is the needle centered when you are on the ground and not moving with the gyro spinning? That is a rate gyro and it only indicates yaw in progress. When yawing stops the needle centers itself. If it is centered when you're not moving and off center in flight, then you are turning, banking or not. If it is off center on the ground when you're not moving, then the centering springs are out of adjustment. ...Clyde Davis

TJ what in the heck are you doin' workin' on them nice cream puff's? Lot easier on the nerves workin' on them beaters. I think the part about bending the vertical stab is in the rigging SI. I think that was written in the days of dope. Luckily I haven't messed one up yet but I still get a little nervous when I do it.

Hi Clyde,
It's off when sitting idle on the ground, & I thought that was OK, but while it moves in flight it still shows a left yaw while my ass/nose (& ball) show me flyin' straight, level & happy. Are these springs adjustable (by me) to center it on the ground? I'd like that.

Hi TJ,
I don't have a 140K cub, but it flies straight. I think the center springs on my turn/bank thingie need adjustment... but I'll trade ya!!

I think it's a job for an instrument shop. ...Clyde

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Another thing to check is the compression of the engine mount rubbers. Make sure they're all the same. If tightened too much (compressed) on one side verse the other, you'll have a thrustline (left to right) deflection that can cause it to fly slightly off center.

While we're on that point...If correcting the fin angle doesn't do the trick, an 1/8" shim (.0125 machine washer) under the engine mount firewall attachment points (top and bottom, one side) on the opposite side of the pull will usually give some good results.

On a lot of older airframes, the firewall is not perfectly square to the rest of the plane, especially if it's ever been crashed or stood on it's nose.

Take care.

Crash


So Crash , Let me get this straight ,if the nose of the plane yaws to the left (need right rudder for straight flight) I should put the 1/8" washers on the left side of the motor mount ??

Thanks, Erik

Yes, that is correct.

First I'd try to tweak (hard to do with fabric on, I've tried) the front of the fin (to the left). Then the only other two things I can think of is a trim tab on the rudder or shimming one side of the engine mount.

I don't like to looks of a trim tab on the rudder but I see a few Cubs with them.

Before I tackled the yaw problem I would double check the rigging starting back at dihedral and washout. If this is right and you have no roll when you let go of the stick and you don't have a dragging low flap or aileron on one side or the other (hinges mounted too low), then you need to correct it at one end or the other of the fuselage (i.e. fin or left/right engine thrust line)

Take care.

Crash