Rudder vertical alignment

[Meyertheflyer]

After attaching the rudder to the vertical stab, I noticed a very slight left alignment near the upper 20% of the rudder trailing edge. I bent the top frame to try and make it perfect but still has a little left of center line. It this so critical? The lower remaining 80% is perfect. I'm thinking this a 100 kt airplane and I won't notice, but I wonder what other builders thoughts are.

[Meyertheflyer]

I took a picture looking at the rudder from behind.

[Dave S]

Robert,

The rudder on my series 7 is about the same. I didn't do anything with it, assembled/covered it that way and have been flying it for 4 years with absolutely no noticeable effect.

Sincerely,

Dave S
KF7 Trigear - Flying
912ULS, Warp Drive

[Esser]

My gut feeling is that is there from the factory to counter act torque in flight.

[jtpitkin06]

Yes, you need to do a little straightening. It's not uncommon for welded frames to warp and twist a bit as they cool from the welding process.

Fortunately, it is not difficult to align the rudder. Simply note where you need to bend and mark it with a piece of tape. I see in the photo you have a long level which you can use as a straight edge to check your progress. To straighten the rudder and control how much you bend for correction, place it on the floor and put wood blocks or shims under the tubing then using gentle foot pressure make the corrections. 4130 tubing can be a bit springy so don't be surprised if it takes a lot of opposite bend to get things straight. Just start small and use more only if necessary.

Sometimes you will only need a single block or shim under one corner. Other times you may need four. When you think you have it straight place it on the flat floor to check for any rocking motion. Then flip it over and check it again.

To answer the question of whether you need to fix it for a 100 knot airplane... just hold a clipboard out the car window at 55 mph and see how much lift it gets. Now double that for 100 knots. You might look at some of the pictures posted on rudder trim tabs to see how small they are. They are very powerful; so yes, the rudder needs to be straight.

You're on the right track... just spend a little more time on the details.

John

[n85ae]

My vertical stab was worse, it's just the normal 4130 cooling and moving
after welding. Nothing a 2x4 and some muscle couldn't fix. Don't be afraid
to bend it. It's a lot tougher than you realize and you won't hurt it.

Regards,
Jeff

[Meyertheflyer]

I bent the rudder and made it straight. Not too hard. Then today I looked at 3 other foxes to compare tails and none is perfectly straight , but now mine is (or at least as straight as I could get it). I tend to think a little bend is necessary to compensate for the P-factor but ill find out when the flight day finally arrives. Thanks!

[ken nougaret]

mine was off too. i ended up welding material to the tabs and filing new holes to properly align the rudder.
ken

[jtpitkin06]

I tend to think a little bend is necessary to compensate for the P-factor but ill find out when the flight day finally arrives. Thanks!

Just wondering... how do you plan to correct the bend after the airplane is completed if it is too much compensation? Would it be better to build the airplane straight and simply angle the engine mount as needed with shim washers during the testing phase?

John

[Meyertheflyer]

Yes, that would be the better plan. I'm bonding the rudder fiberglass leading edge today. Gotta love this weather

[jrevens]

Just wondering... how do you plan to correct the bend after the airplane is completed if it is too much compensation? Would it be better to build the airplane straight and simply angle the engine mount as needed with shim washers during the testing phase?

John

That might mess up your alignment with the cowl. It would probably be more noticable with the streamlined cowl & a large spinner.

[jtpitkin06]

Prior planning prevents....PPP

If you want to use side and down thrust; then, plan for it in advance. A one degree offset is 1/2 inch in 30. That's if you do all the shimming at the firewall. If you do your final fitting of your cowling after you mount the engine it is no different than mounting the engine with no offset.

Or you can do the offset at the shock mounts with even less movement at the prop hub. About 1/4 inch per degree.

The offset is usually not even visible to the eye on the finished airplane.

My Corvair engine has 1 - 1/2 degree offset and 1 degree down. The offset was compensated when I built the engine mount so the spinner is on the center line. If you measure the gap on one side of the 13 inch spinner to the other you will see a difference of 1/4 inch.

all in all it's not a big deal if you plan in advance.

John

[jrevens]

I'm a big fan of offsetting the engine as necessary. Also, in my opinion, there's nothing wrong with rudder trim tabs, or trim tabs on any control surface for that matter. We're talking Kitfox here, but on the first airplane I built, I felt that 1/4" misalignment of the spinner to cowling was a big deal, but your point is well taken.

[HighWing]

Just wondering... how do you plan to correct the bend after the airplane is completed if it is too much compensation? Would it be better to build the airplane straight and simply angle the engine mount as needed with shim washers during the testing phase?

John

Being a pretty fussy builder, I tend to notice these things, but other things slip past only to be noticed after the the paint starts going on. Then, there is the tendency to point out all my mistakes to other builders who are looking at the airplane for the first time. They usually laugh saying they would never have noticed. Check the folks looking at airplanes at an airshow and see if you can find anyone looking straight on looking at alignment issues - except maybe another builder. I agree that a lot of the distortion we see is a welding phenomenon, but not much of an issue. I have found that sequencing of the weld joints can exacerbate or minimize the warpage. A guy building in the 90's saw the same thing and did some cutting and welding to fix his Series 5, so the issue is not new and it was with the 5 and later that the fixtures were the big beefy ones with certification in mind. Don't know if the effort was worth it, but the builder sure felt better.

Regarding P factor and thrust offset. None of the Kitfox engine mounts have thrust offset. Early explanations for that had to do with the two stroke's props turning opposite from the 4 stroke's, That might be a reason to keep the fuselages straight as possible, but I doubt that was the reason for the straight engine mounts as each mount was distinctly different and could easily have had thrust offset engineered into it - and in the day, there were engineers on staff. I don't know the reason but suspect simplicity. When I designed and built my engine mount for my current Model IV, I tried very hard to provide a thrust line exactly as the factory did, but with the engine 2.5 inches further forward. When I flew it for the first time, it flew exactly like my first one did before I added rudder trim. With a touch of rudder trim on the new one, it flies straight as a string. I don't think the effort made modifying the kit as it comes from the factory helps significantly, though it can make us the builders feel a bit better. I tend to defer to Dave S on this one.
Lowell

[n85ae]

Exactly, what to do. People agonize over these things, and the end
solution is some times just a simple "Grab hold and bend it". Actually
the biggest problem isn't hurting it, it's how to bend it! 4130 is some
tough stuff.

If there's a lot of crooked Kitfox tails out there, it's because people
are afraid to do the obvious ... (In my opinion.)

My hangar neighbors spent TWO days agonizing over their Murphy
Moose engine mount, because a bolt hole was off 1/4" inch. I walked
over after being asked if I could reweld it, I took one look, pulled hard
and said - Shove in a bolt! They did, and voila problem was solved.
They both stood their looking at each other feeling really stupid (made
worse because of my laughing at them.) :)

Regards,
Jeff



I bent the rudder and made it straight. Not too hard. Then today I looked at 3 other foxes to compare tails and none is perfectly straight , but now mine is (or at least as straight as I could get it). I tend to think a little bend is necessary to compensate for the P-factor but ill find out when the flight day finally arrives. Thanks!