Rudder vertical alignment

[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!