Yes, I know I should know this but the more I try to figure it out, the more confused I get. Low wing airplane flying upright. Elevator has a bit of nose-down trim. That is, the rear of the elevator is deflected down about a degree from being in line with the stab. Do I shim the trailing edge of the wing up (towards the fuselage center line) or down. Along with the correct answer, I would appreciate the reasoning!

Before I can answer that question I have to ask a couple questions of you. Are you sure the CG is where you want it? Have you measured the airplane and know where your incidences and thrust angles are? Properly trimming and airplane starts with having it built strait and having the CG set where the airplane is flying close to neutral then the fun can begin.

Simple answer is to raise the trailing edge to reduce the wing incidence, this should get rid of the down elevator trim.

However as speedracerntrixie pointed out, this may affect other areas of the flight envelope.

My F3A ship only has an adjustable stab and needed a heap of up elevator from out of the box, so I wound in a heap of negative incidence on the tailplane to get rid of most of it, which fixed some other issues it was having and created a few more that took some twiddling of the motor to sort out. Running up or down elevator trim isn't a bad thing unless it's noticably causing bad things to happen...

The plane is trimmed and flies nose down. The plane is trimmed and flies nose up. Either case, what you're interested in doing is to get the plane to fly level, right.

So move the fuselage relative to the wing to level it out. The elevator trim will change unless you change it's incidence angle the same amount you change the wing incidence angle, of course.

I think the CG is pretty close. It is at the recommended location and the plane (World Models Chipmunk) flies inverted with slight down elevator pressure. It slows well for landing and is easy to flare to a 3-point landing. I take this to mean the CG is about right. Haven't measure the plane. I bought it used a long time ago and finally decided to try it out and decide if it is a keeper. It flies really well and it's a keeper. Downthrust must be pretty close as when I throttle up, it just speeds up with little change in pitch. I just realized that all of this has been done with the gear down and should be done with the gear up (except the landing ).

I realize it is not a pattern plane, like my Focus Sport, but it flies surprisingly well and is a lot of fun.

As near as I can judge, the fuselage is level when flying. However, it is hard to see a small difference.

A good thing to remember! The deflection is pretty small and would not be noticed except the ends of the elevator extend to the front of the stab (not sure what they call that) so you can see a small offset ~1/16".

If you need down trim for the model to fly straight and level then the wing has too much positive incidence, therefore you need to raise the trailing edge of the wing to reduce it. It will be pretty much a trial and error operation so do it in small increments. But considering you said that the model flies quite well if it was mine I would more than likely leave it alone for just a 1/16" out of line at the outer ends of the counter balance.

There is a lot of mis-understanding when it comes to trimming a model. One has to remember that one change or one thing out of whack will influence several things. personally I find it funny that people offer suggestions without knowing some important information. The first thing to do is measure the airplane so that you know where your starting point is. The other thing to consider is that this airplane has a semi-symmetrical airfoil. The stab is not adjustable so what I would do in this situation is to put a level on the stab and block up the tailwheel until the stab levels. Next take a strip of something strait ( I use a peice if 1/8" X 1/2" aluminum ) and clamp between thrust washer and prop washer. This will give you a good flat surface to place the level and see if your thrust angle is 90 degrees to the stab. If not shim until it is. Having this 90 degree relationship will show up during verticals and knife edge. Now if the airplane had a symmetrical airfoil I would say set it to +.5 degree. With this particular airplane I would start at zero degrees. Reason for this is that the semi-symmetrical does not require positive AOA to produce enough lift to support the airplane. Once you have these angles set test fly the airplane and see where the pitch trim ends up at 3/4 throttle. here is where CG comes into play. If it takes up trim at 3/4 throttle and climbs at full throttle then you are nose heavy. If it takes down trim at 3/4 throttle and dives at full throttle then you are tail heavy. The idea would be to adjust CG until no elevator trim is required at all and the airplane flys the same line from 3/4 to full power. Once that is acheived fly some verticals to see if it pulls or tucks. If it pulls you need to adjust the wing positive and reset CG for neutral trim. If it tucks you need to adjust the wing positive and reset CG for neutral trim. Once you are close you can fine tune this with engine thrust. Most airplanes will tuck in knife edge. An airplane with an aft CG will do it worse. If you move the CG forward enough to eliminate the tuck then the forward CG will throw other things out. The fix is to run a mix but if the mix gives more then 1/8" up elevator travel with full rudder then the CG is aft. Lots to digest but in reality just a frtaction of trimming an airplane. Being that you are in Sutter Creek I would be happy to invite you flying this Sunday. I know it is fathers day and for most that is an issue but I will be at the SAM's field in Rancho Cordova Sunday moring to afternoon.

Thanks for the offer speedracertrixie, but the "problem" has been solved. I'd put some sections of fuel tubing on the wing hold down screws to keep them from falling out. They were holding the TE of the wing up a bit (away from the fuse). Noticed that this morning, shortened the sections of tubing and snugged the wing into place. Re-trimmed and the elevator is now aligned with the stab.

I'm not sure I follow your suggestions as it seems you are saying to put in upthrust. Generally every plane I've flown needed down thrust unless the crankshaft, wing and stab were on the same plane. If we ever get a day when the air here is less turbulent, I'll dig out my trimming chart and check it out, remembering that this is a semi-scale sport plane and not a pattern plane. But I have to say it sure is responsive and fun to fly. The limit is still my skills, not the plane!

Some of my aerobatic models have actually ended up with a tiny amount of up thrust. I however am not recommending that to you. The difference here is the airfoil. My race planes have a semi- symmetrical airfoil like your Chippy. I run those at zero/ zero/zero. If you have given it down thrust to compensate for climbing at full throttle your wing to stab incedences are off. Have you measured the airplane yet?

First of all, I am quite happy with the way the plane flies and now that I found why I had ~1/16" down elevator, I'm satisfied.

However, just for grins, I checked the stab to prop alignment as you suggested. To within 0.25 degrees, after about 10 measurements, the prop is at 90 degrees to the stab. However, I do have to note that walking across the floor of my shop from the tail of the plane to the front, with the gauge on the stab, I can get up to a 0.20 degree random variation. I get about the same from one side of the stab to the other (I use a block of wood across the stab to reduce deflections from the weight of the gauge.) I've not done a thorough statistical analysis of the data but think this is good enough.

Bottom line is if you are happy with the way it is flying then it is all good. I have to admit that after 100+ aerobatic contests I have very high expectations on my airplanes. It has taken some fun out of the hobby but gotta feed the OCD. LOL.

I know some of what your mean. I started flying the Focus Sport over a year ago to try to improve my skills. A month or so ago I was getting rather tired of the continual practice of "making it better" so decided to get out a "fun" plane that had been sitting in my garage unused since I bought it maybe 12 years ago. It is a lot of fun with the retracts and flaps but I know that in a week or two, I'll be flying the Focus sport again. This time though I will trade back and forth. I know that continual and constant practice with the same plane is the way to do well at contests but I'm not into competition (other than with myself!). I do want to improve my skills and the Focus sport has definitely helped with that.