View Full Version : Fine Tuning C.G.

04-19-2006, 04:01 AM
If your plane, while gliding, goes swoop, swoop, swoop, is that indicative of tail heaviness or nose heaviness?

04-19-2006, 04:25 AM
Your description is too vague. I'll describe how I fine-tune my CG. Unfortunately, if your motor is out of trim it can cause similar symptoms, but it's really tough to figure out a motor alignment problem if your CG is off. It's helpful to do these tests on a day with very little wind. I normally take 15-20 flights on a bird to finally get it trimmed perfectly.

Step 1: Basic trimming.
Fly like you usually do. Make sure you fly without any weird roll or yaw tendencies across a wide variety of throttle settings. In particular, make sure your motor thrust angle is not pulling you left or right at full throttle. Pitching up and down we will deal with after we fine-tune the CG, as long as it's not a huge, obvious problem. Like on my P-47, the moment I throttled up the nose would go down, even if I'd picked up no speed yet. That was a pretty obvious thrust-angle issue I had to correct before I could fine-tune my CG. If you require more than a couple of clicks of trim in any direction to stay in trim, land your bird and adjust the mechanical linkages themselves so that your plane is just about perfectly in "trim" without needing trim adjustments on the transmitter.

Step 2: Floating.
At about "three mistakes high", idle down to as slow as you can go without losing altitude and maintain level flight without a significant nose-up attitude. Do a few passes back and forth to make sure that you are trimmed perfectly to fly level, hands-off, at a very slow speed, just a few knots above stall speed if possible.

Step 3: Shallow dive test
From about "three mistakes high", going at the same flat, level, slow speed as in step 2, pitch your bird down about twenty degrees while maintaining throttle. This is a fairly shallow dive, but it should be steep enough that your bird is obviously picking up speed. Watch the nose.
If the nose pitches up (the dive flattens), you are nose-heavy.
If the nose pitches down (the dive steepens), you are tail-heavy.

I know this sounds counter-intuitive, but what you are doing with this test is increasing the air-flow over your horizontal stabilizer. If your plane is nose-heavy, your elevator will be trimmed with a significant amount of "up" to keep flat and level at low speed. Increasing speed makes the nose pitch up, since the elevator gains extra authority at higher speeds.

Repeat the shallow dive test until the plane tracks along the glide path perfectly. Some people prefer a slight bit of nose-heaviness; if you are in that camp, then trim it so that rather than tracking straight in, it slowly pulls out of the dive over 2-5 seconds.

Once you have this done, if you have a flat-bottom airfoil you may actually be slightly tail-heavy. Aerodynamics at work :) It's your choice to adjust it where you want it from here, but you know you're in the right ballpark for good flight performance.

Now that we have straight and level slow-flight characteristics, and no tendency to pitch into or out of a shallow dive at low speed, let's adjust the thrust angle.

Step 4: Thrust angle and lateral CG.
You should have made sure your plane was balanced laterally on the bench. Balancing on the spinner and the tip of the rudder with the tips of your fingers, it should have no tendency to tip either way more than the other. Aerobatic aircraft will tend to tip out of this test, but as long as one side isn't favored more than the other, you're fine.
At the field, put your plane into a steep climb, preferably 45 degrees. If your plane doesn't have the power to hold this for a few seconds, you may need to enter the climb from a dive. Do this 45 degree climb repeatedly (at least 5-10 times), with enough power to fall out of the climb after 2-5 seconds. Does the plane tend to fall out in a particular direction each time?
If it tips on the right wing consistently, either you are not laterally balanced or you have too much right thrust on your motor. Use washers or some sort of shim to correct the thrust angle to the left.
If it tips to the left, do the same but angle the motor right.
If it falls on the nose consistently from 45 degrees, you probably have too much down-thrust on your motor. Use a shim or washer to trim it upwards.
If the steepness of the climb increases on its own until the plane falls out towards the canopy, you are either tail-heavy or have too much up-thrust on your motor. Adjust your CG or use shims to add more down-thrust.

There are more trim tests you can do to adjust roll characteristics and other things, but that will do to get you a good-flying sport plane for now.

04-20-2006, 03:06 AM
You have done a great service today for a lot of intermediate pilots. Your explanations are concise and easily understood. I'll copy your post off and take it with me to the field until I can memorize it. I just wish such instructions came with aircraft. We really need a guy like you. Thanks so much.
Bud (the cracker)
Oh, and by the way...swoop, swoop, swoop means balloon up to a mild stall, and then again, and again, etc.

Doc Pete
04-20-2006, 03:34 AM
Oh, and by the way...swoop, swoop, swoop means balloon up to a mild stall, and then again, and again, etc.

If you are serious about the hobby, you should invest in an incidence meter. The wrong incidence on the wing or elevator can make a good model fly poorly.

04-20-2006, 02:34 PM
The wrong incidence on the wing or elevator can make a good model fly poorly.
Dude, I TOTALLY forgot about that possibility in my instructions for balancing CG. If your incidence and/or build angles are off, ALL BETS are off when it comes to tuning for performance!

I have a little J-3 cub with an elevator badly canted to one side. Pull up, and she wants to nose right as well! She fails all the glide tests and thrust-angle tests.

I realized after my earlier post that this is, in fact, a sailplane forum and not so much about powered flight necessarily :) For most sailplanes which aren't powered, of course you can't really do the "climb and fall out" test. That really mostly measures thrust angle and lateral balance, anyway.

Doc Pete
04-20-2006, 03:25 PM
Dude, I TOTALLY forgot about that possibility in my instructions for balancing CG. If your incidence and/or build angles are off, ALL BETS are off when it comes to tuning for performance!
measures thrust angle and lateral balance, anyway.

Sorry, I guess that's the curse of being in this hobby for 50 years.:D I hate remembering "too much";)
Now it they just brought back those great adventures of "Sky King and his Songbird".......:p

04-23-2006, 03:46 AM
How does one use an incidence meter? How does one know what the target incidence is?

04-25-2006, 09:14 AM
You just need to worry that the indicence of your wing and elevator match (by and large). For instance, on my Great Planes Spirit ARF, the elevator has some positive incidence compared to the wing. When the plane is running with a good CG, this means there's actually a little bit of down elevator in order to keep the elevator "level" relative to the wing.

If you lack an incidence meter, don't sweat it. Get your CG issues ironed out, and then you can tell a good deal about your incidence by looking at the trim of your elevator once you have your plane reasonably dialed in. Even with down or up elevator, if it's just compensating for incidence, you'll still pass the trim dive tests. Your plane will just be a bit draggy is all.