Precision Trimming

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CONTENTS

There are three basic styles of flying in R/C.

Free style

3D

and precision or pattern flying.

     This article is directed at dialing in a model for precision flight using a computerized radio system.
     To really get your aircraft dialed in expect to make numerous flights and re-adjustments.
     If your plane is not correctly constructed all the trimming in the not make it fly with optimal precision.

     The following items should be checked and corrected if possible.

Hinges Control surfaces need to be centered and straight with the    airfoils (wings etc) they are attached to

Control horn pivot points should be setup exactly on the hinge line to assure equal travel up and down.

Servo Arms should be parallel to the hinge line.

Hinge lines should be sealed with covering on the bottom surface so no air can pass through.

Push rod slop should be minimized as much as possible.

Balance is extremely important. Check you manual or plan for the proper balance point.

Lateral Balance the aircraft by adding small amounts of weight to a light wingtip to correct it.

Initial Transmitter Setup

Select the model number you want to use
Clear the memory for the model selected
Select REV from the menu and set all the controls for the correct direction.
Select the high rate
Set all channels to the highest setting in both directions up to 150%. It is always best to use full travel on your servos. (maximum resolution of servos)

Control Surface Setup

Aileron:

	Mechanically set linkages perfectly centered with maximum throw recommended by the manufacturer.
	You should have exactly the same throw in either direction.
	If they're not exactly equal mechanically setup the lower of the two to be the correct deflection and reduce the higher direction in the ATV setup. Do this for both ailerons independently of each other.

Elevator

The procedure for elevator setup is the same as the ailerons.

Rudder Setup:

Setup the rudder to for maximum available throw.

First Flight

Trim for level flight.

Trim the rudder

Fly directly up wind

With wings level pull the nose up to the vertical.

Check for any loss of heading or roll

If there are no major heading changes, continue the pitch-up into a series of loops. Do NOT correct them but take note of which direction they fall off center. Add a click or two of rudder trim to correct and try it again. Repeat this several times until you're sure you've got the rudder trimmed as well as possible. Now land.

Ground Trim

You'll want to adjust these mechanically once you land.

Write down the trim offset for each control. Now get out your CRC Throw Meters and measure the offset of each surface in degrees. What you want to do now is adjust the linkages such that you have this offset in the surface with the trims centered. Aileron and rudder trim are really limited in what can be done to solve a problem other than linkage adjustments, lateral balance changes, or SLIGHT thrust adjustments.

For elevator though, we have much more available. If you required some up trim, you can move the CG back, change the incidence in the stab, or just adjust the linkages. For now, stick with either balance or linkage adjustments, as we'll be working on incidence and thrust changes later.

Before we get too far into flight trimming, I want to emphasize one thing. As you make trim changes, or other adjustments, it is important that you change only one thing at a time, and then fly again and retest. Each change can affect other properties, and multiple changes can only confuse the issue you're trying to fix.

Balance Setup:

Roll the airplane inverted and see how it feels. Did the nose drop dramatically during the roll? Pushing too much elevator to hold level? I use balance to adjust this generally to get the feel I want. Move the CG forward or back and retest. This should be done in SMALL increments, retesting the aircraft after each adjustment. Once you're happy with the way the airplane feels in both upright and inverted level flight, move on to the next phase.

Thrust and Incidence:

You'll hear a lot of different ideas on this, but simply put, thrust controls the vertical up lines. Flying directly into the wind, wings level, smoothly pull to the vertical and let it go. Does it pull to the belly or canopy? Does the nose pull left or right? If it pulls to the canopy, land and add a slight amount of down thrust. You could also move the CG back slightly as well. If the nose pulls right or left, add side thrust to counteract this tendency.

Note: Left or right thrust requirements can change depending on the prop used. If you change props, you may have to readjust side thrust. This is especially noticeable when changing from a 2 blade to 3 blade prop, which generally require more right thrust due to the increased spiral slipstream.

Once you're getting consistently good, straight up lines, move onto the down lines. Climb the airplane up to 500 feet or so and get into a vertical down line and let go. Does it pull out or tuck under? I generally like to correct slight down line issues with SMALL balance adjustments. If the airplane pulls out of a long down line, move the CG back slightly. This part of setup is a big juggling act, as each change affects something else. It takes awhile, but eventually you'll narrow it down.

Roll Differential Setup:

Any time you roll an airplane, the downward moving aileron generates more drag than the upward moving aileron due to the induced drag caused by the down aileron lifting that wing panel. With modern aerobatic airplanes using fully symmetrical airfoils, this is usually a very small force. When you roll most airplanes, the drag on the down aileron actually pulls the nose offline. So even though you're rolling right, the nose is going left.

From level flight, pull the nose up to 45 degrees and put in full right aileron. Does the nose go offline? Differential can help this. Select Wing Type (code 22) and adjust the differential to about 4% to start with so that the down aileron travels less than the up aileron. Now fly it again and retest. This should also be tested on a vertical up line and down line to make sure the airplane rolls axially.

Knife-Edge Coupling Setup:

Almost all aircraft exhibit some coupling between yaw, pitch, and roll. Basically, we're going to mix for moderate to high-speed flight. Slow speed knife-edge generally isn't encountered in precision aerobatics. The airplane must be properly balanced to get anything useful out of this part of setup. Airplanes like the Cap232, Extra 300S and so forth will generally pitch towards the belly of the airplane with application of rudder, while some mid-wing airplanes like the Extra 260, or Edge 540 may actually pitch to the canopy. You may or may not get some roll coupling as well.

At full throttle, level flight, roll to knife-edge and hold altitude with the rudder. Try to keep the airplane flying straight. Do you have to hold much elevator to keep it straight? What about aileron? Make a mental note of how much input is required. Keep in mind these are for small to moderate rudder inputs. Exaggerated rudder inputs will have to be mixed out differently.

Select the dedicated Rudder - Aileron, Rudder - Elevator mixer (code 64) and deflect the rudder hard over. Now add about 5% up elevator mixing (or down if needed) and re-fly. Does it need more or less? Work on one axis, and one direction at a time. Once you have one rudder direction fixed, move on to the other. We still haven't fixed any roll coupling. Just fly the correction for now, once the airplane will hold a straight knife-edge on either side for the entire length of the field. Work on the roll coupling in the same manner. Start out with a correction of only 2-3% though as aileron isn't usually needed as much as rudder. I leave this mixing on as it's needed in all rudder inputs.

Throttle Setup:

Yep, you read right. Throttle setup is just as important to a smooth flight as anything else. I use the throttle curves (code 18) to make the throttle response as linear as possible. I want to hear an rpm change with every click on the stick. Most gas engines seem to deliver most of the power in the initial 50% of the carb movement, so this requires an initially flat curve, which then climbs sharply.

It takes some playing to really get it perfect, but when done it makes it SO much easier to get a smooth, constant speed flight. Those of you without dedicated throttle curves can use a programmable point mixer and mix throttle to throttle to get the same effect.

Other Tricks You Can Try:

These are just some slight modifications to the setup that I use as a personal preference. First off, I don't like to push as hard to get good outside performance, so I generally run about 5% more down elevator than up elevator. I also run about 5% less expo on down elevator as well. For most maneuvers, I run the normal aileron rates, but for rolling circles, I knock that down to about 30-40%. Yes, I'm giving up resolution but this lets me move the stick more making it easier to control the roll rate.

Aircraft setup is a constant process really. Every time something is changed, there is the chance it will affect something else. Take your time, and work through it, you'll find yourself fighting the airplane less during a sequence, and that makes it much easier to score well.

Throughout this process, you'll be making several flights, and then changing and adjusting. It's important that you change only one thing at a time and then completely retest the airplane each time.

Keeping the aircraft in perfect trim is an ongoing process.