The Voodoo Magic of Aircraft Setup

by Doug Cronkhite

Sitting at a recent IMAC contest, Mike Caglia and I were discussing aircraft setup and both of us came to the conclusion that few people were really setting up their airplanes well. I think much of this is simply due to not knowing how. Much of this article deals with setup before flying the airplane. After basic setup, it will take anywhere from 30 – 50 flights to really dial in the airplane. This seems like a lot but read on, and you’ll see why. This article is based upon the JR PCM-10X radio and it’s programming references but the information is applicable to any modern computer radio. The setup described here is for a precision setup (not 3D or freestyle) but again.. the same theory can be applied.

Probably the most critical item needed to setup an airplane properly is a control surface throw guage. I use Ivan’s Magic Gadgets, available from Central Hobbies I think. You need to accurately determine how much your surface moves.

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.

The Airplane:
The setup of the airplane really starts during the building process. Hinge lines should be straight and centered on the surface. Control horns should be setup such that the pivot point of the horn is exactly on the hinge line to avoid building in a differential. The arm on the servo should be exactly perpendicular to the servo itself. Servo arms should be switched around until you get the spline alignment correct. Avoid using the radio to center the servos. All hinge lines should be sealed so no air can pass through. It doesn’t matter how tight you think it is.. seal it on the bottom with some covering. You want to minimize pushrod slop as much as possible. Use high quality ball link attachments and machined aluminum servo arms for the best setup. The model should also be balanced properly before any of this starts. Try to find out about other people’s experience with your airplane to get the best balance point. The aircraft should also be balanced laterally if possible. Add small amounts of weight to a heavy wingtip to correct it.

The Radio:
First off.. let’s start with a fresh model memory (or reset the current memory) to insure there isn’t anything left over in the airplane. Now setup the reversing switches such that the controls move in the correct direction. The amount they move isn’t important right now.

OK.. now we have a fresh, very basic setup. What we want to do now is insure you’re getting the maximum resolution out of your servos. You should have your high rate selected since this is a fresh memory location.

Select ATV (code 12) and set all used channels up to 150% in both directions. Don’t forget the flap and aux channels if your using multiple aileron or elevator servo setups.

Select Trace Rate (code 14) and set all 3 primary controls up to 150% in both directions.

These 2 steps insure you’re getting the maximum travel out of your servos and therefore maximum servo resolution. Most modern computer radios are 1024 radios.. meaning there are 1024 steps of servo resolution for it’s full range of travel. By running your ATV and Trace Rate up to their maximum, you utilize all 1024 steps to command the servos.

Now mechanically adjust your linkages so that your ailerons are perfectly centered and get the maximum throw recommended by the manufacturer. You’ll probably have to move higher up on the control horn, and closer to the center of the servo arm. Here is where the Magic Gadgets come in.. If all your initial building was straight and true, 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 screen (code 12) so they’re EXACTLY the same. Do this for both ailerons independantly of each other.

The procedure for elevator setup is the same as the ailerons. Pay close attention to the center and throw. I setup the maximum throw for precision flying (not 3D or freestyle). Again this is probably going to require long control horns, and short servo arms. Make sure both throw is exactly equal in both directions.

To fly freestyle, I change servo arms to a long arm, and use a different model memory with specific freestyle setup information.

Initially setup the rudder to use it’s maximum available throw. It’s important to setup the rudder with the best mechanical advantage possible to insure good resolution and power. You might adjust the throw later on, but for now get as much as you can.

Initial Flights:
OK.. 1st flights are sometimes nerve wracking experiences. Just get the airplane up to some reasonablly high altitude and get it basically trimmed for level flight. Initial trim on the rudder should also be performed now. Fly directly up wind, and straight away from you if possible. Insure wings are level and pull the nose up. Note if there is any loss of heading or roll to the airplane. You MUST be wings level to properly evaluate rudder trim. Repeat this several times until you’re sure you’ve got the rudder trimmed as well as possible. Now land.

Ground Trim:
Once you’ve flown the airplane and got it basically in trim, you need to go back and get everything mechanically neutral again. Write down the trim offset for each control. Now get out your Magic Gadgets 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 it other than linkage 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 trim, as we’ll be working on incidence and thrust changes later.

Now that you have the trims centered again. It’s time to fly and verify your changes. Again.. follow the same procedure and note if any trim changes are needed. If all went well you should have an airplane that is nicely trimmed for straight and level, upright flight. Roll the airplane inverted and see how it feels. Pushing too much elevator?? I use balance to adjust this generally to get the feel I want. Too much push.. move the CG back and retest.. This should be done in SMALL increments. Time to 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 uplines. 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 downthrust. If the nose pulls right or left add side thrust to counteract this tendency. NOTE: Left or right thrust changes depending on the prop used. If you change props, you may have to readjust side thrust. Once you’re getting consistently good, straight uplines, move onto the downlines. Climb the airplane up to 500 feet or so and get into a vertical downline and let go.. Does it pull out or tuck under? I generally like to correct slight downline issues with SMALL balance adjustments. If the airplane pulls out of long downlines, 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:
Any time you roll an airplane, the down travelling aileron generates more drag than the up travelling 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 a 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 upline and downline to make sure the airplane rolls axially.

Knife-Edge Coupling:
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 midwing 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. Are you having 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. Exagerated 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 1 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 hand fly the correction for now. One 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 with 2-3% though as aileron isn’t usually needed as much. I leave this mixing on all the time as it’s needed in all rudder inputs.

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 a initially flat curve that then steepens 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.

These are just some slight modifications to the setup that I use as a personal preference. First off, I don’t like to push very 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. I reduce the elevator travel down to about 60%. Again.. it just helps me to smooth things out.

Aircraft setup is a constant process really. Everytime 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.

If you have any questions or need help.. feel free to email me..


-Doug Cronkhite