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 youll see why. This article
is based upon the JR PCM-10X radio and its 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 Ivans Magic Gadgets, available from Central Hobbies I think. You need to accurately determine how much your surface moves.
Throughout this process, youll be making several flights, and then changing and adjusting. Its 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 doesnt 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 peoples
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.. lets start with a fresh model memory (or reset the current
memory) to insure there isnt anything left over in the airplane. Now setup
the reversing switches such that the controls move in the correct direction.
The amount they move isnt important right now.
OK.. now we have a fresh, very basic setup. What we want to do now is insure youre 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. Dont 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 youre 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 its full range of travel. By running your ATV and Trace Rate up to their maximum, you utilize all 1024 steps to command the servos.
Ailerons:
Now mechanically adjust your linkages so that your ailerons are perfectly centered
and get the maximum throw recommended by the manufacturer. Youll 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 theyre 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 theyre EXACTLY the same. Do this for both ailerons independantly
of each other.
Elevator:
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.
Rudder:
Initially setup the rudder to use its maximum available throw. Its
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 youre sure youve got the rudder trimmed as well as possible.
Now land.
Ground Trim:
Once youve 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 well be working on incidence and thrust changes later.
Now that you have the trims centered again. Its 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:
Youll 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 youre
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 youll
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 youre 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,
were going to mix for moderate to high speed flight. Slow speed knife-edge
generally isnt 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 havent 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 isnt usually
needed as much. I leave this mixing on all the time as its needed in all
rudder inputs.
Throttle:
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.
Misc:
These are just some slight modifications to the setup that I use as a personal
preference. First off, I dont 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.. Im 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.. youll 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..
C4@san.rr.com
10s..
-Doug Cronkhite