Trimming Open Class planes
So you are the proud owner of a new or used open class plane and have a computer transmitter to fly it. Several people have asked me about my trimming process. Here is the method I use in trimming out a plane. Many other methods are just as valid, but using this routine gets the plane to my liking in a few hours so that I can concentrate on the overall performance envelope.
What we are trying to accomplish is to have a responsive plane that will signal lift, turns efficiently in small thermals at slow speed, is stable enough to fly at a distance, and can cruise at moderate speed by itself.
First step is at home, make sure the plane is straight and square. In order of importance here are the checks:
Program the radio to the manufacturer’s recommendation of control throws. Set the CG as on the plans. Make sure the flaps and ailerons are even, all surfaces move the right direction. Set 2 degrees positive incidence in the stab. Don’t mount the towhook.
Step one is best performed on the slope, eliminates knee knocking on a winch line with an untested plane. This trim method will only work with a non-lifting stabilizer. If you have a stab with separate elevator, each adjustment will require the stab to be shimmed so that the elevator is neutral after re-trimming. The aerodynamic effect of a lifting stab cannot be adjusted out with this dive test.
Choose a nice day with less that 15 kph wind at your favorite slope. We are going to get the CG set first. Throw the plane and climb out, adjust the elevator trim so the plane flies slow but safely above the edge of a stall. Repeat shallow dives from 100 feet up at 40 kph, removing nose weight until the plane will barely pull out. I mean the pullout should be hardly noticeable. Adjust the elevator trim each time to be slow flight on the edge of a stall. We’re close now.
Do some inverted flying and note how much down elevator is required to hold level flight. I’m looking for a plane that does not pull out or tuck. Dead neutral. This usually requires another 1/16 oz of nose weight out of the plane. Now some airfoils have hysteresis due to the center of pressure moving backwards at high speed. This is OK because the plane will be stable at low speed and neutral at high speed which is what we want. Once this is done, the CG never moves again. Now when doing this dive test, keep the speeds reasonable, within the range encountered in thermal flying. At higher speeds and tail loads, the flexibility of your tail boom will become apparent., likely resulting in a tuck at higher speeds. The Bird of Prey will tuck severely at high dive speeds, but will pull out if in a gentle dive, so use your judgment. By the way CG affects every other adjustment so it needs to be set first.
While we are at the slope do the following test. Trim plane for level slow flight into the wind, hands off. Flaps and ailerons should be even with wing. Now do a cruising speed pass into the wind. If the plane rolls consistently to one side at high speed, the wing is warped. Now do consecutive inside loops into the wind. If the plane rolls to one side repeatedly, the stab is crooked.
If you have a V-tail, the next step is to set up the differential. Fly the plane directly into the wind and apply full rudder. There will likely be some pitch effect along with the rudder deflection and we want to take this out. Using whatever adjustment is available on the transmitter, set the throws of the rudder/elevator such that the pitch change is eliminated when rudder is applied. Alternatively, you can program in a custom mix with down elevator slaved to the rudder. Remember that down elevator is for both right and left rudder! With coupled aileron/rudder, this mix will have to be added for the aileron as well. Lastly, the mix needs to be off when the aileron/rudder coupling is off, such as in speed mode. Fortunately the Stylus (and I think the Multiplex) can handle all of this. I had to resort to the custom mixes when the V-tail differential program alone did not have enough throw adjustment to remove the unwanted pitch change. As a last resort you can angle the control arms on the servos to give more down throw. Double check your V-tail differential by doing medium speed rolls, they should be completely axial with coupling on. For rolls viewed from the aft, any barrel roll tendencies due to pitch effects will be obvious.
All of the above can be done in less than 1 hour and will affect the handling of the plane more than anything else. The next steps are best done in very light wind, either on the slope or field.
Next on the list is landing settings, easiest to do on the slope. Adjust flaps to droop 85 degrees and no crow. Do repeated landing approaches, 10 feet high and over the crest of the hill. Fiddle with elevator compensation until the plane will not stall when full flaps are slowly deployed. Variable rate elevator mixing and delayed rate on the elevator help a lot to get predictable landing behavior. Once that is OK, start adding crow to the ailerons until the roll response is affected. Reverse differential helps here, if your radio can do it. Adjust elevator compensation with the added crow, usually less down elevator compensation is required. The plane should be nose down and stable at well above stall speed with full flaps, while still being controllable in roll, although sluggish.
We’re done trimming at the slope, so off to the field.
Set the towhook at the CG, perpendicular to the wing zero incidence line. It helps to have a plot of the airfoil to see what the bottom surface of the wing is relative to the zero incidence line. Try NE Sailplane catalog for typical airfoils. If you are using a winch retriever or need more stability on launch, move the towhook forward 1/8".
Next adjustment is the aileron differential. Start with kit recommendations, or 50% more up than down. Decouple the rudder from the ailerons. Flying at slow speed overhead, rock the wings 30 degrees either side and watch the fuselage for yaw. Add more differential if the plane has adverse yaw, decrease differential if the plane has proverse yaw. A small amount of adverse (turn opposite the roll) yaw is OK. Couple the rudder and mix in enough rudder to get a true roll response.
Repeat the rocking at cruising speed overhead. If there is adverse yaw now, increase rudder coupling and decrease differential. If there is small amount of proverse yaw, that’s ok. In speed mode, decouple the rudder. My planes do not require different mix ratios for rudder or differential and handle well at low and cruising speeds. Now make the final adjustment to the CG by noting how many elevator inputs are required to hold smooth thermal turns. Add or remove small amounts of nose weight until the sensitivity is to your liking. I seldom make any further adjustments.
Now we will have some differing opinions on whether rudder coupling should be mixed by the radio or by the pilot. The pilot can always tweak the rudder to achieve coordinated turns when the plane is close enough to see. I do so much of my flying at distances where it is impossible to see accurately whether the plane is in a coordinated turn, either it is too far away or you are looking at it mostly from the side. Bias your turn coordination mixing to slow speed handling and you will know that the plane is turning pretty efficiently when you are way out there and milking the handlaunch size thermal to get home. I challenge anyone to be more efficient than the computer mix when you can't see the tail. I've put in flights over an hour long without seeing the tail, and left it to the computer to worry about turning efficiently.
Next set the camber for slow speed flight. Droop the trailing edge about 3/16", both flaps and ailerons. Fly and see if the plane stalls in level flight when camber is added. Add down elevator mix until the plane does not stall when camber is added at low speed. Try some slow and big thermal turns, the plane should require no aileron and slight up elevator. Try tight thermal turns, the plane should require slight opposite aileron to prevent a death spiral. Predictable handling at low speed and in tight turns is the only way to be able to circle out from 20 foot altitude.
Control location (switches and sliders) on the transmitter is a matter of personal preference but I highly recommend that the camber be placed on a slider, and be available at all times. It is very easy to control the landing time when you can hover the plane on final by dialing in some camber. With a small amount of wind, the final touchdown can be adjusted by plus or minus 5 seconds with 10 to go, by hovering with camber. The other use is when you are short on final, scoot the plane in the ground effect until it runs out of speed, then hit the camber slider to keep the plane flying. The plane will fly for another 40 to 50 feet by doing this maneuver, and can save a landing. I've tried to fly the Futaba transmitters, but it is just too difficult to reach the camber knobs. JR's with either camber or landing mode on the throttle stick is unusable.
Last to be set is launch mode. This is a matter of feel, and will depend on the speed you like to launch. Higher speeds need less droop. With the towhook set as I’ve suggested, no elevator compensation will be needed, just dial in as much flaps as possible to get the maximum launch height. I droop the ailerons 1/8" less than the flaps, it’s supposed to prevent tip stalling on the initial throw. By the way, you should not fly the ailerons at all on launch, use the rudder to control the plane’s direction until the very top, when you level the wings for the zoom. I’m sure most snap rolls on launch are caused by overloading the inboard wing with up aileron when trying to correct a turn.
At this point you should have a very stable and predictable plane. It should slow right down for thermalling and landing. Is should cruise upwind without having to hold down elevator. It will not porpoise, and will be just barely stable enough to fly waaaay out there. The downside is it will be sensitive to control inputs and will not pull out of dives without pilot input. It will happily signal lift and circle smoothly to climb. All the things we asked for and were told in the advertising.
Trimming is not difficult and is well worth the effort. Open class planes with computer radios really push the envelope so that the pilot becomes the limiting factor. Almost any of the top designs today are capable of winning in normal conditions, getting to know how to fly YOUR plane is far more important. I’ve been accused of having too many planes but I also get out to fly each of them enough to know how they react and perform. It’s lots more fun to be able to cover a lot of ground and work small lift, and that’s what the properly trimmed open class plane is capable of doing.
See you in the winners circle, if you can beat me that is.
Copyright W. Man-Son-Hing Sept. 1998