Many Thanks to Michael O'Reilly from Model Flight Adelaide for this text

Trimming Techniques for Multi Channel Gliders

By: Mike O’Reilly

Many people make the mistake of thinking that because their glider is flying OK that there is nothing more to do with regard to trimming their model. Nothing could be further from the truth. Subtle changes to CG, surface deflection and towhook position can drastically change the way your model launches and flies. Listed below are the ways I approach trimming out a new 4-6 channel glider.

The Model

Before flying the model you should check it out to be sure that there are no warps or mis-aligned surfaces. A model with a twisted wing, or one where the tail is not perpendicular to the wings will be much harder to trim than a ‘straight’ model. Similarly, check the lateral balance, one heavier wing is not uncommon. At low speed these problems are not so evident, but at high speed they can be very pronounced and require a different trim to correct them. Make every effort to have a ‘straight’ model.

The whole purpose of the trimming process is to make your glider easy to fly in a ‘hands off’ situation. The easier it is to fly, the better it will fly.

First launches

Having checked the model for warps etc, put the CG where the manufacturer suggests (or having done your own calculations) checked that the controls all move in the correct direction, you are ready for that first launch. The object of the first few launches is to obtain a neutral trim setting for ailerons, elevator and rudder. Fly the model into wind and adjust the trims to obtain straight and level flight. Adjust the elevator trim to the point where the model feels ‘soft’ or ‘mushy’ and then go forward about 2 clicks of down trim.

Centre of Gravity

This subject is probably the object of more debate than any other with regard to model gliders. Why, because changes to CG can drastically affect the way your glider flies. There is no single ‘perfect’ CG for any model, your flying style is part of the equation. The following technique is widely used and will get the CG close to your ideal position.

Launch the model, level off after the zoom and head into wind. Dive the model at about 300 with wings level for about 2 seconds. Then release the elevator stick and carefully observe the behavior of your model. If the model pitches up (climbs) then the model is nose heavy. Land the model, remove some nose weight (5-7g). Push in 2 clicks of down trim and then launch the model again. Fly straight and level into the wind and adjust the elevator trim to be just forward of that ‘mushy’ point. Now perform the dive test again and observe the pitching motion of the model. If you remove too much weight the model will continue to dive at ever increasing downwards angles. In this situation the elevator may not respond quickly so don’t perform the test close to the ground. (If the model will not pull out with up elevator, don’t panic, just feed in some aileron input and the model should come out of the dive. If all else fails push in full down elevator and perform a bunt or ½ outside loop.) The aim is to achieve a model which does not pitch up or down when you release the elevator stick. i.e. is neutrally stable in pitch. (If the surfaces are all straight it should already be neutrally stable in roll and yaw)

Control Surface Deflection

Elevator: Moving the CG rearwards makes the elevator control more efficient or effective. That means that less movement is required to obtain a given amount of pitch movement (the stick becomes more sensitive) The cure for this is not to add lead to the nose of the glider, but to reduce the amount of travel you have on the elevator. Assuming that you are using a computer radio it is a simple matter to reduce the travel (dual rates or travel adjust). How much elevator movement do you need? The 2 situations that require the most elevator movement are the down elevator at the top of the zoom after launch and the amount of up elevator in a tight, slow thermal turn. Reduce your elevator movement so that the stick is almost bottomed out in that tight thermal turn. If you are using equal throws for up and down elevator, test the amount on launch. If you find that you need more throw at the top of the zoom then dial in some more and consider dialing in some exponential as well. A softer elevator input makes the glider much easier to fly, especially at greater distances.

Ailerons: The roll rate of your model is not affected by CG, but increases with the speed of the model. This should be born in mind if you are setting up flight modes in your Tx. So how much aileron control do you need? This is a bit of a ‘feel’ thing, but for mind the most demanding situation is the downwind turn onto base leg when setting up a landing. If you have done everything correctly you will have plenty of speed on and it is not difficult but we have all made that turn too low and too slow at some stage and felt the sluggish response of the model. Set the aileron rates such that you are happy with the response whilst flying slowly downwind and see what it is like in all other situations. A well trimmed thermal glider should not be able to perform 3 rolls per second, one roll may take up to 2 seconds to complete. Any faster roll rate than this may get you in trouble when you are thermalling a kilometre downwind at low altitude. Minor inputs on the sticks should result in minor outputs at the model. Don’t forget that you have exponential control options in your Tx. Aileron differential can have a large bearing on how flat your model will perform thermal turns. All model gliders require some aileron differential. 8 units up and 5 units down is a good starting point for most gliders. Aileron differential is best tested in calm neutral air, strong thermal activity can mask a poor set-up. Launch the model then switch off the aileron-rudder mix. Roll the model to the left and closely watch the angle of the fuselage to the horizon. If it goes nose high (yawing to the right) you have too much differential. If it goes nose down (yawing to the left) you have too little differential. Make the necessary adjustments on your Tx and test again. Aileron – rudder mixing does not cure the nose high or low situation described above, changing the differential is the only cure. Aileron – rudder mixing is used to stop the model skidding in the turns. Imagine looking down on your model (plan view) as it performs the perfect thermal turn. In an ideal world the fuselage would follow the perimeter of that circle. If it is nose in or out there is an increase in drag that will reduce the rate of climb. This is not an easy thing to observe from the ground so using some well proven settings is about as good as you can do. On a 3 metre, 2200g, RG15 equipped model try 10-15% mixing percentage. The faster the model flies use less mixing, the slower it flies use more.

Rudder: The rudder control is not used independently in a thermal glider a lot. As described above it is mixed in automatically in conjunction with the aileron most of the time. Exceptions to this are on launch where corrections to launch direction should be made with rudder not aileron. If you have flight modes then set up a lot of rudder travel in ‘Launch’ mode. The other consideration is rudder differential in V tail models. This should be tested in calm neutral air. Launch the model and fly into wind with your normal trim settings. Feed in full left rudder and observe the angle of the fuselage to the horizon. Ideally it will remain horizontal as the fuselages yaws to the left. Test with right rudder. If you have the same movement for both left and right rudder then the fuselage should do the same to each direction. If it climbs then you need to reduce the amount of movement on the upgoing side or increase the movement on the downgoing side. This will require some programming input on your part and depending on the brand of radio you use may not be easy. Look in the travel adjust section for rudder to begin with. If the nose drops then the correction is the reverse of that described above.

Flaps: On a modern 6 channel glider the flaps serve 3 purposes. On launch they are used to raise the CL of the wing. In thermal mode the whole of the trailing edge can be raised or lowered to decrease or increase the camber of the wing. This will increase or lower the flying speed of the model. On landing the flaps are lowered to increase the drag and reduce the lift that the wing creates to increase the rate of sink.

Launch. This is a complete and complex subject in itself. Suffice it to say that about 200 positive (down) deflection is a good starting point.

Thermal. The complete trailing edge can be lowered about 3mm (on a 3 metre 250mm root chord wing) to enable the model to fly slower in small weak thermals. This should be set on a switch function rather than a slider if possible so that you always get the same deflection and can easily return the trailing edge to a neutral setting. Similarly the complete trailing edge can be raised 2-3mm to improve penetration in strong winds, between thermals or for speed tasks. The ideal settings vary from model to model and should be tested carefully. Again, calm, neutral air is ideal.

Landing. To really slow the model down and increase the rate of sink, the flaps should be lowered at least 600. At the same time it is normal practice to raise the ailerons about 300. Ideally, this braking action (commonly called crow or butterfly braking) should be operated by the throttle stick. The ideal settings will be determined by a number of things, how much downwards flap movement is mechanically available on your model, what mixing is available on your brand of Tx, and your flying style. There are however, some basic rules to follow.

1. It is better to drop the flaps than raise the ailerons (it effectively washes out the tips lowering the stall speed)

2. Dropping the flaps will cause an upwards pitching movement which should be corrected with down elevator mixing. Have lots of practice landings to fine tune the amount of down elevator mixing. Once properly set up landings become a breeze.

3. Raising the ailerons will reduce the roll rate of the model, making directional changes on final slower than you might expect. If you have flight mode switching on your Tx then dial in as much rudder as you can in landing mode.

4. Remember that the ‘crow’ action is proportional. Half stick movement should result in half the rate of descent etc. Raising the flaps quickly may result in the model losing airspeed and ‘dropping’ out of the sky. Try and move the throttle stick in a smooth and controlled manner.

Towhook position

With few exceptions the towhook should be placed in front of the CG. If your model has an adjustable towhook then start about 6mm in front of the CG with the initial launches. Chances are that you have moved the CG rearwards during the trimming process and once you have completed all the steps above you can move the hook back. The ideal position also depends on flap and elevator presets so there is no definitive position but suffice it to say that if the trailing edge of the towhook is 2mm in front of the CG the model will launch well. Moving the hook rearwards increase the steepness of the climb, but ultimately with a reduction in directional stability. Go too far back, get a poor release of the model and before you know it your pride and joy is buried 300mm deep in the ground. Tread warily as the towhook approaches a position near the CG!

Winches, drum diameter, line diameter, flap, elevator and hook settings and release techniques coming soon.

Many thanks to Michael O'reilly From Modelflight for this text on his trip to fly F3B competition in Germany.

Matt Wood and myself journeyed to Berlin for an F3B competition and to catch up with friends and the latest trends. 2003 in Kirchheim was my last F3B competition in Europe and what a change has taken place. The new Crossfire emerged from the 2005 World Championships in Finland. It featured a new thinner, lower camber airfoil than any previous glider and that trend has continued. In fact so much so that the Crossfire is now almost old news. There is a host of new models with 8% or thinner sections with 1.5% camber or less. These models accelerate so easily and hold their speed through turns better than the models with thicker airfoils. It was really an eye opener to see the consistent speed of the Ceres, Evolution, Radical, Thresher and Tool to name a few. There were not many Caracho 3000's or Estrella's being flown as the newer models seem to be demonstrably faster. The performance of all the models seem similar in Distance and it takes good flying and calling/team work to come out a winner here. Duration was interesting at Sputendorf. In the past both Matt and I have struggled at times with German conditions which often have weak thermals that are hard to pick. For the first time ever in Europe we actually had wind and it made life easier. We got our normal good launches (Matt was one of the highest all weekend) and the conditions were similar to a good 4-6 m/sec day at Jerilderie. Matt got close to 10 minutes in all 4 flights and I was 10 or 10.01 in 3 of the 4, but in the last round my normal tactics just did not work and I found myself really struggling from the 4 minute mark and finished with 5.25 and no landing. In the previous heat, the current World Champion Martin Herrig, flying on his own club field flew a 7.30 flight with no landing points. The sink was massive at times. What also showed up was that the new thinner models struggled without a clear thermal to do 10 minutes. On the Saturday the conditions were (as I said) like Jerilderie where you launched high and did long straight runs into and then cross wind for several hundreds of metres at a time looking for the slightly better air. Matt and I (and other Caracho flyers) did the 10 minutes easily by using about 1.5mm of camber and elevator to flap mixing, whilst a lot of the newer models did 8-8.30 flights. What this means in Australia (I think) is that these new thin models may not be all that suitable for our Task A based thermal competitions. They don't launch any higher, they just do everything faster, including come down without thermal assistance. Time will tell.

So why didn't we win, or even come close? No problem with our equipment although I did have a loose aileron servo for 7 flights, but after I fixed it my scores got worse. Lack of practice; lack of top level competition, and ultimately lack of speed. Matt had a cut at Base Bin Rd.4 speed which it could be said cost him 5-7 places. He flew really well with no practice. I flew an 18.5, a 19.5 and two 21 second speed runs and when the winner is doing low 14's you are getting hammered. Add the bad Rd.4 duration score and a 5 lap crunch in one Distance Task where I went right to hurt 2 guys who launched before me, only to be crunched by the last 2 guys who launched 40 seconds later into a strong thermal 150 metres to my left. Poor decisions and lack of speed will hopefully be turned around next weekend in Lunen.

A common German launching style. Effective.

Peter Hubbertz, Martin Webershock and Tobias Knoblauch flying Distance

58 competitors, about 100 winches in a corridor 50 meters wide. Field is 70 meters x 600 meters.

Aust. 1 Germany 0; Matt had a midair in Rd. 1 dura- tion but survived to do a 10.01 and 3 metres. The German lost his plane in a corn crop and had to fly an Ellipse 2 for the rest of the weekend. He was one guy both Aussies beat in the final results.