Checking The Centre Of Gravity (CoG) And Reflex.
You should run through the Thrust Line and Wing Balance tests to ensure that your set up is still valid - in fact you should continue to run all previous tests as you proceed through the trimming procedure to verify that any modifications do not invalidate any previous work. Establishing the correct position for the CoG involves a number of checks. Assuming that you have set the CoG in accordance with the instructions that came with the model or plan, you should be in the correct ball park. Generally speaking, the position of the CoG shown on models is conservative - it's too far forward for most aerobatics. The reason for this is obvious - test flying a model is exciting enough without the CoG being too far to the rear. Once the model has been successfully flown and "sorted" the CoG can then be moved progressively to the rear as required.
If you've never come across reflex before, reflexing the wing is done by setting both ailerons slightly up from the neutral position. The effect of this is to decrease the lift generated by the wing. What??? Sounds stupid but....
If you reflex a wing, you are altering the fundamental aerodynamic properties of the wing section, particularly with a symmetric one. Flaps are an example of this but in reverse- when the flaps go down, there is increased lift on the wing. Reflex is a permanent negative flap so the lift on the wing is reduced (when the model is right way up) but more importantly, it is increased when the model is inverted. The trick is to set the ailerons to the correct reflex angle to minimise the difference in elevator input required for normal and inverted flight. What you are trying to achieve is the best balance between the lift generated in normal flight with that generated in inverted flight. At the end of the day, this will be a compromise for the simple reason that a wing requires to fly at an angle of attack to generate lift.
Take the model up to a safe height, fly straight into wind and execute a spin. Make sure that you use only elevator and rudder to initiate and stay in the spin. If the model goes into a slow spiral dive with the rudder and elevator held on, there are two possibilities. The first probable cause is the CoG is too far forward, the second is that there is insufficient up elevator and/or rudder to keep the model in a spin.
Obviously, the quick solution is to try increasing the elevator and/or rudder throw. Do this by using one of the positions on the elevator and rudder rate switches - you can then switch the increased throw off if the model becomes too "twitchy" for you. You may have to resort to permanently switching to the high rates for spinning (and perhaps landing) and the low rates for normal flying if you find the required high movement too high. If this doesn't do the business for spinning then the chances are that the CoG is too far forward.
Before you alter the position of the CoG there are a couple of other checks to be carried out.
Take your model to a good height (sufficient for a two or three second dive!) and head straight into wind. Cut the throttle, push in elevator to go into a vertical dive then release all the controls. The model will do one of three things. Remember that to avoid wasting all your building and trimming efforts, pull out of the dive in plenty of time!
- The model will continue in a vertical dive.
- The model will pull out towards the cockpit side.
- The model will push out to the wheels side.
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Make a note of what the model does then check the wing reflex. Fly the model horizontally at about half throttle then roll it inverted. Release the elevator and watch what happens.
- The model will continue horizontally.
- The model will gain height quickly.
- The model will lose height quickly.
Make a note of what happens and land.
If your model spins easily, continues in a vertical dive without pulling out and flies inverted with no elevator control - perfect CoG position and reflex!
If all this good stuff isn't happening, then either the CoG position or the reflex (or both) are incorrect. Since setting the CoG and reflex are going to be a compromise, change either individually, re-fly the tests and re-evaluate what happens.
To increase the reflex, disconnect each aileron in turn. Depending on how the aileron servo/s are mounted and how the linkage to the aileron is set up on your specific model will dictate whether you have to increase or decrease the length of the mechanical connection from the servo to the aileron. Whatever the set up, turn the clevis at the aileron two full turns so that the aileron sits slightly up from the top wing surface when the clevis is reconnected. Ensure that the aileron is securely re-attached.
To decrease the reflex, adjust the mechanical connection from the servo to the aileron so that the aileron sits slightly down from the top wing surface.
Altering the CoG should be carried out in small increments - no more than 6mm (¼") at a time. Commercially available self-adhesive weights are ideal TEMPORARILY. When you finally achieve the correct CoG position for flight, make a note of it's position. Remove the self adhesive weights and do something usefull inside the fuselage to change the CoG position - move the battery pack or servos around to suit. You may need to have the battery pack in the rear fuselage. (I've sat here for ages trying to make a sentence using "add", "lead" and "weight" but there's no way I can fit these words together!)
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If everything has been sorted out to your satisfaction, the next step you need to take is to sort out all the servo movements and control throws.