Checking The Wings.
Once you have set the thrust line up, the next stage is to check the model for balance and wing warp. Hopefully, you'll have built your model straight and true. These tests should prove it. Any deviation from straight loops and rolls is generally not excessive although we have witnessed an ARTF "pattern ship" which was so bad that it was in knife edge flight after the first quarter of a loop! (I guess you shouldn't try to make a decent model that includes the words "cheap", "ARTF", "pattern" and "ship".)
This is a two stage test and you have to be able to execute several continuous loops followed by several continuous bunts. The easiest (and most effective) way of doing the bunts is to "cheat" by rolling inverted and bunt upwards into wind. Since you are trying to establish whether the model rolls (or "screws") out of a loop and what's causing the rolling, you must not use aileron or rudder to correct tracking through the manouevres. It should also be fairly obvious that incorrect side thrust will also cause the model to screw out.
If you're new to trimming a model properly, it's a good idea to get a friend to help you with this by taking a note of what the model's doing as you carry out the manouevres.
The first step is to execute several loops so head straight into wind and start the loops. Take great care not to input rudder or aileron to correct any deviation from the loop. Ensure that you have enough airspeed going over the top of the loops - dropping a wing because it has stalled will only confuse the issue. As usual, one of three things can happen.
- The model will fly superimposed loops.
- The model will screw out (in a spiral) to the right.
- The model will screw out (in a spiral) to the left.
Note what happened in this first part of the test.
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Now for the part requiring "bottle"!
Execute several bunts. Unusually do this into wind as it makes it much easier to work out what's happening. As with the loops, ensure that rudder and aileron are not used while maintaining sufficient airspeed over the top of the bunts. As with the loops, the model will do one of three things can happen.
- The model will fly superimposed bunts.
- The model will screw out (in a spiral) to the right.
- The model will screw out (in a spiral) to the left.
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Repeat this sequence several times to make sure you are getting consistent results. That's the test flying done - now for the thinking part! You need to combine the results of the two tests to find out what the solution is. The best way to analyse the results is to use a series of tables.
| Loops Bunts Problem Solution |
- Tracks Straight - Tracks Straight - No Problem! - No Solution To No Problem |
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| Loops Bunts Problem Solution |
- Screws Left - Screws Right - Left Wing Heavy - Add Weight To Right Wingtip |
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| Loops Bunts Problem Solution |
- Screws Right - Screws Left - Right Wing Heavy - Add Weight To Left Wingtip |
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| Loops Bunts Problem Solution |
- Screws Left - Screws Left - Wing Warped - Add Trimming Tab To Left Aileron |
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| Loops Bunts Problem Solution |
- Screws Right - Screws Right - Wing Warped - Add Trimming Tab To Right Aileron |
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If you have to add weight to either wingtip, do so in small increments then repeat the flying tests.
If you have a warped wing, there is no good solution. That is why it is absolutely essential to check the accuracy of wing panels (particularly foam or ARTF models) before you start to build and reject them if they are not up to scratch. There should be at the absolute maximum 2mm (0.080") mismatch at the trailing edge of the inner rib (or root) when the wingtip ribs (or tips) are parallel - anything else is junk. Adding a trim tab may help but the amount of screwing out that will occur will depend on airspeed, something that an accurate wing avoids. Start with a trim tab of approximately 25mm (1") long by 19mm (3/4") wide made from 1.6mm (1/16") ply as near the wingtip end of the aileron as posiible. You are trying to increase the lift on the "dropping" wing. It might, just might, be possible to do some very nifty mixing programs on a computerised transmitter but I wouldn't bet on it!
It is possible that you may not be able to entirely eliminate screwing out of both loops and bunts due to minor wing warp. In this case, the best option is to set the model up so that it tracks correctly when executing loops and use rudder to correct when executing bunts. The reason for this is that you will inevitably pull more looping or positive 'g' manouevres and so you will have to correct tracking less often.