We want to trim our model so that at level flight, or “cruise”, it uses minimum power. This is an oversimplification, but it’s not a bad way to look at it.
We want to first achieve a slow, steady cruise with a 20-25′ circle. Using a half motor, wind to 0.15 in-oz and back off to cruise torque, roughly .07 in-oz. Adjust the wing incidence and stab tilt until the cruise “looks good” to you. But you’re not there, yet. This is just the starting point.
It is a common mistake to fly a F1D under-elevated, or with insufficient decalage. The F1D flies slowly and gracefully even when it’s not trimmed correctly, and it’s easy to misjudge its nice behavior as optimal. I have a very good eye for motion, yet, I have a difficult time judging whether a good looking flight has the best trim. However, after an adjustment to increase the decalage, it is easy to see if the model has slowed down and whether it’s stalling. Our eyes and brains are very good at seeing differences in motion. “The model flies more slowly now,” or “the model now is stalling” are qualitative judgments that we are good at making. We want the model to cruise as slowly as possible without stalling.
To make more accurate assessment of trim, I like to take the model for longer “test hops.” I wind the half motor to .20 in-oz and back down to .12. This will simulate the model’s behavior in late climb, cruise, and early descent. I launch the model at shoulder height, and the model will climb to about 15′. You can judge how the model flies for an extended time, for about 5 minutes, with the model flying above ground turbulence. In each of flight phases, I often see that the model behaves differently. I find that when the cruise looks just right, the climb might look a little stally. Or, if the descent is smooth and looks efficient, the cruise and climb will look (a little) stally.
Make multiple test hops, increasing the decalage slightly each time. I like to adjust the wing’s incidence instead of the stab’s. For the wing, each .030″ (or .75 mm) adjustment is roughly .25 degrees. Judge the trim based on the entirely of the flight, not just on one phase.
I usually choose the trim setting where the climb is a teeny bit stally, the cruise “floaty”, and the descent smooth. I believe that when the model flies closer to the ceiling, the air will be calmer, and the little bit of stall experienced at 15′ may not happen. I also believe a full motor will put slightly more tension on the motor stick resulting in a little decrease in decalage,
In any case, the trim setting chosen here is still preliminary. We still have to fly the model at full launch torque, using a half motor, to judge the model’s trim in initial climb and in late descent. Then, we have to fly the model on a full motor because the model climbs differently with a full motor. In each of these test flights, we will judge the trim based on the entirety of the flight and not just on any one of the flight phases. The trim we choose is one that maximizes flight duration, even though it may not have the best trim in any flight phase.
And, in reality, trimming never stops. In a contest, I’m always judging the quality of every flight and will make adjustments when necessary.
You can easily lose 10% of your performance if your model is not well trimmed. So, don’t assume that your model is well trimmed because it looks good. Methodically trim your model.
In an upcoming segment, we will discuss how to trim for high-torque launches and for long descents. Lakehurst is 176′ high, and your F1D needs to be very well trimmed to make it all the way to the top AND float down very slowly.