Glider
(GLID)
Section
– –
80
Sailplane Trimming and Adjusting
The following chart gives procedures that may be followed when trimming a new
sailplane. The flights should be made in near-calm conditions, and repeat them several times
before making adjustments. If any changes are made, go back over the previous steps and verify,
or further adjust as necessary.
One of the most critical steps is the center-of-gravity (CG)/decalage testing (Step 3).
Decalage is a fancy term describing the relative angle difference between the wing and horizontal
tail. Although the control neutrals have been set in Step 1, there are differing combinations of
elevator trim and CG that produce stable flight. In general, by moving the CG back you get
better performance and you reduce the stability, making the model more difficult to fly and
requiring more attention from the pilot. Moving the CG back lessens the download on the
model’s tail, which means the wing and tail are working more together and less against each other
as they do with a forward CG. Many contest flyers use a CG position located between 35 and
40% of the mean wing chord, which is near the back limits for stability (the mean chord is just
about the same as the average chord, which is calculated by dividing the area by the wing span).
How you set you model up really depends on your preferences. A nose-heavy model will be
easier to fly but will lack the performance of the back-CG model.
You should also set differential and/or rudder coupling carefully. Incorrect settings will
result in needless increased drag, and may be checked fairly easily. If you practice keeping the
fuselage straight while gently rocking the wings back and forth, you’ll learn how to coordinate
turns and won’t need coupled rudder any more. You can also learn about the proper amount of
differential or rudder coupling by studying the figures of the model circling in the “coordinating
turns” section, Chapter 5. Too much differential can make the model sluggish when entering or
exiting turns and banks.
Setting up butterfly can be tricky. The reader is referred to the section earlier in this
chapter which describes the instructions contained in the chart’s line 4, 5, and 6. The elevator
delay can be particularly useful to eliminate trim changes with the application of butterfly.
Whatever you do, be sure to spend a lot of time trimming your sailplane. If you have a
nearby slope, practice flying on very light days, where you can just barely keep the model
airborne. It is under these conditions that you learn whether your model is really trimmed
properly.
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