BASIC TRIMMING for AEroBATICS

BASIC TRIMMING for AEroBATICS
Please read the preamble and end notes fully.
This chart assumes your aircraft was built accurately and you have set the Centre of Gravity close to the manufacturer’s recommendation. Your aircraft has been designed to fly at or below a given weight and a heavy aircraft may never fully satisfy the conditions of this chart. Working through the chart you may have to accept some compromises, but time spent trimming will be well worth the effort. These tests should be carried out in reasonably calm weather. The Basics
1
Trim your model for straight and level flight with the engine set at just over half
throttle. Models smaller than two metres may need a slightly higher throttle setting but
full throttle should not be required. If you have tailplane incidence adjusters it is
assumed that you have trimmed out the inaccuracies to leave the tailplane and elevators
level. Check that all servo throws are matched to recommended settings and there is
no play in the control linkages.
Engine Trust Line
2
Hold straight and
Model climbs
Increase down thrust
level flight at just over
half throttle then
Model dives
Decrease down thrust
smoothly increase the
throttle to full.
3
Fly straight and level
Model pulls to the left
Add more engine right thrust.
pull to the vertical
Model pulls to the right Decrease engine right thrust.
Balance: Centre of Gravity
4
Fly straight and level, After rolling inverted
increase the throttle to down elevator has to be
full and pull to a 45°
used to maintain the 45°
climb. Hold the 45°
line.
Add weight to the tail.
line then roll to
inverted.
If the model climbs
Add weight to the nose.
5
Go back to 1.
Wing Incidence. The degrees of incidence should be related to the datum line of the model.
For ease of comparison it is sometimes related to the centre line of the tailplane airfoil. The
two reference lines may be different.
6
Start high and reduce
throttle to tick over,
dive in a straight line.
Model pulls to canopy
Reduce wing incidence.
Model pulls to belly
Increase wing incidence
OR
7
Model pulls to canopy
Model Pulls to belly
If any incidence is changed go back to 1
Increase tailplane incidence
Decrease tailplane incidence
Page 2
Lateral balance
8
Fly model towards
Wing drops at exit
you and pull a tight
loop.
Repeat for outside
loop
Roll model inverted at Wing that drops is the
OR
half throttle
heavy wing
Aileron differential …. to help achieve axial rolls
9
Fly model towards
If after the half roll your
you and pull into a
model changes heading.
vertical climb. Then
In the same direction as
half roll
the roll. (i.e. If the roll
is to the right and after
the half roll the models
heads to the right).
Add weight to high wing tip
Add weight to other wing
Increase aileron differential. (up
going aileron to move further than
down going aileron)
Opposite direction to the Decrease aileron differential
roll.
Dihedral .. To change - the centre joint, or the wing tube sockets would need to be
repositioned and a control mix may be considered to be an easier compromise here.
10
From straight and
If the model rolls to
Increase dihedral
level flight, roll to
inverted.
knife edge, hold top
rudder to maintain
Model rolls back to
level flight
upright
Decrease dihedral
Trimming should be a constant concern to make the most of your model and it is expected that you may have to revisit the above chart to improve your models flight characteristics. Always make one adjustment at a time and check the effect thoroughly before making any further adjustment. If all the above suggestions do not achieve the desired results, electronic mixing of the controls must be considered, for instance if to maintain a flat turn through 360° the nose drops, mixing some up elevator to react to 80% rudder may solve the problem without affecting other manoeuvres like knife edge fight where less rudder will be used, or you may wish to program a switch to apply the mix when required. It may be a lot less work to apply a mix than change the dihedral is another instance, but remember the objective is to reduce the workload on the sticks whilst flying a schedule. There are some highly regarded pilots who are said to rely more on electronic mixing than mechanical trimming to achieve their ‘required feel’. So trimming is an individual art which you need to develop with time and experience. This guide is intended as a starting place. For those wishing to correct a basic design fault it is worth knowing the first action should be to decide on a fuselage datum line, this is the line you wish the fuselage to assume when flying straight and level, the way it sits in the air. All other incidences, (main wing, engine down thrust and tailplane) are then set with reference to this line. Rudder hinge line angle and proportions of the rudder are also very important. The whole concept of going to these lengths to trim your model is to reduce the workload to correct your model in flight so you can concentrate on flying the schedule. Thanks to many members, past and present, and reference from other sources to compile this chart. A C Hoyland. PRO GBR/CAA March 2007.

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