Trimming Techniques for Multi Channel Gliders

1
Trimming Techniques for Multi Channel Gliders
a gliding retrospective

• Originally discussion by Mike OReilly, October
  1993.
• Further Notes added January, 2009

Photos Brett Anthony, F3J World Championships,
Turkey, 2008
2
The Model

• Make every effort to have a straight model.
• This was important when you built your own model
  from balsa and foam, but modern molded gliders
  rarely have warps or heavy panels.

3
Trimming

• The whole purpose of the trimming process is to
  make your glider easy to fly in a hands off
  situation. The easier it is to fly, the better it
  will fly.
• I like my gliders to be neutrally stable so they
  will fly hands off. This helps you to detect
  thermals in the first instance and to be
  comfortable flying your glider a long way
  downwind.

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 Tow hook position

• With very few exceptions the tow hook should be
  placed in front of or on the CG.
• The ideal position also depends on flap and
  elevator presets so there is no definitive
  position.
• Moving the hook rearwards will increase the
  steepness of the climb, but ultimately with a
  reduction in directional stability.

6
Centre of Gravity

• This subject is probably the object of more
  debate than any other with regard to model
  gliders. Why, because changes to CG can
  drastically affect the way your glider flies.
• There is no single perfect CG for any model,
  your flying style is part of the equation.

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The Dive Test

• Before you do this test the elevator trim must be
  set for normal gliding flight.
• Dive the model into the wind at about 30 with
  wings level for about 2 seconds. Then release the
  elevator stick and carefully observe the behavior
  of your model.
• If the model pitches up (climbs) then the model
  is nose heavy.
• If the model dives at ever increasing downwards
  angles it is tail heavy.
• The aim is to achieve a model which does not
  pitch up or down when you release the elevator
  stick. i.e. is neutrally stable in pitch.  (If
  the surfaces are all straight it should already
  be neutrally stable in roll and yaw).

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A new take on the Dive test

• I noticed in Turkey that a lot of pilots set up
  their lightweight models with a more rearward CG.
  Their logic was that they only fly these models
  in light conditions so the model does not need to
  be as stable as the windy weather model.
• When it is calmer the model will not fly as far
  away from you so it doesnt need to be as
  neutrally stable as the model you fly in the wind
  that may end up 2000 metres down wind.

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Control Surface Deflection 
10

• With the advent of modern computer radios and the
  common use of flight phases it is now possible to
  set the preferred amount of travel for each
  channel for each phase launch, speed, thermal,
  landing and more.

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Aileron Differential

• Aileron differential can have a large bearing on
  how flat your model will perform thermal turns.
• Aileron differential is best tested in calm
  neutral air, strong thermal activity can mask a
  poor set-up.
• Roll the model to the left and closely watch the
  angle of the fuselage to the horizon. If it goes
  nose high (yawing to the right) you have too much
  differential. If it goes nose down (yawing to the
  left) you have too little differential.
• Aileron rudder mixing does not cure the nose
  high or low situation described above, changing
  the differential is the only cure.
• Aileron rudder mixing is used to stop the
  model skidding in the turns. Imagine looking down
  on your model (plan view) as it performs the
  perfect thermal turn. In an ideal world the
  fuselage would follow the perimeter of that
  circle.

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• I struggled with the setup of my first Pike
  Perfect for over 6 months and actually preferred
  my older Pike Superior until I reduced the
  amount of aileron differential to 7mm up and 6mm
  down, reduced the rudder mixing and it suddenly
  turned like it was on rails.
• Thinner airfoils flying faster need less
  differential.

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Rudder
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• Glider fliers are inherently poor users of rudder
  control when they try to fly a helicopter or a 3D
  model. I now use rudder when making slow thermal
  turns (with very little aileron input) to keep
  the wings level during the turn, and to make
  minor corrections on landing approach as again
  you want to keep the wings level so you dont
  touch a wing tip on the ground.

15
Flaps

• On a modern 6 channel glider the flaps serve 3
  purposes.
• Launch
• Thermal
• Landing

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Launch

• This is a complex subject in itself. Suffice it
  to say that about 20 positive (down) deflection
  is a good starting point.
• The ailerons should be level with the flaps.
• A good launch setup is a combination of hook
  position, flap and aileron settings and elevator
  presets

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Thermal

• The complete trailing edge can be lowered about
  2mm to enable the model to fly slower in small
  weak thermals.
• Similarly the complete trailing edge can be
  raised 2-3mm to improve penetration in strong
  winds, between thermals or for speed tasks.
• The ideal settings vary from model to model and
  should be tested carefully. Again, calm, neutral
  air is ideal.
• Elevator to Flap mixing is increasingly used in
  Thermal mode

18
Landing

• To really slow the model down and increase the
  rate of sink, the flaps should be lowered at
  least 70. At the same time it is normal practice
  to raise the ailerons about 20.

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• Most modern computer radios have a multi point
  mixing curve for butterfly-elevator. It is rare
  for this mixing curve to be a straight line.
• I spend a lot of time making sure that as you
  move the throttle stick back and forward the
  fuselage does not change attitude quickly. This
  will make your throttle stick a proportional
  device to control your landing approach.

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How to land on the spot on time.

• The simple answer is practice, practice,
  practice. But there must be some method to your
  practice.
• I find I need to have my glider at certain
  marker points at certain times to get the
  landing right.
• David Hobbys approach is a shorter, faster and
  lower final leg with strong braking in the last
  few metres.
• Any method will work but to be able to produce
  landings inside 1 metre and 1 second regularly I
  think you need some physical markers so that
  you can repeat the process over and over.

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...others prefer different spot landings
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How to make your glider stay up.

• Have a plan of where you will fly before you
  launch.
• This plan may be determined by birds, insects,
  changing wind vectors i.e. the conditions that
  prevail in the 90 seconds or so before you
  launch.
• Believe in your plan.
• If the plan involves flying upwind to search for
  a thermal consider where thermals have been
  popping in earlier flights.
• Do not fly back through air you have already
  flown through. If it was sink then, it is
  probably still sink.
• If your plan is not working, have your helpers
  observe the other models in the air and go to a
  known thermal if a model is indicating good air.
• Maximise good air when you get it by circling in
  the core of the thermal and gain as much height
  as possible in the first few minutes in that
  thermal. Do it right and you wont need a second
  thermal
• Remember that to do a 10 minute flight you do not
  need the glider to go up, merely not to come
  down. Air that will keep you at a constant height
  may well be good enough. Dont leave it unless
  you are going to a known thermal.