Conventional Wisdom
About Trimming
Conventional wisdom (CW) is a term used to describe ideas or explanations that are generally accepted as true by the public or by experts in a field. The term implies that the ideas or explanations, though widely held, are unexamined and, hence, may be reevaluated upon further examination or as events unfold.

Source: Wikipedia

Conventional wisdom for trimming a collective pitch model helicopter is:

  1. Level the control arms of the three servos controlling the cyclic with the body of the model, and adjust the tail rotor servo so its control link and the link on the tail rotor pitch adjustment are parallel.

  2. Adjust the links between servo and swash plate so the swash plate is level when servos are centered and stays level when all three servos move, in unison, to change the collective pitch of the rotor.

  3. Once the servos and swash plate are squared away, adjust the swash-to-rotor linkages so the pitch of the two rotor blades are equal when servo is centered (0 blade pitch) and when raised (+ collective pitch) or lowered (- collective pitch).

  4. Static balance the rotor blades so they are the same weight.

  5. Check and equalize the tracking of the two blades when + collective pitch is applied to correct for any variation in thrust/lift between the two blades. This is done by adding weight to the blade which is tracking higher than the other by applying tape to it.

Those steps are only the starting baseline for adjusting a collective pitch model into hover trim. Changing the collective pitch of the rotor requires the three servos to react at exactly the same rate as the left stick on the transmitter is moved from minimum-to-maximum, a mechanical feat on par with juggling three bowling balls. If there is any variance between the three servos as they move the swashplate up/down to change COLLECTIVE pitch the swashplate will tilt, which changes the CYCLIC feathering pitch, tilting the rotor and causing the model to drift out of hover.

When flying a full-scale helicopter the pilot sitting in it uses the horizon as a reference and can sense and correct manually if an imbalance between servos controlling the rotor cause drift when collective is increased or decreased. There are other forces which will cause the rotor to tilt and drift so constant stick input by the pilot is standard operating procedure for flying in a hover state.

An RC pilot lacking the "seat-of-the-pants" sensory input of a real pilot and can only react to what is seen and the smaller the model gets the quicker it reacts and the more difficult it is to see the changes in attitude from an external point of view. In a nutshell, while the forces acting on the helicopter are similar on full-size and scale models the scale models are more difficult to control.

CCPM

Collective-Cyclic-Pitch-Mixing is term used to describe how the transmitter for a RC helicopter controls the behavior of the servos and attitude of the rotor when collective and cyclic stick inputs are made. If after performing the five preliminary trimming steps above the RC pilot detects an imbalance between the three servos moving the swashplate when collective is applied to lift the model off the ground into a hover, swashplate mixing menu settings on the transmitter can be used compensate for it. This stage of trimming is usually performed using a set of X shaped training skids balls on the ends to prevent the helicopter from tipping over and getting damaged.

3-Axis Gyro Stablization

A recent development in RC helicopters is the use of a 3-axis gyro which can sense and automatically correct any mechanical imbalance between the servos controlling the swashplate. Some more sophisticated systems add a camera aimed at the ground or horizon to provide the same "seat-of-pants" orientation awareness as a pilot in a full-scale helicopter has.

Most of that conventional wisdom does not apply to the trimming of the Blade mSR and 120SR

The significant differences between the mSR, 120SR and collect pitch models is that the collective pitch of the main rotor is fixed a positive angle. Because it is not possible to change collective pitch from positive to negative the shape of the rotors is also different; asymmetrical rather than symmetrical. Instead of using three servos spaced 120°apart the Blade models use two spaced 90° apart. On the mSR the servos and control arms of the swashplate are forward the main shaft with a floating alignment pin at the back 135° from the two arms. On the 120SR the servos control arms are behind the main shaft with the floating alignment pin in back between the two arms.

Eliminating adjustable collective pitch allows for a simpler, more user friendly design better suited for beginners. Another design feature which makes the the Blade models easier to fly is the self-stabilizing 45° flybar which combines traits of the Hiller-Matic self-stabilizing paddles and the heavy stabilizer bar first used on Bell Aviation helicopters in the 1930s.

I've researched the origins of the 45 flybar design and found that in the patent application the rationale for the design isn't really clear. It appears that Hirobo, a Japanese helicopter model company found that on small fixed pitched models which only use two servos to change swashplate angle and cyclic feathering pitch of the rotor and flexible plastic rotors, a 45 Hiller a flybar design, with Bell influenced heavy airfoils on the end resulted in more stability. As with many inventions it appears Hirobo tried a lot of different configurations for the two-servo fixed-pitch models until it found something which worked.

While it might seem the tilting of the swashplate by one or both of the servos is controlling the feathering pitch of the rotor blades the actual cause and effect is more complicated an counter-intuitive. Because the flybar is suspended by the two pairs of links and can tilt freely, and has heavy paddles, it acts like a gyroscope when spinning and will, absent of any momentum influences of the helicopter body, try to stay level with the horizon. That is way when the sticks are released the Blade models will self-stabilize into a hover, but only after rocking back and forth like a pendulum.

When cyclic input is applied the servo(s) tilt the swash plate. That changes the angle of the airfoil paddles on the flybar when it passes 90 to the control arm. What Stanley Hiller discovered in the 1940s is that an airfoil shaped flybar literally flies it into a new orientation and the aerodynamic force, leveraged by the length of the arm, acts to amplify the initial input that caused the flybar to change its orientation. By way of analogy the flybar spinning level in a hover state is like a car sitting on the top of a hill. A slight nudge in either direction will send the car speeding down the hill with gravity doing the work. But in the case of the Hiller flybar it is aerodynamic forces doing the work.

Authur Young, an engineer working for Bell Helicopters in the 1930s devised a way to stabilize a two-blade rotor by mounting a heavy stabilizing rod at a 90 right angle to the rotor. References to the Blade models using a "Bell-Hiller" design are referring to the fact the airfoil blades (the Hiller effect) are also heavy enough to have a stabilizing effect on the flybar.

So what happens as the flybar spins is that it gets tilted and flown into a different plane when it passes 90 degrees to the control arm tilting the swashplate and then the gyroscopic effect of its weight counteracts the tilting and tries to keep the flybar level with the horizon. It's all rather complicated and difficult to wrap one's head around. I explain it in greater detail in the cause and effect tutorial

Don't Worry - Be Happy

So if all the conventional wisdom about creating a trim baseline leveling the swash, setting pitch angles, etc. doesn't apply to the Blade models what is the baseline for trimming for hover?

Horizon Hobby markets the Blade models as "Flight Tested" but that simply means all the parts function correctly not that it will pop out of the box into a perfect hover. There is no guidance in the mSR user manual regarding how long the servo links should be. The 120SR manual (page 13) shows the factory setting for the links should be 37mm. Beyond that its simply a matter of finding a wide-open space, getting it up into the air, letting go of the right cyclic stick and observing how it drifts.

The tail rotor will spin the body and cause the model to pitch nose down and drift forward, so if the body is spinning correct that first using the trim switch or sub-trim on the transmitter first, before adjusting the servo links. After the tail is staying in-line tackle any sideways drift by adjusting the aileron servo one 360° turn if the link at a time. Finally after any sideways drift is eliminated trim out and forward/backwards drift. Don't expect it to hover in one spot forever. That's an unrealistic expectation, even for the self-stabilizing Blade models because of the tolerances of the parts.

Don't obsess over whether the swash is level and the blades equal in weight and tracking UNLESS you find for some reason it is impossible to trim the servos and tail motor to achieve hover. If you find you can't trim a new Blade model into a hands-off hover by adjusting the links and trims with the procedures above, and it hasn't been damaged by a crash the problem may be the result of a defective main or tail rotor, or less frequently a defective 5-in-1 board. There is no warranty beyond the fact Horizon says it will function when it leaves the store but it goes out of its way to accommodate new customers who encounter mechanical problems. So if you find the normal steps above are not working call the Horizon support number, explain the problem, and more often than not it will send free replacement parts. In some cases it may ask to have the entire model shipped back for evaluation.

Post-Crash Troubleshooting:

See the DX6i tutorial for guidance on trimming a new model. More likely than not the first few flights will end in crashes so it is important to allow plenty of space when flying the modes for the first time. A don't recommend flying the 120SR in any indoor space smaller than a gym because it moves very fast and needs a lot of room to self-stabilize if over-controlled by the pilot.

If you have previously trimmed the model but trim changes after a crash try the following:

  • mSR and 120SR: The tail spinning around CCW is an indication of insufficient tail motor thrust. Check for bent rotor (try a new spare) and damage to the motor or its wiring. Having a spare tail boom assembly simplifies troubleshooting

  • mSR and 120SR: The tail spinning around CW is an indication of the tail motor overpowering the main motor and is usually an indication that the main motor or its wiring is failing. The main motors have brushes which wear out and you should expect to replace it. Check for bent rotor (try a new spare) and damage to the motor or its wiring.

  • mSR: The mSR has a two-piece swashplate and the top part with the ball-links can pop up of and cause trim to change. An orange tool is supplied for reseating it. Insert the tool as shown in the user manual and while holding the top of the rotor shaft pull up on the bottom part of the swash plate to reseat the top half.

  • mSR: The white drive gear can slip down on main shaft causing vertical movement of the shaft and swash plate which will cause severe handling problems. First check to see that the guide pin for the swashplate is in the vertical guide posts then reseat the gear by supporting the drive shaft and pressing it up until there is no free play. Check due to slippage of the main gear which is just held in place by pressure.

  • mSR Sticking servos: The small gears on the servos of the mSR are easily damaged and if binding will cause the servo link to stick in one place. With the model unplugged move the gear by hand to find any sticking. I've found a thumbnail works well to reshape the gears, but if that does not work replacement gear sets are available. The mSR servos also have a tiny black collar on the top of the lead-screw shaft that can get dislodged or lost in a crash. Make sure it is in place and seated correctly. The spare parts bag that comes with the mSR contains spares.

  • 120SR Sticking servos: The gears of the 120SR are more robust than the mSR but left rear landing skid post sticks up and can damage the servo gear above it in a hard landing. I cut a canopy grommet in half and place it on the landing gear posts in the rear to prevent that from occurring.

  • mSR and 120SR Chattering Servos: The servo links are positioned via feedback from a resistor strip under the moving link. Dirt can get on the strip or under the contacts of the link causing it to hunt in vain for the correct spot, causing the link to chatter. A blast with a can of Dust-Off will usually remedy the situation. If that does not work carefully remove the servo, clean the board under it, and replace it taking care not to damage the contacts on the link. Replacement servos are available but require soldering to replace them.

  • TBE - Toilet Bowl Effect

    The root cause of TBE is periodic harmonic cyclic pitch change in the main rotor. Since the flybar controls the rotor blade feathering it is a likely culprit when TBE occurs after a crash. A bent flybar will affect the phasing of the stabilization. Tightening the blades in the grips can also affect stability. The blades should be loose enough to freely move sideways, which allows them to lead and lag when feathering pitch is changed and bow upwards (dishing) which helps to stabilize a helicopter. If the blades are too tight the rotational forces are not dampened and the body of the helicopter will vibrate. Every moving part on the Blades will eventually wear out with extended use. Too much free-play due to wear on the balls and links which can cause the flybar rotor feathering pitch to change randomly and affect the ability to hover. After a few hundred flights replacing all the parts in the drive train, which costs about $40, will improve performance.

RC Helicopters
Flying Up The Learning Curve

This tutorial is copyrighted by © Charles E. Gardner.
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You can contact me at: Chuck Gardner

For other tutorials see the Tutorial Table of Contents