Helicopter Cyclic Trim

Flying a helicopter with a cyclic trim is very simple provided a person understands the system and follows a couple of basic principles. One of the most basic forms of a cyclic trim system is simply two electrically driven motors that are activated by a switch located on the grip of the cyclic pitch lever. This switch is commonly known as a “Chinese hat” and is designed to be manipulated by the pilot’s thumb. The Chinese hat can only move forward, aft, left and right so in other words no diagonal movement is possible. The switch is self centering and a very important aspect to remember is to never activate the switch whilst the cyclic friction controls are still on.

When manipulating the cyclic trim switch powers up either one of the two motors. One motor positions the cyclic control in the fore and aft position (pitching plane) and the other motor positions the cyclic control left and right the rolling plane.

For the sake of clarity and for our unfortunate friends that fly fixed wing aircraft it must be noted that no trimming is taking place on the blades itself unlike with fixed wings where the trimming takes place via tabs on the control surfaces. So the cyclic trim system in a helicopter simply positions the cyclic control in a desired position therefore alleviating forces on the stick but it does not mean that the helicopter can be flown hands off as that requires an autopilot system which is far more advanced than the basic trim system.

The basic principle of flying with a trim system is to make a change then check, adjust and trim. For example if the speed of the helicopter needs to be increased from say 60 knots to 80 knots the first action will be to make an attitude change with the cyclic control alone by selecting a new attitude outside. This change of attitude has to be checked at some stage when the pilot perceives the attitude for the new speed has been attained. After the airspeed indicator has settled minor adjustments can now be made to the attitude to refine the actual speed that needs to be flown. Only now does the trim system come into play by manipulating the Chinese hat until no forces can be felt on the cyclic control. In other words the trim system should not be used to make the initial attitude change.

Most small helicopters with this form of basic trim system can be flown without using the trim but obviously a lot of forces will be felt on the cyclic control depending on the phase of flight. The trim system can be a pilot’s biggest friend but if used incorrectly can be a worst enemy in so far as a person can trim the helicopter completely out of a desired attitude for example.

As said before the aforementioned dealt with a basic trim system as there are more advance systems for more sophisticated helicopters that incorporates Stability Augmentation Systems, Helipilots, Force Trim and Autopilots for example but the method of flying these systems apart from an autopilot in essence remains the same.

© Harry Helicopter Training Syllabus 2008

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