I recall having read somewhere that “helicopter in forward flight behaves just like a badly trimmed airplane”. So I'd like to ask:
- Do helicopters have trim?
- If so, how does it work?
- And is the helicopter stable when properly trimmed?
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It depends on what you mean by trim.
If you mean a a mechanism that allows the pilot to reduce the control forces in order to allow the helicopter to be flown hands off, then the answer is a qualified yes.
If you mean a trim tab attached to a control surface, then the answer is no. Many airplanes which have a trim control do not use a trim tab. For example, Pipers have an anti-servo tab on the elevator, which the pilot uses to reduce control forces.
Since helicopters are unstable -- meaning if you take your hand off the cyclic the aircraft will be in an unusual attitude within a second -- nothing short of electronics and actuators will allow a heli to be flown hands off.
But every heli except the smallest ones, have considerable stick forces, which make them difficult to impossible to handle without some kind of assistance.
The simplest setup is what you find on the original 4-seat R-44 Astro, which lacked hydraulic assist. On top of the cyclic you will find a 4-way "hat" switch which uses electrical actuators on the cyclic to reduce the control forces necessary to fly it.
Larger helicopters have hydraulic assist, which allows the copter to be controlled with just two fingers. Inop hydraulics is considered an emergency.
Big helicopters have a stability augmentation system, which allows the helicopter to be flown hands off. An SAS is required for IFR flight.
Helicopters with hydraulic actuation of the swash plate usually have an artificial feel spring. The neutral position (where the stick returns to if released) of this feel spring is the trim position. The pilot holds the cyclic at the position where the helicopter has a steady flight path, and can then trim the feel force away by adjusting trim position in two possible ways:
Beep trim. A 4-way china hat switch at the control is deflected by the pilot, resulting in slow continuous adjustment of the neutral spring position. With a stick held in position, the feel spring forces are gradually reduced until the pilot stops deflecting the switch when feel force is zero.
Trim release. A push button that effectively removes all feel spring force: neutral spring position always follows stick position. With trim release ON the stick can easily fall over if friction is low. Pilot places the stick at steady flight path, then switches trim release OFF. The feel spring is now effective again.
Is the helicopter stable when properly trimmed. We need two answers here, for forward flight and for hover.
In forward flight: Directionally, yes. Pitch - often, if the heli has a horizontal tail. Depends on position of horizontal tail relative to rotor downwash, which varies with airspeed and gross weight. Roll - usually not. That's considering aerodynamic stability only, an AFCS with heading & altitude hold would make for a docile trim situation.
In hover: it's all unstable everywhere, like trying to keep your balance when standing on top of a large inflatable ball. "Trimmed" has very little meaning in the hover.
The R-22 helicopter has both cyclic/collective friction, as well as a trim. When you are hovering, you are using a lot of tail rotor to counteract the spinning of the blade, and there isn't any force to counteract it. The tail rotor, while countering your rotation, will also push you in a direction (like a propeller), so in a hover, with a helicopter that has a counter clockwise rotation, the tail rotor will keep pushing you to the right. To counter that, you have to put in left cyclic. To help reduce the effort required to keep it a hover, Robinson put in a trim, which just adds a constant left input to the cyclic. This greatly reduces the workload for the pilot while taxiing and hovering. The amount of trim, in the case of Robinson helicopters, is not pilot adjustable, but it is a trim setting, technically speaking, which is separate from the friction settings, which helicopter pilots can/do use in the same way fixed wing pilots would use a "trim". For example, I've used the cyclic/collective friction settings in the R-22 to tighten up the movements of the cyclic and collective. This makes it much easier to read a navigation map or use the radio. It allows me to take my hands off some of the controls for a whole second or two to allow me to adjust a map or tune the radio :)