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Cyclic trim


r22butters
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I've read (old forum post) that in the R22, the "right-trim" knob is to counter dissymetry of lift, but what about helicopters who have that "Chinese-hat" cyclic trim.

 

Since it moves in other directions, does it counter other forces? (in the Enstrom the CFI had me using it to turn my way around the pattern)

:huh:

 

Also, what's happening mechanically when you pull that knob in the R22?

:unsure:

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Been awhile since I've flown the 22 but if I remember right I think it's because the 22 is set up so that centered (or neutral) cyclic gives a left input to the main rotor to help compensate for translating tendency (drift) and the trim knob takes that correction out. I'm sure someone can correct me or elaborate better.

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In the H300 it only moves left and right for trim

 

Not true. It moves forward and aft too, just like the 500. Not sure about the 22 question though as I have never really flown one.

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In the 22 it's literally a bungie cord that when tensioned pulls forward on the cyclic to relieve some of the required forward pressure the pilot needs to apply during flight.

 

With the frictions barely applied and the rotors stopped, when the trim knob is pulled you should see the cyclic go forward

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I've read (old forum post) that in the R22, the "right-trim" knob is to counter dissymetry of lift, but what about helicopters who have that "Chinese-hat" cyclic trim.

 

Since it moves in other directions, does it counter other forces? (in the Enstrom the CFI had me using it to turn my way around the pattern)

:huh:

 

Also, what's happening mechanically when you pull that knob in the R22?

:unsure:

The main function of the trim system is to allow flight loads to be trimmed out. Thereby, relieving the pilot from holding increased cyclic pressures especially in cruise flight.

 

The aircrafts you're asking about (like the R22 and S300C) have no hydraulic irreversible control boost (servos) to isolate the pilot from the aerodynamic forces of the main rotor. Therefore, the forces resulting mainly from dissymmetry of lift and blowback are fed back into the cyclic channel. Without a trim system the pilot would have to apply increasing cyclic pressures from the point of hover to cruise flight. Once at the desired cruise speed the pilot, in most cases would need to hold constant forward and left cyclic pressures. These forces are manageable; however, not desirable most of all on long flights. Note the required cyclic control shift in the attached figure.

 

The R22 bungee trim system is a simple setup with limited trim range. Mechanically functioning basically as stated in Pohi's post. The S300C and MD 500 have electrical trim motors allowing full two-axis adjustment, longitudinal and lateral. The MD 500's also have a self-contained hydraulic unit, One-Way Lock, in the longitudinal channel that prevents aft rotor system feedback forces (blowback) from moving the cyclic aft. Trim systems were installed to reduce pilot workload.

 

Scan-1-1.jpg

Edited by iChris
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As iC says there is trim fore & aft + lateral on both 300 & all 500.

Trim runaway was one thing I was taught, comes as a nasty suprise, as stick forces multiply,

Makes you remembert where the trim breaker is though. :blink:

The FI changed the switch on the 300 to the other seat and held reght trim till stop it grabed my attention real fast, then he trimed full back the combination made me sweat, was a leson ?I did not forget.

On the 500s you need serious mucles to counteract it.

I have noticed that a lot of pilots leave trim in, was taught to center stick on run down

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