can anyone give a detailed explanation on the cyclic control stick plot?

[Sigmund alcover]

http://i118.photobucket.com/albums/o95/ichris7/Scan-1-1.jpg

i know the use of the trim button on the uh-1 helicopter but i just get confused why i heard that i have to "press the trim release and fly the aircraft to a new altitude and then release, the new altitude will then be held."

how do you fly an aircraft to a higher altitude when you depress your trim release button?(on a uh-1 huey) i thought that the trim release button is used to relieve the force on the cyclic?

[John Eacott]

i know the use of the trim button on the uh-1 helicopter but i just get confused why i heard that i have to "press the trim release and fly the aircraft to a new altitude and then release, the new altitude will then be held."

how do you fly an aircraft to a higher altitude when you depress your trim release button?(on a uh-1 huey) i thought that the trim release button is used to relieve the force on the cyclic?

That should be 'attitude', not altitude.

[Ascend Charlie]

It isn't "altitude", it is ATTITUDE. Dang, John beat me by a minute.

And it doesn't even hold the attitude, it just holds the cyclic in one spot - the attitude can change but the cyclic will not correct itself - unless it is fitted with an autopilot.

The cyclic plot shows that at low speeds, there is a bit of left cyclic needed to overcome inflow roll, and more forward stick is needed to maintain speed against flapback.

[Sigmund alcover]

oh yeah, i totally got lost. i was saying altitude because i and my friend were talking about the autopilot. in some helicopter the trim release will also reset any coupled flight director datums, to those occuring at the time of re-engaging the trim. For example, if i have altitude hold selected, if i press the trim release and fly the aircraft to a new altitude and then release, the new altitude will then be held. but some helicopters return to what the original settings were. thanks for the time! i got it

[Shackman]

I vaguely remember - in the dim and distant past - having to fly an air test to confirm the stick plot after someone else complained 'it didn't feel right'. I think it was in a Whirlwind (but could have been a Wessex) and someone 'downstairs' had a calibrated measuring stick to check the position at different speeds.

PS- They were quite right, it didn't feel right, but I can't remember why (rigging?)

[Ascend Charlie]

For example, if i have altitude hold selected, if i press the trim release and fly the aircraft to a new altitude and then release, the new altitude will then be held. but some helicopters return to what the original settings were.

If the "Altitude Hold" caption is still lit, and you press the force trim and fly to a new altitude, when you release the trim, the machine will head back to the original altitude. Same with heading, if you don't turn by using the heading bug, it will go back to the bug.

[Paul Cantrell]

i know the use of the trim button on the uh-1 helicopter but i just get confused why i heard that i have to "press the trim release and fly the aircraft to a new altitude and then release, the new altitude will then be held."

how do you fly an aircraft to a higher altitude when you depress your trim release button?(on a uh-1 huey) i thought that the trim release button is used to relieve the force on the cyclic?


As John and AC mentioned, we're talking ATTITUDE, not altitude. That appears to be a typo on your part or whatever you were reading from. I believe your setup is probably similar to the IFR setup on the 206L I fly. The trim release is not actually part of the autopilot, it is simply a cyclic centering device.

The cyclic is held in position by springs and magnetic brakes. In order to move the cyclic, you have to hit the trim release button on the cyclic, move the cyclic to change the attitude of the aircraft, and then let go of the button. All the system does is basically hold the cyclic for you. It is not part of the autopilot system.

I hate this system (but it's required for IFR). You can actually move the cyclic enough even with the magnetic brakes engaged to do small attitude changes. However this causes a problem when you reach the limit of the play in the cyclic and you want to go a little further. You're holding a little force at that point, so when you hit the trim release button the cyclic jumps a little, and everybody on board the aircraft can feel it. So the alternative (and what I generally do) is to hit the button every time I move the stick which is basically all the time trying to hold a perfect attitude.

Meanwhile you also have the coolie hat trim on the cyclic. This is a totally different system, and IS part of the autopilot. It allows you to use the coolie hat to set the attitude, and then the autopilot will hold it. This system is very smooth and works well IMC. However, most of the time if I'm going to turn on the autopilot, I just put it in heading or nav mode with altitude hold.

In altitude hold mode, anytime you pitch the aircraft with the coolie hat, it will cancel the altitude hold mode on the autopilot. You pitch to climb or descend, and when you reach your new altitude you can then hit altitude hold again to resume the autopilot altitude hold mode.

[chevvron]

Stir the pudding.

[212man]

If the "Altitude Hold" caption is still lit, and you press the force trim and fly to a new altitude

In altitude hold mode, anytime you pitch the aircraft with the coolie hat, it will cancel the altitude hold mode on the autopilot

Depends on the autopilot, and whether you are in 3-axis or 4-axis I'd suggest, if these are meant to be generic statements.

[Ascend Charlie]

The trim release is not actually part of the autopilot, it is simply a cyclic centering device.

BONG! WRONG!

The autopilot moves the cyclic via the force trim system - if force trim is not engaged, the autopilot is feeding inputs to the force trim, but if it is not connected to the cyclic, nothing will happen.

Force trim without autopilot is simply "stick position hold", autopilot without force trim is just using up its computing power, but put the 2 together and the machine responds to autopilot inputs.

[[email protected]]

I think what Paul is describing is a simple SAS system that has a trim system to stop the cyclic flopping over and to give a datum to work from as well as feedback for the pilot when pushing against the springs - all the AP is giving is rate damping rather than attitude hold.

What AC is describing is a more modern and complex autopilot where the trim system is an integral part of the AP so that when the series actuators reach their limit of authority, the parallel actuators in the trim system can move the controls (open loop) to redatum the series actuators, thus enabling attitude hold and other higher AP functions.