Pitch Trim problems S5

[Auster]

It's a standard electric pitch trim on an S5 with just over 600 hours. On several occasion recently, in flight, the position has remained in the nose-up situation, failing to respond to the up-down switch. On the ground the problem disappears, that is the full movement is available.

Not sure if this is relevant, but in the past 6 months the noise from the motor when testing on the ground has increased - that is, it rumbles when travelling into the nose-up position.

Before removing the whole mechanism I will check all connections, especially earthing at the switch.

Has anyone experienced something similar or does anyone have a diagnosis which might help? Thanks in advance.

[PapuaPilot]

Sounds like you are checking everything properly. I had a bad rocker switch and had to replace it.

[Eric Page]

This is a long shot, but the trim actuators in the S5 used brushed motors (maybe they still do; I don't know). It seems unlikely that brushes could be worn out even in the lifetime of the plane, but maybe. Or, perhaps some tarnishing of the commutator from lack of use if this airplane has been sitting. You could try disassembling the motor, cleaning the commutator with a pencil eraser and, if the brushes are worn, replace with new. Warning: be careful; the brush material is soft and fragile.

When I rescued my plane and later tried to test the trim actuator, it wouldn't move. I found that the motor's commutator was black with tarnish. Once cleaned, it worked fine.

BDCM Commutator.jpg
Brushed DC motor commutator

Re the rumbling sound, I'm thinking of three things. The first is easy to check: the linkage at the connection between the actuator and horizontal stab could be worn a little so that it has free play and bounces when moved. The other two would be harder to check: wear or corrosion of a bearing inside the actuator or of the acme nut or leadscrew that moves the actuator's ram.

Linear Actuator.jpg

[Auster]

This is a long shot, but the trim actuators in the S5 used brushed motors (maybe they still do; I don't know). It seems unlikely that brushes could be worn out even in the lifetime of the plane, but maybe. Or, perhaps some tarnishing of the commutator from lack of use if this airplane has been sitting. You could try disassembling the motor, cleaning the commutator with a pencil eraser and, if the brushes are worn, replace with new. Warning: be careful; the brush material is soft and fragile.

When I rescued my plane and later tried to test the trim actuator, it wouldn't move. I found that the motor's commutator was black with tarnish. Once cleaned, it worked fine.

BDCM Commutator.jpg
Brushed DC motor commutator

Re the rumbling sound, I'm thinking of three things. The first is easy to check: the linkage at the connection between the actuator and horizontal stab could be worn a little so that it has free play and bounces when moved. The other two would be harder to check: wear or corrosion of a bearing inside the actuator or of the acme nut or leadscrew that moves the actuator's ram.

Linear Actuator.jpg

Eric thank you that is very comprehensive and helpful. If I establish that the switch and wiring are not the problem then I think I will remove the unit, check the gearbox/drive and if that's working freely I will take it to a shop specialising in electric motors and have them deal with it. It just feels like the motor is not able to overcome the aerodynamic forces in flight - which could be gearbox or motor. Thanks for taking the time. I will post the result.

[Auster]

Eric thank you that is very comprehensive and helpful. If I establish that the switch and wiring are not the problem then I think I will remove the unit, check the gearbox/drive and if that's working freely I will take it to a shop specialising in electric motors and have them deal with it. It just feels like the motor is not able to overcome the aerodynamic forces in flight - which could be gearbox or motor. Thanks for taking the time. I will post the result.

Today I dismantled and cleaned the switch, the contacts were very dirty and could easily have been the problem. However on one final cycle of the trim before taking a test flight the problem occurred again i.e. it stopped. I walked to the tailplane, grasped it and it was solid (usually some movement) then it suddenly moved and the trim seemed to operate normally.

I think I have a gear drive problem and it has been trying to tell me this. On the down run i.e. the leading edge running downwards (nose up trim) the motor and gears buzz smoothly as they always have. On the upwards run, there is a loud rattle which sounds very much like the geared drive.

Next step will be to remove the unit and inspect it. Not looking forward to this as that lower mounting bolt looks terribly inaccessible. I will report back.

[alexM]

Today I dismantled and cleaned the switch, the contacts were very dirty and could easily have been the problem. However on one final cycle of the trim before taking a test flight the problem occurred again i.e. it stopped. I walked to the tailplane, grasped it and it was solid (usually some movement) then it suddenly moved and the trim seemed to operate normally.

I think I have a gear drive problem and it has been trying to tell me this. On the down run i.e. the leading edge running downwards (nose up trim) the motor and gears buzz smoothly as they always have. On the upwards run, there is a loud rattle which sounds very much like the geared drive.

Next step will be to remove the unit and inspect it. Not looking forward to this as that lower mounting bolt looks terribly inaccessible. I will report back.

I practiced reaching that lower bolt several times before I covered my fuselage, and I did my final installation with it covered. It is awkward but totally possible.

[Navycposd]

Buy a digital endoscope on Amazon ... a few mm in diameter, lighted and a camera on the subject! I needed it to get the weights out of my tail so I could see what I was feeling around for. Now that I have it, I find more uses for a skinny flexible camera!

[jiott]

I put one of those round access hole rings in the fabric in line with that trim servo lower bolt. I haven't yet, but when I need to I will cut the center fabric out of it and install one of those (pre-painted) access hole covers.

[Auster]

I put one of those round access hole rings in the fabric in line with that trim servo lower bolt. I haven't yet, but when I need to I will cut the center fabric out of it and install one of those (pre-painted) access hole covers.

Jim, my access hole is not really in line with the bolt but at least I can see the nut and should be able to get a socket on it. Will tackle this later in the week.

[Auster]

After numerous ground tests it finally failed, jamming completely. Obviously that rattle/rumble in the unit signified something loose which would jam in flight, but not on the ground without aerodynamic loads.

The unit is now removed and I've written to Kitfox for a new actuator. Whatever the problem with the old, it won't be repaired and re-installed as it has done 600 hours.

If the factory does not have one in stock (they're not in the catalogue) and there's someone reading this who has one sitting on the shelf - a Motion activator for Kitfox S5, 6 or 7, - and wants to sell, I would be very happy. They must be a specific Kitfox item because of the tab welded on to the actuator rod in the correct position for the scissor link.

I want to be back in the air asap as the summer is slipping away and I have some long Kitfox trips planned. Thanks in advance and thanks for all the helpful advice so far.