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Flap board ?

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Yes, you should have it. I don’t have a pic, but you can follow the wires from the first circuit board to the board on the inside of the firewall. You’ll have to go through the same opening that you opened for the first circuit board. It’s kinda tough to get back to it. Mine had a bunch of wires in front of it, but moving the wires I could see it through the opening.

Its close to the size of the primary circuit board, but you’ll see 2 larger boxes on it (relays).

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The board on the firewall looks like this.  Your board won't have the carbon fiber backing panel, it will be attached to the cabin side of the firewall with a couple of screws and rivnuts.  To see it, remove the lower center panel (the one with the flap switch) and look straight forward.  (my plane is a 2006 CTsw).


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Tom's observation is correct.  I had intermittent flaps.  I removed the board (and had one of the rivnuts holding it to the firewall come loose, hence the new mounting plate) and found a broken soldered connection at one of the relays (common problem).  I cleaned the laquer off the board at the failed solder joint and re-soldered.  Worked for a few more years. 

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23 minutes ago, Vic said:

is there any logic on this board or is it simple relays and capacitors.  For those of use planning on future flap failures and parts collecting :)

This board takes power from the aircraft, and sends it to the motor via the relays. These relays are controlled by a low power signal coming off of the flap logic board (i previously called this a flap controller) which are routed through the limiting microswitches on the actuator column. These microswitches are normally closed. If they are pressed, the circuit opens and the control signal feeding the relay board is cut.

As long as there is a control signal, the relays switch into position to send power one way or the other. They are designed so that it's not possible to inadvertently short out, and the small rheostat on it is supposed to be an overcurrent cutout (that never seems to work very well).

If you wanted to use your own controller, you would need 8 wires to consider:

  • 3 wires to the potentiometer to sense position (it is configured as a voltage divider)
  • 3 wires with one common to control up and down movement signal to flap relay board
  • 2 for powering your board

Collectively, the parts for the old flap control layout are more expensive than the new flap logic board which integrates everything into one board. The new flap logic board is much more reliable too, with a proper functioning overcurrent system that works well even in cold weather. However when the new one fails, it's just so rediculously expensive, over 2k last time I had checked. Each component of the old flap system is around 1k when they were still being sold (no longer available).

Honestly, I would have placed the relay control somewhere else. The firewall vibrates too much. On my aircraft I installed rubber isolators to help with the issue.

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Cessna's design is actually pretty clever, it's entirely mechanical but you still get the automation of setting a flap position and it will seek it.

To be flat out honest, what I would do is use off the shelf replaceable parts to replace the flap system but keep the general design idea. The actuators don't seem to have a very long life in terms of cycles so I'd put something else in place of that. This is the biggest issue.

But as for flap control itself: an arduino board, a couple of relays, a 1 pole 7 throw rotary switch, and a display is all you need for your own system (actually the rotary that we already have is a very common off the shelf item, you just need to have a way to mount it). An arduino board that has a watchdog is especially recommended in case it freezes, it will power cycle itself.

The potentiometer is a weak point and I don't have a clean answer for that yet. Maybe just a higher quality pot that is vibration rated is all that is needed (I installed one and are waiting to see how long it lasts and how well it retains its positioning). But if not, I've toyed with the idea of a hall effect position sensor system.

The old flap control boards are *very* simple and just use a bunch of off the shelf parts. Might even be able to dump the eeprom memory to see what it's doing if I had the interest, but I'd rather just replace it with a different PLC that costs a few dollars to swap out the day mine goes out.

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I don't know why flap control has to be reinvented here, there are inexpensive commercial solutions available from Aircraft Spruce and other sources that would eliminate all of the FD electronics except for the limit switches and linear actuator, itself.  See, for one example,   https://www.aircraftspruce.com/catalog/elpages/electronicsflapcontroller.php (look at the PDF instructions that are available with a link on that page). Programmable to four user-set flap positions.  Under $300.  There are several other such solutions on the market.

Even simpler solutions could be designed that would use (i) a double pole momentary-on switch that can handle the current required by the linear actuator, (ii) the existing limit switches, and (iii) a simple visual indicator on the flap hinge or an electronic indicator using common position sensors (even the one by Ray Allen).

Corey, my actuator was making noise and needed to be disassembled, lubricated, and returned to service after >4000 trips around the pattern (yes, I have more than 4000 landings on my plane).  And, I fly in 15 degreeF weather.  Maybe I got a good actuator.


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Fred I probably have 20,000 landings on my aircraft and that's a conservative number. It was used as a trainer.

The current actuator pisses me off because it was a replacement after the last one failed, but also after I pulled it out of being a trainer.

I have replaced probably 3 over the lifespan of this aircraft, untold numbers of lubrications, and several more on others.

They motors are pieces of dogshit. Forgive my french but that's always the part that fails. The rest of the actuator always looks to be in fantastic condition and never had issues with it. It's always the motor.

The manufacturer of this actuator is well known in Germany, according to a friend over there who works in industrial controls. When I sent him pictures he IDed them right away and said the motors specifically are chinese imports but he doesn't know the model, it would like looking for a specific needle in a needle stack.

As for ordering replacements from the manufacturer, they refuse to do business with end users.

As for re-engineering the flap system: I like modding things to make them last and last. If that's not possible or prohibitively expensive, then I want them to be easily serviceable with parts that have prices that reflect their value. I am sick and tired of cheap, shitty parts being used in everything (especially some light sport aircraft) and you will find a lot of things I own are custom built or customized for minimal maintenance and long lasting.

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Tracked by tach, somewhere around 4k hours. Hobbs will be a good bit higher.

Typo though, it should have said 20,000. 2 and 5 are next to each other.

I really feel like I'm estimating way low. I've got 400 alone, and for all the years it flew, around 95% of it was primary training and it wasn't uncommon to see it flying 600 HOBBS hours a year when it wasn't down for maintenance after someone banging something up (which was all the damn time and I finally got tired of it).

it wasn't uncommon to pull off close to 10 landings in an hour with a student, so 2-3 landings per hobbs hour average wouldn't be unreasonable. So maybe 25-30k?

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The screws holding the flap relay board to the firewall on my 2006 CTsw were screwed into rivnuts attached to the firewall.  One of mine spun.  The only way I could get the screw out was by using pliers to hold the rivnut on the engine side of the firewall and then turning the screw.  It was a pain.  To reinstall, I made a piece of carbon fiber plate just a bit bigger than the relay board and installed rivnuts on it.  I then glued the plate to the inside of the firewall (with RTV silicone rubber) and attached the board to it.

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