Mechanical trim indicator

[AK Mongo]

OK fellow builders. Mongo needs some help.

I am building a generic single engine Cessna pit. (Mostly 172, but with prop controls, and a 185 style flap handle, instead of the dash mounted switch.)

I have procured a cessna trim wheel from ebay. I know how I am going to do the mechanical interface to a pot to make it work in FSX, but I really want to have the needle position indicator functional in the pedestal too.

Anyone have any ideas how this works in a real cessna, or has anyone done this would be willing to share what their solution was?

Thanks in advance....

[No Longer Active]

If you look at the simkits trim wheel.



The indicator is attached to the actual wheel itself. The trim wheel here only has 5 positions which is UP, POS 2, TAKEOFF, POS 3 & DOWN.

Just connect the wheel to a 5 position encoder.

Many builders however would use LED's as a position indicator, others use a servo to move the indicator up and down (just like the simkits flap switch indicator).

Don't use a potentiometer as it will be too sensitive, use an encoder (believe me on this one! I know!)!

Good luck

[Ronson2k9]

You have to gear the rotation down. This will give you the correct indicator location and pot position. In the real Cessna it's 8 rotations from top to bottom I believe. So that's 8 times around to move it from full trim up to full trim down. Check out this site on a fellow Cessna 58 simpit builder Beech Baron Rudder Trim.

If you look at the trim wheel you have you'll notice the spiraling slots on the side. This is used to move the trim actuator.

Number of slots = Number of turns to move trim from top to bottom (Left to Right for Rudder Trim)...

Cessna 180 Trim Wheel


I'm working on a drawing showing the gearing for both the pot and the indicator. It's a bit of clock movement somewhat but not to bad. The indicator has to move to the limits of the space for it so it's geared as well and therefore it's a different gearing then that of the pot.

The pot is a pretty obvious 8 to 1.

So that's number of teeth on the wheel sprocket x 8 for the pot sprocket

The indicator depending on it's sweep is about 6 - 8 times that high as it's only moving about 1/6 or 1/8 turn to show you where the trim is at. So about 48 or 56 to 1. This could be attached to another 'wheel' small piece of black painted ply with a needle being made from the tin of a veggie or pop can painted white. The indicator wheel will need to be reversed at least once as it's movement is in the same direction as the wheel itself. This will be an advantage though. A physical brake could be inserted into the indicator wheel to stop the movement.

Now you may want to gear the movement also so that you use of the pot fully (to it's maximum rotation). I'm not that much of a mechanical engineer so I'm still working on how to integrate that into the math hehe..

One other note - Electric Trim.

If you are building an electric trim into the mix. It can be done with a DC motor. I would add this onto the trim wheel with the same number of teeth as the pot sprocket. A DC motor would run off a rectifier and passed through pair of push buttons.. Guess where you would find that...??? Model Railroad shops.


You can get a cheap DC transformer and just wire and reverse wire a DC motor from the track leads on the transformer to the motor. No need for fancy servos in this one.. You can do both the Stabilizer and Rudder trim this way. In the Cessna the Electric trim works like a remote control moving both the wheel and indicator. I don't know about the actual speed so can use the MR transformer throttle (rheostat) to adjust the speed of electric trim movement.

Now... As if that wasn't enough..hehe

A third level of complexity. Guess what happens if you add Auto Leveling or Autopilot? The aircraft (auto leveling) or AP actuates the trims to keep the plane level/on course. So.. You will need feed back to the sim from the trim wheel positions and then a way to activate the electric trim from sim to move them should they require adjustments during AP usage. This will require a bit of thought as it's a indicated position that the computer needs to move to a new position. I'm sure it can be done though.

Doh.. Just thought of it.. The POT on the trim wheel can pull double duty on this one. It's indicated position will tell the computer where the trim is. I don't know if there is an actual movement of the trims - IN FSX. That is if there is an indicated trim position during AP operation? Or more to the point if there is a variable in FSUIPC for that? If not this could be a mute point. It could be pretty cool though. Perhaps the lads over on the heavies could have some input on this?

BTW Alex.. Don't know if I showed you this before but?? There is this device made for manual flap controls to make flying much easier.. Seems a bit of a bother to make a manual flap johnson rod then add another rod to make it work easier but this would make the aircraft prototypical and yet easy to fly.. It's the EZ Flap. I'm thinking one can be easily made out of a wooden plunger or broom handle and bracket + U bolt. This was one of the sticking points in my build as I was going ... I have to move this to move the flaps?? This EZ-Flap saved me from a non prototypical or plane change situation. I haven't made the drawings for the floor mounted controls yet but I know this will make it much easier in any case.

[No Longer Active]

Hi Ron....

My flaps gauge is a 5 position encoder that is wired with 3 LED's. Up position doesn't have an LED. Position 2,3 and 4 has the LED's. My friend on here sent it to me, the wiring looks complicated so hard to explain.....

EZ Flaps gauge would look abit odd in my pit as it is desktop based!

Kind Regards,

Alex

[Ronson2k9]

Sorry bout that Alex.. It's Mongo that is need of the manual flap handle extension perhaps?

Oh and on the 185 the rudder trim is above the flap handle. It has a speed handle attached although I don't know why.. Emergency Trim hehe.. Anyway that's why I want to make it electric as it would be even hard to make that adjustment. Seems that if your leaning forward to made a rudder trim adjustment you would naturally want to move your foot (rudder peddle ?) making that trim even harder to make. Like picking up a CD that had fallen on the floor of your car while driving.

[AK Mongo]

Thanks gents....

Couple of thoughts:

Alex--Are you saying that the TRC trim wheel is only a 5 pos encoder? That surprises me, because it appears like realistic function is important to them. For the amount of money they charge, I would be very upset if that was how it worked had I purchased one. Attaching to a pot is not at all hard to do, if you use some simple gearing. Gears can be as simple as wooden disks with the proper diameters that are connected with heavy rubber rings/string/chain as a drive train.
That kind of stuff is much easier for me to understand than sioc programming, so that is the route I will use. Will report back on pitfalls and progress.

Ronson-- I have noticed the spiral slots in the wheel, and figured it had something to do with the indicator, but am really curious how Cessna interfaces with it. That is what I would like to do eventually, even if it is in "Trim Wheel 2.0".

Not sure it would be that hard to trim via a standard wheel for Rudder, as the goal is Neutral controls. Neutralize your rudder, trim to coordinated flight.... Since I will not be building an enclosure, I will be taking liberties with my console to allow the flaps handle to be accessible. I can make it a little higher than floor mounted, and so will not need an EZ flap. Would be handy with a more realistic pit/enclosure though.

[Ronson2k9]

I know the 185 I'm building has the 5 things on it.

Trims - Rudder and Stabilizer
Water Rudder and Emergency Gear Pump
Flaps.

The later 185's have the cowl flap on there too. So that's 6.

I don't plan on an enclosure but need to have the controls on there and working. One of the most complex parts of the build. However it's mostly mechanical so not to much electronics.

The spiral does move the indicator and the cog/gear moves the control surface. The maximum movement though is in the indicator.

You should grab one of the free Parts / POH manuals from Free Cessna Manuals Especially the parts manuals. They will show you what all the parts do and what they look like. Many of the components are in exploded view. Even though your pit is generic you'll find that many Cessna's are built from the same components.

[AK Mongo]

Thanks for the link Ronson. It appears to be a very simple mechanical arm that slides in the spiral grooves to move the indicator. Should not be too hard to replicate.

[iwik]

Hi Ronson,
Thanks for the link to free Cessna manuals,im building a C177RG and so will be very helpful.
Les

[kermit]

Hi les, i have 177 rg panels for sale, search for underpanels
with some orginal brakers