Thorp Air Command - T18.net

I installed electric elevator trim ala Bill and Bob (grainger motor, limit switches). For those that have electric elevator trim installed, did yall also install a trim position indicator in your panel? If so, how did you wire it into your trim system?

I find that I would like a position indicator, especially when preparing for takeoff.

Most of us have found that you don't need one due to the light controls of the Thorp. Take off trim can vary widely with cg such as fuel state and load. No one setting will be correct for all situations. I think you will find a trim indicator virtually useless.

_________________
Bob Highley
N711SH
SN 835
KLAL

Ryan : Go to Trim Systems on the main forum page and then hit horizontal trim system and there a whole pile of stuff to look at regarding trim position for T/O .

RB

Hi Ryan,

I've been working on a new panel for my Thorp and I think I've worked out a simple and light way to display trim position on my Dynon EFIS and EMS. I agree that trim position on the Thorp isn't absolutely necessary, it is however the only plane I have flown in the past 17,000 hours that didn't have a trim position indicator of some sort in the cockpit. I think it will be worth including.

I purchased a Space Age Control string potentiometer P/N 173-0241. The potentiometer, mount, and wiring, will add about 2 oz to the airframe. Here's a specification sheet for the potentiometer I purchased. I found a vendor that had a new supply on eBay for $10 each.

http://www.servocontrolsindia.com/wp-co ... r/170x.pdf

Here's a photo of the potentiometer I'm using. The cable will attach to the aluminum arm that contacts the limit switches on the trim drive.





Marshall

well, I guess I should just keep flying and see if I still think i need the position indicator at next annual. Thanks for the feedback.

I agree with Bob, & others who don't think a trim indicator is necessary on the Thorp. That being said, I think that is a really neat way to do it Marshall. Thanks for sharing it!

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Agreed. I wasted a lot of time on an indicator because I thought "all airplanes have them."

Just run trim full nose up and then run it back down a second or two and go flying.

Focus on more important things.

D

I concur with Dick; I end up with near full aft trim after landing so I run it forward about 4-5 seconds or so on the roll if I'm going up again or taxiing back to the hangar, either way for me about 4-5 seconds forward trim from fully aft works 100% of the time...
I have an indicator but its useless and I would have to get my head in the cockpit to use it even if it worked.

Taking off with a full tank of fuel w/or without a pax , w/or without baggage I use the same trim setting on my LED indicator . Landing with full flaps (90%+ of the time) I end up with approx. the same trim setting I had for T/O . I vary the trim setting for take off only slightly with less than a full tank . About 1/2 a bar on the indicator . The trim setting is one of the last items on my before take off check list to avoid the OH CRAP (or words to that effect) at rotation ! BOO YAH !

RB

I think a pitch trim indicator can add value. Here's why:

I had an interesting flight a few weeks ago, destination San Luis Obispo- about 200 miles to the south. The takeoff felt a bit different, and during the climb, I noticed I could not get enough down trim- the plane was not responding to the electric pitch trim commands. For a while, I thought maybe I could still make the trip- I could sort this out once in cruise. But that wasn't the case. The trim motor just wasn't affecting the trim- As speed increased, the forward stick force became considerable. Very considerable!! The autopilot couldn't handle it (I tried.) So I had to pull power back close to idle, and turn back to home base. It was a pretty uncomfortable, albeit brief flight. The upside, is this was a daytime vfr flight. Had it been a takoff into IMC... ugh. My airplane is 180 hp, constant speed. Maybe it's a more manageable problem with smaller engines and fixed pitch props. But still...

It turned out the pitch trim motor was open-circuit. I took it apart, re-worked the brushes, put it back together, and it seems okay. I have a few flights on it after the repair, and it's back to normal operation. Had a trim gauge been in the panel, I would have noticed the trim was not moving- and was stuck in the full up position that I had used for the last landing.

Before takeoff, to set the pitch trim, I used the technique others have described- run the trim fully one direction until it stops, and then the opposite way for X number of seconds. In my case, the full range takes about 14 seconds. So, about 7 seconds from either stop is in the ball park for takeoff. As long as the trim motor is spinning, and everything is connected. Without a trim indicator, to verify the electric trim is functioning requires rotating your neck a bit beyond what is comfortable for this aging aviator. Then you can hold approximately neutral elevator as the trim is actuated, and watch the tab move. (Hopefully, it moves.) I had stopped watching for the tab movement, as my neck couldn't take it! Instead, I relied on trust in the trim switch, motor, and everything related. It had always worked. Until it didn't.

So, I think it makes a lot of sense to install some sort of indicator, and I did just that. For not much money, I found a marine stern drive trim gauge, and a compatible sending unit. Fitting the sending unit in the tail, and connecting it to the trim drive was a bit of a challenge- there is only a small inspection plate for access. But it was doable.

Now I use the same count method for setting the trim before takeoff, and IMPORTANTLY, can confirm the action via the gauge. So, once again, life is good (very good!) with my Thorp!

Peter

Interesting with the stern drive indicator.

If the important thing to detect is whether the trim is working. Would a simple on/off light for energizing the trim circuit be used. Sure you would not know the position. But you could still determine that from full position and count in the opposite direction. If the light went on during those two directions, then you would have a reasonable assurance that the trim was moving, and might be a simpler installation. Or even two lights if you could get one light to turn on for nose up, and another light for nose down.

I admit I know not even enough to be dangerous when it comes to anything electrical.

The problem with just indicator lights wired into the trim circuit, is that you will only know if the circuit is being energized. You still have no confirmation the trim motor is actually running. In my case, the circuit was fine- the trim motor relay was energized, and sending voltage to the trim motor. But the motor was open-circuit.

By physically connecting a variable resistor (in my case, the boat stern drive trim sending unit) to the pitch mechanism, I'll see the needle move on the gauge, which affirms the pitch trim is actually changing.

Lights for the trim motor might still be a good idea- they could warn of a stuck trim switch or relay, if nothing else.

From the peanut gallery -- electric trim, no indicator in N118BC. I use the full-up followed by 2-3 nose-down before takeoff method, and it works great. Until, to quote @peterjaypriest above, it didn't. Here's what happened:

I used the aforementioned method, assuming all was well. Took off but had to use forward pressure on the stick to keep from pointing the nose to the sun, more and more as the aeroplane accelerated. I kept trimming nose-down using the switch, but no joy. Strange because before takeoff, I always run the switch and listen for motor noise, which was present, but nonetheless unable to get it off full nose-up trim.

Landed, pulled it apart with my A&P/IA, found one of the roll pins in the universal joint had failed. So she spun round and round with no effect on the physical trim where you need it.

Ergo, per discussion above, lights telling you that you are energizing the motor would have fooled me because I was energizing the motor just fine. Better lights (or more likely "light" (singular)) would be a full-up light, meaning the "full up" limit switch has been activated.

Then the pre-takeoff check is:

1. Full nose-up trim via the switch -- confirm full nose-up light ON.
2. Trim down and count the seconds, roughly -- confirm full nose-up light goes OFF within "x" seconds.
3. Take off and FLY...or repeat 1. and 2. a time or two for confirmation, if desired...THEN fly.

_________________
Philip Mandel
N118BC
S/N 957
Beaverton OR

Just remember, the neutral or slightly nose down position varies with each cg situation. There is no one perfect take off trim position.

_________________
Bob Highley
N711SH
SN 835
KLAL